Evaluating Generator Mechanical and Electrical Specifications

Engineering Encyclopedia Saudi Aramco DeskTop Standards

EVALUATING GENERATOR MECHANICAL AND ELECTRICAL SPECIFICATIONS

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical File Reference: EEX-215.04

For additional information on this subject, contact PEDD Coordinator on 874-6556

Engineering Encyclopedia

Generators Evaluating Generator Mechanical and Elec tr ic al Spec if ic ations

CONTENT

PAGE

INTRODUCTION ............................................................................................................. 6 FACTORS AFFECTING GENERATOR SELECTION ..................................................... 8 Standards ............................................................................................................. 8 NEMA MG-1............................................................................................... 8 17-SAMSS-510 .......................................................................................... 8 SAES-P-114, Chapter 5 ............................................................................. 9 ANSI/IEEE Standard C37.101-1985 ........................................................ 10 ANSI/IEEE Guide C37.102-1987 ............................................................. 10 Types of Systems ............................................................................................... 10 Standby Systems ..................................................................................... 10 Primary Supply......................................................................................... 12 Operational Parameters...................................................................................... 13 Rated Voltage .......................................................................................... 13 Rated Frequency...................................................................................... 14 Rated Speed ............................................................................................ 15 Voltage and Frequency Variations During Operation.............................. 17 Overspeed Rating .................................................................................... 18 Overload Capability.................................................................................. 19 Short Circuit Withstand ............................................................................ 19 Unbalanced Capabilities .......................................................................... 21 Site and Environmental Conditions..................................................................... 22 Dirt ........................................................................................................... 22 Ambient Temperature .............................................................................. 23 Humidity ................................................................................................... 24 Elevation .................................................................................................. 24

Saudi Aramco DeskTop Standards

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SELECTING GENERATOR TECHNICAL CONSTRUCTION REQUIREMENTS ......... 25 Environmental Protection and Enclosure Types ................................................. 25 Open, Drip-Proof, Weather-Protected Types I & II ................................. 25 Totally Enclosed....................................................................................... 27 Stator .................................................................................................................. 33 Mush Wound............................................................................................ 33 Form Wound ............................................................................................ 35 Rotor ................................................................................................................... 37 Laminated-Type ....................................................................................... 37 Solid-Type................................................................................................ 38 Bearings.............................................................................................................. 40 Sleeve ...................................................................................................... 40 Anti-Friction .............................................................................................. 42 Insulation Class and Temperature Rise.............................................................. 45 Exciter Types ...................................................................................................... 47 Direct Current........................................................................................... 47 Alternating Current ................................................................................... 47 Automatic Voltage Regulator .............................................................................. 49 Field Excitation for Exciters...................................................................... 49 Direct alternator excitation ....................................................................... 51 GENERATOR MINIMUM PROTECTION REQUIREMENTS ........................................ 53 Introduction ......................................................................................................... 53 Electrical Protection ............................................................................................ 53 Overload Protection ................................................................................. 53 Phase Fault Protection............................................................................. 57 Ground Fault Protection ........................................................................... 59


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Instrumentation and Alarms................................................................................ 61 Small Generators ..................................................................................... 61 Large Generators..................................................................................... 62 WORK AID 1: RESOURCES USED TO DETERMINE IF GENERATOR TECHNICAL CONSTRUCTION SPECIFICATIONS ARE CORRECT............................................................................................. 63 Work Aid 1A: NEMA MG-1 ................................................................................ 63 Work Aid 1B: 17-SAMSS-510............................................................................ 67 Work Aid 1C: Applicable Evaluation Procedures............................................... 67 WORK AID 2: RESOURCES USED TO SELECT GENERATOR MINIMUM PROTECTION REQUIREMENTS................ 71 Work Aid 2A: ANSI/IEEE Standard C37.101-1985............................................ 71 Work Aid 2B: ANSI/IEEE Standard C37.102-1987............................................ 73 Work Aid 2C: SAES-P-114 (25 APR 94), Chapter 5.......................................... 75 Work Aid 2D: 17-SAMSS-510............................................................................ 75 Work Aid 2E: Applicable Selection Procedures ................................................. 75 GLOSSARY................................................................................................................... 85

List of Figures

Figure 1. Single Unit Stand-by Generator.....................................................................11 Figure 2. Multi- Unit Stand-by Generators ....................................................................11 Figure 3. Synchronous Generator Voltage Ratings ......................................................14 Figure 4. Speed Ratings for 60 Hertz Generators ........................................................16 Figure 5. Overspeed Capabilities ..................................................................................18 Figure 6. Maximum Overload Current Capabilities........................................................19


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Figure 7. Maximum Short Circuit Capabilities................................................................20 Figure 8. Maximum Unbalanced Capabilities ................................................................21 Figure 9. 3600 RPM Open, Air-Cooled Generator.........................................................26 Figure 10. Self-Contained Recirculating Generator Cooling System.............................27 Figure 11. Air-Cooled Generator Labyrinth Seals..........................................................28 Figure 12. Shaft-Mounted Blowers ................................................................................29 Figure 13. Hydrogen-Cooled Generator ........................................................................31 Figure 14. Hydrogen Seal Rings....................................................................................32 Figure 15. Mush Wound Stator Wiring Schematic ........................................................33 Figure 16. Mush Wound Generator Stator Core............................................................34 Figure 17. Form Wound Coil .........................................................................................35 Figure 18. Laminated Salient Pole Rotor.......................................................................37 Figure 19. Round Rotor Forging....................................................................................38 Figure 20. Round Rotor Retaining Ring and End Turns ................................................39 Figure 21. Cylindrical Sleeve Bearing (Single Insulated)...............................................40 Figure 22. Spherical-Seat Bearing (Double Insulated) ..................................................41 Figure 23. Anti-Friction Bearings ...................................................................................43 Figure 24. Single-Shielded and Double-Shielded Anti-Friction Bearings.......................44 Figure 25. NEMA MG-1 Insulation Class Temperature Ratings ...................................46 Figure 26. Brush and Slip Ring Excitation .....................................................................47 Figure 27 Brushless Excitation ......................................................................................48 Figure 28. Automatic Voltage Regulator (Rotating Exciter) ..........................................50 Figure 29. Static Excitation System ...............................................................................52 Figure 30. Medium Direct-Connected Generator Protection Scheme ..........................55


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Figure 31. Large Direct-Connected Generator Protection Scheme..............................56 Figure 32. Small Standby/Emergency Generator Phase Fault Protection....................57 Figure 33. High Resistance Grounding.........................................................................60 Figure 34. Kilovolt-Ampere and Kilowatt Ratings for Synchronous Generators ...........63 Figure 35. Voltage Ratings for Synchronous Generators .............................................64 Figure 36. Speed Ratings for Synchronous Generators...............................................65 Figure 37. Temperature Rise for Synchronous Generators..........................................66 Figure 38. Low Resistance Grounding Scheme ...........................................................71 Figure 39. IEEE Ground Fault Protection Scheme 10..................................................72 Figure 40. IEEE Ground Fault Protection Scheme 16..................................................72 Figure 41. Direct-Connected Generators .....................................................................73 Figure 42. IEEE Supplementary Sensitive Ground Fault Protection Scheme ..............74 Figure 43. Emergency Generator One-Line Diagram...................................................76 Figure 44. Standard MCCB Ampacity Ratings (NEC Article 240-6) .............................77 Figure 45. MCCB Interrupting Ratings..........................................................................78 Figure 46. LVPCB Frame and Sensor Ratings.............................................................79 Figure 47. LVPCB Interrupting Ratings ........................................................................80 Figure 48. Medium, Direct-Connected Generator Protection Scheme .........................82 Figure 49. Large, Direct-Connected Generator Protection Scheme.............................84


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INTRODUCTION When selecting generators, the principle factors that affect the selection are: •

The standards to which the generator selection must adhere.

•

The type of system for which the generator is being selected.

•

The parameters for which the generator is designed to operate.

•

The service conditions and type of loading under which the generator will operate.

Generators can be divided into two categories based on the type of service for which they will be used. Generators that are to be used as base load equipment will have economy of operation and low maintenance requirements as primary factors affecting selection. Generators that are to be used as stand-by equipment must have auxiliary systems and a prime mover that have fast start capabilities and a high degree of short term reliability. Generators are also selected on the basis of the service conditions under which they will be operating. These are divided into two categories, usual and unusual, depending on the environmental and operating conditions to which they will be subjected.


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A pertinent factor in the selection of a generator for either of the above-mentioned purposes is the type of loading to which the generator will be applied. The type of loading will affect the power factor (PF) rating of the generator. A generator that will be used where there is a large inductive or capacitive component in the load requirements, also called reactive load, must be designed to operate under these conditions. Designing a generator with the capability to operate under these conditions can greatly increase the initial cost. On the other hand, a generator that will not see much reactive loading can be designed with a PF close to unity, which will make the generator less expensive. The excitation system selected for a generator that is going to be applied to a high reactive load must also be designed with a higher rating. With regard to protection, generators, like all other types of electrical equipment, must be protected against many different types of abnormal conditions; for example, overcurrents, overvoltages, unbalance, etc. Where generators are in unattended installations, they must be protected against all possible damaging conditions. In those installations where an operator is always on site, it is usually preferable to initiate an alarm on many different abnormal conditions rather than remove the generator from service. Each generator protective scheme will depend on the desired objectives (alarm or shutdown) to be achieved. Saudi Aramco (SAES-P-114) specifies protection requirements based on the size and/or application of the generator in the plant electrical system. This Module will also explain the factors and develop the procedures for selecting overload, phase, and ground fault protection for the following classes of generators: •

Small standby/emergency generators

•

Medium size direct-connected generators

•

Large size direct-connected generators

Note: Selecting protection for unit-transformer connected generators is beyond the scope of this Module.


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FACTORS AFFECTING GENERATOR SELECTION Standards NEMA MG-1

Two parts in the NEMA MG-1 Standards Publication contain information on the factors applicable to the selection of synchronous generators. •

Section II, Part 16 defines General Purpose Synchronous Generators (6.25 to 500 kVA).

•

Section III, Part 22 defines Large Synchronous Generators (1.25 to 75000 kVA).

Each of these parts contain descriptions of the applicable synchronous generators in terms of kilovolt-ampere (kVA) and kilowatts (kW) available at the terminals at 0.8-power factor lagging (overexcited). These descriptions are given in tabular form along with voltage ratings and speed ratings. Both Parts 16 and 22 contain standards for tests and performance, manufacturing, and application data. Part 22 in Section III contains tables on frequency and excitation voltage standards for large synchronous generators in addition to the abovementioned factors. 17-SAMSS-510

Saudi Aramco Materials System Specification 17-SAMSS-510 defines the minimum technical requirements for synchronous generators rated 125 kVA (100 kW) through 1250 kVA (1000 kW), three phase, 60 hertz for further incorporation into a skidmounted, packaged generating set. The specification sets minimum requirements for the field experience of these synchronous generators and describes the reference material needed for verification of that field experience. 17-SAMSS-510 also lists the modifications (additions or exceptions) required to be used together with NEMA MG-1 and additional requirements for bearings, lubrication and voltage regulators that Saudi Aramco includes in the specifications for synchronous generators that are to be used in a skid-mounted, packaged generating set.


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SAES-P-114, Chapter 5

Saudi Aramco Engineering Standard (SAES)-P-114 is the mandatory standard that explains the system and equipment protection requirements for the electrical power systems in Saudi Aramco installations. In particular, Chapter 5 of SAES-P114 explains the protection requirements for generators. SAES-P-114, Chapter 5 defines four categories of generator protection: •

Section 5.2, Large Direct-Connected Synchronous Generators: Direct-connected synchronous generators with a voltage rating of 13.8 kV or above and a kVA rating greater than 12,500 kVA.

•

Section 5.3, Large Unit-Transformer-Connected Synchronous Generators: Large unit-transformerconnected synchronous generators with a voltage rating of 13.8 kV or above and a kVA rating greater than 12,5000 kVA.

•

Section 5.4, Medium Size Direct-Connected Synchronous Generators: Medium size direct-connected synchronous generators with a voltage rating of 600 to 13,8000 volts and kVA ratings greater than 1000 but not exceeding 12,5000 kVA.

•

Section 5.5, Small Standby/Emergency Generators: Small standby/emergency, diesel-engine driven generators rated at 480 volts.

Figures 5.1, 5.2, and 5.3, which are part of the Supplements Manual to SAES-P-114, are one-line diagrams of the protection schemes for the synchronous generator protection requirements explained in Chapter 5. Note: The current revision of SAES-P-114 dated 25 April 94 replaces the latest revision dated 1 July 91. The key difference between the two revisions is that the latest revision now specifies the use of solid-state relays versus electromechanical relays.


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ANSI/IEEE Standard C37.101-1985

ANSI/IEEE Standard C37.101-1985, IEEE Guide for Generator Ground Protection, specifies the application of relays and relaying schemes for the protection of synchronous generators for single phase-to-ground faults in the stator winding. The guide is not intended for the selection of generator or ground connection schemes. Section 5.1 of SAES-P-114 specifically requires that ANSI/IEEE Standard C37.101 be consulted for additional guidance, explanation, and definition of protection schemes. ANSI/IEEE Guide C37.102-1987

ANSI/IEEE Guide C37.102, IEEE Guide for AC Generator Protection, presents a review of the generally accepted forms of protection for the synchronous generator and its excitation system. It also summarizes the use of relays and devices and serves as a guide for the selection of equipment to obtain adequate protection. The guide is primarily concerned with protection against faults and abnormal operating conditions for large hydraulic, steam, and combustion-turbine generators. Section 5.1 of SAES-P-114 also specifies that ANSI/IEEE Guide C37.102 be consulted for additional guidance, explanation, and definition. Note: Although often referred to as a standard, C37.102 is not a standard; it is a guide. Types of Systems Standby Systems

Single Unit Installation - Single unit, stand-by generator systems are used primarily for emergency back-up systems where the generator will automatically start on a loss of AC power. Many critical primary supply auxiliary systems such as the lube-oil system or generator cooling system are dependent on a backup source of AC power. When used as a back-up power source, these generators are sometimes assigned a stand-by rating allowing temperature rises up to 25°C above the rating of a continuous-duty operation generator. Figure 1 shows a schematic diagram of a power system utilizing a 500 kVA generator as a single unit, emergency back-up power source.


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Generators Evaluating Generator Mechanical and Electrical Specifications

Title: 21504-1.EPS from CorelDRAW! Creator: CorelDRAW! CreationDate: Mon Jun 19 09:24:09 1995

Figure 1. Single Unit Stand-by Generator

- Stand-by systems can require a multi-unit installation in situations where redundancy is required to ensure the reliability of the back-up system or, as shown in Figure 2, where the load requirements may dictate that more than one generator be operated in parallel to meet the demand. Multi-Unit Installation

Figure 2. Multi- Unit Stand-by Generators


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Primary Supply

A generator that is to be used for primary supply must be of such a design that it functions well in parallel operation with other generators. Factors that affect successful parallel operation are: •

Synchronizing torque coefficient (P r)

•

Moment of inertia (Wk2 ) of the generator rotor

The synchronizing torque coefficient (P r) is defined as the change in shaft power expressed in synchronous kilowatts per electrical radian of change in displacement angle. Synchronous kilowatts is the power corresponding to the product of torque and synchronous speed, while electrical radians are defined as: Electrical radians = Electrical degrees X (2π /360) Synchronizing torque coefficient (P r) should correspond to a pulsation frequency of one-half the generator's synchronous speed unless otherwise specified. The torque requirements of various synchronous motor applications can vary depending on the design of the particular machine and its operating conditions. Lower values may be adequate or higher values may be required. The P r of the generator should match the load characteristics to which it is connected. The moment of inertia (Wk2 ) of the generator rotor is a factor that could affect the torsional vibration of the generator. Excessive torsional vibration may result in over stressed shafts, couplings, and other rotating parts. While factors that affect torsional vibration are primarily contained in the design of the prime mover, when requested, the generator manufacturer must furnish the Wk2, the weight of the generator rotor, and any other information that may be required to allow the correct design of the combined unit.


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Operational Parameters The following operational parameters are factors that must be considered when selecting a generator for service on a particular power system. Failure to give careful consideration to each of these factors will result in unsatisfactory operation and could cause damage to the generator, the prime mover, and the system to which they are connected. •

Rated Voltage

•

Rated Frequency

•

Rated Speed

•

Voltage and Frequency Variations During Operation

•

Overspeed Rating

•

Overload Capability

•

Short Circuit Withstand Capability

•

Unbalanced Capabilities

Rated Voltage

Rated voltage on all three-phase, wye-connected generators is measured from phase to phase at the output terminals T1, T2, and T3. These are called the line leads and are the high voltage ends of each of the three phases of the generator's stator winding. The opposite end of each phase is called the neutral lead, and these are designated T4, T5, and T6 respectively. The neutral leads are normally tied together in the lead box of the generator. The generator's rated voltage must be matched to the transformer or the power system that it is to supply. Many generators come equipped with variable tap transformers that provide some flexibility in matching the generator to different voltage requirements. Generators are normally connected to a delta-delta or wye-delta step-up transformer for moderate voltages and to a delta-wye step-up and wye-delta step-down transformer for high voltage transmission systems.


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Figure 3 shows information from Table 22-3 of Part 22 of the NEMA MG-1 Standards Publication for Large Synchronous Generators that lists generator voltage ratings.

Figure 3. Synchronous Generator Voltage Ratings

Rated Frequency

With regard to the selection of rated frequency of the generator, NEMA MG-1 Standards Publication Part 22, Section MG 1-22.14 states that all frequencies shall be 50 or 60 hertz.


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Rated Speed

The rated speed of the generator and the number of poles on the rotating field determine the frequency at which a synchronous generator will operate. This speed is defined as its synchronous speed. The factor that will determine the speed at which a generator will operate is primarily the size or kVA rating of the generator. This rating will determine what type of prime mover will be used to drive the generator. The speed at which a prime mover is most efficient is usually the most important factor in determining a generator's rated speed. The shaft system of the entire unit for both the generator and prime mover must be designed with the unit's critical speed in mind. Both lateral and torsional critical speeds must be considered. Lateral critical speed is the speed corresponding to the natural frequency of the shaft system in response to lateral or transverse forces such as a residual unbalance. Torsional critical speed relates to the response of the shaft system to torsional forces. Torsional critical speeds are excited by external forces such as sudden load changes or a short circuit on the system. Critical speeds, especially lateral, are affected by shaft support, including the foundation and by internal and external damping. It is preferable that the operating speed be at least 20% away from the nearest critical speed. Low speed rotors usually operate below the first critical speed. High speed rotors such as two and four pole rotors often operate above the first critical speed. It is important that these rotors be carefully balanced so that the stresses on them are not excessive while passing through the critical speeds on start-up and shutdown. Figure 4 shows information from the NEMA MG-1 Standards Publication for Large Synchronous Generators that lists the speed ratings for 60 hertz generators based on the number of poles on the rotating field.


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Figure 4. Speed Ratings for 60 Hertz Generators


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Voltage and Frequency Variations During Operation

The NEMA MG-1 Standards Publication for Large Synchronous Generators states on Page 6 of Part 22 that "Synchronous generators shall operate successfully at rated kVA, frequency, and power factor at any voltage not more than 5 percent above or below the rated voltage of the generator." The publication gives no allowed margin for frequency variation because this can have an adverse affect on frequency-sensitive equipment to which the generator is supplying power. The prime mover must have a governor capable of maintaining it at a constant synchronous speed over the entire load range of the generator. This form of control is called speed regulation. As the load is increased or decreased on the generator, the amount of fuel being fed to the prime mover must be increased or decreased by the same amount. The purpose of the governor is to provide speed regulation, thereby keeping the generator frequency constant. As previously mentioned, the kVA rating and type of service for which the generator is being ordered will determine the type of prime mover best suited for the application. The degree of sophistication of the governor supplied with the prime mover is dependent on a number of factors. These factors include: •

Generator size

•

Type of service

•

Load range

•

Type of load

These factors influence the cost of the governor. The more sophisticated the system requirements, the more expensive the governor and, therefore, the more expensive the entire generator set .


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Overspeed Rating

When a generator is operating at or close to its rated capacity (commonly called full load), the generator's prime mover is at full throttle. If something on the system should cause the generator to trip off-line, the load on the prime mover is instantly removed. This action causes the rotational speed of the prime mover, the generator, and the rotating exciter (if one is used to supply the generator) to rapidly increase until the governor can reduce the flow of fuel or steam to the prime mover and slow the unit down. All rotating components must be designed to withstand the centrifugal forces created by this overspeed to prevent damage to the unit. The NEMA MG-1 Standards Publication for Large Synchronous Generators, Part 22, Page 6 specifies that synchronous generators shall be constructed in such a manner as to be able to withstand an overspeed of not greater than one minute duration without mechanical damage. Figure 5 shows the percent overspeed required by the NEMA standard based on the synchronous speed of the generator.

Synchronous Speed (RPM)

% Overspeed of Synchronous Speed

1801 and over

20

1800 and below

25

Figure 5. Overspeed Capabilities


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Overload Capability

When a generator is on-line, a fault such as a short circuit anywhere in the system will cause an overload on the generator. This overload can cause the current in one phase or all three phases, depending on the system to which it is connected, to increase above the generator's rated capacity. To protect against this condition, relays are used to trip the generator in extreme cases. In addition, NEMA standards specify the capability to which generators must be built to withstand such overloads and not incur damage to any of their components. The NEMA MG-1 Standards Publication for Large Synchronous Generators, Part 22, Page 4 specifies that synchronous generators shall be capable of carrying a one minute overload with the field set for normal rated load excitation in accordance with the information shown in Figure 6.

Synchronous Speed (RPM)

Armature Current % of Normal Rated Current

1801 and over

130

1800 and below

150

Figure 6. Maximum Overload Current Capabilities

Short Circuit Withstand

A short circuit occurring directly at the terminals of a generator will place maximum stress on the generator's stator winding, field winding, and excitation system. This type of short circuit could result during generator operation from a conductive coating contaminating the main lead bushings of the generator or from a catastrophic failure in a three-phase transformer connected directly to the generator main leads. Under controlled conditions, a shop test employing this type of short circuit is used to determine generator parameters.


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With regard to this type of short circuit, the NEMA MG-1 Standard states that a synchronous generator shall be capable of withstanding, without injury, a thirty second, three-phase short circuit at its terminals when operating at rated kVA and power factor at five percent over-voltage with fixed excitation. The generator shall also be capable of withstanding, without injury, any other short circuit at its terminals of thirty seconds or less, provided: •

The machine phase currents under fault conditions are such that the negative phase sequence current (I2), expressed in per-unit of stator current at rated kVA, and the duration of the fault in seconds (t) are limited to the values which give an integrated product [(I2)2t] equal to or less than the values shown in Figure 7. and

•

The maximum phase current is limited by external means to a value which does not exceed the maximum phase current obtained from a three-phase fault.

Type of Generator

Permissible [(I2)2t] Product

Salient Pole Machines

40

Air-Cooled Cylindrical Rotor Machines

30

Figure 7. Maximum Short Circuit Capabilities


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Unbalanced Capabilities

A three-phase generator is designed to operate with equal current flow in each phase of the stator winding. If the impedance of one phase was to change, an unbalanced current flow would result. This unbalance would induce double frequency currents on the rotor surface and field windings, which in turn would produce pulsating air gap torques that could eventually fatigue the mechanical components of the generator and prime mover. The NEMA MG-1 Standard states that a synchronous generator shall be capable of withstanding, without injury, the effects of a continuous current unbalance corresponding to a negativephase-sequence current (I2) of the values shown in Figure 8, providing the rated kVA is not exceeded and the maximum current does not exceed 105 percent of the generator's rated stator current in any phase. Note: The negative-phase-sequence currents shown in Figure 8 are expressed as a percentage of rated stator current.

Type of Generator

Permissible (I2) Percent

Salient Pole with Connected Amortisseuir Winding

10

Salient Pole with Non-Connected Amortisseuir Winding

5

Air-Cooled Cylindrical Rotor

10

Figure 8. Maximum Unbalanced Capabilities


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The values shown in Figure 8 also express the negative-phasesequence current capability at reduced generator kVA capabilities as a percentage of the stator current corresponding to the reduced capability. Continuous performance with non-connected amortisseur windings is not readily predictable. Therefore, if unbalanced conditions are anticipated, machines with connected amortisseur windings should be specified. Site and Environmental Conditions Synchronous generators should be properly selected with regard to the site and environmental conditions, called service conditions in NEMA MG-1, under which they will be operating. These service conditions are divided into two categories, usual or unusual. Machines conforming to NEMA MG-1 Part 22 are designed for operation in accordance with their ratings under usual service conditions. The following factors can constitute unusual service conditions, requiring the utilization of a generator designed for operation in these conditions: •

Dirt

•

Ambient Temperature

•

Humidity

•

Elevation

Dirt

Dirt or other forms of airborne contamination can be drawn into an open, air-cooled generator by the machine's ventilation system, causing the conducting components of the stator and rotor to be covered with this contamination, thereby decreasing the cooling efficiency of the ventilation system. This contamination can, under certain circumstances, become conductive, causing a breakdown of the generator's insulating components that could cause significant damage to the generator.


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In many applications where airborne contamination is present, filters are used at the intakes of the ventilation system to remove dirt and contaminants. When filters are employed for this purpose, care must be taken that the filters do not become clogged, causing a reduction in the flow of cooling air that will result in the generator overheating. Ambient Temperature

In applications where there is the possibility of the generator having to operate in a high ambient temperature environment, three systems require special attention: •

The generator's cooling or ventilation system

•

The generator's lube-oil system

•

The generator's electrical insulation system

High ambient temperatures require a cooling system with oversized capacity and a greater than normal volume of cooling air to compensate for the higher inlet air temperatures. Oil flow to the bearings may need to be increased to keep the bearings within normal operating temperatures, and the generator's lube-oil system may need extra cooling to keep the oil temperature within normal operating limits. The third system that may require special attention in a high ambient temperature environment is the electrical insulation system. Material Specification 17-SAMSS-510 specifies that Class F insulation, which is rated for a temperature rise of over 100°C, be used on generators being manufactured for Saudi Aramco. If a generator is consistently operated at temperatures above that of its insulation rating, the useful life of that insulation will be greatly decreased.


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Humidity

Synchronous generators that operate in areas of high humidity should be equipped with space heaters to keep the temperature of the stator and rotor windings above the dew point when the unit is not in operation. Moisture forming on these components could cause low insulation resistance values resulting in a flashover on either the rotor or stator winding that could cause a forced outage of the unit. Elevation

Air-cooled synchronous generators operating in the reduced air density of higher altitudes would require a more efficient, higher volume cooling system to compensate for the reduced cooling capacity of the thinner air.


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SELECTING GENERATOR TECHNICAL CONSTRUCTION REQUIREMENTS Environmental Protection and Enclosure Types Open, Drip-Proof, Weather-Protected Types I & II

Open, drip-proof, weather-protected generators, also called guarded generators, are defined as machines that are constructed in such a way that all openings giving direct access to live metal or rotating parts (except smooth rotating surfaces) are limited in size by the structural parts or by screens, baffles, grills, expanded metal, or other means to prevent accidental contact with hazardous parts. Openings giving direct access to such live or rotating parts shall not permit the passage of a cylindrical rod 0.75 inches or greater in diameter. weather-protected generators are open machines with ventilating pas-sages constructed to minimize the entrance of rain, snow, and air-borne particles to the electric parts. Type l

ll weather-protected generators have their ventilating passages at both intake and discharge arranged so that high velocity air and air-borne particles blown into the machine by storms or high winds can be discharged without entering the internal ventilating passages leading directly to the electric parts of the machine itself. Type II generators also use the same weatherprotected enclosures as Type I generators. As shown in Figure 9, the normal path of the ventilating air which enters the electric parts of the Type II machine is diverted by baffling or separate housings to provide at least three abrupt changes in direction, none of which are less than 90 degrees. In addition, an area of low velocity not exceeding 600 feet per minute is provided at the intake air path to minimize the possibility of moisture or dirt being carried into the electric parts of the machine. Type


25




Figure 9. 3600 RPM Open, Air-Cooled Generator


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Totally Enclosed

A totally-enclosed machine as shown in Figure 10 is a generator constructed to prevent the free exchange of air between the inside and the outside of the frame, but not sufficiently enclosed to be termed air-tight. Labyrinth seals with extremely close clearances shown in Figure 11 are commonly used between the rotating and the stationary components to prevent oil vapors from the bearing housings from entering the stator frame and to minimize the exchange of internal cooling air and outside ambient air.

Figure 10. Self-Contained Recirculating Generator Cooling System


27



Figure 11. Air-Cooled Generator Labyrinth Seals


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A totally-enclosed, air-to-air-cooled generator is cooled by circulating the generator's internal air through a heat exchanger that, in turn, is cooled by circulating external air. A fan or fans, integral with the rotor shaft as shown in Figure 12, or separate from the shaft, circulate the internal air within the enclosed generator and through the heat exchanger, and a fan or fans external to the generator frame and integral with the rotor shaft or separate, circulate external air through the heat exchanger. Air-to-Air -

Figure 12. Shaft-Mounted Blowers

A totally-enclosed, air-to-water-cooled generator is cooled by circulating the generator's internal air through a heat exchanger that, in turn, is cooled by water that is pumped through finned tubes in the heat exchanger. The generator's internal air is circulated across the finned tubes by a fan or fans, integral with the rotor shaft or separate. Air-to-Water -


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In contrasting an open-type generator to a totally-enclosed, aircooled generator, the former requires no supplementary cooling system, but is prone to pick up dirt and contamination from the cooling air passing through the components of the generator. The open-type is obviously the most economical generator in terms of initial cost, but the likelihood of increased maintenance is much greater with an open cooling system, especially if it is operating in an area prone to airborne contamination that can be ingested by the cooling system and deposited on the components of the generator. A totally-enclosed system, with its recirculated internal cooling air eliminates the problem of airborne contamination. However, this type of system costs more due to the heat exchanger and external air circulating equipment. This system also limits the mobility of the generator and requires more space dedicated to the machine and its auxiliaries. A gas-cooled generator as shown in Figure 13 uses hydrogen gas (H2) for its internal cooling medium. H2 has two advantages over air that makes it a more efficient internal cooling medium in synchronous generators: Gas-Cooled -

•

H2 is lighter than air, causing less friction and less energy expended to circulate it through the generator.

•

H2 readily picks up heat in the generator and gives up heat in the heat exchangers or coolers.

Conversely, H2 has the following disadvantages: •

H2 is explosive when mixed with air in a ratio of 5 to 70 percent air.

•

H2-cooling requires a sophisticated seal oil system, shown in Figure 14, to keep the gas from escaping around the openings in the frame for the rotor.


30




Figure 13. Hydrogen-Cooled Generator


31




Figure 14. Hydrogen Seal Rings


32



Stator Mush Wound

Figure 15 shows a schematic wiring diagram of a mush wound stator. The mush wound stator is composed of a series of wire conductors wrapped into an iron core. The conductors are insulated from each other and from the core. The core is normally at ground potential. The insulation is of relatively low dielectric strength. 17-SAMSS-510 contains an addition to the NEMA MG1 Standard for Synchronous Generators, Section 22.40, specifying that Class F insulation be used in applications where the NEMA standard allows insulation with a lower temperature rating and that limits be placed on the temperature rise to which these machines can be subjected in normal operation. Figure 16 shows a typical mush wound coil being placed in the slots of the laminated core of a small generator stator. Many coils will be wound in the same manner and then connected together to make the stator winding.

Figure 15. Mush Wound Stator Wiring Schematic


33



Figure 16. Mush Wound Generator Stator Core


34



Form Wound

Form wound coils are generally used in stators with higher voltage and power ratings. Form wound coils can be manufactured as full coils as shown in Figure 17, or as half coils and consist of a number of individually insulated copper strands that are then separated and insulated into groups in the case of multi-turn coils or coils with transposition connections on one end. The groups of strands are then insulated from the generator core by ground wall insulation. The ground wall insulation is of very high dielectric strength.

Figure 17. Form Wound Coil


35



The advantage of mush wound stators is that they are smaller and lighter than form wound stators, but they have the disadvantage of being limited in their power capabilities. Mush wound stators are more prone to be adversely affected by dirt and contamination deposited on them by the cooling air. Form wound stators have the disadvantage of being heavier in construction and are more expensive than mush wound stators, but they have the advantage of much greater voltage and power capabilities and will withstand abuse much better than mush wound stators. Some manufacturers provide a unique frame designation number for each stator as a form of identification. The frame designation number gives pertinent information about the stator. For example, Westinghouse gives each stator that it manufacturers a frame designation that consists of three sets of numbers. An example frame designation is 8-75.5X64. In this example, the first number indicates the number of poles on the rotating field (8 poles). The second number indicates the diameter of the stator core in inches from through bolt to through bolt (75.5 inches). The third number indicates the length of the stator core in inches from finger plate to finger plate (64 inches). These points of reference are indicated on the cross-section drawing of the hydrogen-cooled generator shown in Figure 13.


36



Rotor Laminated-Type

A laminated-type rotor is a salient pole rotor, the poles of which are made of laminated steel to improve the flux pattern emanating from the field winding that is wound around the laminated pole. This style of rotor, shown in Figure 18, is typically found on smaller, slow speed generators. The advantage of a salient pole rotor is that the laminated poles reduce the eddy-current losses at the pole faces, a problem that can manifest itself in the slower speed machines that have a large number of poles and a relatively small air gap.

Figure 18. Laminated Salient Pole Rotor


37



Solid-Type

Solid-type rotors, also called round rotors, are used on larger, high speed generators with synchronous speeds of 1500 or 3000 rpm for 50 hertz machines and 1800 or 3600 rpm for 60 hertz machines. The solid rotor is designed to have the field windings wound into slots that are machined into the rotor body as shown in Figure 19. This design provides a more rigid support for the longer, heavier copper conductors used in the field winding of larger generators. The round design of the rotor also has the advantage of creating less friction at higher speeds. The endturns of the field winding are supported by the use of retaining rings shrunk onto each end of the rotor. This type of rotor construction is shown in Figure 20. Generators with solid-type rotors are normally horizontal as shown in Figures 13 and 20. Generators with solid-type rotors also have a larger air gap that reduces the problem of eddy-current losses on the pole faces.

Figure 19. Round Rotor Forging


38



Figure 20. Round Rotor Retaining Ring and End Turns


39



Bearings Sleeve

Sleeve bearings are generally used on generators that have heavier, larger diameter rotors. There are two types of sleeve bearings: cylindrical and spherical-seat type. Cylindrical bearings are the most common type of sleeve bearing. Figure 21 shows a cylindrical bearing consisting of a cast iron shell lined with babbitt, which is a soft, porous metal alloy that retains the bearing lubricating oil. In a sleeve bearing the shaft rides on a film of oil approximately .003 inches thick, eliminating any metal-to-metal contact.

Figure 21. Cylindrical Sleeve Bearing (Single Insulated)


40



Spherical-seat bearings are self-aligning and are commonly used in smaller, single-bearing generators. As shown in Figure 22, they differ from sleeve bearings in that the rotor rides on two babbitted shoes and not on a continuous sleeve. The babbitted surfaces of both of these sleeve-type bearings are oil-lubricated by either a self-lubrication or forced-flood lubrication system.

Figure 22. Spherical-Seat Bearing (Double Insulated)


41



Self-lubrication systems rely on oil rings that are fitted loosely around the shaft journals and, due to the rotation of the shaft, rotate through an oil bath in the bottom of the bearing housing. The rings pick up the oil and deposit it on the journal to lubricate the bearing. The housing has an oil gauge to monitor the level of the oil. Forced-flood lubrication systems have the oil supplied under pressure to the bearing surfaces. This pressure can be supplied by a shaft-driven oil pump or by a separate motor-driven oil pump, usually mounted on an oil reservoir. Both bearings rest on insulated bearing seats. Figure 21 shows a cross-section view of the cylindrical bearing resting on a singleinsulated bearing seat. Figure 22 shows the spherical seat bearing resting on a double-insulated seat. Double insulation allows the bearing insulation to be tested while the generator rotor is coupled to the prime mover. When the generator and prime mover are coupled, the generator rotor and the frame are both at ground potential. This condition prevents testing of the insulation when a single-insulated bearing is used. Anti-Friction

Anti-friction bearings, used on smaller generators, come in two varieties, ball bearings and roller bearings. Ball bearings, as the name implies, are composed of round, steel balls contained in circular guides called rings. The rings have grooves called raceways machined into them in which the ball bearings roll. The inner ring is pressed onto the rotating shaft, and the outer ring is pressed into the frame of the generator. If kept clean and properly lubricated, the ball bearings will roll in the raceways with little or no friction when the shaft rotates. The single row, deep groove ball bearing shown in Figure 23A will sustain a substantial thrust load in either direction in addition to the radial load. Roller bearings are cylindrical in shape as shown in Figure 23B, but other than that, are constructed and function in the same manner as ball bearings. They are generally used in heavier radial loading applications.


42



Figure 23. Anti-Friction Bearings


43



Anti-friction bearings are usually lubricated with grease applied by a grease gun directly to the bearing through a grease fitting on the housing. This procedure should be performed only when the unit is not running. Many styles of anti-friction bearing housings are supplied with shields to prevent foreign matter from contaminating the bearing and to retain bearing lubrication. Sealed bearings should be specified for applications where there is the possibility of airborne contaminants getting into the bearings. Figure 24 shows an example of an anti-friction bearing with various shield combinations that are available. Note: 17-SAMSS-510 requires anti-friction bearings to be of the double shielded type.

Figure 24. Single-Shielded and Double-Shielded Anti-Friction Bearings


44



When comparing the advantages and disadvantages of sleevetype bearings to anti-friction bearings, sleeve-type bearings are able to withstand more loading and punishment than anti-friction bearings if kept properly lubricated. However, sleeve-type bearings require a more sophisticated and costly lubrication system than anti-friction bearings. Insulation Class and Temperature Rise There are four standard classes of insulation listed in the NEMA MG-1, Standards Publication for Synchronous Generators. The insulation class determines the temperature rise that an insulation system can withstand on a normal, continuous duty basis. Figure 25 lists these temperature rise ratings along with the component on which the temperature is being measured and cautions that should be observed. Due to the high ambient temperatures likely to be encountered by Saudi Aramco, 17-SAMSS-510 supersedes the NEMA standard by specifying that Class F insulation will be used in all applications. The 17-SAMSS-510 specification also places limitations on generator temperature rise for normal operation that are more stringent than the NEMA Standard limitations.


45



Class of Insulation System (See MG 1-1.65) A B F* H* Time Rating-Continuous Temperature Rise, Degrees C 1. Armature windings-resistance 60 80 105 125 2. Field windings-resistance 60 80 105 125 3. The temperatures attained by cores, amortisseur windings, collector rings, and miscellaneous parts (such as brushholders, brushes, etc.) shall not injure the insulation or the machine in any respect.

Suggested Standard for Future Design 11-12-1981, NEMA Standard 11-20-1986. *Where a Class F or H insulation system is used, special considerations should be given to bearing temperatures, lubrication, etc. Note 1: The foregoing values of temperature rise are based upon operation at altitudes of 3300 feet (1000 meters) or less. For temperature rises for generators intended for operation at altitudes above 3300 feet (1000 meters), see MG 1-14-04. Note 2: The temperature rises given in the above table are based upon a reference ambient temperature of 40°C. Generators intended for use in higher ambient temperatures should have temperature rises not exceeding the value calculated from the formula below, rounded off to the nearest 5 degrees: temperature rise = 0.9 (Tc - Ta) where Ta = ambient temperature Tc = 105°C for Class A insulation system 130°C for Class B insulation system 155°C for Class F insulation system 180°C for Class H insulation system When a higher ambient temperature than 40°C is required, recommended values of ambient temperature are 50°C, 65°C, 90°C, and 115°C. Note 3: Temperature rises in the above table are based upon generators rated on a prime power continuous duty basis. Synchronous generators may be rated on a standby duty basis (see MG 1-16.85). In such cases, it is recommended that temperature rises not exceed those in the foregoing table by more than 25°C under continuous operation at the standby rating.

Authorized Engineering Information 11-12-1981, revised 11-20-1986.

Figure 25. NEMA MG-1 Insulation Class Temperature Ratings


46



Exciter Types Direct Current

Direct current exciters use a mechanical rectifier called a commutator to convert the AC voltage developed on the rotating armature to usable DC field current for the generator rotor. The commutator is a high maintenance, carbon dust-producing device that limits the power that can be obtained from an excitation system. For this reason, DC exciters are no longer manufactured. Alternating Current

Alternating current exciters are available in two styles. One style, as shown in Figure 26, uses slip rings to put DC current on the rotating field of the exciter and then uses a stationary solid state rectifier to convert the AC voltage induced into the stationary armature to DC. The DC current is then supplied to the generator rotor through carbon brushes and collector rings.

Figure 26. Brush and Slip Ring Excitation


47



The second style of AC exciter is called brushless. As shown in Figure 27, this exciter uses a rotating rectifier to convert the AC voltage developed on the rotating armature to usable DC field current for the generator rotor. This style of exciter eliminates the need for slip rings on the exciter armature or collector rings on the generator rotor.

Figure 27 Brushless Excitation


48



Automatic Voltage Regulator Field Excitation for Exciters

The AC voltage produced on a generator stator is a function of the number of kilolines of flux emanating from the field winding on the rotor and crossing the stator winding as the rotating field is spun by the prime mover. This flux density is proportional to the amount of field current flowing in the field windings of the rotor. The stationary field poles on the exciter, as illustrated in Figure 28, control the voltage on the rotating armature, and the voltage, in turn, controls the field current in the generator rotor. The function of the voltage regulator is to provide the operator with a means of control and regulation of the current in the exciter's stationary field poles, which in turn, controls the amount of current flowing through the generator field winding. This function can be performed in either a manual or an automatic mode. The voltage regulator also contains limiters and other protective devices to trip the generator to prevent it from being damaged by a fault on the power system to which it is connected.


49



Figure 28. Automatic Voltage Regulator (Rotating Exciter)


50



Direct alternator excitation

Direct alternator excitation, also called static excitation, uses power fed back from the output of the main generator through an excitation transformer to the voltage regulator as shown in Figure 29. In the voltage regulator, the voltage which is typically 600 volts, 3 phase, 60 hertz, is rectified to DC and regulated to meet the voltage demands on the generator. The voltage is then fed to the generator's rotating field windings through carbon brushes and collector rings that are mounted on the end of the generator rotor.


51




Figure 29. Static Excitation System


52



GENERATOR MINIMUM PROTECTION REQUIREMENTS Note: Refer to Work Aid 2 for the procedures to select minimum generator protection requirements. Introduction EEX 215.01 explained each of the various generator electrical protection requirements in detail, regardless of generator size. This Module will describe specific ANSI/IEEE Standard C37 and SAES-P-114 protection schemes for small standby/emergency generators (480 V, 1000 kVA or less), medium size directconnected generators (600 - 13,800 V, kVA greater than 1000 kVA but less than or equal to 12,500 kVA), and large size directconnected generators (greater than 13,800 V and 12,500 kVA). Protection of large unit-transformer-connected generators are beyond the scope of this Module. Electrical Protection This section will describe the following protection requirements for generators: •

Overload Protection

•

Phase Fault Protection

•

Ground Fault Protection

Overload Protection

Most generators are equipped with embedded detectors (resistance temperature detectors - RTDs) that may be used in a bridge circuit to provide sensing intelligence to an indicator or a relay (e.g., an ABB DT-3 relay). The relay has contact-opening torque when the resistance is low, indicating low machine temperature. When the temperature of the machine exceeds, for example, 130°C for Class B insulated machines at 50°C, the bridge becomes unbalanced and the contact closes, which subsequently trips an alarm circuit. The operator then has a choice to shed load or initiate shutdown.


53



RTDs are built into the generator during construction at points which are inaccessible after the generator is built. NEMA MG-1 requires, as a minimum, one RTD per phase, i.e., three detectors for a three-phase machine. The RTDs protect the stator winding of the generator. SAES-P-114 implies that all protection for small standby/emergency generators, not just overload protection, should be built-in and provided by the generator manufacturer. Small Standby/Emergency Generators -

17-SAMSS-510 requires 2 RTDs per phase for generators rated above 500 kVA (400 kW at 0.80 PF). The same specification also requires that the RTDs shall be platinum, 100 Ω at 0°C. All medium size generators require two RTDs per phase for overload protection. When the temperature of the generator exceeds the specified limit, i.e., 130°C in a 50°C ambient environment, the contacts close on the relay, and an alarm is activated. The RTDs shall also be platinum similar to the RTDs specified for small standby/emergency generators. See Figure 30. Medium Size Generators -

Large Size Generators,

similar to medium size generators, require two platinum RTDs per phase, and when the winding temperature exceeds the specified limits, an alarm is activated. See Figure 31.


54



Figure 30. Medium Direct-Connected Generator Protection Scheme


55




Figure 31. Large Direct-Connected Generator Protection Scheme


56



Phase Fault Protection

Small Standby/Emergency Generator phase fault protection is provided by molded case circuit breakers (MCCB) rated at 125% of the generator’s rated full-load amperes, or low voltage power circuit breakers (LVPCB) rated at 100% of the generator’s fullload amperes. See Figure 32.

Figure 32. Small Standby/Emergency Generator Phase Fault Protection


57



internal phase fault protection is provided by differential relays (ANSI Device 87G) (Figure 30). Generator differential relays are usually arranged to trip the generator, field circuit, and neutral breakers (if used) simultaneously using a manually reset lockout auxiliary relay (ANSI Device 86G1). In some applications the differential relay also trips the throttle and admits CO2 to the generator for fire protection. See Figure 30. Medium Size Generator

Overcurrent relays (ANSI Device 51V) that are used for backup phase fault protection are specifically constructed to make the operating characteristics a function of voltage as well as current. Ordinary relays cannot be used because if set low to protect the generator for sustained fault currents (possibly less than FLA), the generator would trip on normal loads or small overloads. If set high, the relay would not respond at all for sustained fault currents under “stuck regulator” conditions. The overcurrent relay also make use of a manually reset lockout relay (ANSI Device 86G2), that simultaneously trips the same devices as the ANSI Device 86G1 relay except for CO2 fire protection. See Figure 30. phase fault protection is identical to the protection provided for medium size generators except that an additional level of protection is provided by an overall differential protection relay (ANSI Device 87U). See Figure 31. Large Size Generator


58



Ground Fault Protection

Ground fault protection schemes for generators are determined by the type of grounding system that is used for the generator. Generators - 17-SAMSS-510 specifies that standby/emergency generators be solidly-grounded wyeconnected. SAES-P-114 does not specify a type of ground fault protection scheme for these solidly-grounded standby/emergency generators; however, SAES-P-114 does specify that ground fault protection schemes for standby/emergency generators must be reviewed by the Manager, Power Distribution Department. Unless separate ground fault protection is provided by the generator manufacturer, the phase fault protective device, an MCCB or LVPCB as illustrated in Figure 32, also provides the ground fault protection; however, 17-SAMSS-510 does specify that solid-state trip (SST) ground units should be provided when zone-selective interlocking is installed on the feeder circuits supplied by the generator. Solidly-Grounded

The SAES-P-114 specified method of grounding for both medium and large direct-connected generators is low resistance grounding. Low Resistance-Grounded Generators -

Medium size direct-connected generators use a simple overcurrent relay (ANSI Device 51N) installed in the neutral of the generator (Figure 30). Large size direct-connected generators use the same ground fault device (51N) as the medium size generators plus two additional sets of ground fault relays. One set (ANSI Device 50GS) provides start-up ground overcurrent protection and the other set, ANSI Device 87GN, provides more sensitive ground fault differential protection than the phase fault differential device 87G. See Figure 31. The SAES-P-114 specified method for grounding large unit-transformer-connected generators is high resistance grounding (Figure 33). High Resistance-Grounded Generators -


59



Figure 33. High Resistance Grounding


60



Referring to Figure 33, the resistor (R) limits the available fault current to very low magnitudes (3 - 25 amperes). Because of this low ground fault current magnitude, ordinary overcurrent relays may not be sensitive enough to detect the fault. ANSI Device 87 (not shown) is also not usually sensitive enough to detect low-level ground faults. For these reasons, very sensitive overvoltage relays (ANSI Device 59GN) are used to detect generator ground faults. ANSI Device 59S as illustrated in Figure 33 is only intended for ground fault protection during the time that the generator is disconnected from the system, i.e., during startup. The relay operating circuit is connected by means of an auxiliary switch (52/b) on the circuit breaker and, therefore, the protection is in service only during the time that the circuit breaker is open. Note: This section on high resistance grounding is for informational purposes only; this Module will not provide procedures for selecting relays for unit-transformer-connected generators. Instrumentation and Alarms There are no standards that specify the minimum recommended types of instrumentation and alarms for generators. The generator manufacturer will supply the generator with their own standard package of instrumentation and alarms; additional instrumentation and alarm packages are limited only by the imagination of the user. The function of the generator instrumentation and alarms with a multitude of sensors and displays is to give the operator reliable, direct, and continuous information about a variety of critical generator operating factors. These operating factors include measuring and displaying a full range of mechanical, electrical, and thermal data. Small Generators

Small standby/emergency generators are usually modularpackaged units that include basic sets of electrical instruments (frequency meters, voltmeters, ammeters, event recorders, etc.). Alarms, such as temperature, cooling apparatus, fuel supply lubrication, etc., are also part of the modular packages.


61



Large Generators

For large generators, the electrical system alarms and instrumentation are relatively minor in nature compared to the massive requirements for monitoring, measuring, recording, etc., information about the generator mechanical system such as fuel, temperature, vibration, etc. As with small generators, the manufacturer will typically recommend a minimum package of instrumentation and alarms.


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WORK AID 1: RESOURCES USED TO DETERMINE IF GENERATOR TECHNICAL CONSTRUCTION SPECIFICATIONS ARE CORRECT Work Aid 1A: NEMA MG-1 The following figures (Figures 34, 35, 36, and 37) show information given in NEMA MG-1, Part 22 that is used to determine correct technical specifications for synchronous generators. kVA

kW @ 0.8 PF

kVA

kW @ 0.8 PF

kVA

kW @ 0.8 PF

1.25 2.5 3.75 6.25 9.4 12.5 18.7 25 31.3 37.5 50 62.5 75 93.8 125 156 187 219

1 2 3 5 7.5 10 15 20 25 30 40 50 60 75 100 125 150 175

250 312 375 438 500 625 750 875 1000 1125 1250 1563 1875 2188 2500 2812 3125 3750

200 250 300 350 400 500 600 700 800 900 1000 1250 1500 1750 2000 2250 2500 3000

4375 5000 5625 6250 7500 8750 10000 125000 15625 18750 25000 31250 37500 43750 50000 62500 75000

3500 4000 4500 5000 6000 7000 8000 10000 12500 15000 20000 25000 30000 35000 40000 50000 60000

Figure 34. Kilovolt-Ampere and Kilowatt Ratings for Synchronous Generators (Reference: NEMA MG-1, Table 22-1)


63



Three-Phase Voltage 208Y / 120 240 480 600 2400 4160Y / 2400 4800 6900 13800

Figure 35. Voltage Ratings for Synchronous Generators (Reference: NEMA MG-1, Table 22-3)


64



Speed, rpm No. Poles

60 hertz

50 hertz

2

3600

3000

4

1800

1500

6

1200

1000

8

900

750

10

720

600

12

600

500

14

514

429

16

450

375

18

400

333

20

360

300

22

327

273

24

300

250

26

277

231

28

257

214

30

240

200

32

225

188

36

200

167

40

180

150

44

164

136

48

150

52

138

Figure 36. Speed Ratings for Synchronous Generators (Reference: NEMA MG-1, Table 22-2)


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Item

1

Method of Temperature Determination

Machine Part

Temperature Rise (°C) per Class of Insulation System A

B

F

H

Resistance

60

80

105

125

Embedded detector

70

90

115

140

(1) 7000 volts and less

Embedded detector

65

85

110

135

(2) Over 7000 volts

Embedded detector

60

80

105

125

Resistance

60

80

105

125

Armature winding (a) All kVA ratings (b) 1563 kVA and less (c) Over 1563 kVA

2

Field winding

3

The temperature attained by cores, amortisseur windings, collector rings, and miscellaneous parts (such as brushholders, brushes, pole tips, etc.) shall not injure the insulation of the machine in any respect.

Note: Temperature rises in the above table are based upon generators rated on a continuous duty basis. Note: Temperature rises in the above table are based on a reference ambient temperature of 40°C. However, it is recognized that synchronous generators may be required to operate in an ambient temperature higher than 40°C. For successful operation of the generator in ambient temperatures higher than 40°C, it is recommended that the temperature rises of the generators given in the above table be reduced as indicated by the following information, for the ranges of ambient temperature given.

Ambient Temperature (°C)

Values by Which the Temperature Rises in the Above Table Should be Reduced (°C)

Above 40 up to and including 50

10

Above 50 up to and including 60

20

Figure 37. Temperature Rise for Synchronous Generators (Reference: NEMA MG-1-22.40)


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Work Aid 1B: 17-SAMSS-510 For the content of Work Aid 1B, refer to Handout 1.

Work Aid 1C: Applicable Evaluation Procedures Step 1:

kVA - Evaluate the generator technical specification given for kVA and determine if it is correct. Evaluate this specification in accordance with NEMA MG-1. The technical specification for the kVA must be equal to the value required by the proposed or given operating conditions for which the generator will be applied, and the kVA specification must be equal to one of the approved ratings shown in NEMA MG-1, Table 22-1 (refer Figure 34, Work Aid 1A).

Step 2:

Stator Volts - Evaluate the generator technical specification given for stator volts and determine if it is correct. Evaluate this specification in accordance with NEMA MG-1. In accordance with NEMA MG-1-22.13, voltage ratings for synchronous generators must be in accordance with NEMA MG-1, Table 22-3 (refer Figure 35, Work Aid 1A).

Step 3:

Stator Amps - Evaluate the generator technical specification given for stator amps and determine if it is correct. The stator amperes must be equal to the value determined by using the rated kVA, rated stator volts, and the following equation:

IT = where: I T = stator amps VT = rated stator volts


67



kVA3Φ = rated kVA Step 4:

Power Factor - Evaluate the generator technical specification given for power factor and determine if it is correct. Evaluate this specification in accordance with NEMA MG-1. In accordance with NEMA MG-1-22.10, synchronous generators shall be rated on a continuous duty basis, and the rating shall be expressed in kilovolt-amperes available at the terminals at 0.8-power-factor lagging (overexcited).

Step 5:

Synchronous Speed - Evaluate the generator technical specification given for synchronous speed and determine if it is correct. Evaluate this specification in accordance with NEMA MG-1. In accordance with NEMA MG-122.12, speed ratings shall be as shown in NEMA MG-1, Table 22-2 (refer Figure 36, Work Aid 1A).

Step 6:

Exciter Volts - Evaluate the generator technical specification given for exciter volts and determine if it is correct. Evaluate this specification in accordance with NEMA MG-1. In accordance with NEMA MG-1-22.15, the excitation voltages for field windings shall be 62.5, 125, 250, 375, and 500 volts direct current. These excitation voltages do not apply to generators of the brushless type with direct-connected exciters.

Step 7:

Frequency - Evaluate the generator technical specification given for frequency and determine if it is correct. Evaluate this specification in accordance with NEMA MG-1. In accordance with NEMA MG-1-22.14, frequencies shall be 50 and 60 hertz.

Step 8:


Cooling System Type - Evaluate the generator technical specification given for the type of cooling system and determine if it is correct.

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The cooling system type should be either open-cooling type or closed-cooling type depending on service operating conditions. In accordance with industry recommended practice:

Step 9:

(a)

Select open-cooling type for service conditions where no serious interference with the ventilation of the generator exists.

(b)

Select closed-cooling type for service conditions where serious interference with the ventilation of the generator does exist.

Stator Winding Type - Evaluate the generator technical specification given for the type of stator winding and determine if it is correct. The stator winding type should be either mush wound or form wound depending on generator rating, design, and/or application. In accordance with typical industry practice: (a)

For generators rated greater than 500 kVA, the stator winding type should always be form wound.

(b)

For generators rated 500 kVA and below, the stator winding type may be either form wound or mush wound depending on individual design and application.

Step 10: Rotor Type - Evaluate the generator technical specification given for the type of rotor and determine if it is correct. The rotor type should be either salient-pole or round. In accordance with industry recommended practice: (a)


Round-type rotors should be used for two-pole, 3600 rpm generators and four-pole, 1800 rpm generators.

69



(b)

Salient-pole rotors should be used for generators with eight or more poles and speeds less than 1800 rpm.

Step 11: Bearing Type - Evaluate the generator technical specification given for the type of bearing and determine if it is correct. The bearing type should be either anti-friction, cylindrical-sleeve type, or shoe-sleeve type. In accordance with industry recommended practice: (a)

Generators rated less than 500 kVA may use either anti-friction or cylindrical-sleeve type bearings depending on design and application.

(b)

Generators rated 500 kVA or greater and designed as two-bearing machines should use cylindrical-sleeve type bearings.

(c)

Generators rated 500 kVA or greater and designed as one-bearing machines, should use shoe-sleeve type bearings. (Note: One-bearing machines are designed to have the drive end of their rotor supported by the prime mover.)

Step 12: Insulation Class and Temperature Rise - Evaluate the generator technical specification given for the insulation class and temperature rise and determine if they are correct. Evaluate these specifications in accordance with NEMA MG-1 and 17-SAMSS-510. In accordance with NEMA MG-1-22.40, the observable temperature rise under rated load conditions of each of the various parts of the synchronous generator above the temperature of the cooling air shall not exceed the values given by the table provided in NEMA MG-122.40 (refer to Figure 37, Work Aid 1A). The temperature of the cooling air is the temperature of the external air as it enters the ventilating openings of the machine, and the temperature rises given in the table are based on a maximum temperature of 40°C for this external air.


70



Refer to 17-SAMSS-510 (Work Aid 1B, Handout 1) to determine any additional Saudi Aramco requirements for insulation class and temperature rise rating. WORK AID 2: RESOURCES USED TO SELECT GENERATOR MINIMUM PROTECTION REQUIREMENTS Work Aid 2A: ANSI/IEEE Standard C37.101-1985 (1)

Neutral grounding for large and medium size directconnected synchronous generators is in accordance with Table 1, Method III (Figure 38).

Figure 38. Low Resistance Grounding Scheme


71



(2)

The minimum required generator protection scheme for large and medium size direct-connected synchronous generators shall be Scheme 10 (Figure 39). Both IEEE Schemes 10 and 16 shut down the generator under fault conditions.

Figure 39. IEEE Ground Fault Protection Scheme 10

Figure 40. IEEE Ground Fault Protection Scheme 16


72



Work Aid 2B: ANSI/IEEE Standard C37.102-1987 (1)

The generator neutral or grounding transformer neutral will generally be grounded through a low ohmic value resistor as illustrated in Figure 41.

Figure 41. Direct-Connected Generators


73



(2)

Supplementary sensitive ground fault protection shall be provided as illustrated in Figure 42.

Figure 42. IEEE Supplementary Sensitive Ground Fault Protection Scheme


74



Work Aid 2C: SAES-P-114 (25 APR 94), Chapter 5 For the content of Work Aid 2C, refer to Handout 2. Work Aid 2D: 17-SAMSS-510 For the content of Work Aid 2D, refer to Handout 1. Work Aid 2E: Applicable Selection Procedures Step 1:


Collect the following minimum nameplate data (as applicable) from the generator specifications: a.

Manufacturer’s name, serial number, or other suitable identification -

b.

Kilovolt-ampere output -

c.

Power factor -

d.

Time rating -

e.

Temperature rise for rated continuous load -

f.

Rated speed in rpm -

g.

Voltage -

h.

Rated current in amperes-per-terminal -

i.

Number of phases -

j.

Frequency -

k.

Maximum ambient temperature -

l.

Insulation system designation -

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Step 2:

Small Standby/Emergency Generators: Note: This step applies to diesel-driven standby/emergency generators rated 480 V, 1000 kVA or less as per SAES-P-114, Section 5.5. a.

This Module assumes that protection of small standby/emergency generators is built-in protection provided by the manufacturer. See Section 5.5.1 of Work Aid 2C.

b.

Sketch a simple one-line diagram of the system as illustrated in Figure 43.

Figure 43. Emergency Generator One-Line Diagram

c.

Select an ampacity rating for the molded case circuit breaker at 125% of the emergency generator’s full-load amperes (FLA). Note: If using a low voltage power circuit breaker (LVPCB) for main breaker protection, proceed to Step 2F. MCCB ampacity rating = 1.25 FLA


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d.

Select the ampacity rating of the next standard size MCCB from the following list (Figure 44).

Standard Ampere Ratings

Standard Ampere Ratings

200 A 225 A 250 A 300 A 350 A 400 A 450 A 500 A

600 A 700 A 800 A 1000 A 1200 A 1600 A 2000 A

Figure 44. Standard MCCB Ampacity Ratings (NEC Article 240-6)


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e.

Line No.

Frame Size (amps) (AF)

Select a line number MCCB from the following list (Figure 45) based on the size of the MCCB ampacity rating from Step 2d and the available short circuit current (SCA). Note: The ratings in Figure 45 are typical from one MCCB vendor; other vendors may have different ratings.

Rated Continuous Current (amps) (AT)

Interrupting Current Rating (AIC) (amps) 480 Volts Sym Asym

1

225

125-200

18,000

20,000

2

225

70-225

22,000

25,000

3

225

70-225

35,000

40,000

4

225

70-225

100,000

5

225

70-225

25,000

30,000

6

400

200-400

35,000

40,000

7

400

200-400

100,000

8

400

200-400

30,000

9

600

300-600

100,000

10

800

300-800

30,000

35,000

11

800

300-800

35,000

40,000

12

800

600-800

100,000

13

1000

600-1000

30,000

35,000

14

1200

700-1200

30,000

35,000

15

1600

800-1600

100,000

--

16

2000

800-2000

100,000

--

--

-35,000 --

--

Figure 45. MCCB Interrupting Ratings


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f.

Frame Size (amperes) 800 1600 200

Where a low voltage power circuit breaker (LVPCB) is used as main circuit breaker protection, the LVPCB shall have a continuous current rating not less than the generator maximum rated load current. When a solid-state trip (SST) device is provided, it shall have long-time and short-time phase units only, except that a ground unit shall also be provided when zone-selective interlocking is installed on the feeder circuits supplied by the generator. Select the LVPCB frame, sensor size, and interrupting ratings from Figures 46 and 47 based on the generator’s fullload amperes (FLA) and available short circuit current (SCA).

Available Sensor Ratings (amperes) 50, 100, 150, 200, 300, 400, 600, 800 100, 150, 200, 300, 400, 600, 800, 1200, 1600 100, 150, 200, 300, 400, 600, 800, 1200, 1600, 2000

Figure 46. LVPCB Frame and Sensor Ratings


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Frame Size (amps)

Interrupting Ratings, RMS Symmetrical Amperes With Instantaneous Trip 480 Volts

Short Time Ratings 30 cycles (With Short-Delay) 480 Volts

800

30,000

30,000

1600

50,000

50,000

2000

65,000

65,000

Figure 47. LVPCB Interrupting Ratings

g.


Select a solidly-grounded neutral (wye-connected) grounding system.

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Step 3.

Medium, Direct-Connected Synchronous Generators

Note:

This step applies to medium, direct-connected synchronous generators with a voltage rating of 600 to 13,800 volts and kVA ratings greater than 1000 kVA but not exceeding 12.5 MVA as per SAES-P-114, Section 5.4.


a.

Select a low resistance grounding scheme per ANSI/IEEE Standard C37.101-1985, Table I, Grounding Method III. See Figure 38 of Work Aid 2A.

b.

Select generator ground fault protection per ANSI/IEEE Standard C37.101-1985, Table I, Generator Connection E, Schemes 10 and 16. See Figures 39 and 40 of Work Aid 2A and Figure 42 of Work Aid 2B.

c.

Select generator minimum protection per Drawing No. SAESP114.094 of SAES-P-114 (Figure 48 of this Work Aid).

d.

Select required generator protection per SAES-P114 (Work Aid 2C), Sections 5.6, 5.7, and 5.8.

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Figure 48. Medium, Direct-Connected Generator Protection Scheme


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Step 4.

Large Direct-Connected Synchronous Generators

Note: This step applies to large, direct-connected synchronous generators with a voltage rating of 13,800 volts or above and a kVA rating greater than 12.5 MVA, as per SAESP-114, Section 5.2.


a.

Select a low resistance grounding scheme per ANSI/IEEE Standard C37.101-1985, Table I, Grounding Method III. See Figure 38 of Work Aid 2A.

b.

Select generator ground fault protection per ANSI/IEEE Standard C37.101-1985, Table I, Generator Connection E, Schemes 10 and 16. See Figures 39 and 40 of Work Aid 2A and Figure 42 of Work Aid 2B.

c.

Select generator minimum protection per Drawing No. SAESP114.092 of SAES-P-114 (Figure 49 of this Work Aid).

d.

Select required generator protection per SAES-P114 (Work Aid 2C), Sections 5.6, 5.7, and 5.8.

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Figure 49. Large, Direct-Connected Generator Protection Scheme


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GLOSSARY brushless exciter

A main field exciter consisting of a rotating armature within a stationary field. The AC voltage produced on the armature is rectified to a DC voltage by rotating rectification components assembled onto the shaft of the exciter, thus eliminating the need for collector rings and brushes.

commutator

A mechanical rectifier used on DC exciters to rectify the AC voltage on the rotating armature to DC current for the field winding of the generator.

directly-connected generator

A generator whose output terminals are connected to the power distribution system without the interposition of a power transformer.

excitation

The process of establishing and maintaining the magnetic field of a generator.

excitation system

The source of field current for a generator, including its means of control.

exciter

The machine which supplies DC current to the generator field winding.

fault

An insulation failure or breakdown in the continuity of a conductor, usually causing an electrical device to fail to perform as required.

field

The magnetic field of a generator; synonymous with rotor.

flux

Magnetic lines of force.

generator

A machine that converts rotational mechanical energy into electrical energy.

governor

The automatic control element of a prime mover that regulates it to a constant rotational speed.

grounding system

A system comprising all interconnected grounding facilities in a specific area.


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high-resistance grounded system

A system with the intentional insertion of resistance between a generator or transformer neutral and ground, with resistive ground fault current greater than or equal to the system charging current, and with total ground fault current generally less than 10 amperes.

inner-cooled machine

A generator in which the stator coil coolant flows internally through the coils, parallel rings and lead bushings with heat transfer consequently occurring by means of direct contact between the conductors and the cooling medium.

insulation class

A rating given to the insulation system of a generator that identifies the maximum temperatures at which the insulation is designed to operate.

kilowatt (kW)

The value obtained when the sum of the products of output phase voltages and resistive components of output line currents of a generator are divided by one-thousand.

low resistancegrounded system

A system with the intentional insertion of resistance between a generator or transformer neutral and ground, thus limiting fault current within the range of 100 and 1200 amperes.

low system

A system with line-to-line voltage of 1000 V and less.

voltage

medium voltage system

A system with line-to-line voltage in the range of 1001 and 15,000 V.

megavolt-ampere (MVA)

The value obtained when the sum of the products of phase voltages and line current outputs of a generator are divided by onemillion.

megavolt-ampere reactive (MVAR)

The value obtained when the sum of the products of output phase voltages and the reactive components of output line currents of a generator are divided by one-million.

megawatt (MW)

The value obtained when the sum of the product of output phase voltages and resistive components of output line currents of a generator are divided by one-million.


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motoring

The condition of a generator when it is driven by the power from other generation sources of the distribution system. In common usage, this term usually implies the special case of motoring when the fuel or steam supply of the prime mover has been interrupted and the generator remains connected to the system.

overcurrent relay

A relay that functions when the current in an AC circuit exceeds a predetermined value. Such a relay is referred to as ANSI Device 51.

overexcited generator

A generator whose stator voltage is higher than that of the distribution system and is supplying reactive power (VARs) to the system.

overvoltage relay

A relay that functions on a given value of overvoltage. Such a relay is referred to as ANSI Device 59.

power factor

The real power to apparent power ratio; the decimal fraction by which volt-amperes must be multiplied to obtain watts.

prime mover

The engine or machine that drives the generator.

rated MW

The maximum continuous MW output of a generator which can be delivered at rated power factor without exceeding the temperature limits of winding components.

rated power factor

The value of lagging power factor at which the stator-windinglimited capability curve meets the rotor-winding-limited capability curve.

reactance grounded system

A system with the intentional insertion of reactance between a generator neutral and ground with total ground fault current between 25 percent and 100 percent of three-phase fault current.

regulator

The excitation system controlling mechanism which senses a change in the voltage or current at each phase of the generator terminals and that corrects for this change by adjusting the field current or tripping the generator off-line.

resistance temperature detectors (RTD)

Instrumentation elements that are made of a material for which the electrical resistivity is a known function of temperature, that are located throughout the machine, and that indicate various strategic winding and iron temperatures.


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short circuit

An abnormal electrical connection that is of extremely low resistance and that usually results in excessive current flow.

solidly-grounded system

A system with an intentional and direct connection to ground, generally exhibiting very high ground fault current magnitudes.

stator

The stationary element of the generator magnetic circuit and its associated electrical windings and leads.

synchronism

The condition of an AC generator when its output voltages are the same frequency and zero-degree phase angle difference as are the voltages of the generator bus.

temperature rise rating

The temperature rise rating given on a generator nameplate describing the limit of expected rise in temperature over ambient.

thermocouple (TC)

A temperature sensing device used in a manner similar to an RTD but composed of two dissimilar metals which produce a measurable voltage that varies with temperature.

ungrounded system

A system with no intentional connection to ground but with coupling through the stray capacitance of the phase conductors.

unit-connected generator

A generator where the output terminals are connected to the distribution system through an interposing transformer.

unit-transformer

A transformer designed and constructed especially to interpose and transform voltages between a generator and its distribution system.

voltage regulator

The automatic control component of the excitation system through which the output voltage of the generator is established and maintained.

winding

A grouping of inter-connected coils, all of the same phase or polarity.


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Evaluating Generator Mechanical and Electrical Specifications

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