Introduction to Protection Switchgear & Protection Unit 1

Padmasri Dr.B.V.Raju Institute of Technology

Switchgear & Protection Prepared by

Prof.P.Paulclee And N.Ramchander Asst Professor ,

Department of

Electrical and Electronics .Engineering

PADMASRI DR. B.V.RAJU INSTITUTE OF TECHNOLOGY DEPARTMENT OF

ELECTRICAL AND ELECTRONICS ENGINEERING

Course Content

Switchgear and Protection Prepared By

Prof.P.Paul Clee

Mr.N.Ramchander

Assistant Professor

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SWITCH GEAR AND PROTECTION (SYLLABUS)

Objective : This course introduces all varieties of Circuit Breakers and Relays for protection of Generators, Transformers and feeder bus bars from over voltages and other hazards. It emphasis on Neutral grounding for overall protection. UNIT – I

Circuit Breakers-1

Circuit Breakers: Elementary principles of arc interruption, Recovery, Restriking Voltage and Recovery voltages.- Restriking Phenomenon, Average and Max. RRRV, Numerical Problems - Current Chopping and Resistance Switching - CB ratings and Specifications : Types and Numerical Problems. – Auto reclosures. UNIT –II

Circuit Breakers-2

Description and Operation of following types of circuit breakers: Minimum Oil Circuit breakers, Air Blast Circuit Breakers, Vacuum and SF6 circuit breakers. UNIT – III

Electromagnetic and Static Relays

Principle of Operation and Construction of Attracted armature, Balanced Beam, induction Disc and Induction Cup relays. Relays Classification: Instantaneous, DMT and IDMT types. Application of relays: Over current/ Under voltage relays, Direction relays, Differential Relays and Percentage Differential Relays. Universal torque equation, Distance relays: Impedance, Reactance and Mho and OffSet Mho relays, Characteristics of Distance Relays and Comparison. Static Relays: Static Relays verses Electromagnetic Relays. UNIT – IV

Generator Protection

Protection of generators against Stator faults, Rotor faults, and Abnormal Conditions. Restricted Earth fault and Inter-turn fault Protection. Numerical Problems on % Winding Unprotected.

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4 UNIT –V

Transformer Protection

Protection of transformers: Percentage Differential Protection, Numerical Problem on Design of CT s Ratio, Buchholtz relay Protection. UNIT –VI

Feeder and Bus-Bar Protection

Protection of Lines: Over Current, Carrier Current and Three-zone distance relay protection using Impedance relays. Translay Relay. Protection of Bus bars – Differential protection. UNIT – VII Neutral Grounding Grounded and Ungrounded Neutral Systems.- Effects of Ungrounded Neutral on system performance. Methods of Neutral Grounding: Solid, Resistance, Reactance Arcing Grounds and Grounding Practices. UNIT – VIII Protection against over voltages Generation of Over Voltages in Power Systems.-Protection against Lightning Over Voltages - Valve type and Zinc-Oxide Lighting Arresters - Insulation Coordination -BIL, Impulse Ratio, Standard Impulse Test Wave, Volt-Time Characteristics. TEXT BOOKS: 1. Switchgear and Protection – by Sunil S Rao, Khanna Publlishers 2. Power System Protection and Switchgear by Badari Ram , D.N Viswakarma, TMH Publications

REFERENCE BOOKS: 1. Fundamentals of Power System Protection by Paithankar and S.R.Bhide.,PHI, 2003. 2. Art & Science of Protective Relaying – by C R Mason, Wiley Eastern Ltd. 3. Electrical Power Systems – by C.L.Wadhwa, New Age international (P) Limited, Publishers, 3nd editon 4. A Text book on Power System Engineering by B.L.Soni, Gupta, Bhatnagar, Chakrabarthy, Dhanpat Rai & Co.

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5 Objectives of this Course

In this course, we plan to teach the following:

1. Fundamental principles of fuse and overcurrent protection and application to feeder and motor protection. 2. Fundamental principles of distance relaying and application to transmission system protection. 3. Fundamental principles of differential protection and application to transformer, busbar and generator armature winding protection. 4. Role of Current and Voltage transformers in power system protection. 5. Relay co‐ordination in transmission and distribution system. 6. Introduction to Numerical relaying. DSP fundamentals like aliasing, sampling theorem, Discrete Fourier Transform and application to current and voltage phasor estimation. 7. Numerical relaying algorithms for overcurrent, distance and differential protection with application to transmission system, transformer and bus bar protection.

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Introduction to Power System Protection

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INTRODUCTION 1.1. Need for protection: A power system is not only capable to meet the present load but also has the flexibility to meet the future demands. A power system is designed to generate electric power in sufficient quantity, to meet the present and estimated future demands of the users in a particular area, to transmit it to the areas where it will be used and then distribute it within that area, on a continuous basis. To ensure the maximum return on the large investment in the equipment, which goes to make up the power system and to keep the users satisfied with reliable service, the whole system must be kept in operation continuously without major breakdowns. This can be achieved in two ways:

• The first way is to implement a system adopting components, which should not fail and requires the least or nil maintenance to maintain the continuity of service. By common sense, implementing such a system is neither economical nor feasible, except for small systems.

The second option is to foresee any possible effects or failures that may cause longterm shutdown of a system, which in turn may take longer time to bring back the system to its normal course. The main idea is to restrict the disturbances during such failures to a limited area and continue power distribution in the balance areas. Special equipment is normally installed to detect such kind of failures (also called ‘faults’) that can possibly happen in various sections of a system, and to isolate faulty sections so that the interruption is limited to a localized area in the total system covering various areas. The special equipment adopted to detect such possible faults is referred to as ‘protective equipment or protective relay’ and the system that uses such equipment is termed as ‘protection system’.

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1.2. Basic requirements of protection: A protection apparatus has three main functions/duties: 1. Safeguard the entire system to maintain continuity of supply 2. Minimize damage and repair costs where it senses fault 3. Ensure safety of personnel.

These requirements are necessary, firstly for early detection and localization of faults, and secondly for prompt removal of faulty equipment from service. In order to carry out the above duties, protection must have the following qualities:

• Selectivity: To detect and isolate the faulty item only. • Stability:

To leave all healthy circuits intact to ensure continuity or supply.

• Sensitivity:

To detect even the smallest fault, current or system abnormalities and operate correctly at its setting before the fault causes irreparable damage.

• Speed:

To operate speedily when it is called upon to do so, thereby minimizing damage to the surroundings and ensuring safety to personnel.

To meet all of the above requirements, protection must be reliable which means it must be: • Dependable: It must trip when called upon to do so. • Secure: It must not trip when it is not supposed to.

1.3 Basic components of protection Protection of any distribution system is a function of many elements and this manual gives a brief outline of various components that go in protecting a system. Following are the main components of protection. • Fuse is the self-destructing one, which carries the currents in a power circuit continuously and sacrifices itself by blowing under abnormal conditions. These are

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9 normally independent or stand-alone protective components in an electrical system unlike a circuit breaker, which necessarily requires the support of external components.

• Accurate protection cannot be achieved without properly measuring the normal and abnormal conditions of a system. In electrical systems, voltage and current measurements give feedback on whether a system is healthy or not. Voltage transformers and current transformers measure these basic parameters and are capable of providing accurate measurement during fault conditions without failure.

• The measured values are converted into analog and/or digital signals and are made to operate the relays, which in turn isolate the circuits by opening the faulty circuits. In most of the cases, the relays provide two functions viz., alarm and trip, once the abnormality is noticed. The relays in olden days had very limited functions and were quite bulky. However, with advancement in digital technology and use of microprocessors, relays monitor various parameters, which give complete history of a system during both prefault and post-fault conditions.

• The opening of faulty circuits requires some time, which may be in milliseconds, which for a common day life could be insignificant. However, the circuit breakers, which are used to isolate the faulty circuits, are capable of carrying these fault currents until the fault currents are totally cleared. The circuit breakers are the main isolating devices in a distribution system, which can be said to directly protect the system. • The operation of relays and breakers require power sources, which shall not be affected by faults in the main distribution. Hence, the other component, which is vital in protective system, is batteries that are used to ensure uninterrupted power to relays and breaker coils. The above items are extensively used in any protective system and their design requires careful study and selection for proper operation.

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1.4 Summary Power System Protection – Main Functions 1. To safeguard the entire system to maintain continuity of supply. 2. To minimize damage and repair costs. 3. To ensure safety of personnel. Power System Protection – Basic Requirements 1. Selectivity: To detect and isolate the faulty item only. 2. Stability: To leave all healthy circuits intact to ensure continuity of supply. 3. Speed: To operate as fast as possible when called upon, to minimize damage, production downtime and ensure safety to personnel. 4. Sensitivity: To detect even the smallest fault, current or system abnormalities and operate correctly at its setting. Power System Protection – Speed is Vital!! The protective system should act fast to isolate faulty sections to prevent: • Increased damage at fault location. Fault energy = I2 × Rf × t, where t is time in seconds. • Danger to the operating personnel (flashes due to high fault energy sustaining for a long time). • Danger of igniting combustible gas in hazardous areas, such as methane in coal mines which could cause horrendous disaster. • Increased probability of earth faults spreading to healthy phases. • Higher mechanical and thermal stressing of all items of plant carrying the fault current, particularly transformers whose windings suffer progressive and cumulative deterioration because of the enormous electromechanical forces caused by multi-phase faults proportional to the square of the fault current. Sustained voltage dips resulting in motor (and generator) instability leading to extensive shutdown at the plant concerned and possibly other nearby plants connected to the system.

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11 Power System Protection – Qualities Dependability Security Reliability Dependability: It MUST trip when called upon. Security: It must NOT trip when not supposed to. Power System Protection – Basic Components

1.Voltage transformers and current transformers: To monitor and give accurate feedback about the healthiness of a system. 2.Relays: To convert the signals from the monitoring devices, and give instructions to open a circuit under faulty conditions or to give alarms when the equipment being protected, is approaching towards possible destruction. 3. Fuses: Self-destructing to save the downstream equipment being protected. Circuit breakers: These are used to make circuits carrying enormous currents, and also to break the circuit carrying the fault currents for a few cycles based on feedback from the relays. DC batteries: These give uninterrupted power source to the relays and breakers that is independent of the main power source being protected.

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Unit-I [Circuit Breakers –I]

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Contents 1.

Overview of Power system

2.

Why protect?

3.

Causes and types of faults

4.

Factors influencing protection system design

5.

Aspects of protection system

6.

Zones of protection

7.

Protection types and classes

8.

Important consideration while applying protection

9.

ANSI reference numbers EEED-BVRIT

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Contents 1.

Overview of Power system

2.

Why protect?

3.

Causes and types of faults

4.

Factors influencing protection system design

5.

Aspects of protection system

6.

Zones of protection

7.

Protection types and classes

8.

Important consideration while applying protection

9.

ANSI reference numbers EEED-BVRIT

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Components of a power system

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Components of a power system Generating power

Phase A

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Phase B

Phase C

_ 120° 240° 360°

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Exporting power

Transmission System

Load

AC Generator

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Step-Up Transformer

Step-Down Transformer


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Transmission System

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Transmission & Distribution

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Load

Sub Station

Transmission Voltage From Power Company

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Generation to Load

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Distribution connection

Utility Transformer

Load Center

Load Center

Meter

Meter Transformer From Utility Service

Overhead Service

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Domestic load

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Substation SLD at Generation



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Substation SLD at load centre



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MV Indoor distribution


Protection – Why Is It Needed ?

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All Power Systems may experience faults at some time. PROTECTION IS INSTALLED TO : Detect fault occurrence and isolate the faulted equipment. SO THAT : Damage to the faulted equipment is limited; Disruption of supplies to adjacent un-faulted equipment is minimized. PROTECTION IS EFFECTIVELY AN INSURANCE POLICY - AN INVESTMENT AGAINST DAMAGE FROM FUTURE FAULTS. EEED-BVRIT N.RAMCHANDER-BVRIT


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Faults on power systems risk : Severe damage to the faulted equipment : Excessive current may flow; Causes burning of conductors or equipment windings; Arcing - energy dissipation; Risk of explosions for oil - filled switchgear, or when in hazardous environments.

Damage to adjacent plant : As the fault evolves, if not cleared quickly; Due to the voltage depression / loss of supply. RAMCHANDER-BVRIT

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Faults on power systems risk : Danger to staff or the public : Risk of shock from direct contact with the faulted equipment; Danger of potential (voltage) rises in exposed metalwork – accessible to touch; Fumes released by burning insulation; Burns etc.

Disruption to adjacent plant : Prolonged voltage dips cause motors to stall; Loss of synchronism for synchronous generators / motors.

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Summary : Protection must : Detect faults and abnormal operating conditions; Isolate the faulted equipment.

So as to : Limit damage caused by fault energy; Limit effect on rest of system.

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Power System Faults - Causes

Lightning

Wind

Ice and Snow storm

Flying objects

Contamination of insulators

Physical contact by animals

Human errors

Falling trees

Insulation aging

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Underground Cables

Diggers Overloading Oil Leakage Ageing



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Overhead Lines

Lightning Kites Trees Moisture Salt Birds Broken Conductors

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Machines

Mechanical Damage Unbalanced Load


Types of Fault

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Ø/E

a b c e

Ø/Ø/E

a b c e

Ø/Ø

a 3Ø b c

a b c

3Ø/E

a b c e

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Types of Fault

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Cross country fault a

a'

b c

b'

e

e

c'


Types of Fault

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Open circuit + ø/e a b c e

Fault between adjacent parallel Lines


Types of Fault

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Changing fault in cable

a

b

c


Voltages And Currents During Faults

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Fault Fault Currents 5000 4000

Amps

3000 2000 1000

Ia Ib

0

Ic

-1000 -2000

In

-3000 -4000 -5000 Time


Voltages And Currents During Faults

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Fault Fault Voltages 600 400 200

Volts

Va Vb

0

Vc

-200 -400 -600 Time


Factors Influencing Protection System Design42 Types of fault and abnormal conditions to be protected against Quantities available for measurement Types of protection available Speed Fault position discrimination Dependability / Reliability Security / Stability EEED-BVRIT N.RAMCHANDER-BVRIT

Factors Influencing Protection System Design43

Overlap of protections

Phase discrimination / Selectivity

Instrument transformers (CTs & VTs)

Auxiliary supplies

Back-up protection

Cost

Duplication of protection EEED-BVRIT

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Aspects of Protection System

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Reliability Speed Discrimination (Zones, Phases) Simplicity


Protection Aspects - Reliability R2

R1

G1

R3

45

R4

G2 F

Reliability Security Dependability


Protection Aspects - Reliability

Security

Dependability

AND

1

46

OR

2

1

2


Protection Aspects - Reliability R2

R1

G1

R3

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R4

G2 F

Reliability Dependability Security

D

S



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Dependability / Reliability Protection must operate when required to Failure to operate can be extremely damaging and disruptive Faults are rare : Protection must operate even after years of inactivity Improved by use of :

Back-up protection

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Security / Stability Protection must not operate when not required to, e.g. due to: Load switching Faults on other parts of the system Recoverable power swings EEED-BVRIT N.RAMCHANDER-BVRIT

Protection Aspects - Speed

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Speed

Milliseconds Count EEED-BVRIT N.RAMCHANDER-BVRIT


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Speed Fast operation : Minimizes damage and danger

Very fast operation : Minimizes system instability Discrimination and security can be costly to achieve as it generally involves additional signaling / communications equipment. EEED-BVRIT N.RAMCHANDER-BVRIT


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Speed

D a m a g e

Catastrophic Damage Steel Copper Cable

100

200

300

500

Time (ms) EEED-BVRIT N.RAMCHANDER-BVRIT

Fault Discrimination - Zones Of Protection

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Fault Position Discrimination Power system divided into PROTECTED ZONES Must isolate only the faulty equipment or section


Fault Discrimination - Zones Of Protection Busbar Protection

G

Trf

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Busbar Protection

G

Line Protection

Generator Protection

Generator Protection Motor Protection

M


Zones of Protection - Protection Overlap

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Overlap of Protections No blind spots Where possible use overlapping CTs

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Zones of Protection - Protection Overlap

BBP ‘1’

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BBP ‘2’

J

H

‘Z’ G

LP ‘H’

LP ‘J’

L

K

LP ‘K’

LP ‘L’ EEED-BVRIT

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Fault Discrimination - Phases

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Phase Discrimination Correct indication of phases involved in the fault Important for single phase tripping and autoreclosing applications


Example 1 – Dependability / Security

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R6 B6

R7

R2

R1 B1

F

B2

R3

R4

B3

B5

R5

B7 B4

Directional relays provided as shown Breakers marked as shown Fault at F EEED-BVRIT N.RAMCHANDER-BVRIT


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R6 B6

R7

R2

R1 B1

F

B2

R3

R4

B3

B5

R5

B7 B4

Resulted in operation of R1, R2, and R5, which in turn tripped their respective breakers Was there loss of dependability or security?



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R6 B6

R7

R2

R1 B1

F

B2

R3

R4

B3

B5

R5

B7 B4

Resulted in operation of R1, R2, and R5, which in turn tripped their respective breakers Was there loss of dependability or security? Yes, relay R5 lost its security for this fault EEED-BVRIT N.RAMCHANDER-BVRIT

Example 2 – Zone discrimination

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R6 B6

R7

R1

R2

B1

B2

R3

R4

B3

B5

R5

B7 B4

System as shown with relays and breakers marked



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R6 B6

R7

R1

R2

B1

B2

R3

R4

B3

B5

R5

B7 B4

System as shown with relays and breakers marked A single fault has resulted in the operation of breakers B1, B2, B3 and B4. There was no loss of security or dependability Identify the location of the fault N.RAMCHANDER-BVRIT

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R6 B6

R7

R1

R2

B1

B2

R3

R4

B3

B5

R5

B7 B4

Fault in the overlap zone at breaker B2 as shown


Cost

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The cost of protection is equivalent to an insurance policy against damage to plant, and loss of supply and customer goodwill. Acceptable cost is based on a balance of economics and technical factors. Cost of protection should be balanced against the cost of potential hazards. There is an economic limit on what can be spent. MINIMUM COST : Must ensure that all faulty equipment is isolated by protection. EEED-BVRIT N.RAMCHANDER-BVRIT

Cost

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TOTAL COST should take account of : Relays, schemes and associated panels and panel wiring Setting studies Commissioning CTs and VTs Maintenance and repairs to relays Damage repair if protection fails to operate Lost revenue if protection operates unnecessarilyEEED-BVRIT N.RAMCHANDER-BVRIT

Cost

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DISTRIBUTION SYSTEMS Large numbers of switching and distribution points, transformers and feeders Economics often overrides technical issues Protection may be the minimum consistent with statutory safety regulations Speed less important than on transmission systems Back-up protection can be simple and is often inherent in the main protection Although important, the consequences of mal-operation or EEED-BVRIT

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failure to operate is less serious than for transmission systems

Cost

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TRANSMISSION SYSTEMS Emphasis is on technical considerations rather than economics Economics cannot be ignored but is of secondary importance compared with the need for highly reliable, fully discriminative high speed protection Higher protection costs justifiable by high capital cost of power system elements protected Risk of security of supply should be reduced to lowest practical levels High speed protection requires unit protection Duplicate protections used to improve reliability Single phase tripping and auto-reclose may be required to EEED-BVRIT maintain system stability N.RAMCHANDER-BVRIT

Types of Protection

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Fuses For : LV Systems, Distribution Feeders and Transformers, VTs, Auxiliary Supplies Direct Acting AC Trip For : LV Systems, Pole Mounted Reclosers Overcurrent and Earthfault Widely used in all Power Systems Non-Directional Voltage Dependant Directional N.RAMCHANDER-BVRIT

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Types of Protection

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Differential For : Feeders, Busbars, Transformers, Generators, etc. High Impedance Restricted E/F Biased (or low-impedance) Pilot Wire Digital EEED-BVRIT N.RAMCHANDER-BVRIT

Types of Protection

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Distance For : Distribution Feeders and Transmission and Sub-Transmission Circuits Also used as Back-up Protection for Transformers and Generators Phase Comparison For : Transmission Lines Directional Comparison For : Transmission Lines EEED-BVRIT N.RAMCHANDER-BVRIT

Types of Protection

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Miscellaneous Under and Over Voltage Under and Over Frequency Special Relays for Generators, Transformers, Motors, etc. Control Relays Auto-Reclose, Tap Change Control, etc. Tripping and Auxiliary Relays EEED-BVRIT N.RAMCHANDER-BVRIT

Classes of Protection

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Non-Unit, or Unrestricted Protection : No specific point downstream up to which protection will protect Will operate for faults on the protected equipment; May also operate for faults on downstream equipment, which has its own protection; Need for discrimination with downstream protection, usually by means of time grading.


Classes of Protection

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Unit, or Restricted Protection : Has an accurately defined zone of protection An item of power system plant is protected as a unit; Will not operate for out of zone faults, thus no back-up protection for downstream faults.


Important Considerations When Applying Protection

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Current and Voltage Transformers These are an essential part of the protection scheme to reduce primary current and volts to a low level suitable to input to relay. They must be suitably specified to meet the requirements of the protective relays. Correct connection of CTs and VTs to the protection is important. In particular for directional, distance, phase comparison and differential protections. VTs may be electromagnetic or capacitor types. Busbar VTs : Special consideration needed when used for line protection. N.RAMCHANDER-BVRIT

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Instrument Transformer Circuits

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Never open circuit a CT secondary circuit, so : Never fuse CT circuits; VTs must be fused or protected by MCB. Do wire test blocks in circuit (both VT and CT) to allow commissioning and periodic injection testing of relays. Earth CT and VT circuits at one point only; Wire gauge > 2.5mm2 recommended for mechanical strength. EEED-BVRIT N.RAMCHANDER-BVRIT

Auxiliary Supplies Required for :

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TRIPPING CIRCUIT BREAKERS CLOSING CIRCUIT BREAKERS PROTECTION and TRIP RELAYS

AC AUXILIARY SUPPLIES are only used on LV and MV systems. DC AUXILIARY SUPPLIES are more secure than AC supplies. SEPARATELY FUSED SUPPLIES used for each protection. DUPLICATE BATTERIES are occasionally provided for extra security. MODERN PROTECTION RELAYS need a continuous auxiliary supply. During un-operated (healthy) conditions, they draw a small ‘QUIESCENT’ load to keep relay circuits energized. During operation, they draw a larger current which increases due to operation of output elements. EEED-BVRIT N.RAMCHANDER-BVRIT

Relay Outputs

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TRIP OUTPUT CONTACTS : Check that these are rated sufficiently to make and carry the circuit breaker trip coil current. If not, a heavier duty tripping relay will be needed. Use a circuit breaker normally open (52a) contact to interrupt trip coil current. This extends the life of the protection relay trip contacts. TYPE OF CONTACTS :

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Make (M) / Normally Open (NO)

Close when energised, typically used for tripping.

Break (B) / Normally Closed (NC)

Close when de-energised.

Changeover (C/O)

Can be break before make (BBM) or make before break EEED-BVRIT (MBB).

ANSI Reference Numbers 2 21 25 27 30 32 37 40 46 49 50 51 51N 52 52a 52b

Time Delay Distance Synchronising Check Undervoltage Annunciator Directional Power Undercurrent or Under Power Field Failure Negative Sequence Thermal Instantaneous Overcurrent Time Delayed Overcurrent Time Delayed Earthfault Circuit Breaker Auxiliary Switch - Normally Open Auxiliary Switch - Normally Closed

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59 60 64 67 67N 79 81 85 86 87

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Overvoltage Voltage or Current Balance Instantaneous Earth Fault (High Impedance) Directional Overcurrent Directional Earthfault Alarm Auto-Reclose Frequency Signal Receive Trip / Lock-Out Differential

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1

THE PHILOSOPHY OF PROTECTIVE RELAYING WHAT IS PROTECTIVE RELAYING? We usually think of an electric power system in terms of its more impressive parts–the big generating stations, transformers, high-voltage lines, etc. While these are some of the basic elements, there are many other necessary and fascinating components. Protective relaying is one of these. The role of protective relaying in electric-power-system design and operation is explained by a brief examination of the over-all background. There are three aspects of a power system that will serve the purposes of this examination. These aspects are as follows: A. Normal operation B. Prevention of electrical failure. C. Mitigation of the effects of electrical failure. The term “normal operation” assumes no failures of equipment, no mistakes of personnel, nor “acts of God.” It involves the minimum requirements for supplying the existing load and a certain amount of anticipated future load. Some of the considerations are: A. Choice between hydro, steam, or other sources of power. B. Location of generating stations. C. Transmission of power to the load. D. Study of the load characteristics and planning for its future growth. E. Metering F. Voltage and frequency regulation. G. System operation. E. Normal maintenance. The provisions for normal operation involve the major expense for equipment and operation, but a system designed according to this aspect alone could not possibly meet present-day requirements. Electrical equipment failures would cause intolerable outages. There must be additional provisions to minimize damage to equipment and interruptions to the service when failures occur. Two recourses are open: (1) to incorporate features of design aimed at preventing failures, and (2) to include provisions for mitigating the effects of failure when it occurs. Modern

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THE PHILOSOPHY OF PROTECTIVE RELAYING

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80 power-system design employs varying degrees of both recourses, as dictated by the economics of any particular situation. Notable advances continue to be made toward greater reliability. But also, increasingly greater reliance is being placed on electric power. Consequently, even though the probability of failure is decreased, the tolerance of the possible harm to the service is also decreased. But it is futile-or at least not economically justifiable-to try to prevent failures completely. Sooner or later the law of diminishing returns makes itself felt. Where this occurs will vary between systems and between parts of a system, but, when this point is reached, further expenditure for failure prevention is discouraged. It is much more profitable, then, to let some failures occur and to provide for mitigating their effects. The type of electrical failure that causes greatest concern is the short circuit, or “fault” as it is usually called, but there are other abnormal operating conditions peculiar to certain elements of the system that also require attention. Some of the features of design and operation aimed at preventing electrical failure are: A. Provision of adequate insulation. B. Coordination of insulation strength with the capabilities of lightning arresters. C. Use of overhead ground wires and low tower-footing resistance. D. Design for mechanical strength to reduce exposure, and to minimize the likelihood of failure causable by animals, birds, insects, dirt, sleet, etc. E. Proper operation and maintenance practices. Some of the features of design and operation for mitigating the effects of failure are: A. Features that mitigate the immediate effects of an electrical failure. 1. Design to limit the magnitude of short-circuit current.1 a. By avoiding too large concentrations of generating capacity. b. By using current-limiting impedance. 2. Design to withstand mechanical stresses and heating owing to short-circuit currents. 3. Time-delay undervoltage devices on circuit breakers to prevent dropping loads during momentary voltage dips. 4. Ground-fault neutralizers (Petersen coils). B. Features for promptly disconnecting the faulty element. 1. Protective relaying. 2. Circuit breakers with sufficient interrupting capacity. 3. Fuses. C. Features that mitigate the loss of the faulty element. 1. Alternate circuits. 2. Reserve generator and transformer capacity. 3. Automatic reclosing.

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81 D. Features that operate throughout the period from the inception of the fault until after its removal, to maintain voltage and stability. 1. Automatic voltage regulation. 2. Stability characteristics of generators. E. Means for observing the electiveness of the foregoing features. 1. Automatic oscillographs. 2. Efficient human observation and record keeping. F. Frequent surveys as system changes or additions are made, to be sure that the foregoing features are still adequate. Thus, protective relaying is one of several features of system design concerned with minimizing damage to equipment and interruptions to service when electrical failures occur. When we say that relays “protect,” we mean that, together with other equipment, the relays help to minimize damage and improve service. It will be evident that all the mitigation features are dependent on one another for successfully minimizing the effects of failure. Therefore, the capabilities and the application requirements of protective-relaying equipments should be considered concurrently with the other features.2 This statement is emphasized because there is sometimes a tendency to think of the protective-relaying equipment after all other design considerations are irrevocably settled. Within economic limits, an electric power system should be designed so that it can be adequately protected.

THE FUNCTION OF PROTECTIVE RELAYING The function of protective relaying is to cause the prompt removal from service of any element of a power system when it suffers a short circuit, or when it starts to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system. The relaying equipment is aided in this task by circuit breakers that are capable of disconnecting the faulty element when they are called upon to do so by the relaying equipment. Circuit breakers are generally located so that each generator, transformer, bus, transmission line, etc., can be completely disconnected from the rest of the system. These circuit breakers must have sufficient capacity so that they can carry momentarily the maximum short-circuit current that can flow through them, and then interrupt this current; they must also withstand closing in on such a short circuit and then interrupting it according to certain prescribed standards.3 Fusing is employed where protective relays and circuit breakers are not economically justifiable. Although the principal function of protective relaying is to mitigate the effects of short circuits, other abnormal operating conditions arise that also require the services of protective relaying. This is particularly true of generators and motors. A secondary function of protective relaying is to provide indication of the location and type of failure. Such data not only assist in expediting repair but also, by comparison with

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82 human observation and automatic oscillograph records, they provide means for analyzing the effectiveness of the fault-prevention and mitigation features including the protective relaying itself.

FUNDAMENTAL PRINCIPLES OF PROTECTIVE RELAYING Let us consider for the moment only the relaying equipment for the protection against short circuits. There are two groups of such equipment–one which we shall call “primary” relaying, and the other “back-up” relaying. Primary relaying is the first line of defense, whereas back-up relaying functions only when primary relaying fails. PRIMARY RELAYING

Fig. 1. One-line diagram of a portion of an electric power system illustrating primary relaying.

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83 Figure 1 illustrates primary relaying. The first observation is that circuit breakers are located in the connections to each power element. This provision makes it possible to disconnect only a faulty element. Occasionally, a breaker between two adjacent elements may be omitted, in which event both elements must be disconnected for a failure in either one. The second observation is that, without at this time knowing how it is accomplished, a separate zone of protection is established around each system element. The significance of this is that any failure occurring within a given zone will cause the “tripping” (i.e., opening) of all circuit breakers within that zone, and only those breakers. It will become evident that, for failures within the region where two adjacent protective zones overlap, more breakers will be tripped than the minimum necessary to disconnect the faulty element. But, if there were no overlap, a failure in a region between zones would not lie in either zone, and therefore no breakers would be tripped. The overlap is the lesser of the two evils. The extent of the overlap is relatively small, and the probability of failure in this region is low; consequently, the tripping of too many breakers will be quite infrequent. Finally, it will be observed that adjacent protective zones of Fig. 1 overlap around a circuit breaker. This is the preferred practice because, for failures anywhere except in the overlap region, the minimum number of circuit breakers need to be tripped. When it becomes desirable for economic or space-saving reasons to overlap on one side of a breaker, as is frequently true in metal-clad switchgear the relaying equipment of the zone that overlaps the breaker must be arranged to trip not only the breakers within its zone but also one or more breakers of the adjacent zone, in order to completely disconnect certain faults. This is illustrated in Fig. 2, where it can be seen that, for a short circuit at X, the circuit breakers of zone B, including breaker C, will be tripped; but, since the short circuit is outside zone A, the relaying equipment of zone B must also trip certain breakers in zone A if that is necessary to interrupt the flow of short circuit current from zone A to the fault. This is not a disadvantage for a fault at X, but the same breakers in zone A will be tripped unnecessarily for other faults in zone B to the right of breaker C. Whether this unnecessary tripping is objectionable will depend on the particular application.

Fig. 2. Overlapping adjacent protective zones on one side of a circuit breaker.

BACK-UP RELAYING Back-up relaying is employed only for protection against short circuits. Because short circuits are the preponderant type of power failure, there are more opportunities for failure in short primary relaying. Experience has shown that back-up relaying for other than short circuits is not economically justifiable.


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84 A clear understanding of the possible causes of primary-relaying failure is necessary for a better appreciation of the practices involved in back-up relaying. When we say that primary relaying may fail, we mean that any of several things may happen to prevent primary relaying from causing the disconnection of a power-system fault. Primary relaying may fail because of failure in any of the following: A. Current or voltage supply to the relays. B. D-c tripping-voltage supply. C. Protective relays. D. Tripping circuit or breaker mechanism. E. Circuit breaker. It is highly desirable that back-up relaying be arranged so that anything that might cause primary relaying to fail will not also cause failure of back-up relaying. It will be evident that this requirement is completely satisfied only if the back-up relays are located so that they do not employ or control anything in common with the primary relays that are to be backed up. So far as possible, the practice is to locate the back-up relays at a different station. Consider, for example, the back-up relaying for the transmission line section EF of Fig. 3. The back-up relays for this line section are normally arranged to trip breakers A, B, I, and J. Should breaker E fail to trip for a fault on the line section EF, breakers A and B are tripped; breakers A and B and their associated back-up equipment, being physically apart from the equipment that has failed, are not likely to be simultaneously affected as might be the case if breakers C and D were chosen instead.

Fig. 3. Illustration for back-up protection of transmission line section EF.

The back-up relays at locations A, B, and F provide back-up protection if bus faults occur at station K. Also, the back-up relays at A and F provide back-up protection for faults in the line DB. In other words, the zone of protection of back-up relaying extends in one direction from the location of any back-up relay and at least overlaps each adjacent system element. Where adjacent line sections are of different length, the back-up relays must overreach some line sections more than others in order to provide back-up protection for the longest line. A given set of back-up relays will provide incidental back-up protection of sorts for faults in the circuit whose breaker the back-up relays control. For example, the back-up relays that trip breaker A of Fig. 3 may also act as back-up for faults in the line section AC. However,

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85 this duplication of protection is only an incidental benefit and is not to be relied on to the exclusion of a conventional back-up arrangement when such arrangement is possible; to differentiate between the two, this type might be called “duplicate primary relaying.” A second function of back-up relaying is often to provide primary protection when the primary-relaying equipment is out of service for maintenance or repair. It is perhaps evident that, when back-up relaying functions, a larger part of the system is disconnected than when primary relaying operates correctly. This is inevitable if back-up relaying is to be made independent of those factors that might cause primary relaying to fail. However, it emphasizes the importance of the second requirement of back-up relaying, that it must operate with sufficient time delay so that primary relaying will be given enough time to function if it is able to. In other words, when a short circuit occurs, both primary relaying and back-up relaying will normally start to operate, but primary relaying is expected to trip the necessary breakers to remove the short-circuited element from the system, and back-up relaying will then reset without having had time to complete its function. When a given set of relays provides back-up protection for several adjacent system elements, the slowest primary relaying of any of those adjacent elements will determine the necessary time delay of the given back-up relays. For many applications, it is impossible to abide by the principle of complete segregation of the back-up relays. Then one tries to supply the back-up relays from sources other than those that supply the primary relays of the system element in question, and to trip other breakers. This can usually be accomplished; however, the same tripping battery may be employed in common, to save money and because it is considered only a minor risk. This subject will be treated in more detail in Chapter 14. In extreme cases, it may even be impossible to provide any back-up protection; in such cases, greater emphasis is placed on the need for better maintenance. In fact, even with complete back-up relaying, there is still much to be gained by proper maintenance. When primary relaying fails, even though back-up relaying functions properly, the service will generally suffer more or less. Consequently, back-up relaying is not a proper substitute for good maintenance. PROTECTION AGAINST OTHER ABNORMAL CONDITIONS Protective relaying for other than short circuits is included in the category of primary relaying. However, since the abnormal conditions requiring protection are different for each system element, no universal overlapping arrangement of relaying is used as in short protection. Instead, each system element is independently provided with whatever relaying is required, and this relaying is arranged to trip the necessary circuit breakers which may in some cases be different from those tripped by the short-circuit relaying. As previously mentioned, back-up relaying is not employed because experience has not shown it to be economically justifiable. Frequently, however, back-up relaying for short circuits will function when other abnormal conditions occur that produce abnormal currents or voltages, and back-up protection of sorts is thereby incidentally provided.

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FUNCTIONAL CHARACTERISTICS OF PROTECTIVE RELAYING SENSITIVITY, SELECTIVITY, AND SPEED “Sensitivity,” “selectivity” and “speed” are terms commonly used to describe the functional characteristics of any protective-relaying equipment. All of them are implied in the foregoing considerations of primary and back-up relaying. Any relaying equipment must be sufficiently sensitive so that it will operate reliably, when required, under the actual condition that produces the least operating tendency. It must be able to select between those conditions for which prompt operation is required and those for which no operation, or time-delay operation, is required. And it must operate at the required speed. How well any protective-relaying equipment fulfills each of these requirements must be known for each application. The ultimate goal of protective relaying is to disconnect a faulty system element as quickly as possible. Sensitivity and selectivity are essential to assure that the proper circuit breakers will be tripped, but speed is the “pay-off.” The benefits to be gained from speed will be considered later. RELIABILITY That protective-relaying equipment must be reliable is a basic requirement. When protective relaying fails to function properly, the allied mitigation features are largely ineffective. Therefore, it is essential that protective-relaying equipment be inherently reliable, and that its application, installation, and maintenance be such as to assure that its maximum capabilities will be realized. Inherent reliability is a matter of design based on long experience, and is much too extensive and detailed a subject to do justice to here. Other things being equal, simplicity and robustness contribute to reliability, but they are not of themselves the complete solution. Workmanship must be taken into account also. Contact pressure is an important measure of reliability, but the contact materials and the provisions for preventing contact contamination are fully as important. These are but a few of the many design considerations that could be mentioned. The proper application of protective-relaying equipment involves the proper choice not only of relay equipment but also of the associated apparatus. For example, lack of suitable sources of current and voltage for energizing the relays may compromise, if not jeopardize, the protection. Contrasted with most of the other elements of an electric power system, protective relaying stands idle most of the time. Some types of relaying equipment may have to function only once in several years. Transmission-line relays have to operate most frequently, but even they may operate only several times per year. This lack of frequent exercising of the relays and their associated equipment must be compensated for in other ways to be sure that the relaying equipment will be operable when its turn comes. Many electric utilities provide their test and maintenance personnel with a manual that experienced people in the organization have prepared and that is kept up to date as new

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87 types of relays are purchased. Such a manual specifies minimum test and maintenance procedure that experience has shown to be desirable. The manual is prepared in part from manufacturers’ publications and in part from the utility’s experience. As a consequence of standardized techniques, the results of periodic tests can be compared to detect changes or deterioration in the relays and their associated devices. Testers are encouraged to make other tests as they see fit so long as they make the tests required by the manual. If a better testing technique is devised, it is incorporated into the manual. Some organizations include information on the purpose of the relays, to give their people better appreciation of the importance of their work. Courses may be given, also. Such activity is highly recommended. Unless a person is thoroughly acquainted with relay testing and maintenance, he can do more harm than good, and he might better leave the equipment alone. In some cases, actual field tests are made after installation and after careful preliminary testing of the individual relays. These field tests provide an excellent means for checking the over-all operation of all equipment involved. Careful maintenance and record keeping, not only of tests during maintenance but also of relay operation during actual service, are the best assurance that the relaying equipment is in proper condition. Field testing is the best-known way of checking the equipment prior to putting it in service, but conditions may arise in actual service that were not anticipated in the tests. The best assurance that the relays are properly applied and adjusted is a record of correct operation through a sufficiently long period to include the various operating conditions that can exist. It is assuring not only when a particular relaying equipment trips the proper breakers when it should for a given fault but also when other relaying equipments properly refrain from tripping.

ARE PROTECTIVE PRACTICES BASED ON THE PROBABILITY OF FAILURE? Protective practices are based on the probability of failure to the extent that present-day practices are the result of years of experience in which the frequency of failure undoubtedly has played a part. However, the probability of failure seldom if ever enters directly into the choice of a particular type of relaying equipment except when, for one reason or another, one finds it most difficult to apply the type that otherwise would be used. In any event, the probability of failure should be considered only together with the consequences of failure should it occur. It has been said that the justification for a given practice equals the likelihood of trouble times the cost of the trouble. Regardless of the probability of failure, no portion of a system should be entirely without protection, even if it is only back-up relaying.

PROTECTIVE RELAYING VERSUS A STATION OPERATOR Protective relaying sometimes finds itself in competition with station operators or attendants. This is the case for protection against abnormal conditions that develop slowly enough for an operator to have time to correct the situation before any harmful consequences develop. Sometimes, an alert and skillful operator can thereby avoid having

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88 to remove from service an important piece of equipment when its removal might be embarrassing; if protective relaying is used in such a situation, it is merely to sound an alarm. To some extent, the preference of relying on an operator has a background of some unfortunate experience with protective relaying whereby improper relay operation caused embarrassment; such an attitude is understandable, but it cannot be supported logically. Where quick and accurate action is required for the protection of important equipment, it is unwise to rely on an operator. Moreover, when trouble occurs, the operator usually has other things to do for which he is better fitted.

UNDESIRED TRIPPING VERSUS FAILURE TO TRIP WHEN DESIRED Regardless of the rules of good relaying practice, one will occasionally have to choose which rule may be broken with the least embarrassment. When one must choose between the chance of undesired or unnecessary tripping and failure to trip when tripping is desired, the best practice is generally to choose the former. Experience has shown that, where major system shutdowns have resulted from one or the other, the failure to trip–or excessive delay in tripping-has been by far the worse offender.

THE EVALUATION OF PROTECTIVE RELAYING Although a modern power system could not operate without protective relaying, this does not make it priceless. As in all good engineering, economics plays a large part. Although the protection engineer can usually justify expenditures for protective relaying on the basis of standard practice, circumstances may alter such concepts, and it often becomes necessary to evaluate the benefits to be gained. It is generally not a question of whether protective relaying can be justified, but of how far one should go toward investing in the best relaying available. Like all other parts of a power system, protective relaying should be evaluated on the basis of its contribution to the best economically possible service to the customers. The contribution of protective relaying is to help the rest of the power system to function as efficiently and as effectively as possible in the face of trouble.2 How protective relaying does this is as foIlows. By minimizing damage when failures occur, protective relaying minimizes: A. The cost of repairing the damage. B. The likelihood that the trouble may spread and involve other equipment. C. The time that the equipment is out of service. D. The loss in revenue and the strained public relations while the equipment is out of service. By expediting the equipment’s return to service, protective relaying helps to minimize the amount of equipment reserve required, since there is less likelihood of another failure before the first failure can be repaired.

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89 The ability of protective relaying to permit fuller use of the system capacity is forcefully illustrated by system stability. Figure 4 shows how the speed of protective relaying influences the amount of power that can be transmitted without loss of synchronism when short circuits occur.4 More load can be carried over an existing system by speeding up the protective relaying. This has been shown to be a relatively inexpensive way to increase the transient stability limit.5 Where stability is a problem, protective relaying can often be evaluated against the cost of constructing additional transmission lines or switching stations. Other circumstances will be shown later in which certain types of protective-relaying equipment can permit savings in circuit breakers and transmission lines.

Fig. 4. Curves illustrating the relation between relay-plus-breaker time and the maximum amount of power that can be transmitted over one particular system without loss of synchronism when various faults occur.

The quality of the protective-relaying equipment can affect engineering expense in applying the relaying equipment itself. Equipment that can still operate properly when future changes are made in a system or its operation will save much future engineering and other related expense. One should not conclude that the justifiable expense for a given protective-relaying equipment is necessarily proportional to the value or importance of the system element to be directly protected. A failure in that system element may affect the ability of the entire system to render service, and therefore that relaying equipment is actually protecting the service of the entire system. Some of the most serious shutdowns have been caused by consequential effects growing out of an original failure in relatively unimportant equipment that was not properly protected.

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HOW DO PROTECTIVE RELAYS OPERATE? Thus far, we have treated the relays themselves in a most impersonal manner, telling what they do without any regard to how they do it. This fascinating part of the story of protective relaying will be told in much more detail later. But, in order to round out this general consideration of relaying and to prepare for what is yet to come, some explanation is in order here. All relays used for short-circuit protection, and many other types also, operate by virtue of the current and/or voltage supplied to them by current and voltage transformers connected in various combinations to the system element that is to be protected. Through individual or relative changes in these two quantities, failures signal their presence, type, and location to the protective relays. For every type and location of failure, there is some distinctive difference in these quantities, and there are various types of protective-relaying equipments available, each of which is designed to recognize a particular difference and to operate in response to it.6 More possible differences exist in these quantities than one might suspect. Differences in each quantity are possible in one or more of the following: A. Magnitude. B. Frequency. C. Phase angle. D. Duration. E. Rate of change. F. Direction or order of change. G. Harmonics or wave shape. Then, when both voltage and current are considered in combination, or relative to similar quantities at different locations, one can begin to realize the resources available for discriminatory purposes. It is a fortunate circumstance that, although Nature in her contrary way has imposed the burden of electric-power-system failure, she has at the same time provided us with a means for combat.

Fig. 5. Illustration for Problem 2.

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PROBLEMS 1. Compare protective relaying with insurance. 2. The portion of a power system shown by the one-line diagram of Fig. 5, with generating sources back of all three ends, has conventional primary and back-up relaying. In each of the listed cases, a short circuit has occurred and certain circuit breakers have tripped as stated. Assume that the tripping of these breakers was correct under the circumstances. Where was the short circuit? Was there any failure of the protective relaying, including breakers, and if so, what failed? Assume only one failure at a time. Draw a sketch showing the overlapping of primary protective zones and the exact locations of the various faults. Case

Breakers Tripped

a

4, 5, 8

b

3, 7, 8

c

3, 4, 5, 6

d

1, 4, 5, 6

e

4, 5, 7, 8

f

4, 5, 6

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Arc Extinguishers

A common method used to extinguish an arc. In general, it confines, divides and cools the arc.

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•An An Arc Extinguisher is the component of the circuit breaker that extinguishes an arc when the contacts are opened opened. •An arc is a discharge of electric current crossing a gap between two contacts contacts. • Circuit breakers must be designed to control them because arcs cannot be prevented. •There are four techniques to extinguish an arc and there are several arc control methods. methods EEED-BVRIT

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What is an Arc?

Do you ever recall pulling a plug from a wall socket and seeing what appeared to be sparks? What you were observing, observing on a very small scale, was an attempt at arc formation between bet ee tthe e wall a co contacts tacts a and d tthe ep plug ug contacts in your hand. For the sake of this discussion,, let's define an arc as a discharge of electric current crossing a gap between two contacts. EEED-BVRIT

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Arcs are formed when the contacts of a circuit breaker are opened p under a load. Arcs can be very destructive and vary greatly in size and intensity. The size of the arc depends on the amount of current present when the contacts are pulled apart. F example, For l an arc th thatt fforms when h normall load current is broken is insignificant compared t th to the arc th thatt fforms when h a short h t circuit i it iis broken. Because arcs cannot be prevented, circuit breakers must be designed to control them. EEED-BVRIT

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The heat associated with an arc creates an i i d gas environment. ionized i t The more ionization, the better the conditions are for an arc to be maintained and grow. The bigger the arc, the more heat created, which increases ionization. Arcing is a condition that must be dealt with quickly and effectively by a circuit breaker.

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The important thing to remember here is that the ability of the circuit breaker to control the arc is the key to its short circuit interrupting capability. This is a critical factor for selecting circuit breakers breakers. A short circuit is the most devastating overcurrent condition. condition

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Current Zero or Zero Point is a very important p aspect p to arc extinguishing. g g At current zero, conditions are optimal for preventing p g an arc from continuing. g The current is said to be "Current Zero" when 180° and 360° 360°. the sine curve is at 0° 0°, 180°

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Voltage is also a very important consideration because it is the pressure that keeps the current moving. L ft unchecked, Left h k d voltage lt will ill keep k pushing hi the current through current zero and give new lif life tto th the arc. Voltage does not take kindly to being stopped in its tracks during the extinguishing of an arc. If it reignites, it can damage the whole electrical system. y EEED-BVRIT

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Circuit breakers take this process into account by simultaneously opening the contacts and extinguishing the arc. arc The successful extinguishing of the arc depends on the Dielectric Strength of the gap between the contacts. The dielectric strength is the maximum voltage a dielectric can withstand without breaking down. A Dielectric is any insulating material between two conductors conductors. In these discussions, the circuit breaker contacts are the conductors and the insulating material can be air, gas or a vacuum. If the dielectric strength is greater than the voltage trying to re re--ignite the arc, the arc extinguishing will be successful.

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The invention of a device called DEDE-ION® arc extinguisher in the early 1900s by Westinghouse was a revolutionary advance in arc interruption. Improved versions were used for years with a majority of circuit breakers and continue to be used today with low voltage circuit breakers.

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A c Cont Arc Control ol Techniques Techniq es

Each approach has made improvements to its initial concept p in an effort to extinguish g arcs more efficiently. Arc control methods utilize one or more of the following general techniques:

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St t hi A Stretching Arc

The arc is produced when the contacts part. As the gap widens, widens the arc is stretched and cooled to the point where it is extinguished EEED-BVRIT

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B ki Arc Breaking A into i t Smaller S ll Pieces Pi

The arc is produced when the contacts part. The arc moves up into the arc divider and splits, cools and is extinguished

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Blowing g Out Arc

In this method, a highhigh-pressure gas blows the arc into an arc divider to be extinguished

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Enclosing Contacts

In this method, the contacts are housed in an oxygenyg -free enclosure with a dielectric oxygen such as a vacuum, gas or cooling oil. Without oxygen, oxygen the arc cannot sustain itself and the arc is extinguished.

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Enclosing Contacts

In this method, method the contacts are housed in an oxygen oxygen-free enclosure with a dielectric such as a vacuum, gas or cooling oil. Without oxygen, the arc cannot sustain itself and the arc is extinguished. EEED-BVRIT

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Arc A c Control Cont ol Methods

There are six methods used around the world todayy to deal with arc control. The two most commonly used methods are arc chute and vacuum interrupter. interrupter The other four methods are SF6, minimum oil magnetic coil and puffer. oil, puffer

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A c chute Arc ch te method

The arc chute method only uses the Breaking Arc into Smaller Pieces technique. Arc chutes are normally associated with low voltage circuit breakers due to efficiency and cost cost. In general, an arc chute will confine, divide and cooll an arc, resulting lti in i the th arc being b i unable bl to t sustain itself. There is one arc chute for each set of contacts. EEED-BVRIT

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Vacuum Vac m Interrupter Inte pte Method

The vacuum interrupter p method uses the Enclosing g Contacts technique to extinguish arcs. The vacuum enables the contacts to be smaller and eliminates the divider, divider making this method the most cost effective and efficient above 1000V. One vacuum interrupter is provided for each set of contacts. t t

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Arcing takes place within a sealed evacuated enclosure. The contacts are located inside and arcing occurs when the contacts are separated. Because the environment inside the interrupter envelope is a vacuum, an arc cannot be easily easil sustained. It will not reach the intensity possible with an arch chute. EEED-BVRIT

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SF6 method

The SF6 method also uses the Enclosing Contacts technique. It was a precursor to the vacuum interrupter and used SF6 gas as the dielectric. The heat energy gy created byy the arc works to break apart the SF6 molecules. The larger the arc, the greater the breakdown of the gas which aids in extinguishing the arc. The technology is related more to European manufacturers of medium and higher voltage circuit breakers. EEED-BVRIT

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minimum minim m oil method

The minimum oil method also uses Enclosing Contacts with oil as the dielectric. The arc energy is absorbed as it rips hydrogen away from the oil molecule. The oil itself also helps p to cool the arc. As current zero is approached, more oil is drawn into the system, further cooling and Deionizing the arc. It is used today in low voltage situations and potentially explosive environments where an arc chute is not desirable. EEED-BVRIT

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magnetic coil method

The magnetic coil method uses the g Arc into Smaller Pieces Breaking technique. It is very similar to the arc chute method method. The natural movement of an arc is upward in this instance, upward, instance into an arc chute. chute A coil, called a blowout coil, is located in th center the t off th the arc chute. h t EEED-BVRIT

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The arc is broken into two. The arcs are lengthened and cooled as they rise higher. The cooling reduces the rate of ionization When the ionization drops below the level necessary to sustain the arcs, they extinguish at current zero zero. Prior to vacuum interrupter technology becoming the method of choice with medium voltage power breakers for extinguishing arcs arcs, the magnetic coil method served well for many years. EEED-BVRIT

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p ffe method puffer

The puffer method uses the Blowing Out g Contacts techniques. q Arc and Enclosing It uses SF6 gas as the dielectric. It is the most efficient and cost effective method above 38 kV.

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This type interrupter is basically a pair of separable contacts, a piston and a cylinder, mounted in a reservoir of gas. As the contacts part, the piston moves up to drive the g gas through g the arc to interrupt it. It also utilizes coils and takes advantage of natural magnetic affects to create a force sufficient to extinguish the arc.

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CIRCUIT BREAKERS The circuit breaker is the most important and p of all types yp of power p circuit interruption p complicated equipment. This is due to its highly important p y of interrupting p g the ppowerful short circuit capability current, over and above its normal role of g, isolatingg and interrupting p g nominal load conducting, currents. Circuit Breakers have two basic functions •Switching • F lt interruption Fault i t ti EEED-BVRIT

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• Close the current path and carry the steady state load current without overheating. • Maintain sufficient contact pressure when closed to prevent a high resistance path between contacts. • Rapidly open the contacts under fault condition so that current interruption does not resulting excessive burning of the contacts. • Always Al provide id adequate d t phase h and d phase h tto ground insulation.

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ARC Phenomena in Circuit Breakers • When the contacts are being separated arcing is possible even when the circuit emf is considerably below the minimum cold electrode breakdown voltage, voltage because of large local increase in voltage due to the circuit self inductance. • The arc is extinguished every time the current passes through zero and can restrike only if transient recovery voltage across the electrode already separated and continuing to separate reaches a sufficiently high value know as breakdown voltage.

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Arcing Chamber • It is a closed volume containing a fixed contact, a moving contact and the interrupting medium. medium • An arc is created when the contacts part. The i t interrupting ti medium di i is responsible ibl f for quenching the arc and establishing the nominal l l off isolation level i l ti between b t th open contacts. the t t • Several chambers may be connected in series to serve higher voltage levels. In this case a grading capacitor is installed in parallel with each chamber to balance the voltage across the contacts when parting. EEED-BVRIT

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Insertion resistor • The sudden modification of circuit characteristics,, when circuit breakers operate, p , produces peak voltage impulses where the level is determined byy the circuit characteristics. These impulses may reach very high levels and must be reduced. A well-known method is closing or opening in two or three steps on resistors. EEED-BVRIT

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• On trip: the voltage impulse levels are generally acceptable when interrupting nominal or short circuit currents, currents but they can be dangerously high when interrupting small capacitive or inductive currents. • On close: sudden energizing of a circuit always generates voltage impulses with moderate levels levels, with the exception of closing or reclosing on long unloaded lines where the impulses, impulses function of the line length, instant of closing or reclosing and discrepancy of the three poles, poles can reach extremely high levels. EEED-BVRIT

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Operating Mechanism • The operating mechanism is where the needed energy to part the contacts and to extinguish the arc is developed. developed • The most common operating mechanisms in circuit i it breakers b k are • Spring operated • Hydraulically operated • Pneumatically operated. operated EEED-BVRIT

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Control • Closing and tripping coils • Control relaying system • Pressure switches and gauges • Surveillance and alarm system g system y to restore the • Re-inflating energy spent on the operation.

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Functioning Characteristics • Correct function • The circuit breaker control must ensure correct closing action, whatever the closing current value, and ensure breaking (opening) at the required moment by releasing, releasing by mechanical action or via a relay, the energy stored in the accumulators.

• Operation cycles • The circuit breaker has to be capable of executing different operation cycles and achieve fast breaking of short circuit currents -- the faster, the better for the network. EEED-BVRIT

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Classification Depending Upon the interrupting medium • A circuit i i breaker b k h has to interrupt i weak k capacitive or inductive currents, up to high short h t circuit i it currents t and, d as a result, lt to t extinguish powerful electric arcs. The main problem bl i then, is th essentially, ti ll an arcing i problem. bl Another problem is over voltage impulses; this i related is l t d to t the th nature t off the th circuit i it where h it is i installed. EEED-BVRIT

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Interrupting Medium • Mineral oil • Compressed air • Sulfur hexafluoride (SF6)

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Fault Current PLOT

additional information

Graph +35

IFAULT

VFAULT

peak 32 pu (41 kA) I [pu], V [pu]

annotations

+12.5

line thickness

Fault Inception

label and unit

notes section

-10 0

0.02

0.04

0.06

0.08

Fault 0.1Interruption

Time (Sec)

Power System Transients

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Circuit Breakers (CB) • CB: interrupting highest short circuit currents needs sophisticated arcing chamber technology UR [kV] IB [kA] 36 160 36 275 245 63 362 50

Type SF6 Air Blast Oil SF6 (one chamber)

• Load switch: load currents – routinely switching • Disconnector: very small currents – safety switch Power System Transients

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Live Tank HV breaker


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Dead Tank HV breaker


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Breaker with Switching Resistors


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Short Circuit Interruption (1) Equivalent Circuit RL

LL

IS

Fixed Contact

Moving Contact

US

CL

U1

UN

U2

US

Fault!

10

IS

UN

5

rated current

Us

0 US

Fault!

-5 -10

-40

-30


-20

-10

0

10

20

30

t [ms]

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LL

IS

Fixed Contact

Moving Contact

US

CL

U1

UN

U2

US

Fault!

10

IS

UN

5

rated current

Us

0 US

Fault!

-5 -10

-40

-30


-20

-10

0

10

20

30

t [ms]

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LL

IS

Fixed Contact

Moving Contact

US

CL

U1

UN

U2

US

Fault!

10

IS

UN

5

rated current

Us

0 US

Fault!

-5 -10

-40

-30


-20

-10

0

10

20

30

t [ms]

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LL

IS

Fixed Contact

Moving Contact

US

CL

U1

UN

U2

US

Fault!

10

IS

UN

5

rated current

Us

0 US

Fault!

-5

f =

-10 -40

-30


-20

-10

0

10

20

1 2 ⋅ π ⋅ LLCL

30

t [ms]

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Curent Zero: Current Interruption Current

Fixed Contact

Arcing Voltage

100 V 1

10 A

2

Residual Current

t [µs] US

Transient Recovery Voltage (TRV)

Arc allways extinguishes at current zero (voltage peak),

Moving Contact

enegy dissipation high enough --> Arc cease conducting – high resistivity --> Current interruption - TRV e


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Curent Zero: No Current Interruption Current

Fixed Contact

Arcing Voltage

100 V 1

2

10 A

Arc allways extinguishes at current zero (voltage peak),

t [µs] US

Moving Contact

but contact distance is too small --> Arc keeps conducting --> Current continous to flow undisturbed Power System Transients

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Curent Zero: Thermal Re-Ignition Current

Fixed Contact

Arcing Voltage

100 V 1

10 A Transient Recovery Voltage (TRV)

2

t [µs]

Residual Current

US

New Current Rise

Voltage Collapse

Moving Contact

Loosing the „Thermal Race“ The TRV causes a residual current which heats up the arc plasma => arc gains conductivity => voltage collapses => current rises again Power System Transients

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Curent Zero: Dielectric Re-Ignition Current

Fixed Contact

Current stops flowing!

Arcing Voltage

100 V 10

50 A Transient Recovery Voltage (TRV)

NOTE: time scale! 20

t [µs]

Residual Current

US

New Current Rise

Moving Contact

Dielectric Strength insufficient

Voltage Collapse

The TRV reaches a value which causes a dielectric breakdown of the (still opening) contact gap Power System Transients

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CB: modeling • Mechanical delay time (from command to contact separation): min 10...20 ms w w

“opening time” = contact separation current interruption: default at zero crossing

• Arcing time: 1...5 cycles w w

depending on CB type and current wave form include arcing time in “opening time”

• Arc modeling w

w w

Arc voltage – current relationship highly complex (parameters: CB design, thermodynamics) generic models mostly insufficient – special modeling necessary zero arcing voltage for high voltage systems constant value (10...100 V) for low voltage systems


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Introduction to Protection Switchgear & Protection Unit 1

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