Write short note about Power System structure with diagram. Power System structure with diagram:
Electrical power system can be divided into following regions regions : Generating stations Transmission systems
• •
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Receiving station
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Distribution system
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Load points
Generating stations:
In all these regions, there are switchgears. Bus bars are connecting bars to which a number of local feeders are connected. Bus bars operate at constant voltage. Besides the bus bars, there are other equipment in the electrical schemes such as circuit breakers, CTs, PTs, etc. These equipments can be installed according to various schemes depending upon requirements. Equipment in electrical scheme of Transmission system;
The power transformers are installed between two bus bars of different voltage levels. A powe po werr tran transf sfor orme merr is the the cost costlilies est, t, he heav avie iest st,, an andd mo most st impo import rtan antt eq equi uipm pmen entt in substation.
Typical station is distribution system.
In 11 kV distribution system, the cost of elaborate protection may not be justified for protection transformer up to about 500 kVA. The substations are generally unattended. The H.V fuse is the only protection provided on H.V. side.
Receiving system & load points for a medium size industrial work:
The switch gear is installed in the substation of local points, such as industrial works, railway substation, cinema house, large building, foundries etc. The substation has following items: - Incoming line section - Transformer section - Secondary switching section
When we connect the large capacitor bank in series?
We connect large capacitor bank in series to improve the voltage power supply at the load end in balanced transmission line when there is considerable voltage drop along the balanced transmission line due to high impedance of the line. So in order to bring the voltage at the load terminals within its limits (i.e. (+ or - %6) of the rated high terminal voltage) the large capacitor bank is used in series.
What is the usual apparatus in a substation?
A substation usually contains the following major apparatus: 1. Transformers 3. Horn-gap switches 5. Grounding switches 7. Current-limiting reactors
2. Circuit breakers 4. Disconnect switches 6. Surge arresters 8. Instrument transformers
Why we are use 11KV / 22KV / 33KV / 66KV / 110KV / 230KV / 440KV this type of ratio. Why can’t we use other voltage ratio like 54KV / 99KV etc?
When an alternator generates voltage, we always use a multiple of 1.11 because for a pure sine wave the FORM FACTOR is the ratio of rms value of voltage or current with the avg. value of voltage or current and for pure sine wave rms value of current is Imax/root '2' and avg. value is 2Imax/pie and which comes out to be 1.1;
We can't have a combination of other then a multiple of 1.11*. So we can see all the voltages are made inevitably multiple of this value (1.1, which is the form factor of ac wave). Also it provides us the best economic construction of step up and step down transformers. * In the case of a Square Wave ie. a digital wave, the RMS and the average value are equal; therefore, the form factor is 1.
Why, when birds sit on transmission lines or current wires doesn't get shock?
Answer: It’s true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock. So if a human touch single one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral) and in the most of electric lines the neutral is grounded..So that means that human who touch the line closes the circuit between phase and neutral.
What are the advantages of static relay over electromagnetic relay?
o Low power consumption as low as 1mW o No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts o No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc. o A single relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional blocks. This is not possible in electromagnetic relays o Static relay is compact o Superior operating characteristics and accuracy o Static relay can think , programmable operation is possible with static relay o Effect of vibration is nil, hence can be used in earthquake-prone areas o Simplified testing and servicing. Can convert even non- electrical quantities to electrical in conjunction with transducers.
What are the types of protection of a power system? Types of Protection: 1. Apparatus Protection • • • • •
Busbar Protection Motor Protection Generator Protection Transformer Protection Transmission Line Protection
2. System Protection •
Under-frequency Relays
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Out-of-Step Protection
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Islanding Systems
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Rate of Change of Frequency Relays
Why do we need protection of a power system?
Electrical apparatus operates at various voltage levels and may be enclosed or placed in open. Under abnormal operating conditions protection is necessary forSafety of electrical equipments. Safety of human personnel. • •
What are the purpose of ground wires in overhead transmission lines?
Ground wires are bare conductor supported at the top of transmission towers. They serve to shield the line and intercept lightning stroke before it hits the current-carrying conductors bellow. Ground wires normally do not carry current. Therefore they are often made of steel. The ground wires are solidly connected to ground at each tower.
Give the limitations of Merz Price protection.
Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT’s. In addition to this, there always an inherent phase difference between the primary and the secondary quantities and a possibility of current through the relay even when there is no fault.
What are the methods of capacitive switching?
Methods of capacitive switching: Opening of single capacitor bank. Closing of one capacitor bank against another. • •
What are the problems arising in differential protection in power transformer and how are they overcome?
1. Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.
2. Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil. 3. Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this. 4. Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.
What do you mean by Positive Sequence, Negative Sequence and Zero Sequence network?
When an unbalanced fault occurs, we need a way to easily analyze the network. Positive Sequence, Negative Sequence and Zero Sequence network is three methods which used to determine and calculate fault in a three phase AC transmission and distribution. Positive sequence with positive frequency, negative sequence with negative frequency, and zero sequence which are constant.
Figure: Typical Sequence Network Diagram
Write short notes on Inductive interference in a transmission line. Inductive Interference in Transmission lines:
Inductive interference depends upon gradient of voltage at surface of conductor or corona. The inductive reactance affected from conductor attenuation at 1000 kHz varies from 0.3 to 0.9 dB/mtr.
Describe the primary and back-up protection features that are provided for transmission lines. A Primary and back-up protection:
The primary protection is essential protection provided for protecting a machine. As a precautionary measure, an additional protection is generally provided and is called a “Back up protection”. The primary protection is the first to act and back-up protection is the next in the line of defence. Therefore, if primary protection fails, the back-up protection comes into action and removes the faulty part from one healthy system. When main protection is in-operative for purpose of maintenance, testing etc, the backup protection acts like main protection. The back-up protection can be classified as: - Relay back-up:
Same circuit breaker is used by both main and back-up protection, but protective systems are different. Separate trip coils may be provided for the same-breaker. - Breaker Back-up:
Different breakers are provided for main and back up protection; both the breakers are being in the same station. - Remote back-up:
Main & back up protection provided at different stations and are completely independent. - Centrally coordinated back-up:
Here system is having central control and it can be provided with centrally controlled back-up. The central coordinating station receives information about the abnormal conditions through high frequency carrier signals. The stored programme in digital computer determines the correct switching operation as regard severity of faults, system stability etc. Main protection at various stations and back-up protection for all stations is at central control centre.
What are the advantages of high voltage transmission and its limitations. Advantages and Limitations of high voltage transmission line:
The choice of transmission systems and rated voltages for a transmission line is made from HV AC (up to 220 kV) EHV AC (400 kV – 750 kV) UHVAC (above 760 kV AC) depending upon technical and economic consideration. Advantages: • •
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High power transferability of AC lines P α V 2. Line losses decrease with increase of transmission voltage and improvement of power factor for same power transfer. Bulk power transfer from large group of generating stations upto main transmission network.
Limitations: • •
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Higher voltage gives lesser current, lesser I 2R losses. Short circuit levels: In case of very long lines of above 500 km, intermediate switching sub-stations are necessary to install the shunt reactors for compensation. Right of way: In some cases of big cities, industrial localities, it is impossible to acquire right of way for EHV AC lines. Line insulation: The creep age distance (leakage distance) determined on the basis of required impulse with stand level. Corona: The critical value of voltage stress depends upon pressure, temperature, humidity, pollution level in air.
The three phase system has only two types of connections viz. star connection and delta connection. Why?
In the three phase system, the three phases should be so connected that we get a system that has equal line voltages displaced 120 degrees electrical from one another and acting simultaneously in the circuit. Only star and delta connections can satisfy these conditions. Any attempts made to connect the three phase in a manner other than these two schemes will result in a system of line voltages which are neither equal nor 120 degree displaced. Such a system will not serve the function as if three phase circuits are being connected.
What are the criteria for the classification of transmission lines as short, medium and long lines? Classification of transmission lines (Power):
The network of transmission lines is formed by three phase AC system. This is required for Bulk power transfer from large group of generating stations to main transmission network. Short power transmission line :
Short distance power transmission line has voltage ≤ 11000 volts. These lines are required to carry power from main sub-station to local distribution area. Further power supply is distributed through pole mounted sub- station or plinth mounted sub-station. Medium power transmission line:
Medium distance power transmission line has voltage range 33-132 kV. For the economic voltages for medium lines an approximate rule is 1 kV/mile or 0.6 kV/km. kV= 5.5 √km x 0.6 + kW/100. These lines supply power directly to big organization or industry, which establishes own sub-station. Long power transmission line:
A long distance power transmission line has voltage range 220 kV to 750 kV. These lines are used for transfer of power from sending end to receiving end or for system interconnection for exchange of power between independently controlled networks. For longer transmission lines, higher power transmission voltages are necessary (p α v^2). Higher voltage gives lesser current, lesser I^2R losses, higher power transferability.
What is the role of the distribution substation? Distribution Substation:
A bank of step-down transformers near the end users. The distribution substation converts voltage closer to the required usable levels. A typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side. Role of the Distribution Substation: •
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A distribution substation transfers power from the transmission system to the distribution system of an area. These substations are located near the consumers localities, receive electrical power from a high-voltage transmission system and convert it to voltage levels suitable for industrial, commercial, or residential use.
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Distribution substations are typically the points of voltage regulation, although on long distribution circuits (of several miles/kilometers), voltage regulation equipment may also be installed along the line. In addition to transforming voltage, distribution substations also isolate faults in either the transmission or distribution systems.
Distribution substation - 20/04kV MCset metal-clad switchgear (Schneider Electric)
What are the essential differences between H.V. and L.V. switchgears? H.V. Switch gears: Circuit breaker: Switching
during normal and abnormal conditions, interrupt the fault
currents. It is disconnecting switch to disconnect the system from line parts under no load condition. Isolators:
This is used to discharge the voltage on the lines to earth after disconnecting them. Earth switch:
This is used to divert the high voltage surge to earth and maintaining continuity during normal voltage. Surge arrester:
Current Transformer:
To step down the current for measurement, protection and
control purposes. Potential Transformer:
To step down the voltage for the purpose of protection,
measurement and control. L.V. Switchgears: MCBs (Miniature Circuit Breakers):
Switching OFF during abnormal conditions to
interrupt the fault current. A short length wire having low melting point, connected in series with circuit. In the event of fault, the circuit current rises abruptly and fuse wire melts to interrupt the circuit. Fuses:
these are used to ON /OFF the power of a circuit. These are used in power/ control circuits. The switches are specified as per voltage rating, current rating, number of poles, duty cycle and fault interruption capacity. Switches:
What is PLC? How does it work? Distinguish SCADA & PLC controlled system. What is PLC?
The PLC (Programmable Logic Controller) is a small industrial computer usually found in factories. Its main use is to replace the relay logic of a plant or process. Today, the PLC is being used in SCADA systems to do its very good programmability. Earlier PLC’s have no serial communication ports for interfacing to radio for transferring of data. Nowadays, PLC's have extensive communication features and a wide support for popular radio units being used for SCADA system. In the near future we are seeing the merging of the RTUs(Remote Terminal Unit) and the PLC’s. What is SCADA?
SCADA (Supervisory Control And Data Acquisition) system refers to the combination of telemetry and data acquisition. It consists of collecting information, transferring it back to a central site, carrying out necessary analysis and control, and then displaying this data on a number of operator screens. The SCADA system is used to monitor and control a plant or equipment. Control may be automatic or can be initiated by operator commands. Components of SCADA System
Components of a SCADA System a SCADA system is composed of the following: 1. Field Instrumentation 2. Remote Stations 3. Communications Network 4. Central Monitoring Station
SCADA & PLC
Traditional PLC are migrating toward to traditional DCS for instants Fisher Delta V. For application in small or medium plant many people use a PLC (Allen Bradley) and also use DCS (Delta V, PlantScappe). If you have a small plant, it is better to set PLC or DCS system which has a system starting at 5 I/O and going up to 500 I/O. SCADA is just that supervisory and data acquisition. It usually is not in plant but it used to supervise multiple small sites. In industry SCDA often do monitoring systems with little RTU at each well site.
Discuss the criterion for choice of voltage for transmission and distribution.
Criterion for Choice of Voltage For Transmission And Distribution: We know that the power P =VI, so for the same power if voltage is increased then current will decrease. If suppose power required is 500 W and voltage is 250V then current I will be equal to 500/250 =2A. Now if voltage is increased from 250V to 500V and the power required is same then I = 500/500 =1 A Now, if the voltage is still increased to say 1000V for the same power then I = 500/1000 =0.5 A
From the above examples, we see that if the voltage is doubled, the current will be halved and when it is quadrupled then current is reduced to 1/4th of its initial value. Thus we conclude that if voltage is increased to n times then the current will be reduced to 1/nth times for the same power. With the reduction of the current to 1/nth times, the conductor area will also be reduced to 1/nth times of its original area for the same current density, Hence less material is required when the voltage is increased. We also know that when the current passes through any conductor, there is loss of power in that particular conductor according to the relation, I^2R. As the loss is proportional to the square of current, so, if the current is reduced to ½ value then the loss will be reduced to 1/4th its original value. Hence the efficiency of the transmission line and all others equipments associated with the line will increase and more power will be available for use. When current is passing through a conductor there will be a voltage drop according to the relation V=IR. So, when the current is reduced the drop of the voltage is less in the line, of course with the same cross sectional area of the conductor. With the reduction of cross sectional area, considered the main advantage of transmitting electrical energy at very high voltage via 132kV, 220kV or even 400kV. But in case of distribution system such high voltage is dangerous, so distribution voltage is generally 400/230V.
Draw the Three-Phase Diagram of the Protection Team.
Three-Phase Diagram of the Protection Team What is circuit breaker? Write the classification of circuit breakers based on the medium used for arc extinction? Circuit breaker:
It is a piece of equipment used to break a circuit automatically under fault conditions. It breaks a circuit either manually or by remote control under normal conditions and under fault conditions. Classification of circuit breakers based on the medium used for arc extinction: • •
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Air break circuit breaker Oil circuit breaker Minimum oil circuit breaker Air blast circuit breaker
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SF6 circuit breaker
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Vacuum circuit breaker
What is the main problem of the circuit breaker?
When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current interruption process and also generates enormous heat which may cause damage to the system or to the breaker itself. This is the main problem.
What are the advantages of air blast circuit breaker over oil circuit breaker? • •
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The risk of fire is diminished The arcing time is very small due to rapid buildup of dielectric strength between contacts The arcing products are completely removed by the blast whereas oil deteriorates with successive operations
What are the characteristic of SF6 gas?
It has good dielectric strength and excellent arc quenching property. It is inert, non-toxic, noninflammable and heavy. At atmospheric pressure, its dielectric strength is 2.5 times that of air. At three times atmospheric pressure, its dielectric strength is equal to that of the transformer oil.
What is meant by electro negativity of SF6 gas?
SF6 has high affinity for electrons. When a free electron comes and collides with a neutral gas molecule, the electron is absorbed by the neutral gas molecule and negative ion is formed. This is called as electro-negativity of SF6 gas.
What is Electrical Fault? Write down the causes & classification of fault. Fault:
A fault in electrical equipment is defined as a defect in its electrical circuit due to which the current is diverted to the un-intended path. Causes:
Generally caused by breaking of conductors or failure of insulation Other causes: • Mechanical failure • Accidents • Excessive stresses Fault Classification: • •
Open circuit - most common fault Single line to ground fault - most common fault
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Line to line fault
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Double line to ground fault
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Three phase fault – Dead short circuit
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Three phase to ground fault – Rare fault
* Faults cannot be eliminated but can be minimized
Can a transformer be used to transform direct voltage and direct current? Justify your answer.
The transformer cannot be used for dc supply system (dc voltage & current). According to working principle of the transformer, it works on Faraday laws of Electromagnetic Induction. Therefore, induced emf e = (-dφ/dt). The changing rate of magnetic flux is responsible for e.m.f generation, which opposes the change of magnetic flux that is indicated by negative sign. But in case of dc supply system (d.c Voltage & Current), there is no change of magnetic flux, d φ = 0, so that induced emf at secondary side of transformer is zero. Therefore, we cannot use transformer for dc supply system.
