Transmission of power: The huge amount of power generated in a power station (hundreds of MW) is to be transported over a long distance (hundreds of kilometers) to load centers to cater power to consumers with the help of transmission line and transmission towers as shown in figure 2.5.
To give an idea, let us consider a generating station producing 120 MW power and we want to transmit it over a large distance. Let the voltage generated (line to line) at the alternator be 10 kV. Then to transmit 120 MW of power at 10 kV, current in the transmission line can be easily calculated by using power formula circuit (which you will learn in the lesson on A.C circuit analysis) for 3-phases follows:
I = 3 LP V cos θ where cos θ is the power factor
= 63 120×10 3×10×10 ×0.8
∴ I = 8660 A
Instead of choosing 10 kV transmission voltage, if transmission voltage were chosen to be 400 kV, the current value in the line would have been only 261.5 A. So sectional area of the transmission line (copper conductor) will now be much smaller compared to 10 kV transmission voltage. In other words, the cost of the conductor will be greatly reduced if power is transmitted at higher and higher transmission voltage. The use of higher voltage (hence lower current in the line) reduces voltage drop in the line resistance and reactance. Also, transmission losses are reduced. Standard transmission voltages used are 132 kV or 220 kV or 400 kV or 765 kV depending upon how long the transmission lines are.
Therefore, after the generator, we must have a step up transformer to change the generated voltage (say 10 kV) to desired transmission voltage (say 400 kV) before transmitting it over a long distance with the help of transmission lines supported at regular intervals by transmission towers. It should be noted that while the magnitude of current decides the cost of copper, level of voltage decides the cost of insulators. The idea is, in a spree to reduce the cost of copper one can not indefinitely increase the level of transmission voltage as the cost of insulators will offset the reduction copper cost. At the load centers, voltage level should be brought down at suitable values for supplying different types of consumers. Consumers may be (1) big industries, such as steel plants, (2) medium and small industries and (3) offices and domestic consumers. Electricity is purchased by different consumers at a different voltage level. For example, big industries may purchase power at 132 kV, medium and big industries purchase power at 33 kV or 11 kV and domestic consumers at a rather low voltage of 230V, single phase. Thus we see that 400 kV transmission voltage is to be brought down to different voltage levels before finally delivering power to different consumers. To do this we require obviously step down transformers.
Substations
Substations are the places where the level of voltage undergoes change with the help of transformers. Apart from transformers, a substation will house switches (called circuit breakers), meters, relays for protection and other control equipment. Broadly speaking, a big substation will receive power through incoming lines at some voltage (say 400 kV) changes the level of voltage (say to 132 kV) using a transformer and then directs it outwards through outgoing lines. Pictorially such a typical power system is shown in figure 2.6 in a short of a block diagram. At the lowest voltage level of 400 V, generally 3-phase, the 4-wire system is adopted for domestic connections. The fourth wire is called the neutral wire (N) which is taken out from the common point of the star connected secondary of the 6 kV/400 V distribution transformer.
Power Station step-up transformer
Step down transformer 400 kV/33 kV
Step down transformer 33 kV/11 kV
Step down transformer 11 kV/6 kV
Step down transformer 6 kV/ 400 V
3-phase, 4 wire 400 V, power
Big industries
Medium industries
To Small industries 400 kV HV transmission line Generator 10 kV
Domestic consumers
R Y BN
Some important components/equipments in substation
As told earlier, the function of a substation is to receive power at some voltage through incoming lines and transmit it at some other voltage through outgoing lines. So the most important equipment in a substation is a transformer(s). However, for flexibility of operation and protection transformer and lines, additional equipment is necessary.
Suppose the transformer goes out of order and maintenance work is to be carried out. Naturally, the transformer must be isolated from the incoming as well as from the outgoing lines by using special type of heavy duty (high voltage, high current) switches called circuit breakers. Thus a circuit breaker may be closed or opened manually (functionally somewhat similar to switching on or off a fan or a light whenever desired with the help of an ordinary switch in your house) in substation whenever desired. However, unlike an ordinary switch, a circuit breaker must also operate (i.e., become opened) automatically whenever a fault occurs or overloading takes place in a feeder or line. To achieve this, we must have a current sensing device called CT (current transformer) in each line. The ACT simply steps down the large current to a proportional small secondary current. Primary of the CT is connected in series with the line. A 1000 A/5 A CT will step down the current by a factor of 200. So if primary current happens to be 800 A, secondary current of the CT will be 4 A.
Suppose the rated current of the line is 1000 A, and due to any reason if the current in the line exceeds this limit we want to operate the circuit breaker automatically for disconnection.
In basic scheme is presented to achieve this. The secondary current of the CT is fed to the relay coil of an overcurrent relay. Here we are not going into constructional and operational details of an overcurrent relay but try to tell how it functions. Depending upon the strength of the current in the coil, an ultimately an electromagnetic torque acts on an aluminum disc restrained by a spring. Spring tension is so adjusted that for normal current, the disc does not move. However, if current exceeds the normal value, torque produced will overcome the spring tension to rotate the disc about a vertical spindle to which a long arm is attached. To the arm, a copper strip is attached as shown in figure 2.8. Thus the arm too will move whenever the disk moves.
The relay has a pair of normally opened (NO) contacts 1 & 2. Thus, there will exist open circuit between 1 & 2 with normal current in the power line. However, during a fault condition in the line or overloading, the arm moves in the anticlockwise direction till it closes the terminals 1 & 2 with the help of the copper strip attached to the arm as explained pictorially in figure 2.8. This short circuit between 1 & 2 completes a circuit comprising of a battery and the trip coil of the circuit breaker. The opening and closing of the main contacts of the circuit breaker depending on whether its trip coil is energized or not. It is interesting to note that trip circuit supply is to be made independent of the A.C supply derived from the power system we want to protect. For this reason, we expect batteries along with battery charger to be present in a substation.
Apart from above, there will be other types of protective relays and various meters indicating current, voltage, power etc. To measure and indicate the high voltage (say 6 kV) of the line, the voltage is stepped down to a safe value (say 110V) by transformer called potential transformer (PT). Across the secondary of the PT, MI type indicating voltmeter is connected. For example, a voltage rating of a PT could be 6000 V/110 V. Similarly, Across the secondary, we can connect a low range ammeter to indicate the line current
To give an idea, let us consider a generating station producing 120 MW power and we want to transmit it over a large distance. Let the voltage generated (line to line) at the alternator be 10 kV. Then to transmit 120 MW of power at 10 kV, current in the transmission line can be easily calculated by using power formula circuit (which you will learn in the lesson on A.C circuit analysis) for 3-phases follows:
I = 3 LP V cos θ where cos θ is the power factor
= 63 120×10 3×10×10 ×0.8
∴ I = 8660 A
Instead of choosing 10 kV transmission voltage, if transmission voltage were chosen to be 400 kV, the current value in the line would have been only 261.5 A. So sectional area of the transmission line (copper conductor) will now be much smaller compared to 10 kV transmission voltage. In other words, the cost of the conductor will be greatly reduced if power is transmitted at higher and higher transmission voltage. The use of higher voltage (hence lower current in the line) reduces voltage drop in the line resistance and reactance. Also, transmission losses are reduced. Standard transmission voltages used are 132 kV or 220 kV or 400 kV or 765 kV depending upon how long the transmission lines are.
Therefore, after the generator, we must have a step up transformer to change the generated voltage (say 10 kV) to desired transmission voltage (say 400 kV) before transmitting it over a long distance with the help of transmission lines supported at regular intervals by transmission towers. It should be noted that while the magnitude of current decides the cost of copper, level of voltage decides the cost of insulators. The idea is, in a spree to reduce the cost of copper one can not indefinitely increase the level of transmission voltage as the cost of insulators will offset the reduction copper cost. At the load centers, voltage level should be brought down at suitable values for supplying different types of consumers. Consumers may be (1) big industries, such as steel plants, (2) medium and small industries and (3) offices and domestic consumers. Electricity is purchased by different consumers at a different voltage level. For example, big industries may purchase power at 132 kV, medium and big industries purchase power at 33 kV or 11 kV and domestic consumers at a rather low voltage of 230V, single phase. Thus we see that 400 kV transmission voltage is to be brought down to different voltage levels before finally delivering power to different consumers. To do this we require obviously step down transformers.
Substations
Substations are the places where the level of voltage undergoes change with the help of transformers. Apart from transformers, a substation will house switches (called circuit breakers), meters, relays for protection and other control equipment. Broadly speaking, a big substation will receive power through incoming lines at some voltage (say 400 kV) changes the level of voltage (say to 132 kV) using a transformer and then directs it outwards through outgoing lines. Pictorially such a typical power system is shown in figure 2.6 in a short of a block diagram. At the lowest voltage level of 400 V, generally 3-phase, the 4-wire system is adopted for domestic connections. The fourth wire is called the neutral wire (N) which is taken out from the common point of the star connected secondary of the 6 kV/400 V distribution transformer.
Power Station step-up transformer
Step down transformer 400 kV/33 kV
Step down transformer 33 kV/11 kV
Step down transformer 11 kV/6 kV
Step down transformer 6 kV/ 400 V
3-phase, 4 wire 400 V, power
Big industries
Medium industries
To Small industries 400 kV HV transmission line Generator 10 kV
Domestic consumers
R Y BN
Some important components/equipments in substation
As told earlier, the function of a substation is to receive power at some voltage through incoming lines and transmit it at some other voltage through outgoing lines. So the most important equipment in a substation is a transformer(s). However, for flexibility of operation and protection transformer and lines, additional equipment is necessary.
Suppose the transformer goes out of order and maintenance work is to be carried out. Naturally, the transformer must be isolated from the incoming as well as from the outgoing lines by using special type of heavy duty (high voltage, high current) switches called circuit breakers. Thus a circuit breaker may be closed or opened manually (functionally somewhat similar to switching on or off a fan or a light whenever desired with the help of an ordinary switch in your house) in substation whenever desired. However, unlike an ordinary switch, a circuit breaker must also operate (i.e., become opened) automatically whenever a fault occurs or overloading takes place in a feeder or line. To achieve this, we must have a current sensing device called CT (current transformer) in each line. The ACT simply steps down the large current to a proportional small secondary current. Primary of the CT is connected in series with the line. A 1000 A/5 A CT will step down the current by a factor of 200. So if primary current happens to be 800 A, secondary current of the CT will be 4 A.
Suppose the rated current of the line is 1000 A, and due to any reason if the current in the line exceeds this limit we want to operate the circuit breaker automatically for disconnection.
In basic scheme is presented to achieve this. The secondary current of the CT is fed to the relay coil of an overcurrent relay. Here we are not going into constructional and operational details of an overcurrent relay but try to tell how it functions. Depending upon the strength of the current in the coil, an ultimately an electromagnetic torque acts on an aluminum disc restrained by a spring. Spring tension is so adjusted that for normal current, the disc does not move. However, if current exceeds the normal value, torque produced will overcome the spring tension to rotate the disc about a vertical spindle to which a long arm is attached. To the arm, a copper strip is attached as shown in figure 2.8. Thus the arm too will move whenever the disk moves.
The relay has a pair of normally opened (NO) contacts 1 & 2. Thus, there will exist open circuit between 1 & 2 with normal current in the power line. However, during a fault condition in the line or overloading, the arm moves in the anticlockwise direction till it closes the terminals 1 & 2 with the help of the copper strip attached to the arm as explained pictorially in figure 2.8. This short circuit between 1 & 2 completes a circuit comprising of a battery and the trip coil of the circuit breaker. The opening and closing of the main contacts of the circuit breaker depending on whether its trip coil is energized or not. It is interesting to note that trip circuit supply is to be made independent of the A.C supply derived from the power system we want to protect. For this reason, we expect batteries along with battery charger to be present in a substation.
Apart from above, there will be other types of protective relays and various meters indicating current, voltage, power etc. To measure and indicate the high voltage (say 6 kV) of the line, the voltage is stepped down to a safe value (say 110V) by transformer called potential transformer (PT). Across the secondary of the PT, MI type indicating voltmeter is connected. For example, a voltage rating of a PT could be 6000 V/110 V. Similarly, Across the secondary, we can connect a low range ammeter to indicate the line current
Transmission of power
Reviewed by Unknown
on
June 13, 2018
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Reviewed by Unknown
on
June 13, 2018
Rating:
Insightful post
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