Transmission

                  

           Power transmission

After generating the power it should be transmitted and distributed. The difference between transmission and distribution is transmission deals with high voltage (such as 220kv, 132kv) and transmission is done from generating station to the load centers. In distribution it deals with low voltage (such as 230v). In distribution section it distributes power to the loads.

So there will be a question, why high voltage for transmission? Answer is to reduce the power loss. For transmission it is used high voltage DC or high voltage AC. Ac or DC choice is depend on the transmission distance. Refer the below graph. 

Therefore it is clear that up to a certain distance AC transmission is chief than DC. But after that DC is chief although its initial cost is high.

In power systems there is places called switchyards. In this places the main function is step down/up voltages, provide protection, distributing power etc. the arrangement of switchyard is shown below. 

Arrangement of a switch yard

[Image source: http://en.wikipedia.org/wiki/Electrical_substation]

Let’s discuss about components and their functions in switchyard.

Components in switchyard

Power lines

There are mainly two categories primary power lines and secondary power lines. If we consider a switchyard at the generating station primary power lines conduct power from generators to the switchyard. So secondary lines conduct power from switchyard to the distribution/load centers.

Instrument transformers

• Voltage transformer (VT) /capacitive voltage transformer (CVT)

This is used to step down high voltage into low voltage signal for measurement or to operation of protection relays. 

VT operation is same as usual potential transformer. But operation of CVT is different. 

[Image source: http://en.wikipedia.org/wiki/Capacitor_voltage_transformer]

There are two capacitors. Value of C2 is greater than the C1. Because we take voltage across the C2 as shown in below.

measured voltage=(V*C1)/(C1+C2)

Hence we will able to reduce the measured voltage successfully. In addition it can be seen that there is an inductor L1. We know that combination of capacitor and inductor makes a kind of filter circuit (here low pass filter). By changing inductor value CVT can be tuned to the line frequency. After that there is a step down transformer which is used to further reduce the voltage level and to provide isolation. Hence CVT provide filtered low voltage signal.

• Current transformers (CT)

Current transformers have more windings in secondary and hence it step down the current value. This value is used for measuring and for operation of protection devices. When we look at a CT it has two sides named as P1 (incoming side) and P2 (outgoing side). Outgoing terminals are also named as S1 and S2. 

Surge/lightening arrester

These are used to protect power systems from abnormal high voltages such as lightning strikes. There are two terminals, one end is connected to the power line and the other end is connected to the ground. Between these two ends there is a nonlinear resistor. For normal voltage it is very high and hence no current flows through it. But with the voltage increasing resistance become decreasing. So for very high voltages resistance is very low and hence power start flows through the resistor. Hence power system protect.

Isolators

Isolators can be treated as a kind of switch. These are used to isolate a portion of power system or some equipment for maintenance or similar purposes. This can be operated either manually or by using motors. But it should be noted that isolators can only operate at no load conditions, means when operate an isolator the power line should be completely dead. Therefore before operating an isolator circuit breakers should be off. 

Wave trap

Wave trap is an inductor. We know that inductor with a capacitor can arrange as a high pass filter. Here also wave trap with a coupling capacitor is used as a high pass filter. Usually capacitor will be a capacitor inside a CVT. So, why it is used this type of high pass filter. Because usually high voltage transmission lines are used as a communication medium. It means power lines conducting 50/60 Hz power in three phases while High frequency communication signals (usually in the range of 50Hz-500Hz) are transmitting through a one conductor. So usually you can see that in switchyards wave trap is only apply to a one conductor (for only a single phase). But it is just according to my experience. If you have problem, what is for this communication? The answer is communication between generation point and switch yard or another two points in power system. 

Grounding transformer (Zig Zag transformer)

This is a special kind of a transformer which consist of only primary windings. For three phases there are three legs and each leg consist of two windings. In each leg two leg two windings are wound so that their current flow is in opposite directions. Hence total flux become zero and there is no current flow through the neutral. But if some fault occurs the flux is not in balance and therefore a current flows through the neutral. 

Importance of the grounding transformer are as follows.

• ·         It provides earth reference to delta connections.
• ·         It removes triplen harmonics. (3^rd , 9^th etc. ).