What are the different operating voltages used for generation, primary and secondary transmission in AC power supply systems in India?

a) What are the different operating voltages used for generation, primary and secondary transmission in AC power supply systems in India?

b) What are the limitations of Kelvin's law?

c) Discuss about the factors affecting choice of transmission voltage level.

i) Generation: At the generating station, electrical power is generated with the help of three phase alternators. The voltage level is 11 KV but the voltage level may be 6.6 KV, 22 KV or 33 KV depending upon the capacity of the generating station. After the generating station, actual transmission and distribution starts.

(ii) Primary transmission: The generated electrical power is transferred with the help of overhead transmission lines. The voltage level is increased to 132 KV/220 KV or more, with the help of step up transformer at generating substation. Hence this transmission is also called high voltage transmission. The primary transmission uses 3 phase 3 wire system.

(iii) Secondary Transmission: The primary transmission line continues via transmission towers till the receiving stations. At the receiving stations, the voltage level is reduced to 22 KV or 33 KV using the step down transformer. There can be more than one receiving stations. Then at reduced voltage level of 22 KV or 33 KV, the power is then transmitted to various substations using overhead 3 phase 3 wire system. This is secondary transmission.

4) Distribution: At the substation the voltage level is reduced to 6.6 KV or 3.3 KV with the help of step down transformers.

b) Limitations of Kelvin's Law:

Kelvin's law is theoretically true. A number of difficulties are involved in its application. Some important limitations are given below:

1.  The amount of energy loss cannot be estimated accurately because load factors of losses and the load itself are different and future load conditions and load factors cannot be predicted accurately. 
2. It is difficult to calculate the cost of energy loss. The cost of losses per unit is greater than the generating cost per unit. Prices of conductor material and the rates of interest are changing continuously.
3.  The assumption that annual charge is of form (P+Pa) is strictly speaking not true. For example, in underground cables neither the cost of laying the cables nor the cost of cable dielectric and sheath vary in this manner.
4.  The Kelvin's law does not take into account factor like mechanical strength, safe current density, corona loss etc.
5. The most economical conductor size may not be suitable to carry the required amount of current due to its thermal rating and temperature rise limits.
6. Interest and depreciation on the capital outlay cannot be accurately determined.

c) Factors affecting choice of transmission voltage level:

1. Type and size of the load to be served - Nominal voltage supplied to the system depends on the type and rating of the loads to be supplied. In industries, generally below 200 kW all motors are supplied with 415 V supply and motor rated above 200 kW are supplied with 6.6 kV voltage level. Motors above 200 kW can also supply with 415 V supply however this results in increase in the cost of the motor and size of the motor which is not economical.
2.  Distance to which power is to be transmitted Power transfer capability of a line depends on the nominal operating voltage at which power is transmitting in the system. For example 220 kV transmission line can evacuate power about 200 to 250 MW with temperature rise in permissible limits for a distance of about 150 to 200 Km. 400 kV transmission line can able to evacuate power about 500 MW. In order to transmit high power for long distances higher operating voltage is chosen.
3.  Equipment availability for fault isolation - Short Circuit capability of switching equipment increases with increase in voltage rating. The short circuit capability generally determines the amount of load that can be connected to the bus. Selection of the voltage levels depends on the total load and system fault levels.
4.  Permissible voltage regulations System voltage regulation and short circuit capability both contradict with each other. Short circuit current in the system is limited by the impedance present in the system. Cables and transformers are the impedances present in the power system. In order to reduce the short circuit, transformer with high percentage impedance can be chosen. However, because of the high impedance high voltage drop exist during normal opération results in poor regulation. If short circuit constraint and voltage regulation does not satisfy for a particular voltage level, the short circuit capability of the switchgear shall be increased by selecting a higher nominal system voltage. The increase in the short circuit capability will permit the use of lower impedance transformer, thereby improving voltage regulation.
5. Future Load Growth - While choosing the voltage for a system, future load addition to the system should also be considered. Addition of new load will increase the voltage drop in the system and also increases the short circuit level of the system.