The control of transmission voltage through adjustments in generator reactive output and transformer taps, and by switching capacitors and inductors on the transmission and distribution systems.
Importance of voltage control
In case of lighting load, the lamp characteristics are very sensitive to changes of voltage. For instance, if the supply voltage to an incandescent lamp decreases by 6% of rated value, then illuminating power may decrease by 20%. On the other hand, if the supply voltage is 6% above the rated value, the life of the lamp may be reduced by 50% due to rapid deterioration of the filament.
In case of power load consisting of induction motors, the voltage variations may cause erratic operation. If the supply voltage is above the normal, the motor may operate with a saturated magnetic circuit, with consequent large magnetising current, heating and low power factor. On the other hand, if the voltage is too low, it will reduce the starting torque of the motor considerably.
Too wide variations of voltage cause excessive heating of distribution transformers. This may reduce their ratings to a considerable extent.
Location of voltage control equipment
The voltage control equipment is used at more than one point in the system for two reasons.
- Firstly, the power network is very extensive and there is a considerable voltage drop in transmission and distribution systems.
- Secondly, the various circuits of the power system have dissimilar load characteristics.
voltage control equipment is used at :
- generating stations
- transformer stations
- the feeders if the drop exceeds the permissible limits
Methods of voltage control
- By excitation control
- By using tap changing transformers
- Auto-transformer tap changing
- Booster transformers
- Induction regulators
- By synchronous condenser
Method (i) is used at the generating station only whereas methods (ii) to (v) can be used for transmission as well as primary distribution systems. However, methods (vi) is reserved for the voltage control of a transmission line.