HVAC Transmission System (Electric Power Transmission Lines)
Introduction to HVAC Transmission System
The increased demand of electricity needs more generation of electrical power. As the generation takes place at remote places, an efficient distribution system is necessary. The problems of AC transmission particularly in long distance transmission has led to the development of DC transmission.
HVAC Transmission system can be divided into two parts:
(a) Transmission System
(b) Distribution Sysyem
Each part is again subdivided into two parts:
1. Primary Transmission
2. Secondary Transmission
Generation and transmission now a days is almost exclusively three phase. The secondary transmission is also three phase whereas the distribution to the customer may be three phase or single depending on the requirement of the consumer.
HVAC Transmission System
in fig the central station CS generates power using three phase alternators at 6.6 or 11 or 13.2 or 32 kV. The voltage is then stepped up by suitable three phase transformers, to 132 kV as shown.
Such a high voltage of transmission requires conductors of smaller cross section which results reduction in the cost of copper or aluminium.
However the cost of insulation increases. The transmission voltage is therefore determined by economic considerations. Higher transmission voltage also reduces the line losses and improves the efficiency.
The three phase three wire overhead high voltage transmission line gets terminated in the step down transformers in a sub station known as Receiving Station (RS).
Receiving station is usually situated outside the city to ensure safety. Here the voltage is stepped down to 33 kV from 132 kV.
From The receiving station, power is then transmitted at 33kv by underground cables located at various points in the city. This is known as secondary or low voltage transmission. At substation this is further reduced from 3kV to 3.3 kV, using step down transformers.
The output of substation at 3.3 kV can be directly given to a customer whose demand exceeds 50kVA using special feeders. This is primary distribution.
The secondary distribution is done at 440 or 230 V. The reduction in voltage level from 3.3 kV to 440/230 V is done by the step down transformers at the distribution substations.
The most common system for secondary distribution is 440/230 V, three phase, four wire system. The single phase residential load is connected between any one line and the neutral where as three phase 440 V , motor load is connected across thee phase lines directly. The standard frequency for a.c. working is 50 Hz in India and 60 Hz for U.S.A. For single phase traction applications, frequencies as low as 25 or 16.67 Hz are also used.