WHAT IS EHT, HT, & LT LINE?
Indian Electricity Rules, 1956, sec.-2 says:
- Low Tension (LT) or Low Voltage (LV) line: Upto 250 Volt.
- Medium Tension (M.T. or M.V.): Lines from 250 volts up to 650 Volts.
- High Tension (HT) or (HV) : Lines from 650 volts upto 33KV.
- Extra High Tension (EHT): Lines above 33KV.
THE "VARIATION" IN THE VOLTAGE LEVELS OF THE ABOVE IS REGULATED BY SEC.84 OF INDIAN ELECTRICITY RULES 1954 AS FOLLOWS:
(a) For LT & MV supply
The supply voltage shall not vary more than ( ± 6%) of the declared voltage.
- For LT, declared voltage is 250 volts.
The permitted variation is 250 x ( ±6%) = 235 volts to 265 volts
- For MV, declared voltage is 650 volts.
The permitted variation is 650 x (±6%) = 611 volts to 689 volts.
(b) For HT lines
The supply voltage shall not vary more than 6% in higher side and shall not vary less than 9% in lower side.
- For 11KV Lines
Lower side, 11KV x (-9%) = 10.01KV
Higher Side, 11 KV x (+6%) 11.66KV
Thus the permitted 11KV power supply range is from 10.01 KV upto 11.66 K
- For 33KV.
Lower side, 33KV x (-9% )= 30.03KV
Higher Side, 33KV x(+6%) -34.98KV
Hence, the permitted 33KV power supply range is from 30.03 KV to 34.98 KV
(c) For EHV lines
The Supply voltage shall not vary by more than 10% in higher side or less than 12.5% on lower side.
- For 132KV:
Lower side, 132KV x (-12.5%) = 115.5KV
Higher side, 132KV x (+10% ) = 145.2KV
Thus, the permitted 132KV supply range is from 115.5 KV up to 145.2KV
- For 220KV:
Lower side, 220KV x (-12.5%) = 192.5KV
Higher side, 220KV x(+10% )=242KV
Thus, the permitted 220 KV supply range is from 192.5 KV to 242 KV
PERMITTED FREQUENCY RANGE
INDIAN ELECTRICITY RULES, 1956, SEC.65′ REGULATES,THE FREQUENCY OF POWER SUPPLY SYSTEM AS FOLLOWS: The permitted frequency shall not vary from the declared frequency by ± 3%.
In India, our frequency is 50 cycles per second (Hertz).
50x (±3%)=48.5 to 51.5 cycle per second.
Thus, the permitted range of frequency is 48.5 to 51.5 cycles per second.
Why such restrictions are imposed? Do you guess?
It is because all the electrical appliances & equipments are designed upon such declared supply voltage & declared frequency. If these parameters would vary beyond tolerance range, then the insulation would fail and the equipments would be damaged. Even fatal accident may also occur.