Thermal Plants


Electrical energy management system ensures supply of energy to every consumer at all times at rated voltage, frequency and specified waveform, at lowest cost and minimum environmental degradation. The switchgear, protection and network automation are integral parts of the modern energy management system and national economy.

The modern 3-ph, 50 Hz, AC interconnected system has several conventional and non-conventional power plants, EHV AC and HVDC Transmission system, Back to Back HVDC coupling stations, HV Transmission network, substations, MV and LV Distribution systems and connected electrical loads. The energy in electrical form is supplied to various consumers located in vast geographical area, instantly, automatically, and safely with required quality at all times. The service continuity and high quality of power supply have become very important.

For fulfilling the foresaid purpose, a state of the art, scientifically and technologically advanced SUBSTATION is required. Sub-Station is the load control center of the thermal plant where power at rated voltage, frequency and waveform is exported/imported as per requirements.

The substation at GHTP Lehra Mohabat has only one 220 KV switchyard. There are two output units each having a capacity of 210 MW. The generating voltage is limited to 15.75 KV and this voltage as stepped up by two-generator transformer 15.75/220 KV manufactured by BHEL. A part of 15.75 KV supply is fed to unit auxiliary transformer, which is stepped down to voltage 6.6 KV which is used to run the major auxiliaries of the plant.

After step up, the 220 KV output from the generator transformer is fed to either of the two bus bars through relays and circuit breakers and these are connected to various feeders through various equipments.

1. BUS-BARS: –

Bus bar is a term used for main bar of conductor carrying an electric current to which many connections may be made. These are mainly convenient means of connecting switches and other equipments into various arrangements.

At GNDTP there are two 220 KV bus bars. These are made of aluminium and all the incoming and outgoing supplies are connected through the bus bars.


These are equipments designed to protect insulators of power lines and electrical installations from lightening surges by diverting the surge to earth and instantly restoring the circuit insulation to its normal strength with respect to earth.


The main purpose of current transformer is to step down the current to a level that the indicating and monitoring instruments can read. When rated current flows through its primary winding, a current of nearly 1 amp will appear in its secondary winding.

The primary is so connected that the current being passes through it and secondary winding is connected to an ammeter. The CT steps down the current to the level of the ammeter.


These are used to step do the voltage to a level that the potential coils of indicating and monitoring instruments can read. These are also used to feed the potential coils of relays. The primary winding is connected to the voltage being measured and the secondary winding to a voltmeter. The PT steps down the voltage to the level of the voltmeter.


These are used to step up down the voltage from one a.c voltage to another a.c voltage level at the same frequency. Shunt reactor in EHV substations is to provide reactive power compensation during low loads.


Wave trap is used to prevent high frequency signals from entering other zones.


Ammeters, voltmeters, watt meters, KWH meters, KVAR meters are installed in sub-station to watch over the currents flowing in the circuit and the voltages and the power loads.


One of the cardinal measures for ensuring full safety in carrying out work on equipment in electrical installations is to disconnect reliably the unit or the section on which the work is to be done from all other live parts of the installation. To guard against mistakes, it is necessary that apparatus, which makes a visible break in the circuit such as isolators, should do this.

Isolators do not have arc control devices therefore cannot be used to interrupt currents at which the arc will be drawn across the contacts. The open arc in these is very dangerous, in that it will not only damage the isolator or the equipment surrounding it but will also cause the flashover between the phase in other words, it will result in short circuit in the installation i.e. why isolators are used only for disconnecting parts after de-energizing them by opening their respective circuits by use of their circuit breakers.


Earthing switch is used to discharge the voltage on dead lines to earth. An auxiliary switch to provide interlock always accomplishes it.


Circuit breakers are mechanical devices designed to close o open contact or electrical circuit under normal or abnormal conditions. CB is equipped with a strip coil directly attached to relay or other means to operate in abnormal conditions such as over power etc. In GNDTP three types of CB are used. SF6 CB is used to control 220 KV in switchyard.
VCB are used to control 6.6kv in switchgear & ABCB are used to control 415 KV in switchgear.

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