NEHU Grid Sub-station is located at Umjarain; under the control of the Resident Engineer, NEHU Grid S/s, Umjarain which is under the Office of the Executive Engineer, Transmission and Transformation Division Shillong. At present there is 1(One) Assistant Engineer, 3(Three) Junior Engineers and 4 staffs (which include 2 Jugali, 1 Lineman-2 and 1 Casual Jugali) performing shift duty on rotation basis. Besides these, there are 5 Maintenance staffs (Comprising of 1 Lineman-2 and 4 nos. Casual Jugalis) that are assisting the Resident Engineer in day to day maintenance work to maintain uninterrupted Power supply. NEHU Grid substation is connected to 4 Grid Substations namely Umiam Grid s/s by Umiam Feeder, Mawlai Grid S/s by Mawlai Feeder, NEIGRIHMS Grid S/s by NEIGRIHMS Feeder and Khliehriat S/s by Khliehriat Feeder which are further interconnected to other S/s and Power stations of the state of Meghalaya. The transmitting voltage is 132 KV. The Power received at NEHU S/s at 132 KV voltage level is transfer to the 33 KV Bus after the voltage is stepped down from 132 KV to 33 KV by 2 nos. 20 MVA transformer located in the 132 KV switchyard of the S/s. These two Transformers operate in parallel. From the 33 KV Bus the power is transfer to the different 33/11 KV sub-stations by the 33 KV Feeders viz, Happy Valley Feeder, Meter factory feeder, SE Falls Feeder, Umiam feeder and NEHU-Mawiong Feeder. There is also one 33 KV Tie-Line namely 33 KV Tie-Line-2 which connects the NEHU Grid S/s and Mawlai Grid S/s. This line facilitates the transformers at both Sub-stations to operate in parallel when the CBs and isolators of the line are closed at both ends. Some of the power received at NEHU S/s is consumed as energy in the Sub-station itself as station service for lighting, supply to the charging motors, Battery charger, cooling fans. etc. For this there is one 33/ 0.433 KV, 630 KVA transformer located at 33 KV switchyard.

The closing and opening of breakers of the transformers and different feeders (132 KV and 33 KV) can be done remotely from the control room or locally from the switchyard. For this there is a control circuit for closing and tripping of the breaker. The breaker can also be tripped automatically by the relaying protective system in case of fault is detected by the relay. The transformers and different feeders are protected through different relay protection schemes.

There are different equipments and devices at the Sub-station besides the transformers. These are the Circuit Breakers, Isolators, Instrument Transformers, the Lightning Arrester or Surge Arresters and Insulator, R&C Panels, Battery Charger and Battery Bank and LT Panel. There are also Communication Equipments and data transfer equipments like RTU, PLCC Panels and Wave traps.

Circuit breaker: It is a switching and current interrupting device. It serves two basic purposes: 1. Switching during normal operating conditions for the purpose of operation and maintenance. 2. Switching during abnormal conditions such as short circuits & interrupting the fault current.

Isolator: It is a switch which can be used for disconnecting a circuit under no current condition.

Lightning Arrester or Surge Arrester: This is a protective device which conducts the high voltage surges on the power system to the ground.

Instrument Transformer: These are devices which transfers high voltages or currents in power lines to values which are convenient for the operation of measuring instruments and relay since measuring instrument and protective device are designed for low voltages (generally 110 V and current (about 5 Amp/ 1 Amp). There are two types of Instrument Transformers viz.

1. Current Transformer C.T. : A current transformer is essentially a step-up transformer which steps down the current in a known ratio. The primary of this transformer consist of one or more turns of thick wire connected in series with the line. The secondary consist of a large no. of turn of fine wire and provide for the measuring instruments and relays a current which is a constant fraction of the current in the line.

2. Potential Transformer P.T. : A Potential transformer is essentially a step-down transformer which steps down the voltage in a known ratio. The primary of this transformer consist of a large no. of turn of fine wire connected across the line. The secondary winding consist of a few turns and provide for measuring instruments and relays a voltage which is a known fraction of the line voltage.

3. Metering and Indicating Instrument: There are several metering and indicating Instrument e.g. Watt Meter, Ammeters, Voltmeters, Energy meters installed in a sub-station to maintain watch over the circuit quantities. The instrument transformers are invariably used with them for satisfactory operation.

R & C Panel: These panels house the Relays, Breaker Control Switches, and Different Meters (Ammeters, Voltmeters, Megawatt meters, Energy meter etc.

Protective Relay: A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.

Different accessories of the Transformer:

Conservator: Conservator is used on transformer for providing adequate space for expansion of oil when transformer is loaded or when ambient temperature changes substantially. It also conserves the insulating property of oil from deterioration and protects the transformer against failure on account of bad quality of oil.

Buchholz Relay: Buchholz relay is a very sensitive, gas and oil operated instrument which detects formation of gas or development of sudden pressure inside the oil filled transformer. It is connected to protection circuits to give an early audible alarm in case of gas collection and to disconnect the transformer from supply in case of severe fault inside the transformer.

Explosion Vent/ Pressure Relief Valve: The purpose of the Explosion Vent/Pressure Relief Valve is to prevent damage to transformer tank by releasing any excess pressure that may be generated inside the tank.

Radiators: These are provided for cooling down the oil inside the transformer.

Silica Gel Breather: Whenever there is a change in the ambient temperature or in the load of an oil immersed transformer, there is a change in the volume of oil. This change, therefore, forces the air above the oil level in the conservator to be either pushed out or breathe in from outside atmosphere. Whenever the air is breathed in, there is a possibility of moisture and dust from the atmosphere to be sucked in. This is dangerous to the insulating properties of oil. Silica gel breathers are provided to prevent this and to ensure that dry air is breathed in and out through the surface.

Magnetic Oil Indicator: This is a direct oil level indicating device provided on large transformers with conservator at relatively high levels from the ground.

Marshalling Box: The transformer is protected by certain fittings directly mounted on the transformer at different locations. These fittings usually are of electrical nature and have contacts or terminals which are connected to the protection scheme. The connection to protection scheme gives various types of alarm indications and also disconnects the transformer from incoming and outgoing supply in caser of severe fault conditions. In order to facilitate connections of all such devices to the protection scheme, the leads of all contacts are wired to a weather proof terminal box. This box also houses indicators of oil temperature, winding temperature and heater.

Oil temperature Indicator: It serves for monitoring the oil temperature of the transformer.

Winding temperature Indicator: It serves for monitoring the winding temperature of the transformer.