Government of India
Ministry of Power

North Eastern Regional Power Committee

FAQ - Welcome to NERPC

NERPC welcomes you to the frequently asked question section. Here we provide clarifications on most of the items used in power system parlance. Please feel free to suggest more topics that can be put in this section.

Generator or Generating Station (group of generators) generates electrical power from other sources of primary energy. A generator forces electric current to flow through an external circuit. These are generally located at the sub-urban regions or several kilometers away from the cities or the load centres, because of its requisites like huge land and water demand, along with several operating constraints like the waste disposal etc. Generators inject electrical power to the Grid which ultimately reaches to the consumer through transmission /distribution system. Presently, in Eastern Region Thermal (coal based) generators contributes about 87% of generation while Hydro-electric generators supports during the evening peak hours.

Other than embedded generators of the states, Eastern Region has thermal Central Generating Stations (CGS) such as Farakka Stage-I,II&III, Kahalgaon Stage-I &II, Talcher Stage-I&II and Barh Stage-II of NTPC. Also, there are Hydro CGS such as Rangit HPS and Teesta HPS of NHPC. Other than this the region acts as receiving point for generation coming in from neighbouring country of Bhutan such as Tala HPS, Chukha HPS, Kurichu HPS and Dagachu HPS. There are major private players also such as GMR Kamalanga Energy Ltd, Jindal India Thermal Power Ltd in Odisha, Adhunik Power & Natural Resources in Jharkhand, Chuzachen HPS and Jorethang Loop Hydro in Sikkim.

Transformer: A transformer transforms an alternating (A/C) current of a certain voltage to an alternating current of different voltage, without change of frequency, by electromagnetic induction (magnetically). A ‘step up’ transformer receives a low voltage and converts into a higher voltage, and a ‘step down’ transformer does just the reverse.

An auto transformer transforms the energy electrically and also magnetically. For a certain range of transformer ratio, the use of autotransformer is economically cheaper than a two winding transformer. The transformer capacity gives a fair idea of the load of a state/utility. The higher the transformation capacity, the higher the demand that can be met. Power transformers which operate near to full load have a higher efficiency than distribution transformers as they may be partly loaded much of the time.

Good maintenance practices are recommended for a trouble free life of the transformer. Dissolved gas analysis must be routinely undertaken to gauge the health of the windings and core.

ISTS: Inter State Transmission System (ISTS) is the network of wires for conveyance of electricity across the territory of an intervening State. The ISTS is planned by states, Central Transmission Utility and Central Electricity Authority. It is developed by PGCIL and other TBCB companies. ISTS systems are generally planned and built for long term access.

The cost of Inter State Transmission System is recovered through the point of connection charges which is based on hybrid of marginal participation and average participation. The slab rates are calculated by National Load Dispatch Center and approved by CERC. The regional transmission accounts are prepared by RPCs based on approved rates and approved Long Term Access/Medium Term Access quantum. Total charges for ISTS is recovered by PGCIL and the same is further transferred to other ISTS licensees.

Bus Reactor: Bus reactor is a type of air core inductor, or in some cases, oil filled, connected between two buses or two sections of the same bus in order to limit the voltage transients on either bus. It is installed in a bus to maintain system voltage when the load of the bus changes.

The permanent connection of the shunt reactors, however, may lead to reduced voltage levels and decreased transmission capacity of the lines during full load conditions. This problem is addressed by breaker controlled shunt reactors in many parts of the world. In such cases, the dynamic over voltages and problems evolving from breaker switching of reactors have to be solved by other technical innovations.

Line Reactor: A line reactor is placed in line at the point of use or just after a transformer to maintain stable amperage to the user. When a line is disconnected from the system, the line reactor is also disconnected from the system. Line reactors are often used to compensate line capacitance, mitigate voltage transients due to switching, and to limit fault currents, especially in case of underground transmission lines.

Due to their inductive nature of the Shunt Reactor, it is used whenever there is need for compensation of capacitive reactance. Power System loads are predominantly inductive in nature and Capacitor banks are used to compensate for the inductive loads. During system light load condition, often voltages increase beyond the normal operating levels and such a condition demands additional inductive loads to maintain system voltage levels within the normal range.

As mentioned above, during light load condition there is risk of system instability due to generated VAr larger than system can absorb. When system VAr generation is higher than the required VAr load, Generators tend to go to under-excitation. Under excitation limit of AVR is used to prevent level of under- excitation below stability limit.

Special Protection Schemes (SPS) are designed to detect a particular system condition that is known to cause unusual stress to the power system and to take some type of predetermined action to counteract the observed condition in a controlled manner. SPSs are designed to detect a system condition that is known to cause instability, overload, or voltage collapse. The action prescribed may require the opening of one or more lines, tripping o generators, ramping of HVDC power transfers, intentional shedding of load, or other measures that will alleviate the problem of concern.

Special protection schemes have been successfully implemented in Talcher hvdc-Talcher STPS system where tripping of hvdc pole initiates tripping of Talcher STPS units. It has also been implemented in hydro generators in Sikkim so that in normal circumstances more power could be transmitted through the grid. However, there is some reservation on the part of the generators, with respect to incorrect operation of SPS, which have not been substantiated.

Islanding scheme is to devise a defence mechanism as a final stage remedial measure under power system defence plan during power blackout in which a healthy part of the system is islanded from a disturbed grid so that this sub-part could survive in isolation from rest of grid. Islanding scheme is that it helps us in saving from total blackout during a major grid disturbance. A successfully survived island also helps in quicker restoration of grid.

The logic of islanding scheme is prepared by the concerned utility and vetted by ERLDC. Thereafter the scheme is implemented after approval of ERPC. During the July, 2012 grid disturbances, CESC system in Kolkata and also many small CPP systems successfully islanded from the grid with some load. Successful islanding in coordination with black start of hydro/gas stations is the backbone of system restoration in case of major disturbances.

The guidelines adopted to evolve a common methodology so that the settings of the various relays are suitably coordinated and the protection system operates in an efficient manner. The objective is to clear power system faults selectively without disturbing the healthy part of the network. This common philosophy helps in better coordination of the protection relays.

The power system consists of various types of relays such as distance, over current, earth fault, etc which have different operating conditions. The protection philosophy for a 400 kV system will be different from a 200kV system. In a 200 kV system main distance protection has to be coordinated with an over current back up protection. Deciding a protection scheme and calculation of the settings is a very specialised task done by protection/testing engineers of STU’s.

Automatic under frequency load shedding scheme (AUFLS) is a most effective defence mechanism to improve the frequency and save the system under emergencies. The loads at distribution network will automatically shed by the under frequency relays during low frequency conditions to maintain the stability of the grid intact. The loads to be shed during emergencies are predetermined and uniformly distributed throughout the grid.

AUFLS is one of the demand side initiatives which are used to bring the load generation into balance. However, for certainty of getting relief when required, the UF relays must be tested periodically. ERPC carries out inspection tours in its constituent substations for checking UFR healthiness. For this purpose a team of protection engineers is formed from two-constituents and approved by ERPC. In future, this demand side response may become eligible for participation in reserve regulation ancillary service also.