ERPC 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.

What is a generator?

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.

What is a transformer?

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.

What is meant by ISTS?

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.

What is meant by Bust Reactor and Line Reactors?


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.

What is Power Supply Position Report?

Power Supply Position (PSP) report covers the monthly power requirements for all States/ UTs in terms of peak demand and availability (in MW) as well as energy requirement and availability (in MU). PSP is assessed considering the projections made by the constituent states, past data and the trend analysis. This information may be useful for the utilities likely to face shortages, to tie-up bilateral exchanges/ purchase of power from the states having surplus power.

The power supply position gives a detailed break up of the sources of power available to the states. The source of power may be internal to the state or external to the state, central sector, IPP, etc. Also, the sources are segregated into hydro sources and thermal sources. Thus, a careful study of the report will enable the observer to get the thermal-hydro mix of the state also.

Information about power cuts/load shedding as collected from the state are also provided in the report. This gives an estimate about the quantum of power cuts being resorted. However, the PSP cannot take into consideration the demand not met due to system constraints.

What are Special Protection Schemes?

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

What are Islanding schemes?

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.

What is Protection Philisophy?

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.

What is automatic under frequency load shedding scheme?

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.

What is the procedure for approved shutdowns?

Maintenance of power system elements is very important to maintain the reliability of the equipment. Coordinating shutdown of generator and transmission elements is difficult when more than one constituent is involved. Therefore, constituents place the requisition to RPCs/RLDCs for shutdown of generators and transmission elements for maintenance. All the shutdowns are checked for network operation constraint and accordingly placed in monthly operation meeting for approval.

The actual availing of the shutdown is coordinated by ERLDC in real time. In case the shutdown could not be availed due to system conditions, the shutdown gets deferred. Shutdowns of emergency nature are directly taken up with ERLDC. For important inter-regional lines the shutdown process is monitored by NLDC also. The data of shutdown is in turn used for the calculation of availability of transmission system.

What are Regional Energy Accounts?

Regional Energy Accounts (REA) for eastern region is prepared from Inter State Generating Station (ISGS) (whose 100% power is allocated)  availability, injection schedules & constituent drawal schedule published by ERLDC. The REA Statement is prepared as per the specified procedure in CERC (Terms & Conditions of Tariff), Regulations. REA is prepared on the monthly basis stating therein the Share Allocation, indicative Capacity Charges payable by the beneficiaries, Drawal Schedule, indicative Energy Charges Billing etc.

The Regional Energy Accounts also show the portion of bundled coal power scheduled under JNN Solar Mission. Although the other energy figures in REA are based on schedule, the REA for Bhutan Hydro stations is based on actual generation of the generating stations and other contractual adjustments. Normally the scheduled generation of a generating station should match the total drawal schedule of beneficiaries from the generating station. However, in case of regulation of power the scheduled generation may be more than the drawal schedule of the beneficiaries as some power may be sold on exchange.

What are Bilateral Accounts?

Bilateral Statement is prepared for the purpose of billing transactions of energy of non ER ISGS generators and may be engaged in by traders, generating stations, discoms, bulk cousumers, etc. These include short term trades, medium term trades, long term trades and also over the counter trades in energy exchanges. However, anonymous trades on exchanges is not reflected in these accounts and settled in exchange itself.

Bilateral accounts include the schedule of solar generating stations also. Over the years as the power market has expanded, the number of transactions on the bilateral markets and also on the exchange have increased steadily. Short term bilateral trades are conducted by the constituents to meet short term power requirement whereas medium term trades are for meeting medium term requirement. Wise estimation of power requirement and contracting the least cost power for meeting the requirement will result in economic savings for the constituents.

What is URS Account?

The ISGS generating stations give their declared capability to RLDS and beneficiaries are accordingly informed of their entitlement based on share. Now, all the beneficiaries may not get their full quantum scheduled. If all the power available in ER ISGS stations is not scheduled due to less than entitlement schedule given by the beneficiaries some power remains available for scheduling. This power is called Un-requisioned surplus (URS) power. As per CERC guidance, this power may be availed by any other beneficiary having share allocation from the specific ISGS. Due to requisition of URS by another beneficiary, the capacity charges payable are adjusted through the URS account.

URS power can also be commanded by RLDC under Regulation Reserve Ancillary Service for load generation balance. Also, the generating station may now sell the URS power through the exchange and share the profits with the original beneficiaries. However, these competing demands on the URS unless prioritized will lead to coordination issues.

What is Entitlement/Schedule report?

It is a statement which shows the monthly entitlement, schedule & actual drawal of various constituents in eastern region and other regions as a whole. The statement gives an idea as to how much of energy was available to the ER constituents through ISGS entitlement and how much they have got scheduled. Generally, the scheduled energy is equal or less than the entitlement. However, in special cases such as URS, the scheduled energy may become more than entitled energy. It also shows the requirement met through bilateral trades in energy except through anonymous bidding on exchange.

For generating stations of Bhutan, the settlement is on the basis of actual figures and is accordingly reflected in the report. Comparison of actual figures with schedules gives an indication of the grid discipline adhered to by the constituents. The power scheduled to regions other than eastern region from ER ISGS is also available for comparison.