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.

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.

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.