The operation of a modern power system has become very complex. Frequency and voltage is to be maintained within limits in addition to ensuring reliability of power supply and for maintaining the frequency and voltage within limits it is essential to match the generation of active and reactive power with the load demand. For ensuring reliability of power supply it is necessary to put additional generation capacity into the system in the event of outage of generating equipment at some station. Over and above this, it is also necessary to ensure the cost of electric supply to the minimum.
In large power systems, a central load dispatcher is necessary to assign loads to various stations and units in accordance with a predetermined schedule, modified from time to time as the actual load differs from predicted load or as emergencies arise owing to loss of generating units or tie-lines. The load dispatching may be by telephone, remote telemetering and signaling, or both.
Load assignment to a particular station varies with the type and function of the station and its relation to the system. Frequency control is sometimes assigned to one of the largest generating station, station feeding local load and a tie load carry their local load plus or minus a schedule tie load, whereas stations with no local load follow a schedule tie-line load.
In a manual load dispatch system, the control engineer at the area control centre obtains the telemetered information, calculates the station outputs for different generating stations and advises the different stations to increase or decrease generation. With systems becoming more and more complex, it is very difficult to satisfy all the system constraints with manual control, and it has become therefore, necessary to make use of on-line computing techniques. The use of on-line computers for load dispatching is increasing day by day. The on-line computer aided control system can be either open-loop or closed-loop type.
Up to date information is supplied to the control engineer by the open-loop systems so that the corrective action could be taken by the control engineer. The computer is kept up to date with information such as a basic load prediction, restrictions on plants, forecasted switching changes, synchronising time of sets, and merit-order costs of plants and power outputs of individual sets.
By reading the present state of the systems, the computer calculates outputs and line flows, and provides a statement of the expected load on each set in half an hour’s time. The outputs of the sets are adjusted manually in accordance with the findings of the computer with respect to economy and system security i.e., computer is employed simply as a tool for high speed security checking and load prediction.
The closed-loop on line computer systems performs all the above functions of data storage and security checking and in addition takes corrective action. The closed-loop operation permits a great variety of emergency conditions to be catered successfully and more efficiently. Automatic control involves the provision of a machine controller connected to the governor of each turbo-generator unit.
Thus outputs of the different generating units are altered in compliance with requirements (to meet the demand with maximum economy subject to system security). Examples of closed loop applications include automatic starting, loading and shut down of generators, automatic switching at substations.
The total interconnected ac network, called the National grid, is controlled from National load centre, called the National load dispatch centre. The National load control centre allocates- (i) the MW generation to each regional grid depending upon the prevailing MW demand in that regional grid and required MW export/import from the regional grid (ii) amount of MW power through tie lines between neighbouring regional grids.
The National grid operates at a common prevailing frequency (50 Hz in India).
Each regional load control centre controls load and frequency of its own by matching generation in various generating stations with total regional MW demand plus MW losses plus/minus amount of tie-line power flow.
The network controller installed in each regional load control centre is in communication with the national load control as well as with control rooms in generating stations and major substations in its zone through power line communication channels, microwave communication channels, telephone communication channel, Fax etc.
Load/generation controller installed in generating station control rooms ensures that the station frequency is within prescribed limits. The settings of the turbine input are adjusted by the station load controller automatically depending upon required generation allocated by the regional grid and the turbine governor of each generating unit operates to control the speed and frequency automatically.
The task of the load control centre is to keep the exchange of power between various zones (Electricity Boards or areas) and system frequency at desired values. Each zone may have its own load control centre for regulating the generating stations and loads in its own zone. The National load control centre regulates the exchange of power between different regional zones. The function is performed automatically by network controller installed in the load control centre.