Automatic Voltage Regulator — System Level Control
The function of an electric power system is to convert energy from one of the naturally available forms to electrical form and to transport it to the points of consumption automatic voltage regulator.
A properly designed and operated power system should, therefore, meet the following fundamental requirements.
- Adequate ‘spinning reserve’ must be present to meet the active and receive power demand.
- Minimum cost with minimum ecological impact.
- The power quality must have certain minimum standards within the tolerance or limit such as.
- Constancy of frequency.
- Constancy of voltage (Voltage magnitude and load angle).
- Level of reliability.
Factor affecting power quality
The factors affecting power quality are:
- Switching surges.
- Flickering of voltages.
- Load shedding.
- Electromagnetic interference.
- Line capacitance and line inductance.
- Operation of heavy equipment.
- Welding machine operation, etc.
The three main controls involved in power systems are:
- Plant Level Control (or) Generating Unit Control.
- System Generation Control.
- Transmission Control.
Plant Level Control or Generating Unit Control
The plant level control consists of:
- Governor control or Prime mover control.
- Automatic voltage regulator (AVR) or excitation control.
Governor Control or Prime Mover Control
Governor control or Prime mover controls are concerned with speed regulation of the governor and the control of energy supply system variables such as boiler pressure, temperature and flows. Speed regulation is concerned with steam input to turbine. With variation in load, speed of governor varies as the load is inversely proportional to speed. The speed of the generator varies and the governor senses the speed and gives a command signal, so that, the steam input of the turbine is changed relative to the load requirement.
Automatic Voltage Regulator (AVR) or Excitation Control
The function of Automatic voltage regulator (AVR) excitation control is to regulate generator voltage and relative power output. As the terminal voltage varies the excitation control, it maintain the terminal voltage to the required standard and the demand of the reactive power is also met by the excitation control unit.
System Level Control
The purpose of system generation control is to balance the total system generation against system load and losses, so that, the desired frequency and power interchange with neighbouring systems are maintained. The comprises of:
- Load frequency control (LFC)
- Economic dispatch control (EDC)
- System voltage control.
Load Frequency Control (LFC)
The involves the sensing of the bus bar frequency and compares with the tie-line power frequency. The difference of the signal is fed to the integrator and it is given to speed changer which generates the reference speed for the governor. Thus, the frequency of the tie-line is maintained as constant.
Economic Dispatch Control (EDC)
When the economical load distribution between a number of generator units is considered, it is found that the optimum generating schedule is affected when an incremental increase at one of the units replaces a compensating decrease at every other unit, in term of some incremental cost. Optimum operation of generators at each generating station at various station load levels is known as unit commitment.
System Voltage Control
The involves the process of controlling the system voltage within tolerable limits. This includes the devices such as static VAR compensators, synchronous condenser, tap changing transformer, switches, capacitor and reactor.
The controls described above contribute to the satisfactory operation of the power system by maintaining system voltages, frequency, and other system variables within their acceptable limits. They also have a profound effect on the dynamic performance of power system and on its ability to cope with disturbances.
