Automatic Voltage Regulator – System Level Control

Automatic Voltage Regulator

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.

1. Adequate ‘spinning reserve’ must be present to meet the active and receive power demand.
2. Minimum cost with minimum ecological impact.
3. 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:

1. Governor control or Prime mover control.
2. 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.

Security Control

The main objective of real time power system operation requires a process guided by control and decisions based on constant monitoring of the system condition. The power system operation is split into two levels.

Level 1: Monitoring and Decision

The condition of the system is continuously observed in the control centres by protective relays for faults or contingencies caused by equipment trouble and failure. If any of these monitoring devices identifies a sufficiently severe problem at the sample time, then the system is in an abnormal condition. If no such abnormality is observed, then the system is in a normal condition.

Level 2: Control

At each sample, the proper commands are generated for correcting the abnormality on protecting the system from its consequences. If no abnormality is observed, then the normal operation proceeds for the next sample interval.

Central controls also play an important role in modern power systems. Today systems are composed of interconnected areas, where each area has its own control centre. There are many advantages to interconnections. The interconnected areas can share their reserve power to handle anticipated load peaks and unanticipated generator outages. Interconnected areas can also tolerate larger load changes with smaller frequency deviations at spinning reserve and standby provides a reserves margin.

The central control centre monitors information including area frequency, generating unit outputs, and tie-line power flows to interconnected areas. This information is used by automatic load frequency control in order to maintain area frequency at its scheduled value.

Overview of System Operation – Automatic Voltage Regulator

Load forecasting

The load on their systems should be estimated in advance. This estimation in advance is known at load forecasting. Load forecasting based on the previous experience without any historical data.

Classification of load forecasting:

Forecast	Load Time	Application
Very short time	Few minutes to half an hour	Real time control, real time security evalution
Short term	Half an hour to a few hours	Allocation of spinning reserve, unit commitment, maintenance scheduling
Medium term	Few days to a few weeks	Planning or seasonal peak-winter, summer
Long term	Few months to a few years	To plan the growth of the generation capacity

Need for load forecasting:

Need for load forecasting are:

• To meet out the future demand
• Long term forecasting is required for preparing maintenance schedule of the generating units, planning future expansion of the system.
• For day-to-day operation, short term load forecasting is needed in order to commit enough generating capacity for the forecasting demand and for maintaining the required spinning reserve.
• Very short term load forecasting are used for generation and distribution. (i.e.,) economic generation scheduling and load dispatching.
• Medium term load forecasting is needed for predicted monsoon acting and hydro availability and allocating spinning reserves.

Unit Commitment

The unit commitment problem is to minimize system total operating costs while simultaneously providing sufficient spinning reserve capacity to satisfy a given security level. In unit commitment problems, we consider the following terms.

• A short term load forecast
• System reserve requirements.
• System security
• Startup costs for all units.
• Minimum level fuel costs for all units.
• Incremental fuel costs of units.
• Maintenance costs.

Load Scheduling (Load Dispatching)

Loading of units are allocated to serve the objective of minimum fuel cost is known as load scheduling.

Load scheduling problem can be divided into:

• Thermal scheduling.
• Hydrothermal scheduling.

Thermal scheduling:

The loading of steam units are allocated to serve the objective of minimum fuel cost. Thermal scheduling will be assumed that the supply undertaking has got only from thermal or from steam stations.

Hydrothermal Scheduling:

Loading of hydro and thermal units are allocated to serve the objective of minimum fuel cost is known as hydrothermal scheduling.

Scheduling of hydro units are complex because of natural differences in the watersheds, manmade storage and release elements used to control the flow of water are difficult.

During rainy season, we can utilize hydro generation to a maximum and the remaining period, hydro generation depends on stored water availability. If availability of water is not enough to generates power, we must utilize only thermal power generation. Mostly hydroelectric generation is used to meet out peak loads.

There are two types of hydrothermal scheduling.

Long Range Hydro-Scheduling:

Long range hydro scheduling problem involves the long range forecasting of water availability and the scheduling of reservoir water releases for an interval of time that depends on the reservoir capacities.  Long range hydro scheduling involves from 1 week to 1 year or several years. Long range hydro scheduling involves optimization of statistical variables such as load, hydraulic inflows, and unit availabilities.

Short-range hydro-scheduling:

Short-range hydro scheduling involves from one day to one week or hour by hour scheduling of all generation on a system to achieve minimum production cost for a given period. Assuming load, hydraulic inflows and unit availabilities are known.

For a given reservoir level, we can allocate generation of power using hydro plants to meet out the demand, to minimize the production cost.

The largest category of hydrothermal system includes a balance between hydroelectric and thermal generation resources.  Hydrothermal scheduling is developed to minimize thermal generation production cost.

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