Understanding the Automatic Voltage Regulation of Alternator or Generator

The Automatic Voltage Regulator (AVR) component of the excitation system monitors generator output voltage to determine the strength of DC amperage applied to the main exciter windings. If generator output voltage changes, the Automatic Voltage Regulator (AVR) increases or decreases the flow of excitation current directed to the generator field windings. Generator output voltage is increased or decreased by changing the current supply (number of flux lines) to the generator.

Automatic Voltage Regulation of Alternator or Generator – how it works

Parameters of Electrical voltage in Alternator or Generator needed for voltage regulation:
For generation of electrical voltage in a circuit, three things are required- magnetic flux, conductor and relative motion between the magnetic flux and conductor. The magnetic circuit placed on rotor which is rotating during operation. Conductor is placed in the stator circuit. Now, the amount of voltage depends on three things

1. Flux density, which is the number of magnetic flux lines per a certain area.
2. Rotor speed, which determines the flux variation.
3. Conductor length, which means the length of the machine and the number of coils in series with each other.

How to Regulate or change the Voltage in Alternator or generator:

Under normal operating condition of an alternator,

• Flux density- can be adjustable.
• Rotor speed- fixed or constant. That is cannot be changed.
• Conductor length- fixed or constant. Cannot be changed.

Under normal operating conditions, synchronous generators run at constant rated speed. The geometric arrangement of rotor and stator windings is fixed by its design. Hence, the only way to change voltage during synchronous operation is to change the density of the rotating flux. This is accomplished by changing the generator’s field current or the excitation current.

Excitation current or field current generates magnetic flux in the rotor. Thus, by adjusting the excitation current, the magnetic field can be adjusted. And by adjusting the magnetic field the voltage in alternator or generator terminal can be adjusted or regulated. In simple words, by adjusting the excitation current, the voltage regulation can be possible.

Voltage regulation during No-load and On-load condition:

No-load means, there is no current in the stator. That is generator is running, there is full voltage in the terminal but outgoing circuit breaker is open. thus there is no current generation and no current supply.

1.Voltage regulation during no-load condition:

With the generator main breaker OPEN (no-load condition), stator voltage is a function of the applied field current and speed. As field current increases in an open-circuit condition, stator voltage increases. This relationship is linear up to the point at which core saturation takes place.

In no-load condition the stator voltage is a function of speed, excitation current.

Effect of core saturation on stator voltage:

Core saturation changes the reluctance of the magnetic circuit. Reluctance in magnetic flux circuits corresponds to resistance in electrical resistance circuits. As field current increases, flux density increases, and the iron circuit becomes further saturated. It means when the flux density reaches a certain value (e.g. 1,8 Tesla) the relationship between field current and produced flux is no longer linear. The more the circuit is saturated, the more field current is required to change stator voltage.

2.Voltage regulation during on-load condition:

In on-load condition the stator voltage is a function of speed, excitation current and load connected to the stator terminals. In on-load condition the magnetic field generated by the excitation current is distorted by an opposing magnetic field. When the generator is connected to the grid, stator current will flow. Thus there is additional magnetic flux created in the stator due to this load current. This additional magnetic flux is in opposite direction of the magnetic flux of the rotor. Thus it distorts the magnetic flux in the rotor, this terminology is called the armature reaction.

With the lower magnetic flux , the voltage will be reduced unless excitation current is increased to maintain the magnetic flux.

• In no load condition: Effective magnetic flux=Rotor magnetic flux created by excitation current.
• In on-load condition, Effective magnetic flux=Rotor magnetic flux created by excitation current- stator magnetic flux created due to load current in the stator.

Explanation of armature reaction:

This flux field rotates around the stator at synchronous speed. The field acts in such a way that it distorts the flux produced by the field current in the rotor and changes the number of flux lines crossing the air gap and linking the armature coils; this is known as armature reaction.

Maintaining desired terminal voltage; field current required at load

In order to maintain the desired stator or terminal voltage, the field current must be continually adjusted to produce the same number of flux lines crossing the air gap and linking the armature coils. In other words, the field current required at load is a function of the field current required to produce rated voltage at no-load, plus an additional amount to overcome or cancel out the effects of armature reaction.

What is Automatic voltage regulation

We already know about voltage regulation. What about automatic voltage regulation? Automatic voltage regulation system performs the voltage regulation automatically. During continuous operation, this device captures the different parameters of the system such as terminal voltage status, load condition, reactive power etc. Through capturing the parameters value, there is complex calculation based on the algorithm. As an outcome of the calculation, it adjusts the amount of excitation current to regulate the terminal voltage in an alternator.