Brushless Excitation of an Alternator

Excitation systems have a powerful impact on generator dynamic performance and availability, it ensures quality of generator voltage and reactive power, i.e. quality of delivered energy to consumers. Brushless excitation of alternator is the method of excitation through which we can avoid the problem with brushes.

Following are common methods of excitation:

1. Brushless excitation systems, with rotating exciter machines and Automatic Voltage Regulator (AVR), or
2. Static Excitation Systems [SES], feeding rotor directly from thyristor bridges via brushes

Brushless Excitation of an Alternator:

For an alternator brushless Excitation has two ways that is either using Permanent Magnet Generator or without using PM Generator.

Brushless Excitation System Without Using PM Generator:

• The excitation is being given through the output of the generator itself.
• Initially when the generator is not started its generation, the power is being given through the external sources such as the d-c generator set or the grid.
• The supply from the external source is then feed to the excitation transformer where the 11KV output is being converted to the 230 V A.C
• The output of the Excitation Transformer is being send to the A.V.R where the 230 A.C is converted into the 110 V D.C
• In the A.V.R the thyristor bride circuit is being used to control the excitation.
• The controlled D.C output is being feed to the exciter field
• The Exciter produces the controlled A.C supply which is further converted into controlled D.C through the Rotating Diode and given to the field of the main generator.

Brushless Excitation System With Using PM Generator:

For the larger machines the PMG generators are being used instead of the excitation generators and the controlled field is being produced at the generators.

The Permanent Magnet Generator (PMG) with its rotating field and stationary armature supplies high frequency AC power to the voltage regulator.

The voltage regulator receives voltage and reactive current feedback provided by potential and current transformers to provide voltage and reactive current feedback. Comparing these signals to a reference setpoint in the voltage regulator, the voltage regulator provides a controlled variable DC current to the stationary field of the rotating exciter. With its stationary field and rotating armature, the exciter generates three phase high frequency AC output.

This output is rectified by using rotating rectifiers.This DC current is fed via conductors to the center of the rotor shaft and it is carried by a special lead bar in the hollow shaft area under the bearing journal, which is then applied to the main generator field winding.

The rotating rectifier is a three phase full wave diode bridge.

Automatic Voltage Regulator: AVR

• AVR is a device used to maintain a constant voltage at the Alternators terminals. It functions by maintaining the Excitation required hence maintaining the terminal voltage of the Alternator with change in the load.
• AVR is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.
• AVR increases or decreases exciter current for a more linear voltage and frequency.

Effect of AVR on Generator KVAR and Power Factor:

• Increase in excitation voltage increases reactive volt-ampere (KVAR) which decreases power factor (cos (φ)).
• Generally, voltage (V) of the alternator is kept constant so the rest of the components in VICos (φ), i.e. I and Cos (φ) have to vary according to the load.
• Increase in I causes decrease in Pf and Decrease in I causes increase in Pf. so it is clear that Excitation V is proportional to KVAR and load current (I) and inversely proportional to power factor Cos(φ)) Increase in excitation voltage increases reactive volt-ampere (KVAR) which decreases power factor (Cos (φ)).

The other type of excitation system is

Static Excitation Systems [SES], feeding rotor directly from thyristor bridges via brushes.