Excitation Systems: Types, Drawbacks, Advantages

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Excitation systems
Excitation systems

The generated electrical energy is distributed from the urban areas and delivered to the ultimate consumers through a network of transmission and distribution. There may be a variation of voltage at the consumer’s terminal which may cause erratic operation or even malfunctioning of the consumer’s appliances such as motors, lamps, and other loads. The main cause of the voltage variation is the variation in the load on the supply of the system. With the increase in the load on the supply system, the voltage at the consumer premises falls due to an increase in voltage drops in.

  1. alternator synchronous impedance
  2. transmission lines
  3. transformer impedance
  4. feeders and
  5. distributors

The voltage limit must be ±6% of the declared supply voltage to operate the appliances satisfactorily. We can install the voltage controlling equipment such as

  1. generating stations
  2. transformers stations supplying feeders

In a dc supply system voltage can be controlled by using over compound generators in case of feeders of equal lengths fed from it, but in case of feeders of different lengths being fed by one generator the voltage at the end of each feeder is kept constant by means of feeder booster.

But in the case of the ac supply system, the voltage can be controlled by various methods:

  1. Excitation control and voltage regulators in generating stations.
  2. using tap transformers at the sending as well as receiving end of the transmission lines.
  3. use of booster transformers.
  4. use of induction regulators.
  5. use of series capacitors in long EHV transmission lines.
  6. use of synchronous condensers.

Excitation systems

The first use of the primary excitation system was the use of an amplifier in the feedback path which amplified the error signal. The excitation system has become more and more complex with the increase in the unit and growth in the interconnection of the system. The excitation system is required to provide the necessary field current to the rotor winding of synchronous machines. The amount of excitation depends on the load current, load power factor, and speed of the machine. The larger the load current, the lowers the speeds and lagging power factors, and the more the excitation required.

The main requirements of excitation systems are:

  • Reliability
  • simplicity
  • ease of maintenance
  • stability and
  • fast transient response

Mainly there are three types of excitation systems:

  1. DC excitation system
  2. AC excitation system and
  3. Static excitation system

DC Excitation system

DC Excitation system
DC Excitation system

Dc excitation system has two exciters:

  • the main exciter: a separately excited dc generator providing the field current to the alternator.
  • pilot exciter: a compound wound self-excited dc generator providing the field current to the main exciter.

The output of the exciter is regulated by an automatic voltage regulator(AVR) for controlling the output terminal voltage of the alternator. The current transformer input to the AVR limits the alternator current during fault. when the field breaker is opened, the field discharge resistor is connected across the field winding so as to dissipate the stored energy in the field winding which is highly inductive. The main and pilot exciters can be either driven by the main shaft or separately driven by the motor. Direct-driven exciters are usually preferred as these preserve the unit system of operation and the excitation is not affected by the external disturbances.

Drawbacks

The main difficulties of the dc excitation system are large time constant(3 sec) and commutation difficulties.

AC Excitation System

This system consists of an alternator and thyristor rectifier bridge directly connected to the main alternator shaft.

Rotating Thyristor Excitation system

                                 Block diagram for rotating thyristor excitation system

This system comprises an ac exciter having a stationary field and a rotating armature. Full-wave thyristor bridge rectifier circuit rectified exciter output which is supplied to main alternator field winding. The alternator field winding is also supplied from the exciter output through another rectifier circuit. The power supply and rectifier control, as a part of the rotating machine, generates the required phase-controlled triggering signals in response to a dc control level fed by the voltage regulator.

The alternator voltage signal is averaged and compared directly with the operator’s voltage adjustment in the auto mode of operation. In the manual mode of operation, the excitation current of the alternator is compared with a separate manual voltage adjustment and supplied to the rotating thyristor bridge through separate regulating elements. The exciter voltage is averaged, compared with the exciter voltage references, and applied to the rectifier control and rectifier.

This system also provides over-voltage protection, over-current protection in the exciter field control,lead-lag compensation for stabilization of voltage control, watt and VAR signals for regulating the voltage and field discharge resistor.

Brushless Excitation system:

Brushless excitation systems are the most popular system. Silicon diodes and thyristors made it possible to have a compact rectified system converting ac to dc at higher power levels. The excitation system comprises an alternator rectifier main exciter and a permanent magnet generator(PMG) pilot exciter. Both are driven directly from the main shaft. The main exciter has a stationary field and a rotating armature directly connected through silicon rectifiers to the field of the main alternator. The pilot exciter is a shaft-driven permanent magnet generator having rotating permanent magnets attached to the shaft and 3 phase stationary armature which feeds the main exciters field through 3-phase full wave phase controlled thyristor bridges.

Advantages

  • Eliminates the use of commutators, collectors, and brushes.
  • The short time constant
  • short response time of less than 0.1 sec.
  • Eliminates the problem of brush maintenance.

                                                 Brushless Excitation system

Static Excitation system

This system uses SCRs. The excitation supply is taken from the alternator itself through a 3-phase star/delta indoor type step down transformer and a rectifier system having mercury arc rectifiers or silicon controlled rectifiers. The star-connected primary is connected to the alternator bus, the delta-connected secondary supplies power to the rectifier system and delta connected tertiary feeds power to grid control circuits and other auxiliary equipment. The rectifiers are connected in parallel to give sufficient current carrying capacity.

Advantages

  • Each Leg of rectifier is protected with series fuse,surge protection and fault indicating light.
  • Has very small response (20 millisecond).
  • High power gain.
  • Elimination of exciter windage loss and commutator wearing.
  • Reduces the operating costs.
  • Electronic speed response.

  static Excitation using SCRs

References

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