• Introduction to Excitation Systems


    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. Excitation Systems includes excitation power and excitation device. The main body of excitation power is exciter or excitation transformer.

    The excitation device is an electrical control device that controls and adjusts excitation current besides excitation power in excitation systems of synchronous generator. According to different specification, type and operating requirement, it consists of regulating panel, control panel, de-excitation screen and rectifier separately.

    Excitation system is the important part of generator and is a must-have equipment for power stations. It also has significant influence and effect on the safe and stable operation of the power system and the generator itself.

    Input to the Alternator:

    1.Mechanical Energy to rotate the rotor
    2.Excitation current in field winding.


    “Excitation System is a system which provides excitation power, regulation, control and protection for a synchronous machine. ” 


    The main function of the excitation system

    1. Maintain voltage for a given value of machine side according to the generator load changes to adjust the excitation current corresponding.

    2. Control reactive power allocation between parallel operation of the generator.

    3. Improve static stability of the generator parallel operation.

    4. Improve transient stability of the generator parallel operation.

    5. Deexcitation to reduce loss fault when internal generator failure.

    6. According to operational requirements to limit the maximum excitation and the minimum excitation of generator.


    Functions of an excitation system

    The process of generating a magnetic field by means of an electric current is called excitation.    


    The functions of an excitation system are

    1. to provide direct current to the field winding of synchronous generator.
    2. to perform control functions, limiting functions and protective functions for the satisfactory operation of the power system. 


    These functions are accomplished automatically by appropriate changes in the level of machine excitation.


    Control functions regulate specific quantities at the desired level.


    1. Voltage control
    a. Tap changing transformers
    b. Shunt reactors/capacitors
    c. Synchronous phase modifiers
    2. Reactive power control
    a. Static var compensators
    3. System stability control


    Limiting  functions

    1. Prevent certain quantities from exceeding set limits.
    2. It ensure that constraints of synchronous machine, excitation system and other equipments are not violated.


    Protective functions

    1. If any of the limiters fail, then protective functions remove appropriate components or the unit from service.
    2. These functions ensure that the capability limits of the synchronous machine, excitation system, and other equipment are not exceeded.
    Capability Curve of Synchronous Generator

    It defines the bounds within which it can operate safely. Various bounds imposed on the machine are:


    1. MVA-loading cannot exceed the generator rating. This limit is imposed by the stator heating. Stator Heating Limit: Sets |S|max and |IA|max
    2. MW-loading cannot exceed the turbine rating which is given by (MVA rating) x (p.f. rating). Prime Mover Limit: Sets Pmax
    3. The generator must operate a safe margin away from the steady-state stability limit (δ = 90°). This can be laid down as a maximum allowable value of δ. Static Stability Limit: δ =90°
    4. The maximum field current cannot exceed a specified value imposed by rotor. Rotor Heating Limit: Sets |E|max.
    Performance requirements of the excitation system

    The performance requirements of the excitation system are determined by

    a) Generator considerations: Excitation system should be able to
    1. supply field current.
    2. automatic adjustment of field current as the generator output varies within its capability.
    3. respond to transient disturbances
    – rotor insulation failure due to high field voltage
    – rotor heating due to high field current (OEL)
    – stator heating due to high VAR loading (UEL)
    – heating due to excess flux (volts/Hz)


    b) Power system considerations: Excitation system should be able to
    1. effective control of system voltage
    2. improve  power system stability
    – Transient stability
    # sudden load change
    # fault on/ loss of transmission line
    – Steady state stability
    – Dynamic stability


    What is De-excitation ?

    De-excitation quickly cuts off power flow to the rotor and consumes the energy of magnatic field stored in excitation winding.

    Special measures have to be taken to break the DC current and discharge the energy stored in the field winding of the machine. If not, very high voltages occur which can damage both the excitation equipment and the rotor winding. If a generator electrical fault should occur, it is also very important to de-excite the generator as fast as possible.

    De-excitation is performed by a field circuit breaker connected on the DC side of the converter, and a controlled thyristor discharge circuit. The breaker disconnects the rectifier from the field winding, and the discharge thyristor closes a circuit through the de-excitation resistor for the field current.


    Types of Excitation Systems

    Classified into three broad categories based on the excitation power source:

    1. DC excitation systems
    2. AC excitation systems
    3. Static excitation systems