The functions of an excitation system are to provide direct current to the synchronous generator field winding, and to perform control and protective functions essential to the satisfactory operation of the power system.

The performance requirements of the excitation system are determined by:

1. Generator considerations:

It is used to supply and adjust field current as the generator output varies within its continuous capability. It also responds to transient disturbances with field forcing consistent with the generator short term capabilities:

2. Power system considerations:

It contributes to effective control of system voltage and improvement of system stability

Elements of an Excitation System

• Exciter: It provides dc power to the generator field winding.
• Regulator: It processes and amplifies input control signals to a level and form appropriate for control of the exciter.
• Terminal voltage transducer and load compensator: It senses generator terminal voltage, rectifies and filters it to dc quantity and compares with a reference; load comp may be provided if desired to hold voltage at a remote point.
• Power system stabilizer: It provides additional input signal to the regulator to damp power system oscillations.
• Limiters and protective circuits: It ensures that the capability limits of exciter and generator are not exceeded.

Types of Excitation Systems can be classified into three broad categories based on the excitation power source:

1. DC excitation systems

• utilize dc generators as source of power
• driven by a motor or the shaft of main generator
• self or separately excited
  • represent early systems (1920s to 1960s)
  • lost favor in the mid-1960s because of large size; superseded by ac exciters
  • voltage regulators range from the early non-continuous rheostatic type to the later system using magnetic rotating amplifiers
  • self-excited dc exciter supplies current to the main generator field through slip rings
  • exciter field controlled by an amplidyne which provides incremental changes to the field in a buck-boost scheme
  • the exciter output provides rest of its own field by self-excitation

2. AC excitation systems
   • use ac machines (alternators) as source of power
   • usually, the exciter is on the same shaft as the turbine-generator
   • the ac output of exciter is rectified by either controlled or non-controlled rectifiers
   • rectifiers may be stationary or rotating
   • early systems used a combination of magnetic and rotating amplifiers as regulators; most new systems use electronic amplifier regulators

2.1       Stationary rectifier systems:

• dc output to the main generator field supplied through slip rings

2.2   Rotating rectifier systems:

• the need for slip rings and brushes is eliminated; such systems are called brushless excitation systems
• they were developed to avoid problems with the use of brushes perceived to exist when supplying the high field currents of large generators
• they do not allow direct measurement of generator field current or voltage

1. Static Excitation Systems:

• all components are static or stationary
• supply dc directly to the field of the main generator through slip rings
• the power supply to the rectifiers is from the main generator or the station auxiliary bus

3.1  Potential-source controlled rectifier system:

• excitation power is supplied through a transformer from the main generator terminals
• regulated by a controlled rectifier
• commonly known as bus-fed or transformer-fed static excitation system
• very small inherent time constant
• maximum exciter output voltage is dependent on input ac voltage; during system faults the available ceiling voltage is reduced

3.2  Compound-source rectifier system:

• power to the exciter is formed by utilizing current as well as voltage of the main generator
• achieved through a power potential transformer (PPT) and a saturable current transformer (SCT)
• the regulator controls the exciter output through controlled saturation of excitation transformer
• during a system fault, with depressed generator voltage, the current input enables the exciter to provide high field forcing capability

3.3  Compound-controlled rectifier system:

• utilizes controlled rectifiers in the exciter output circuits and the compounding of voltage and current within the generator stator
• result is a high initial response static system with full “fault-on” forcing capability

A modern excitation control system includes a number of control, limiting and protective functions which assist in fulfilling the performance requirements identified earlier. Any given system may include only some or all of these functions depending on the specific application and the type of exciter.

• control functions regulate specific quantities at the desired level
• limiting functions prevent certain quantities from exceeding set limits. If any of the limiters fail, then protective functions remove appropriate components or the unit from service.

Fig: Excitation system control and protective circuits

• AC Regulator: maintains generator stator voltage.
• DC Regulator: holds constant generator field voltage (manual control).
• Excitation System Stabilizing Circuits: series or feedback compensation is used to improve the dynamic response and most commonly used form of compensation is a derivative feedback.
• Power System Stabilizer (PSS): It uses auxiliary stabilizing signals (such as shaft speed, frequency, power) to modulate the generator field voltage so as to damp system oscillations.
• Load Compensator: It is commonly used with hydro units and cross-compound thermal units. It ensures proper sharing VARs between generators bussed together at their terminals.
• Under excitation Limiter (UEL): It is intended to prevent reduction of generator excitation to a level where steady-state (small-signal) stability limit or stator core end-region heating limit is exceeded.
• Over excitation Limiter (OXL) : Its purpose is to protect the generator from overheating due to prolonged field over current. OXL detects the high field current condition and, after a time delay, acts through the ac regulator to ramp down the excitation to about 110% of rated field current.
• Volts per Hertz Limiter and Protection: It is used to protect generator and step-up transformer from damage due to excessive magnetic flux resulting from low frequency and/or overvoltage.