1. EXCITATION SYSTEMS
Copyright © P. Kundur This material should not be used without the author's consent

2. Excitation Systems Outline Functions and Performance Requirements
Elements of an Excitation System
Types of Excitation Systems
Control and Protection Functions

3. Functions and Performance Requirements of Excitation Systems
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
Generator considerations:
supply and adjust field current as the generator output varies within its continuous capability
respond to transient disturbances with field forcing consistent with the generator short term capabilities:
rotor insulation failure due to high field voltage
rotor heating due to high field current
stator heating due to high VAR loading
heating due to excess flux (volts/Hz)
Power system considerations:
contribute to effective control of system voltage and improvement of system stability

4. Elements of an Excitation System
Exciter: provides dc power to the generator field winding
Regulator: processes and amplifies input control signals to a level and form appropriate for control of the exciter
Terminal voltage transducer and load compensator: 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: provides additional input signal to the regulator to damp power system oscillations
Limiters and protective circuits: ensure that the capability limits of exciter and generator are not exceeded

5. Types of Excitation Systems
Classified into three broad categories based on the excitation power source:
DC excitation systems
AC excitation systems
Static excitation systems
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represent early systems (1920s to 1960s); lost favor in the mid-1960s because of large size; superseded by ac exciters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

6. AC Excitation Systems:
Figure 8-2 shows a simplified schematic of a typical dc excitation system with an amplidyne voltage regulator
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
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

7. Figure 8.2: DC excitation system with amplidyne voltage regulators

8. 2.1 Stationary rectifier systems:
dc output to the main generator field supplied through slip rings
when non-controlled rectifiers are used, the regulator controls the field of the ac exciter; Fig shows such a system which is representative of GE-ALTERREX system
When controlled rectifiers are used, the regulator directly controls the dc output voltage of the exciter; Fig. 8.4 shows such a system which is representative of GE-ALTHYREX system
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

9. Figure 8.3: Field controlled alternator rectifier excitation system
Figure 8.4: Alternator supplied controlled-rectifier excitation system

10. Figure 8.5: Brushless excitation system

11. Figure 8.6: Potential-source controlled-rectifier excitation system
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
Figure 8.6: Potential-source controlled-rectifier excitation system

12. 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
An example is the GE SCT-PPT.
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
An example is the GE GENERREX system.

13. Fig. 8.7: Compound-source rectifier excitation system
Figure 8.8: GENERREX compound-controlled rectifier excitation system ©IEEE1976 [16]

14. Control and Protective Functions
A modern excitation control system is much more than a simple voltage regulator
It includes a number of control, limiting and protective functions which assist in fulfilling the performance requirements identified earlier
Figure 8.14 illustrates the nature of these functions and the manner in which they interface with each other
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

15. Figure 8.14: Excitation system control and protective circuits

16. Excitation System Stabilizing Circuits:
AC Regulator:
basic function is to maintain generator stator voltage
in addition, other auxiliaries act through the ac regulator
DC Regulator:
holds constant generator field voltage (manual control)
used for testing and startup, and when ac regulator is faulty
Excitation System Stabilizing Circuits:
excitation systems with significant time delays have poor inherent dynamic performance
unless very low steady-state regulator gain is used, the control action is unstable when generator is on open-circuit
series or feedback compensation is used to improve the dynamic response
most commonly used form of compensation is a derivative feedback (Figure 8.15)
Figure 8.15: Derivative feedback excitation control system stabilization

17. Power System Stabilizer (PSS):
uses auxiliary stabilizing signals (such as shaft speed, frequency, power) to modulate the generator field voltage so as to damp system oscillations
Load Compensator:
used to regulate a voltage at a point either within or external to the generator
achieved by building additional circuitry into the AVR loop (see Fig. 8.16)
with RC and XC positive, the compensator regulates a voltage at a point within the generator;
used to ensure proper sharing VARs between generators bussed together at their terminals
commonly used with hydro units and cross-compound thermal units
with RC and XC negative, the compensator regulates voltage at a point beyond the generator terminals
commonly used to compensate for voltage drop across step-up transformer when generators are connected through individual transformers

18. Figure 8.16: Schematic diagram of a load compensator
The magnitude of the resulting compensated voltage (Vc), which is fed to the AVR, is given by

19. Underexcitation Limiter (UEL):
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
control signal derived from a combination of either voltage and current or active and reactive power of the generator
a wide variety of forms used for implementation
should be coordinated with the loss-of-excitation protection (see Figure 8.17)
Overexcitation Limiter (OXL)
purpose is to protect the generator from overheating due to prolonged field overcurrent
Fig shows thermal overload capability of the field winding
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; if unsuccessful, trips the ac regulator, transfers to dc regulator, and repositions the set point corresponding to rated value
two types of time delays used: (a) fixed time, and (b) inverse time
with inverse time, the delay matches the thermal capability as shown in Figure 8.18

20. Figure 8.17: Coordination between UEL, LOE relay and stability limit
Figure 8.18: Coordination of over-excitation limiting with field thermal capability

21. Volts per Hertz Limiter and Protection:
used to protect generator and step-up transformer from damage due to excessive magnetic flux resulting from low frequency and/or overvoltage
excessive magnetic flux, if sustained, can cause overheating and damage the unit transformer and the generator core
Typical V/Hz limitations:
V/Hz limiter (or regulator) controls the field voltage so as to limit the generator voltage when V/Hz exceeds a preset value
V/Hz protection trips the generator when V/Hz exceeds the preset value for a specified time Note: The unit step-up transformer low voltage rating is frequently 5% below the generator voltage rating
V/Hz (p.u.)
1.25
1.2
1.15
1.10
1.05
Damage Time in Minutes
GEN
0.2
1.0
6.0
20.0 
XFMR
5.0