The document discusses different types of excitation systems used in synchronous generators including DC, AC, and static excitation systems. It describes the components and operating principles of various excitation system configurations. Key topics covered include AC and DC voltage regulators, stabilizing circuits, power system stabilizers, load compensation, and over-excitation and under-excitation limiters used to protect the generator.

1. Excitation Systems

2. Block diagram of Synchronous Gen. Excitation System

3. Types of Excitation Systems
1) DC Excitation Systems
utilize a DC generator with commutator
2) AC Excitation Systems
utilize an AC generator (Alternators) with stationary or
rotating rectifiers
3) Static (Non Rotating) Excitation Systems
utilize a transformer and Rectifier

4. DC Excitation Systems

5. AC Excitation Systems
a) Stationary Rectifier Systems
b) Rotating Rectifier Systems

6. a) Stationary Rectifier Systems

8. b) Rotating Rectifier Systems

9. Static Excitation Systems
a) Potential Source Controlled Rectifier Systems
b) Compound Source Rectifier Systems
c) Compound Controlled Rectifier Excitation Systems

10. a) Potential Source Controlled Rectifier Systems

11. b) Compound Source Rectifier Systems

12. • Field Flashing of Static Exciters
Compound Controlled Rectifier Excitation Systems

13. Excitation System Control and
Protection Circuits

15. AC and DC Regulators
 AC Regulator – Maintain the generator stator voltage
 DC Regulator – holds constant generator field voltage – Manual Control
 Primarily – testing, start up and AC regulator is faulty
 Minimize voltage and reactive power excursions
 During abrupt removal of AC regulator
 Caution - During Manual control – machine should not exceed the limits

16. Excitation System Stabilizing Circuits
 Excitation systems – elements with significant time delays – poor inherent
dynamic performance – particularly for dc and ac type
 Excitation control – becomes unstable – generator – open circuit
 To improve dynamic performance – compensation is essential
 Compensation – minimizes the phase shift – introduced by time delays
 Results in stable offline performance
 Can also be adjusted – online performance

17. Excitation System Stabilizing Circuits
 Depending on type of excitation systems – Many levels
 Major Outer Loop
 Minor Inner Loop
 Static Excitation systems – negligible time delays – don’t require
stabilization unit

18. Power System Stabilizers
 Uses auxiliary stabilizing signals to control – excitation system –improve
power system dynamic performance.
 Input signals may be – Shaft speed, terminal frequency and power
 Damping system oscillations – dynamic performance
 Enhance small signal stability performance

19. Load Compensation

20. Load Compensation
 Automatic voltage regulator (AVR) – control – generator stator terminal
voltage.
 Load compensation – control voltage – w.r.t to voltage at a point – within or
external to the generator.
 Compensator – Adjustable resistance (Rc) and inductive reactance (Xc) –
impedance between generator terminals and the point at which the voltage
need to be controlled.

21. Load Compensation
 Using a) Measured Impedance, b) Current, voltage drop is computed, added or subtracted
from the terminal voltage. Compensated voltage fed to AVR
𝑉
𝑐 = 𝐸𝑡 + (𝑅𝑐+𝑗𝑋𝑐) 𝐼𝑡
 If RC & XC are positive – Terminal Voltage + Voltage drop
 Compensator regulates the voltage within the generator provides voltage droop
 Ensures power sharing reactive power between the generators
 Commonly used in hydro power plants
 Reactor power compensator – artificial coupling between the generator

22.  Without load compensator –
 One generator – tries to control terminal voltage slightly higher than the other
 One Generator – supply reactive power,
 another generator – absorb reactive power – allowed till under-excitation limits

23. Load Compensation
 When Rc and Ic are negative – regulates the voltage beyond the terminals
 Compensates the voltage drop in across the step-up transformer
 Typically 50 – 80 % of the transformer impedance is compensated
 Ensure the voltage droop – for satisfactory parallel operation of the
generators
 Also called as Line drop compensator

24. Under-excitation limiter
• Under-excitation limiter is intended to prevent the reduction of
generator excitation to a level where
• Small signal (steady-state) stability limit
• Stator core end region heating limit
are exceeded

25. Under-excitation limiter
• Also called as
• under excitation reactive-ampere limiter
• Minimum excitation limiter

26. Over-excitation limiter
• Overexcitation limiter – protect from overheating – due to prolonged field
overcurrent
• Also called as maximum excitation limiter
• Generator Field winding – designed to operate continuously at a value
corresponding to rated loading conditions
• Permissible thermal overall of the field winding of rotor – ANSI standard
C50.13-1977

27. Over-excitation limiter
• Overexcitation limiting function – detects high field current
• After a time delay – ac regulator to reduce excitation – within 100% to 110%
• Unsuccessful – trips ac regulator – change to dc regulator
• If excitation is not within safe value – limiter initiates breaker and unit trip

28. Over-excitation limiter
• Two types of time delays used
• Fixed time limiters
• Inverse time limiters – includes field
thermal capability
• For high ceiling voltage exciters
• Additional fault current limiter
• Acts instantaneously through ac regulator
• Limits current to short time limit (160%)

29. 𝐸 = 4.44 ∅𝑚 f N
𝐸 = 𝑘 ∅𝑚 f
∅𝑚 = k
𝐸
𝑓
• Field voltage is controlled to control the generator terminal voltage
Volt-per-Hertz Limiter and Protection

30. Field Shorting Circuits
Rectifiers can’t conduct in reverse direction – ac and static exciters. Induced current is
negative under pole slipping and system short circuits – causes high voltage
• Thyristor and
• Field Discharge Resistor (FDR)
• During overvoltage – created by induced
current – not having path initially
• Non linear resistor
• Offers high resistance during normal
operation
• During overvoltage - resistance decreases
• Provides path to current flow and limits
the voltage