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

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

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

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

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self or separately excited represent early systems (1920s to 1960s); lost favor in the mid-1960s because of large size; superseded by ac exciters voltage regulators range from the early non- continuous rheostatic type to the later system using magnetic rotating amplifiers

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

Figure 8.2: DC excitation system with amplidyne voltage regulators

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. 8.3 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

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

Figure 8.5: Brushless excitation system

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

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.

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

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

Figure 8.14: Excitation system control and protective circuits

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

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

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

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. 8.18 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

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

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