Text 2 Operating Principle of an avr
The way in which an AVR controls the excitation of a generator or exciter varies, however all of them fall under two basic categories namely:
The Excitation Supply AVR
The AVR supplies the whole of the required excitation current under normal operating conditions.
The Compounding Control AVR
The generator is compounded to produce an excessive excitation current at all times. The function of the AVR is to trim down the current to the correct value.
Excitation Systems
The excitation system has to both supply and control the direct current for the rotor's pole windings in the case of a rotating field, stationary armature alternator. The level of the excitation current and resulting field strength is automatically adjusted by the voltage component.
Self-excited AVR Controlled Generator
The main stator provides the input for excitation via the AVR, which governs the level of excitation provided to the exciter field. The AVR responds to a voltage sensing signal derived from the main stator winding. By controlling the low power of the exciter field, control of the high power requirement of the main field is achieved through the rectified output of the exciter armature.
The AVR senses the average voltage on two phases thereby ensuring effective regulation. In addition it detects the engine speed and provides voltage fall-off (i.e. reduction) with speed, below a pre-selected speed (frequency) setting, which could be + 5% under permanent conditions and +10% under transient conditions, preventing over-excitation at low engine speeds and softening the effect of load switching in order to relieve the burden on the engine. The recovery time may be as high as 5 seconds.
The AVR may also possess features like three-phase root mean square sensing, also providing for over-voltage protection when used in conjunction with an external circuit breaker that is switchboard mounted. A block diagram of the same is depicted in Figure 1.
Self-excitation
System
Permanent Magnet Generator (Excited -AVR Controlled Generators)
Newer generators employ six separate windings. The additional two windings are identical in operation to any pair of field and armature windings. These extra windings provide external excitation for the generator in the same way the four-winding generator provided for its own self-excitation.
The magnet is mounted on the rotor and is located inside the permanent magnet generator's armature. When the generator is running, the magnet generates an EMF in the armature, providing current directly to the automatic voltage regulator for control of the exciter field.
The permanent magnet provides power for excitation of the exciter's field (while starting) via die AVR which governs the excitation provided to the exciter's field. It facilitates greater voltage control under extreme load conditions.
The AVR responds to a voltage-sensing signal derived via an isolating transformer connected to the main stator winding. By controlling the low power of the' exciter field, 'control of the high power requirement of the main field is achieved through the rectified output of the exciter armature. The PMG system provides a constant source of excitation power irrespective of the main stator's load. It ensures a high motor-starting capability as well as immunity to waveform distortion on the main stator output that is created by non-linear loads, e.g., thyristor controlled d.c. motors. A block diagram of the system is depicted in Figure 2.
The AVR senses average voltage on two phases ensuring close regulation. In addition it detects engine speed and provides an adjustable voltage fall-off with speed, below a preselected speed (frequency) setting, preventing over-excitation at low engine speeds and softening the effect of load switching to relieve the burden on the engine. It also provides over-excitation protection which acts following a time-delay, to de-excite the generator in the event of excessive exciter field voltage.
Fig.2
PMG-excited AVR Controlled Generator
Additional Important Features in a Modern AVR
The latest AVRs follow the three-phase root mean square sensing method for superior voltage regulation and form part of an excitation system for brushless alternators. The permanent magnet generator in the circuit isolates the AVR control circuits from the effects of non-linear loads and alleviates the effects of radios frequency interference. Adjustable soft start circuits provide a smooth controlled build-up of voltage in a time span of about 0.4 to 4 seconds. A frequency measuring circuit continually monitors the shaft speed of the generator and provides under-speed protection of the excitation system by reducing the generator output voltage proportionally with speed below a pre-set threshold. The other features are in the following:
Stability Adjustment
Under Frequency Roll Off
Excitation Trip
Over Voltage Protection
Transient Load Switching Adjustments
Ramp
Droop
Over-voltage De-excitation Breaker
The Block Diagram (the functions of the blocks are explained briefly):
Power Supply
The Potential Divider and Rectifier
The DC Mixer
The 3-Phase Rectifier
The Amplifier (Amp)
Block Diagram of a Modern AVR
The Stability Circuit
The Low Hz Detector
The Synchronising circuit
Power Control Devices
The Circuit Breaker
The Over Excitation Detector
The Over Voltage Detector