MAIN AND PILOT EXCITER

Main Exciter

The exciter (sometimes called the main exciter) is a synchronous generator that has its stator and rotor windings inverted. Its field winding is fixed in the stator, and the rotor carries the armature or AC . In addition the rotor carries the semiconductor bridge rectifier that converts the armature voltages to a two-wire DC voltage system. The AC voltages and currents in the armature are often alternating at a higher frequency than those in the main generator, e.g. 400 Hz. The higher frequency improves the speed of response of the exciter. The DC power circuit is coupled to the field of the main generator by the use of insulated conductors that pass coaxially inside the rotor of the exciter and the rotor of the main generator. This eliminates the use of slip rings, which were traditionally used before shaft mounted rectifiers were developed. A slight disadvantage of this technique is that the derivative feedback cannot be taken from the output of the exciter. However, with modern electronic devices used throughout the AVR, this can be regarded as an insignificant disadvantage.
The time constant Te of the exciter is mainly related to its field winding. The saturation block in Figure 4.1 accounts for the magnetic saturation of the iron core of the exciter, and it is important to represent this because the expected range of the performance of the exciter is wide. Its terminal voltage may have a value of typically 3.0 per unit when the generator is fully loaded. This may increase to about 6.5 per unit when the generator needs to maintain a full short circuit at or near to its terminals. The maximum excitation voltage is called the ‘ceiling voltage’ of the exciter.

Pilot exciter

The AVR system requires a source of power for its amplifier, its reference voltage and other electronic circuits that may be involved e.g. alarms. There are several methods of obtaining this necessary power,

• An external power supply.
• Self-excitation.
• Pilot exciter.

An external supply could be an uninterrupted power supply (UPS) that is dedicated to the generator. Although this is feasible it is not a method that is used, the main reason being that it departs from the requirement of self-containment. The equipment involved would require external cables and switchgear, both of which add a factor of unreliability to the scheme.

The self-excitation method relies upon the residual magnetism in the iron core of the main generator that remains in the core after the generator is shut down. When the generator is started again and run up to speed a small emf is generated by the residual magnetism. A special circuit detects the residual emf at the main terminals and amplifies it to a predetermined level. This amplified voltage is rendered insensitive to a wide range of emf values and has sufficient power to feed all the auxiliary requirements of the AVR. The advantage of this method is its low cost compared with using a pilot exciter. Its main disadvantage is an inferior performance when a short circuit occurs at or near the main generator. The detected emf, or terminal voltage, when the generator is connected to the busbars, falls to near zero when the short circuit exists. The AVR may lose its supply during this period or perform in an unpredictable manner. The excitation of the generator may collapse, which is not desirable.

The pilot exciter method is highly reliable and has a fully predictable performance. A small alternator is mounted on the same shaft, and often within the same frame, as the main exciter. It receives its excitation from a shaft mounted permanent magnet rotor system. Hence its level of excitation is constant and dependable. The AC output from the pilot exciter is rectified and smoothed by components within the AVR cubicle. It can be seen that this method is completely independent of the conditions existing in the main generator. This is the method usually specified in the oil industry.