Synchronising with the aid of Lamps

MODULE 2

UNIT 3. PARALELLING ALTERNATORS

AUTOSYNCHRONIZING ACGs

AUTO SYNCHRONIZING OF ACGs

General.When it is intended that two or more generators be operated in parallel, means are to be provided to divide the reactive power equally between the generators in proportion to the generator capacity.

3.13.3(b) Reactive load sharing.The reactive loads of the individual generating sets are not to differ from their proportionate share of the combined reactive load by more than 10% of the rated reactive output of the largest generator, or 25% of the smallest generator, whichever is the less.

3.13.3(c) kW load sharing.In the range between 20% and 100% of the sum of the rated loads of all generators, the kW load on any generator is not to differ more than ±15% of the rated output kW of the largest generator, or 25% of the rated output kW of the individual generator, whichever is the less, from its proportionate share. The starting point for the determination of the foregoing load-distribution requirements is to be at 75% load with each generator carrying its proportionate share.

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The Basics

Main generator units (steam turbine, diesel driven and shaft driven) have to be run in parallel to share a total load that exceeds the capacity of a single machine. Changeover of main and standby generator units require a brief parallel running period to achieve a smooth transition without a blackout situation.

For the sake of simplicity and security, it is normally not possible or advisable to run a main generator in parallel with either the emergency generator or the shore supply. Circuit breaker interlocks are incorporated to prevent it. Parallel running is achieved in two stages -synchronising and load sharing. Both can be carried out automatically but manual control is still in common use and is generally provided anyway as a back-up to the auto control mode.

The generator already 'on the bars' is called the running machine and the generator to be brought into service is the incoming machine.

In order to parallel the incomer smoothly, it must be synchronised with the running generator (or the bus bars); the following conditions are essential:

1. Same voltage

2. Same frequency

3. Same phase sequence

In practice, one may find it difficult to adjust the speed of the incoming machine so that the pointer of the synchroscope is stationary at 12 o'clock. Such a condition is not essential and a more practical proposition is to have the pointer rotating slowly in the 'Fast' direction and to close the paralleling switch at about 11 o'clock. Due to the time lag of the operating mechanism and human response, actual synchronising will thus take place closer to the 12 O'clock position, and the machine, running fast will be slowed down slightly while taking a small proportion of the load.

If the incoming machine is synchronised when it is running slow, it would slow down further and draw a motoring current, which may operate its reverse-power relay and 'trip' the circuit-breaker of the machine already on the 'bars' due to overloading.

The likely consequences of attempting to close the incomer's breaker when the generators arenot in synchronism are that at the instant of closing the breaker, the voltage phase difference causes a large circulating current between the machines; this results in a large magnetic force to 'pull' the generators into synchronism. This means rapid acceleration of one rotor and deceleration of the other. The large forces may physically damage the generators and their prime movers, which may include deformation of the stator windings, movement between the stator core and frame, failure of the rotor diodes in brushless machines, twisted rotor shafts, localised crushing of shaft-end keyways and broken couplings. Tie large circulating current may also trip each generator breaker. Severe vibration of is also at symptom of loss of synchronism. This will be accompanied by flashover at the slip rings in the case of alternators that have a rotating armature. The ultimate result is a blackout, danger toe embarrassment!

Check-Synchronising Unit

This unit uses an electronic circuit to monitor the voltage, phase angle and speed (frequency) of the incoming generator with respect to the bus bars i.e., it prevents faulty manual synchronising. 'Check Synchronising' modules are often provided with a manual over-ride switch for use in an emergency. This can lead to problems if the over-ride is left activated after the emergency.

Auto-Synchronising

This does everything an operator would do. It senses and controls the voltage and frequency then initiates a circuit-breaker 'close' signal (of the incoming alternator) at the correct instant. The auto-synchronising equipment uses electronic circuits to monitor the magnitude of voltage, frequency and phase angle difference, and then acts to regulate them until they are equal to the existing parameters of the bus bar. (Refer Figure 10.2). Usually, one set of either check or auto synchronising units is switched between a set of generators as and when required. When an incoming generator has been successfully synchronised, the synchronising equipment should be switched off. The total bus bar load can now be shared between generators or totally transferred to the new machine. In a parallel operation, thegovernor of the alternator's prime mover directly controls power (kW) while its AVR trimmer or hand voltage regulator controls reactive volt amps (kVAr) or power factor. Figure 10.3(a) is similar to Figure 10.1(b) except for the fact that while synchronising is automatic, the process has to be manually initiated. However Figure 10.3(b) depicts a completely automatic system's flow chart.

alternator starting and synchronizing (automatic operation)

Synchronising with the aid of Lamps

As a back-up or alternative to the synchroscope, a set of lamps may be used. The lamp method of synchronising makes use of filament lamps, so connected across the contacts of the paralleling switch that the intensity of the illumination varies continuously i.e., in each case the lamps are connected between the incoming generator and the bus bars. The correct synchronised state may be indicated by the 'Sequence' method that utilises 3 lamps.

The 'Sequence' method displays a rotation of lamp brightness, which indicates whether the incoming machine is running fast (clockwise) or slow (anticlockwise). As with the synchroscope in Figure 9.41, the lamps' sequence must appear to rotate slowly clockwise. Correct synchronisation occurs when the top or 'key' lamp is dark and the two bottom lamps are equally bright. The error in the frequency of the incoming machine as compared with bus bar frequency is shown by the rate at which the lamps 'darken' or 'brighten'. Figure 10.4 depicts the usual 'Sequence' or 3-lamp method. For three-phase systems, although the direct connection of three lamps across the contacts of each line or cross-connecting of the lamps are methods which can be used, the Siemens-Halske arrangement as shown in Figure 10.4 and explained in Figure 10.5, is favoured.

arrangement of synchronizing lamps