Phasor Rotation while Synchronising
This method not only indicates the correct instant for synchronising but also indicates when the incoming alternator is running fast or slow relative to the bus bar voltage. From the superimposed phasor diagrams of Figure 10.5 it will be seen that when running 'Slow', the lamps will glow in the order Li, L3, L2 and so on. If the incoming machine is running 'Fast' the lamps will glow in the order Li, L2, L3 and so on.
When the machines are in phase, then vectors 'VR' and 'VRi' will be aligned and therefore 'L,' will be dark, 'VY' and 'VBi' will be 120° apart and therefore '1^' will be approaching maximum luminosity, and the same will be for 'L3' with 'VY!' and 'VB' 120°.
So, if the lamps were arranged in a triangular pattern they would tend to brighten in a clockwise direction when the incoming generator is running faster than that which is already running; in short the frequency will be slightly higher. The moment for synchronising is with the 'key' lamp Li 'dark' and the other lamps L2 and L3 glow equally but not at full brilliance. Alternatively (or in addition) synchronising instruments may be used as shown in Figure 10.6. After successful synchronisation, the generator load should be shared equally, provided the alternators are similarly rated.
Synchronising with the Aid of a Voltmeter
To monitor the correct instant for synchronising without the aid of a synchroscope or synchronising lamps, connect a pair of 500V voltmeter probes across one phase of the incoming machine circuit breaker. Adjust the generator speed until the voltmeter slowly fluctuates from zero to maximum. Close the breaker when the voltmeter passes through zero.
In order to parallel generators, the prime movers must be in proper working order. For example, the diesel engines need to be mechanically sound and properly tuned. The governors must be set properly. Before you ever consider major adjustments on the distribution switchboard, you must consult the operation and maintenance manual of the prime mover.
If the prime movers do not operate with the expected speed characteristics, then there is no possible way for you to compensate for their inaccuracies at the switchboard. For a paralleled alternator to take its share of the load, it is necessary to study the effects of two possible adjustments possible - namely:
1. Operation of thefield regulator i.e. excitation control
2. Operation of throttle or steam valve i.e. speed control.
With two alternators in parallel, an increase in excitation of one machine raises the generated EMF and should tend to make it bear a greater share of the load. However, the machine cannot slow down since it is "tied" synchronously to the system and thus the governor of the prime-mover is unaffected. No action results in causing the machine to bear greater loads. As will be seen, the operation of the excitation control system merely causes a wattles current, which circulates in the paralleled machines and the bus bar system. This current lags the generated EMF by an angle <p and the load can be equated to EI Cosф. The kW load thus remains constant to maintain an unvaried governor setting. To change the distribution of load between alternators in parallel, the throttle valves must be manipulated.
We thus see that for two alternators in parallel, since the speeds (frequencies) must be identical, the kW loading on each machine must be related to the prime-mover input power i.e. to the amount of operation of the throttle valve and cannot be controlled by the excitation. The effect of excitation and throttle control will now be considered in detail. The parallel operation of alternators may be studied under two distinct considerations:
The first would be parallel working with an 'infinite bus bar', as constituted by shore-based power stations linked through a national transmission grid system. An ideal case of infinite bus bars is one where the system is so large, in comparison with a single alternator, that its voltage and frequency are unaffected by the behaviour of the alternator.
The second consideration is of importance to the marine engineer, since it relates to working on board a ship. Here, bus bar voltage and frequency can be altered by local conditions and the more common case, of two or more alternators running in parallel, is therefore stressed upon in this chapter.
AUTO SYNCHRONIZER T4500
• Automatic synchronization with relay outputs for speed control
• Adjustable delta frequency and delta voltage
• Adjustable breaker make time
• Visual indication of bus voltage, generator voltage, closing signal, delta voltage, increase and decrease signals
• Automatic voltage matching
• Cost effective and highly reliable design
• 50 hours burn-in before final test
• Operating temperature range:
-20°C to +70°C
• Vibration test up to 4g (5 - 100Hz)
• Certified by major classification societies
• Flame retardant enclosure
• DIN rail or screw mounting
The T4500 Auto Synchronizer provides automatic synchronization of an incoming generator to a busbar in a minimum of time, by controlling the speed via the electric servomotor on a conventional speed governor, or by controlling an electronic speed controller via an intermediate motorized potentiometer.
Together with the T4800 Load Sharer, the T4500 provides the optimal solution for generator control, both in marine and land-based applications. The T4500 is type approved by major marine classification societies.
The T4500 measures the voltage across two phases on either side of the circuit breaker in order to obtain data on voltage, frequency and phase difference for closing the circuit breaker at exact phase accordance.
The synchronization function will become active when the difference between the bus voltage and the generator voltage is within limits, which is indicated on the DVOLT LED.
The voltage difference is selectable between 2% to 10% (see the resistor table on page 2 for selecting the DVolt window value). If the voltage difference is too high, the voltage matching function of the unit can be used (see the separate section on voltage matching).
When the synchronization function is active, the T4500 will automatically adjust the speed of the generator through the governor in order to match the frequency to the busbar. Two built-in relays provide the increase and decrease pulses for a conventional governor. The length of the pulses is proportional to the frequency difference.
The E7800 Motorised Potentiometer can be used to adapt the contact pulses to a signal, suitable for the speed trim input of an electronic speed controller.
The T4500 will continuously adjust the generator speed until the frequency difference is within limits. The frequency difference is adjustable on the front dial DIFF. FREQ from 0.1Hz to 1.0Hz.
Before the breaker can be closed this frequency difference must be positive. The reason is that in order to protect the generator against reverse power, the generator should come in at a slightly higher frequency than the frequency of the busbar.
When the voltage and frequency difference are within limits, the closing signal will be activated just before the next phase accordance, anticipating the circuit breaker make time.
The circuit breaker make time should be set on the front dial C/B MAKE TIME according to the specifications of the circuit breaker. The T4500 compensates for this make time so that the circuit breaker will close exactly at zero phase.
The circuit breaker closing signal is a pulse signal of 0.7 seconds duration at terminals 9 and 10 (CLOSE). A connection between terminals 11 and 12 (DISABLE) will disable the closing signal, but will not influence the automatic frequency alignment.
When commissioning, it is recommended to disable the closing signal with this connection. Check that the closing signal indicated on the RELAY LED is at phase accordance.