2 types of synchronisers required on a power system with two supplies

Synchronisers are required at points on a power system where two supplies (either generator and grid, or two grid supplies) may need to be paralleled.

They are more than just a measuring device, as they will provide contact closures to permit circuit breaker closing when conditions for paralleling (synchronising) are within limits.

Syncronisers

However, they are not regarded as protection relays, and so are discussed in this article for convenience. There are two types of synchronisers:

1. Check synchronisers and
2. Auto-synchronisers

1. The function of a check synchroniser

The function of a check synchroniser is to determine if two voltages are in synchronism, or nearly so, and provide outputs under these conditions. The outputs are normally in the form of volt-free contacts, so that they may be used in CB control circuits to permit or block CB closing.

When applied to a power system, the check synchroniser is used to check that it is safe to close a CB to connect two independent networks together, or a generator to a network, as in Figure 1.

Synchronism occurs when two a.c. voltages are of the same frequency and magnitude, and have zero phase difference. The check synchroniser, when active, monitors these quantities and enables CB close circuits when the differences are within pre-set limits.

While CB closure at the instant of perfect synchronism is the ideal, this is very difficult to obtain in practice and some mismatch in one or more of the monitored quantities can be tolerated without leading to excessive current/voltage transients on CB closure.

The check synchroniser has programmable error limits to define the limits of acceptability when making the comparison.

The conditions under which a check synchroniser is required to provide an output are varied. Consider the situation of a check synchroniser being used as a permissive device in the closing control circuit of a CB that couples two networks together at a substation.

It is not sufficient to assume that both networks will be live, situations where either Line A or Busbar B may be dead may have to be considered, leading to the functionality shown in Table 1(a).

Table 1 – Check Synchroniser function set

When the close signal is permitted, it may be given only for a limited period of time, to minimise the chances of a CB close signal remaining after the conditions have moved outside of limits.

A check synchroniser does not initiate any adjustments if synchronising conditions are not correct, and therefore acts only as a permissive control in the overall CB closing circuit to provide a check that conditions are satisfactory.

In a substation, check-synchronisers may be applied individually to all required CBs.

Alternatively, a reduced number may be installed, together with suitable switching arrangements in the signal input/output circuits so that a single device may be selected to cover several CBs.

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2. Auto-synchroniser

An auto-synchroniser contains additional functionality compared to a check synchroniser. When an auto- synchroniser is placed in service, it measures the frequency and magnitude of the voltages on both sides of the circuit breaker, and automatically adjusts one of the voltages if conditions are not correct.

Application of auto-synchronisers is normally restricted to generators – i.e. the situation shown in Figure 1(a), replacing the check synchroniser with an auto- synchroniser.

The auto-synchroniser will check the voltage of the incoming generator against the network voltage for compliance with the following:

1. Slip frequency within limits (i.e. difference in frequency between the generator and network)
2. Phase difference between the voltages within limits
3. Voltage magnitude difference within limits

The CB close command is issued automatically when all three conditions are satisfied. Checks may also be made that the network frequency and voltage is within pre-set limits, and if not the synchronising sequence is locked out. This prevents synchronising under unusual network conditions, when it may not be desirable.

This facility should be used with caution, since under some emergency conditions, it could block the synchronising of a generator that was urgently required in service to help assist in overcoming the condition.

If (a) above is not within limits, signals are sent automatically to the governor of the generating set to adjust the speed setpoint appropriately.

In the case of (c) not in limits, similar signals are sent to the Automatic Voltage Regulator to raise or lower the setpoint. The signals are commonly in the form of pulses to raise or lower the setpoint, but could be continuous signals if that is what the particular equipment requires.

The possibility of tripping due to reverse/low forward power conditions and/or field failure/under-excitation is avoided. Use of an auto-synchroniser also helps avoid human error if manual synchronising were employed – there is potential for damage to equipment, primarily the generator, if synchronising outside of permitted limits occurs.

To ensure that the CB is closed at the correct instant, the CB close time is normally a required data item. The auto-synchroniser calculates from a knowledge of this and the slip frequency the correct time in advance of phase coincidence to issue the CB close command. This ensures that the CB closes as close to the instant of phase coincidence as possible. Upon receipt of the signal indicating ‘CB closed’ a further signal to raise frequency may be sent to the governor to ensure stable export of power is achieved.

Conversely, failure of the CB to close within a set time period will reset the auto-synchroniser, ready for another attempt, and if further attempts are still unsuccessful, the auto-synchroniser will lock out and raise an alarm.

Practice in respect of fitting of auto-synchronisers varies widely between Utilities. Where policy is flexible, it is most common when the time to synchronise is important – i.e. emergency standby and peak lopping sets. Many Utilities still relay on manual synchronising procedures.

It is also possible for both an auto-synchroniser and check-synchroniser to be fitted in series. This provides protection against internal failure of the auto-synchroniser leading to a CB close command being given incorrectly.

Table 2 – Additional functions of Auto-Synchroniser

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Reference // Network Protection & Automation Guide by Alstom