Short Term Infrequent Paralleling Issues

09/04/2010 by David Slade.

Short term infrequent paralleling is usually applied to standby power supplies that are designed to support a customer’s islanded network and allows for seamless transfer following restoration of supply or for load transfer during routine testing.

Acceptable number of, and duration of paralleling time.
G59/1 allows for the infrequent connection of generation with, (at the DNO’S discretion,) a relaxation in protection requirements. No hard and fast definitions are given for “infrequent” but a figure of once per month is suggested. The number of parallel operations should be at the discretion of the individual DNO based on the capability of the network. However if it is required to test more than once a week then this would no longer be considered as infrequent paralleling.

Once operating in parallel, the generator is allowed to remain in parallel for a maximum of 5 minutes. The duration of the parallel should be kept as short as is reasonably practical for change over to take place. Less than 1 minute is normally achievable with longer times only being required if the equipment is incapable of shorter change over. After this time the parallel has to be broken automatically by a timer. This timer should be a separate device from the changeover control system such that failure of the auto changeover system will not prevent the parallel being broken.

Summary: In order for the generator to be considered as operating as “short term infrequent paralleling”, it must not be allowed to connect in parallel for more than 5 minutes in any month, or more frequently than once per week. If the duration of parallel connection exceeds this period, or this frequency, then the generator must be considered as if it is, or can be, permanently connected.

Protection Requirements
Short term occasional parallel operation requires only basic under /over voltage and under /over frequency protection.

A timer separate from the normal changeover control system should be provided to break the parallel if the normal means of changeover should fail.

This protection only needs to be in operation for the time the generator is operating in parallel.

Loss of mains protection in the form of ROCOF and Vector shift are not required, although many G59 multifunction relays now have this function built in as standard.

Similarly additional requirements such as NVD, intertripping and reverse power are not required.

This is based on the assumption that during the year the generator is only likely to be in parallel for a maximum of 1/8760th of the time and therefore the chance of a genuine loss of mains event coinciding with the parallel is unlikely.

If a coincidence did occur, the possibility of the generation supporting the Island also becomes a factor. Under voltage / under frequency is likely to trip the generation off if the load is greater than the generation capacity. Consideration could be given here to applying different settings for short term parallel connection. As this generation will not be expected to give grid Support or contribute to P2/6 security, more sensitive settings e.g. 49.5 Hz -6%V would compensate for lack of LOM protection?

Ultimately if an island was established the situation would only persist for the duration of the parallel operation timer setting before generation was tripped.(Auto Reclose excepted)

Generator Star point Earthing
For HV connected generation ETR113 Fig 5.6 shows that for short term parallel the star point of the generator should remain connected to earth.
It is recommended that for LV generation operating in short term parallel the same should apply for the following reasons.

• Having switches in the generator earthing circuits, that for the majority of the time will need to remain closed creates an unnecessary complication / risk of failure of leaving an unearthed system.

• Multiple earths on the system could result in circulating third harmonic current around the neutral earth path. This could result in heating of the cables, however as this is a thermal rating issue with a relatively long time constant the short period of parallel operation is unlikely to result in any serious overheating.

Very Short Term Parallel
Some manufacturers are now installing their standby machines with Fast acting Automatic Transfer Switches. These are devices that only make a parallel connection for a very short period of time, typically 100 - 200mS. Under these conditions installing a conventional G59 relay with an operating time of 500mS is not appropriate when the parallel will normally be broken before it has a chance to operate. There is however the risk that the device will fail to operate correctly. Therefore a backup timer should be installed to operate a conventional CB if the parallel remains on for more than 1 Sec.

Contribution to fault level
For short term infrequent parallel there is the need to consider the effect of the generation contribution to fault level. If any problems are identified, then the process for controlling this risk will need to be agreed with the Network Operator.

Voltage Rise / Step Change
Networks should be designed such that the connection of a generator under normal operating conditions does not produce voltage rise in excess of the statutory limits. In general this should not be an issue with most short term parallel operation as at the time of synchronising with the mains most sites will normally be generating only sufficient output to match the site load. Therefore the power transfer on synchronising should be small, with the generator ramping down to transfer site load to the mains. If the generator tripped at this point it could introduce a larger voltage step change than would normally be acceptable for loss of a long term parallel generator but in this event it could be regarded as an infrequent event and a step change of up to 10% would be acceptable*
(* assuming this is the figure the DNO agree to).

Breaker out of phase switching capability
For a new metering point the metering circuit breakers should have an out of phase switching capability.

For an existing installation that does not require replacement of the metering breaker for any other reason, then for short term parallel the risks of out of phase opening are low. Therefore replacement of the metering circuit breaker should not be necessary. It is assumed however that the generator synchronising breaker would have out of phase capability.

Posted in intertripping, P2/6 security, Multiple earths, parallel operation, Voltage Rise / Step Change, embedded generator, Generator Star point Earthing, fault level, G59 | No Comments »