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Advanced - Power Management & G59 Principles

Advanced - power management - national grid, generator - mains failure

Common Generator Related Questions

Why do some sites or buildings have more than one generator running in parallel?

The most common reason is redundancy; if one generator fails the other is capable of supporting site load. This is described as N+N redundancy where N=site nominal load and the number following the ‘+’ sign is the level of redundancy, in this case also nominal. Where multiple units are required due to other factors, X number of units could be required to cope with nominal load.

Some redundancy could be afforded by an additional unit to cater for a single unit failure. For example, 5 x 400kVA units would be required to supply a theoretical 2MVA load, an additional 400kVA unit would give immunity to a single unit failure (effectively a spare unit), described as N+1 redundancy.

Space factors and noise also dictate to a certain degree if multiple sets are to be used in lieu of one single large generating set but often load profile is overlooked. It is highly undesirable that a diesel generator should run at below 30% of its maximum capability as engine degradation becomes evident; similarly, overloading is to be avoided.

Generally a 70% duty average over any 24hr period, with a maximum single load step of 60%, is recommended and accepted as being the norm, giving spare power headroom for motor starts and system inrush whilst maintaining demand well above the 30% low power damage threshold. If the site is such that power demand falls to little or negligible load during the night or at weekends, a long term outage could lightly load the generator for a similarly long period.

A site with the intention of joining the STOR scheme may match embedded generation capability to the maximum allowable site export capability (usually the substation rating) to maximise STOR revenue. The site may only have a site demand of 25-30% of substation capacity due to expansion allowance resulting in an oversized generator. Multiple generators with lower capacity, which are intelligently brought online to match demand in these scenarios, will better maintain gen-set utilisation.

What is STOR?

Short Term Operating Reserve is a strategy whereby a site, which has embedded generating equipment of 3MW or above, can offer to provide additional active power to the National Grid distribution infrastructure to reduce demand or even contribute to the power available on the Grid. This is typically required at certain times of the year and would be required to provide power within four hours of instruction by “the Grid”. To qualify for STOR status, the power would also have to be maintained for at least a two hour period. A generator-to-mains synchronising (no break transfer) system is required if site disruption or energy exportation is required.

What is “Island Mode”?

Any site and electrical load system can be considered “islanded” when no mains utility supply is being used to power the site electrical load, just as if the site was on an island. The generators supply all customer loads and neither the load nor the generators are connected to the utility supply.

What is "Embedded Generation"?

Embedded generation (also referred to as “distributed generation”) enables the user to generate and use their own electricity via renewable sources, green energy and other on-site power generation equipment. If you do produce surplus electricity, this can be sold back to the local provider and then distributed back into the National Grid.

Whether utilising embedded generation for self-sufficiency to lower bills, or as a method of providing power in a mains failure, the Engineering Recommendation G59/2 provides guidance and technical requirements for allowing producers of surplus electricity to make this power available to other Grid users.

What is G59, G59-1 or G59-2?

The proper title is ER G59 or Engineering Recommendation G59. G59 is a generic term, G59-1 is the old recommendation and G59-2 is the current requirement. The document refers to the recommendations and guidelines for connecting embedded generation to utility supplies for long and short term duration. Power distribution was traditionally one way, from the power station source at high voltage and distributed in lower voltages to end users.

Government policy, environmentally friendly power sources and advantages to the national grid balancing mechanism, led the way to it being desirable to allow small embedded generation sources to be able to feed and contribute power into the grid infrastructure, an infrastructure almost exclusively controlled upstream by the national grid.

If an unpredictable and non-controlled quantity of wind turbines and embedded generation were to be allowed to feed into the grid, some regulated basic protocols would be needed.

ER G59 attempts to specify basic functionality requirements to protect both the grid and the embedded power generation.

One of the recommendations within the requirement is for mains failure detection whilst the generator system is in parallel with the mains.

A traditional phase failure relay, sensing voltage only, may not detect a mains failure due to the generator operating in parallel and supplying power at the detection point.

Many manufacturers have specific G59 detection relays that will supervise the system when in parallel, constantly monitoring the mains/generator common voltage and generator output current to detect rapid changes in characteristics which would identify that the mains is no longer present and a mains failure has occurred.

A common misconception is that the G59 relay protects workers upstream if they isolate at MV/HV and commence to work on the network, assuming several downstream generators will keep the equipment live and in a dangerous condition.

Firstly, any planned outage, where possible, would be re-fed via an alternative path on the grid to minimise outage and as a minimum you will be informed if this is not possible. Secondly, a robust, safe working practice will test and ensure the bus is dead before commencement of work and thirdly they will earth the power rail at the point on the network on which they are working.

Predominantly the G59 relay protection guards against unplanned system failure for both parties (grid and generator owner).

The mains disconnection has to occur quickly for two reasons:

1. If the dead utility network is not disconnected quickly, the site embedded generation will attempt to supply the local surrounding network and fail on over current, making the point of having the generators useless.

2. Depending on where geographically the site is on the distribution network, an attempt to bypass the network fault will be made or auto re-closers on the network will attempt to reconnect power a number of times to see if the fault was temporary (a falling tree for example).

Either way, power would reappear on the utility mains causing a crash synchronisation if embedded generation was still attached.

Click the PDF image to view the technical information in full


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