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In sync: Connecting generating sets to the grid

Engineers often just assume they need standby generation, but if they want to generate revenue by selling power to the grid then it is worth investing a little more in a prime mover generator. David Hatherill, engineering manager for Finning Power Systems, outlines the process of generator synchronization.

David Hatherill, Finning Power Systems, UK

If you have watched enough films, you will inevitably have seen a few chase sequences and stunts in which someone jumps from one moving vehicle to another.

In many ways this is similar to the process of generator synchronization. Fortunately, though, we have the assistance of accurate instrumentation and control equipment, which means that the process is rather less risky, and in the majority of cases fully automatic.

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However, we cannot see whether two electrical circuits are in phase or at the same frequency, so it has always been necessary to have some form of visual aid. Initially these took the form of two (or in some cases three) synchronizing lights à‚— the set being in phase and at the correct voltage when both lights were extinguished.

More recently, electronic synchroscopes have been used, which give a much better visual interpretation of what is going on. These evolved into having a combined check-sync relay, and finally into the modern auto synchronizer used today, which has some limited indication, but has no opportunity for manual intervention.

The fact that the process has been automated to the point of the system “pressing the button” for you, does not mean that the fundamentals of what it is doing does not matter anymore. The design of much of the rest of the system is much the same, so we’ll now look at some of the other design aspects of synchronizing and connecting to the grid.

The secret of sync success

To synchronise successfully we first need some form of common reference point, and generally this is the neutral on a low voltage (LV) system, which is why we normally want a three pole synchronizing circuit breaker. We then need to match the voltage to the supply by adjusting the regulator settings, and make sure the frequency is the same and in phase which we do by adjusting the generator speed.

Finning LIMA control system engine overview screen
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Assuming that these are all correct we can close the synchronizing breaker. Once the synchronizing breaker is closed we must then increase the engine fuelling to export electricity.

One of the first parts of designing any system is to determine the functionality that we desire, and in this case that is whether we want to synchronise generators to each other, to the grid, or both.

If we want to connect in parallel to the grid, then the regional electricity company (REC) should be an early port of call to check that they will be happy to connect you. This is important as they may have limitations caused by infrastructure which will limit how much can be connected or impose other design limits. A good example of this may be fault level, which even if you do not export, still has an impact on their network if parallel running is required.

Caterpillar 3516 1150 kWe biogas generator set
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In the UK, a document called Engineering Recommendation G59/1 covers the requirements for connection and specifies what protection devices should be fitted to enable parallel running. For larger installations, above 5 MW, you need to apply the requirements from the document Engineering Recommendation G75/1.

For a generator that synchronises at LV these protections will include: under and over voltage, under and over frequency and loss of mains. The latter incorporates rate of change of frequency (ROCOF) and vector shift of the voltage vector.

If the connection is to be at high voltage (HV) further protections are required. These may include: neutral voltage displacement, earth fault, over current, and reverse power, although these are not “prescribed” and are to some extent down to the discretion of the local REC’s engineer.

Regulation Regulation Regulation

Strictly speaking G59/1 applies according to the connection voltage to the grid of the site, but the spirit of the regulation and the general implementation is that it is the synchronization voltage that is considered.

A further document, Engineering Technical Report (ETR) 113 was issued to “clarify” some of the requirements of G59/1, but it should be noted that some of these clarifications actually impose additional requirements. Also it does not take into account the strength or otherwise of the local network, so the document is not a clarification in the usual sense.

In the longer term there is intent to combine G59/1 and G75/1, which will make ETR 113 redundant, but this has not reached fruition at the present time.

In many ways paralleling to the grid is the easy option because the grid does not interact: you simply have to fit in with what it is doing. The synchronised generator also cannot see load, even on its own part of the system, so can run in a very controlled situation with no transients.

The fact that the generator cannot see load in this situation probably requires a little more explanation as it is a concept that is not at first obvious. Quite simply the generator set load is controlled by the engine governor; the governor looks at throttle input and compares this to the desired setting.

If the engine speed slows it increases fuelling, and if the engine speed rises it decreases the fuelling. When the generator set is running paralleled to the grid the speed does not vary as the frequency does not vary, so the governor does not sense the load. Further to this, since the generator cannot see load, it also cannot see other generators, so no particular instrumentation is required to coordinate the activities of other units, even if they are on the same leg of the network.

A common myth dispelled

There is also a common misconception that load can be controlled when running in parallel by changing the excitation of the generator, and that if the generator output voltage is raised the generator will carry more load. This is not so.

If the generator voltage is increased beyond the bus voltage this causes the voltage vector to shift which alters the power factor, and all this achieves is to make the power factor worse. In fact tight control of excitation is how generators control their own power factor. They cannot correct load power factor.

Finning LIMA control distribution management system screen at Norfolk & Norwich Hospital, UK
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When running in island mode with multiple generators the scenario is much more complicated. Firstly generators do see load, and they also see each other, so some form of load share governing system is required. This essentially allows the sets to communicate so that as load is applied or removed from the system they all react together in a coordinated way.

This is particularly important if dissimilar generators are connected together. The reason that the generators see load in this situation, and each other, is that as load is applied to the system there is no infinite bus to buffer these changes. As such the load on, and therefore the speed of, the generator increases or decreases, and this is reacted to by the engine governor.

All of the above are the very bare essentials of what you need to connect a generator to the grid. Today’s customers expect so much more than just a generator which can be started and synchronised to the grid. Modern schemes such as hospitals, banks, data centres, sports stadiums, air traffic control centres and supermarket wharehouses all require systems that can not only start and pick up load, but also reconfigure the site distribution system, carry out building management activities, switch high voltage switchgear and the list goes on.

When it comes to generator synchronization it is best to approach a supplier that has the relevant experience and can demonstrate packaged solutions that are reliable and easy to use. After all it is a legal requirement to show compliance with the regulations when synchronizing generator sets to the mains and there are always the financial benefits of peak shaving. It is likely that such benefits will only increase in the future with the continued strain placed on the utility grid.