How to select a standby generator

GREGORY DIANA, DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING, UNIVERSITY OF KWAZULU-NATAL AND THE SOUTH AFRICAN INSTITUTE OF AGRICULTURAL ENGINEERS (SAIAE)

This article highlights important practical factors for selecting standby generator plants in the range of 10 - 200 kVA.

The current lack of generation capacity and increasing economic growth is expected to necessitate load shedding in the near future and it would be wise to plan for such an eventuality. It is however important to carefully consider certain important factors.

First things first

The first thing to consider is safety and compliance as all electrical equipment and appliances only guarantee operation within specified tolerances or quality of supply; failing this results in either underperformance, failure or damage to equipment. Hence, standby generation must also guarantee and maintain operation within such equipment tolerances – for example, an approximate 5% variation in voltage and an approximate 1% variation in frequency. Failing this, electric motors might overheat or stall due to under-voltage.

Other not too obvious issues are those of safety and protection, as one does not have the downstream electrical protection that either Eskom or the municipality supplies. Hence, any faults experienced by the standby generator will have to be "cleared" by your own protection; otherwise the system will simply stall, creating your very own load shed or blackout!

To summarize, the following should be noted:

• Magnitude of voltage variation – rated approximately 5% (within 0,5 seconds).
• Magnitude of frequency variation – 50 Hz approximately 1% (within 5 seconds).
• Waveform quality <2% sinusoidal with <2% imbalance for three phase.
• Rather run at a slightly higher than lower voltage.
• Check speed regulator droop from no load to full load.
• What protection is provided, how and who sets it and how reliable is this?
• Note and take care of these issues, because if and when an insurance claim is necessary, these factors will be considered.

Down to business

The sizing and selection are quite straightforward issues and one presumes an existing house, factory, workshop etc. has a distribution board with main circuit breakers with the rated amperage written on them.

For example, if your main circuit breaker for your house or pole fuse is rated at 80 amps, then this should suffice plus an additional 20% of say a 100 amps. If your supply voltage is single phase 220 v, then the rating of the standby generator should be 220*100 = 2,2 kVA. If one has a 400 v three-phase supply, then it should be 1,73*400*100 = 70 kVA.

Several facilities, each with its own main distribution board all operating at the same voltage levels, would for example require one to sum the individual main circuit breaker ratings together, add 10% and multiply by the rated voltage. Therefore, if we have 40 A, 100 A and 80 A, we have 220*1,1 ~ 250 A or 250*220 = 55 kVA.

This only allows one to determine the static rating, whereas a more important consideration is the dynamic rating. The static rating is the rating required once the system is running at constant speed with a constant load. However, what happens if the generator set is suddenly loaded by the starting of a motor or a pump takes additional load? Then one will require a significantly larger amount of power in order to recover and keep the speed, voltage and frequency within the specified tolerances. The dynamic rating may significantly increase the overall rating, depending on the largest load that one expects to either come onto or drop off the system.

You might therefore have a static rating of 44 kVA, but due to a load of 20 kVA being imposed on the system and being removed at a regular interval, the dynamic rating might have to be doubled to 88 kVA to account for this and allow the engine to recover quickly enough and remain within the specified tolerances. In some instances some units employ a smaller unit equipped with a flywheel which serves to utilise the kinetic energy stored in the flywheel to do the same. However, in such cases the units will take some time to get up to speed from a standing start – a case of "horses for courses".

It is not generally known that any standby unit is unable to suddenly take on full load. After a loss of supply, the unit will either be manually started or start automatically. Once it has run up to speed and has settled down, providing the correct voltage and frequency, it is ready to be loaded. Generally most standard units may be loaded in incremental steps of ~25% full load.

In such a case one might have to design a digital controller to bring load on line in an incremental fashion or else run the risk of stalling the engine. In many instances one thinks of the system being "self-starting", but very few consider what happens if the systems stalls! In most cases, people then have to go outside and manually restart each system and to overcome this, one should also consider auto-reclosing.

Many remote farms already have this, where the system trips due to lightning or some other fault and the system is automatically reclosed trying to re-establish the supply, stopping after three or four unsuccessful attempts. Eskom uses this to prevent having to send technicians considerable distances just to reset a switch! In farms or remote areas this might be an important safety consideration.

Another critical issue is phase sequence. Three phase systems have three phases and a neutral commonly labelled as A, B, C, N or U, V, W, N or R, S, T, N, depending upon country of origin. The phase sequence in which the motor is connected determines the direction of rotation. Hence, one may be connected to the Eskom supply whose phase sequence is ABC and upon load shedding, the generator provides ACB, which will cause all motors to run "backwards" and have severe consequences.

One must ensure during commissioning that the phase sequence is correctly determined to avoid any mishap. It is normal that during load shedding the standby units "kick in" within 3 - 5 sec, in which case large motors might still be running down and if suddenly supplied with reverse phase sequence, large currents may be drawn, causing the system to stall and even damaging the shaft of the motor if connected to a pump, fan or non-reversible mechanical load.

It must also be noted that it will cost some 3 - 4 times more to operate compared with Eskom or the municipality and that if you wish to store more than 1 000 litres it requires an environmental impact assessment. As this applies to systems of any size, these factors should be used as a selection guide to ensure the solution being proposed is workable. All too often one rushes out and buys a unit only to realise it does not work properly.

To summarise, the following should be noted:

• Do you need to run your entire operation?
• Determine essential items to reduce running costs.
• Can a unit comply with previous points?
• Determine static rating in kVA.
• Determine dynamic rating kVA (larger unit or flywheel).
• How much fuel is required and how and where may it be stored safely?
• Manual or auto starting with auto reclosing for remote units?
• Single unit or multiple units?
• What protection is provided and who does the protection settings?
• Check speed regulator droop from no load to full load.
• What protection is provided, how and who sets it and how reliable is this?
• Ensure correct phase sequence with normal and standby units.
• Can engine and generator be repaired locally?
• Does service provider provide this service or third party?
• Availability of spares?
• What is turnaround time for service?
• Where is the unit manufactured?
• Who assumes legal responsibility if a person is injured or killed?
• Are you and/or your staff competent to operate a standby unit?

Conclusion

Hopefully this article highlighted some of the not so obvious issues related to standby generators. The issues of compliance, safety, professional accountability and operational competence are key, if and when things go wrong. When opting for a standby generator, one not only buys a solution, but also assumes the total responsibility and accountability that are normally assumed by Eskom or the local municipality. Unfortunately there is no quick fix!

This article was made possible through the South African Institute of Agricultural Engineers.