Size matters

Geoff Halliday, Head of Sales at Power Electrics considers generator selection in the context of the functional building environment of today.

In recent years the loads required to be handled by generating sets have become much more complex. Consequently more care has to be exercised in selecting the generating set to ensure optimum performance. It was quite common and it is still often the case even today that the generator was over sized. Given the current economic situation where budgets are very tight, a more scientific approach needs to be adopted.

Load types

Modern day loads are typically very complex and can include an array of specialist equipment and devices with very specific demands on both mains and generators supplies. Today most building loads will include lighting and IT as well as mechanical loads which include motor loads for air movement plant and lifts.

One of the most demanding environments for the installation of a generator is a hospital where the loads protected are diverse in the extreme and range from lighting, large air handing and filtering plant to the complex non linear loads of intensive care equipment and the various types of scanning equipment.

Diesel generator standards

European generating set manufacturers typically produce their product to EN/ISO 8528 and it is to this standard that I will refer in this article.

The standard offers a number of definitions for generating sets dependant upon the type of application; the most commonly used definitions are Continuous Power, Prime Power and Standby.

Definitions

A Continuous rated generating set would typically be used for base load applications. The stated rating for a Continuous rated set indicates the maximum power which the generating set is capable of delivering continuously whilst supplying a constant electrical load. A typical application would be base load generation or as the source of power in the absence of a grid supply.

A Prime Power rated generator is typically used in lieu of the mains supply and is capable of delivering its full rating plus a 10% overload for one hour in twelve. The generator must feed a varying electrical load who’s demand shall not exceed 70% of its full rating over a given 24 hour period.

A Standby rated generating set is designed as an emergency power source in the event of utility power failure. It is able to supply power into a varying load for the duration of the emergency outage. The average power output shall typically not exceed 70% of standby power rating and the set will typically run for no more than 200 hours per year.

Performance classes

ISO8528 – 5 identifies four classes of performance G1 to G4; G4 being much more onerous than G1. The four performance classes define the permitted tolerances of operation under both steady state and transient conditions for voltage and frequency. When an electrical load is applied to a generator it will have an effect on both the output voltage and frequency, reflecting the engine and alternator reaction to load.

The applied load acts like a break on the engine and instantaneously the engine will slow causing the output frequency to fall. The performance criteria specify the level of permitted frequency excursion and recovery period. When a load is removed from the output the braking effect of the load is reduced and this will result in a momentary increase in speed. This section of the specification dictates the permitted excursion period and recovery time.

The application or removal of a load from the generator will also cause a fluctuation in the output voltage; a reduction in voltage as the load is applied, and an increase in voltage when the load is rejected. As with the frequency the standard clearly defines the maximum permitted change of voltage and the time allowed for recovery.

It is important to know that any generating set can meet the requirements of G1, G2, G3 and G4; the main consideration is the amount of load that can be applied or removed in one step.

Most manufacturers will offer a generating set and provide load step data based on a G2 level of performance. In the UK this typically reflects something close to the performance of the mains supply.

Load step

A common misconception is that when the generator contactor/breaker(s) closes onto the building load, that all of the connected loads are picked up at that moment. In many cases generators are sized on this basis and it is very important to undertake a full load analysis before selecting the most appropriate generator for your application.

The second phase of analysis is to understand the many different types of load present in the system and to consider the variation of methods used for controlling these devices and how they are energised. In certain types of load, and for some motor starting methods such as a star delta changeover, the highest transient loading may not occur at the moment of switch on. It is necessary to consider how the transient varies until the normal operating load point is reached.

A common piece of equipment in frequent use in many commercial organisations today is an Uninterruptible Power Supply (UPS) and in some environments a UPS could represent over 30% of the total protected load. In this case, once the generator connection has been made to the distribution network, the UPS will monitor the supply for up to four seconds in order to determine its stability. Over a further period of several seconds, it gradually ‘walks in’ progressively transferring from battery to generator supply. This typically provides a period of more than ten seconds from the generator coming on line to it managing the full UPS load (see Figure 1).

A further similar example would be when a motor is operated and managed by a variable speed drive. It is also common place for chiller plant and other large mechanical plant to be ‘sequenced in’ either via internal controls, a BMS system or a load management system.

Other considerations

Many of today’s building loads typically include a high proportion of non linear loads such as UPS, inverter drives and high frequency lighting systems. Where such loads are present an assessment of their impact is a necessity. High levels of harmonics present in the load can lead to overheating of alternator windings. An over sized alternator is one option open to help mitigate this problem and permanent magnet excitation fitted to the alternator will help significantly in the reduction of voltage distortion in the system when the load is running on generator.

In conclusion

With a full range of load analysis data in hand those old sizing pitfalls can be avoided.

Specifying a generator with an over-engineered first load step requirement will almost invariably result in an over sized generating set. Not only increasing the capital cost of the generator itself but also adding significantly to other installation costs such as acoustic treatment, larger fuel systems, switchgear and cable installation.

Generators which are over sized against their running load also run less efficiently resulting in burning more fuel. In turn, this makes the generator prone to maintenance and reliability issues linked with light load running (slobber) and will invariably result in both higher emissions and maintenance costs. Many of the top generator manufacturers have software programmes available to help the consultant and contractor with sizing their generator solution.

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