ELECTRICAL SYSTEMS
Managing complex loads on standby generators
WB Power Services Business consultant, Geoff Halliday, considers the issue of transferring complex loads onto a standby diesel generator. Today, standby generating sets are seeing an increase in the scope and variation of loads being connected to such plant.
Historically, during the early decades of power generation, standby generating sets were mostly used for applications such as emergency lighting, providing power to key military installations and other similar applications. As the capabilities of the diesel-powered standby generating sets increased, so also did the type of loads supported by standby power. In the 1970s and 1980s, the industry saw dramatic growth in motor and pump-type applications such as water treatment works, process control, and lifts, etc. These types of loads placed additional requirements on the coordination of power transfer from utility and back to utility, and vice-versa. Simple double-throw contactor arrangements were no longer sufficiently reliable to transfer loads with high levels of electromechanical inertia. It became necessary to undertake a full assessment to understand what happened when transferring from one source to another, particularly when transferring from a standby source to a utility source.
Scope of modern-day connected loads on the standby generating set Once again, standby generating sets are seeing an increase in the scope and variation of loads currently being connected to standby power plant. The massive growth in use of switched electronic conversion techniques in things like battery charging systems, switch mode power supplies, and other such systems used in PCs and IT servers, has led to a massive increase in the level of capacitive loads being connected. The growing use of downstream transformers has become more prevalent, as facilities are becoming more complex in their design, and as the data service industry (data centres) continues to grow exponentially. The problems associated with both leading power factors and downstream transformer loads are well understood, such that the specifying engineer can analyse the application and correctly coordinate equipment. The ‘traditional’ electrical network of a commercial building or facility would largely comprise resistive (lighting, heating) or inductive loads (pumps, motors). In the last decade or so this has changed significantly with the introduction of switched electronic devices – for example LED lighting, motor soft starters, variable speed drives, desktop PCs, file servers, and UPS etc. These switched electronic devices predominantly operate at close to unity power factor, but on ‘switch on’ can present a largely capacitive/leading power factor load which, when operating on a standby generating set, brings some unique problems.
Application of a classical resistive/ inductive load When applying a classical resistive/inductive load to a standby generating set, it would respond with a dip in both output voltage and frequency (dip in engine speed). Where the rate of frequency change is beyond preset limits, the modern electronic voltage regulator helps compensate by reducing the output voltage, effectively reducing the active power being delivered by the set (sometimes called a load acceptance or V/Hz feature). When a leading power factor/capacitive load step is applied to the generator, it will respond by increasing the output voltage, while at the same time the engine speed will reduce (reduction in frequency). The alternator on the generator set is changing from an under-excited state to an overexcited state, while the engine is seeing an increase in active power, and responding by reducing speed (reduction in output frequency). In this scenario, the voltage
A common hospital generator power arrangement.
Two generating sets sit raised on fuel tanks, in standby mode.
April 2025 Health Estate Journal 55
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