In this article, Alan Luscombe, director at Uninterruptible Power Supplies Ltd., a Kohler company, discusses the factors to be considered when adding a generator to a UPS system. These range from designing in electrical compatibility to addressing environmental and physical constraints.
Most businesses of any size now rely on clean, continuous power from an uninterruptible power supply (UPS) to prevent loss of service, data loss and damage to sensitive IT and communications equipment. A correctly-specified UPS from a reputable supplier will provide this protection during short-term power disturbances and blackouts, but is not designed to deliver backup power indefinitely. If a business’s IT must remain online rather than shutting down gracefully during an extended blackout, a generator will be needed to complement the UPS.
While UPSs today provide excellent protection from utility mains transients and short-term blackouts, they’re not a complete solution for applications that must remain online 24/7, without interruption. A power outage that exceeds the battery autonomy is always possible, yet a system shutdown, even if managed gracefully, is not acceptable. Additionally, running the application from the UPS battery is pointless if there is no power for the air conditioning system that cools it.
The solution is to complement the UPS with a generator that starts up if the battery autonomy is threatened, then runs indefinitely if necessary. The UPS bridges the power gap between the mains failing, and the generator starting up and reaching synchronisation. It also cleans all power, whether from the mains or the generator.
A number of factors apply when planning a generator installation; these relate to maintenance and fuelling, compatibility with the UPS and power system, correct generator sizing, and physical integration into the target environment. All must be taken into account to ensure a generator/UPS pair that truly guarantees uninterrupted availability, even during extended power outages.
Generator characteristics and care
A generator comprises a fuel supply, an engine and an alternator. The fuel can be gas or diesel, although gas-powered generators are used mainly for combined heat and power (CHP) applications. Diesel, used for both base load and standby applications, is the usual complement to UPS systems. Generator engines resemble those found in large trucks, and have the same maintenance requirements. An adequate fuel supply should always be available, together with a healthy battery for reliable starting. Regular maintenance is essential, with frequent checks on oil and coolant levels.
The alternator converts the engine’s mechanical power into an AC electrical supply, usually at either 230 V (single-phase) or 400 V (three-phase) levels; these levels depend on how the alternator is wound. Output voltage amplitude and stability are controlled by an Automatic Voltage Regulator (AVR), while its frequency – usually 50 Hz in the UK – is set by the engine speed. The speed/frequency relationship usually depends on the alternator design; typically an alternator speed of 1500 rpm equates to a 50 Hz output frequency. Engine speed and output frequency are controlled by a governor which regulates the amount of fuel reaching the engine. More fuel means higher engine speed and electrical output frequency.
Simple, mechanical governors based on springs and spinning weights are available as a low-cost way of fuel supply regulation, however they lack the fast response and frequency regulation stability essential for use with UPSs. Instead, generators intended for use with UPS systems almost always rely on electronic governors, which operate by counting the teeth on the alternator flywheel and regulating the fuel accordingly. They are highly responsive and offer very stable engine speed regulation.
Compatibility with UPSs
Achieving tight engine speed control is critical in these applications, as it allows synchronisation between the UPS and the generator. Synchronisation problems between the UPS and the utility mains supply are rare, because utility companies’ generators are huge, so their inertia can absorb any sudden increase in load. By contrast a small local generator subjected to the same load change will slow down until the governor compensates.
Overall, the generator’s frequency range may be too wide for the UPS to accept. In the worst case, synchronisation may not be possible, either because the frequency is out of limits or because its rate of change (slew rate) is too fast for the UPS to follow without endangering the load. If this happens, most UPSs will flag an alarm to warn the operator that if a fault occurs the load will not be transferred from the UPS to the raw generator feed.
These problems can be prevented by ensuring that the generator manufacturer is aware of their product’s intended use with UPSs, and fully testing it during commissioning. As mentioned, the use of electronic governors will minimise or completely eliminate potential problems.
Generators also need to be prepared in other ways for their standby power source role. They must always be ready to start on demand, as mains power failures are unpredictable. Accordingly they must always be kept warm, have a fully-charged battery and sufficient fuel. They must be aware of mains failure and restoration status so that they can start and stop as appropriate. This status information is usually supplied by signals from an Automatic Mains Failure (AMF) panel. This shows arrangements not only for detection, but also for switching to the generator and back during mains failure and restoration.
Nuisance starts every time the mains is disturbed must be avoided, so the AMF signal is usually set to operate only after an extended mains failure time; delays of 2 s to 10 s are typical. Stopping the generator and switching the UPS back to mains as soon as supply is restored must equally be avoided, as the underlying fault may not have been cleared. Accordingly, most AMF controlled generators continue running for at least two minutes after detecting that mains has been restored.
If a mains failure is detected, there is an inevitable time delay before the generator can be started up and synchronised. The UPS covers this by powering the critical load from its battery, then switching to generator power when available. It also uses generator power to recharge the depleted battery. Similarly, the UPS uses battery power to maintain uninterrupted supply during transfer back to mains, then uses the mains power to recharge its batteries again.
Planners should also consider how to switch the neutral conductor during mains changeovers. Four-pole switching is normal in the UK, as short interruptions in the neutral plane are accepted, due to today’s proliferation of on-site generators. The solution finally chosen depends on the requirements of both the UPS and other connected loads, and on local electricity supply regulations.