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Using UPS with Generators

Published by Jason Koffler on 28 January 2019

A UPS-generator set protects computer-stored data as well. While power surges can overload a system, power outages can damage equipment. The following information can keep these events from happening.

The Meaning of UPS

UPS represents an acronym that stands for Uninterrupted Power Supply. Both UPS systems and generators provide back-up power when power outages occur. However, that is where the similarities end.

Generators and UPS Systems Work Together to Provide Backup

Generators provide temporary or standby power and can keep you running indefinitely during a power outage. UPS systems maintain operations for only a limited time. The concept behind a UPS is to keep a system running until a switch can be made to generator-supplied power, or a proper shutdown can be performed. Generators and UPS systems, therefore, work in concert until your prime power is restored.

Generators come in various sizes and operation time configurations. The size you choose depends on your operations, the size of your plant or building, and the amount of equipment used. To ensure the compatibility of your UPS system and generator, you need to calculate the generator size first.

Calculating a Generator’s Size

Proper sizing of a generator is not as clear-cut and simple as you might think. When sizing generators, make sure the generator size exceeds the UPS power rating. Generators must support the power rating, including an excess of 20% to 30% to cover power line conditioning.

Brands, such as SDMO, offer power line conditioning systems that maintain productivity and uptime while correcting voltage sags using automatic voltage regulation. Power conditioning circuits smooths out voltage fluctuations as the generator warns up and or due to the load characteristics.

Sizing a Diesel Generator and UPS

Maybe you need to size a diesel generating set for a UPS. SDMMO and FG Wilson are a good example of brands which work very well with our Eaton, Riello, APC or GE UPS. Sizing a diesel generating set for a UPS can be further calculated as follows:

KVA gen = KVA UPS + KVA battery charge divided by 0.8 x ∩ UPS

Where: ∩ is the efficiency of the UPS 0.8 is the deration factor

UPS manufacturers lay down frequency parameters that require diesel generators to feature an electronic governor. This part is sometimes referred to as a speed controller. While older generator sets employ mechanical or electrical controls, electronic controls power newer generator sets. By fitting a diesel generator with an electronic governor, frequency is maintained within minus or plus 1%.

Developing a Sizing Strategy

To develop a generator sizing strategy, you need to do the following:

- List all the electrical equipment
- Ask specific questions about each piece of equipment, such as – What would be the effect on your reputation and profits if the equipment failed? How would it affect employee safety? Would it impact the smooth running of operations?
- Keep in mind that not all your electrical equipment needs UPS support. This type of equipment can remain non-operational until the power is restored.
- Ask if a power outage would cause a mild upset or if you could be exposed to extreme financial loss or ruin? How much of an inconvenience would be posed if your organisation could not access the Internet, correspond by email, or access business applications during the day?

You will need to prioritise your choice of a UPS, as it serves as the basis for establishing critical power supplies. For example, some machines in a plant, such cooling plant devices, should be included on your power needs list. This equipment keeps the servers from shutting down as the result of higher temperatures. Non-essential loads can be protected by the generator that accept a break in the mains while the generator starts.

Well-known UPS Brand Solutions

While brands, such as Riello, Eaton, Trippltie and APC provide a great range of UPS’s solutions which include features to inhibit the UPS charger while on generator.

Account for the Surge

Also, review your health and safety needs, such as lifts and emergency lighting. Once you have listed and identified the electrical equipment in your facility, you need to consider the surge current or electrical inrush. This phenomenon takes place when an electrical device starts up and the start-up power usage greatly surpasses a regular load. Therefore, the generator you choose should work with a UPS that regulates frequencies. As the power load changes, so do the generator’s frequencies.

A Double-conversion Online System

If the UPS senses that the frequency modulations are out of tolerance, it instantly reverts to battery generation. This event can happen with both line interactive and off-line UPS systems. If you choose an on-line UPS, featuring frequency regulation, you can completely avoid the issue. A double conversion online UPS system usually operates at 90% efficiency on full loads.

However, you can still have problems with harmonics – distortions that can reduce the working life of a generator’s components. Harmonics also reduce the power output. Although this type of unwanted side effect can be reduced by the use of filters, a larger-sized generator will reduce the effect overall.

When a generator initially starts, it needs to synchronise with UPS-supplied current. If the generator works close to capacity, the chance exists that the generator will drop a load. Since you do not want this to happen, you need to increase the generator size.

Avoid a Problem with Overheating

Temperature also plays a major role in backup. For example, when a generator runs, a rise of 10 degrees Celsius is typical. If the ambient temperature is around 40 degrees Celsius, the added heat can damage the generator and many generators will need to be derated to operate above this. Larger generators with sufficient electrical capacity typically operate at higher temperatures by oversizing them. Therefore, heat becomes less problematic.

When choosing your UPS and generator size, you also need to provision and scale your generator for more power. Oversize the generator between 1.25 and two times the UPS output when the generator only supports the UPS. Increase this amount to the required rating when the generator needs to support extra electrical equipment, such as air conditioning, emergency lighting, or lifts.
Bottom Line

When deciding on UPS systems and generators, you need to identify the various levels of power protection needed for different pieces of equipment as well as calculate the power loading for the generator you select. Generators should be chosen so they meet the requirements for surge current, UPS power, harmonics, operational temperature, synchronisation, and future electrical loads. Typically, the generator size should not surpass the UPS power rating by around 1.25 to 3.00 overall.