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How to Size A Generator for A UPS System

Generators provide a source of backup power and can help to keep your server room or datacentre applications running during a mains power failure. The most commonly used generating set for this type of application is a diesel-powered generator with an alternative being a liquid petroleum gas or gas-powered generator.

How Generators Interface with UPS Systems

Generators by themselves cannot protect a server room or datacentre from mains power problems. The issue is that the generating set is normally in the ‘off’ or powered down state and requires a period of running when the mains power supply fails to come up to full speed. The start-up period is typically covered by an uninterruptible power system and its battery set.

The two systems are connected by an automatic mains failure (AMF) panel. This is installed upstream of the UPS system. When the AMF panel senses a mains power failure, a signal contact initiates the generator to start. Once the generator is up to full cycle speed and capable of supplying power, the UPS system input synchronises to the output supply to the generator. When this is achieved the UPS system draws on the supply from the generator and its batteries go into recharge.

When the mains power supply returns, the generating set will continue to operate for several cycles to ensure the mains power supply is stable. The UPS system then synchronises back to the mains power supply and the generator is issued a signal contact from the AMF panel to power off.

Sizing UPS Generators

When sizing UPS systems and generators it is important to classify the loads to be protected into critical, essential and non-essential loads. Critical loads are generally IT servers and networks. Essential loads include air conditioning, lighting and security systems whose operation must be maintained during a mains power supply failure. Non-essential services may include printing which may not be required during a power outage.

When sizing the generator system the load normally consists of the:

Critical loads + Essential Loads

The critical load is generally supported by the UPS system and so the sizing becomes:

UPS system + Essential Loads

For each of the essential loads to be supported it is important to note their start-up and running currents and AC voltage and phase requirements. These may be available on system rating plates, on datasheets or within installation and operation manuals or manufacturer’s website pages. Where they are not available the most accurate way to size a generator is through a site electrical survey performed by a qualified electrician or electrical contractor.

Some loads may only be sized in Amps. These figures can be converted into Watts as follows:

  • For resistive load: Wattage = amperes x volts
  • For reactive load: Wattage = (amperes x volts) x load factor

It is important with the UPS system to consider the full load applied to the generator by the uninterruptible power supply. UPS systems are not 100% efficient and so a 100kVA UPS at full load will draw more than 100kVA in Apparent Power. In addition the battery may require charging adding more load to the generating set by the UPS rectifier circuit. The input voltage waveform and harmonics may also have to be considered to ensure compatibility between the UPS system and its generating set.

As a rule of thumb most generators are sized at 1.25 to 2 times the size of the UPS system if the generating set has a modern electronic governor and the UPS is a high efficiency system with a high input power factor greater than 0.9pF. Where this is not the case a factor up to 3 times may be required but this is rare.

Single and Three Phase Load Balancing

When listing the loads it is important to note whether they require single or three phase loads. A three-phase generator can support single phase loads but the individual loading per phase must be balanced across the three phases. This means that you should have 33% of the total load for each phase of a three-phase output generating set.

Prime or Standby Power Modes

A further consideration is the mode of operation of the generating set. Most generators can operate in prime or standby power mode. In a UPS server room or datacentre application, the generating set will typically operate in a standby power mode as a supplementary power source to the UPS system.

A Prime mode of operation is a continuous operation. This mode may be chosen where the mains supply is continuously unstable or not present. An example would be a remote application.

Generator Power Outputs

Generators sizes may be shown as in kVA (Apparent Power) or kW (Real Power). Care should be taken when sizing generators in kVA as the output Power Factor (pF) may not be equal to 1 (Unity). This means that a generator of 100kVA at 0.8pF will deliver 80kW of power.

Tier Rating and Redundancy

A final point to note is the level of redundancy required within the generator-UPS system combination. This is defined by the Uptime Institute in their Tier-rating for server rooms and datacentres.

Most organisations install a single generating set referred to as N. For the higher Tier ratings redundancy is required for maintainability during maintenance or set downtime. Parallel generating sets can be installed to achieve N+X redundant configurations where X is the number of additional generating sets.

The Advantages Sizing a UPS Generating Set

Calculating the right size for a UPS generator combination has several advantages.

  • Ensure continuous operation during a power outage
  • Ensure synchronisation between the UPS system and generating set
  • Prevent overload conditions on both the generator and UPS system
  • Increased working life for the generating set and UPS system
  • Regular planned preventative maintenance routines without downtime
  • Overall the primary benefit of correctly sizing a UPS generating set is a secure power protection plan and one that can give years of valuable backup service.

Please contact the projects team at Server Room Environments if you need assistance sizing or planning a generator installation for your uninterruptible power supply. We supply a wide range of standby generators and provide preventative maintenance visits and generator maintenance contracts in addition to decommissioning and trade-in services.