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Standby Generator Basics

Standby Generator BasicsStandby Generator Basics: How to choose a generator for a data center

Standby generators are a critical element to any high availability power system.  Data centers, network closets, hospitals, campuses, manufacturing floors – all of these loads require maximum uptime.  Uninterruptable Power Supply (UPS) systems provide battery power for a few minutes, but on-premise power generation is required for long-duration outages.  In most cases, that power source is a standby generator.

Three Most Common Generator Types

The first step in selecting a generator is to understand which type of generator you need.  The three most common types of generators are Standby, Prime, and Continuous.

Standby power generators are most commonly used in emergency power applications when another continuous power source, such as the utility, is the primary source of power.  As the name suggests, the generator is on standby ready to take over in the event that the utility or other primary power source is no longer available.  Standby generators are recommended for use only during a power outage or for required maintenance and testing.  Standby generators are the most common generators for Tier I and Tier II data centers.

Prime generators are considered to have an unlimited run time and can be used in both standby applications or as the primary source of power.  Prime generators usually include an overload capacity of around 10% for short durations and are durable enough to provide maximum power to a variable load for an unlimited number of hours.  Tier III and Tier IV data centers require prime generators.

Continuous power generators are similar to prime, but generally supply power to a constant load.  Continuous generators are not rated for overload conditions or variable loads.  Because of these limitations, continuous power generators are rarely used in data center or critical applications where utility power is available.

The Standby Generator System

The generator system, often referred to as the genset, includes the standby generator, the automatic transfer switch (ATS), and the output power distribution.  The ATS is fed by both the utility and the generator with the utility as the preferred source.  When the ATS detects a problem with the utility power source, it automatically commands the generator to start and switches the power feed to generator.

Because the generator requires anywhere from a few seconds to several minutes to supply consistent power to the load, an uninterruptable power supply (UPS) protects the load with battery back up from the time utility power is lost until the generator can supply clean and consistent power.

How a Standby Generator System Works

When the UPS detects an interruption in power, within milliseconds, it provides immediate battery back-up to the load.  So, as far as the protected equipment is concerned, there is no power interruption and existing processes and computation are not interrupted.  Meanwhile, the ATS has detected the utility outage, has commanded the generator to start and has switched the power source to generator.  Once the UPS recognizes the generator as a stable source, it starts to transfer the load off battery and onto the generator.  This is done in sequence and carefully managed by both the switchgear and UPS firmware, so the load is not simply dumped onto the generator risking overload.  When the utility power is restored, the ATS and switchgear transfer the load back to utility and the generator is commanded to power down.

Three Most Common Standby Generator Systems

In most cases, the standby generator system will fall into one of three categories – Emergency Standby Generator System, Multiple Isolated Standby Generators, Multiple Generators Operating in Parallel with Utility System.

Emergency Standby Generator System

The emergency standby generator system is the simplest type of genset.  There is a single utility source, a single generator source, and one ATS.  In this case, the load is either supplied from the utility or the generator.  The generator and the utility are never connected together.  This type of generator system improves reliability but does not provide any redundancy in the event the generator doesn’t start properly.  This basic emergency standby generator system may or may not include a UPS and is most common in smaller, less critical applications.

Multiple Isolated Standby Generators

In critical applications when redundancy is required, the multiple isolated standby generator system is more common.  In this configuration, multiple generators are connected to a paralleling bus feeding multiple transfer switches.  This configuration is often used when N+1 or 2N redundancy is required.  Multiple generator systems have a more complex control mechanism as the units need to share the load under normal operating conditions and be synchronized and paralleled together.  This is typically done through programmable logic controllers (PLCs) in the switchgear.  Because of the high criticality of the IT load, multiple isolated generator systems almost always include at least one UPS.

Multiple Generators Operating in Parallel with Utility System

While the emergency standby generator system and multiple isolated standby generator systems both require that power be supplied by either the utility or the generators through operation of one or more ATSs, when multiple generators are operating in parallel with the utility system, the generators are in parallel with the utility.  This is most common in applications where load shedding or peak shaving is required.  The electric utility will likely have strict interconnection requirements for paralleling with the utility and the system requires a complete and complex protection and control scheme.

Standby Generator Basics: How to Choose a Generator for a Data Center

While you will need an electrical engineer to assist you in determining the exact specifications for your generator, ATS and/or switchgear, in most data center applications, you will find either standby or prime diesel generators in either single or multiple isolated systems.  In other words, not in parallel with the utility.  While gasoline, natural gas, liquid propane, steam and other fuels are available, diesel generators remain the top choice because of the quick start-up and power of the internal combustion engine.  Additionally, diesel remains one of the most readily available and economic fuel types.

Factors to consider when choosing a generator – How to size a generator

In most commercial applications, the general rule of thumb is to size the generator at 2x the UPS load while factoring in the PUE.  The rationale is that the generator capacity beyond what is required for the load connected to the UPS will provide enough power for the mechanicals, cooling, and other critical building systems.

More specifically, start by calculating the target power usage effectiveness (PUE) of the data center.  This is the ratio of power delivered to the facility to power delivered to the IT load.  A PUE of 1.2 is considered very efficient.  Let’s say the site has a 500kVA UPS with a PUE of 1.2.  If you take the UPS load and multiple times 2, you get 1000kVA.  Multiply that by the PUE of 1.2, you have 1200kVA.  From there, you would round up to the next nominal generator capacity which may vary by manufacturer, but in this case, would likely be 1250kVA.

Factors to consider when choosing a generator – Generator System

When deciding whether to use a generator system with a single generator, or one with multiple generators, the biggest question to ask is whether or not you need redundancy.  While generally this is a question left to the data center operator and senior management, some facilities such as hospitals or college campuses have regulatory rules in place than mandate a certain level of redundancy.

If a single generator system is adequate, it is worth considering sizing the battery capacity of the UPS for some additional run time to allow extra time to initiate graceful shut-down of non-critical loads in the event the generator doesn’t come online as quickly as expected.

Factors to consider when choosing a generator – Generator Type

If the design calls for a single emergency generator, then a standby generator is likely the most appropriate for your application.  For highly critical loads and redundant configurations, a prime generator may be preferred or even required depending again upon industry regulations and the Uptime Institute data center tier you are striving for.  Tier III and Tier IV data centers require prime generators and a minimum of N+1 redundancy.

Fuel source is also an important consideration.  As mentioned earlier, many fuel types are available, but diesel remains the gold standard for data center applications because of its short start-up time, power, and reliability.  For highly critical applications, duel redundant fuel sources may be considered.

Factors to consider when choosing a generator – Environmental

Aside from choosing the generator and generator system itself, other factors to consider when designing a power system are:

  • Exhaust and local EPA regulations
  • Noise and local noise ordinances
  • Aesthetics and any zoning regulations that may require hiding the generator from plain site
  • Foundation or padding size and type
  • Genset derating due to high altitudes or excessive ambient temperatures
  • Fuel storage tanks and hazardous waste considerations
  • Environmental hazards that can block or contaminate the intake and/or exhaust such as heavy snow or salty air near the coast

Standby Generator Basics: Conclusion

There are many different things to consider when selecting a generator.  Each configuration is unique and requires careful planning and consideration.  It’s worth emphasizing that this article is intended as a general guideline only.  An electrical engineer familiar with the building power distribution system is the most qualified professional to properly size the generator for your requirement.  At a minimum, the engineer will require an electrical one-line of the power system to begin sizing and configuration of the generator and ATS or switchgear.

Power Solutions LLC can assist with the process.  Our team has successfully engineered, furnished and installed generators and power systems for customers all over the country.  Power Solutions LLC is a national vendor-agnostic solutions provider of power products and services for IT, manufacturing facilities, and telecommunications applications. As a value-added reseller for more than 20 different manufacturers, we help you configure the best solution for your specific application. With access to such a wide range of power products and services for both AC and DC applications, we offer customized turn-key solutions that simplify project execution and procurement with a single point of contact.

For assistance with the configuration of an emergency standby generator system,
call 800-876-9373 or email