Protecting standby generators for mission critical facilities


Standby, prime vs. continuous

Generator rating must also be considered when designing standby power systems for mission critical facilities. The International Organization for Standardization (ISO 8528-1 standard) generator ratings are:

Emergency/standby: Emergency/standby rating is the maximum power for which an engine-generator is capable of delivering for up to 200 hr/yr. The allowable average power output over a 24-hr period is limited to 70% of the nameplate rating.

Prime: Prime rating is the maximum power for which an engine-generator is capable of delivering continuously with a variable load for an unlimited number of hours. The allowable average power output over a 24-hr run period is limited to 70% of the prime rating.

Continuous: Continuous rating is the maximum power for which an engine-generator is capable of delivering continuously for a constant load for an unlimited number of hours. Typically, continuous rating is used for exporting power to a utility.

An additional key difference between standby and prime is that the prime rating does allow for a 10% overload and the standby rating has no overload allowance. Because of power output and run time limitations, emergency/standby-rated generators are not generally used for mission critical (Tier 3 and Tier 4) type applications. When sizing and specifying the generators for mission critical facilities, engineers must evaluate the expected load profile of the facility, the number of redundant units operating, and the expectancy of the system to operate for longer than 24 hr.

Many generator manufacturers have recently developed their own rating (mission critical standby or data center continuous), which basically falls between the ISO standby and prime ratings to correlate with the operation of data centers. Although the rating definitions vary among manufacturers, the result is an increased average power output and an increased limitation on the run hours. These ratings generally do not change the generator size or cost, but they may affect the system warranty.

Emission requirements

The U.S. Environmental Protection Agency (EPA) regulates emissions from diesel powered equipment based on engine horsepower rating. The generator ratings range from Tier 1 to Tier 4 (different from the ANSI/TIA 942 Tier ratings). Since 2011, most nonemergency diesel generators have been required to comply with Tier 4 requirements, which include some type of aftertreatment, such as selective catalytic reduction and particulate filters.

The EPA does allow an exemption to the Tier 4 requirements if the diesel generators are used only for emergency applications such as during the loss of utility power. This exemption also allows the generator to be operated for up to 100 hr/yr for maintenance and exercise. The 100 hr/yr limit is an important number for mission critical facilities because those types of facilities tend to test and exercise the generators more often to ensure they will operate when called upon. In addition, if the engine generator is used to support the load when normal power is present (peak shaving, load curtailment, or storm avoidance), it is not exempt and must comply with the appropriate Tier requirements.

Testing and maintenance

Because facilities depend on generators and standby power systems to keep operating, all generator systems should be fully tested and commissioned before they are put into service. For mission critical facilities, testing and commissioning involves a five-step process:

  • Level 1—design review: Design plans are evaluated to confirm they meet the intended operation
  • Level 2—factory acceptance testing: Key pieces of equipment are powered up and tested at the factory to ensure that they perform according to specified parameters
  • Level 3—installation inspection and verification: Equipment is inspected and verified on site
  • Level 4—component testing: Individual components and systems are tested to verify operation
  • Level 5—integrated systems testing: The complete system is tested with all components operating. 

Some of the tests performed on generators during factory and component testing include load steps, transient response, and heat runs (see Figure 5).Figure 5: Generators for mission critical facilities are tested and commissioned before they are put into service. Part of that process is factory acceptance testing, which includes load steps, transient response, and heat runs.

Another important aspect of ensuring standby power system operation is implementing a preventive maintenance program. The National Electrical Testing Association recommends generator maintenance testing every 12 mo. Generator systems that require maintenance include lubrication, fuel, exhaust, cooling, and electrical/control systems. Additional standby power system maintenance recommendations include:

  • Keep an inventory of spare parts
  • Properly train all operators
  • Exercise the generator regularly
  • Load test the generator
  • Analyze lube and fuel oil periodically
  • Arrange for annual or semiannual manufacturer checks/service.

Lessons learned

During Hurricane Sandy, many mission critical facilities in the Northeast relied on their standby power systems for multiple days to keep their facilities operating. Some of the lessons learned involve generator location, fuel supply, and, filter changes.

Generator location: Locate generators and other standby power distribution equipment above the flood plain. In general, standby systems can’t keep the facility operating if they are under water. Therefore, make sure the generators and any distribution equipment are located above the flood plain to ensure they will operate when required. During Hurricane Sandy, some areas did see flooding above the 100 yr flood level, so consider locating the standby power equipment above the 500 or even the 1,000-yr flood level.

Fuel supply: Maintain an adequate supply of on-site fuel storage. Although the water subsided in about a day, the debris and damage caused by the hurricane made it difficult to deliver fuel—even for those facilities that had contracted emergency deliveries. On average, it was about three days before facilities could get fuel deliveries. A minimum of 72 hr of fuel storage is required by NEC Article 708 for critical operation of power supply systems.

Changing filters: Provide means to change filters while operating. In addition to the difficulty of getting fuel delivered, the fuel that was delivered was often contaminated with water and debris. During the extended power outage caused by Hurricane Sandy, the demand for fuel was very high. Fuel delivery companies were delivering everything they had, which meant sometimes getting fuel that was less than desirable (bottom of the tank). Many facility operators said that they were replacing filters every couple of hours to keep the system from clogging. Recommend installing a dual-header fuel filter system with a transfer valve that allows filter replacement while the engine is running. In addition, consider adding a fuel polishing system to clean the fuel before it gets to the generator.

The goal of the generator and standby power system is to provide power when there is a loss of utility power. Mission critical facilities are required to remain operational under all conditions. The generator and standby power systems for mission critical facilities require a higher level of reliability and availability.

Kenneth Kutsmeda is an engineering design principal at Jacobs Engineering in Philadelphia. For more than 18 years, he has been responsible for engineering, designing, and commissioning power distribution systems for mission critical facilities. His project experience includes data centers, specialized research and development buildings, and large-scale technology facilities containing medium-voltage distribution.

Standby power system codes and standards references

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