Commissioning, testing gensets using resistive/reactive load banks
Consulting engineers can help their clients by conveying to them the importance of including reactive load bank testing during commissioning and periodically during normal operations.
When examining the key differences between resistive and resistive/reactive load testing—and why the latter is necessary—it is important to focus on addressing a facility’s emergency power generation system as a whole by testing the entire system to identify system-wide weaknesses at the time of commissioning and at periodic test intervals to comply with regulatory agencies (see Figure 1).
Reactive load testing is primarily important at health care facilities, data centers, life safety, and mission critical applications where the need to demonstrate the capability to provide electrical power as intended is prescribed by regulatory standards and codes specified by the designers. Examples of typical emergency power sources include gas- and diesel-fueled reciprocating engine generators, liquid- and gas-fueled turbine generators, rotary UPS, and battery UPS systems.
Understanding the standards for certification
While understanding the key benefits of reactive load testing is not necessarily a primary focus, it is important for consulting engineers and facility managers to understand the specific code requirements for installation, performance, and testing of emergency power systems.
The National Fire Protection Association (NFPA) publishes and updates these standards on a regular basis with input provided by professionals, engineers, and members of industries that provide related equipment and services. Applicable NFPA resources include:
- NFPA 101: Life Safety Code (2012)
- NFPA 99: Health Care Facilities Code (2012)
- NFPA 110: Standard for Emergency and Standby Power Systems (2013)
- NFPA 37: Standards for the Installation and Use of Stationary Combustion Engines and Turbines (2010)
- NFPA 70: National Electrical Code (2011)
- NFPA 70B: Recommended Practice for Electrical Equipment Maintenance (2010)
- Joint Commissions (formerly JCAHO).
Specific regulations such as NFPA 101, Article 22.214.171.124 require that emergency generators be installed, tested, and maintained in accordance with NFPA 110. Provisions dealing with maintenance and testing of emergency generators can be found in NFPA99, Article 4.4, which deals with issues such as:
- Test criteria
- Test conditions
- Test personnel
- Maintaining and testing circuitry
- Battery maintenance.
Specifying engineers and facility managers should have access to the latest versions of these NFPA standards. They are available online at www.nfpacatalog.org. Individual states and localities also have standards, codes, and regulations pertaining to mission critical facilities.
Key reason for load testing
Typically, gensets seldom run under full-load conditions after the manufacturer’s factory testing. Although they may be tested in compliance with the regulatory requirements that permit the use of actual loads, over time, this practice can lead to conditions that could affect performance and reliability. Modern diesel gensets designed to meet the stringent U.S. Environmental Protection Agency Tier Level emission standards are designed to be operated at loads of more than 50% for optimum life and performance. In addition, the use of after-treatment particulate matter filters that depend on a certain exhaust temperature to facilitate regeneration can be compromised by low-load operation, and consequently can restrict exhaust gas flow due to buildup, causing higher than recommended exhaust back pressure, which can limit the performance of a reciprocating engine genset and/or increase the need for unscheduled maintenance.
When multiple units are installed, they are often run individually for periodic and annual testing using a test load much less than the manufacturer’s recommended levels. The use of a large capacity resistive/reactive load bank can allow testing of multiple units simultaneously, thus reducing the time required to perform and document mandatory testing. Resistive/reactive load banks allow the paralleling controls to be exercised under realistic conditions.
Again, load bank testing is a critical component to meeting regulatory requirements. Today’s diesel gensets that use electronic engine and emission controls to meet current and future EPA emission requirements depend on the engines operating at the manufacturer’s recommended load levels and temperatures.
NFPA testing guidelines refer to minimum load levels of 30%, or as recommended by the manufacturer. Industry associations such as EGSA and the major engine-generator manufacturers recommend load testing at higher levels to ensure that the maximum benefits of load testing can be achieved.
As with regular maintenance, periodic testing is required by code in all health care applications to maintain compliance with the regulatory agency. It is common for health care facilities to perform regular genset testing during off-peak times when loads are at their lowest. While this practice prevents the possibility of serious interruptions to large and/or critical loads, it does not adequately test the genset under worst-case conditions.