Commissioning, testing gensets using resistive/reactive load banks

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:

Specific regulations such as NFPA 101, Article 7.9.2.4 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:

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.

The case for reactive load bank testing

The ability to simulate varying reactive loads, which are more realistic, is the most essential benefit for a load bank that provides both kVA (resistive) and kVAR (reactive) loads. The critical differences between testing with a resistive-only load bank and a resistive/reactive load bank are compared in Table 1. A resistive-only load bank can provide adequate testing of the individual prime mover and load sharing (including load add/load shed) controls of a multiple unit facility. However, a reactive load bank allows testing of the alternator, load sharing, and transient responses because it can apply loads that approach those experienced during normal genset operation (see Table 2).

Genset engine governors respond to loads by reducing engine speed. Figure 2 compares the transient response for a large diesel standby genset when applying a block load using restive-only and resistive/reactive load banks. The resulting initial synchronous voltage dip (Vdip1) using the 75% load at 0.80 power factor results in a voltage dip that is approximately 25% greater when compared to the equivalent resistive-only load applications. The engine speed related voltage dip (Vdip2) is similar, in both cases, due to the manufacturer’s standard V/Hz-type voltage regulator.

During testing with a resistive-only load bank, a system that is sensitive to transient voltage dips would not necessarily provide an indication of a power supply or system condition that would lead to a potential problem during operation. Solid-state controls and power supplies are particularly sensitive to transients and can shut down unexpectedly during load changes unless specifically backed up with a dedicated power source capable of riding through the voltage and frequency transients associated with block loading of the gensets.

When testing multiple unit generator systems, the ability to share reactive loads (kVAR) equally is critical to achieving the maximum-rated power system output. When load sharing controls are not properly configured (i.e., droop settings, cross-current compensation, and measurement and control device polarities), resistive-only testing can fail to determine how the reactive load is accepted by an individual generator. In addition, the paralleling switchgear and protective relays may perform adequately under resistive load applications, but the reactive load bank testing will provide load acceptance and rejection that simulates real-world conditions more closely (see Figure 3).

Choosing the right resistive/reactive load bank

When selecting a resistive/reactive load bank it is important to consider key features including ease of operation, onboard diagnostics, metering, the ability for an operator to control multiple units from a single controller, and data download capabilities. Load banks offering automatic step loading and duration, and data collection and reporting capabilities are beneficial in providing the necessary records to demonstrate compliance with the facility and regulatory requirements.

Connecting load banks to a facility normally involves temporarily connecting the 3-phase power conductors to the load bus. Electrical testing and load bank rental companies can supply the necessary load banks, transformers, and cables to provide the correct service voltage for the equipment being tested.

The proof is in the testing

Specifying engineers can and should promote reactive load bank testing because it is the best way to test the entire system to identify system-wide weaknesses during commissioning and at periodic test intervals to be in compliance with the regulatory agencies. Without proof of testing, the design remains hypothetical. Testing systems with the correct size and type of resistive/reactive load bank validates the power generation system design (see Table 3).

For existing installations, proper reactive load bank testing provides real-time data and factual evidence of reliability, functionality, and reduction of capacity resulting from aged equipment. Additionally, reactive load bank testing provides a best-case simulated real-world condition where voltage-drop, thermal heating, harmonics, and efficiency can be analyzed more effectively than with a resistive load only.

Full system integration testing of critical systems during commissioning establishes an accurate baseline for ongoing operational performance and is a valuable tool in providing a higher level of confidence in the emergency power system.

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