Commissioning, testing gensets using resistive/reactive load banks
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.