In the Know

Load Bank Testing | Part 3 | Choosing the Right Load Bank

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 unexpectedly shutdown 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 generator sets.

When testing multiple unit generator systems the ability to share reactive loads (kVARs) equally is critical to achieving the maximum rated output of the power system. When load share controls are not properly setup and configured (i.e. droop settings, cross current compensation and polarity of the measurement and control devices) resistive only testing can fail to determine how the reactive load is accepted by the 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 more closely simulating real world conditions. A single-line diagram showing the components used for a load test of a large facility is shown in Figure B.

Choosing the Right Load Bank

When selecting a resistive/reactive load bank it is important to consider key features including ease of operation, onboard diagnostics and 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, along with data collection and reporting capabilities, are beneficial in providing the necessary records for the demonstration of compliance with the facility and regulatory requirements.

Table B: Types of Resistive and Resistive/Reactive Load Banks

100 kVA | Resistive Only | Portable, Small Generators and UPS System (120/208, 240 VAC, 60 Hz)
200 kVA | Resistive Only | Small Generators and UPS Systems (480VAC, 60Hz)
200 kVA | Resistive/ Reactive | Single Units (480VAC, 60 Hz)
>1MVA – 6MVA | Resistive/ Reactive | Single Units (480/5kV/15kV, 60 Hz)
>6 MVA | Resistive/ Reactive | Multiple Units Combined (480/5kV/15kV, 60 Hz)

The connection of load banks to a facility normally involves temporary connection of the three-phase power conductors to the load bus. Electrical testing and load bank rental companies can provide the necessary load banks, transformers and cables to provide the correct service voltage(s) for the equipment being tested.

Yes to Reactive Load Bank 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 at time of commissioning and at periodic test intervals to be in compliance with the regulatory agencies. Without proof of testing, the design remains hypothetical in nature. Testing systems with the correctly sized and type of resistive reactive load bank proves out the design of the power generation system.

For existing installations, proper reactive load bank testing provides real-time data and factual evidence of reliability, functionality and any reduction of capacity which is the result of aged equipment. Additionally, reactive load bank testing provides a ‘best case’ simulated real world condition where voltage drop, thermal heating, harmonics, efficiencies, etc. can be analyzed more efficiently than with just a resistive load.

Furthermore, 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 for the owner and tenants.

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