Commissioning electrical systems in mission critical facilities
Engineers should follow best practices to overcome the inherent challenges of electrical system functional performance testing.
The overall goal of commissioning must be to ensure that a facility meets the design intent and the owner’s requirements. For critical facilities, this goal is generally achieved by proving to the owner that the reliability, redundancy, and resiliency that he or she paid for is indeed present and operational in the finished facility.
Because there are so many failure scenarios and variables, it is rarely possible or cost efficient to reasonably test each one, but the commissioning authority has an obligation to provide a level of testing that will allow the owner to feel confident that each system is working and capable of maintaining a proper planned operational state during common external events.
As expected, the owner will want to use the commissioning process to be certain that the installation, performance, and operation of new equipment is acceptable before it supports critical load, and he or she will strive to do this as cheaply and as quickly as possible.
This article explores the best practices for testing several electrical systems, as well as some of the challenges encountered. It also presents selected case studies observed during the functional performance testing phase of the commissioning process, as detailed in ASHRAE Guideline 0. Implementing these best practices and lessons learned on future projects will improve the quality of the product provided to the owner.
Including the generators in the commissioning scope for a critical facility is imperative because they are the only source of long-term standby power when the utility becomes unavailable (see Figure 1).
When testing a generator, it is best practice to ensure that the load for step loading and endurance testing has a power factor rating that matches the nameplate power factor on the generator, as the generator will be tuned and calibrated to operate best at its rated conditions. The manufacturer also will not likely be able to provide documentation on how the generator is expected to perform if the load used for testing deviates from the name plate conditions. The tuning and calibration is especially important when attempting a 0% to 100% step load, and often the system will not respond properly within acceptable tolerance if the power factor of the load does not match the nameplate rating.
Due to new EPA regulations, generators are now limited regarding the amount of pollution that they can emit under all running conditions, including when responding to step loads. This has been a challenge for generator manufacturers who in the past simply allowed the system to call for more fuel, which resulted in billows of black smoke entering the environment. In an effort to minimize pollution, manufacturers have had to finely tune the generators, resulting in the increased importance of testing the generators at rated power factor. In addition, because the generators are typically exercised under load for routine maintenance and testing, the owner often buys a permanent resistive load bank (unity power factor) sized for the rated capacity of the generator. It is important to explain to the owner that the permanent load bank that will be used for future load testing may not be appropriate to use during commissioning if it is rated at unity power factor.
Generator commissioning case study: Two 13.8 kV 3 MW generators that were rated for 0.8 power factor were each tested using a 3 MW unity power factor load bank. In each case, when conducting the 0% to 100% load step, the generators were able to support the load during only one out of seven attempts. The load was resistive, but the voltage drop induced by the step load caused the load bank controller to lose power, which shut down the load bank. Even when the load was maintained, the voltage and frequency deviated beyond the published criteria for 100% of the step load because the generator performance data was not based on a unity power factor load. This problematic operation was not observed for the same generators when they were tested at the factory using a 0.8 power factor load bank.
Automatic transfer switch (ATS)
The ATS is an important component of the critical facility because it is used commonly in critical facility designs to transfer power from a primary source to a secondary source after the loss of the primary source.
Open transition ATSs are designed to allow for an interruption to the load using a break-before-make transfer. Because of this, loading the ATS during open transition transfers during functional performance testing is not required. Load is also not required when testing an ATS’s ability to perform closed transition transfers. During closed transition transfers, the ATS will parallel the primary and secondary sources prior to transferring. It is important to ensure that the ATS can properly conduct closed transition transfers and will handle the transition in the same manner, regardless of whether it is carrying load or not. A power quality meter must be connected to the output of the ATS to confirm that the transfer is completed within the specified time for closed transition applications. It should be noted that load is required for all ATSs when conducting infrared scanning. It is recommended that all components of the ATS are infrared scanned under full load on all primary, secondary, and bypassed power paths after final installation is complete. Load is also required for closed transition applications when the secondary source of the ATS is a generator. This testing is usually conducted as an integrated system test to prove that the generator and ATS work properly together under full load. The integrated system testing is conducted after functional performance testing for the ATS, generator, and other integral systems is completed.
In most cases, for an ATS to be functionally tested, both sources must be available because the ATS will usually inhibit any transfer if there is only one source. This problem can arise in situations where ATSs are added to existing live facilities. Because of their integral role in the power distribution system, they often can’t be tied into the electrical system without bringing down the loads that they will serve. In an effort to minimize disruption to the live facility, the ATS testing will likely occur prior to connecting it to the live facility. However, the ATS can be connected to the secondary source if the secondary source is a generator. When the primary source serving the load is restored, there is usually limited time for testing the ATS as it will immediately be required to provide power to critical loads.
ATS commissioning case study: An ATS manufacturer was required to start up and test the ATS on a project before it was tied into the electrical system. To do this, the ATS vendor required both the primary and the secondary sources to be available for the start-up. The electrical contractor added a jumper between the two sources and connected the secondary source of the ATS to the generator. When the generator was started, the ATS saw both the primary and secondary sources as available. A major drawback was that there was no way to disconnect only the primary source during start-up without also simulating the loss of the secondary source, so it was not possible to verify automatic transfer operations without simulation techniques. The ATS also had a much easier time performing closed transition transfers because the two sources were perfectly synchronized, as they both came from the same generation point. All of the functionality was retested after the final tie-in during functional performance testing to ensure the system was operating properly in the actual design configuration.