Commissioning electrical systems in mission critical facilities

12/09/2013


Main electrical switchgear

Main electrical switchgear is an important component to a critical facility because it distributes power to all of the downstream electrical distribution equipment.

Circuit breaker settings must be inputted, coordinated, tested, and verified throughout all main electrical distribution equipment. If there is a fault in the system, it is imperative that selective coordination is implemented so that the fault is isolated as far downstream as possible. Main circuit breakers must be properly set up to ensure that they will stay closed during fault conditions and wait for downstream equipment to clear the fault. This will be ensured by implementing proper National Electrical Testing Association-recommended circuit breaker testing including instantaneous pickup, short time pickup, short time delay, long time pickup, long time delay, ground fault pickup, ground fault time delay, contact resistance tests, and insulation resistance tests. 

While main electrical switchgear is an integral part of the electrical distribution system, the system’s current carrying capacity may increase the arc flash hazard. To avoid injury, main electrical switchgear should be disconnected before it is opened or worked on. Because the owner will often not own a means of disconnect ahead of this equipment, it usually requires involvement from the utility provider, which can be problematic and difficult to schedule. 

Main electrical switchgear commissioning case study: Modifications were required to be made to the main electrical switchgear that serves a data center site. To ensure that all modifications were made correctly, infrared scanning had to be conducted. Due to the current carrying capacity of the main electrical switchgear, it was not safe to be within 6 ft of the equipment when it was open, and opening it could be done only when the main electrical switchgear was not energized. This required the lengthy process of shutting down all of the loads in the building, opening the main electrical switchgear, and restarting all of the systems so the main electrical switchgear could be scanned at a safe distance under load. The same procedure had to occur to replace the covers on the main electrical switchgear after the infrared scanning was completed. 

Static transfer switch (STS)

Figure 4: STSs are capable of seamlessly transferring load automatically or manually from one source to another within a few msec. Courtesy: ESDAn STS is an important and useful component for a critical facility because it provides the ability to seamlessly transfer load during both failure and maintenance situations (see Figure 4). 

STSs behave similarly to ATSs, but because they are designed to transfer within a few msec, there are several settings that must be coordinated. STSs are commonly fed from UPS systems. These UPS systems are present to prevent interruptions to the downstream STSs. During a planned maintenance event or during a utility power failure, the UPSs are designed to perform transfers to bypass or battery within a certain time frame. Because the STSs are set up to transfer on a loss of the primary source for a certain duration, the time frame must be longer than the allowable interruption seen from the UPS. If not coordinated properly, a routine transfer to bypass at the UPS level can cause the downstream STSs to transfer to their secondary source. 

On several occasions, phantom voltage and current readings have been observed at the STS screens with no connected load. Rebooting the system typically corrects this problem. While the manufacturers generally indicate that there are no operational risks, this anomaly is puzzling. 

STS commissioning case study: At a site containing eight STSs, one unit displayed current values on a single phase with open load breakers and no current was measured using portable power monitoring equipment. Another STS unit showed 160 A in this scenario while 0 A was measured with portable power monitoring equipment. The manufacturer assured the team that simply rebooting the screen would correct the problem and would not jeopardize the load in any way. Rebooting the screen did correct the problem, and the unit was monitored to ensure that the problem did not return. 

Electrical power monitoring system (EPMS)

Figure 5: The EPMS allows the operator to view the electrical status of every system in the critical facility from a single location. Courtesy: ESDThe EPMS allows all of the electrical systems within the critical facility to be monitored from a single location, giving the operator visibility to ensure that all systems are not generating any alarms and are operating properly and efficiently (see Figure 5). 

When confirming that the EPMS is monitoring systems correctly, multiple states must be checked for each point. Points must be modified in the field and checked to ensure that the same values or statuses observed in the field are properly reported back to the EPMS. 

One difficulty encountered in this area has to do with discrepancies with points. Design engineers typically specify points to be monitored by the EPMS, but they often approve equipment submittals that are unable to provide these points. To avoid this problem, it is best to meet with the design engineer and the equipment manufacturers prior to the acceptance of the submittals to ensure that the points that are important to the design engineer can be provided by the equipment. 

EPMS commissioning case study: Many points monitored by the EPMS, including voltage spikes and sags, are very difficult to simulate. To simulate real voltage sags on a project, the electrical system was placed on generator and large step loads were added with a load bank. The generator struggled to maintain the voltage when required to carry the large step load, which resulted in voltage sag alarms and generation of waveforms captured at the EPMS. 

Conclusion

The equipment in the electrical distribution system of mission critical facilities must operate dependably. After commissioning challenges have been resolved and best practices have been employed, these systems will meet the original design intent and owner’s requirements, ensuring the owner that the facility embodies reliability, redundancy, and resiliency. 


Joshua J. Gepner is a senior associate at Environmental Systems Design Inc. He has more than 10 years of engineering experience focusing on design, consulting, and commissioning. He specializes in commissioning mission critical facilities and is knowledgeable in commercial, residential, and industrial electrical design as well as LEED and building energy code standards.


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EDDIE , GA, United States, 01/03/14 05:10 PM:

The anecdotal information contained in the case study presented in this article is doing a great disservice to the reader as it is technically inaccurate. The fact that the voltage dip caused the load bank controls to drop out on site, but not at the factory has everything to do with how the load bank controls were powered and nothing to do with the power factor of the load bank being used. IF all other factors/variables were identical between the two tests, the engiens would have performed better (less voltage dip) on the resistive only test than they did on the 0.8 pf resistive/reactive test which requires more torque / Hp from the engine to overcome.

The value of testing at a power factor that is similar to the actual building load is completely valid and worthwhile, but the data used to try to support the point are invalid.
Anonymous , 01/03/14 07:20 PM:

Torque is needed for power production, less so for VAR production. With a PF of 0.8, the power production is less than with a unity PF so wouldn't the torque required be less, not more?
BHAVESH , MAHARASTRA, INDIA, 01/03/14 11:13 PM:

nice article .

Pls present same type of article for HVAC System .

Bhavesh Mehta
Reliance Industries Limited
nAVI mUMBAI
INDIA
+91 98 676 13136
Anonymous , 01/06/14 08:36 AM:

Never heard of a diesel engine-generator set being "tuned and calibrated" to operate at its rated conditions. It's supposed to meet its published ratings over the entire range of 0.8>pf>1.0
PHILIP , LA, United States, 01/17/14 12:38 PM:

Interesting and Informative.
Tell Eddie to consider using
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