Why Does My System Fail When Each Component Works?
on 10-02-200912:25 PM - last edited on 10-14-200909:11 AM by woody
A few years ago I was involved with a project for a new hospital. During the initial commissioning, testing showed that the ten second power restoration (NFPA-110 regulation) was not being met. The bottom line was this: separately the components met their requirement, but when combined, the system failed. Let me walk you through it.
High level, the site consisted of:
Two 12,470V utility feeds with switchgear
Generator paralleling switchgear
Two 2250 kW, 480V generator sets
Transfer switches to transfer power between utility switchgear and paralleling switchgear
The sequence of operations was:
Transfer switch detects loss of utility, and after a one second delay, it signals the generator paralleling switchgear to start.
The generator paralleling switchgear then starts the generator sets.
As the first generator set reaches rated speed and voltage, the generator circuit breaker (located in the paralleling switchgear) is closed.
The transfer switch detects generator power and transfers load.
Total time from lights out to lights on was eleven seconds. There was a big scramble by the various suppliers involved to prove their equipment was not at fault. And every one of the suppliers was right – not one piece of equipment in the system was operating outside of the expected parameters. Still, the system was failing to meet the requirement. Weeks were lost troubleshooting functional components to come to this conclusion.
It was not until the site was looked at as a complete system that the problem was identified. We supplied data acquisition equipment for the site, and the local Cat dealer connected it to multiple points in the system. This allowed for real time monitoring and logging of exactly what was happening with the equipment. The focus was on the control signals – how long did each of these steps take:
ATS to detect loss of utility
Generator set to start
Switchgear to detect proper voltage and close the breaker
ATS to transfer to the emergency source – powering the load
It was only after this data was collected and analyzed that the root cause was determined.
The switchgear was not initiating the closure of the generator circuit breaker until after the generator set reached rated speed and voltage. Typically load is applied to the generator set when it reaches 90% rated speed and voltage. This added a full second to the overall time. This was further compounded by the fact that that it took between one and a half to two seconds after the generator circuit breaker was closed for the transfer switch to transfer load to the generator sets.
The switchgear settings needed to be adjusted so that the circuit breaker closed when the genset reached 50% rated speed and voltage, improving the time to provide power to the transfer switch by nearly two seconds. The installation realized an overall power restoration time decrease of one and a half seconds – putting the site within the ten second power restoration requirement.
While problems on site can often be attributed to a single piece of equipment, it should not automatically be assumed that any single component is at fault. Taking a system level approach to troubleshooting can save valuable time and resources. In the above example, weeks were spent troubleshooting the problem. Once the data acquisition equipment was brought in and the data collected, it took less than a day of analysis to discover the root cause and identify solutions.
This is just one example I have personally been involved in. Have you experienced a similar situation? Have you used different techniques to analyze and troubleshoot installations? Can you offer any tips to others out there? Post your comment below.