Safety Corner System Grounding Changes by Paul Hartman Power Testing and Energization O ver the years there has been a multitude of utility voltage schemes throughout the US, and as technology has improved so have the voltage schemes. This improvement in technology occasionally requires the utilities to reconfigure their systems. Unfortunately, this reconfiguration process can have undesirable safety issues if all of the possible negative consequences are not thought out. The guilty scheme in this article is a 15 kV grounded wye which in reality was a solidly grounded 12,470 / 7,200 system. I called up a friend to set up a lunch meeting. He told me to swing by his office at 11:30 the next day as there was not much on his schedule. He is in the electrical testing business and is responsible for coordinating the testing crews. Lo and behold when I walked into his office the next day it looked like the control room for a space shuttle launch. He was on his cell phone and his desk phone at the same time. There were photos of a switchgear failure on the big screen on the wall and various other photos and drawings scattered around the room. People were coming and going out of his office. It was clear that lunch was not going to happen. In between phone calls, he explained that a water treatment plant was operating in a normal mode when all of a sudden, as the customer put it, their switchboard “A” blew up. In reality there was a contained fault in the main circuit breaker section of one of their 15 kV switchgears. Either way this was not a good thing. Fortunately, or so the customer thought, the facility had an identical switchgear as an alternate feed to all of their electrical systems, so they could continue to run without the “blown up” switchgear. When the technicians arrived at the site they secured the equipment and the surrounding area and then proceeded to remove covers and commence troubleshooting. It didn’t take very long to determine what components had initiated the fault from the switchgear. It was quite evident that the surge arresters had failed. There was damage to the arresters, the arrester conductors, insulators, and the phase power conductors that traveled through the surge arrester compartment. Now the question was why had a perfectly good system, which had been operating for years, failed? My friend was coordinating getting new 15 kV cable and terminations delivered to the site and getting a team of electricians to install and terminate the cables. Also on the list of things to do was to get three new surge arresters. www.netaworld.org In the midst of this one of the senior techs arrived at the office. After listening to the situation he said that he had seen this type of failure before, about a year ago at a mill. He said back then what occurred was that the local utility changed their system from a solidly-grounded wye to a highresistance wye. This change allowed the phase-to-ground reference point to shift when there was a ground fault on one of the phases. In the mill situation the existing system had a ground detection system that would normally trip when there was a preset amount of ground current flowing. When the mill surge arresters failed, the high resistance ground limited the current to a level below the preset trip value; therefore, the 15 kV fault persisted for about 15 seconds before it was finally extinguished. Due to the damage and cost, it was months before the mill equipment was back in service. It didn’t take long to confirm that this was a similar situation as the utility had recently changed their grounding scheme on the system to a high resistance ground, creating a worst case scenario of 12,470 V applied to the 7 kV surge arresters. It is not known what the actual level of phase to ground voltage was on the switchboard, only that it was more than the 7 kV surge arresters could handle. Spring 2009 NETA WORLD Remember earlier when I said that the customer was not very concerned because they had an identical alternate switchgear. Now there was real concern that the alternate with its 7 kV surge arresters could have a similar fault. The area around switchgear “B” was secured just in case, and the crews worked round the clock to get the damaged switchgear “A” back in service so they could take “B” off line and remove the surge arresters. Within 24 hours both switchgears were back in service minus their 7 kV surge arresters. Fortunately the failure was contained without injury. Limiting access to energized equipment is a must for one never knows what kind of ticking time bomb could be inside one of the big grey metal boxes. As for lunch, we rescheduled for the following week. Paul Hartman has over 20 years experience in start-up, commissioning, maintenance, and training in power generation, including international projects in Pakistan, Indonesia, Thailand, Brazil, and Korea. He has been an instructor for state certified continuing education programs. Paul is currently Project Manager for Power Testing and Energization. He is a regular contributor to NETA World and a frequent speaker at PowerTest Electrical Maintenance and Safety Conference. NETA WORLD Spring 2009 www.netaworld.org