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