Special Feature
High Resistance Grounding…
Is It More Than Just a Big Heater?
by Chris Gingras
Magna Electric Corp.
A
s part of our standard testing and inspection routine during maintenance
outages and commissioning, we inspect and test the neutral grounding
resistor, commonly referred to as a NGR. A NGR is connected to the
X0 point or star connection point of a wye-connected transformer. The purpose
of the NGR is to control the amount of current flow into a ground fault in the
event of insulation failure of one of the normally energized conductors to ground.
When a ground fault occurs the current path back to the source (which in this
case is the wye connected power transformer) contains a high resistance. In many
cases the ground fault current is limited to 10 amperes or less. When the ground
fault current exceeds a specified pickup point, a NGR monitor or ground-fault
relay is used to alarm the presence of the ground fault, or trip the upstream
breaker to clear the faulted circuit. The Canadian Electrical Code states that any
system rated 5 kV or less that does not serve single-phase loads is permitted to
sustain a ground fault if the available current is limited to 10 amperes or less and
an audible or visual alarm is present to enunciate the presence of a ground fault.
When properly applied, a NGR can prevent catastrophic damage to equipment
and injury to personnel.
When specifying, designing, or commissioning a system which is to contain
a NGR, a few important items should be considered. First and foremost, is the
system going to service single-phase loads? Secondly, what is the phase-to-ground
voltage rating required for the NGR? This needs to be known in order to properly
select the required resistance, which is point three. What is the maximum ground
fault current the system should be designed to handle?
I have come across errors in each of these design steps during commissioning,
maintenance, and call outs. First, does the system service single-phase loads? If a
system services single-phase loads and uses a high-resistance grounding system,
nuisance trips may occur and the single-phase loads will receive lower voltage
than expected. Energized with no load, the single-phase system will appear
normal. Once load is applied, the voltage at the load will drop considerably and
www.netaworld.org the upstream ground-fault protection
may trip. This is due to the fact that the
single phase load is now connected in
series electrically with a relatively large
resistance. Once the load is turned on,
the voltage drop across the resistance
is large and the voltage at the singlephase load is limited. Next, what is the
voltage rating and resistance required
of the NGR? Instances have come
up where the designer has taken the
phase-to-phase rating of the system
and chosen the resistance of the NGR
based on that voltage. While this may
not seem to be a huge oversight, the
ground-fault current will not reach the
expected value. A system improperly
designed may not detect a ground
fault. For example, a 4160 volt wyeconnected system has a phase-toground voltage of 2400 volts. A proper
resistance rating for a five ampere
NGR is 480 ohms. If the phase-tophase voltage is used for the resistance
calculation the NGR would be specified to be 832 ohms. In this case the
maximum permissible ground fault
current would be 2.9 amperes, which
may be well below the ground-fault
alarm or trip setting. If this ground
fault is left unchecked, it will only be
a matter of time before another phase
faults to ground and the system will
now have a phase-to-phase fault.
Fall 2008 NETA WORLD
Another very commonly missed design and commissioning step when working with high resistance grounded
systems is the insulation coordination. Now that we have
inserted a high resistance between the X0 bushing and system ground, the phase-to-ground voltage on the unfaulted
phases may now measure as high as the phase-to-phase
voltage in the event of a ground fault. This is due to the
fact that the neutral point voltage is shifted from ground
potential by the voltage drop across the grounding resister.
Since the resister limits ground-fault current to a low value,
there is little voltage drop in the faulted phase. Thus, the
faulted phase is at ground potential while the other two
phases remain close to the normal phase-to-phase voltage
raising their voltage to ground to that value. If this system
has surge arresters installed, the maximum continuous operating voltage (MCOV) of the arrester may be exceeded
during the ground fault. For example, suppose that a 4160
volt phase-to-phase system has a 100 ampere NGR installed
on the neutral bushing (X0) of the source transformer. For
a solidly grounded system, a surge arrester would be chosen
which would have a rating in the neighborhood of 10-15
percent higher than the phase-to-ground voltage, which in
this case is 2400 volts. If this arrester were installed on a
system as described previously, the MCOV of the arresters
would be exceeded on the unfaulted phases in the event
of a ground fault. I have come across situations in the past
where a system was changed from a solidly-grounded system to a high-resistance grounded system and the surge
arresters were not changed to ones having a higher voltage
rating. When you receive a call out and someone tells you
that they had two of three surge arresters fail in a particular
location, I would first investigate the ratings of the surge
arresters before continuing on the trail for the source of the
problem. The reason for having only two surge arresters fail
is due to the fact that only two phase-to-ground voltages
have exceeded the arrester MCOV rating during a single
phase-to-ground fault.
In a recent maintenance outage we were asked to test
the two main transformers at an industrial plant. Each
transformer was rated for 20 MVA and they were connected in delta on the primary and wye on the secondary
with an NGR on the star point; the NGR’s are rated for 100
amperes. During the routine inspection of the NGR’s, we
noticed that the connection between the transformer and
NGR was no longer made. The crimped lug on the cable
from the NGR was found disconnected from its intended
connection point and hanging loose inside the NGR enclosure. Not only was the one NGR not connected to the
X0 bushing, but the second transformer which is commonly
paralleled with the first had a broken connection from the
NGR to the main ground grid, which left this NGR floating
from ground as well. In the event of a ground fault in this
case, the ground fault relays would not detect any problem.
The system disturbance would need to advance to the point
of a phase-to-phase fault before any relaying would detect
a problem since there is no path back to the source. This
NETA WORLD Fall 2008
poses a serious personnel and equipment risk, and this is
one of the many reasons why the NETA Maintenance
Testing Specifications should be followed when performing
maintenance or acceptance testing.
We depend on the proper operation of our electrical
systems to provide continuity of service, proper personnel
and equipment protection in the event of system abnormalities, and proper isolation of faulted systems. It is key that
grounding systems are inspected and tested on a regular
basis along with the source transformers.
Chris Gingras, Technical Services Manager for Magna Electric
Corporation in Regina, SK, has 10 years of experience in plant start-up,
commissioning, maintenance, and design of relay protection and control
systems rated up to 365 kV. Chris has performed arc-flash hazard assessment studies and designed and installed numerous turn key arc-flash
mitigation relay systems to protect personnel and equipment from the
dangers of arc-flash hazards. He is a NETA Certified Level IV Test
Technician.
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