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August 2012
Zigzag Grounding Transformers
What’s wrong with this picture?
I was taking my daily walk through the production floor a few years ago looking for photo
opportunities to include in a training presentation. I came across a padmount transformer in the Final
Prep area, opened the compartment doors to set up for a picture. Imagine my surprise when I
discovered that the transformer had made it all the way to the shipping floor without anyone noticing
that there were no secondary bushings! How could this be? How did this transformer make it past
Quality Control? How is it possible that it was even tested?
After bringing my finding to the attention of Engineering, I was sat down and explained that what I
had discovered was not an “oversight”, but rather was a “grounding” transformer. What the heck is a
grounding transformer? – Hence the topic of this paper (by the way, there is nothing wrong with the
picture…).
Pacific Crest Transformers
300 West Antelope Road – Medford, Oregon 97503
Tel : (541) 826 – 2113
Fax : (541) 826 - 8847
Here is the nameplate drawing for the above transformer.
Still a bit confusing isn’t it?
Simply put, a grounding transformer is used to provide a ground path to either an ungrounded "Y" or
a delta connected system. Grounding transformers are typically used to:
•Provide a relatively low impedance path to ground, thereby maintaining the system neutral at or
….near ground potential
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•Limit the magnitude of transient over voltages when re-striking ground faults occur
•Provide a ground fault current during line-to-ground faults
•Permit the connection of phase to neutral loads when desired
If a single line-to-ground fault occurs on an ungrounded or isolated system, no return path exists for
the fault current, thus no current flows. The system will continue to operate but the other two unfaulted lines will rise by the square root of 3 above ground, resulting in overstressing of the
transformer insulation and other associated components on the system by 173%. MOV lightning
arresters, which are connected between line and ground are particularly susceptible to damage from
heating by leakage across the blocks even if the voltage increase is not sufficient to flash over. A
grounding transformer provides a ground path to prevent this.
One common configuration of grounding transformers is the zigzaq connection. Consider a threephase Y connected transformer with a ground connection at the neutral. Cut each winding in the
middle so that the winding splits into two. Turn the outer winding around and rejoin the outer
winding to the next phase in the sequence (i.e. outer A phase connects to inner B phase, outer B
phase connects to inner C phase, and outer C phase connects to inner A phase) as depicted below.
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Here is a schematic wiring diagram of the above pictorial view.
Large multi-turbine wind farms use Zig-Zag Grounding Transformers for fault protection on
ungrounded lines because many wind farm layouts use the substation transformer to provide the
sole ground source for the distribution system. When a ground fault on a collector cable causes the
substation circuit breaker for that cable to open, the wind turbine string becomes isolated from the
ground source. The turbines do not always detect this fault or the fact that the string is isolated and
ungrounded; thus the generators continue to energize the collector cable, and the voltages between
the un-faulted cables and the ground rise far above the normal voltage magnitude. A grounding
transformer placed on the turbine string provides a ground path in the event the string becomes
isolated from the system ground.
When a zigzag transformer is connected to a stable electrical system, very little current flows in the
connection between the zigzag's neutral point and ground. By design, the magnetic fluxes in the split
windings cancel each other out.
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If one phase of the delta faults to ground, the voltage applied to each phase of the zigzag is no
longer in balance and flux in the windings no longer oppose. Zero sequence (ground fault) current
flows between the zigzag’s neutral to the phase affected by the fault. Hence, the purpose of a zigzag
transformer - to provide a return path for ground faults on delta-connected systems.
Unless the grounding transformer is used to provide auxiliary power, there is no kVA rating, because
the grounding transformer does not function as a power source. During "normal operation" no
current flows in the grounding circuit because the system is balanced and no neutral current occurs.
During a fault, the duration is limited to seconds in the extreme and a few cycles in most cases. Some
designers talk about a "fault power" rating but this is time sensitive and not a true "kVA" power
rating. A grounding transformer will be labeled for grounding use and rated by the continuous
current and fault current it is designed to carry.
Design consideration
•Continuous Neutral Current - The continuous neutral current is defined as three times the phase to
current, or in other words, the zero sequence current. This is usually considered to be zero if the
system is balanced. However, for the purposes of designing a grounding transformer, it is a value
that is expected to flow in the neutral circuit without tripping protective circuits (which would force
the current to be zero) or the leakage current to ground that is not a symmetrical function. Again this
value is needed to design for thermal capacity of the grounding transformer.
•Fault current and duration - This value is needed to calculate the short time heating that results
from a fault on the system and should be determined from an engineered system study. Typical
values for this range from a few hundred amps to a few thousand amps with duration times
expressed in seconds and not cycles. For instance, a value of 400 amps for 10 seconds is typical. The
fault duration is a critical parameter for the transformer designer. Where protection schemes use the
grounding transformer for tripping functions, a relatively short time duration is specified (5 -10
cycles). On the other hand, a continuous or extended neutral fault current duration would be required
when the grounding transformer is used in a ground fault alarm scheme.
•Impedance - The impedance can be expressed as a percentage or as an ohm value per phase. In
either case it should be chosen so that the un-faulted phase voltages during a ground fault are within
the temporary over-voltage capability of the transformer and associated equipment, such as arresters
and terminal connectors. Because of this description, the values can vary from as low as 8% to almost
100%. This value must come from the system designer.
Basic overall construction features - note the following features as they apply to each transformer...
•Compartmental Padmount transformer with integral tamperproof compartment or
…design
•Outdoor or indoor
•Fluid type- mineral oil, silicone, Envirotemp FR3
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…substation
•Temperature rise is assumed to be 65'C
•Site elevation or environmental concerns
•Special paint as required
Construction
Given that grounding transformers primary purpose is the management of system faults, the
construction of the cores and coils are significantly different than conventional distribution
transformers.
Copper conductor, round coils, and cruciform cores are standard for PCT. The round coils include
360 degree cooling ducts to minimize localized heating and deterioration of the insulation system.
Under fault conditions, radial and axial forces are exerted on the coils. Radial forces are equalized
via the round construction. Axial forces are managed via top and bottom pressure plates.
Round coils with 360 degree cooling ducts
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In this picture note the routered pressure
plates which facilitate the 360 degree
cooling. The plates are secured via top to
bottom threaded rods which provides
the necessary containment of axial forces.
The heavy duty core frame secures the
core/coil assembly and compresses the
core laminations.
Pacific Crest Transformers – specific designs
for specific applications
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