Simply put, a grounding transformer is used to provide a ground path to either an
ungrounded “Y” or a delta connected system and is used to…….
1.Provide a relatively low impedance path to ground, thereby maintaining the
system neutral at or near ground potential.
2.Limit the magnitude of transient over voltages when re-striking ground faults
3.Provide a source of ground fault current during line to ground faults
4.Permit the connection of phase to neutral loads if 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 un-faulted lines will rise in voltage by the square root of 3 resulting in overstressing of the transformer insulation, and other associated components on the system, by 173%.
MOV lightning arresters 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.
Large multi-turbine wind farms provide an example of the use of grounding transformers for fault protection on ungrounded lines. In many wind farms the substation transformer provides 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 as described above. A grounding transformer placed on the turbine string provides a ground path in the event the string becomes isolated from the system ground.
Grounding transformers are normally constructed either with (1) a Zig Zag (Zn) connected winding with or without an auxiliary winding or (2) as a Wye connected winding with a delta connected secondary that may or may not be used to supply auxiliary power.
The geometry of the Zig-Zag connection is useful to limit circulation of third harmonics and can be used without a Delta connected winding or, 4 or 5 leg core design normally used for this purpose in distribution and power transformers. Eliminating the need for a secondary winding can make this option both less expensive and smaller than a comparable two winding grounding transformer. Furthermore, use of a Zig-Zag transformer provides grounding with a smaller unit than a two winding, Wye-Delta, transformer providing the same zero sequence impedance.
Wye connected grounding transformers, on the other hand require either a delta connected secondary or the application of 4 or 5 leg core construction to provide a return flux path for unbalanced loading associated with this primary connection. Since, it’s often desirable to provide auxiliary power from the grounding transformer secondary winding, this benefit can sway the end user to specify a 2 winding grounding transformer in lieu of a Zig-Zag connection. The current trend in Wind Farm designs is toward the Wye connected primary with a delta secondary.
It’s important to understand that both Zig-Zag and 2 winding grounding transformers can be provided with the ability to provide auxiliary power, and this can be either a Wye or Delta connected load.
A solidly grounded system using a grounding transformer offers many safety improvements over an ungrounded system, however, the grounding transformer alone lacks the current limiting ability of a resistive grounding system. For this reason, neutral ground resisters are often used in conjunction with the grounding transformer to limit neutral ground fault current magnitude. Their ohmic values should be specified to allow high enough ground fault current flow to permit reliable operation of the protective relaying equipment, but low enough to limit thermal damage.
How to specify a grounding transformer
The basic parameters required for quoting a grounding transformer are…..
❖ Primary voltage
❖ Rated KVA or continuous primary phase current
❖ Continuous Neutral current
❖ Available neutral fault current and duration
❖ Impedance as a % or as an ohms/phase value
❖ Primary winding connection
❖ Secondary connection. If applicable
❖ Basic overall construction features
❖ Neutral ground resisters if required
This is the system voltage to which the grounded winding is to be connected. Don’t forget to specify the BIL also. In some cases the BIL will be dictated by equipment considerations, such as 150 kV BIL ratings on 34500 volt wind farms because of the limitation on dead front connectors.
Because the grounding transformer is normally a short time device, its size and cost are less when compared with a continuous duty transformer of equal kVA rating. For this reason, grounding transformer are often not sized by “KVA” but by their continuous and short time current ratings. Regardless of how you rate it, the grounding transformer must be sized to carry the rated continuous primary phase current without exceeding its temperature rating. This load includes the magnetizing current of the core, the capacitive charging current for the cables, and any auxiliary load if applicable. The higher this value, the larger and more costly the transformer will be. Typical continuous current values can be as low as 5 amps to as high as a few hundred. Be sure to include any auxiliary loading requirements.
Continuous Neutral Current
The continuous neutral current is defined as three times the phase 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. (Note: see comment below next section to tie this point in.)
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 a 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 seconds). 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.
Most often, it is the fault duty and duration that will establish the size of the grounding transformer, but it may also be established by the continuous current rating. Thus it is
important to provide both pieces of information.
The impedance can be expressed as a percentage or as an ohmic 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.
Primary winding connection
Specify the type of primary connection, either Zig-Zag or grounded Wye.
Specify the secondary voltage and connection when applicable.
Specify the size of auxiliary loading to be connected for either Zn or Wye connected primary windings.
If the option is to have a two winding transformer with no secondary load, advise if the delta winding can be “buried” (that is not brought out) or if only one bushing is to be brought out for grounding to the tank or testing.
Neutral Ground Resisters
The rated voltage of the NGR should be equal to the line to ground
voltage of the grounding transformer. The current rating and duration should match the
grounding transformer ratings. Remember to set the current rating high enough to be above the cable charging current and grounding transformer magnetizing current.