Grounding Transformer FAQs

By November 8, 2013Articles

1. What is the purpose for a grounding transformer

 

Simply put, a grounding transformer is used to provide a ground path to either an ungrounded “Y” or a delta connected system and are used to….

  • Provide a relatively low impedance path to ground, thereby maintaining the system neutral at or near ground potential.
  • Limit the magnitude of transient over voltages when re-striking ground faults occur.
  • Provide a source of ground fault current during line to ground faults.
  • Permit the connection of phase to neutral loads is 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.

 

2. What is the kVA rating for my grounding transformer?

 

Unless you are using the grounding transformer 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.

 

3. What current rating do I need to order my grounding transformer?

 

You need to know the available neutral 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.

 

4. What is meant by “continuous current”? Are there guidelines for this?

 

You will also be asked to supply the continuous current. You can choose to provide the continuous neutral current or the continuous phase 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. When the continuous current is not known, ANSI/IEEE Std. 32-Reaffirmed 1990 , provides a guideline based on the fault current magnitude. If the value is not specified, the designer will assume the continuous current to be 3% of the short time fault current (based on a 10 second rating)

 

5. Why is the neutral current 3 times the phase current.

 

When an ungrounded system experiences a ground fault event, the grounding transformer provides the return path for the fault currents. The transformer, “sees” this fault current as a zero sequence fault current, meaning it occurs on all three phase simultaneously. In a three phase transformer with equal impedances to ground on all three legs, the current will divide and flow equally in all three phases simultaneously with 1/3 the fault current in each.

 

6. Why is the impedance so critical in my grounding transformer?

 

When current flows in the grounding transformer windings a voltage will be developed by the well known formula ( E = IR) where the resistive component is the impedance. Clearly this is different for every location and application, however we can say that because of the magnitude associated with fault currents, if too high a value is given for the impedance, during the fault the resulting voltage can exceed design limits. It is important to remember that one function of the grounding transformer is to provide voltage support for the faulted leg, thus holding that leg above ground and limiting “neutral shift”. In all cases 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.

7. What is a Zig-Zag grounding transformer? Why would I use one?

 

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.

 

8. Can I also provide auxiliary power from my grounding transformer?

 

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.

 

9. What data do I need to know in order to size and order a grounding transformer?

 

Primary voltage – 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.

 

Rated KVA – 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 rater it, the grounding transformer must be sized to carry the rated continuous primary phase current without exceeding its temperature limit. 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 a 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 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 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.

 

Impedance – 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.

 

Secondary connection – 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.

 

Basic overall construction features – note the following features as they apply to each transformer….

  • Compartmental Padmount transformer with integral tamperproof compartment or substation design?
  • Outdoor or indoor
  • Fluid type- mineral oil, silicone, Envirotemp FR3
  • Connectivity – dead front, live front, spade terminals, location of terminals – cover or sidewall , exposed or enclosed, etc
  • Temperature rise is assumed to be 65°C
  • Site elevation or environmental concerns
  • Special paint as required

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.

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