At one of our substation, we have two 40MVA, 88/33kV, star/delta transformers with 8.4% impedance voltage. These units are earthed through zigzag earthing transformers on 33kV side. Recently, we had a fault on 33kV side of one of these transformers (33kV surge diverter shuttering in the process). Investigations and tests revealed that the earthing transformer on this bay got burnt during the fault. The earthing transformers are rated for 6A continous and 300A short time for 10s. Our feeling is that these transformers could have been underspecified. We want to do fresh specifications for replacement transformers. Could anyone with experience in this subject please help me with information on how to specify an earthing transformer especially with regard to current ratings and KVA rating?
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Specification of Neutral Earthing Transformers 3
- Thread starter vidima
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The first step is to decide how much ground fault current you want to allow.
If you plan to solidy ground the transformer, you will need to specify the grounding transformer impedance (normally given in **ohms** for grounding transformers)as well as how long the transformer must be able to handle this (normally 10 seconds). For a solidly-grounded system, the grounding transformer impedance largely determines the ground fault current.
If you want to resistance-ground the system, then you will need to specify the maximum fault current along with the duration.
300A sounds very low for a solidly-grounded system.
The primary determination of transformer size will be the short-circuit current it has to produce and the duration. You do not need to specify a kVA rating. Transformer manufacturer needs to know:
Operating voltage/BIL
Desired impedance in ohms
Maximum short circuit current
Short circuit current duration
Maximum continuous current
At the risk of running this thought into the ground, keep in mind that a conventional delta-wye transformer can also be used as a grounding transformer. Zig-zag transformers are somewhat hard to find, especially from larger transformer suppliers.
Good luck.
If you plan to solidy ground the transformer, you will need to specify the grounding transformer impedance (normally given in **ohms** for grounding transformers)as well as how long the transformer must be able to handle this (normally 10 seconds). For a solidly-grounded system, the grounding transformer impedance largely determines the ground fault current.
If you want to resistance-ground the system, then you will need to specify the maximum fault current along with the duration.
300A sounds very low for a solidly-grounded system.
The primary determination of transformer size will be the short-circuit current it has to produce and the duration. You do not need to specify a kVA rating. Transformer manufacturer needs to know:
Operating voltage/BIL
Desired impedance in ohms
Maximum short circuit current
Short circuit current duration
Maximum continuous current
At the risk of running this thought into the ground, keep in mind that a conventional delta-wye transformer can also be used as a grounding transformer. Zig-zag transformers are somewhat hard to find, especially from larger transformer suppliers.
Good luck.
dpc gave a good overview. One clarification—a conventional "grounded"-wye/delta transformer can be used but often rated for shorter-than-continuous duty if it can be reasonably assure that relaying will operate to isolate the faulted circuit. For resistance grounding, the MV resistor can be placed at the grounding-transformer wye point. The delta winding can be left floating, and one CT in the loop used as a polarizing quantity.
An alternative is application of an LV resistor inserted into the delta winding—a so-called broken-delta [not open-] connection. Reference IEEE C62.92.4.
An alternative is application of an LV resistor inserted into the delta winding—a so-called broken-delta [not open-] connection. Reference IEEE C62.92.4.
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- Thread starter
- #4
Thanks for the useful informatiion. As additional information, the short circuit current on the 33kV busbar is 3300A. The 33kV bus is feeding the distribution network. Our system is solidly grounded. I will be more confortable if you took me step by step through the specification/sizing process. Meanwhile I am trying to get hold of the suggested reference standard. Thanks again.
I'm not clear on the configuration of the 33 kV system. If it is already solidly-grounded and you have 3300 A of available fault current (line-to-ground?)I am not sure why you need a grounding transformer.
The purpose of the grounding transformer is to establish a ground reference for your neutral. To do this, it must have a fairly low impedance to ground under ground fault conditions.
The grounding transformer impedance is determined by engineering judgement and the nature of your 33 kV system. If it is a small, compact system, you can get by with a higher impedance (maybe quite high). If you are trying to establish a ground for a long utility distribution feeder that is 20 miles long, then you will want a solidly-grounded system with relatively low impedance.
The purpose of the grounding transformer is to establish a ground reference for your neutral. To do this, it must have a fairly low impedance to ground under ground fault conditions.
The grounding transformer impedance is determined by engineering judgement and the nature of your 33 kV system. If it is a small, compact system, you can get by with a higher impedance (maybe quite high). If you are trying to establish a ground for a long utility distribution feeder that is 20 miles long, then you will want a solidly-grounded system with relatively low impedance.
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