CT Type for Earthing Transformer Protection
CT Type for Earthing Transformer Protection
(OP)
I would appreciate any comment on the type of CT to be used for a Z-transformer (NEC / Earthing Transformer) protection scheme. The Earthing Transformer is connected to a generator bus and therefore needs separate winding protection. I will use delta connected CTs to prevent zero sequence currents going to the earthing transformer O/C protection relay (as recommended by most literature). However, I have seen an existing installation using a Class PX CT while our experienced Protection Engineer believe a Class 5P is O.K. My fear is that with the O/C protection set at a very low current (10%) of the CT nominal current, the unbalanced CTs might cause spill currents in the Delta during a single phase to earth fault and cause a false O/C trip of the earthing transformer protection. I can not find any literature giving a CT requirement on this arrangement. Is a Class PX ct a requirement or will a Class 5P ct be adequite. References to literature will also be appreciated.






RE: CT Type for Earthing Transformer Protection
Regarding the low pick-up setting (10%):
Actually you want to check for over-current conditions in the zig-zag trsf, but the CTs are to big?
and
Is the zig-zag trsf solidly earthed, or earthed through a resistance?
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RE: CT Type for Earthing Transformer Protection
Thanks for your reply. The problem is that going to low ratio will make it difficult to prevent the CTs not to saturate under fault conditions. I will use power frequency current only (prevent inrush current to cause trips because of the low setting) and can therefore not let the CT saturate even under 3 phase fault conditions.
The neutral of the NEC is solidly earthed. Earth fault current 1000A - 2500A (depending on the number of generators in service). Continuous current rating (Io = 27A per phase) with a CT ratio of 100/1.
I could not obtain any additional info and have therefore settled on a class 5P design as I could not convinced the more experienced guys. I would still appreciate any input should I ran into the same request in future.
RE: CT Type for Earthing Transformer Protection
I've checked through some of my protective relaying books, but the only place I could find some information was in IEEE Guide for Protective Relay Applications to Power Transformers, IEEE C37.91-1985. (sorry, I only do have this edition, there might be a newer one) This might be because zig-zag transformers are seldomly switched by themselves.
From the book:
When a grounding transformer with a low or no neutral impedance is used, a phase-to-ground fault is normally not allowed to persist as it might when the current is restricted to a very low value by a high neutral impedance. Therefore, the selection of a CT associated with the grounding transformer is more dependent on the pick-up of the ground relay than the rating of the grounding transformer. However, if a fault is allowed to persist then the CT-ratio must be selected with the continuous current in mind.
According to the figure it is for overcurrent protection. As noted in your first post, the CTs must be connected in delta.
Also:
a sudden pressure or fault-pressure relay should be considered to be the first line of protection, even though it may be marginal.
Regarding the type of CTs, in my opinion, I would say class 5P, (even class 10P) will be fine if you can use an O/C-unit and a highset-unit with it.
Harmonic restrained O/C relays may be used to prevent inadvertent tripping upon energizing.
Have you also considered a differential scheme, like REF-protection?
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RE: CT Type for Earthing Transformer Protection
Rather check for a CT with a high last number, like 5P20 or 10P20.
5P or 10P is just the Accuracy Class, while the last number represents the Accuracy Limit Factor. (ALF)
A 5P20 CT will be within 5% accurate at 20 times the rated current.
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RE: CT Type for Earthing Transformer Protection
RE: CT Type for Earthing Transformer Protection
REF and buchholtz trips were experienced once when a internal fault occured in one of the zig-zag transformers.
Will your zig-zag transformer be switched alone?
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RE: CT Type for Earthing Transformer Protection
REF protection is quite fast (in the order of 20 ms). It does provide added sensitivity, but I think in your scenario, you'd need an added CT input.
I agree with Ralph that 5P20 (or even 10P20) sufficient for OC or EF applications, and is standard industry practice.
Ralph,
Please clarify "REF and buchholtz trips were experienced once when a internal fault occured in one of the zig-zag transformers."
RE: CT Type for Earthing Transformer Protection
Sorry about that one, should always double-check, even more since English is not my first language.
What I actually wanted to say:
We use a lot of combined transformer and zig-zag transformer schemes. An internal fault occurred once in one of the zig-zag transformers, and it was only the REF and Buchholtz relays that operated. But, it is a combined system - no overcurrent CTs on the zig-zag transformer. You do not find it many times, because zig-zag transformers are seldomly switched on their own.
My question still remains: will the zig-zag transformer be switched alone?
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RE: CT Type for Earthing Transformer Protection
So I assume my fears for spilling current from the circulating zero sequence current in the delta connected CTs during a phase to ground fault in other parts of the network is unfound (even with a 0.1A O/C setting) and an higher accuracy CT is not required.
Ballenden, I aggree that 10P CT's are O.K. for normal O/C and E/F. I am a bid unsure in the delta connected CT scheme.
RE: CT Type for Earthing Transformer Protection
For single phase (or multiphase) faults in the Z-transformer, positive, negative and zero sequence currents shall flow into the transformer. Positive and negative sequence currents circulate to O/C relays detecting the internal fault.
To size your CT´s you could consider the X/R ratio of your power system at Z-transformer installation.