Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations KootK on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

CT Type for Earthing Transformer Protection

Status
Not open for further replies.

KJvR

Electrical
Feb 20, 2006
56
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.
 
Replies continue below

Recommended for you

KJvR:

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?


[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376
 
Hi Ralph,

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.
 
KJvR

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?




[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376
 
Also:

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.

[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376
 
I discarded the idea of a type of REF on the windings based on the cost of an earthing transformer - actual continuous rating is only 750kVA / 11kV. Will this type of protection provide the same sensitivity as the standard REF used to protect transformer windings.
 
The differential scheme was just a thought - we use it on a combined system. (zig-zag solidly earthed + transformer)
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?




[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376
 
KJvR,

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."




 
Yikes!

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?



[red]Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon[/red]

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376
 
Ralph, the Z-Tranformer is connected to a 11kV bus fed by a multiple of generator transformers with delta 11kV windings. The earthing transformer can be individually switched but is not required as the generators must be tripped if a fault is detected on the Z-Transformer (only earthing point of the network).

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.
 
At least, theoretically, in case of an external single phase to ground fault, only Io current shall circulate in the three phases of the Z-transformer and then no current to flow outside of the delta connected CT´s, towards O/C relays.

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.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor