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elecsun (Electrical) (OP)
3 May 05 8:31
Presently we are upgrading one of the feeder and connecting 4 Nos. 3CoreX300 Sq.MM cables, so we have problem of installling core balance CT.Whether we can install core balance CT in one of the cable so that it will serve the purpose of zero sequence earth fault protection?What about residual type of earth fault connection using O/C CT's in resdual format(Advnatage and deisadvantage over core balance ct.Experts guidence required.....
elecsun (Electrical) (OP)
3 May 05 8:37
Addition to baove question:We have solidly grounded system of earthing.
Helpful Member!  DiscoP (Electrical)
3 May 05 18:19
A residual connection means that some of the current that should go through the earth fault protection relay, is shunted away to the three CTs as magnetising current. This means that the current the relay measures is less than what the actual primary fault current is, so the protection is desensitised.
The amount of current shunted, depends on the Voltage that the CT needs to produce - so if you have a high burden relay, this will be less sensitive for earth faults.
The advantage of the core balance CT is that there is only one CT to magnetise, so there is more of the primary current going to the relay - therefore it is more sensitive.
The disadvantage of the Core Balance CT is you have another CT to install and care is required to earth the sheath on the line side of the CT.
RalphChristie (Electrical)
4 May 05 2:25
On which voltage do you operate? Do your system mainly use cables or also overhead conductors?

Earth fault protection using the residual connection:
Use an earth-fault relay with overcurrent-relays. (If you have overcurrent protection, you can easily add earth fault protection) An earth fault relay responds to the residual current in the system, which exists if there is a phase to earth fault. Normally the setting is between 10% and 40% of full load current. Also, like DiscoP noted, especially on electromechanical relays, the relay can be less sensitive on lower settings, due to the burden it puts on the CTs during fault conditions. (For electromechanical relays, the lower the setting, the higher the impedance of the relay) Another factor is the possible predominance of a third harmonic component in the residual current even under quiescent conditions. Third harmonics appear as zero sequence currents and could cause mal-operation of the relay. If the setting is to low, you can find a lot of nuisance trips.

Sensitive earth fault protection:
Used mainly on systems with high-impedance neutral earthing devices. The load current demands high ratio CTs, while the neutral current has been limited to a low value for safety reasons. Normal earth fault protection using the residual connection would be unable to detect earth faults, sensitive earth fault protection should then be considered. Also, on overhead rural distribution systems it is possible for high resistance earth faults to occur, especially if a conductor breaks and falls on very dry ground. This presents a hazard to human live and livestock if left undetected for any lengh of time. The setting of this relays can be very low, down to 1% and even lower. Use of core-balance CTs many times essential. The problem sometimes is to take all three phases through the window of the CT, it is especially a problem on bigger size cables.
raghun (Electrical)
4 May 05 3:13
With four nos 3cx300sqmm cables for the feeder, finding a single CBCT that can admit all the cables through its window may be impractical, to say the least. Your idea of passing only one cable through the CBCT window seems interesting. May be it will work too (if you take care of the sensitivity as only 1/4 the fault current is seen by the CT during faults). I would like to see if some one has experience with such a method!! The alternative that is widely practiced under such circumstances is use of Residual connection.

raghunath_n00@rediffmail.com

elecsun (Electrical) (OP)
8 May 05 0:00
Mr.RalphChristi -Thanks for your resposne.Our proposed feeder modificatin voltage level is 13.8kV,60Hz system.Yes we have overhead lines in the downstream side as well.Bascically these overhead lines are connected to wellhead ESP motors (Oil field electrifications).We plan to provide latest model(GE) multi function feeder protection relay for O/C and earthfault protection(Iknnow in these relays there are provision to provide both residual as well as sensitive earth fault protection).
RalphChristie (Electrical)
9 May 05 5:24
During most ground faults on solidly grounded systems the current is usually high. The exception can be if a overhead conductor breaks and falls on a surface with a high resistance. If there is not a load connected on that part of the line or if the load is very low, this fault can go undetected.

1.You can connect a ring-CT to SEF-input. (sensitive earth fault element) Like rraghunath stated, you'll have a hard time finding this size ring-CT to accomodate your cables.

2.You can connect the SEF-input in series with the ground-fault element. Eskom (local South African electricity supplier) does it on a lot of their rural feeders. The setting on the ground-fault element is like normal (usually 10% setting with an IDMT-curve) while the SEF-element's setting is very low with a direct trip time. (usually 6 seconds) To obtain the lowest possible SEF-setting can be sometimes difficult due to some nuisance trips, and you have to play around until you find a satisfactory setting.

Hope it helps.

Regards
Ralph
raisinbran (Electrical)
9 May 05 6:32
Elecsun,
Your initial proposal of passing one cable through a CT will probably not work because of the way that the four "Phase A" conductors share current.  Due to slight differences in length and resistance, the four conductors making up one phase will not share 25-25-25-25 in all cables.  The same cable will probably have different currents is all three phases.  These differences in current sharing will show up as a ground fault when there is not a problem.  A residual method withe the four "phase A" wires passing thru one CT, and the four "Phase B" wires passine thru another CT, etc. should give satisfactory results.
Regards,
Raisinbran  
hmchi (Electrical)
21 May 05 18:26
ABB has long had a solution to the need for large opening core balance CT [or current sensors] --- been there since 1974 !  I used to sell tons of it ...

See page 6 of http://library.abb.com/GLOBAL/SCOT/scot229.NSF/VerityDisplay/44D954733A5AEC1885256EDE004C1FE7/$File/71172a.pdf
sksimonsen (Specifier/Regulator)
29 Jun 05 20:52
My apologies for being so tardy, I have just returned from holiday and am starting to get caught up.

Primary conductors should be grouped together and fed through the current transformer aperture. It is essential that each conductor passes through the device in the same direction. Each phase conductor (and neutral if present) must pass through the current transformer. The current transformers sum the currents flowing into and back from the load. Ideally, the load will have no leakage current, so current flow through the CT will completely cancel out. For example, 100 Amps flowing into load and 97 Amps flowing back provides an output of 3 Amps.
The equipment grounding conductor must always bypass the current transformer. The connections between the current transformer and protector should be kept as short as possible to minimize signal noise. For best results, use screened cable, with the screen grounded at the protector.

The above information is from a Crompton Instrument brochure which can be viewed at http://www.crompton-instruments.com/protectors/p_cbt_94f.pdf

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