Zero sequence earth fault protection
Zero sequence earth fault protection
(OP)
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.....






RE: Zero sequence earth fault protection
RE: Zero sequence earth fault protection
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.
RE: Zero sequence earth fault protection
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.
RE: Zero sequence earth fault protection
raghunath_n00@rediffmail.com
RE: Zero sequence earth fault protection
RE: Zero sequence earth fault protection
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
RE: Zero sequence earth fault protection
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
RE: Zero sequence earth fault protection
See page 6 of http
RE: Zero sequence earth fault protection
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