Earth fault Indicator (EFI) 1

[Mr zezo]

IN (delta system) how RMU earth fault indicator operates , and what is the function of the used medium voltage cable toroid (only one toroid allaround the 3core cable), does it only measures the surrounding or the sum of magnetic field of the 3 ph or what it actually sensing? and in another types of EFI it uses 3 toroids for each ph and in another types which used foe EFI+SC it uses the combination 3toroids +allover toroid, what is the difference?
and for medium voltage cable does the earth fault indicator with CBCT operate with ph to ph faults and unbalance conditions such as open conductor (like only one fuse is open so there is only 2 ph operating so there will be unbalance)?
what is the difference between earth leakage relay and earth fault relay and earth fault indicator?thanks

 

[Kiribanda]

During an earth fault the zero seq current has to be captured to activate an earth fault protection relay.
For that CTs can ban be used in several ways.
1) CBCT/ ZCT: This a single toroidal CT encircling all phase conductors (in case of 4-wire it will encircle all phase conductors plus the neutral excluding any earth conductor).
2) Three CTs in Nicholson connection: The three CTs are connected and the residual connection is passing through an earth fault relay.
3) Earth leakage relay: This is used for human protection with a low setting typically 30mA. It needs a ZBT.
4) Earth fault relay: This is used in a coordinated earth fault protection scheme of mains & feeders which needs high current settings.
Hence 3CT Nicholson connection is used here to capture the zero seq current during an earth fault relay.

 

[che12345]

I have the following opinion, for your consideration.
1. The question is " In (delta system) how RMU earth fault indicator operates..."[/color]. I take it that the delta system is NOT earthed. Therefore, no neutral.
2. In a delta system, a single line dead-short to the earth causes no problem. No over-current and the vector sum of three lines equal to zero. Attention: if another line, a second dead-short occurs, over-current operates. The vector sum of three lines is zero.
3. This configuration is widely practised on board of vessels/ships, where insulation monitor is mandatory. Note: when only one line is dead-short to the earth/hull, NO over-current in any lines. Therefore, immediate tripping is NOT required.
Che Kuan Yau (Singapore)

 

[Mr zezo]

thanks for your response kirbanda,
but do you mean that the EFI ct will capture the zero sequence current only as in all cases the sum of the magentic field is just zero but at earth fault the sum of the magnetic field is not a zero?and hence can the EFI operates at high unbalance condition like open conductor! or not?
what is the difference bet CBCT/ ZCT and ZBT? is it its sensitivity only or what?
che12345, its a delta system with earthing transformer,thanks for your response.

 

[waross]

The CT or CTs are arranged so that under normal conditions the sum of the currents is zero.
In the event of current to ground, the ground current by-passes the CT(s) and the CT(s) register the unbalanced current.
The unbalance may be in the range of milli-Amps, for human protection) or it may be a major fault current, limited by the supply impedance.
In the instance of a grounding transformer, the zero sequence CT(s) must be downstream of the grounding transformer.
Consider a loop formed by the grounding transformer, the conductors to the load, an inadvertent connection to ground and back to the grounding transformer via the ground.
This ground current may be detected by a CT anywhere on the loop.
The CT may be connected to monitor current in the grounding conductor of the grounding transformer.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kiribanda]

CBCT: Core Balanced CT
ZCT: Zero seq CT
All above works on spill over current (residual current) theory. When the three phase conductors are encircled, during a single-ph to earth fault
there is s spill over current which is the zero seq. current which can be used for tripping a relay.
230V, 30A, 30mA current operated earth leakage circuit breaker (ELCB/ RCCB) used for human protection in the IEC world or a Ground Fault Circuit Interrupter
(GFCI)in the ANSI world are working on spill over current principle.
It looks like you need to read or some coaching about basics of ground fault detection.

 

[Mr zezo]

the moral of the question is to know the way of operation of the efi device it self and its ct , meaning how it detect beacuse there was afult occurs on mv system which consits of number of kiosks and we reconnect the feeder and its ok, but there was a blown mv fuse of incoming cable , and i noticed that the efi didnot work .and an opinion was that it shouldnot work as there wasnot an earth fault , but i think that it should operate as there is unbalabce of the cable phases . noting that the feeder ampere was 200A,and the efi is adjusted to 20 A.
so what is your opinion should efi operate or not?

thanks.

 

[Kiribanda]

Could you please upload your Single Line Protection dwg and mark where the fault has occurred?
Otherwise for me it is difficult to answer your questions.

 

[davidbeach]

If all the current stayed in the conductors then there's nothing for it to detect. Currents in two phases involved in a phase-phase fault still sums to zero.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Mr zezo]

SLD, it is nearly like that .
green leds are EFI.
it is open conductor fault means ampere of phases was 200A;0A;200A.
thanks

 

[davidbeach]

But the two 200As were 180degrees apart and summed to zero.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Mr zezo]

davidbeach, why 180 ? the angle bet each phase should be 120 Not 180 shouldnot it?
thanks.

 

[davidbeach]

It was 120 when there were three, but it can’t be 120 with only two wires. Where would the rest of it flow? 120 degrees between current angles requires 3 conductors and when the fuse opens you have only two. In two conductors the currents are 180 apart.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Kiribanda]

You have not marked the transformer connection symbols.
Is your 22kV system is an UNGROUNDED DELTA system or otherwise?

 

[Mr zezo]

as i said up its a delta system with earthing transformer;means the 22 kv is delta and there is a nother grounding transformer used for grounding the delta system.

 

[Kiribanda]

Your 22kV system cannot be considered as a DELTA ungrounded system because 22kV system is having a grounding transformer.
During a single line to ground,
1) The Single toroidal CT encircling the 3-core cable will be sensing the zero seq current and energize the connected ground fault relay
2) If a relay is connected to the residual connection of the three toroids then it will energize its connected relay

 

[Mr zezo]

that is not my question !
the fault here is open conductor not earth fault , should EFI work with open conductor fault or not?
thanks.

 

[Kiribanda]

No. Because during an open conductor fault no zero seq components. Therefore an EFI cannot work
unless there is something else triggering the EFI.

 

[Mr zezo]

sorry , sequence components are imaginary , so what do you really mean by there is no zero sequence here? do you agree with davidbeach that the 2 ph will be 180 apart so the magnetic sum will be zero! or what?
thanks.

 

[Kiribanda]

Yes