ZCT setting in ungrounded systems

[charz]

How do you set the current setting coming from the ZCT? Does ZCT produce any secondary current at all?
If it is a single feeder in a power system (against the radial) the ZCT balances the charging current 3Ic in case of single phase to ground fault and produces zero flux. On the other hand, in case of radial feeders there would be some small current from the ZCT? Has my understanding right?

 

[7anoter4]

ZCT current transformer is not proper for ungrounded system. See[for instance]:
Ground Fault Protection for an Ungrounded System

http://www.ece.mtu.edu/faculty/bamork/EE5223/EE5223TermProj_Ex3.pdf

https://cdn.selinc.com/assets/Liter...roundFaults_NF_20061010.pdf?v=20170217-161017

GROUND FAULT PROTECTION ON UNGROUNDED AND HIGH RESISTANCE GROUNDED SYSTEMS APPLICATION GUIDE

http://www.postglover.com/download/high-resistance-grounding-application-guide/

 

[Parchie]

@charz,
The trick there is to wire the shielding earth wire back through the ZCT. Keep in mind that any earth wire should never pass through the ZCT. That way the summation of the currents flowing on the phase conductors in the ZCT does not include the ground currents flowing in the shielding screen.

 

[charz]

My question is how to make the ZCT sense the fault current even if it is very small

 

[waross]

Use three CTs or three PTs connected in grounded wye.
Secondaries connected in broken delta. (Open delta in IEC land?)
The CTs will show the unbalanced current to ground.
You may use a loading resistor to convert the current output into a voltage output.
The PTs will show any event that causes the voltage to ground of any phase to drop.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[charz]

Hi Waross,

Isn't using residual connection of 3 CT's same as using ZCT?

 

[krisys]

charz (Electrical)

It looks like the application of protection scheme is not appropriate to the system, i.e; ungrounded system.

Detection of earth fault becomes difficult using a ZCT and a non-directional relay. The capacitive current introduces some a 90° phase shift. This confuses many people. I would suggest you to go with the time tested ground fault detection and protection scheme for the ungrounded system.

At least in the IEC world, the ungrounded systems are now becoming rare and used only in some special cases.

 

[stevenal]

Great illustration, which shows an obvious answer. Move the CT so it lies between the cable capacitance and the fault. Since this may be a bit impractical, a voltage based scheme may be in order.

 

[Fuse4]

For ungrounded systems with only 1 feeder is true that the ZCT (window transformer) do not see earth fault current because the current of the faulted phase is the sum of the other phases (opossite direction) and always the ZCT see zero amper. (IEEE papaer "Considerations for ground fault protection in medium-voltage industrial and cogeneration systems D.J. Love and N. Hashemi)

For the small network can use the 3V0 element for detection (PTs connected in grounded wye indicated by Waross).
The phase to earth capacitive charging produces more high values of residual voltage (3V0) (opposite to big network that for the same fault type have low values of 3V0)