differential rely

[bejoyengineer]

Hello ,
I am confused a bit with these questions

1. What happens in differential protection , if CT reaches it?s knee voltage point? When do such cases occur and what is the solution for stopping the relay from picking up in such cases?

++++++++++++++++++Relay++++++++++++++++++++++++++++++++++++

 

[ohmhead]

Well were not a expert but if you pass the knee point which is the voltage induced in that ct by the pri conductor they have a rating of knee point when designed . Its that saturated flux Thats beyond what the ct needs its not doing you any good so you must limit voltage .
Drops in voltage and spikes in voltage can effect trips .

It may not trip relay or it may trip it all the time .
Adjustable resistor in series with ct circuit to relay .

I ve seen folks use series resistors on current relays from ct this limits false trips but lets see what others have to say .

Because i just run pipe all day !

 

[Hameedulla-Ekhlas]

Hello ,
I am confused a bit with these questions

1. What happens in differential protection , if CT reaches it?s knee voltage point? When do such cases occur and what is the solution for stopping the relay from picking up in such cases?

++++++++++++++++++Relay++++++++++++++++++++++++++++++++++++

CT and VTs are the sensors for the relay.
CT and VT function like ?ears' and the ?eyes' of the protection system. They listen to and observe all happening in the external world. Relay itself is the brain which processes these signals and issues decision commands implemented by circuit breakers, alarms etc. Clearly, quality of the relaying decision depends upon ?faithful' reproduction on the secondary side of the transformer.

when the CT reaches it's knee voltage point there is problem of going the CT to saturation point. One of the major problems faced by the protection systems engineer is the saturation of CT on large ac currents and dc offset current present during the transient. When the CT is saturated, primary current source cannot be faithfully reflected to the secondary side. In other words. Also, the magnetizing impedance falls down during saturation. Then the transformer behaves more like an air core device, with negligible coupling between the primary and secondary winding. The high reluctance due to the air path implies that the magnetizing impedance (inductance) falls down.

So, the main engineering job is to not let the CT to go in saturation point in calculation and design work.

 

[SG-1]

I am thinking of a bus differential circuit. If one CT reaches the knee point the differential relay may call for a trip to clear the zone. This only happens when fault current is passing through the differential zone, no fault in the zone itself.

The prevention is coordination with the rest of the system, by choosing CTs with the proper knee voltage ( C200 vs C400 etc... ) & especially by the differential relay setting.

 

[erickench]

Differential protection is usually used for transformers but it can be used for feeders too. What happens is that the conductor or transformer plus conductor between the CTs is the protection zone and the protection circuit consist's of two CTs in parallel with a relay. Let's say you have a feeder, if the current flowing through both CTs is the same then there would be a balance and no current would flow through the protection relay. If there is a fault between the CTs then current flowing through the CTs would not be equal and so a current would flow through the relay and then trip the CB. For the feeder the number of turns for the two CTs would be the same. For the transformer the ratio of the turns of the two CTs would be the same as the turns ratio of the transformer that you are protecting.