CT polarity for High impedance differential protection motor

[elecpl123]

Hi Guys,

Can any body give CT Polarity for a High impedance differential protection for HT Motors.CT polarity in the Line side of a motor and in the neutral side of a motor.

 

[lansford]

Look at the schematic drawing for the differential rely and wire it that way.
JIM

 

[elecpl123]

Hi Lansford,

In my schematic they have not shown the neutral side CT's
in line side P2 is towards motor winding,What will be the polarity of neutral side of the motor

 

[dpc]

You need to refer to an instruction book for the relay in question. There is no way to know for sure because it will be specific to the relay. There must be documentation on CT wiring if this is a new relay.

 

[ScottyUK]

Current 'in' to the P1 side of the CT is accepted to result in a current 'out' of the S1 terminal. Knowing how the secondaries are wired and that under normal conditions the relay current should be zero should be enough for you to work out the rest.


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If we learn from our mistakes I'm getting a great education!

 

[elecpl123]

Hi ScottyUK,

Here with i have attached the sketch diagram of my circuit can u give me the current flow during the healthy condition and also during the fault condition.relay used is spae010

 

[tsknan]

for all differential protections the circuit diagram is as shown,no dubt at all offcourse specific to relay,but the principle is same.

 

[slavag]

Hi.
What Tsknan saied.
Your connection is correct.
But you need relay documentation for right calculation of varistor and resistor and right secondary wiring dimention.
Best Regards.
Slava

 

[DiscoP]

The convention used in our organisation is the ct star points towards the zone of protection.

It is very easy to apply and it doesn't matter which way P1 and P2 are facing.

 

[elecpl123]

Hi Slavag,

With the above attached documentwhat is your opinion whether diff current flows in to the relay during healthy conditions of motor.

 

[slavag]

Hi Elecpl123.
Isn't important what is a connection.
Rules: P1 "in" current-S1 "out"current
P2 "in" current-S2 "out" current. Isn't important if it's star point of CT's.
You are protect a motor. Current is only "in" from line/bus
(OK, we don't take in account contribution of motor in case of external SC). In case of SC into a motor, current "in" from the line/bus, flow via line CT, but don't continiue flow via neutral CT (or partly only). This disbalance or diffirential current flow via varistor/resistor and make voltage on the protective relay terminals. Relay will trip.
Hope that help you.
Good Luck.
Slava

 

[DiscoP]

I added the current flow for a fault, and normal load (or external fault).

If current flows in P1, it flows out S1

 

[elecpl123]

Hi Guys,

i have the connection as per attacment and i also do the stability test and the result also found ok.i have done stability by applying 415 V supply to the motor for a short duration and stability is also found ok.the motor rated voltage is 6600V.Applying 415 v supply doing stability is right procedure,Any limitation in this procedure or any body can give any other method of doing stability test for motor