High Voltage in Hi-Z Protection ?

[YERUK123]

Hi All,

In Hi-Z diff Protn we calculate Vpeak and if its greater than the Impulse Voltage rating of the TRM we go for Non-linear Resistor.
I wanted to the physics behind , How High voltage is generated in Hi-Z differential Protection during internal faults? Can anyone explain?

Thank you in advance.
Yeruk

 

[jghrist]

The CTs are connected in parallel and the sum of the currents goes through a resistor. If there is an internal fault, the source CT currents add together and the current through the resistor causes a voltage. If there is a through fault, the current in the source CTs equals the current in the CT of the faulted line and the sum of the currents is zero, so there is no voltage across the resistor.

 

[YERUK123]

Hi Jghrist,

I agree partially . when there is internal fault due to imbalance of current Voltage is developed but why is it "High after the instance of Fault" and later settles. in other words There is an Impulse later it settles ? I am not completely convinced.

Thanks in advance
Yeruk

 

[purplepete]

Work from:
An internal fault in a switchboard fitted with bus zone prot (HiZ)
-maximum fault current available at the switchboard(amps)
-causes x amps in the CT summated secondary circuit because it's coming in but not going out.
The x amps is flowing through the resistor and the prot relay.
x amps times whatever ohms is volts (lots usually).
hence the metrosil or whoever''s Non-linear resistor to clamp the volts

 

[davidbeach]

Typical installation also include a lockout relay that bypasses the resistor once the relay issues its trip, so the high voltage only lasts from fault inception to relay trip.