SHUNT RESISTORS IN BUS BAR PROTECTION

[arjun_athi]

Dear All,

Is there any special reason of using Shunt resistors with Metrosil in parallel (Non Linear Resistors) in the bus bar protection relay schemes, though series resistance are connected in the system?

Most of the cases we are using metrosil in parallel with series resitors on the bus bar protection schemes...

But some cases, shunt resistors in parallel with metrosils are used along with series resistors in the schemes.....

Please explain is there any reason for using such shunt resistors in bus bar protection?

Regards,

Alex @ Arjun...

 

[LMAO]

Dear All,

Is there any special reason of using Shunt resistors with Metrosil in parallel (Non Linear Resistors) in the bus bar protection relay schemes, though series resistance are connected in the system?

Most of the cases we are using metrosil in parallel with series resitors on the bus bar protection schemes...

But some cases, shunt resistors in parallel with metrosils are used along with series resistors in the schemes.....

Please explain is there any reason for using such shunt resistors in bus bar protection?

Regards,

Alex @ Arjun...

Are you talking about MOVs (metal oxide varistors) used as SPD (surge protection device)? They are used for transient over-voltage protection.

 

[petersonra]

Dear All,

Is there any special reason of using Shunt resistors with Metrosil in parallel (Non Linear Resistors) in the bus bar protection relay schemes, though series resistance are connected in the system?

Most of the cases we are using metrosil in parallel with series resitors on the bus bar protection schemes...

But some cases, shunt resistors in parallel with metrosils are used along with series resistors in the schemes.....

Please explain is there any reason for using such shunt resistors in bus bar protection?

Regards,

Alex @ Arjun...

if you were to put resistors in parallel with the bus bars they would likely get pretty warm unless they were very high resistance.

 

[Julius Right]

See:
http://www.metrosil.com/applications/relay2

 

[Haji]

But some cases, shunt resistors in parallel with metrosils are used along with series resistors in the schemes.....

Please explain is there any reason for using such shunt resistors in bus bar protection?

Regards,

Alex @ Arjun...

Check the setting of your relay. It would be set more than rated current in the bus bar. So if one of the two CT's of the high impedance differential protection is open circuited, there would not be much current in the secondary of the other CT to operate to trip the relay and so to provide sufficient current for operation of relay in such cases, the shunt resistor is provided.

 

[Haji]

Oh! Sorry. Actually your relay current setting might be lower than the rated current of the bus bar. So if one of the two CT's of the high impedance differential protection is open circuited, there would enough current in the secondary of the other CT to operate to trip the relay and so to bypass the current through the relay coil to prevent relay operation in such cases, the shunt resistor is provided.

 

[Bugman1400]

LMAO is correct in that this product is a type of MOV used to clamp the high voltages that are seen on the terminals of a high-impedance bus differential relay. Basically, the relay measures voltage instead of the typical current to protect the bus against an internal bus fault. The relay is supposed to avoid tripping for faults that are outside the bus zone. When all the CTs associated with the bus are paralleled together and wired to the bus diff relay, the resulting voltage can be greater than 1000V during a fault. The MOVs help clamp this voltage to a manageable level. A calculation must be done to determine how many joules the MOV must shunt to provide the over-voltage protection. Otherwise, the MOV could burn up. Sometimes, two MOVs are wired in parallel on each phase to increase the joules rating.

Please read the 587Z_ApplicationGuidelines_SZ-DC_20090323 document found on the SEL website for details that explain this concept. This is an excellent document.