VT Fuse Supervision (function)

Some Protections are voltage sensitive such as Impedance Protection (21) i.e. V/I. When a fault occurs on a line or a generator, voltage V is reduced and at the same time (note the word same) current is increased and Impedance Protection operates as the V to I ratio is reduced and the Impedance Z lies in the range of tripping zone. Let us suppose a case in which fuse or miniature circuit breaker MCB of the PT is blown or tripped. The relay will sense zero voltage i.e. 

V = 0

Which implies that Z= V/I = 0.

VT Fuse Failure Protection is meant to block unwanted tripping and this function monitors Zero and Negative Phase Sequence currents (I2> and I0>), phase voltage V<, Pole status, Residual voltage Vn, change in the line current delta-I> and magnitude of the line current I>. If zero and negative phase sequence current are below the set value, any pole is not dead, line current is less than 2.5 times normal and the voltage is dropped than it means that it is healthy situation of the line and the MCB/ Fuse is tripped/ blown and thus Impedance Protection is blocked to avoid unwanted tripping. Another way is to compare I current vs V voltage i.e if phase voltage V< is suddenly dropped but there is no change in I> or less than setting value than fuse supervision blocks the tripping as its the case of PT failure. 

FFP is DETECTED if all are fulfilled:

{

1. Residual voltage Vn> is greater than setting value of 0.75V-nom

2. I2> is NOT more than setting value

3. I0> is NOT more than setting value

4. I is NOTgreater than 2.5 I-nom

5. Any pole is NOT dead

6. delta-I> is NOT more than setting value 

7. Phase voltage V< is dropped ie less than setting value

} 

the detection of the vVT fuse failure is sent to announce through a some define delay. The same signal of fuse failure protection is ORed with normal closed Aux-contact of the MCB or Fuse Blown Signal through OPTO.

FFP is reset when either the network voltage is normalized OR when the main circuit breaker is open AND all poles are dead.