Zone Protection or Restricted earth fault (REF)Protection for Transformer

Zone Protection or Restricted earth fault (REF) Protection for Transformer.

This relay is operative only for the internal faults of the transformer and thus fast operating time can be achieved. An external fault on the star side will result in current flowing in the line CT of the affected phase and a balancing current in the neutral CT and current in the relay is zero and hence relay is stable. During an internal fault, the line current on the line CT gets reversed and hence relay operates. The arrangement of residually connected CTs on the delta side of a transformer is only sensitive to earth faults on the delta side because zero sequence currents are blocked by the delta winding.

For external faults, no current flows through REF unless a CT gets saturated. Hence minimum pickup current setting is adopted (10% or 20% In) on REF relay. Based on the fault current, the stabilizing resistor is set such that the relay will not operate for the external fault when a CT gets saturated. This relay operates only for internal earth faults, instantaneously.

Fault current for external fault If = 2500 A (assume)

·         C.T. Ratio (line and neutral) = 300/1 A

              2500

·         Secondary fault current = ------ = 8.33 A (Sec.)

             300

·         RCT = C.T. Resistance

·         TL = Lead Resistance = 7.41 Ohms/Km (2.5 sq mm Cu)

·         Voltage developed across CT (Saturated)

             (Vk) = If (RCT + 2RL)

            = 8.33 (5 + 3)

            = 66.64 Volts

·        Relay burden = 1 VA

·        Relay Operating Current = 0.2 A (Set value)

·        Relay Operating Current(VR)

             VR =  Relay burden/ Relay Operating Curren               
              = 1/0.2 = 5 Volts

             VK-VR 

             Stabilizing Resistor SR = ( VK-VR )/I Set

             = (66.64-5.0) /0.2

            = 308.2 Ohms

            Set SR = 310 Ohms

If the calculated value of SR exceeds the existing range, the current settings can be raised accordingly and arrived at suitable SR value.