Author: Engr. Jahangir Amir
PROPERTIES OF DIFFERENTIAL RELAYS:
- Use for power transformers protection
- It responds to vector differences (phase/magnitude) between two or more similar electrical quantities
- The difference flows through the operating coil during any abnormal fault conditions
- Differential relay asses only the amount of current flowing, when ‘I’ exceeds it gives out tripping signal.
- It is non directional relay
- It works on the basis of difference of current
- It differentiate between fault and healthy condition
- Differential Zone is between HV-CT to LV-CT
WORKING OF DIFFERENTIAL RELAYS:
- IT SHOULD OPERATE:
- It Should Operate If Current Is Increased From Its ‘Present Value’, Immediately
- It Should Operate When Fault in Differential Zone.
2. IT SHOULD NOT OPERATE:
- It should not operate (stable) on inrush current, when transformer is energized/switching
- It must not operate during through fault (fault outside the differential zone)
- It must not operate in any overload conditions
3. IT CAN CLEAR THE FOLLOWING FAULTS:
- Any sudden fault on the jumpers from 132 kV CT to breaker, lighting arrestors
- 132 or 11kV lighting arrestor may damage
- Any phase to phase or phase to earth short circuit faults within the body of the transformer
- There may be fault in 11 kV PT/CT or any incoming cables
TESTING OF DIFFERENTIAL RELAYS:
- PICK-UP TEST:
- For testing the performance of relay
- Test the tripping on setting values
2. SLOP or BIASING TEST (BALANCING):
Checking the balancing of differential relay
- Is It Operating On Zone Fault?
- Is It Stable (Not Operate) On Through Fault (Out Of Zone Fault)?
- Is It Stable On On-Load Tap Operation?
3. 2ND HARMONIC RESTRAINT TEST (INRUSH CURRENT):
- Check The Relay Stability On Inrush Current/ 2nd Harmonic
- When Power Transformer Is Energized, Inrush Current Produced, Due To Inrush Current 2nd Harmonic Produced In The Transformer.
4. BEHAVIOR ON INRUSH CURRENT:
- At the time of switching heavy inrush current flow into the primary of t/f, secondary side is still open and no current flow in it.
- It may be 6 times of rated current
- Bias winding provide protection from inrush current
5. HARMONIC RESTRAINED:
- This relay also provides a harmonic restraint features.
- During inrush currents several harmonic currents may also flow. (2nd to 7th ) Restrained Coil/Biasing Winding Is Used In Differential Relay For Preventing The Tripping On Inrush Current
TYPES OF DIFFERENTIAL RELAY:
- Simple Over Current Differential Relay
- %Age Biased Differential Relay
EXAMPLES OF DIFFERENT DIFFERENTIAL RELAYS:
- SIEMENS SIPROTEC, 7UT7250, 7UT5125,
- ABB SEAD346C
- GEC DMH,DTH,
- ASEA RXD3B4, RXDSD4,
- AEG RX4A
- BBC D21SE2