This course covers Restricted Earth Fault (REF) protection and its installation, connections & schematics (for transformer, motor, generator, shunt reactor and autotransformer), calculations and protection requirements. You will learn the REF schematics, to draw primary and secondary currents and many other topics. An example of a real Simens 7UT6 REF relay is included.
The course consists of 19 lessons in 3h 14 min. total length.
It’s important to know that conventional earth fault protection using overcurrent elements fails to provide adequate protection for transformer windings. This applies particularly to a star-connected winding with an impedance-earthed neutral. This is where REF protection comes into the first place.
The following topics are covered in this training:
- Introduction to REF protection.
- Why it is required to have REF protection along with differential protection?
- Comparison of differential & REF protection.
- Requirement of phase, ratio, tap-changer compensation in transformer differential protection.
- Operating, restraining current, slope in differential protection.
- Comparison of overcurrent earth fault protection and restricted earth fault protection explained the working principle and limitations.
- Where REF protection can be installed?
- Zero sequence current philosophy in star connected system with isolated star point.
- Zero sequence current philosophy in star connected system with solidly / NGR grounded system.
- Why delta connected windings are required for the transformer?
- No formation of zero sequence current at phase-to-ground fault in delta connected windings.
- Why Zigzag transformers are used in power systems.
- Explained philosophy of power flow to source only.
- High impedance fault in a transformer.
- Understand simple REF scheme, draw primary and secondary currents.
- Current transformer dot notation.
- Current transformer P1, P2 notation at the primary side and S1 & S2 notation for secondary side.
- Current transformer star point formation at the secondary side of the circuit.
- The direction of current flow as per KCL.
- Star point grounding & its application.
- The scheme, current flow logic for REF relay at star-connected winding ( transformer, motor, generator).
- Can REF protection be installed at delta, isolated star point winding?
- Zone of REF protection.
- In Zone and out of Zone faults.
- Does REF protection operate at through fault?
- Does REF protection operate for phase to phase and three-phase faults?
- The current flow direction of phase to phase, three-phase faults, in the zone and out of zone faults.
- Which fault can not be sensed by differential protection?
- Importance of polarity of CT, if wrong then REF protection will operate at through fault.
- Comparison of high impedance vs low impedance REF protection, advantages & disadvantages.
- REF connections of DY transformer.
- REF connections of YD with Zigzag transformer.
- REF connections of the motor, generator.
- REF connections of shunt reactor.
- REF connections of the autotransformer.
- Working principle of Simens 7UT6 REF relay.
- Single line diagram for YD transformer with the Zigzag transformer connected to delta side.
- Calculations for transformer rated current and mismatch factor.
- Calculations for y connected winding REF relay pickup & slope.
- Calculations for DY Z connected winding REF relay pickup & slope.
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