Stability Matters: A Deep Dive into Proving Transformer REF Protection

Introduction

Restricted earth fault protection is a type of transformer protection that is designed to detect and isolate faults that occur between the transformer winding and the earth. This type of protection is important because faults that occur in this location can cause serious damage to the transformer and present a significant risk to people and equipment.

The basic principle of restricted earth fault protection is to measure the current that flows through the transformer winding and compare it to the current that flows through a sensing resistor that is connected between the transformer winding and the earth. If there is a fault between the winding and the earth, the current through the sensing resistor will be different from the current through the winding, and the protection system will trip the transformer to isolate the fault.

In addition to detecting faults, restricted earth fault protection can also be used to provide protection against phase-to-phase faults, as well as over current and overload conditions. It is an important component of transformer protection systems and is widely used in a variety of applications, including power generation, transmission, and distribution.

Stability testing of REF Protection

Stability test of REF is carried out to ensure that all CT polarities and related wiring is correct. 4 CT's are involved for REF protection, three for phase and one for neutral.

Normally during commissioning, single phase current sources are available at testing site. So, we'll look into proving stability using single phase source.

Following Equipment's are required for testing REF stability of Power transformer.

  • Variable power source with high current capacity,
  • Cable loop to short phase on secondary side,
  • Clamp on Ammeters,
  • Shorting cables / leads,
  • Multi meter,
  • Banana and crocodile wire loops,
Prepare set up as per connection diagram diagram below :

Step for Stability Testing

  1. Temporarily short stabilizing resistor with wire loop. It's not mandatory, but a good practice to avoid current flow for long duration as process may take quite some time.
  2. Corresponding phase to phase terminal shorting to be temporarily shorted on delta side. Refer diagram below for other phases. (Considering Dyn1 vector group).  
  3. Connect any one phase on star side to phase output of single phase variac with neutral to star point. As shown in connection diagram, R phase and neutral connected to variac output.
  4. Inject current of magnitude Is*CT ratio (where Is is current pickup setting of relay). To inject this current, variac and source must be capable of supplying VA of minimum capacity of square of injected current * impedance of transformer star winding. If current source in not capable, reduce relay setting to match with variac rating.
  5. Note down current Iph, In and Id. If wiring and CT polarities are correct, then Iph = In and Id will be 0A.
  6. We check cross check this with one CT polarity reversed. This will make Id current to flow through relay. If Id > Relay setting, then relay will issue REF trip. Remember to restore CT polarity to normal.
  7. Repeat same procedure for remaining phases.
  8. Remove all temporary shorting and normalize relay settings, if modified for testing.

Conclusion  

By following these steps, the stability of the REF can be thoroughly tested, ensuring the correct operation of the protection system for the power transformer. It is important to conduct these tests meticulously during the commissioning process to prevent any potential malfunctions or incorrect operation of the REF system.
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