Principle and Commissioning Method of Transformer Differential Protection Device

Principle of Transformer Differential Protection

The basic principle of differential protection adopted by protection relays for transformer and relays for transformer protection is to compare the magnitude and phase of currents on both sides of the transformer. Under normal operating conditions, the currents on both sides are equal in magnitude and identical in phase. When an internal fault occurs inside the transformer, a deviation appears between the currents on the two sides. Based on this characteristic, differential protection is implemented by these relays to isolate the transformer and protect it against internal faults.

Commissioning Methods

Fully understand transformer parameters and protection device principles before commissioning

Check and record parameters: Record the transformer nameplate parameters (rated capacity, voltage level, connection group) and the transformation ratio of current transformers (CTs).

Read technical manuals: Master the differential protection principle of the protection device and phase compensation modes (Y→Δ or Δ→Y).

Familiarize with operating equations: Understand the operating logic of percentage differential protection, differential instantaneous overcurrent tripping, and harmonic restraint.

Setting calculation

Calculate basic parameters in accordance with transformer specifications and relay protection setting criteria:

Balance coefficient: Take the high-voltage side as the reference to calculate the balance coefficient of each side.

Differential starting current: Generally set to (0.3~0.5) times of Ie​.

Differential instantaneous trip setting: Generally set to (5~10) times of Ie​.

Percentage restraining characteristic parameters: inflection point current and restraining coefficient.

Input the calculated protection settings into the transformer differential protection device.

Carry out tests on the differential protection device by using a relay protection tester

Accuracy Test

Verify the sampling accuracy of the protection device, including voltage, current, power factor, frequency and other measured quantities.

Protection Functional Test

Balance Current Calibration

Ensure that under normal operating conditions, the differential current displayed by the transformer differential protection device is close to zero and within the allowable error range.

Percentage Differential Protection Test

Apply through current: The differential current shall be lower than the starting value, and the protection shall not operate.

Apply internal short-circuit current: The differential current exceeds the starting value, and the protection shall operate correctly.

Inject current containing second harmonic components to verify the harmonic restraint function.

After the above tests, verify the actual starting value and inflection point value of the transformer differential protection.

Record key data after commissioning

Measured value of differential starting current

Measured value of differential instantaneous trip current

Test points of percentage restraining characteristic curve (at least 3~5 points)

Measured harmonic restraint coefficient

Results of overall linkage test

Compile and issue the test report.

Conclusion

Transformer differential protection constitutes transformer primary protection, which operates only when a fault occurs inside the transformer. Transformer overload protection and transformer ground fault protection act as secondary protections, covering transformer bushings and external adjacent areas.

The above is an introduction to transformer differential protection and is provided for reference only.

FAQ

Q1: What is the core working principle of transformer differential protection?

A1: Transformer differential protection is based on current balance principle. It compares currents from transformer HV and LV side CTs. Balanced currents produce nearly zero differential current in normal operation and external faults. Internal faults break current balance and generate large differential current, triggering trip to isolate faults.

Q2: Why is transformer differential protection different from line differential protection?

A2: Unlike line differential protection, it must handle three transformer-specific unbalanced currents: CT ratio mismatch (amplitude error), YNd11 winding 30° phase shift, and magnetizing inrush current with harmonics. Thus, it is equipped with phase compensation, ratio braking and harmonic blocking functions to prevent maloperation.

Q3: What faults can transformer differential protection effectively detect, and what are its blind spots?

A3: It protects against major internal faults: phase/ground short circuits and turn-to-turn faults. Blind spots include minor turn-to-turn faults and high-resistance grounding faults with ultra-low fault current. These subtle faults are covered by transformer gas protection.

Q4: Why is phase compensation required for YNd11 transformer differential protection?

A4: YNd11 transformers have a natural 30° current phase difference between two sides, which causes false differential current. Numerical relays adopt algorithm-based phase compensation to eliminate phase deviation and ensure current balance under normal operation.

Q5: What is the purpose of ratio compensation in differential protection?

A5: Ratio compensation corrects amplitude errors caused by mismatched transformer and CT ratios. The device calculates compensation coefficients to balance two-side current amplitudes, eliminating steady-state unbalanced current and avoiding protection maloperation.

Q6: How to deal with zero-sequence current in transformer differential protection?

A6: Zero-sequence current on the Y side cannot transfer to the delta side, causing false differential current. The protection filters out zero-sequence components and only compares positive/negative sequence currents to ensure accurate fault judgment.

Q7: What are the mandatory commissioning items for transformer differential protection before putting into operation?

A7: Key pre-commissioning tests: CT circuit inspection, two-side current balance test, differential startup value calibration, ratio braking characteristic test, and second harmonic blocking anti-inrush test.

Q8: How to conduct the on-site current balance test for differential protection?

A8: Test under no-load or light-load conditions. Qualified criteria: opposite current phases, matched compensated amplitudes, and differential current below 5% rated value. Excessive differential current indicates wrong wiring, incorrect parameters or CT saturation.

Q9: What is the testing standard for harmonic braking function during commissioning?

A9: The standard second harmonic blocking threshold is 15%–20%. The protection should be blocked for inrush current with excessive second harmonics and trip reliably for pure fundamental fault current, preventing no-load closing maloperation.

Q10: What are the common causes of differential protection misoperation during normal operation?

A10: Main maloperation causes: incorrect CT wiring/polarity, mismatched compensation parameters, heavy-load CT saturation, failed harmonic blocking, and sampling circuit interference.

Q11: Why does differential protection refuse to operate when an internal fault occurs?

A11: Protection refusal is mainly caused by overhigh startup threshold, unreasonable braking coefficient, CT saturation distortion, and trip circuit or logic faults.

Q12: How to judge whether the differential current is caused by internal fault or external interference?

A12: Internal faults produce pure fundamental high-amplitude differential current and trigger tripping. External interference (inrush, CT saturation) contains obvious second harmonics and causes protection blocking. External faults also generate larger braking current.

Q13: What key parameters need to be set for transformer differential protection?

A13: Core parameters: differential startup current, ratio braking coefficient, second harmonic threshold and zero-sequence compensation coefficient. All parameters are configured according to transformer and CT rated parameters.

Q14: What routine maintenance items are required for differential protection devices?

A14: Routine maintenance includes daily sampling data inspection, regular CT circuit checks, annual parameter verification, periodic trip loop tests and harmonic function inspection.

Q15: What is the difference in commissioning methods for traditional electromagnetic and modern numerical differential protection?

A15: Traditional electromagnetic protection needs manual hardware adjustment with complicated commissioning and low precision. Modern numerical protection adopts full digital compensation, requiring only parameter configuration and calibration, with higher accuracy and self-diagnosis ability.