Advanced Differential

Comprehensive three-phase differential relay testing

Advanced Differential is a set of test modules which form a complete testing solution for differential schemes. It is particularly suitable for transformer differential schemes with up to three windings and up to nine currents to be injected.

Extensive modeling of the protected object (e.g. power transformer), the secondary equipment (CTs, CT connection) and the relay characteristics provides the data for the calculations required to facilitate testing. The automatic calculation of the test currents eliminates the most time consuming and error-prone manual tasks. Testing the correct operation of the relay becomes simple, time saving, and cost efficient.


This test solution provides:

  • Testing with all fault types (L-N, L-L, L-L-L)
  • Shot tests at pre-defined test points or search tests
  • All shots synchronizable to GPS or IRIG-B for end-to-end testing (e.g. line differential protection)
  • Evaluation and assessment of results against nominal characteristics and tolerances
  • Report generation including graphical representation of the results in the characteristic diagrams
  • No blocking of voltage related functions required (important for testing of multifunctional relays)


For transformers, automatic calculation of currents to be injected are based on:

  • CT ratios and connections
  • Fault type
  • Fault/supply side (primary, secondary, etc.)
  • Load current
  • Magnitude and phase correction


For the protective relay, the assessment of the measured values is based on:


  • Operating characteristic
  • Bias calculation
  • Zero sequence elimination


If a suitable combination of a CMC and an additional amplifier unit is used, the modules can control up to nine currents for comfortable testing of three-winding transformer protection.

For non-transformer applications, such as testing generator differential protection, the current calculations are done without the transformer model.



Details of the four test modules in Advanced Differential:


Diff Configuration

This module simulates through-faults to verify that the protection is stable for faults outside the protected zone. Since investigation of the stability may require the observation of multiple measurements, the module gives the tester the option to check the readings before proceeding with the test. The actual values read from the relay under the fault conditions (operating or restraint currents of the different phases) can be entered in the report for full documentation.


Diff Configuration

  • Secondary wiring and interposing transformers (electromechanical and numerical relays)
  • Correct parameter setting of digital relays (specification of protected object)
  • Zero-sequence elimination


Diff Operating Characteristic

Diff Operating Characteristic tests the operation of the protection for faults inside the protected zone.

The currents injected into the relay are calculated from ldiff/Ibias value pairs specified in the ldiff/ Ibias plane. This relates directly to how manufacturers commonly specify the operating characteristic. The correct reaction of the relay, either trip or no trip, is assessed against the specified characteristic.


Diff Trip Time Characteristic

This module tests the dependency of the trip time from the magnitude of the differential current.

Diff Trip Time Characteristic measures tripping times at specified differential currents. The actual test currents for the specified differential currents are automatically calculated. The test points are defined in the trip time characteristic diagram and the measurements are assessed against this characteristic.


Diff Harmonic Restraint

Diff Harmonic Restraint tests the inrush and CT saturation blocking function of a differential relay. The test points are defined in the harmonic restraint characteristic diagram, where the differential current is drawn over the harmonic content of the test current.

For simulating different inrush conditions, the initial phase shift between fundamental and harmonics can be specified.


This module is part of the Test Universe software. For more information please refer to the Test Universe website.

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