The transformer differential protection belongs to the unit protection scheme. A unit protection confines itself within a defined zone. A differential protection scheme compares the vector sum of currents entering and leaving the protected equipment. In other words, it uses Kirchhoff’s current law. Under healthy and external fault conditions, the currents entering and exiting the zone are balanced. So, the differential relay senses zero differential current. Only an internal fault in the zone causes a large differential or unbalanced current. Then the relay activates the tripping relay of the respective boundary circuit breakers.
In a power transformer, the protected zone typically includes all windings and connections (bushings, neutral) between the HV and LV CTs. Under normal load or external fault, the equal secondary currents of the HV and LV CTs cancel each other. Hence, the relay is stable. For inter-faults, a net differential current operates the differential relay.
Differential protection provides very fast and complete coverage of internal faults. We utilize transformer differential protection in conjunction with backup overcurrent protection or restricted earth fault protection to provide comprehensive coverage of the power transformer.
Applicability and Standards
Differential protection is mandatory for large power transformers. The CEA guideline recommends differential protection for transformers of 10 MVA and above. In practice, most utilities apply differential protection for transformers above 5MVA capacity. As per standards, relay 87 implies a differential relay.
Current Transformer (CT) Selection
Choosing the proper CT ratio is the most essential part of the transformer differential protection. Depending on the CT ratio, the HV and LV CT secondaries produce equal currents. To create so, the ratio of the ratio of HV and LV CTs should match the current ratio of the power transformer (or an integer multiple thereof).
\[\frac{\text{HV CT Ratio}}{\text{LV CT Ratio}} = \frac{\text{HV Current of Transformer}}{\text{LV Current of Transformer}} = \frac{1}{\text{Turns Ratio of Transformer}}\]
The above relation cannot always hold exactly well. This happens when the tap changer positions other than the normal tap. Also, there is always a small mismatch due to ratio errors in the CTs. Therefore, the differential relay setting must allow a few percent of mismatch.
CT Accuracy Class for Transformer Differential Protection
The CTs for transformer differential protection must remain linear even under very high fault currents. This linearity must match for both HV and LV side CTs. Mismatched linearities may cause maloperation of the differential relay. The standard 5P10 or 5P20 CTs can introduce errors due to mismatched linearities.
The PS class (also called the PX Class) solves this problem. The knee point voltage, CT resistance, and excitation current define a PS class. We choose the CTs for both HV and LV CTs with matching knee point voltage, CT resistance, and excitation current. Therefore, the saturation curve of all CTs becomes similar. So they do not produce any mismatched current. This prevents spill current, which can cause a false differential operation. In practice, we choose CTs with a high VA so that even at high fault currents, the curves remain linear similarly.
Percentage Biased Differential
Modern numerical differential relays use a biased percentage restraint characteristic. The relay computes the differential current \(I_{diff}=|I_1\pm I_2|\) and the through (restraint) current \(I_{restraint}=\frac{1}{2}(I_1+I_2)\) (for two-winding). It trips if\[I_{diff} > I_{bias} + k \cdot I_{restraint}\]Where, k stands for the bias slope (often 20–30%) and \(I_{bias}\) for a fixed offset or pickup.
Equivalently, the relay requires \(I_{diff}\) to exceed a set percentage of \(I_{restraint}\). This percentage restraint makes the relay insensitive to small mismatches. For example, a 25% slope means the differential current must exceed 25% of the through current to trip.
Percentage differential schemes are usually accompanied by harmonic restraint or blocking to prevent false tripping on magnetizing inrush. Transformer energization produces second-harmonic-rich currents (up to 8–30× rated) that look like an internal fault. Modern relays detect the second-harmonic content of the differential current. If the 2nd-harmonic exceeds a threshold, the relay is restrained.
CT Secondary Connections in Star–Delta Transformers
Power transformers with a transformation ratio of more than 2 are often Δ/Y (delta-star) or Y/Δ connected. These connections introduce a 30° phase shift between HV and LV line currents. The CT secondary connections must compensate for this phase shift. So the differential relay sees properly phasor-aligned currents from both sides. This is why we connect the secondary of CTs in Δ (delta) form on the star side of the transformer and Y (star) form on the delta side of the transformer. This opposite connection makes an overall 0° shift between the CT secondary currents at the same phase.
Although the modern numerical relays can also compensate for this phase shift internally. But is a good practice to follow the CT secondary wiring in the reverse manner with respect to the transformer winding connections.
For Y–Y transformers (auto transformer), there is no phase shift between the same phase currents of the sides. However, to avoid zero-sequence current, we connect all three CTs on both sides in a delta configuration. Obviously, this connection blocks zero-sequence from entering the differential circuit.