What is Transformer Differential Protection Relay? What Does It Do, How Does It Work and Why Is It Used?

Transformer differential protection relay is the main protection relay that detects internal faults by comparing the currents entering and exiting the transformer's protection zone. Briefly, the answer to the question of what is a transformer differential protection relay is; It is a protection device that monitors whether the current on two or more sides of the transformer is in balance and gives a very rapid tripping command when an internal fault occurs. In ANSI numbering, this protection function is often referred to as 87T.

Selective and fast protection lies at the heart of the question of what a transformer differential protection relay does. When a winding fault, phase-to-phase short circuit, winding-to-ground fault or some internal insulation problems occur in a transformer, the aim is only to remove the faulty transformer from the circuit. Differential protection does this based on the basic principle that the current entering and exiting the protection zone of the transformer must be equal. If this balance is disrupted, the relay may interpret this as an internal malfunction.

Although the basic logic of transformer differential protection seems similar to Kirchhoff's current law, things are a little more complicated in transformers. Because the currents on both sides of the transformer may naturally differ in terms of both magnitude and phase angle. This is due to structural features such as voltage ratio, vector group, delta-star connection difference and OLTC. Therefore, the differential relay does not collect raw currents directly; It first applies compensation to make them comparable.

To explain the question of how the transformer differential protection relay works simply, current information is first obtained from the CTs on all sides of the transformer. These currents are then normalized within the relay, taking into account the ratio, phase shift and, if necessary, zero sequence regulation. If there is a normal load or external fault, these rectified currents largely balance each other and the differential current remains small. If there is an internal fault, the incoming and outgoing current equality is disrupted and the differential current seen by the relay increases significantly.

Although the power at the input and output of the transformer is close to the same line in normal operation, the currents are not always the same. For example, the current magnitude on the high voltage side and low voltage side is different depending on the conversion ratio. Phase angle shift also occurs in connections such as delta-star. Therefore, transformer differential protection requires more engineering than straight line differential protection. In order for the relay to work correctly, the vector group and ratio compensation must be defined correctly.

The most commonly used approach in this protection is the biased differential characteristic. Here the relay does not only look at the magnitude of the differential current; It also takes into account the through current level, that is, the current level passing through the transformer. At high external fault currents or in situations where there is a risk of CT saturation, the relay applies restrain logic to remain more stable. Thus, the possibility of false tripping during an external fault is reduced.

More than one slope can be used in the percentage differential characteristic. It is desired to provide more sensitive operation in low current regions and higher stability in high through-fault currents. For this reason, double-slope or multi-zone differential characteristics are commonly used in modern transformers, rather than single-slope. The goal is to both remain sensitive to internal fault and avoid false tripping on CT saturation or transient instability.

One of the most critical special situations for transformer differential protection relay is the magnetizing inrush current. When the transformer is first energized, very high inrush current may occur, especially at certain switching moments and depending on the core remanence. This current often appears on only one side of the transformer and may appear to be an internal fault from the perspective of the differential relay. That's why differential protection requires special logic that can distinguish inrush from real internal fault.

The most well-known method for this separation is second harmonic based blocking or restriction. While the second harmonic component can be significantly high in inrush current, in true internal short circuits this component is usually low. Therefore, if the relay detects a high second harmonic in the differential current, it can block or restrain the protection. Some modern relays also feature advanced fourth harmonic and waveform based logic.

Excessive magnetization, that is, overexcitation, also requires special consideration in differential protection. Because in this case, a differential-like current may occur due to the magnetic behavior of the transformer. For this reason, some relays provide additional security or generate alarms via the fifth harmonic. Thus, differential protection becomes safer in events that are not faults but produce differential current-like behavior, such as inrush and overexcitation.

In transformers with OLTC, tap change is also important for differential protection. Because OLTC changes the transformer's effective transformation ratio, a differential function operating under the constant ratio assumption may experience pseudo-differential current. For this reason, OLTC compensation or appropriate bias adjustment is used in modern transformers. Properly implemented OLTC compensation can make differential protection both more sensitive and safer.

The transformer differential protection relay alone does not undertake all protection; It often works together with other functions. Restricted Earth Fault, or REF, can provide more sensitive protection, especially for ground faults close to neutral. In addition, additional protection functions such as Buchholz, pressure relay, temperature protection, overcurrent backup and instantaneous differential complete the general protection philosophy of the transformer. Differential protection generally stands out as the main internal fault protection.

Transformer differential protection relay and overcurrent relay are not the same thing. The overcurrent relay makes decisions based on the magnitude of the current in the system and generally offers a more general protection logic. Transformer differential relay focuses on the question of whether the current balance in the transformer is disturbed. Therefore, it can detect internal faults much more selectively and quickly. Especially in large power transformers, differential protection is considered as the main protection and overcurrent is considered as backup protection.

In summary, transformer differential protection relay; It is the main protection relay that detects internal faults quickly and selectively by comparing the currents entering and exiting the transformer's protection zone. Structures such as percentage differential characteristic, CT and vector group compensation, second harmonic security for inrush, additional logic for overexcitation and high-set differential when necessary form the basis of this protection. A correctly selected, correctly adjusted and correctly tested transformer differential protection system is one of the most critical security layers of the transformer. In the next step, if you want, I can also prepare an article on the tests and maintenance that should be done in transformer differential protection relays with the same pattern.