Power Transformer Differential Protection

A power transformer’s main task is to transform an electrical power from one voltage level to the other voltage level. A power transformer is the most important part of the electrical system as well as well the most expensive part. The function of all other electrical equipment (e.g. circuit breakers, instrument transformers, etc.) is to protect the power transformer. Considering the importance of the transformer and its high cost compared to other equipment, it is reasonable to install high-quality systems for protection against external failures from the network or internal power transformer failures.

Power Transformer Protection Systems

The external failures which appear somewhere in the network (overvoltage, short circuit, overload, atmospheric discharge, etc) can cause the troubles for the transformer (which is part of that network). E.g. short circuits in the network can cause the significant heating of the transformer busbars and windings.

The copper losses I²R are increased with the square of the current and dissipated as heat. Also, failures can appear inside the power transformer, such as windings short circuit, inter-turns short circuit, a short circuit between phases, faults in the core, transformer tank, and breakthroughs on the transformer bushing. When it comes to the failure location, the power transformer protection systems can be divided into external and internal protections.

The main task of the protection system is to separate the transformer from the energy supplying as soon as possible, thus preventing unintended consequences and major transformer damages.

 The protection system is designed to be able to signalize if irregularities occurred in the electrical system which could lead to the transformer failure.

After a preset relay blocking time (operation time delay), the protection system sends the signal to the circuit breaker which will turn off the transformer from the system before the failure affects them.

The power transformer substation with the protected transformer, circuit breaker and measurement current transformers is illustrated in Figure 1. The different transformer protection systems according to the operation criteria are listed in Table 1.

Figure 1. The transformer substation

The different protection systems can detect the different faulty conditions in the transformer. The Table 2 shows which failures can be detected with corresponding protection.

Differential Protection of Power Transformer

The transformer differential protection (ΔI) is a reliable and safe protection as well as the most important and most commonly used transformer protection. It is used for protecting the power transformer with nominal power above 8 MVA (it is usually not used in case of a transformer with lower nominal power up to 4 MVA).

The ΔI covers almost all short circuits inside the transformer such as short circuit: between phases, inter-turns, between phase and ground. If the transformer neutral is directly grounded, this protection also covers insulation breakthrough through all windings. If the transformer neutral is isolated the ΔI covers only faults between two phases but not single phase failures.

Transformer Differential Protection Principle

The differential protection (ΔI) principle is based on comparing the output and input transformer currents as illustrated in Figure 2.

In the normal network condition, power transformer operates with the nominal current. The current transformers (CT) are selected with corresponding turns ratio that the currents in CT secondary sides are equal. In this case, there is no current flow through ΔI (ΔI=0) because CTs secondary currents have equal amplitude and phase displacement value. The ΔI will not operate.

 In faulty condition, the transformer current value will be much higher than the nominal current which will cause ΔI>0. In this case, the protection will operate and take transformer off from the service.

Figure 2. Transformer differential protection diagram

Theoretically, this protection system seems very simple. But in reality, the protection operating criteria is not as simple. The ΔI challenges are listed below:

• The primary and secondary transformer currents are usually different. The current transformer should be properly selected so that the differential current in normal condition is ΔI=0.
• Different transformer vector groups have different current phase displacement on the primary and secondary side.
• The CTs on both transformer sides should have approximately the same saturation characteristics regarding the knee point and saturation curve.
• The tap changer operation (transformer voltage regulation) can cause the ΔI current through protection circuit because of the changing transformer turns ratio.
• When the transformer is first energized, it causes the current in only one transformer side and disturbs the ΔI balance.
• CT saturation and the DC current component phenomenon cause the current differences.
• The external ground fault in electrical system from the low voltage transformer side can cause the zero-sequent current component which can operate the ΔI.

Nowadays, analog and digital differential protection can be found in the electrical system. The analog system uses the old-fashioned mechanical solutions, while the new digital technology solves the issues by using a software.

New electrical systems are designed according to the digital protection systems. The digital systems resolve the using of the interconnection transformers, the higher threshold ΔI value, chokes (inductors) and capacitors.

 A software process solves all the mentioned requirements on the differential protection. The purpose of the interconnection transformers is to filter the zero-sequence current component which appeared due to external ground fault in the network. Those transformers should be connected in the Yd vector group.

It is very tricky to set the corresponding threshold ΔI current. It should be low enough to detect faulty current and take power transformer off the service in short period. But on the other hand, it should be high enough to avoid the wrong operation in some regular transformer condition such as first energizing (higher current), no-load current (DC current component), etc.

If the remnant flux is present in the transformer core when it is first energized, the inrush current value can reach almost the short circuit current value. Because of that, it is necessary to predict protection operation delay when the transformer is first energized.

The startup power transformer current contains conspicuous second harmonic and DC current component. The slowly reducing DC component and its high value can saturate the CTs and cause incorrect current measurement.

Because of the above the operation threshold ΔI is usually set on the 20-40% of the nominal current value (20% transformers without tap changer and 30-40% with tap changer). It should be low enough to cover internal short circuit failures.

The differential protection should operate as quickly as possible (in practice 25-40 ms) to decrease fault energy which destroys the transformer. When the ΔI has detected the protection, it sends the signal for circuit breaker operation and the sound alarm is triggered. In practice before putting the power transformer in service again, the transformer is tested in detailed and the failure causes are analyzed.

 Figure 3 The digital protection system

A new digital ΔI solution performs the DC current component filtering by using the software. The software gives the signal to the protection system which will block the DC current component protection operation by using electronics circuit.

The software usually uses the algorithms for current wave analysis. The main requirements for those algorithms are even harmonics detection (important for detecting magnetization current), the DC current component and fifth harmonic detection. All of these are important to divide the transient disturbances and faulty conditions. According to the calculated harmonics amplitude, the blocking signals are defined (Relay Logic).

The protection system manufacturer gives a range of blocking threshold, but the customer (electrical engineer) should set the corresponding threshold according to the experience and characteristic of the protected power transformer.