Transformer Magnetising Inrush: Causes, Mitigation, and Differential Protection

• When a transformer is energised at no load, the current drawn may temporarily rise well above its normal magnetising current. This transient current is known as magnetising inrush. It is usually a consequence of energisation and does not, by itself, indicate an internal transformer fault.
• Magnetising inrush can produce a substantial differential current. If the protection scheme is not appropriately configured and set, the transformer may trip during energisation. Engineers therefore need to reduce the impact of inrush where practical while ensuring that differential protection remains stable during inrush and operates dependably for internal faults.

1. What causes transformer magnetising inrush?

When a transformer is energised, its core flux transitions from its initial state to its steady state. The point on the voltage wave at which the circuit breaker closes and any residual flux in the core determine how the flux develops during this transition.

If the transient flux and the residual flux combine to drive the core into saturation, the magnetising current rises sharply. Consequently, the same transformer can draw markedly different inrush currents on successive energisations.

The main influencing factors are:

• Point on wave at closing: Closing at different points on the voltage wave produces different transient flux conditions.
• Residual core flux: Flux remaining after the transformer is de-energised can increase or reduce saturation during the next energisation, depending on its magnitude and polarity.
• System and equipment conditions: Source impedance, energisation voltage, and transformer characteristics affect the magnitude and decay of the inrush current.

The inrush current is often asymmetric, and its waveform may be heavily distorted. It generally decays as the transient subsides, but its magnitude and duration vary with the transformer and the energisation event.

2. Why can magnetising inrush affect differential protection?

• A transformer differential protection relay compares currents measured at the boundaries of the protected zone. It accounts for CT ratios and the phase shift associated with the transformer winding connections to determine whether a fault lies within that zone.
• During no-load energisation, current flows from the source into the transformer to establish core flux, while little or no corresponding current may flow at the other terminals. The differential relay may therefore see a substantial differential current. A large differential current alone does not establish that an internal fault has occurred.
• Second-harmonic restraint is a widely used method for detecting magnetising inrush. However, the second-harmonic content of inrush is not constant. Under some energisation conditions, the second-harmonic content may be too low for a scheme that relies solely on that criterion to distinguish inrush reliably. Lowering the second-harmonic threshold alone is not a complete solution: its effect on dependable operation for internal faults must also be assessed using disturbance records, relay logic, and the project’s protection requirements.
• Other inrush detection methods are also used in industry. The methods available on a particular relay must be confirmed from its technical documentation; they should not be assumed to be features of every model.

3. How can magnetising inrush and its impact be reduced?

3.1 Assess controlled switching

• Where inrush has a significant impact, controlled switching may be considered. By controlling the circuit breaker closing instant, controlled switching can reduce the transient change in core flux.
• Its effectiveness depends on factors such as residual flux, the circuit breaker’s operating time, and variations in that operating time. A closing strategy should therefore be based on the equipment and operating conditions rather than a fixed point on the voltage wave alone.

3.2 Assess pre-insertion resistors and other engineering measures

• For suitable high-voltage circuit breakers and project designs, pre-insertion resistors may be considered. They are inserted briefly before the main contacts complete closing to limit the energisation transient. Their suitability depends on the circuit breaker design, system parameters, and project requirements.
• Where practicable, core demagnetisation may also be assessed to reduce the influence of residual flux on subsequent energisation. These measures require project-specific engineering analysis and are not necessary for every installation.

3.3 Configure differential protection correctly

• Differential protection should be selected and set with reference to the transformer rating, voltage levels, winding connections, current transformer (CT) arrangements, and operating conditions. Percentage-restraint characteristics, inrush detection, and associated settings must be coordinated so that the protection remains stable during energisation and operates dependably for internal faults.
• Simply raising the differential operating threshold to prevent an energisation trip may reduce sensitivity to internal faults.

3.4 Analyse unexpected operations using relay records

• If the differential relay issues an alarm or trips during energisation, review its disturbance records and sequence of events (SOE). Check the current waveforms, operated protection elements, CT secondary circuits, and protection settings.
• A trip at the instant of energisation should not automatically be attributed to magnetising inrush. Internal faults, CT wiring errors, incorrect settings, and other causes must be investigated before settings are changed and the transformer is re-energised.

4. How should transformer and generator–transformer unit differential relays be selected?

• The appropriate relay depends on the boundaries of the differential protection zone and the locations of the CTs.
• For a protection zone covering a two- or three-winding power transformer, the GoWatron GWPR300-TD Transformer Differential Protection Relay can be considered. It is designed for power transformers rated up to 132 kV and provides transformer differential protection (ANSI 87T). Its percentage-restraint differential protection and independently configurable second-harmonic restraint can be applied to improve stability during transformer energisation.
• A generator connected to a step-up transformer forms a generator–transformer unit. If the required differential protection zone encompasses the generator and step-up transformer as one unit, the GoWatron GWPR300-GT Generator–Transformer Unit Differential Protection Relay should be assessed. Unit differential protection and transformer-only differential protection cover different zones; the two applications should not be treated as interchangeable merely because both include a transformer.
• When correctly configured, a differential protection relay can use its inrush detection function to restrain the transformer differential element during energisation. The relay does not reduce the inrush current itself. Available functions, their configuration, and their settings must be confirmed against the selected model’s technical documentation and the project design.
• For a transformer or generator–transformer unit project, provide the single-line diagram, equipment ratings, CT ratios, CT locations, and the intended protection zone so that the appropriate relay, configuration, and quotation can be confirmed.