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Transformer Energization in Power Systems: Overview, Causes and Mitigation Techniques

1. What is Transformer Energization?

Transformer energization and de-energization are planned switching operations in electrical power systems. The transformers can be involved in switching operations due to changes in power system network configurations, equipment maintenance, reconfiguration of power system networks, and operation of protective relays. Figure 1 shows the schematic diagram for transformer energization in power system networks.

Figure 1: Energizing a transformer in power system networks.

2. Causes of Transformer Energization

The switching operation of transformers can generate the following transients in voltages and currents.

High inrush currents & RMS voltage drop.
Harmonic Temporary overvoltage (TOV) due to network resonance.

Mostly, the transients produced by the transformer switching operation are damped within the power system networks. In some cases (for example, uncontrolled random closing of circuit breaker), it can lead to transformer insulation damage, surge arrester failure, mal-operation of protective relays, and power quality issues.

2.1. Inrush Current & RMS Voltage Drop

When the voltage applied to the transformer has sudden changes, the transformer core saturates due to the non-linear nature of flux-linkage (flux linkage is the time integral of applied voltage) vs current relation. It can happen during the switching operations of the transformer for energization, de-energization, faults, and fault clearance, etc. The transformer core saturation produces a high transient current called as Inrush current, which is several times higher than the nominal steady-state current of the transformer.

The magnitude of inrush current mainly depends on the following factors:

Residual flux in the transformer core. The residual flux is the flux that remains in the transformer core after it has been disconnected from the power networks. It is called as DC offset of the flux in the transformer core.
The switching instant (closing time) of the circuit breaker. The switching of the transformer at the instant of voltage zero crossing will produce worst-case inrush currents, whereas, the switching of the transformer at the instant of voltage peak will produce minimal inrush currents.
The network impedance that is connected with the transformer involved in the switching operation.

The inrush current will decay over time, and the decay time depends on the time constant XL/R of the overall connected network. Since the flux-linkage vs current relation has a non-linear nature, the inrush current is not sinusoidal; it contains harmonics (mostly dominant in lower orders) and a DC offset.

The inrush current is supplied from the connected power network (system sources, motor loads, and any other components in the networks) to the transformer through the network impedance. Hence, there is a voltage drop across the network impedance and a drop in the line voltage. Since the current contribution from different networks will be added towards energizing the transformer, the voltage drops across network impedance, and the drop in line voltage will be severe towards energizing the transformer.

Note: The definition of voltage sag/dip is a decrease in system voltage between 0.1 per unit to 0.9 per unit at power frequency for a duration of half a cycle to 1 minute.

2.2. Harmonic Temporary Overvoltage (TOV) due to Network Resonance

The inrush current generated by transformer energization contains harmonics. The interconnected power system networks may have system sources, transformers, transmission lines and cables, loads, and capacitor banks which has series inductance and shunt capacitance. Series inductance depends on the short circuit strength of the networks, whereas shunt capacitance depends on the cables, lines, and capacitor banks. The natural resonant frequency of the power system network depends on the series inductance and shunt capacitance.

The harmonic temporary overvoltage (TOV) occurs when the harmonic component of the inrush current interacts with the natural resonant frequency of interconnected power system networks. [OR]. The harmonic temporary overvoltage (TOV) occurs when the harmonics produced by transformer energization interact with the power system inductive-capacitive resonances. The generated TOV can lead to arrester failure.

The Temporary overvoltage (TOV) produced by transformer energization mainly depends on the following factors:

Switching instant (closing time) of the circuit breaker.
Energizing transformer rating & saturation curve.
Residual flux in energizing transformer core.
Short circuit strength & shunt capacitance of power system networks.

3. Mitigation Techniques

The issues of transformer energization such as inrush current, RMS voltage drop & harmonic TOV can be mitigated by the following methods:

Controlling the switching instant (closing time) of the circuit breaker. The point on wave switching (POW) is an effective way to reduce inrush currents, RMS voltage drop & harmonic TOV. This technique may need independent pole breaker operation.
Apply pre-insertion resistor (PIR) to reduce the inrush current & RMS voltage drop (Refer Figure 2).
To limit harmonic TOV, temporarily detune the network resonance before the energization by temporarily changing the configurations of the power system networks.
Use of surge arresters to limit the harmonic TOV.

Figure 2: Pre-insertion resistor (PIR) in series with the circuit breaker.

References

CIGRE 568, Transformer Energization in Power Systems: A Study guide, 2014.
CIGRE 913, Evaluation of Temporary Overvoltages in Power Systems due to Low Order Harmonic Resonances, 2023.
D. Durbak, Temporary overvoltags following transformer energizing, Siemens PTI Newsletter, nr. 99, pp. 1-3, Sept. 2006.
C. Cheng and S. Chen, Simulation of resonance over-voltage during energization of high voltage power network, Electric Power Systems Research, vol. 76, pp. 650-654, 2006.