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POWER TRANSFORMER

INRUSH AND CONTROLLED

SWITCHING INTRODUCTION Random power transformer energization can lead to large flux asymmetries and as a consequence to core saturation. The saturation of a core can drive currents of high amplitude and harmonics, known as inrush currents, which can result to undesirable effects including equipment damages and loss of life.

The protection engineers when dealing with those currents prefer to de-sensitizing or even blocking the protection schemes. Those tactics are of high risk as it is shown from statistics that failures during energization are quite common occurrences.

THEORETICAL APPROACH

Typical hysteresis curve

In an AC Transformer voltage and current waveforms vary in cycles, means when

the direction of current flow for one half cycle is in one direction and for other half life the direction reverses. So the direction magnitude of Field applied to the core of the Transformer also alters for every half cycle. Consider the core is magnetized by the magnetic field (H) called magnetizing force. The magnetic flux density (B) of the core of Transformer will increase and saturates at the knee point and then starts decreasing as magnetizing force (H) decreases. An instant (D in fig.) will arise where the magnetizing force (H) will become zero i.e, no magnetic field force is applied to the material (This arise due to the current wave during passing from one half cycle to other half cycle it touches zero point) but still magnetic flux density is not zero in the material but have some value. This is called Magnetic Retentivity. The value of H at zero B is called Corrective Force( C in fig.). Coercive Force required making the magnetic flux (B) zero by applying the magnetic field in opposite direction. When the flux applied in the opposite direction to the material in opposite direction in another cycle the same phenomenon takes place. This complete loop is called hysteresis loop. Hysteresis loop is significant because the area under the hysteresis loop gives the total Hysteresis Loss of the Magnetic Material.

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The transformer normal core flux is given by the Faraday low of induction:

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Integrating the applied voltage we can calculate the flux at the specific instance. From the above equation it is obvious that the flux waveform is sinusoidal with π/2 phase shift compared to voltage in such a way that when voltage is in its maximum the flux is zero.

When a transformer is de-energized a permanent magnetization of the core remains due to hysteresis of the magnetic material. This permanent magnetization is called “residual flux” and it is mainly influenced by the core materials and geometry, winding or other external connected capacities as also circuit breaker chopping characteristics.

During transformer re-energization the instantaneous magnitude of the core flux at the instant of energization is the vector summation between residual flux and applied sinusoidal flux. Depending of the instances of energization and de-energization the peak transient core flux can reach values more than two times the rated peak flux which can generate transient currents of high amplitude and rich of harmonics superimposed on a dc offset.

CONTROLLED SWITCHING A common practice of reducing the inrush currents of power transformers is by introducing power resistors in series or specially designed energizing circuits. Those solutions are often too expensive

and depending on the voltage level hard to utilize.

A modern approach is to reduce or even eliminate the transient energizing currents by selecting the instant and sequence of opening or closing the feeding circuit breaker. This method of transient elimination is also applicable for power capacitors and transmission lines.

CASE A, IGNORING THE REMANENCE FLUX In cases where the inrush current phenomenon is not severe or the switching element is a three pole simultaneous interrupting device, we can improve the network transient response by selecting the closing time instant to be when the voltage amplitude has its maxima value. When the voltage passes through the maximum, the induced flux is zero. One of the requirements of this technique is the measurement of the circuit breaker closing time in order to calculate the close command timing.

Here to mention that, most of the circuit breakers out in the market, interrupts the power networks at zero current passing time, so someone can insist that there would be no residual flux at the core at the next energization. This is not actually the case, as the breakers are chopping the current waveforms at values other than exactly zero.

CASE B, TAKING IN ACCOUNT THE REMANENCE FLUX In the cases that severe inrush transient phenomena occurs the residual flux must

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be supervised and measured by integrating the applied voltage at the moment of opening in order to calculate the closing time so that the victorious summation of the sinusoidal and residual flux is equal to zero. This is actually the case when the breaker is opened by a protection. In the case of maintenance or scheduled opening no monitoring is needed if we reassured the opening at zero current crossing time.

This solution is best applied when individual pole opening is available (i.e. three single phase breakers), so that the pole closing is happened after a time delay (120o) in order to avoid asymmetries in the core flux.

Depending on the needed result the above concepts can be utilized via existed numerical protection and control relays or by using dedicated market solutions.

Typical inrush current wave form without taking in account the residual flux

Typical inrush current wave form taking in account the residual flux

REFERENCES • Moraw, G., et al, “Point-ON-Wave

Controlled switching of High Voltage Circuit-Breakers,” CIGRE paper 13-02, pp. 1-6, 28 August-3 September 1988.

• Brunke, J.H., “Elimination of Transient Inrush Currents When Energizing Unloaded Power Transformers” ETH Zurich 1998

• Holmgren, R., Jenkins, R.S., Riubrugent, J., “Transformer Inrush Current,” CIGRE paper 12-03, CIGRE Paris, pp. 1-13, 1998

• John H. Brunke, Klaus J. Frohlich, “Elimination of transformer currents by controlled switching Part I – Theoretical Considerations”