Analysis of the Principle of Second - harmonic Restraint for Overcurrent Protection of Distribution Automation Switches

Substance of Second - harmonic Restraint in Overcurrent Protection

The substance of second - harmonic restraint in overcurrent protection is to use the second - harmonic component to judge whether the current is a fault current or an excitation inrush current. When the percentage of the second - harmonic component to the fundamental - wave component is greater than a certain value, it is judged to be caused by the excitation inrush current, and the overcurrent protection is blocked.

Therefore, the larger the second - harmonic restraint ratio, the more second - harmonic current allowed to be contained in the fundamental wave, and the worse the restraint effect.

Principle of Second - harmonic Restraint for Measures of Overcurrent Protection Against Excitation Inrush Current Waveforms

Deriving Second - harmonic Restraint

In the power system, second - harmonic restraint is used to distinguish between the excitation inrush current of a transformer and an internal fault. When a transformer is switched on no - load or an external fault is restored, an excitation inrush current will be generated, which may cause the transformer differential current protection to malfunction (at this time, it is not an internal fault of the transformer, and the relay protection should not operate). Therefore, it is necessary to distinguish between the transformer excitation inrush current and an internal fault. When an internal fault occurs in the transformer, the relay protection should operate to remove the faulty transformer; when an excitation inrush current is generated, the differential current protection should be blocked to prevent malfunction.

Since the transformer excitation inrush current contains a large number of harmonic components, especially the second - harmonic component, while an internal fault will not generate so many second - harmonic components, it is possible to use the level of the second - harmonic content to distinguish between an excitation inrush current and an internal fault. This is the principle of second - harmonic restraint.

The low - voltage side motor will also generate a large number of harmonics during startup. If there is no blocking of the second and fifth harmonics, the probability of the transformer differential protection malfunctioning is quite high.

The current instantaneous trip protection can operate instantaneously when a line fault occurs, thereby protecting the line.

Deriving Excitation Inrush Current

When a transformer is put into the power grid no - load or the voltage is restored after an external fault is removed, due to the saturation of the transformer core flux and the nonlinear characteristics of the core material, a relatively large excitation current will be generated. This impact current is usually called the excitation inrush current.

The transformer excitation inrush current is: the transient current generated in the winding when the transformer is switched on no - load and put into the power grid. When the residual flux in the core before the transformer is put into operation has the same direction as the flux generated by the operating voltage when the transformer is put into operation, the total magnetic flux far exceeds the saturation magnetic flux of the core, causing the core to be saturated instantaneously. Therefore, a huge impact excitation current is generated (the maximum peak value can reach 6 - 8 times the rated current of the transformer), which is usually called the excitation inrush current.

Deriving the Characteristics of Excitation Inrush Current Waveforms

Biased to one side of the time axis, and the inrush current contains a large DC component;
The waveform is intermittent, and the interruption angle is large, generally greater than 60°;
Contains a large second - harmonic component;
The sum of the three - phase inrush currents at the same moment is approximately zero;
The excitation inrush current is attenuating.
The amplitude of the excitation inrush current is very large

Deriving the Hazards of Excitation Inrush Current

Due to the very large amplitude of the excitation inrush current, it may cause the switch protection to malfunction and trip. Therefore, in the case of excitation inrush current, effective measures must be taken to block the overcurrent protection to prevent malfunction.