The mechanism of harmonic influence on the operation characteristics of earth leakage protection devices is very complex. Under different circumstances, the factors that dominate the reliable operation of harmonic currents on earth leakage protection devices are also different. This article mainly discusses the influence of harmonic waveforms on the reliability of earth leakage protection devices. We do not consider the effects of magnetic saturation and hysteresis, assuming that the leakage current waveform on the primary side can be accurately transmitted to the secondary side, and analyze the impact of low-order and high-order harmonics on earth leakage protection devices.
The impact of harmonics on earth leakage protection devices
Both low-order and high-order harmonics will affect the reliable operation of earth leakage protection devices, but the main factors affecting them are different. The influence of low-order harmonics includes two aspects: harmonics amplitude and harmonics phase. Changes in harmonic phase and amplitude will cause obvious distortion of the leakage current waveform, change the amplitude factor of the leakage current waveform, and cause protection errors or fluctuations. Although the high-order harmonics change rapidly, the waveform changes periodically 49 times within one power frequency cycle, and the phase factor has a small impact on the action value of the earth leakage protection device. However, similar to low-order harmonics, its amplitude will also affect the protection.
The influence of harmonics on different types of earth leakage protection devices (electronic, electromagnetic) is also different. Electronic earth leakage protection devices can effectively reduce the impact of harmonics on the sensitivity of leakage current protection devices, thanks to the filter elements in the electronic components that can filter out the harmonics on the secondary side of the leakage current transformer and then apply the processed leakage current signal to the relay. Therefore, they are more reliable than electromagnetic devices when used.
Countermeasures for the impact of harmonics on earth leakage protection devices
This article proposes some practical measures to reduce the impact of harmonic currents in the power grid on the operation reliability of earth leakage protection devices and improve their reliability.
The harmonic interference generated by single-phase motors during startup and operation, including the amplitude and duration of harmonics, is related to the initial phase angle of the terminal voltage of single-phase motors, the distance between single-phase motors and low-voltage distribution boxes, and the number of single-phase motors connected to a certain phase. To eliminate or reduce the misoperation of protectors due to harmonic interference, the following methods can be taken:
Distribute the load of the single-phase motor as evenly as possible on the three phases, and avoid excessive harmonic content in any phase circuit. Appropriately increase the rated residual operating current of the protector, which can greatly reduce the misoperation of the protector caused by harmonic interference during the startup of the single-phase motor.
For the impact of harmonics generated by variable frequency equipment used in rural low-voltage power grids on earth leakage protection devices, the action current setting value and action time of the total leakage current protection device can be increased to suppress interference caused by harmonics generated by variable frequency equipment. By adjusting the action time and current action setting value of the pulsed total earth leakage protection device and using the coordinated protection of the total protection and the final protection, the tripping of the protection device due to jumping action can be effectively avoided.
For electromagnetic earth leakage protection devices that have been installed in rural networks, because they do not have electronic filter elements, the impact of harmonics on the reliability of protection is relatively large. Therefore, it is advisable to add filter devices at the exit of protection to reduce the harmonic current that flows into the earth leakage protection device.
In the areas where earth leakage protection devices are about to be installed in rural networks, it is recommended to increase the use of electronic earth leakage protection devices and take advantage of their internal filter elements to reduce the impact of harmonics on protection.
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