Abstract Based on the working principle and adjustment characteristics of the biased arc suppression coil, this paper develops an automatic tracking dynamic compensation system for the biased arc suppression coil based on a microprocessor. Automatic tracking and detection of the capacitive current under normal grid operation and dynamic compensation under fault operation are realized. Keywords: biased arc suppression coil, automatic tracking of capacitive current, dynamic compensation 1 Introduction When a single-phase ground fault occurs in a neutral ungrounded system, the magnitude of the grounding capacitive current is equal to three times the phase-to-ground charging current under normal operation, and the grounding current differs from the phase voltage of the grounded phase under normal operation by 90°. When the grounding current crosses zero, the power supply voltage applied across the arc gap is at its maximum value, thus the arc at the fault point is not easily extinguished. When the grounding capacitive current is large, intermittent arc grounding or stable arc grounding is easily formed. Intermittent arc grounding can lead to dangerous overvoltages; stable arc grounding can develop into multi-phase short circuits [1]. If the capacitive current is limited from the perspective of grounding method, grounding the neutral point through an arc suppression coil is the only option [2]. The biased-magnetic arc suppression coil is an arc suppression coil with continuously adjustable inductance. Its internal structure is completely static with no moving parts, resulting in high reliability and fast response. Furthermore, it can adjust the inductance value even when the arc suppression coil is subjected to high voltage, making it a promising arc suppression coil. The working principle of the biased-magnetic arc suppression coil is to achieve continuous change in inductance by changing the permeability of the additional DC excitation magnetized iron core. Since the volt-ampere characteristic of the arc suppression coil is linear in both high-voltage and low-voltage sections, whether the arc suppression coil is subjected to high voltage during a grid fault or low voltage during normal grid operation, the inductance value of the biased-magnetic arc suppression coil is uniquely determined by the excitation current in the excitation winding. We can accurately adjust the inductance of the arc suppression coil by precisely providing the excitation current in the excitation winding [3]. 2 System Structure and Functions 2.1 System Structure The biased-magnetic arc suppression coil automatic tracking dynamic compensation system consists of a grounding transformer, a biased-magnetic arc suppression coil, an excitation power supply system, and a microprocessor-based fully digital controller. The system structure is shown in Figure 1. The grounding transformer is connected to the power grid bus, and an artificial neutral point N is led out. A biased magnetic arc suppression coil is connected between the neutral point N and the ground. The excitation winding of the biased magnetic arc suppression coil is provided with excitation current by the excitation power supply system. The magnitude of the excitation current is determined by the fully digital controller. The fully digital controller consists of a single-chip microcomputer minimum system, a synchronization circuit, a pulse amplification and output circuit, and a signal conditioning circuit, realizing digital triggering and digital PID regulation based on current closed loop. Among them, the single-chip microcomputer minimum system consists of the Intel MCS-96 series 80C196KC single-chip microcomputer [4] and the WSI PSD913F2 field-programmable single-chip microcomputer peripheral chip [5]. 2.2 System Function The primary side of the voltage transformer is applied to both ends of the biased magnetic arc suppression coil. The secondary side voltage ao passes through the isolation transformer, and after passing through the signal conditioning circuit, A/D conversion circuit and opto-isolation, it enters the single-chip microcomputer system. The microcontroller detects the capacitor current by adjusting the control current in the excitation winding of the arc suppression coil and sampling the neutral point displacement voltage. It then uses the high-voltage regulation characteristic curve of the biased arc suppression coil to convert the detected value into the control current value that the control winding needs to apply in the event of a single-phase ground fault in the power grid, thereby determining the conduction angle of the thyristor. The tracking sensitivity is set via the keyboard, and the display shows the capacitor current value and operating status. The microcontroller samples the neutral point displacement voltage value according to the set capacitor current detection sensitivity, determines whether to re-detect the capacitor current based on its change, transmits the normal operating status information of the arc suppression coil to the host computer via serial communication, reads keyboard information and handles routine management tasks, and waits for a ground fault to occur. When a single-phase ground fault occurs, the neutral point displacement voltage rises, the interrupt signal generation circuit issues an interrupt signal, the microcontroller executes the interrupt program, opens the logic interlock in the pulse amplifier and output circuit, the excitation current rises to the required value within the required time, the arc suppression coil performs full compensation, and an alarm signal is issued simultaneously. Grounding status information is also transmitted to the host computer via serial communication. The microcontroller comprehensively judges the interrupt signal, and after multiple comparisons and delay processing, it exits excitation and returns to the original state if it determines that the fault has completely disappeared. If full compensation exceeds a certain time, the excitation is adjusted appropriately to determine whether the ground fault has disappeared. 3. Automatic Tracking Detection and Dynamic Compensation of Capacitor Current 3.1 Capacitor Current Detection By increasing the excitation current, the neutral point displacement voltage U0 reaches its maximum value. Then, based on this value, the power grid imbalance is judged. For grids with good balance, Method 1 is used to detect the capacitor current; for grids with large imbalance, Method 2 is used. During capacitor current detection, the reactance value of the biased arc suppression coil is determined according to the low-voltage regulation characteristic curve. Method 1 [6]: For a well-balanced power grid, regardless of the main cause of the neutral point voltage displacement, we can always adjust the inductance of the arc suppression coil to make the neutral point voltage reach its maximum value. At this time, the inductive reactance of the arc suppression coil is equal to the total capacitive reactance of the power grid to ground. The volt-ampere characteristic of the low-voltage section of the biased magnetic arc suppression coil is linear, and the inductance value is uniquely determined by the control current, which is linearly corresponding to the given voltage. The total capacitive reactance of the power grid to ground can be easily determined using a microprocessor. The only detection quantity in the whole process is the amplitude of the neutral point displacement voltage. Method 2 [2]: For a power grid with a large degree of imbalance, since IC = 1/(D2 - D3B4), where D2, D3 and B4 can be obtained from the four sets of data U2/U1, U2/U3, U3/U1 and U3/U2, the single-phase grounding capacitance current of the power grid can be calculated through this formula. Because the inductor currents I1, I2, and I3 can be precisely given by the excitation current based on the low-voltage regulation characteristics of the biased arc suppression coil, the single-phase grounding capacitance current of the power grid can be calculated simply by detecting the neutral point displacement voltage amplitudes U1, U2, and U3 under different control currents. In both capacitance current detection methods described above, only one quantity is measured: the neutral point displacement voltage amplitude. Method 1 only requires comparing the changes in the neutral point displacement voltage amplitude as the arc suppression coil control current changes; Method 2 only requires knowing the ratios U2/U1, U2/U3, U3/U1, and U3/U2 before and after the change in neutral point displacement voltage. Therefore, it does not require precise detection of the neutral point displacement voltage magnitude, making measurement accuracy relatively easy to achieve. The accuracy of capacitance current detection mainly depends on the measurement accuracy of the low-voltage regulation characteristic curve of the biased arc suppression coil, and accurate measurement of the low-voltage regulation characteristic is very easy to achieve. Furthermore, the hardware implementation of the two methods described above is completely identical. Only the microprocessor needs to be programmed separately to automatically switch between the two methods according to different neutral point displacement voltages, thus achieving accurate measurement of different grid capacitor currents. 3.2 Automatic Tracking of Capacitor Current Analysis shows that the change in neutral point displacement voltage caused by a decrease in capacitor current IC by ΔI is equal to the change caused by an increase in inductor current IL by ΔI. Similarly, the change in neutral point displacement voltage caused by an increase in IC by ΔI is equal to the change caused by a decrease in IL by ΔI. The change in IL depends on the change in the inductance of the arc suppression coil, which is controllable. Therefore, we can let IL change by ΔI in both positive and negative directions, detecting the resulting changes in neutral point displacement voltage ΔUL+ and ΔUL-. Then, keeping IL unchanged, when the grid capacitor current changes, we first determine the direction of the capacitor current change based on the rise and fall of the neutral point voltage. If UN increases, it indicates a negative change; if UN decreases, it indicates a positive change. Then, determine whether the change ΔUC is greater than ΔUL. If ΔUC > ΔUL, it means that the change in capacitor current exceeds ΔI, and the capacitor current value needs to be re-detected. Otherwise, maintain the original value. Therefore, ΔI is the set capacitor current tracking detection sensitivity. 3.3 Dynamic Compensation of Capacitor Current During normal operation of the power grid, the detected capacitor current value is converted into the excitation current required for full compensation according to the high voltage regulation characteristics. Once a single-phase ground fault occurs in the power grid, the neutral point voltage rises, and the microcontroller executes the interrupt handling program. The pulse amplification and output channel is opened, and the excitation current rises to the required value within 20ms. The biased arc suppression coil generates an inductive current to keep the single-phase ground fault capacitor current in a normal state. After ensuring that the ground fault has disappeared, the pulse amplification and output channel is closed, the interrupt handling program is exited, and the main program is returned. 4 Running Experiment 4.1 Response Curve of Control Current At the moment of single-phase ground fault, the response curve of the control current captured by the memory oscilloscope is shown in Figure 2. It can be seen from the response curve of the control current that at the moment of ground fault, the control current rises from zero to the excitation current required for tuning in less than 20ms. 4.2 Waveform of Grounding Residual Current After a single-phase ground fault occurs, the residual current waveform is observed using an oscilloscope, as shown in Figure 3. The actual waveform of the grounding residual current shows that after a single-phase ground fault, the residual current is mainly composed of high-order harmonics, primarily the third and fifth harmonics. This indicates that after a single-phase ground fault, the capacitive current in the power grid is completely compensated by the inductive current generated by the arc-suppression coil in the biased magnetic arc-suppression coil compensation system. 5 Conclusion This device achieves automatic tracking and detection of capacitive current under normal power grid operation, dynamic compensation under fault operation, and self-recovery after the fault disappears. The device uses an 80C196KC microcontroller and a WSI PSD913F2 field-programmable microcontroller peripheral chip as the minimum system, realizing fully digital control of the biased magnetic arc-suppression coil and eliminating the inherent shortcomings of analog control systems. Field operation fully demonstrated that the device is reliable, has high measurement accuracy, fast response speed, and good compensation effect. It also realized the adjustment of inductance value of the arc suppression coil when subjected to high voltage, thus greatly improving the operation of the power grid, effectively controlling the harm of capacitive current, and increasing the reliability of power grid operation. 6 References 1 Griffel D, et al. A New deal for safety and quality on MV networks [J]. IEEE Trans on Power Delivery, 1997, 12(4): 1428-1433 2 Newbould A, et al. Improving UK power quality with arc suppression coils. IEES seventh International Conference on Developments in Power System Protection, 2001: 487-490 3 Wang Hongyan. Fully digital control system of biased arc suppression coil: [Dissertation]. Beijing: China University of Mining and Technology, 2001. 4 Sun Hanfang. Intel 16-bit single-chip microcomputer. Beijing: Beijing University of Aeronautics and Astronautics Press, 1995. 5. PSD9××F Series Datasheet and Application Notes. Wuhan Liyuan Electronics Co., Ltd., 2000. 6. Cai Xu. 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