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Droop control strategy is widely used to control parallel inverters in a microgrid. However, due to the mismatched line impedance, the traditional droop control strategy is difficult to evenly share the reactive power and restrain the circulating current among multiple parallel inverters. In this article, an adaptive virtual impedance-based droop control strategy without communications is proposed to solve the reactive power sharing problem caused by the difference of line impedance. The reactive power output and voltage of each distribution generator are introduced into the adaptive virtual impedance controller to achieve a tradeoff between reactive power sharing accuracy and voltage drop. Small-signal stability analysis of the inverters equipped with the proposed controller is conducted to determine the optimal control parameters. Simulation and experimental results demonstrate the effectiveness of the proposed droop control strategy.

Accurate faulty phase detection under single-line-to-ground (SLG) fault is the basis of the emerging voltage-based fault arc suppression technology. Most conventional faulty phase recognition criteria assume the distributed line-to-ground parameters are symmetrical, therefore suffering invalidity in actual distribution networks especially under high-resistance ground-fault conditions. This paper firstly analyzes the magnitude and phase angle variation models of the phase-to-ground voltages before and after the SLG fault. Considering the asymmetrical distributed parameters and the line impedance, the specific phase-to-ground voltage variation rules corresponding to SLG fault on each phase are then discussed. Based on the voltage variation rules, a robust and practical method for faulty phase identification is proposedwhich does not need the distributed parameters. Finally, the correctness of the variation rules and the availability of the proposed method is verified by simulation. Comparative study shows the proposed method has better accuracy and applicability with the conventional methods when the asymmetry and ground-fault resistance are high.

In the non‐effectively grounding distribution system, residual current under single‐line‐to‐ground (SLG) fault threatens the safety of human being and power supply equipment. Active arc suppression device has been proved to be effective for SLG fault arc suppression when the line impedance is ignored. However, in practice, line impedance varies with the fault location and the load current flowing through the impedance brings about additional voltage drop, which increases the fault current and is not dealt with by the conventional methods. To achieve accurate SLG fault arc suppression with the existence of line impedance, the neutral‐to‐ground voltage reference for full ground‐fault current compensation is firstly derived and a detection method is then proposed. The pre‐fault and post‐fault line currents are used to eliminate the influence of load current on the line impedance voltage drop. A dual‐loop voltage and current controller is then designed. The prototype of active arc suppression device was developed. The results of simulation and prototype experiment validate the effectiveness of the proposed method.

Precise fault recognition of motor rolling bearing fault is playing a significant role in any machinery and equipment. However, conventional decomposition methods fail to completely reveal the fault signal information of motor rolling bearing due to mixed modes problem. To solve the problem, the median-point mode decomposition (MMD) method is presented. The MMD method uses sort-based inversion to sort out each variation of the same time interval for better and specific mode decomposition, with the assistance of the advanced envelope curve formed by the median points between adjacent extreme points. It certainly alleviates the mixed mode during the iteration of intrinsic mode functions (IMFs). Therefore, comparison results are simulated in the proposed MMD method with conventional methods. Experiment of motor rolling bearing fault is operated for fault recognition in order to demonstrate the MMD algorithm.

A single line-to-ground (SLG) fault may lead to a more severe line-to-line fault and power supply interruption if the ground-fault current exceeds a certain value. Arc suppression device (ASD) is a good solution to eliminate the ground-fault current. A novel hybrid ASD is proposed in this article, which consists of a passive device and an active one. The passive device utilizes multiterminal breakers and an isolation transformer to couple a secondary voltage of a zig-zag grounding transformer to the neutral point to compensate the majority of the ground-fault current. The active device uses a single-phase voltage source inverter to eliminate the residual fault current due to the leakage inductance of the zig-zag grounding transformer in the passive device. A dual-loop voltage and current control method for the active device is designed for the accurate residual current compensation. Results of simulation and prototype experiment validate the effectiveness of the proposed hybrid ASD. The proposed hybrid ASD does not need to detect distributed line-to-ground parameters, and it has lower cost, less control complexity, higher reliability, and better performance, compared to other ASDs.

In the non-effectively grounding distribution system, residual current under single-line-to-ground (SLG) fault threats the safety of human being and power supply equipment. Active arc suppression device has been proved to be effective for SLG fault arc suppression when the line impedance is ignored. However, in practical, line impedance varies with the fault location and the load current flowing through the impedance brings about additional voltage drop, which increases the fault current and is not dealt with by the conventional methods. To achieve accurate SLG fault arc suppression with the existence of line impedance, the neutral voltage reference for full ground-fault current compensation is firstly derived and a detection method is then proposed. The pre-fault and post-fault line currents are used to eliminate the influence of load current on the line impedance voltage drop. A dual-loop voltage and current controller is then designed. The prototype of active arc suppression device was developed. Results of simulation and prototype experiment validate the effectiveness of the proposed method.

The power distribution system generally adopts neutral non-effective grounding. Most grid faults are single-line to ground (SLG) faults. As the SLG faults are difficult to monitor online, they severely threaten human health and lives and damage power supply equipment. The Peterson coils are widely used in the SLG fault arc suppression, but it belongs to passive arc suppression method which cannot compensate the active power and harmonic current. The ground residual current is still large. Active arc suppression method makes up for the defect of Peterson Coil. In order to improve the self-healing effect of single-phase grounding fault. This paper summarizes the research status of active full compensation technology, and further studies the traditional arc suppression strategy based on active voltage control. An improvement scheme of injection current control strategy is proposed. The ratio of the zero-sequence current and the zero-sequence voltage is applied to modify the reference value of the injection current instead of measuring the ground parameters. Results of simulation validate the effectiveness of the proposed control strategy.

Single-line-to-ground (SLG) fault arcs cause the intermittent over-voltages, leading to short circuits within large-scale area. In this paper, a flexible arc suppression device is proposed, which adopts a zigzag grounding transformer, multi-terminal breakers and an isolation transformer to regulate zero-sequence voltage. Principle of zero-sequence voltage regulation (ZVR) and arc suppression method is presented. By carefully choosing the secondary side voltages of grounding transformer as the input of isolation transformer, the zero-sequence voltage is nearly constrained to the opposite of faulty phase supply voltage, and the ground-fault current is limited to a very small value. This device has high adaptability for low-resistance ground fault as it doesn't adopt any electronic apparatus. A 10kV prototype was established and multiple fault conditions were experimented to verify the effectiveness of the proposed ZVR device and its arc suppression principle.

As a common control method for blades, variable pitch technology is used to regulate the output power of the turbine in region 3. To cope with the random variation of wind speed, a novel pitch angle controller is designed in this paper. The proposed method applies a selfcoupled PI (SC-PI) to obtain the desired speed tracking performance, and employs an adaptive neuro-fuzzy inference system (ANFIS) to compensate the controller for disturbance damping capability. A contrast experiment with a baseline PI is conducted in Matlab/Simulink. The performance superiority of the proposed hybrid control method is thus confirmed.

Faulty phase recognition under single-line-to-ground (SLG) fault condition is critical for some arc suppression methods in medium voltage (MV) distribution networks. Conventional methods tend to be invalid due to network asymmetry in high-resistance ground-fault condition and the difficulty of detecting phase-to-ground parameters in practice. In this paper, the variation of the three phase-to-ground voltages with the change of SLG fault resistance is analyzed in detail, and the magnitude and phase angle variation rules of the voltages are obtained. Then, the unique variation rules on SLG fault for each phase are separately and strictly discussed in theory and numerical analysis. Furthermore, a faulty phase recognition method is proposed based on the variation rules of three-phase voltages. Simulation results in PSIM verify the proposed variation rules and validate the effectiveness of the proposed faulty phase recognition method, which ensures accurate recognition of faulty phase in high-resistance ground fault.

Neutral-to-ground overvoltage may occur in non-effectively grounded power systems because of the distributed parameters asymmetry and resonance between Petersen coil and distributed capacitances. Thus, the constraint of neutral-to-ground voltage is critical for the safety of distribution networks. In this paper, an active grounding system based on single-phase inverter and its control parameter design method is proposed to achieve this objective. Relationship between its output current and neutral-to-ground voltage is derived to explain the principle of neutral-to-ground voltage compensation. Then, a practical current detection method is proposed to specify the reference of compensated current. A current control method consisting of proportional resonant (PR) and proportional integral (PI) with capacitive current feedback is then proposed to guarantee sufficient output current accuracy and stability margin subjecting to large range of load change. The PI method is taken as the comparative method and the performances of both control methods are presented in detail. Experimental results prove the effectiveness and novelty of the proposed grounding system and control method.

Most renewable energy distribution systems operate with neutral point ineffectively earthed. Traditional arc fault suppression devices, such as Petersen Coil, compensate the single-phase grounding fault caused capacitive current with zero sequence inductive current. They cannot compensate the active and harmonic currents, thus have limited arc suppression performance for single-phase grounding faults. To solve these problems, an arc suppression device based on single-phase inverter is presented in this paper. When single-phase grounding faults occur, the device injects active, reactive and harmonic currents to the neutral point to force the faulted phase voltage to zero, the faulted current will be limited to zero and the fault arcs can thus be reliably extinguished. The proposed voltage control method uses the zero sequence voltage and faulted phase voltage as reference to generate the injection currents. It is not needed to measure the capacitive and fault residual currents of the distribution system, which are difficult to implement. The simulation results validate the effectiveness of theoretical analysis.