As a result of small fault current, high level of noise and a large penetration of distributed generators (DG), in the neutral non-effectively grounded medium-voltage (MV) distribution networks, it is quite difficult to locate the faulted line section for single phase to ground faults. In this paper, using a technique based on synchronized measurements in distribution networks, a fault location method based on the analysis of the energy of the transient zero-sequence current in the selected frequency band (SFB) is proposed. The equivalent impedance of the distribution network with lateral branches is studied with an equivalent network, and the phase-frequency characteristics of the equivalent impedance are analyzed. The SFB, within which the transient energy of the faulty line section is larger than that of the healthy line sections is determined. A combined fault-section location criterion is proposed and the implementation scheme is illustrated with the distribution level phasor measurement units. Numerical simulations of the IEEE 34 node system and the field experiments of a 10kV distribution network validate the feasibility and effectiveness of the proposed method.

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Location of Single Phase to Ground Faults in Distribution Networks Based on Synchronous Transients Energy Analysis

Traditional ground-fault arc suppression devices mainly deal with capacitive component of ground current and have weak effect on the active and harmonic ones, which limits the arc suppression performance. The capacitive-current detection needed in them suffers from low accuracy and robustness. The commonly used large-capacity reactive component may bring about overvoltage because of possible resonance with the distributed phase-to-ground capacitance. To solve these problems, an active ground-fault arc suppression device is presented. It employs a topology based on a single-phase inverter to inject current into the neutral without any large-capacity reactors, and, thus, avoids the aforementioned overvoltage. It compensates all the active, reactive, and harmonic components of the ground current to reliably extinguish the ground-fault arcs. A dual-loop voltage control method is proposed to realize arc suppression without capacitive-current detection. Its time-based feature also brings the benefit of fast response on ground-fault arc suppression. The principle of full current compensation is analyzed, together with the controller design method of the proposed device. Experiment on a prototype was carried out to validate the effectiveness of the device.

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Principle and Control Design of Active Ground-Fault Arc Suppression Device for Full Compensation of Ground Current

The traditional resonance grounding method is hard to completely satisfy the requirement of suppressing normal neutral-to-ground voltage and self-extinguishing grounding fault arcs in asymmetry distribution systems. For improvement, a new grounding method of a Petersen-coil in parallel with a reactor is developed, and some novel techniques for insulation parameters measurement and Petersen-coil control are proposed in this paper. An interpolation algorithm is presented to calculate the distribution systems resonance frequency. The insulation parameters (including capacitance to earth, leakage conductance, damping coefficient, and dissipation factor) are measured by the injecting resonance signal. The measured insulation parameters are then applied in Petersen-coil tuning. The new grounding equipment and the device for insulation parameters measurement and Petersen-coil control are developed. The results of laboratory tests and distribution systems operation show that the novel Petersen-coil control techniques can satisfy the requirement of asymmetry distribution systems. The proposed measurement method of insulation parameters is simple, convenient, precise, and tested in real time without power supply interrupting.

Some Novel Techniques for Insulation Parameters Measurement and Petersen-Coil Control in Distribution Systems

This paper proposes a flexible neutral grounding method based on electromagnetic hybrid Petersen coil (EHPC), which consists of a magnetically controlled reactor and an active power compensator (APC). Under normal conditions, the system's neutral point grounds through a reactor of high reactance. When a single line-to-earth fault occurs, the EHPC can be controlled flexibly to suppress the recovery voltage of faulty phase and to reduce the ground-fault current to almost zero. This paper expounds the two operating modes and corresponding characteristics, the measuring method of insulation parameters of the system (mainly including capacitance to earth and leakage resistance), and the full compensation principle for arc suppression based on the equivalent negative impedance of EHPC. This paper also proposes a compound control method of neutral-to-earth impedance based on neutral-point displacement voltage and a compensating method for the harmonic component of the ground-fault current. Compared to traditional grounding methods, this method can be fast and reliably make the residual current approach almost zero without arc reignition while greatly reducing the output capacity of APC and, thus, decreasing the cost of this device. Simulations and experiments verify the correctness of this measuring method and feasibility of this flexible grounding control strategy.

A Novel Neutral Electromagnetic Hybrid Flexible Grounding Method in Distribution Networks

Neutral-to-ground overvoltage may occur in noneffectively 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 a 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 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.

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Principle and Design of a Single-Phase Inverter-Based Grounding System for Neutral-to-Ground Voltage Compensation in Distribution Networks

Many mine power systems operate with a floating neutral or are high-resistance grounded, and earth fault current is no more than a few tens of amperes. Traditional earth-fault-detection methods based on zero-sequence current have poor sensitivity in this case. For improvement, a fault-detection scheme with fault-current and fault-resistance measurement is presented in this paper, which is capable of detecting high-impedance ground faults. The scheme has been verified by electromagnetic-transient-program simulation, and a fault-detection prototype has been developed and tested in the laboratory. Results show that the scheme has high sensitivity and robustness.

Sheng Su

Papers

Ground-Fault Feeder Detection With Fault-Current and Fault-Resistance Measurement in Mine Power Systems