Neutral Ungrounded System Research Articles

Single-Phase Grounding Fault Identification Based on Transition Conductance Tracking Method

The instability of single-phase grounding faults makes it difficult to track the fault development process accurately. In this paper, a transition conductance tracking method for single-phase grounding fault identification is established to perceive the health state of the distribution network. To solve the situation that the insulation parameters of the neutral ungrounded system are difficult to obtain, a phase-shift method is proposed to measure the insulation parameters accurately. Based on the measurement results of insulation parameters, a transition conductance tracking model is constructed to monitor the changes of transition conductance in real time. Since the transition conductance varies steadily, it can be used as an important parameter to evaluate the development of a single-phase grounding fault. The innovative tracking method is verified by the simulation model, and the error analysis shows that the tracking error of transition conductance is less than 9%. The simulation results demonstrate that the proposed method can track the development of grounding faults effectively and give a quick warning to the operator. The fault identification theory adopted in this paper is not limited by the neutral grounding mode and especially can be applied to situations where the neutral point parameters are not adjusted.

Fault location method for neutral ungrounded system based on C- type travelling wave method

In China, the 10kV distribution network operates with neutral ungrounded. Due to the influence of high transition impedance and branches, the problem of fault location has not been effectively solved. The characteristics of C-type travelling wave method at neutral ungrounded system are analyzed in this paper. Multi-point fault location method based on C-type travelling wave method is proposed. By actively injecting high-voltage pulses into the circuit and using multiple measuring devices to detect the reflected wave of the fault point, the effect of high-impedance- grounded and branch points on the reflected wave of the fault point is effectively reduced, and the success rates of reflected wave detection of the fault point are improved. The validity of the method is verified by simulation.

Mechanism analysis of ferromagnetic resonance of electromagnetic voltage transformer in neutral ungrounded system

In the neutral point ungrounded power distribution system, due to the nonlinear characteristics of the electromagnetic voltage transformer (PT) excitation inductance, ferromagnetic resonance overvoltage is easily generated under certain conditions, which seriously affects the safe operation of the system. This paper takes the Yecheng substation 6 kV distribution network as the model, and uses ATP-EMPT electromagnetic transient calculation software to theoretically analyze the characteristics and mechanism of ferromagnetic resonance caused by electromagnetic voltage transformer, and points out the main problems of ferromagnetic resonance in PT operation. This article discussed the existing harmonic elimination measures, proposed some effective suppression schemes and demonstrated the effectiveness of some mainstream harmonic elimination methods. The author’s work can provide a good reference for the prevention and control of ferromagnetic resonance overvoltage, and it also has high practical value for improving the power supply safety and reliability of the distribution network.

Impact Transient Characteristics and Selection Method of Voltage Transformer Fuse

In a neutral ungrounded system, the high voltage fuses used to protect voltage transformers (VTs) often abnormally blow out, causing unbalanced VT operation. Fuses also fail to blow out in time, resulting in further damage to the VT. This paper reported the results of steady-state current testing, breaking characteristics, X-ray measurements, fuse corona testing, and electromagnetic transient impact testing for VT fuses. This paper comprehensively examines and analyzes the quality and electrical performance of VT fuses and provides new guidance for the use of high voltage fuses in voltage transformers. This paper recommends that 35 and 10 kV systems use fuses rated for a current of 1 A based on a single fuse, which is not easily oxidized and has a wound skeleton composed of an Ag or Ni melt.

Reference voltage calculation method based on zero-sequence component optimisation for a regional compensation DVR

ABSTRACTThis paper describes the design of a dynamic voltage restorer (DVR) that can simultaneously protect several sensitive loads from voltage sags in a region of an MV distribution network. A novel reference voltage calculation method based on zero-sequence voltage optimisation is proposed for this DVR to optimise cost-effectiveness in compensation of voltage sags with different characteristics in an ungrounded neutral system. Based on a detailed analysis of the characteristics of voltage sags caused by different types of faults and the effect of the wiring mode of the transformer on these characteristics, the optimisation target of the reference voltage calculation is presented with several constraints. The reference voltages under all types of voltage sags are calculated by optimising the zero-sequence component, which can reduce the degree of swell in the phase-to-ground voltage after compensation to the maximum extent and can improve the symmetry degree of the output voltages of the DVR, thereby effectively increasing the compensation ability. The validity and effectiveness of the proposed method are verified by simulation and experimental results.

Fault Line Selection in Distribution Networks Based on Parameter Identification in Frequency Domain

The distribution network is mostly the ungrounded neutral system or the resonant grounded system in many countries. Selecting the fault line as soon as possible is of great significance to power systems. This paper proposes a parameter identification method in frequency domain to solve this problem. By using the matrix pencil method to extract frequency-points and their information in zero-sequence voltage and zero-sequence current, spectral leakage in DFT is avoided. With the estimated frequency points, an equation in frequency domain is established which will help to calculate the capacitance value and thus to identify the fault line. This method requires a low sampling rate and has high accuracy. In addition, the frequency-band in this new method is wider than the SFB's frequency-band. Verified by simulation in ATP-EMTP and data from RTDS, the new method is quite effective in fault line selection with an ability to overcome transition resistor and is immune from the impact of arc-suppression.

The Calculation of Earth Capacitance Based on Least Square Method

The grounding capacitive current is one of the most important parameters of ungrounded neutral system. Usually this parameter is obtained with the method of additional capacitor or metal grounding. However, the former is complicated and the additional capacitor has negative effects on measurement results, and the later is dangerous when used in testing. In view of that, the paper proposes a new method to calculate grounding capacitance current based on the data of single-phase-grounding faults in power system with indirectly earthed neutral. This method reduces the error by using the least-square method .The ATP simulation results show that this method is accurate basically.

중성점 비접지 UPS시스템의 부동현상에대한 실험적 연구

In this study we investigate the background of UPS output transformer(△-Y) operation on neutral ungrounded system, and perform field experiment of neutral floating-phenomenon for the inter-locked securing of electric facilities and improvement of neutral floating-phenomenon. Experiment results show that a floating-phenomenon occurs when four-pole input-breaker trip and the neutral to earth voltage occurring concurrently is identified as phantom voltage. And the measures for the transient overvoltage that can occur in floating-state also are investigated. As a result, the risk elements of neutral ungrounded method as well as neutral floating-phenomenon are removed by installation of separate solidly-grounded transformer and switching four-pole input-breaker to three-pole input breaker.

Ground Selector for Ungrounded Three-Phase Distribution Systems

Ever since three-phase distribution has been known, a controversy has been going on with respect to the relative merits of grounded and ungrounded neutral. The inability of locating and isolating a ground fault is the chief disadvantage of the ungrounded neutral system, this being the reason why lately the grounded neutral has been given preference. The purpose of the so-called ground selector is to provide means to permit the selective clearance of a ground fault in an ungrounded neutral system. The device thus eliminates the mentioned disadvantage and raises the ungrounded neutral system to the same operating ating standard as the grounded neutral system, retaining at the same time the many other advantages inherent to the ungrounded system. The paper describes the functioning of the ground selector and compares in detail the two systems. Two ground selector installations on largely different systems are described and the practical operating results are reported and discussed. The conclusion is reached that the ungrounded neutral system equipped with a ground selector combines the advantages of the ungrounded and grounded neutral systems without being afflicted with their respective disadvantages.