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This paper addresses the problem of the power flow analysis of bipolar direct current (DC) networks considering unbalanced loads and the effect of a neutral wire, which may be solidly grounded or non-grounded. The power flow problem is formulated using the nodal admittance representation of the system and the hyperbolic relations between power loads and voltages in the demand nodes. Using Taylor series expansion with linear terms, a recursive power flow method with quadratic convergence is proposed. The main advantage of the hyperbolic approximation in dealing with power flow problems in DC bipolar networks is that this method can analyze radial and meshed configurations without any modifications to the power flow formula. The numerical results in three test feeders composed of 4, 21, and 85 bus systems show the efficiency of the proposed power flow method. All of the simulations were conducted in MATLAB for a comparison of the proposed approach with the well-established successive approximation method for power flow studies in distribution networks.

In the three-phase four-wire system, there are usually multiple grounding points in the primary neutral line due to safety and economic considerations. The rising “neutral to earth voltage (NEV)” caused by a broken primary neutral can threaten the safety of nearby facilities and humans; therefore, the integrity of the primary neutral conductor is of vital importance for the multi-grounded neutral (MGN) system. In this paper, a new passive method is proposed to monitor the integrity of the primary neutral line in the MGN system. The method is based on the measured voltage and current data at the service transformer, and there is no need to install any signal generators. Therefore, it causes no disturbance to the utility and customer. In the paper, the equivalent analysis circuit is established and a new parameter is proposed to reflect the neutral condition. The value of the parameter is estimated based on the measured data, and then, the equivalent impedance of the primary neutral groundings can be obtained. On the other hand, the impedance value for the primary neutral under normal operating conditions can be estimated based on the derived analytical formulas. By comparing the monitored primary neutral impedance with its normal value, the broken neutral condition in the primary system can be detected. Different primary neutral broken cases are analyzed in the paper based on the Monte Carlo simulation. The results indicate that the integrity condition in the primary neutral can be accurately monitored by the proposed method.

Modular multilevel converter (MMC) is troubled by the inherent second harmonic of single-phase ac power. By adding feedforward to the modulation signal, low-frequency harmonic voltages are significantly reduced thus overcoming an important weakness of MMC. Capacitors of sub-modules do not have to be over-sized. If desired for protection, transformers can be wye-connected with grounded neutral. The reduction method is based on harmonic function analysis. In addition to deriving algebraic formulae of harmonic voltage components of ‘open loop’ control, this study makes original contributions by deriving formulae which include feedforward control. Analytical insights from the formulae have shown the way to design feedforward methods to reduce low-frequency harmonics. Validation is by simulations using SIMULINK/MATLAB.

Programmable Logic Device(PLD) has been widely used in hardware circuit, and it has evolved into two types: Complex Programmable Logic Device(CPLD) and Field Programmable Gate Array(FPGA). This paper takes small current grounding in electric power system as background, uses xc95144, a representative CPLD of Xilinx Company in the signal collecting circuit to collect voltage and current signals, and do some other operations to spare circuit board area. This paper also tries to translate the schematic into VHDL-described text and do simulation to the text.

Single-phase ground fault line selection problem has never been satisfactorily resolved in small current neutral grounding system in china. According to such a situation and combining theoretical analysis and simulation research with reports published in literature, existing faulty line selection technologies are generalized, and working principles, application conditions and existing defects of these technologies are analyzed, meanwhile the feasible improvement of these technologies and the up-to-date research achievements, and deficiencies of applications in this field are summarized. Finally, the development trend of faulty line selection technology for small-current neutral grounding system is pointed out.

In order to reduce grounded current and restrain arc-grounded over voltage during single-phase grounded faults in transmission line and realize fault-line selection, the neutral grounded mode with Peterson coil in parallel with big reactor is proposed in the paper. The simulation and comparative analysis of the performance of the 10 kV distribution grid systems in different neutral grounded modes have been done. The simulation results have shown that the neutral grounded mode via Peterson coil in parallel connection with big resistances can restrain the fault current effectively and it is the best mode.

Ferroresonance is a usual internal overvoltage in power system; it mostly occurs in distribution network that neutral hasn’t grounded directly, but it also happens in the high voltage grid that neutral-grounded system frequently. This paper introduced the fundamental theory of ferroresonance. An 110kV substation has been used as the model, to emulate the waveform of its ferroresonance, and also discussed the measure to remove the ferroresonance.

Neutral un-effectual grounded system (NUGS) is widely used in Chinese medium voltage power distribution network. However, existing earthed fault location methods in NUGS are not satisfactory in practical application due to weak fault current and unstable arc. Based on analysis of spectrum characteristics of current signals in single-phase earthed fault feeder, this paper proposed an earthed fault location method based on support vector machine (SVM). Firstly, the zero sequence current signal was sampled, and its spectrum was gotten by discrete Fourier transform. Secondly, the three dimensional feature vector that constituted by fundament, harmonic, inter harmonic component, had been extracted as the input of SVM. Furthermore, the SVM was trained with the samples obtained by simulation under different fault distance, different fault resistance and different fault close angle. Finally, the earthed fault feeder is selected using the trained SVM. The simulation results indicate that the fault location by the presented method is more precise.

The electrical insulation withstand levels of individual pieces of equipment, and power‐system insulation coordination, are essential engineering subjects. However, before discussing these topics, this chapter examines all types of overvoltage phenomena, including steady‐state and temporary overvoltages that occur under nominal or temporary conditions, as well as overvoltages due to lightning or switching. Neutral‐grounding methods can be classified as effective neutral grounding (or solidly neutral grounding) and noneffective neutral grounding. The difference between the two practices is the difference in the zero‐sequence circuit from the viewpoint of power network theory. Therefore, all power‐system behavior characterized by the neutral‐grounding method can be explained as phenomena related to the characteristics of the zero‐sequence circuit. The chapter explains some typical features of neutral‐grounding methods. By using a plain expression for the non‐effective grounded system, grounding fault currents can be reduced considerably, but on the other hand, higher temporary overvoltages are caused during faults.

The selection of the faulty feeder in the case of single phase-to-earth fault in a Neutral Un-effectively Grounded System (NUGS) has been one of the conventional problems of distribution network. The compensation method is introduced to highlight the difference of the zero-sequence admittances between the faulted feeder and the sound feeder. In this way, the setting of the criterion becomes feasible. The on-site requirement to the protection is also involved in this paper. A variety of simulation tests were carried out by virtue of ATP software. The results demonstrate that this algorithm is immune to the variation of network structures and parameters, fault conditions, and fault resistance etc. The successful rate of feeder selection is proved to be improved to a great extent.