Single-phase Short-circuit Currents Research Articles

PROTECTION COORDINATION OF PHOTOVOLTAIC POWER PLANT IN THE TIME DOMAIN

The safety of workers and equipment in the power grid requires the shutdown of power plants in case of maintenance or malfunction. The shutdown relies on protective devices that must be properly coordinated to isolate only the part of the grid affected by the malfunction. Using the DIgSILENT PowerFactory software, an analysis was conducted on a model of an accurate real grid integrating a photovoltaic power plant with a capacity of 390 kW. Protection coordination testing was carried out for three-phase, two-phase, and single-phase short circuit currents at arbitrarily selected locations at medium and low voltage levels. Protection in the time domain is coordinated on lines and busbars to determine the speed and selectivity of protective devices. The analysis results indicate that adequately adjusting the three-phase short circuit at the main transformer output 110/10 kV with an impedance of 0 Ω and an allowed protection operation time of up to two seconds can be correctly addressed within 36.7 ms.

General average model of T-type three-level converter for active compensation circuit of distribution network

The active compensation device, between the distribution network’s neutral point and the ground, compensates for the single-phase short-circuit current. And the T-type three-level circuit is a possible scheme for compensation devices. This paper proposes the T-type average model based on the above topology, with a controlled voltage/current source, regardless of modulation or control methods. Compared with the two-level converter topology, the three-level converter topology improves the rated voltage stress, with few switching devices’ requirements. The modeling and simulation indicate that the T-type’s average model is identical to its switch model. And the T-type three-level converter can achieve the desired active compensation.

Features of asymmetric short circuit currents calculation in the elplek software product

In order to simplify the analysis of the electrical networks operation modes , specialized software tools for mathematical modeling of processes in them are used. There are many software products, but most of them are paid. To carry out research in complex power systems and obtain results in certain sections when performing practical and course work, as well as at certain stages of research, the authors actively use the free software product «Elplek», but in a deeper study of the software encountered certain features some parameters, which differ slightly from the generally accepted domestic method, which led to an error in the calculations in some versions of the schemes of power systems. The purpose of the research is to study the peculiarities of calculating the characteristics of asymmetric short circuits in the software product "Elplek" and to check the adequacy of the formed models. At the first stage, the object of research was chosen – this is a segment of the electrical network containing a small number of nodes and a theoretical calculation of the searched parameters of the short circuit mode in this segment of the electrical network was carried out. In the future, an approach was proposed on the task of parameters of elements in the software product "Elplek" and based on this approach, modeling was carried out as a result of which the results were obtained under the condition of the system's task, as sources of infinite power and calculations for the active part differ by 0.013%, and by reactive – 0.004%; subject to the task of the system, as sources of a certain power and calculations on the active part differ by 0.0046%, and on the jet - 0.0044%, which makes it possible to assert the feasibility of using the proposed approach in the analysis of indicators of emergency modes of complex schemes of electrical supply to consumers of certain segments of electric power systems. Features of calculation of characteristics of asymmetric short circuits in the Elplek software product are considered and the adequacy of the generated models is checked. Key words: power system segment modeling, Elplek software product, element parameter setting, single-phase short-circuit currents

ПРОДОЛЬНОЕ СОПРОТИВЛЕНИЕ ОДНОЦЕПНОЙ ЛЭП С ДВУМЯ ГРОЗОЗАЩИТНЫМИ ТРОСАМИ В ЗАДАЧЕ ДИСТАНЦИОННОГО ОМП

One of the most important tasks in remote fault location in a 110 kV+ network is modeling the power lines taking into account the electromagnetic and electrostatic interaction of their conductive elements. A common approach is to use the mathematical models recommended by the guidelines for calculating short-circuit currents for relay protection. However, these models of power transmission lines take into account the geometric mean distance between the wires and the lightning protection cables, which means they average the values of the mutual induction impedances. In cases where wires and cables are located asymmetrically relative to each other, this leads to an overestimation of the zero-sequence impedance of power lines and, as a consequence, to an underestimation of the single-phase short-circuit current. This situation can affect the results of remote fault location. To eliminate this drawback, the article proposes a combined equivalent circuit for a single-circuit power transmission line with two lightning protection cables, based on a combination of methods of phase coordinates and symmetric components. The given equivalent circuit of power transmission lines allows calcula¬ting the mode parameters using the method of symmetric components and, at the same time, simulating those of interest for calculating power lines using phase coordinates.

Practical Calculation Method for the Short-Circuit Current of Power Grids with High Temperature Superconducting Fault Current Limiters

With the expansion of the power grid scale and the enhancement of network structure interconnection, the short-circuit current of power grids is increasingly close to the upper capacity limit of circuit breakers. The fault current limiter is effective in suppressing the short-circuit current of power grids, and with the mature development of superconducting technology, high-temperature superconducting fault current limiters (HTS-FCLs) have been widely used in power grids. Due to the nonlinear impedance–current characteristic of HTS-FCLs, the traditional short-circuit current calculation method is inapplicable to power grids with HTS-FCLs. First, based on the establishment of the electromechanical transient model and the short-circuit calculation model, the short-circuit current calculation procedure for power grids with HTS-FCLs is proposed. Second, the calculation methods for three-phase and single-phase short-circuit currents at the substation bus in power grids with HTS-FCLs are analyzed. Finally, three-phase and single-phase short-circuit currents at the 500 kV Xijiang substation bus in the Guangdong power grid are calculated as the study case. Simulation results show that compared with the situation where there is no HTS-FCL installed, the short-circuit current on the 500 kV bus decreases from 65.34 to 58.31 kA in terms of the three-phase short-circuit fault, and decreases from 54.56 to 46.62 kA in terms of the single-phase short-circuit fault, thus verifying the effectiveness and practicability of HTS-FCLs in suppressing the short-circuit current, which is significant for the safe, stable and reliable operation of the power grid.

Device for limiting single phase ground fault of mining machines

The paper shows the reasons and consequences of the single-phase ground fault. With all the variety of devices for limiting the current single-phase ground fault, it was found that the most effective are Peterson coils having different switching circuits. Measuring of the capacity of the network is of great importance in this case, a number of options capacitance measurement are presented. A closer look is taken at the device for limiting the current of single-phase short circuit, developed in the Far Eastern Federal University under the direction of Dr. G.E. Kuvshinov. The calculation of single-phase short-circuit currents in the electrical network, without compensation and with compensation of capacitive current is carried out. Simulation of a single-phase circuit in a network with the proposed device is conducted.

Напряжения, наводимые на грозозащитном тросе воздушной линии электропередачи, как фактор риска. Часть 2

To ensure the personnel safety during live working on overhead transmission line (OTL), as well as lightning protection
 cables’ (LPC) isolation integrity maintaining, by the example of three-phase 750 kV OTL has been presented an algorithm
 for calculation of voltages, electro-moving forces and currents induced by single phase short-circuit currents’ magnetic field
 of OTL in LPC1 grounded in one point, and in LPC 2 (with fiber-optic communication line function) grounded at every tower.
 For single-phase short circuit modes has been presented an algorithm for length selection of grounded on one end LPC1
 part on the condition of compliance with the maximum permissible voltage level on a spark gap which bypasses an
 insulator set of LPC1.

Development of power supply devices for limitations of short circuit on the ship's hull

The authors of the paper have analysed the reasons and consequences of single-phase ground faults (hull faults). For all varieties of devices limiting the current single-phase ground faults, the most effective devices were found to be the arc-suppression coils with different switching circuits. In this case the measurement of circuit capacitance takes on a great importance. A number of variants of capacitance measurement is presented in the paper. The authors have had a detailed look at a device, limiting the single-phase short-circuit current. This device was developed on the basis of the Far Eastern Federal University under the direction of Dr. G.E. Kuvshinov. The device is provided with power supply that converts alternating current (AC) to direct current (DC), and is realised due to the use of semiconductor devices - transistors and diodes - in a bridge circuit. The technical outcome of this power supply application consists in the reduction of size and weight parameters (compared to the closest analogues) in order to connect the controlled voltage rectifier to the alternating voltage source, including mass and size of the capacitor bank of the current limiting circuit, and enhancing the dynamic parameters of the stage of uncontrolled charge of the output capacitor of the controlled voltage rectifier.

Research of Installing Small Reactors on Neutral Points of 500 KV Autotransformers and the Analysis of Overvoltage

Because of the extensive application of autotransformer in 500 kV power grid, the value of single-phase short-circuit current is generally greater than that of three-phase short-circuit current, or even greater than the maximum rated through current of the breaker. In order to reduce single-phase short-circuit current and avoid implement of bus split, this paper proposes to install small reactors on neutral points of 500 kV autotransformers. By analyzing and calculating the effect on short-circuit current limitation, the most suitable impedance value of small reactor is proposed. In addition，the overvoltage of the neutral point and insulation coordination are particularly discussed.

The Problem of Coordination of Levels of Short-Circuit Currents in Power Systems

The problem of coordination of short-circuit (fault) currents is considered. Data on the dynamics of variation of the levels of single-phase and three-phase short-circuit currents (their highest values) in networks rated for different voltage are presented for a long-term period. The main factors (integral parameters of the networks) affecting the values of short-circuit currents are listed. Statistics of variation of rated parameters of electrical equipment in past years and in the recent period is presented and predictions for the future are made. Basic methods and means for limiting short-circuit currents are discussed. A method of automatic and stationary network separation and a method of circuit design are described in detail. The efficiencies of different methods are compared. The effect of integral parameters of the network on the maximum level of short-circuit currents is shown. The aspects of the switching life of breakers and of allowance for probabilistic characteristics of faults and for the risk factor of decision-making are considered. It is shown that coordination of the levels of short-circuit currents is a possible means for raising the reliability of power installations and systems.

Grounding Banks of Transformers with Neutral Impedances and the Resultant Transient Conditions in the Windings

The question of grounding transformer bank neutrals through different impedances has recently arisen due to the desire to limit system single-phase and two-phase short-circuit currents. This limitation has been required to ease the duty on circuit breakers and assist in maintaining system stability. The use of resistance, inductance, and various combinations has been studied as to the effect on the short-circuit current and particularly upon lightning transients within the transformer windings and at the neutral. It was found that the use of resistance only may be undesirable on account of the high voltage at the neutral in limiting the short-circuit current. The use of inductance only may result in high voltages within the transformer and at the neutral due to lightning transients, which necessitates that the transformer be fully insulated throughout. Methods using parallel paths with the inductance, these parallel paths being designed primarily to reduce the lightning transients at the neutral, have been studied and found to limit the transients within the transformers to values approximating those for solidly grounded neutral which permit the grading of the transformer insulation. The method to be selected depends upon the individual case, but generally the use of the valve type lightning arrester is the simplest to apply.