A Method Based on Only Currents for Determining Fault Direction in Radial Distribution Networks Integrated with Distributed Generations

Nowadays, more distributed generations (DGs) are connected to a radial distribution network, so conventional overcurrent relays cannot operate correctly when a fault occurs in the network. This study proposes a method to determine the fault direction in a three-phase distribution network integrated with DGs. The obtained pre-fault and fault currents are utilized to extract their phasors by the fast Fourier transform, and the phase angle difference between the positive-sequence components of the pre-fault and fault currents is used. Moreover, the method only uses the local current measurement to calculate and identify the phase angle change of the fault current without using the voltage measurement. Matlab/Simulink software is used to simulate the three-phase distribution network integrated with DGs. The faults with different resistances are assumed to occur at backward and forward fault locations. The simulation results show that the proposed method correctly determines the fault direction.

Influence of Single-Phase Voltage Loss and Load Carrying Mode on Mine Drainage Pump Motor in Vietnam

Mine drainage pump is the most important load in mining which requires high reliabilitywhen operating. Currently, the power supply of a mine drainage pump is connected to the same powerline with many nonlinear loads, and is equipped with power electronic converters, which makes thepower supply nonsinusoidal. During the working process of a mine drainage pump, the load-carryingfactor often changes, and many types of failures occur, among which single-phase voltage loss is themost common problem. In the case of a nonsinusoidal power supply, if a single-phase voltage lossoccurs in different load modes, it will greatly affect the working mode of the mine drainage pumpleading to influences on the working efficiency, the life of the pump, and sometimes it is necessary torecalculate the protection parameters. This paper studies the influence of single-phase voltage loss andload carrying mode on the working mode of mine drainage pump motor in case the of nonsinusoidalpower supply. Research results show that, in the case of nonsinusoidal power supplied with single-phasevoltage loss, copper losses in the rotor and stator circuits increase with increases in voltage totalharmonic distortion (THD) and load-carrying factor, 5th order reverse harmonic increases copper loss inasynchronous motor the most, and higher harmonic components have less effect on copper loss in themotor. At the same time, the speed ripple decreases with the increase of the motor load factor anddecreases in the presence of the 5th order negative sequence harmonic, and increases significantly in thepresence of the 7th order positive sequence harmonic. 5th order negative sequence harmonic increases,the torque ripple increases, while the 7th order positive sequence harmonic reduces the torque ripple inthe case of single-phase voltage loss. The results of the paper will help improve the operationalefficiency of the mine drainage pump in Vietnam's mines.

Analysis of Stationary- and Synchronous-Reference Frames for Three-Phase Three-Wire Grid-Connected Converter AC Current Regulators

The current state of the art shows that unbalance and distortion on the voltage waveforms at the terminals of a grid-connected inverter disturb its output currents. This paper compares AC linear current regulators for three-phase three-wire voltage source converters with three different reference frames, namely: (1) natural (abc), (2) orthogonal stationary (αβ), and (3) orthogonal synchronous (dq). The quantitative comparison analysis is based on mathematical models of grid disturbances using the impedance-based analysis, the computational effort assessment, as well as the steady-state and transient performance evaluation based on experimental results. The control scheme devised in the dq-frame has the highest computational effort and inferior performance under negative-sequence voltage disturbances, whereas it shows superior performance under positive-sequence voltages among the reference frames evaluated. In contrast, the stationary natural frame abc has the lowest computational effort due to its straightforward implementation, with similar results in terms of steady-state and transient behavior. The αβ-frame is an intermediate solution in terms of computational cost.

Setting and Testing of the Out-of-Step Protection at Mongolian Transmission System

Modern distance relays have integrated numerous protection functions, including power-swing blocking and out-of-step or pole-slip tripping functions. The main purpose of the power-swing blocking function is to differentiate faults from power swings and block distance or other relay elements from operating during stable or unstable power swings. Most power-swing blocking elements are based on traditional methods that monitor the positive sequence impedance rate. The required settings for the power-swing blocking elements could be difficult to calculate in many applications, particularly those where fast swings can be expected. For these cases, extensive stability studies are necessary to determine the fastest rate of possible power swings. This paper presents a detailed step-by-step method for settings calculation of out-of-step (OOS) protection, both blocking and tripping functions considering a generic two-source system. Then the method is applied to define the protection relay settings installed at the interconnection between the Russian and Mongolian power systems, as it is crucial to feed the demand-rich Mongolian power system. In this paper, a specific impedance method is used for defining the OOS protection settings. This paper innovates by testing the settings using the recordings of the major events of 15 September 2018 in two approaches: hybrid co-simulation and cyber-physical. Both tests have demonstrated the appropriate performance of the proposed settings and proving the proposed methodology works appropriately.

Positive sequence pilot impedance protection of AC tie line based on MMC-HVDC system

MMC-HVDC has the characteristics of limited amplitude, controlled phase angle and unequal positive and negative sequence impedance. Therefore, the fault characteristics of flexible direct system and AC power grid tie line are quite different from traditional synchronous power supply, which may affect the performance of AC power grid sudden variable protection. Therefore, a new AC protection method based on positive sequence current sudden variable impedance is proposed. The results show that when an in zone fault occurs, the direction of current sudden changes at both ends is the same, and the positive sequence impedance may approach the line impedance; When an out of area fault occurs, the current abrupt variables at both ends have opposite directions and equal sizes, and the positive sequence impedance is much greater than the line impedance. Based on the above characteristics, the criteria of fault start up and fault type are constructed. The simulation results show that the protection method can realize fault discrimination quickly and reliably.

Methodology of Adaptive Instantaneous Overcurrent Protection Setting

This paper proposed a methodology of adaptive instantaneous overcurrent protection (AIOCP) setting that ensures that the protection coverage remains unchanged regardless of the operating condition of the electrical network. The methodology calculates the protection setting parameters based on the real-time estimation of the Thevenin equivalent circuit (TEC). The estimation algorithm uses the positive-sequence voltage and current values and a system of non-linear equations, which is solved iteratively by the Gauss–Newton method. The proposed methodology calculates the IOCP settings in real time by using the real-time estimation of the TEC; therefore, any change in the electric network conditions is represented in the TEC, and the calculated setting keeps the desired protection coverage constant. Simulation results demonstrate that the proposed AIOCP can keep constant the protection coverage overcoming the classical problems of classical IOCP: sub-allocation and over-coverage.

Assessment of Sequence Extraction Methods Applied to MMC-SDBC STATCOM Under Distorted Grid Conditions

Under grid distortions, Modular Multilevel Converters (MMC) must adopt proper control strategies to fulfill the power system requirements and ensure a stable operation. An inappropriate control under such conditions may lead to energy unbalances between the MMC legs, inaccurate current injection, and failure in the synchronization process. In this context, sequence extraction methods play a critical role in enhancing the performance of the control, firstly, by aiding the Phase-Locked Loop (PLL) to maintain the synchronization with the AC grid by following the positive sequence fundamental component of the voltage, secondly, by allowing an accurate active and reactive currents injection via the decoupled Voltage Oriented Control (dVOC), thirdly, by properly managing the internal energy of the MMC through the circulating current control. In prior researches, some sequence extraction methods have been used for MMC STATCOM. However, the sequence extraction was not the core of the performed studies and their impact on the system behavior has not been highlighted or tested in several grid conditions. This work fills this gap by first assessing the performance of a Single Delta Bridge Cell MMC (SDBC-MMC) STATCOM with four well-known sequence extraction methods (i.e., Decoupled Double Synchronous Reference Frame (DDSRF), Dual Second Order Generalized Integrator (DSOGI), Improved DSOGI, and Fortescue matrix-based (FMB) filter) under normal and abnormal grid conditions, then, finding the most suitable one in terms of stability, dynamics, and functionalities.