Development of a Range of Modified Multifunctional Residual Current Systems and Their Industrial Implementation

The developments of the AltSTU have been reviewed in the area of creating a new technology for preventing technogenic hazards based on the residual current devices. The residual current devices are intended for protecting people from electric shock in case of contact with conductive parts of the electric appliances and shall facilitate reduction of fire risks caused by a prolonged flow of leakage currents and fault currents resulting from them. The results of creating different modifications of protective trip circuits and their industrial use are provided.

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.

Design and implementation of an adaptive relay based on curve-fitting technique for micro-grid protection

The micro-grid should operate in a well controlled manner both during grid connected and islanded mode. Also, micro-grid equipped with conventional overcurrent relays has protection issues during faults due to the difference in fault currents when micro-grid operation changes from one mode to the other. This paper proposes suitable relaying scheme based on curve-fitting technique for micro-grids and performs well without using any extensive communication facility. A new algorithm is developed to create large number of fault data for fitting the curve. The fitness of curve is accessed by measuring the value of coefficient R-squared value (R 2). More number of polynomials is added to improve the value of R 2. Results show that the actual tripping time of the relay is very close to desired tripping time by introducing more number of polynomials. Efficiency of the proposed method is evaluated by comparing with conventional over current relay. The protection logic of the curve-fit relay has been implemented on a TMS32F28377S micro-controller based hardware. A laboratory based experimental setup has been established to test the operational performance of curve-fit relay for a five bus system.

Short-Circuit Analysis of DER-Based Microgrids in Connected and Islanded Modes of Operation

Since microgrids should be able to smoothly operate in two distinct modes—grid-connected and islanded, their fault currents can widely fluctuate depending on the operational mode. When the microgrid is connected to the grid, the highest fault current, by far, is supplied by the utility grid. In this mode, the fault current contribution from distributed energy resources (DERs) is less than 20%. However, when the microgrid switches to the islanded mode, the fault current contribution from the utility grid is lost and DERs are the sole fault current sources. Thus, the overall fault current in the islanded mode is multiple times lower when compared to the grid connected mode. Moreover, most of the DERs are inverter-based, with limited fault currents, which further reduces the overall fault current in the islanded mode. With the rapid rise of the microgrid penetration around the globe, this phenomenon can adversely influence the relay protection, and thus the microgrid fault current needs to be precisely analyzed. Therefore, the main purpose of this paper is to thoroughly analyze the fault current differences in two distinct operation modes of a microgrid, and to consequently derive conclusions regarding the required improvements in fault calculations and relay protection analysis in emerging microgrids. A representative microgrid test bed is developed and modelled using the in-house developed software as well as in a state-of-the-art hardware-in-the-loop environment. Several different short-circuit faults were simulated and analyzed in both grid-connected and islanded modes. The results show that the fault currents significantly differ depending on the operating mode, and thus highly influence the protection system. Moreover, test results show that the fault calculation algorithms aimed at radial distribution grids, mostly used for microgrid fault calculations in the available literature, need to be further improved to provide precise and time-efficient results when the emerging microgrids are considered. These results provide a valuable insight into the current state of the microgrids’ fault calculation and protection and reveal several important directions for future research.

Discrimination of Transformer Inrush Currents and Internal Fault Currents Using Extended Kalman Filter Algorithm (EKF)

The discrimination of inrush currents and internal fault currents in transformers is an important feature of a transformer protection scheme. The harmonic current restrained feature is used in conventional differential relay protection of transformers. A literature survey shows that the discrimination between the inrush currents and internal fault currents is still an area that is open to research. In this paper, the classification of internal fault currents and magnetic inrush currents in the transformer is performed by using an extended Kalman filter (EKF) algorithm. When a transformer is energized under normal conditions, the EKF estimates the primary side winding current and, hence, the absolute residual signal (ARS) value is zero. The ARS value will not be equal to zero for internal fault and inrush phenomena conditions; hence, the EKF algorithm will be used for discriminating the internal faults and inrush faults by keeping the threshold level to the ARS value. The simulation results are compared with the theoretical analysis under various conditions. It is also observed that the detection time of internal faults decreases with the severity of the fault. The results of various test cases using the EKF algorithm are presented. This scheme provides fast protection of the transformer for severe faults.

Reducing Fault Current by Using FACTS Devices to Improve Electrical Power Flow

Today, there is a large increase in the demand for electricity. The transmission and distribution networks, however, cannot fulfill unbound demands due to the scarcity of resources. Power lines have losses which make the situation more unfavorable for maximum power transfer. Implementing a flexible AC transmission system (FACTS) is one of the best ways to reduce line losses. This paper proposes a FACTS-based method for minimizing the fault current in the system. Switchgear and protection equipment also perform better when this is done. Moreover, due to the reduced fault current of the switched system, a larger amount of power can be transmitted. Static synchronous series compensator (SSSC), static synchronous compensator (STATCOM), and unified power flow controller (UPFC) are evaluated in this case. With STATCOM and UPFC, fault currents are significantly reduced. Furthermore, STATCOM and UPFC can also reduce the fault currents in the power system in addition to voltage regulation and power flow control. A MATLAB/Simulink model is used to evaluate the model’s feasibility.

Short-Circuit Fault Analysis of the Sen Transformer Using Phase Coordinate Model

The Sen Transformer (ST) provides an economical solution for power flow control and voltage regulation. However, fault analysis and evaluation of the performance of the transmission protection system in the presence of a ST have not been investigated. Hence, a short-circuit model of the ST using the phase coordinate method is proposed in this paper. Firstly, according to the coupled-circuit ST model, the nodal admittance matrix between the sending end and receiving end of the ST was deduced. Subsequently, a fully decoupled mathematical model was established that can reflect three characteristics, including its winding connection structure, electrical parameters, and ground impedance. Thus, with the help of the phase-coordinate-based solving methodology, a short-circuit ST model may be built for various short-circuit faults. The MATLAB and PSCAD/EMTDC software were employed to carry out simulated analyses for an equivalent two-bus system. The short-circuit currents obtained from the time-domain simulation and the analytic calculation utilizing the proposed model reached an acceptable agreement, confirming the simulation’s effectiveness. Moreover, the variation of the fault currents with the variation of the compensating voltage after single-phase-to-ground and three-phase short-circuit faults was demonstrated and used to analyze the effect of the ST on the fault currents.