Digital protective relay

Abstract: Estimating the proximity of power systems to voltage collapse in real-time still faces difficulties. Beside the data management and computational issues, any central-control method is subject to the reliability of long-distance data communications. In the paper, the authors describe a new data-processing method to estimate the proximity to voltage collapse. The method (code-named SMARTDevice, for Stability Monitoring And Reference Tuning Device) employs only local measurements-bus voltage and load current-and calculates the strength of the transmission system relative to the bus. The collapse occurs when the local load approaches this value. The method is simple enough so that it can be implemented in a numerical relay. The performance of SMARTDevice is compared against the conventional undervoltage relays. It is shown that the latter can misoperate while the new device does not. SMARTDevice is in fact a new breed of voltage relay whose setpoint is automatically tuned to the power system condition.

Abstract: Microgrids have been proposed as a way of integrating large numbers of distributed renewable energy sources with distribution systems. One problem with microgrid implementation is designing a proper protection scheme. It has been shown that traditional protection schemes will not work successfully. In this paper a protection scheme using digital relays with a communication network is proposed for the protection of the microgrid system. The increased reliability of adding an additional line to form a loop structure is explored. Also a novel method for modeling high impedance faults is demonstrated to show how the protection scheme can protect against them. This protection scheme is simulated on a realistic distribution system containing a high penetration of inverter connected Distributed Generation (DG) sources operating as a microgrid. In all possible cases of operation the primary and secondary relays performed their intended functions including the detection of high impedance faults. This system is simulated using Matlab Simulink's SimPowerSystems toolbox to establish the claims made in this paper.

Abstract: This paper presents a new algorithm suitable for calculating impedances from digitized voltages and currents sampled at a relay location. Each input is assumed to be composed of a decaying d.c. component and components of the fundamental and harmonic frequencies. Parameters of a digital filter determined by using the least error squares approach are then used to compute the real and imaginary components of the voltage and current phasors. Impedances as seen from a relay location are then calculated. The proposed algorithm was tested using the fault data recorded at the Regina South switching station of the Saskatchewan Power Corporation; some of the test results are included.

Abstract: A new protective relay that solves many of the problems associated with the protection of EHV and future UHV systems is described. Based on a traveling wave approach the relay system operates at ultra high speed and can detect and determine the direction to a fault in a few milliseconds. The relay operating principle, design and application are presented as well as results from computer simulation studies and laboratory tests. The first relay system was installed in the USA in Bonneville Power Administration's 500 kV system and represents a substantial contribution to present efforts to shorten fault clearing times as a method to improve system stability and power transfer capability.

Abstract: This paper introduces the new standard "IEEE standard inverse-time characteristic equations for overcurrent relays". It provides an analytic representation of typical electromechanical relays operating characteristic curve shapes in order to facilitate coordination when using microprocessor-type relays.