Numerical Relays Research Articles

DFT-Based Phasor Estimation for Removal of the Effect of Multiple DC Components

A current sampled signal in a faulted transmission line can contain multiple dc components, namely a primary decaying dc component (DDC), a secondary DDC, an auxiliary DDC, and a bias DDC, and these may be due to a faulted line, current transformer, internal transformer, and analog-to-digital converter in the numerical relay, respectively. The intensity of the primary DDC is much higher than those of other DDCs. The elimination of the phasor-estimation error due to the primary DDC is the main issue in numerical relays, where the presence of other DDCs can deteriorate the phasor-estimation accuracy. This paper proposes a discrete Fourier transform (DFT)-based method using a second-order finite-impulse response (FIR) notch filter to combine multiple DDCs as a certain DDC through DFT. The FIR notch filter is designed to minimize the calculation complexity of the identification. Phasor-estimation errors that are due to the multiple DDCs are corrected by identifying the combined DDC. The proposed method can be applied for both full-cycle DFT (FCDFT) and half-cycle DFT. The feasibility of the proposed FCDFT-based method is compared with those of conventional FCDFT-based methods using simulations on DDCs, harmonics, noise, and a faulted transmission line.

Shunt Capacitor Banks Online Monitoring Using a Superimposed Reactance Method

A new indicating quantity, named superimposed reactance (SR), is presented in this paper to determine failed capacitor elements’ involved phase and number in shunt capacitor banks (SCBs). The proposed quantity is estimated using available measurements to the unbalance protection function of SCBs numerical protective relays. The proposed SR adopts calibrating factors for element failures online monitoring and can provide live report of the number of failed capacitor elements. The proposed method applications are: faster identifying SCBs faulty units, for fuseless and internally fused designs, and planning preventive/scheduled maintenance for all types of unit designs including externally fused units. With integration of these applications into protection intelligent electronic devices that provide captured and time-tagged event records via communication protocols for HMIs and SCADA systems, outage times would be reduced and smart grids reliability would be improved. The developed algorithm supports three different grounding arrangements, Wye-ungrounded, Wye-grounded via a low ratio current transformer, and Wye-grounded via a grounding capacitor at the SCB neutral point. Comprehensive simulation and fault-location sensing in PSCAD and MATLAB have verified the proposed algorithm performance. Advantages of the proposed method reports over conventional unbalance relaying alarms are also demonstrated using a commercial relay test results comparison.

Practical approach to testing the transformer differential protection for internal and external faults, CT saturation and inrush transients

This study presents a methodology that allows examining the differential protection of power transformers through computational simulations and numerical relay tests. Using that purpose, the alternative transient program (ATP) was used as a computational tool to perform several different events, which the power transformers are submitted to, and that can cause mal-operation of its differential protection, such as: external and internal faults, inrush current and saturation of current transformers (CTs). A testing system was implemented in laboratory, which consisted in simulating the power system in ATP (power transformer, CTs and faults modes) and applying the simulated signals through a programmable test set to a commercial relay to evaluate their response to these events. The main results of the behaviour of transformer protection relay under the tested transients are presented and discussed in this work.

New design of ground fault protection

Ground fault protection is widely used to protect transmission and distribution lines in case of ground faults. Combined with a directional element and used in a teleprotection scheme ground fault protection can detect and isolate even high resistive ground faults which are not seen by distance protection. Today in general the directional element of ground fault protection is based on zero sequence components or negative sequence components. There is no clear advice which kind of polarization should be preferred for a special application. However there are a lot of maloperations due to incorrect result of the directional element of ground fault protection using either zero sequence or negative sequence quantities. Analyzing numerous fault records it seems obvious that these problems with the directional elements cannot be solved using either zero sequence or negative sequence because these quantities are sometimes very small or not related to the fault. One great advantage of numerical relays is that these relays measure all the voltages and currents of a three phase system. Analyzing fault records related to complicate cases for ground fault protection it can be seen that there is much more information about the fault than used by today's implementations of ground fault protection. For instance the location of the impedance in the complex plane often gives a clear indication about the direction to fault. This paper suggests a new design of ground fault protection using this additional information given by the numerical relays. The starting condition for the ground fault protection remains a threshold of zero sequence current. Once this threshold is exceeded a multi-criteria phase selector selects the faulted phase. Several criteria based on magnitudes of voltages and currents, changes in voltages and currents, symmetrical components and impedances are applied in parallel. The results of each single criterion are weighted and combined to get a final result for the selection of the faulted phase. Using the information of the faulted phase a multi-criteria directional element is suggested to estimate the direction to the fault. Different criteria based on actual voltages, memorized voltages, symmetrical components and delta quantities are applied in parallel. The final result is obtained by the multi-criteria directional element as a weighted combination of the result of each single criterion. This paper explains the new algorithm in more detail and illustrates the advantages of the proposed method using some real fault records. With the new design the ground fault protection takes a lot of advantages regarding phase selection and directional element from the distance protection. The main difference between distance protection and ground fault protection remains the different grading. For distance protection the sensitivity is limited by the resistive reach. The basic principle for grading of ground fault protection remains a simple threshold of zero sequence current.

A pilot-based distance protection scheme for meshed distribution systems with distributed generation

This paper proposes a pilot-based distance protection scheme for meshed distribution systems with a high penetration of distributed generation. The proposed scheme assumes distance relays installed at the opponent ends of each main line segment. A forward distance element is enabled in each relay to protect the main line segment, while, a reverse distance element is enabled in each relay for protecting adjacent buses and laterals. The relays protecting a main line segment or a bus/lateral communicate in a permissive logic, ensuring protection sensitivity and security at the same time. The scheme provides efficient primary and backup protection against faults of any type, even with a considerable fault resistance, and under weak-infeed conditions. It is also appropriate for both the grid-connected and the islanded mode of system operation. Coordination of the distance protection scheme with the lateral protection means is also addressed. Existing numerical distance relay technology is considered, to enhance the applicability of the proposed scheme. Offline setting of the distance relays is required, which has to be performed once. Only one setting group is ultimately extracted for each relay, suitable for different fault/system conditions, avoiding the need for adaptive protection techniques. The proposed scheme is applied to a test meshed distribution system and conclusions are drawn. Comparison with directional overcurrent and differential protection shows the advantages of the designed distance-based protection scheme.

Impact of CT saturation on overcurrent relays

An overcurrent relay analyses and processes the secondary currents from a set of current transformers (CTs) and if the currents exceed the operating threshold initiates the tripping of a circuit breaker after an appropriate time. An important issue for a protection relay is how it responds when the steel core of the CT is saturated. The exact behaviour of a CT depends on various factors such as core size, CT ratio, primary current magnitude, DC offset, secondary burden resistance and remnant flux. The aim of this study is to investigate the impact of CT saturation on overcurrent relays using both a physical relay test bench that includes actual protection CTs, and a modern relay test set configured to simulate the test bench and its CTs. The relay operating times, obtained under similar fault conditions using both the test bench and the test set, will then be compared. The results obtained will then be used to determine how different levels of CT saturation affect different generations of an inverse time overcurrent relays. The final section of this study will explain how CT saturation affects an overcurrent grading study involving a mixture of electromechanical, static and numerical relays.

Design and implementation of flexible Numerical Overcurrent Relay on FPGA

This paper presents the contemporary design and implementation of an intelligent revelation in the field of the over-current relay to meet the challenges of the modern grid. The unique three-neuron single layered architecture of Artificial Neural Network (ANN) provides flexibility by exploiting its universal function approximation capabilities. The Unique Selling Proposition (USP) of the present development is the simple design of ANN, suitable for low-end, low-cost Field Programmable Gate Array (FPGA) implementation. The nano-scaled internal processing time for three-phase design, with the provision of remotely controlled adaptive relay settings, would definitely an innovative solution for grid connection of renewable energy sources. The proposed design of the universal over-current relay, confirmed by the real-time testing, is a true fusion of electrical power, communication and information technology to meet the global trend of the electrical power industries.

Adaptive quadrilateral distance relaying scheme for fault impedance compensation

Abstract Impedance reach of numerical distance relay is severely affected by Fault Resistance (RF), Fault Inception Angle (FIA), Fault Type (FT), Fault Location (FL), Power Flow Angle (PFA) and series compensation in transmission line. This paper presents a novel standalone adaptive distance protection algorithm for detection, classification and location of fault in presence of variable fault resistance. It is based on adaptive slope tracking method to detect and classify the fault in combination with modified Fourier filter algorithm for locating the fault. To realize the effectiveness of the proposed technique, simulations are performed in PSCAD using multiple run facility & validation is carried out in MATLAB® considering wide variation in power system disturbances. Due to adaptive setting of quadrilateral characteristics in accordance with variation in fault impedance, the proposed technique is 100 % accurate for detection & classification of faults with error in fault location estimation to be within 1 %. Moreover, the proposed technique provides significant improvement in response time and estimation of fault location as compared to existing distance relaying algorithms, which are the key attributes of multi-functional numerical relay

Adaptive phasor estimation technique during off-nominal frequency for numerical relays

ABSTRACTThe use of digital multifunction protection relays and their proper integration on the power system and smart grid is an important element. However, the performance of digital relays depends on many factors and needs to be carefully evaluated. These factors may be caused by correct operations with less performance or completely incorrect operations. Frequency deviation is one of those factors which results from unbalanced demand load and generation levels. Load characteristics and generation control response to demand load changes will effect on amount and duration of frequency deviations. Frequency decreases when demand load is higher than generation, whereas it increases when load is less than generation. Protection relays are provided with frequency tracking to reduce the effects of frequency deviations on accuracy of relay decisions. In this paper, traditional frequency tracking technique used on protection relays is modified to give a better performance (less Total Vector Error, TVE) during off-nominal frequency variations. The performance of the proposed algorithm is compared with traditional tracking using Least Square Error (LSQ), and Discrete Fourier transform (DFT) algorithm without any tracking, which are investigated in MATLAB environment.

Tuning CCVT Frequency Response Data for Improvement of Numerical Distance Protection

A new method to numerically estimate the primary voltage waveform of coupling capacitor voltage transformers is presented. The outcome is a simple digital filter suitable for the frequency range from 10 to 60 Hz, enabling us to recover the transformer primary voltage waveform from its secondary voltage waveform. Unlike the techniques reported in the literature, the method does not depend directly on the transformer topology nor on the operational characteristics of the protected system, once only the coupling capacitor voltage transformer frequency response is required as input. The evaluation of the technique is performed through digital simulations using the Alternative Transients Program and transmission-line data from the Brazilian National Power System Grid. Frequency- and time-domain analysis of two coupling capacitor voltage transformers with different topologies and voltage levels are presented. It is shown that the proposed method is effective for the dynamic correction of low-frequency disturbances that may improve the speed, reliability, and security of the numerical distance relay.

Least error squared phasor estimation with identification of a decaying DC component

A fault current in a power system contains both harmonics and a decaying DC component, which can produce a severe error in phasor estimation of fundamental frequency. Currently, full-cycle discrete Fourier transform phasor estimation is widely used in numerical relays owing to harmonics immunity, which produces easy identification of the decaying DC component. It is desirable to promptly estimate the phasor for a fast protection of the power system. Such promptness is dependent on the data window length of the phasor estimation. The least error squares (LESs) method is effective to achieve this promptness as it can estimate the phasor with a data window length of less than one cycle. This study proposes a phasor estimation method using LES with the data window length less than one cycle. The time constant of the decaying DC is identified through Prony analysis, while the noise effect on the phasor estimation is investigated according to the data window length. The performances of the proposed method are investigated through simulations on time constants of the decaying DC, frequency deviations of the power system, and signal-to-noise ratio. The feasibility of the proposed method is verified to estimate the phasor of a fault transmission line in a MATLAB/Simulink.

Development of nonuniform transmission line protection for accurate distance protection: Computational analysis of an adaptive distance relay characteristic

Numerical distance relays are used in modern power system to boost the performance and reduce the computational burden of relay. Quadrilateral zone of distance relay is more suitable and capable of boundary modification on protection scheme. In this paper, an adaptive distance relay is introduced to tackle the maloperation problems encountered in non-uniform structure of combined transmission system, in which the zone 1 of distance relay is modified. Two boundaries are modified with fifth degree of polynomial equation for quadrilateral zone characteristic. Appropriate objective functions are defined to minimize the increment in relay zone, maloperation rate, and boundary accuracy error. The multiobjective optimization problem is solved by using genetic algorithm. According to the extensive computational analysis results performed by MATLAB, the high accuracy of the proposed adaptive relay was achieved. Furthermore, the proposed method has the ability to encompass all faults and tackles the influence of various power system conditions by finding the optimized equation coefficients for the nonuniform structures.

Experimental Study of Numerical Relay for Over-current Protection in Solar Panel for Securing the Hydrogen production

Every electrical system in solar panel can fail during electrical faults. In this incidence, high fault current can occur. Such current must be interrupted by a protective system. The research was supported by experimental tests. In work conditions close to real, the numerical relay REF542plus was tested for both instantaneous and extremely inverse definite minimum time IDMT over-current protection functions with the help of CMC 365 injection and test equipment associated to Test Universe software. Protecting hybrid solar panels generating by different renewable energy sources for hydrogen production from over-current is very important for improving the energy efficiency in one hand, and securing the function in critical condition from damage of the solar cells in second hand. The contribution of this research is controlling the over-current in the solar panel for securing the continuation of the hydrogen production from renewable energy sources in short time. The obtained results allowed the observation of the relay’s behavior when subjected to certain faults; where the solar panel keeps producing the hydrogen.

Development of Stand-Alone Numerical Relay for The Protection of Three-Phase Induction Motor

Induction motors are highly reliable; they are susceptible to many types of abnormalities like overload, over voltage, under voltage, phase reversal, ground fault and insulation failure. Due to these electrical conditions the winding of motor gets overheated which leads to insulation failure and hence reduces the life of the motor. These can cause production shutdowns, personal injuries, and waste of raw material. Therefore, to protect the motor the fault must be detected in the initial stage. In the proposed protection scheme, a panel has been designed which alone can protect induction motor from various abnormal conditions such as overload, overvoltage, under voltage and phase reversal. A microprocessor based relay is designed and is tested with the setting of various preset values of parameters. The simulation and hardware results are presented to validate the proposed method.

Modeling of distance protection logic for out-of-step condition in power system

Out of step (OOS) is considered an abnormal condition in power system. At this case, the electrical power is not equalized with the mechanical power for the generator. So, OOS leads to variation in three-phase voltage and current for the power system. The distance protection relay is depending on the voltage and current in the protection zone. So, the distance protection calculations for the load impedance may consider that the OOS condition is a three-phase fault in the protection zone for over head transmission line. This paper discusses the requirements from distance protection function in the numerical protection relay at OOS and how to overcome this case to prevent the power system blackout or separating system by the optimum design for out-of-step blocking.

Half‐cycle method for exponentially DC components elimination applicable in phasor estimation

Decaying DC component has considerable effect on both accuracy and speed of convergence in numerical and digital relays for estimating fundamental phasor. Basically, this component causes unwanted and undesired oscillations in the time response of the fundamental phasor. As a result, it extremely affects decision making of the protective relay, which operates according to fundamental phasor component. In this study, a novel hybrid algorithm based on integration and half-cycle discrete Fourier transform (HDFT) is presented. Proposed method of this study is able to efficiently estimate fundamental frequency component by taking different parameters, including DC offset in the fault current, DC offset in a current transformer secondary side and off-nominal frequency condition into account. Moreover, in this study, modified DFT-based method for phasor estimation that is called combined integration and HDFT (CIHDFT) is put forward, which accurately estimates the original value of fundamental frequency phasor in half-cycle by considering the presence of harmonics in the waveforms. The proposed method is successfully applied to different test cases, followed by discussion on the results. Analysing the simulation results, it can be concluded that CIHDFT algorithm has remarkable performance in comparison with previous phasor estimation algorithms.

Protection coordination of directional overcurrent relays: new time current characteristic and objective function

Clearing power grid faults swiftly and selectively offers higher security, reliability and sustainability. Accomplishing this aim by deploying directional overcurrent relays (DOCRs) is one of the major challenges in meshed and multisource distribution networks. To overcome this challenge, the current study elaborates a new coordination strategy which concentrates on minimising overall operation time of relays. In this strategy, an auxiliary variable is added to classical operation time model of each DOCR. All the auxiliary variables are considered as coordination constraints which help to yield new and well-defined time current characteristics (TCCs). In other words, more flexibility is attained in adjusting relay's characteristic which helps to alleviate clearing time of faults. The obtained TCCs can be easily performed by numerical relays. In addition, a new objective function is defined to steer the relay settings towards optimal solutions conveniently. It was shown that the proposed approach not only reduces the operation time of relays, but also prevents miscoordinations. This approach demonstrates a non-linear programming model tackled by particle swarm optimisation algorithm. The effectiveness of the proposed approach is verified via 8-bus and IEEE 14-bus test systems and results are discussed in depth.

An intelligent protection scheme to mitigate the impact of integrating large share wind energy resources in a weak distribution network

Although integration of wind distributed generation directly into the distribution level has considerable advantages, increased penetration of wind distributed generation (renewable distributed generation) alters the network configuration and jeopardizes the protection system operation and system stability; for this reason, necessary changes in power system protection philosophy must be achieved. Modern numerical relays offer extraordinary features and fast and accurate methods for spotting and detecting different unbalanced operating conditions and can be used to mitigate the influence of integrating wind distributed generation into distribution network. In this study, an adaptive directional negative protective scheme was implemented in the medium-level distribution network to investigate and evaluate the performance of protection system and introduce new adaptive protective scheme based on negative overcurrent protection to increase the selectivity and sensitivity of the protection system in case of unbalanced faulty conditions. The medium-level distribution network of Libya Eastern electric network had been implemented in ETAP software to address and evaluate the efficiency of the proposed approach.

Application of artificial neural network (ANN) to enhance power systems protection: a case of the Nigerian 330 kV transmission line

This work investigates an improved protection solution based on the use of artificial neural network on the 330 kV Nigerian network modeled using MATLAB R2014a. Measured fault voltages and currents signals decomposed using the discrete Fourier transform implemented via fast Fourier transform are fed as inputs to the neural network. The output plots of the neural network shows its successful application to fault diagnosis (fault detection, fault classification and fault location). Unlike conventional protection schemes, the neural network can be adapted to distances which can cover the entire length of the protected line. Numerical assessment carried out on the neural network fault locator shows a reduced time of operation of 5.15 ms as compared to the 0.350 s with the use of ordinary numerical relays. This work also investigates the adaptive auto-reclosure scheme implemented using artificial neural network. The adaptive reclosure scheme has been adapted for use in the Nigerian network successfully to distinguish transient and permanent faults. Simulation results prove that the adaptive reclosure scheme was able to detect a line-to-ground transient fault and clear this fault in 0.1 s while the line-to-ground permanent fault is cleared after 0.14 s. The fault diagnostic algorithm designed using artificial neural network (ANN) for the 330 kV network was tested on a 132 kV network. Results show and prove that the algorithm is flexible and can be adopted to other networks.

Study and Analysis of Modern Numerical Relay Compared to Electromechanical Relay for Transmission of Power

Study and Analysis of Modern Numerical Relay Compared to Electromechanical Relay for Transmission of Power