Conventional Differential Protection Research Articles

DC line fault discrimination based on maximum information coefficient

In DC networks, conventional line differential protection has the disadvantages of low reliability, weak resistance to transition resistance, and vulnerable to harmonic interference. In this paper, a fault identification method based on maximal information coefficient (MIC) is proposed for non-high resistance faults in the DC side of a flexible DC distribution network containing a modular multilevel converter (MMC) with single pole grounding DC protection inside and outside segments. The failure prediction is performed based on the reciprocal information at two sides of the line current. At the same time, for single-pole grounded high-resistance faults, the MIC is compared using the zero moduli of the current at two sides of the line for the recognition of high-resistance faults. The MMC-based flexible DC transmission network model is built and validated in the PSCAD/EMTDC platform. The results show that the method can identify faults protection inside and outside segments on the DC line area quickly and reliably. It also has the ability to resist harmonic interference and has an endurance of resistance for single-pole faults on the DC side; meanwhile, for high-resistance faults, it can also be detected accurately.

Novel Differential Protection for a Powerformer Considering Capacitive Current Compensation

A Powerformer is a special type of generator whose peak capacitive current is approximately 30 times larger than that of conventional generators with the same rated apparent power. Powerformer stator fault relays may be misoperated by using conventional differential protection strategies, thus degrading network reliability. Therefore, a novel differential protection scheme for a Powerformer is proposed in this paper. First, the nonlinear distribution laws of the Powerformer stator winding capacitance and induced electromotive force (EMF) are analyzed. Next, an equivalent distributed parameter circuit model of the Powerformer considering cable-windings with graded insulation is established and verified by using FEM-based analysis. Furthermore, to eliminate the influence of the capacitive current on the differential protection, the capacitive current is analyzed, and a novel differential protection scheme considering capacitive current compensation is proposed. A simulation model is established by using MATLAB and PSCAD, and extensive simulation studies verify the effectiveness of the proposed protection approach.

A Statistical Approach to Discriminate the Magnetising Inrush Current and Internal Fault Current of a Transformer

Abstract: The transformer is one of the important elements of electrical power system. So, it is required to provide the good protection scheme for transformer, which enhance the reliability & economy of the system. There are many methods available for the protection of transformer out of which differential protection is the most commonly used method. It is observed that the conventional differential protection scheme mal-operates during the magnetizing inrush current. The mal-operation of differential relay will affect the continuity of supply to customer and hence the economy of our system. To avoid such type of mal-operation of the relays, proper discrimination between the magnetising inrush current and fault current is required. This paper presents Teager Energy Operator (TEO) and Statistical parameters based novel approach for the discrimination between the magnetizing inrush current and the internal fault current of a transformer. In this paper, TEO of the differential current of the transformer is calculated and compared with its threshold value to detect the abnormal condition. When TEO is more than threshold, statistical parameters like variance & standard deviation are calculated and if the calculated value is more than the threshold then it’s an internal fault condition and hence relay gives the trip signal. Else relay does not issue the trip signal. The suggested algorithm is tested using the experimental data. The results demonstrated that the suggested algorithms are capable of accurately differentiating between the transformer's internal fault current and inrush current.

Transient Wavelet Energy-Based Protection Scheme for Inverter-Dominated Microgrid

When faults occur in the microgrids, high frequency transients will be superimposed on the system currents and voltages. The magnitude of those transients will attenuate as it encounters the discontinuity points in the network such as busbars, or any other impedance discontinuity points. This phenomenon can also be quantified by wavelet energy, which provides a useful tool to detect faults and locate the faulted feeder in the microgrid. In this paper, a novel protection scheme based on the transient wavelet energy of the superimposed current extracted by the Maximal Overlap Discrete Wavelet Transform (MODWT) algorithm is developed to detect faults and locate the faulted feeder in microgrids. Compared with existing protection schemes, the proposed protection scheme has the advantage of being largely immune to the changes in system fault level, fault types and positions, microgrid operating status and the control strategies deployed on the inverters, while presenting much lower requirement on the sampling frequency (10 kHz) compared with travelling wave-based methods. Unlike the conventional differential protection, the proposed scheme does not require synchronized measurement or high bandwidth communication channels, and thus, it can be considered as an economical and promising solution for microgrids.

Change detection filter technique-based fault analysis of HVDC transmission line

PurposeThis research paper aims to investigate the change detection filter technique with a decision tree-based event (fault type) classifier for recognizing and categorizing power system disturbances on the high-voltage DC (HVDC) transmission link.Design/methodology/approachA change detection filter is used to the average and differential current components, which detects the point of fault initiation and records a change detection point (CDP). The half-cycle differential and average currents on both sides of the CDP are sent through the signal processing unit, which produces the respective target. The extracted target indices are sent through a decision tree-based fault classifier mechanism for fault classification.FindingsIn comparison with conventional differential current protection systems, the developed framework is faster in fault detection and classification and provides great accuracy. The new technology allows for prompt identification of the fault category, allowing electrical grids to be restored as quickly as possible to minimize economic losses. This novel technology enhances efficiency in terms of reducing computing complexity.Research limitations/implicationsSetting a threshold value for identification is one of the limitations. To bring the designed system into stability condition before creating faults on it is another limitation. Reducing the computational burden is one of the limitations.Practical implicationsCreating a practical system in laboratory is difficult as it is a HVDC transmission line. Apart from that, installing rectifier and converter section for HVDC transmission line is difficult in a laboratory setting.Originality/valueThe suggested scheme’s importance and accuracy have been rigorously validated for the standard HVDC transmission system, subjected to various types of DC fault, and the results show the proposed algorithm would be a feasible alternative to real-time applications.

Bus protection in systems with inverter interfaced renewables using composite sequence currents

Power systems are experiencing noticeable changes in operation with the integration of a large number renewable sources, which are generally connected to the grid through inverters. With significant modulation in voltage and current during fault by the embedded controllers in the inverters, available grid protection schemes are vulnerable. The theoretical analysis and simulated case studies reveal the influencing factors on the malfunction of the conventional busbar differential protection while connected to an inverter based resource. This paper proposes a differential protection method for the busbar, connecting inverter based resources to the grid, based on composite sequence currents. The performance of the proposed method is immune to the variations due to control action in the inverter based resource and change in grid codes. The IEEE 9-bus system interconnecting a PV plant is used to evaluate the proposed method where the comparative assessment reveals its superiority.

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.

Research on the Differential Protection of the SFC Output Transformer for Pumped Storage Hydro Unit

Pumped storage power stations play an great role in promoting new energy consumption, ensuring the safety of power grid, building clean, low-carbon, safe and efficient energy system, and has a broad development prospect. SFC is the most important start-up mode of pumped storage unit, and the output transformer is an important equipment of SFC system, which needs to be equipped with reliable and fast differential protection to ensure its safe and reliable operation. The current frequency varies from 0 to 50Hz during SFC start-up, so the conventional transformer differential protection can not adapt. In this paper, the output transformer differential protection technology is studied according to the electrical characteristics and short circuit fault characteristics of SFC start-up process, the high precision frequency algorithm and the RMS integration algorithm with compensation are proposed to realize the precise measurement of frequency and amplitude, and the ultra-low frequency CT saturation identification technology is proposed to realize the whole process protection of the output transformer from start-up to grid-connection.

A wavelet-based restricted earth-fault power transformer differential protection

This paper proposes a restricted earth fault protection based on the wavelet transform (REFW) for detecting ground faults close to the transformer neutral point (turn-to-ground faults) and supporting the conventional phase differential protection, which presents limitations in this kind of faults. The proposed REFW protection uses only high-frequency components instead of phasor estimation, speeding up the detection of turn-to-ground faults. A performance comparison between the proposed wavelet-based restricted earth-fault differential protection and the respective conventional restricted earth-fault (REF) unit was accomplished considering simulated turn-to-ground faults, involving a few winding turns. The proposed method is more efficient and faster than the conventional one, reaching a success rate of 100%.

Cosine Similarity Based Line Protection for Large Scale Wind Farms Part II—The Industrial Application

With the integration of the large-scale wind farms, the conventional pilot protection is facing new challenges. It suffers from the performance degradation can even refuse operation for some extreme fault scenarios. To address this problem, this article continues the research of the novel pilot protection based on cosine similarity. The protection pickup setting and industrial application testing is investigated. The functional structure is described and the logic of the cosine similarity based protection is evaluated under different fault scenarios. Compared with the conventional differential protection, application of the proposed protection can greatly improve the relay performance. The proposed protection is embedded into an industry relay device, and the standard hardware-in-loop tests are carried out using the real time digital simulator before the industrial application. Now, the protection device based on cosine similarity is installed in the wind farms and put into operation in Inner Mongolia.

Research on Differential Protection of Generator Based on New Braking Mode

In view of the difficulties in coordination between reliability and sensitivity of conventional generator differential protection, this paper presents a novel generator differential protection scheme based on a new braking method. On the basis of the mathematical model of the field-circuit coupling method and the electrical network cascade characteristics, the stator windings were combined and decoupled by the improved six-sequence component method to eliminate electromagnetic coupling between winding coils. In accordance with the basic characteristics of longitudinal impedance, the function of sub-item discrimination was realized. Numerous simulations and the up-to-date dynamic experiments showed that the proposed method has good state discrimination ability and can effectively resist the influence of current transformer saturation. Thus, it has an excellent prospect for engineering promotion.

Fault Detection in DC Microgrid Based on the Resistance Estimation

Evaluation toward the line parameter-based fault detection technique is an attractive option due to the limitations associated with the conventional differential and overcurrent protection schemes, mostly during a high-resistance fault situation. The fault detection strategy based on the line parameter can be exploited using the resistance instead of directly taking the decision from voltage and current. In this article, the proposed technique is focused on estimating the resistance and detecting the fault even during high-resistance fault case. Resistance is estimated using the local measurements available at the bus. After that, the sign of the estimated resistance is compared at both ends of the line segment to detect the fault and the use of the sign of resistance eliminates the time synchronization issues. The proposed method is capable of detecting the fault in radial as well as ring configuration dc microgrid. A ring main dc microgrid system is simulated using the EMTDC software to validate the proposed technique. The performance of the proposed approach is also validated using a dc microgrid hardware setup and tested for numerous situations. The results obtained from simulation and experiment verify that the proposed method accurately detects the faults (close in fault, high-resistance fault) in the dc microgrid.

Current differential relay characteristic for bipolar HVDC transmission line fault detection

This study proposes a new differential relay characteristic to detect internal faults on bipolar HVDC (high voltage direct current) transmission lines and discriminate accurately and in a timely manner both internal and external faults. In comparison with the conventional differential protection characteristic, the proposed relay characteristic avoids the need for an intentional time delay typically required to avoid nuisance fault detection. The sampling currents of the positive and negative poles are obtained from both line ends, and the noise is eliminated from the current signals using a stationary wavelet transform. Initially, a startup unit is utilised to activate the proposed protection scheme. The proposed current differential scheme adopts a biasing technique, which is based on the inherent transient behaviour of the bipolar HVDC system during both external and internal faults. The HVDC transmission line is implemented on the PSCAD/EMTDC platform utilising the frequency-dependent phase model, and it is tested considering numerous internal and external faults. Finally, the proposed current differential relay characteristic is tested considering different fault resistances up to 1000 Ω as well as different fault locations, including internal HVDC line faults and external faults at both rectifier and inverter sides. The obtained results confirm the robustness of the proposed protection scheme for bipolar HVDC transmission lines fault detection.

Swarm intelligence-based differential protection scheme for wind integrated transmission system

Conventional differential protection scheme considers a fixed threshold to detect the fault in a transmission system. In a wind integrated transmission system, any fixed threshold-based protection function is not reliable always due to the dynamic variation of control parameters of the wind turbine. To mitigate this issue, an adaptive differential protection scheme is proposed in this work for the transmission system integrated with wind farm. The method uses a particle swarm optimization (PSO) algorithm to generate an optimum threshold for the system considering the varying system and fault operating conditions. An objective function is designed for PSO considering the fault location, inception angle, and fault resistance. The method is tested by using a standard wind integrated transmission system model, simulated using MATLAB software. Performance of the method is tested for numerous test cases considering the wide variations in fault and system operating conditions.

Ratios-based universal differential protection algorithm for power transformer

For safe operation of power systems, it is essential that the transformer protection operate reliably if a fault occurs. There are many challenges in developing a fast and accurate differential protection scheme that not only disconnects the transformer during internal fault conditions but also restrains during energizing and external fault conditions. A universal algorithm based on the ratios of primary and secondary voltages and currents, supplemented by current direction and wave-shape criteria, for power transformers with any type of winding connection, star or delta, is proposed. The ratios are the ratio of the absolute difference and absolute sum of primary and secondary line currents and the terminal phase voltages. The primary objective is to avoid incorrect operation of the conventional differential protection approach during energization, energizing on an existing internal fault and external faults under current transformer saturation. Performance studies confirm that the proposed algorithm can provide fast, efficient and reliable protection of a power transformer for all types of winding configurations.

Ultra‐high‐speed non‐unit non‐differential protection scheme for buses of MMC‐HVDC grids

Half-bridge modular multilevel converter-based high-voltage direct-current (MMC-HVDC) grids are technically and economically viable solutions to transmit bulk electrical energy generated from geographically dispersed and remote renewable resources. However, due to the half-bridge converter's inability to control fault currents, these grids need specialised protection schemes to increase reliability and equipment safety. This study proposes a non-unit non-differential protection scheme founded on a comparative orientation measure of fault components for buses of half-bridge MMC-HVDC grids. Considering the selected protection strategy, the protection zone of this scheme can also cover a part of the converter station's ac side; in which severe short-circuit faults should be quickly detected and isolated as well since they may cause the entire grid outage. This scheme can be used as the main protection or a backup of the conventional differential current protection, and subsequently, increase the protection system dependability by increasing the functional diversity. The test results disclose that the proposed scheme has superior sensitivity to identify single-pole and double-pole faults with resistances up to 1500 and 2000 Ω, respectively, at the protected dc bus with the maximum delay of 1 ms, while it is robust against all ac and dc faults outside the designated protection zone.

A Novel Structural Similarity Based Pilot Protection for Renewable Power Transmission Line

The wide application of power electronic component in power system with renewable energy sources has changed the fault characteristics of conventional power system, resulting in the performance degradation of conventional protection, and even the risk of maloperation and rejection. This brings challenges to the security and stability of the power grid. Therefore, in order to solve these problems, a novel principle of pilot protection based on structural similarity and square error method is proposed. The structural similarity criterion makes use of the difference of fault characteristics between renewable sources and synchronous generators to identify internal faults, while the square error auxiliary criterion is used to solve abnormal calculation of the existing similarity based protection. The detailed model of grid-connected renewable energy power plant is built in Real Time Digital Simulator (RTDS) and the proposed protection algorithm is solidified in the industrial protection device. The effectiveness of the proposed principle is verified by hardware-in-loop dynamic simulation experiments. Compared with conventional differential protection, the proposed protection shows excellent performance in speed and reliability during various faults. In addition, a field short-circuit test is carried out in a real wind farm to verify the effectiveness of the proposed protection scheme. It is proved that this proposed method has the prospect for industrial application.

A novel algorithm for improving the differential protection of power transmission system

Current differential protection is simple in principle and is immune to power swings, load encroachment and mutual coupling problems. It provides simultaneous tripping at both ends of the transmission line and multi phase auto reclosing can be easily enabled because of its accuracy in determining the faulted phase and its communication channels. Conventional differential protection has certain issues to be dealt with before using it as a best alternative to distance protection for power transmission lines. Many researchers strive to improve the sensitivity of the current differential protection. Sensitivity and safety are opposing elements and increased sensitivity may sacrifice safety. In this paper, a new differential protection algorithm is proposed using the polarity of the terminal current DC transients. Different cases are studied in order to illustrate the applicability of the proposed differential approach. The proposed algorithm is able to detect high resistance faults and is not affected by line charging current, CT saturation. It improves the sensitivity without compromising the safety of the system. Simulations are carried out on 10 Bus system using PSCAD® software and obtained results are used for illustrating the efficiency of the proposed differential protection.

A Setting-Free Differential Protection for Power Transformers Based on Second Central Moment

This paper describes a differential algorithm for the protection of a three-phase transformer to differentiate transient phenomena, such as inrush currents and external faults from solid and turn-to-turn faults. This algorithm is based on the behavior of the second central moment (SCM) to identify the kind of event. Instantaneous differential currents are used as input signals to the algorithm once they are filtered and normalized. The algorithm compares the magnitude of the SCM with an established threshold based on the binomial distribution to achieve a successful identification of the event. If the magnitude of the SCM is above the limit, the algorithm identifies the event as a fault condition. Otherwise, the event is determined as a transient event or a steady-state condition. The threshold is setting-free if the transformer parameters are changed or if the harmonic content of the differential current varies. Also, the threshold does not have to be recalculated if the power system is modified. The D index was introduced to improve the fault-detection time when the current transformers experience saturation. This index uses the magnitude of the SCM as a basis and is compared with an established limit to detect the fault current. Both indices run in parallel, and if any of them does cross its respective threshold, a fault condition is determined. The algorithm is tested in a real-time digital simulator and is implemented in MATLAB. The algorithm was compared against the conventional differential protection with harmonic restraint to evaluate the performance of both indices in critical conditions. The results validate the performance of the algorithm and indicate that it can be the basis of a new algorithm to protect power transformers.

Current-Based Out-of-Step Detection Method to Enhance Line Differential Protection

Line differential protection provides the most selective protection scheme for transmission lines. It has an acceptable performance in situations which are challenging for the legacy distance protection. Following large disturbances, two or more areas of a power system may lose their synchronism and go out of step (OOS) which appears often as unstable power swings in some transmission lines. Existing OOS detection methods are based on the impedance trajectory seen by distance relays and conventional line differential protection cannot detect this condition. This paper introduces a new OOS detection method using the current phasors measured at both ends of a transmission line. Rigorous mathematical analyses and illustrated simulation study results are presented to substantiate the efficiency of the proposed method. The obtained results approve that the proposed method can be served as an efficient OOS detection method for line differential protection schemes.