Challenges, advances and future directions in protection of hybrid AC/DC microgrids

Hybrid microgrids which consist of AC and DC subgrids interconnected by power electronic interfaces have attracted much attention in recent years. They not only can integrate the main benefits of both AC and DC configurations, but also can reduce the number of converters in connection of Distributed Generation (DG) sources, Energy Storage Systems (ESSs) and loads to AC or DC buses. In this paper, the structure of hybrid microgrids is discussed, and then a broad overview of the available protection devices and approaches for AC and DC subgrids is presented. After description, analysis and classification of the existing schemes, some research directions including communication infrastructures, combined control and protection schemes, and promising devices for the realisation of future hybrid AC/DC microgrids are pointed out

Intelligent fault location in MTDC networks by recognising patterns in hybrid circuit breaker currents during fault clearance process

In this paper, a novel, learning-based method for accurate location of faults in MTDC networks is proposed. By assessing the DC circuit breaker currents during the fault clearance process, a pattern recognition approach is adopted from which the fault location is estimated. The implementation of the algorithm is allocated into three main stages, where similarity coefficients and weighted averaging functions (incorporating exponential kernels) are utilized. For the proposed algorithm, only a short-time window of data (equal to 6 ms) is required. The performance of the proposed method is assessed through detailed transient simulation using verified MATLAB/Simulink models. Training patterns have been retrieved by applying a series of different faults within an MTDC network. Simulation and experimental results revealed that the proposed scheme i) can reliably determine the type of fault ii) can accurately estimate the fault location (including the cases of highly resistive faults) and iii) is practically feasible

A predictive control strategy for mitigation of commutation failure in LCC-based HVDC systems

High-voltage direct-current (HVDC) systems are being widely employed in various applications because of their numerous advantages such as bulk power transmission, efficient long-distance transmission, and flexible power-flow control. However, line-commutated-converter-based HVDC systems suffer from commutation failure, which is a major drawback, leading to increased device stress and interruptions in transmitted power. This paper proposes a predictive control strategy, deploying a commutation failure prevention module to mitigate the commutation failures during ac system faults. The salient feature of the proposed strategy is that it has the ability to temporarily decrease the firing angle of thyristor valves depending on the fault intensity to ensure a sufficient commutation margin. In order to validate the performance of the proposed strategy, several simulations have been conducted on the CIGRE Benchmark HVDC model using PSCAD/EMTDC software. Additionally, practical performance and feasibility of the proposed strategy are evaluated through laboratory testing, using the real-time Opal-RT hardware prototyping platform. Simulation and experimental results demonstrate that the proposed strategy can effectively inhibit the commutation failure or repetitive commutation failures under different fault types, fault impedances, and fault initiation times

A controllable thyristor-based commutation failure inhibitor for LCC-HVDC transmission systems

Commutation failure is a serious malfunction in line-commutated high voltage direct current (HVdc) converters which is mainly caused by the inverter ac faults, and results in a temporary interruption of transmitted power and damage to the converter equipment. In this article, a controllable commutation failure inhibitor (CCFI) is developed which obviates the main drawbacks of the existing power electronic based and fault current limiting based strategies. Under normal circumstances, the developed CCFI improves the steady-state stability and the power transfer capability of the inverter ac lines, while it does not cause excessive voltage stress on the converter valves. In addition, it would reduce the risk of commutation failure occurrence, since it does not lead to any voltage drop in the commutation circuit. When a fault occurs at one of the inverter ac systems, its corresponding CCFI limits the fault current depending on the reduced extinction angle. This would not only inhibit the successive commutation failures on the HVdc converter, but also extend the lifetime of components in the inverter ac systems. The practical feasibility of the developed CCFI is assessed through laboratory testing, using a real-Time Opal-RT hardware prototyping platform. The obtained results indicate that the developed CCFI can reliably inhibit the commutation failures during various types of faults

Centralised busbar differential and wavelet-based line protection system for multi- terminal direct current grids, with practical IEC-61869-compliant measurements

This paper presents a method for discriminative detection of DC faults on VSC-powered multi-terminal HVDC transmission systems using two fundamental guiding principles, namely instantaneous current-differential and travelling waves. The proposed algorithm utilises local voltage and current measurements from all transmission lines connected to a DC busbar, and current measurement from the DC side of the converter. The scheme operates at a sampling frequency of 96 kHz which conforms with IEC 61869-9. No long distance communication is involved while measurements and signal exchange within DC substations are enabled by the utilisation of IEC 61850. Performance is assessed firstly through detailed transient simulation, using verified models of modular multi-level converters, hybrid DC circuit breakers and inductive DC-line terminations. Furthermore, practical performance and feasibility of the scheme is evaluated through laboratory testing, using the real time Opal-RT hardware prototyping platform. Simulation and experimental results demonstrate that the proposed protection algorithm can effectively, and within a very short period of time (i.e. less than 1 ms), discriminate between busbar and line faults (internal faults), while remaining stable during external faults. Additionally, it has been demonstrated that IEC 61869-9 is suitable for enabling fast DC protection schemes incorporating travelling waves

Design of DC-line terminating inductors for enhancement of protective functions in MTDC grids

This study presents a detailed DC-side fault analysis considering inductive termination of lines within a high-voltage multi-terminal direct current (MTDC) grid. The analysis aims to provide design guidelines for DC-side inductors, taking into account important aspects of protection such as the required speed of operation of relays and the performance characteristics of current interruption devices (i.e. of DC circuit breakers). Moreover, the impact of current limiting inductors on the fault signatures is investigated. In particular, it has been found that DC-side inductors not only limit the fault current level, but also the resulting signatures in voltage and current, can assist to enhance the speed of operation, stability and selectivity of protective functions for DC-side faults. The analysis has been extended to include the impact of inductive termination on fast transient phenomena known as travelling waves. Specifically, DC-side inductors can form a significant reflection boundary for the generated travelling waves. A deeper insight into the faults has been achieved by utilising wavelet transform

A novel traveling-wave-based protection scheme for LCC-HVDC systems using Teager Energy Operator

Line Commutated Converter (LCC) based High-Voltage Direct Current (HVDC) technology has been in operation with a high level reliability and little maintenance requirements for more than thirty years. The current-source based or classical LCC-HVDC systems are being considered for buried cable transmission as well as overhead transmission. The fault analysis and protection of LCC-HVDC system is a very important aspect in terms of power system stability. This paper proposes a novel protection scheme for LCC-HVDC systems, in which the difference between propagation processes of traveling wave under internal and external faults is used as a criterion for detection of fault incidents in HVDC transmission lines. In order to quantify and intensify this difference, Teager Energy Operator (TEO) is used which has the ability to reflect the instantaneous energy of a signal. The main feature of the proposed scheme in comparison with the existing ones is that it operates faster, since it does not require to extract any harmonic or high frequency component; moreover, a 2-ms sampling window is sufficient for its algorithm which only deals with simple calculations. In order to validate the effectiveness of the proposed protection scheme, several fault events under different fault resistances and fault locations were simulated on a test network using PSCAD/EMTDC software. Also, the performance of the proposed scheme under real fault events was tested using four field data cases. Both simulation data and field data test results indicated that the proposed protection strategy has the ability to accurately discriminate between internal and external faults and detect the faulted pole in the bipolar systems even under high-impedance fault conditions

IMPROVEMENT OF LOADABILITY IN DISTRIBUTION SYSTEM USING GENETIC ALGORITHM

Abstract -Generally during recent decades due to development of power systems, the methods for delivering electrical energy to consumers, and because of voltage variations is a very important proble

Comparison analysis of different classification methods of power quality disturbances

Good power quality delivery has always been in high demand in power system utilities where different types of power quality disturbances are the main obstacles. As these disturbances have distinct characteristics and even unique mitigation techniques, their detection and classification should be correct and effective. In this study, eight different types of power quality disturbances were synthetically generated, by using a mathematical approach. Then, continuous wavelet transform (CWT) and discrete wavelet transform with multi-resolution analysis (DWT-MRA) were applied, which eight features were then extracted from the synthesized signals. Three classifiers namely, decision tree (DT), support vector machine (SVM) and k-nearest neighbors (KNN) were trained to classify these disturbances. The accuracy of the classifiers was evaluated and analyzed. The best classifier was then integrated with the full model, which the performance of the proposed model was observed with 50 random signals, with and without noise. This study found that wavelet-transform was effective to localize the disturbances at the instant of their occurrence. On the other hand, the SVM classifier is superior to other classifiers with an overall accuracy of 94%. Still, the need for these classifiers to be further optimized is crucial in ensuring a more effective detection and classification system