Fault Detection and Protection Strategy for Multi-Terminal HVDC Grids Using Wavelet Analysis

A unified power-flow algorithm of AC power systems integrated with multi-terminal high-voltage direct current (MTDC) grids incorporating the interline DC power-flow controller (IDCPFC) is presented in this paper. The MTDC grids are based on voltage-sourced converter (VSC) technology. Pulse-width modulation control is employed for the VSCs. As against most of the research works published in this area, in this algorithm, the VSC modulation indices are considered as unknowns. The IDCPFC considered for the power-flow management of the DC grid is a generalized one, with an arbitrary number of DC voltage sources. In the proposed model converter losses are also included. All case studies are carried out in MTDC grids embedded in the IEEE-300 bus test system. For all the case studies, the power-flow algorithms were implemented with MATLAB. In all occurrences, a mismatch error tolerance of $$10^{-10}$$ p.u. was selected. Results obtained with case studies on AC–MTDC systems validate the proposed work.

This paper proposes a novel single-end protection scheme to satisfy the operating speed, sensitivity, and selectivity required in the protection of HVDC grids based on the modular multilevel converter (MMC). In this regard, the fault voltage characteristics are analyzed at the early stage of internal and external faults to implement the protection scheme. It is shown that the existence of smoothing inductors at both sides of the MMC-HVDC transmission line causes the attenuation of the high-frequency components of the fault traveling wave head under external faults. Whereas, the generated fault traveling wave under internal faults contains plenty of high-frequency features. Putting this into perspective, an improved complementary ensemble empirical mode decomposition with adaptive noise (ICEEMDAN) is used to extract the highest frequency oscillations of initial fault voltage. Afterward, the high-frequency energy of the initial fault voltage is calculated by adopting the Hilbert transform (HT). As a result, the high-frequency energy calculated via ICEEMDAN-HT forms the fault identification criterion. To assess the proposed protection scheme, a four-terminal half-bridge MMC-HVDC system is modeled in the PSCAD environment. According to simulation results, the proposed protection method is capable of detecting remote faults up to 300 Ω transition resistance within 1.1 ms only by acquiring local voltage data. Furthermore, the proposed method exhibits reliable performance under external faults and noise interferences.

The usage of the high-harmonic restraint is one of the most common ways to ensure the non-operation of the power transformer differential protection when a magnetizing current inrush occurs. The application of this method allows not only to lower the minimum operating current of the protection, which increases its sensitivity to the developing inter-winding faults, but also provides inaction of the protection when unbalance currents increase during a transient caused by an external fault. However, there is no accurate information in the literature on possible ranges of variation of content of higher harmonics in the differential current in the cases when external or internal faults occur. This complicates the calculation of settings of protective relays, ensuring their most effective functioning. In this paper authors analyze the content of the higher harmonics in the differential current in the cases of external and internal faults, taking into account the saturation of the measuring current transformers in order to determine the range of variation of the higher harmonics under these conditions. It is shown that in all cases the ranges of variation of higher harmonics overlap which makes it difficult to create an effective differential protection that recognizes emergency and normal operation conditions operation only by harmonic analysis of the operating signal

A fault protection method is proposed for flexible direct-current (DC) power systems based on the cosine similarity of currents in current-limiting reactors. The typical characteristics of external and internal faults in a flexible DC power system were analyzed. The principles of cosine similarity and the fault current characteristics of current-limiting reactors were used to distinguish between internal and external faults. The ratio of the average positive and negative voltages of the current-limiting reactor was then used to distinguish the fault types and fault line. Finally, a simulation model of a circular flexible DC power system was constructed using MATLAB/Simulink software (2018b). The simulation results show that the proposed protection scheme can quickly and accurately identify the location and type of faults, and has a strong ability to withstand fault resistance and is less affected by distributed capacitance, noise, and communication delay.

With the growing maturity of power electronic devices, the applications of the voltage source converter (VSC) technique on the high-voltage direct current (HVDC) transmissions have attracted great attention. Due to the inherent control characteristics of the VSC, the realization of multi-terminal HVDC (MTDC) grids becomes less complicated. However, one significant issue that restrains the development of VSC MTDC grids is the proper management against DC faults. This paper proposes a DC fault management scheme, delayed-auto-re-configuration (DARC), for VSC MTDC systems. This scheme does not require additional component to be added to the existing system. With appropriate protection and control, the MTDC system can be well protected and after the isolation of the faulted line section, the unfaulted terminals will be controlled and recovered to form a new MTDC configuration. The effectiveness of the proposed scheme is validated by simulations using the real-time digital simulator (RTDS).

DC power flow controllers (DCPFCs) are emerging and promising devices to facilitate power flow in voltage source converter (VSC)-based multi-terminal HVDC (MT-HVDC) grids. In this paper, a novel Newton-Raphson (NR)-based DC power flow solver (DCPFS) is proposed by employing a novel multiport interline DC power flow controller (MIDCPFC) to solve the DC power flow problem (DCPFP) by modifying physical and control state variables of the whole system (MIDCPFC and MT-HVDC grid) simultaneously to obtain the predetermined control objectives. The static model (SM) and power injection model (PIM) of the considered MIDCPFC are derived and their related equations are embedded within the proposed DCPFS. There are no fictitious buses in the proposed DCPFS, the original conductance matrix of the system and its symmetry are preserved, and only minor modifications are needed for the original system Jacobin matrix. The shunt conductance of HVDC lines have also been considered. Furthermore, in the proposed method, the loss of MIDCPFC is modeled for the first time, and the loss of VSCs is considered. Finally, a new 15-bus MT-HVDC grid is proposed for verification purposes. The obtained results verify accuracy and efficacy of the proposed concepts, models and formulations in this study.

The multi-terminal high voltage direct current (MTDC) grids make it possible to connect large scale clean energy to the grid. However, the lack of effective methods minimizing the DC line faults impact limits the development of MTDC grids in some extent. This paper first analyses the propagation characteristics of the fault traveling wave in DC line. Then, the expressions of the initial line-mode fault current traveling wave (ILCTW) under typical internal and external fault are obtained. Moreover, a fast nonunit protection scheme is proposed based on the filtered fault transient current traveling wave. A modular multilevel converter (MMC)-based four-terminal high voltage direct current (HVDC) grid is established in PSCAD/MTDC, and extensive simulation results are conducted to validate that the protection scheme can not only identify the internal fault accurately and rapidly, but is also effective in detecting different fault locations and large fault resistance.

Discrimination of different DC faults near a converter end of a DC section consisting of a filter, a smoothing reactor, and a transmission line is not an easy task. The faults occurring in the AC section can be easily distinguished, but the internal and near-side external faults in the DC section are very similar, and the relay may cause false tripping. This work proposes a method to distinguish external and internal faults occurring in the DC section. The inputs are the voltage signals at the start of the transmission line and the end of the converter filter. The difference in voltage signals is calculated and given to an intelligent controller to detect and discriminate the faults. The intelligent controller is designed using machine learning (ML) and deep learning (DL) techniques for fault detection. The long short-term memory (LSTM-) based relay gives better results than other ML methods. The proposed method can distinguish internal from external faults with 100% accuracy. Another advantage is that a primary relay is suggested that detects faults quickly within a fraction of milliseconds. Nevertheless, another advantage is that a backup relay has been designed in case the primary relay cannot operate. Results show that the LSTM-based protection scheme provides higher sensitivity and reliability under different operation modes than the conventional traveling wave-based relay.

A new differential protection scheme based on Synchro-squeezing transform applied to AC Micro-Grid

Busbar protection is an essential component in power system design, protecting the most important system node for network stability and security. When faults occurs on busbar itself, it takes much time to isolate the bus from source which may cause much damage in the bus system. Faults in power system are classified as internal and external faults. Faults within the zone are termed as internal faults whereas, the faults outside the Zone are called as external faults. Ideally, a relay looking after the protection of a zone should operate only for internal faults. It should restrain from operating for external faults or through faults. In this paper the busbar protection using differential protection scheme has been investigated for internal and external faults. An algorithm has been developed to improve the selectivity of the relay and the same has been tested in IEEE9 bus system for internal and external faults. Separation of De-noised signal from fault signal is made using wavelet transform so that the nature of fault occurs on the system can be identified. In this study Daubechies 4 at level 3 is used to separate original signal and de-noised signal. The entire simulation has been done using MATLAB R2017a.

A reliable DC fault protection system is essential for the development of HVDC grids. Therefore, this paper deals with the voltage derivative ROCOV scheme to locate and detect DC faults. The algorithm is able to differentiate internal and external faults considerably fast. The proposed algorithm is analyzed in a HVDC grid with different fault case scenarios. Finally, the ROCOV protection thresholds are discussed.

This paper presents a simple and practical method which enables to prevent malfunction of protection relay due to differential current caused by one end CT saturation in case of external fault. This method uses difference of magnitude(instantaneous value) between the both end current just before the occurrence of differential current without a separate method to CT staturation detection. One end CT saturation is simulated by current transformer model using type-96 component and the presented method is verified by using EMTP MODELS with respect to internal and external fault with one end CT staturation. The presented method distinguished rightly bewteen external and internal fault with one end CT saturation. This information can be used to prevent malfunction of current differential protection relay in case of external fault. And this method is not affected by sampling rate and has no calculation burden, so it will be applicable to differential current protection relay with ease.

Modern microprocessor based relays used for busbar and transformer protection, typically implement a low impedance percentage differential principle using dual slope characteristics. These relays may sometimes misoperate in response to the CT saturation during close-in external faults, and the subsidence current present after clearing external faults. A primary reason for such misoperations is the fact that the traditional percentage differential principle relies exclusively on current magnitudes rather than directionality for tripping decisions. This paper describes two novel principles that distinguish between external and internal faults so as to enhance the security of differential protection; directional phase angle comparison and rate-of-change of differential current. The paper also discusses mathematical models used for each principle and their interaction during operation as well as their interaction with the traditional dual slope method. In addition, the paper compares the performance of a protective device using the traditional dual slope method vs. one with the new principles. Test cases are presented to illustrate the effectiveness of the new principles in enhancing the security for external faults.

The busbar protection relays typically implements low impedance percentage differential principle using dual slope characteristics. These relays may sometimes misoperate in response to the CT saturation during close-in external faults, and the subsidence current present after clearing external faults. A primary reason for such misoperations is the fact that the traditional percentage differential principle relies exclusively on current magnitudes rather than directionality for tripping decisions. This paper evaluates the performance of an enhanced bus differential relay which uses different principles that distinguish between external and internal faults so as to enhance the security of differential protection. Extensive testing has been carried out to evaluate the effectiveness of the new principles in enhancing the security for external faults.

There is a smoothing reactor and DC filter between the inverter and the direct current line to form a boundary in the HVDC transmission system. Since this boundary presents the stop-band characteristic to the high frequency transient voltage signals, the high-frequency transient voltage signal caused by external faults through boundary will be attenuated and the signals caused by internal faults will be unchanged. A new algorithm based on mathematical morphology is presented for fast discrimination between internal fault and external fault. The method is based on the principle that the waveform caused by internal fault contains much wave peak and wave trough and the waveform caused by external fault changes slightly.