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Abstract Protecting DC microgrids (DCMGs) from faults is critical due to the rapid current changes that occur in milliseconds. However, ensuring fast and accurate protection in DCMGs is more challenging than in AC systems. This study proposes a novel protection algorithm using traveling waves (TWs) for fault detection and localization. The high‐order synchrosqueezing transform (FSSTH) is applied to precisely identify TWs at the relay location. FSSTH offers a sharp time–frequency representation, enhancing the accuracy and speed of fault detection. This method can accurately detect transient phenomena like TWs in DCMGs, even with noise and variable fault resistance. By using the spectral envelope with FSSTH, ridges in time–frequency representations are extracted, improving fault diagnosis. The approach differentiates external from internal faults and recognizes fault direction by assessing TW polarity. Testing on two different DCMGs showed this algorithm's high efficiency and accuracy, with fault location errors ranging from 1 to 50 meters in low‐voltage and 13 to 64 meters in medium‐voltage DCMGs, even under challenging conditions like high resistance (up to 500 Ω) and low signal‐to‐noise ratio (5 dB). These results demonstrate the method's superior accuracy and robustness compared to existing techniques.

Abstract Fast progress of High Voltage Direct Current (HVDC) technology has been considerable over the past decades. Emerging Modular Multilevel Converter (MMC) provides power systems with promising solutions to integrate large‐scale renewable energy sources, as well as multiple asynchronous power grids. Consequently, implementation of a multi‐terminal HVDC transmission system has been easier using MMC converters. However, protection of multi‐terminal MMC‐based HVDC grids under DC fault conditions is a challenge for the industry. This study sheds light on concept of system‐level DC protection coordination for multi‐terminal HVDC grids. Although, there are different protection methods for HVDC transmission systems including multi‐terminal ones, there are no well‐defined protection coordination strategies for transmission links protection and belonging HVDC converters under DC fault conditions. The proposed protection coordination prevents unnecessary tripping of MMCs in the multi‐area HVDC grids and supports the system with minimum effect on healthy parts. For the first time, different HVDC Converter Tripping (HCT) profiles are presented based on a fully selective protection strategy and their characteristics are discussed comprehensively. Simulation results demonstrate the effectiveness of the proposed strategy for multi‐terminal MMC‐based HVDC grids. The proposed coordination can be applied to DC grid code requirements of meshed HVDC grids in the future.

Abstract The protection of traditional high‐voltage capacitor banks relies on an unbalance relay which operates when an internal fuse is blown. However, the unbalance relay cannot indicate the cause of the fault. Thus, an operator wastes time and human resources investigating the fault issues. In this paper, a fault which occurred in a 230‐kV power system of Electricity Generating Authority of Thailand was observed by performing simulations using the Power Systems Computer Aided Design (PSCAD) program. The study system based on the double bus station and 72 MVAR capacitor banks was installed in the form of a back‐to‐back topology. Three scenarios were considered: normal condition, fault occurrence in one capacitor bank and fault occurrence in both capacitor banks. Current characteristics such as the current phase and difference in unbalance current were considered. In addition, discrete wavelet transform was applied to solve the ambiguity of current generated from the PSCAD. The authors’ results suggest that identifying fault events using a coefficient of wavelet is more efficient than relying on the current amplitude. The findings mentioned in this paper can be applied in a traditional power system protection scheme to enhance a system's reliability.

Abstract Modular multi‐level converters (MMCs) are a mainstay in many HVDC transmission projects worldwide. Half‐bridge MMC (HB‐MMC) is mostly used in the current project. The large capacitor of sub modules (SMs) is often required in MMC to ensure the low voltage ripple. Here, the three‐stage switching MMC (TSS‐MMC) topology is proposed, which can greatly reduce the capacitor voltage ripple of SMs by dynamically adjusting the position of inductance on the upper and down bridge arms. The principle of reducing capacitor voltage ripple based on equal capacitance theory is introduced. The selection of the inductance value of each part in TSS‐MMC is introduced. The start‐up process strategy, capacitor voltage ripple reduction strategy in steady‐state operation, and DC fault ride‐through strategy of TSS‐MMC are proposed. The effectiveness and engineering practicability of the proposed TSS‐MMC is verified by transient simulation program built on PSCAD/EMTDC. The simulation results show that compared with MMC with circulating current suppressing controller (CCSC), it can effectively reduce the capacitor voltage ripple by over 50%. Furthermore, the total used inductance of TSS‐MMC can also be reduced. A comprehensive assessment of TSS‐MMC is also carried out.

Abstract Even though shunt compensation of transmission lines (TLs) is a common practice to limit the overvoltages stemming from the Ferranti effect, it may cause transient and steady‐state resonant overvoltages when one phase is left open due to the single‐pole‐open condition following the temporary phase‐to‐ground (Ph‐g) fault. Furthermore, the existing shunt reactors potentially increase the duration of the secondary arc current at the fault point and hence prevent offering fast single‐pole reclosing. To exploit the mentioned issues, in recent studies, a neutral reactor is introduced, which is installed in the neutral point of the shunt reactors; nonetheless, obtaining the appropriate amount of it is remarkably challenging. In this paper, an optimal neutral reactor (ONR) is determined based on an optimization problem, which minimizes the neutral reactor cost subject to limiting both resonant overvoltages and the duration of the secondary arc current. In modelling, the Laplace domain is employed rather than the phasor domain to consider both transient and steady‐state conditions since this technique allows having better control over transient overvoltages as well as transient secondary arc.

Abstract The high current of hybrid DC circuit breaker (HCB) in the process of breaking generates strong transient magnetic field (MF), which may interfere with the normal operation of driver control units (DCUs). Therefore, the analysis for transient MF disturbance is of great significance. Due to the large space scale of HCB, with the large number of power electronic devices, the numerical calculation method has the disadvantages of large consumption of resources and slow calculation speed in solving the transient MF inside the HCB. In this study, the current generation mechanism of a 500 kV HCB is analysed, and the equivalent current path is obtained by considering the skin effect. Combined with the temporal and spatial distribution of transient current, the multi‐process transient equivalent model of transient MF calculation is established, and the model has the advantages of high precision and high speed. Then, the breaking experiment of HCB is carried out, and the transient MF near the DCU is measured. The experiment results verify the validity of the model. Furthermore, the disturbance of transient MF at the DCU of the transfer branch during breaking 25 kA current is analysed.

This study presents a novel multi-rate algorithm for the co-simulation of power system transients using base-frequency dynamic phasor solver for frequency adaptive simulation of transient (BFAST) and electromagnetic transient (EMT) solvers. The BFAST solver alters its solution technique from dynamic phasors to EMT based upon the frequency contents of the waveforms being simulated. A changeover algorithm between the two solvers is also presented. The BFAST solver is then integrated with an industrial-grade EMT solver to develop a BFAST–EMT multi-rate co-simulator. The interface between the two solvers is established using transmission lines at the partitioning locations. The co-simulator combines the benefits of dynamic phasors, frequency adaptive simulation of transients, parallel processing, and multi-rate simulation. The study describes the several solution modes of the proposed co-simulator. Illustrative examples are included to demonstrate the accuracy and computational benefits of the proposed co-simulator.

According to the actual requirements of the project, when the pole-to-pole fault occurs in the DC system, the fault must be cleared during the capacitor discharge phase, so the requirements of the calculation of the current transient expression at the fault point are made. To solve this problem, this study proposes a dual-port network model with fault flags. First, analyse the equivalent circuit diagram of the tree and ring DC system after fault, and compare it with the dual-port network model. The relationship between the parameters of the original system and the parameters of the proposed model is given. Second, put the relationship expression into the short-circuit current expression of the dual-port network model and obtain the final transient expression for multi-ended systems. Finally, the 3-end tree and ring DC system model are set up on the MATLAB/Simulink software platform. The simulation of the model is verified. The error is below 5%, which proves the reliability of the model.

The traditional electromagnetic transient (EMT) model of transmission line is very slow for the large-scale power systems simulation due to the small step. To accelerate the electro-magnetic transient program (EMTP) calculation, a modified line model used for realising multi-step simulation is proposed. This model is based on the timescale transformation of single-phase coordinate which can transform the primary high-frequency signal into a lower one. By this means, the part which needs to be simulated in detail, such as power electronic devices, can be calculated with a small time step, and a large time step can be used for the other part. In this article, the basic theory of timescale transformation is introduced firstly, and then the new line model is deduced in detail. Finally, a simple system is simulated by the new and traditional model, respectively, and the validity of this approach is verified by comparing with the traditional EMTP simulation. Furthermore, due to the large-step simulation, the calculation speed is improved.

In the hierarchically connected ultra-high voltage (UHV) DC projects, the minimum air clearance on the 1000 kV AC side of converter transformer directly affects the layout and area of the valve hall. The minimum air clearance is determined by the overvoltage and the flashover characteristics of different electrodes. Since the UHV DC projects are hierarchically connected to the AC power grid, the overvoltage and the electrode flashover characteristics are different from that of general UHV AC projects, necessitating the relevant overvoltage calculation and test studies. The electromagnetic transients software PSCAD is used for modelling of the ±1100 kV Wannan convertor station and the related AC system. The power frequency overvoltage, switching overvoltage, time-to-crest value, and coefficient α of phase-to-phase overvoltage on the AC side of the 1000 kV converter transformer are calculated. Tests are carried out on air clearances between bundle conductors, tubular buses, and equipment grading rings under switching impulse with 1000 μs long wave-front. The minimum phase-to-earth and phase-to-phase air clearances on the 1000 kV AC side of the converter transformer are given.

As a most widely applied real-time digital simulation system, real time digital simulation (RTDS) is however quite limited on scale of simulation by the number of hardware purchased. Through frequency-dependent network equivalent (FDNE), scale of simulation of RTDS could be largely increased by frequency equivalent on limited hardware. Based on basic principles of FDNE, function limitations and problem are analysed, taking the Shanghai Sijing district power grid as an example; detailed simulation model and FDNE equivalent module are built, transient characteristics under faults before and after FDNE equivalent are analysed. Furthermore, the fault characteristics of high-voltage direct current (HVDC)-Sijing power grid is also studied, which provides a realistic means for real-time digital simulation of AC–HVDC hybrid power grid with limited hardware of RTDS.

Switching AC filter may cause harmonic impact, which may lead to commutation failure of HVDC system. Here, the commutation failure mechanism of HVDC system caused by AC filter switching is studied. The circuit and mathematical model of the transient process simulation of filter switching are established, and the time-domain solution of the overvoltage of each filter including voltage reduction and distortion is given. The commutation failure mechanism of HVDC system is analysed based on the commutation failure analysis method of harmonic voltage and time area. The simulation results verify the effectiveness of the mechanism analysis of commutation failure caused by AC filter switching in HVDC system.

Due to the fast development of UHVDC, the power transferring through HVDCs takes a great part of the power from West China to East China. The response for multiple-coupled HVDCs can only be simulated through EMT programmes. To increase the simulation accuracy, the EMT-TS Hybrid simulation has been used in studies of the power system operation. Currently, the EMT-TS hybrid simulation speed is too slow to be unaffordable. This paper presents a novel power system tearing technology to greatly improve the simulation speed in EMT-ST Hybrid Simulation. The TS network is teared according to obtain network islands as many as possible, while the EMT network is teared through the combined transmission line and electric nodes. An algorithm is supposed to form the EMT-ST sub-network groups, which has no coupling with each other. Then, a novel parallel programme structure is proposed to use this EMT-ST sub-network groups to realise the parallel simulation and speedup the simulation. The case studies of the grids with seven HVDCs and 3000 nodes show that the proposed method could greatly improve the simulation speed without lowering precision.

In order to correctly simulate the simultaneous switching of power electronics circuits and to solve the problems of virtual power loss existing in the traditional interpolation algorithm, this article proposes interpolation algorithm considering simultaneous solution and instantaneous solution. After each integration, it searches for the switching events and determines the simultaneous switching. The simultaneous switching events are solved simultaneously. Instantaneous solution is carried out at forced commutation switching instant. The historical terms calculation method in instantaneous solution under different conditions is given, and the exact power loss of forced commutation switch is obtained. After processing all switching events during one time-step, the half-step interpolation is performed to eliminate numerical oscillations. The proposed algorithm is applied to the electromagnetic transient programme of advanced digital power system simulator (ADPSS), and the correctness and effectiveness of the algorithm are verified by simulation tests for typical power electronics circuits. The simulation results show that the proposed algorithm has high simulation accuracy and can satisfy requirements of power electronics simulation.

Transient processes at neutral bus of ultra-high value direct current (UHVDC) converter station received insufficient attention, because of the lower amplitude compare to poles. However due to the lower insulation level, electromagnetic transient processes at neutral bus may cause serious fault, and become an important issue that should be studied carefully in the design stage. A method to analyse the electromagnetic transient based on time-domain wide-band circuit simulation is proposed in this study. Models of major equipment are described. The electromagnetic transient of a ±800 kV converter station is calculated. The transient under different operation statuses is compared. Effects of suppression method such as arresters and capacitors are analysed.

This study introduces an active distribution network with high penetration of inverter-based distributed generations (IBDGs) to improve the steady state, dynamic and transient performance of the distribution network. The size and location of the IBDGs, capacitor bank, on-load tap changers position and voltage regulator set points have been optimised to minimise the system losses, improve voltage profile and maximise the dynamic reactive power reserve. In addition to that, the adaptive master/slave roles are deployed in planning the active distribution network to achieve the best utilisation of each device based on its capability and response time in steady state, dynamic and transient operation. Indeed, the optimisation algorithm allows the IBDGs to operate in an inductive mode of operation during steady state resulted in maximising the dynamic reactive power reserve. Consequently, the IBDGs react with large dynamic margin in response to grid fault leading to enhanced voltage recovery and improved transient response. A comprehensive system optimisation and transient analysis are carried out to demonstrate the performance of the distribution network for enhancing the voltage profile, increasing the penetration level of renewable energy distributed generation, IBDGs, and improving the transient recovery in response to severe grid fault.

The accessing of distributed generation resources to distribution network leads to a multi power grid structure. The direction of power flow changes with it. The photovoltaic (PV) generation contains a large number of power electronic devices and control links, which causes that the output responses and fault characteristics are significantly different from the synchronous machine. In consideration of the topological structure of the distributed distribution network, this paper analyses the differences of transient characteristics of AC distribution line with PV power supply between internal and external faults. Then, a new principle for constructing directional element using transfer and consumption characteristics of high-frequency transient energy is proposed. This directional element only uses high-frequency transient fault voltage and current in early stage, which can reliably distinguish the fault direction. What's more, the existence of transition resistance will be conducive to the judgement. Finally, corresponding simulation cases were carried out to verify the feasibility and superiority of the proposed directional element.

The distance protection method with parameter identification can distinguish internal fault from external fault correctly and avoid the transient overreach effectively based on the accurate secondary signal data. However, due to the limitation of the frequency band of the electromagnetic transformer, the secondary signal distortion is caused when the transient signal is transmitted, which limits the application of the parameter identification principle. Here, a matching method of line model and transformer type for relay protection is proposed. Analysed the appropriate frequency band of line model and transformer linear transfer frequency band and the electrical signal beyond their intersection should be filtered out. The simulation results show that this method improves the accuracy of the operation. This method not only makes sense to the practical application of distance protection based on the principle of parameter identification, but also has some reference value to other engineering applications.

This study proposes a dual-loop supplementary frequency control (SFC) scheme for back-to-back voltage source converter high-voltage DC (BTB-VSC-HVDC) interconnecting asynchronous AC grids, to provide frequency support for each other after a large disturbance. The proposed dual-loop SFC consists of the frequency-active power (f–P) loop and the frequency-reactive power (f–Q) loop. The former deployed on the P-loop of VSC-HVDC can provide frequency support for the disturbed grid during the primary frequency regulation and improve the steady-state characteristics of the frequency response, while the latter attached to the Q-loop of VSC-HVDC supports virtual inertia to ameliorate the transient characteristics of the frequency response. Simulation studies are conducted based on the equivalently simplified model of Southwest Power Grid and Hubei Power Grid interconnected by Chongqing-Hubei BTB-VSC-HVDC. Simulation results show that the proposed SFC scheme can effectively improve the transient and steady-state characteristics of the system frequency response under a wide range of operating condition.

Transient rotor angle stability assessment and oscillatory rotor angle stability assessment subsequent to a contingency are integral components of dynamic security assessment (DSA) in power systems. This study proposes a hybrid algorithm to determine whether the post-fault power system is secure due to both transient rotor angle stability and oscillatory rotor angle stability subsequent to a set of known contingencies. The hybrid algorithm first uses a new security measure developed based on the concept of Lyapunov exponents (LEs) to determine the transient security of the post-fault power system. Later, the transient secure power swing curves are analysed using an improved Prony algorithm which extracts the dominant oscillatory modes and estimates their damping ratios. The damping ratio is a security measure about the oscillatory security of the post-fault power system subsequent to the contingency. The suitability of the proposed hybrid algorithm for DSA in power systems is illustrated using different contingencies of a 16-generator 68-bus test system and a 50-generator 470-bus test system. The accuracy of the stability conclusions and the acceptable computational burden indicate that the proposed hybrid algorithm is suitable for real-time security assessment with respect to both transient rotor angle stability and oscillatory rotor angle stability under multiple contingencies of the power system.