Fast and reliable DC fault detection is one of the main challenges for modular multilevel converter (MMC) based DC grid with DC circuit breakers (DCCBs). This paper extracts the high-frequency components in transient voltages by wavelet transform and proposes a fault identification method based on the difference of square of transient voltages to identify the faulted lines for DC grids using overhead lines. Meanwhile, a faulted pole discrimination method based on the difference between the change of positive and negative pole voltages is presented. A line protection scheme including detection activation, fault identification, faulted pole discrimination, and post-fault re-closing is designed. Using only the local measurements, the scheme can realize the protection of the whole line without communication and has the capability of fault resistance endurance and anti-disturbance. The proposed method is tested with a four-terminal MMC-based DC grid in PSCAD/EMTDC. The selection methods of threshold values are presented and the impact of DCCB operation on the reliability of DC fault protection is analyzed. Simulation results verify the fast detection and reliability of the designed DC line protection scheme.

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A Transient Voltage-Based DC Fault Line Protection Scheme for MMC-Based DC Grid Embedding DC Breakers

This paper proposes a fast dc short-circuit fault detection method for HVdc grids, which relies on the voltage across the dc fault-current-limiting reactor. In the proposed approach, primary detection uses only the single-terminal dc reactor voltage information and is shown to be able to identify the faulted line as well as the faulted pole within 1 ms. The backup detection is primarily required for high resistance faults and uses communication between the two sides of the line. Its speed is dependent on the travel time of signals between the two ends, but will operate, as shown in the paper, typically in 2 ms or less. The approach is justified using analytical calculations and exhaustively validated via electromagnetic transient simulation for pole-to-pole and pole-to-ground faults, different fault resistance, fault locations, and dc reactor values. The validation shows that the method is able to identify all genuine dc line faults, but is not prone to misoperation with ac line faults, different loading levels, and so on.

A Fast DC Fault Detection Method Using DC Reactor Voltages in HVdc Grids

Fault-current handling capability of the modular multilevel converters (MMCs) under dc-cable short-circuit conditions is a major concern for the MMC applications on the high-voltage direct-current (HVDC) transmission systems, where the MMCs based on half-bridge submodules (SMs) cannot block the fault currents to protect the converter devices. In this paper, a comprehensive review for the fault-ride-through capability of the HVDC transmission systems based on the MMCs adopting different SM schemes is presented, where the MMCs can block the fault currents and compensate the reactive currents to the electric grid during the dc faults. An analysis of the dc short-circuit faults in the MMC is introduced and then the operation principle of different SM circuits building the MMC for blocking the fault currents is highlighted. The fault-tolerant operation of these MMC schemes as static synchronous compensator to enhance the ac grid stability during the dc faults is also investigated. A comparison in terms of investment cost, loss, volume, and controllability for various MMC topologies is performed. Comprehensive simulation results for the most promising topologies of the MMC with a capability of fault-ride through under dc-fault conditions are finally presented.

An Overview of Modular Multilevel Converters in HVDC Transmission Systems With STATCOM Operation During Pole-to-Pole DC Short Circuits

With the capacity of new energy resources such as wind power and photovoltaic in Zhangbei region increasing rapidly, the reliable transmission and consumption capabilities of large-scale and high-proportion new energy resources are urgently improved. Considering the advantages of the VSC-HVDC transmission technology, Zhangbei project is designed as symmetrical bipolar system which adopts half-bridge modular multi-level converter (MMC) topology and overhead lines for power transmission. In this paper, a system scheme is presented, and some results are provided. Meanwhile, a novel modular cascaded hybrid DC breaker concept is proposed, and 500kV VSC valve and DC breaker prototype are developed to meet the requirement of the Zhangbei four-terminal dc grid.

Research on Key Technology and Equipment for Zhangbei 500kV DC Grid

High-voltage direct current (HVdc) transmission systems based on modular multilevel converters (MMCs) are a promising solution for efficient bulk power transmission over long distances. As in grid-connected converters, the occurrence of grid-side faults is rather common, leading to imbalances and distortions of the ac-side voltages. Therefore, operation and control of MMC-HVdc systems under grid imbalances become of great significance in order to satisfy grid codes and reliability requirements of the system. The aim of this paper is to provide a comprehensive review of operation and control methods applied to MMC-HVdc systems with a specific focus on unbalanced ac-grid conditions. The methods are classified based on their main control targets that include ac-side power control, control of the circulating current, and dc-side power ripple suppression control. Special attention is given to the comparison of the different control methods and specific requirements under certain operating conditions that include grid, load, or internal imbalances.

Operation and Control Methods of Modular Multilevel Converters in Unbalanced AC Grids: A Review

This paper proposes a generic pole-to-pole short-circuit fault current calculation method for dc grids. The calculation procedure begins from the simplified RLC equivalent model of a single modular multilevel converter, and then the prefault matrices and faulted matrices are established and modified to calculate the dc fault currents of all the branches. The proposed approaches are validated by comparing with the electromagnetic transient (EMT) simulation results on PSCAD/EMTDC. Besides, two case studies showed that the calculation method can be easily used to evaluate the severity of a dc fault. Moreover, the calculation can be applied to select the parameters of a fault current limiter (to match the circuit breaker capacity. The main contributions of the proposed numerical calculation method are: 1) The proposed method is accurate and much more time efficient than the EMT simulations; 2) the proposed method can handle all kinds of dc grid networks including the ring, radial, and meshed topologies; and 3) the proposed method is applicable to dc grid with multiple dc voltage level areas connected with dc/dc converters.

A Pole-to-Pole Short-Circuit Fault Current Calculation Method for DC Grids

This paper proposes an optimal design method of modular multilevel converter (MMC) levels to approach the reasonable compromise between accuracy and computational efficiency. The entire procedure is benchmarked by a Thevenin equivalent 451-level MMC model. First, the calculation method of the critical maximum simulation time step is proposed to faithfully represent each voltage level of the staircase waveforms. Second, the previously developed improved analytic hierarchy process is adopted to integrate the relative errors of all the main electrical parameters into one comprehensive relative error at each MMC level. Third, the steady state, ac, and dc transient state relative error results as well as the central-processing unit times of all models are normalized and curve fitted based on the benchmark model. Finally, the optimal MMC level is obtained by the plotted curves and the given subjective weights considering the accuracy and efficiency. And the effectiveness and robustness of the proposed method are validated by various simulation scenarios on PSCAD/EMTDC.

Optimal Design of MMC Levels for Electromagnetic Transient Studies of MMC-HVDC

The invention relates to a direct-current power grid bipolar short-circuit fault current calculation method and belongs to the power transmission and distribution technical field. The objective of the invention is to solve the problem of inexistence of a specific fault current calculation method for inter-coupling of the capacitor discharge of various converters after a bipolar short-circuit fault occurs on a direct-current power grid. According to the method of the invention, based on the discharge process of the equivalent capacitors of the converters after a fault, a modular multilevel direct-current power grid is simplified into an RLC equivalent network with the fault feed current of an alternating current side ignored; and according to the characteristics of direct-current power grid nodes, the direct-current power grid nodes are divided into real station nodes and virtual station nodes. According to the technical schemes of the invention, based on Kirchhoff circuit basic principle, matrixes required by the calculation of the whole direct-current power grid bipolar short-circuit fault current are written, and the written matrixes are corrected according to the location of a fault, and fast calculation of the magnitude of fault current of all direct-current lines after a bipolar short-circuit fault occurs on any line of the direct-current power grid can be realized.

Direct-current power grid bipolar short-circuit fault current calculation method

Presents corrected author information for the paper, “A pole-to-pole shortcircuit fault current calculation method for DC grids,” (Li, C.), IEEE Trans. Power Syst., vol. 32, no. 6, pp. 4943–4953, Nov. 2017.

Corrections for “A Pole-to-Pole Short-Circuit Fault Current Calculation Method for DC Grids”

The invention designs a damping circuit optimization design method for inhibiting numerical oscillation in simulation, and belongs to the field of electromagnetic transient. The invention aims to solve the problem of numerical oscillation generated by non-state variables of energy storage elements such as an inductor, a capacitor and the like through an action of a switch device in an electromagnetic transient simulation platform without an interpolation function. The core technical scheme provided by the invention is that through the damping circuit parameter optimization method designed by the invention, for different simulation circuits, different damping circuit configuration schemes are designed according to specific circuit parameters and topological structures. A basic process comprises the steps of 1, determining the selection of an initial value of a damping circuit; 2, searching for feature information representing an oscillation degree through a custom module; and 3, designing an iterative loop by adopting a simplex module, continuously updating R and C values through multi-time nonlinear iterations, and observing characteristic information to determine R and C values when the inhibition effect is optimal, thereby achieving the purpose of damping circuit parameter optimization.

Ji Yuke

Papers

A Pole-to-Pole Short-Circuit Fault Current Calculation Method for DC Grids

Optimal Design of MMC Levels for Electromagnetic Transient Studies of MMC-HVDC

Direct-current power grid bipolar short-circuit fault current calculation method

Corrections for “A Pole-to-Pole Short-Circuit Fault Current Calculation Method for DC Grids”

Damping circuit optimization design method for inhibiting numerical oscillation in simulation