MTDC System Research Articles

Steady state analysis of Multiport Interline DC Current Flow Controller integrated meshed MTDC grids

Flexible DC Transmission Systems (FDCTS) devices offer the most promising solution for Power Flow Management (PFM) problem in complex meshed MTDC grids. This paper proposes a new formulation for steady state analysis of a MTDC grid, integrated with the multiport version of the FDCTS device called Multiport Interline DC Current Flow Controller (MIDCCFC). MIDCCFC can regulate the current flow of two or more lines, thereby achieving line power flow regulation and aid for DC line breaking in a MTDC system. The proposed formulation integrates the steady state operating point of MIDCCFC with the MTDC system. The proposed formulation is tested on a 5 bus 320 kV meshed MTDC grid with 5-port MIDCCFC. A criterion is evolved to ensure the feasibility of the operating point obtained from the proposed formulation. The line current regulation range offered by MIDCCFC for line-12 in the test system is (30-3086) A. The impact of line-12 current variation on the system variables over the obtained range is investigated. The application of MIDCCFC for DC breaking is also investigated. The results are validated by performing steady state simulation of the test system in MATLAB/Simulink and real time simulation of the test system in Real Time Digital Simulator.

DC protection in modern HVDC networks: VSC-HVDC and MTDC systems

Considering the importance that HVDC systems are expected to have in the future of power networks, this paper aims to provide a wide overview of the protection against DC faults. In particular, it analyzes the detection and location techniques that have been proposed to date, focusing on the approaches that are set to be applied in modern HVDC networks, which comprise VSC-HVDC and MTDC systems.

Distributed collaborative optimization DC voltage control strategy for VSC–MTDC system with renewable energy integration

Distributed collaborative optimization DC voltage control strategy for VSC–MTDC system with renewable energy integration

Modified VMD Algorithm-Based Fault Location Method for Overhead-Cable Hybrid Transmission Line in MTDC System

Modified VMD Algorithm-Based Fault Location Method for Overhead-Cable Hybrid Transmission Line in MTDC System

Setting-Less Pilot Protection for MTDC Systems with Weak or No Line Boundary Based on Model Differences Between Forward and Backward Faults

Setting-Less Pilot Protection for MTDC Systems with Weak or No Line Boundary Based on Model Differences Between Forward and Backward Faults

Stability Assessment and Improvement of MTDC System Connected with Offshore Wind Farms

Stability Assessment and Improvement of MTDC System Connected with Offshore Wind Farms

On Resilience and Distributed Fixed-Time Control of MTDC Systems Under DoS Attacks

On Resilience and Distributed Fixed-Time Control of MTDC Systems Under DoS Attacks

A decision-making method for the operation flexibility enhancement of hybrid cascaded MTDC

To enable the integration of large-scale renewable energy, hybrid HVDC technology, which combines the technical advantages of LCC-HVDC and VSC-HVDC, is being gradually deployed in the power grid nowadays. The operation of the Wu-dong-de Hybrid DC Project and the Jian-su Hybrid cascaded MTDC Project has proved its advantages. However, for the simultaneous application of different converter station technologies in the system, the control strategies become complex. Issuing appropriate control instructions to ensure system stability according to operational requirements is an issue that cannot be ignored in decision-making. Even under abnormal conditions, when the topology changes due to various failure scenarios, reasonable decision-making and precise control instruction definitions are required. To achieve flexible planning of the MTDC system, this paper presents a decision-making method for control strategies of a hybrid cascaded MTDC system, which analyzes the control strategy combinations selected for normal and abnormal conditions of the MTDC system. In addition, a control instruction calculating method and decision-making process for precise control in normal and abnormal control conditions is proposed. Simulation results based on a five-terminal hybrid cascaded MTDC in PSCAD/EMTDC have verified the effectiveness of the proposed method.

Distributed Fixed-Time Secondary Control for MTDC Systems Using Event-Triggered Communication Scheme

Multi-terminal DC transmission (MTDC) systems have attracted much attention due to their significant advantages in long-distance and high-capacity transmission. To improve their reliability and operation performance, a distributed fixed-time secondary control of frequency restoration and active power sharing is proposed under event-triggered communication, which only depends on the states of each AC grid and its neighbors. By utilizing Lyapunov theory, we prove that the MTDC system with the fixed-time secondary control can be stable in a settling time, and the conditions of the settling time are established for fixed-time algorithms. In addition, we simulate a five-terminal MTDC system in Matlab/Simulink. Several cases of MTDC systems are exhibited to showcase how well the suggested controller works when dealing with load changes and attacks. The comparison of the number of event-triggered instants shows that the proposed control method can effectively reduce communication resources.

Zero-sequence current suppression control for fault current damper based on model predictive control

In a multi-terminal direct current (MTdc) system based on a modular multilevel converter (MMC), high-speed and large interruption capability direct current circuit breakers (dc CBs) are required for dc fault interruption. However, commercializing these breakers is challenging, especially offshore, due to the large footprint of the surge arrester. Hence, a supplementary control is required to limit the rate of current rise along with the fault current limiter. Furthermore, the operation of the dc CB is not frequent. Thus, it can lead to delays in fault interruption. This study proposes the indirect model predictive control (MPC)-based zero-sequence current control. This control provides dc fault current suppression by continuously controlling the zero-sequence current component using circulating current suppression control (CCSC) and by providing feedback to the outer voltage loop and inner current loop of MMCs. The proposed control is simulated for pole-to-pole and pole-to-ground faults at the critical fault location of an MTdc system. The simulation is performed in Real Time Digital Simulator (RTDS) environment, which shows that the predictive control reduces the rate of rise of the fault current, providing an additional 3 ms after the dc fault occurrence to the dc CB to clear the fault. Besides, the energy absorbed by the dc CB’s surge arrester during the pole-to-pole and pole-to-ground fault remains the same with the proposed control.

An Improved Adaptive Coordination Control of Wind Integrated Multi-Terminal HVdc System

The increasing penetration of the renewables and integration of the power electronic devices leads to lower system inertia, which is changeling the system stability of a multiterminal HVdc (MTdc) system. This article presents an improved adaptive predictive control with the multiobjective targets coordinating the key parameters that the dc voltage, ac frequency and power-sharing among the terminals in MTdc system. Specifically, we contribute two main points to the relevant literature, with the purpose of distinguishing our study from existing ones. First, the proposed method is based on minimal information exchange by only considering neighboring terminals. Second, the adaptive control is achieved by setting a weighted fitness function to adaptively tune the weights with the effective integration of the trust-region and particle swarm optimization. A four-terminal HVdc system built within the IEEE 30-bus ac system is used as the study case to validate the robustness and efficiency of the proposed method. In the case study regarding the multiobjective fitness function, the proposed approach benefits in suppressing the voltage deviation, providing frequency support and establishing an automatically updated power equilibrium leveraging by the adaptive parameters.

Primary Frequency Support Strategy for MTDC System With Enhanced DC Voltage Response

This paper presents a mutual frequency support strategy for ac grids interconnected by multi-terminal high voltage dc transmission system. The proposed scheme has different strategies for ac side disturbance and converter outage. During ac side disturbance, the proposed scheme makes sure that the actual aggregated droop gains of the converters in the same ac grid is equal to the designed aggregated droop gains by effectively utilizing the available headroom (AvHR) of the converters. Moreover, the paper proposes a threshold compensation strategy to eliminate the effect of the frequency deviation threshold that is intentionally imposed on the frequency droop control of converters. Therefore, the proposed scheme can accurately achieve the operator pre-specified ratios among the frequency deviation of the ac grids. The aggregated droop gain of the converters in each ac grid is designed to achieve the pre-specified ratios amongst frequency deviation of the ac grids. Furthermore, during an ac side disturbance, converters in the disturbed ac grid operates in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$P-f$</tex-math></inline-formula> droop control mode and the converters in the supporting ac grids work in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$P-V_{dc}$</tex-math></inline-formula> droop control mode. Therefore, the impact of the frequency deviation on the dc voltage profile can be reduced. During converter outage, the proposed scheme minimizes the frequency deviation of the ac grids by trying to restore the power flow between the ac grids and their converters to a level before the disturbance. The effectiveness of the proposed scheme is verified through rigorous simulation tests.

A protection method for multi-terminal HVDC system based on fuzzy approach

This paper presents a Fuzzy Inference System (FIS)-based method to detect a fault, identify the faulty section, and recognize the faulty pole in the Voltage Source Converter (VSC)-based Multi-Terminal High Voltage Direct Current (MT-HVDC) system. The method uses only rectifier end measurements of voltage and current signals. To achieve complete protection of the MT-HVDC system, three separate frameworks of FIS modules have been developed. The FIS-1 detects the presence of fault in either AC or DC segments. The FIS-2 identifies the fault section and eventually, the FIS-3 recognizes the faulty pole. The proposed method provides a rapid fault detection and it does not require any communication media as it is based on the rectifier-end measurements only. The efficacy of the implemented FIS-based method is evaluated in an MT-HVDC system simulated in MATLAB environment.Important highlights of this method are:-•FIS-based method is deployed using simple if-then rules, therefore very easy to implement.•Effortless means to exploit one end measurements of MTDC system in an environment accustomed to power engineer.•Proposed method provides a rapid fault detection and it does not require any communication link.

Two-layer control structure for enhancing frequency stability of the MTDC system

This paper explores the possibilities of providing fast frequency support as an emergency support service to the disturbed AC system through the MTDC grid. A two-layer hierarchical control structure of the MTDC grid is proposed to assure the minimum cost of the frequency control actions, the minimum voltage deviations, or the minimal impact on the frequencies of not-affected AC systems while ensuring the stable operation of MTDC grid. An optimization algorithm is executed at the secondary control level to find the optimal reference values for the voltage-droop characteristics of the voltage-regulating converters, and consequently their DC voltages and active power references. Then, at the primary control level, the reference values are tuned with the optimization results. Implemented control structure confirms that MTDC can provide set values at its terminals without endangering its stability. The secondary control layer is implemented in MATLAB, while the performance of the controller is successfully evaluated through simulation in RSCAD.

Research on Adaptive Exponential Droop Control Strategy for VSC-MTDC System

To solve the problem of large DC voltage deviation caused by the power fluctuations and poor power distribution characteristics of converters in a voltage source converter multi-terminal DC (VSC-MTDC) system based on traditional droop control, this paper proposes an adaptive exponential droop control strategy. This strategy introduces the relative power deviation factor of the converter, and replaces the traditional linear droop control curve with a nonlinear exponential curve. Under different working conditions, the converter adaptively adjusts the droop control coefficient of the converter according to the relative power deviation factor to realize stability for the DC voltage and a reasonable power distribution for the MTDC system. A simulation model of a three-terminal VSC-MTDC was established in MATLAB/Simulink, and the feasibility and effectiveness of the proposed strategy were verified.

A Fault Detection Scheme in MTDC Systems Using a Superconducting Fault Current Limiter

This article proposes a fault detection scheme for modular multilevel converter-based multiterminal dc (MMC-MTdc) girds based upon a resistive-type superconducting fault current limiter (RSFCL). By analyzing the characteristics of the currents through the faulted and healthy lines in an MMC-MTdc system, the theoretical foundations of the scheme are established. This yields a fault detection scheme with fault identification, fault pole discrimination, and high-resistance tolerance abilities. Extensive simulations confirm that the proposed detection scheme can identify the metallic short circuit fault within 1 ms, with the ability to promptly detect faults with high resistance in less than 3 ms taking the communication delay into consideration. This method does not cause any mal-operation of dc circuit breakers as a result of faults on adjacent lines and power order changes. The proposed scheme has immunity to noise and provides full-length protection of the protected transmission line. Simulation results show significant suppression of both the amplitude and the rate-of-rise of the fault current during the fault detection process, which enhance the reliability and stability of the protected grid.

Impact of System Inherent Characteristics on Initial-Stage Short-Circuit Current of MMC-Based MTDC Transmission Systems

This paper studies the impact of the inherent characteristics on the initial-stage short-circuit current of modular multilevel converter based multi-terminal direct current (MMC-based MTDC) transmission system. A short-circuit current calculation method based on input-output characteristics is proposed. The MMC’s input-output characteristics are factorized into a series combination of a linear and a nonlinear part through the operator approach. The inherent characteristics of the system represent the inherent properties of the MMC-based MTDC system, which are unrelated to the short-circuit fault. Therefore, short-circuit currents are decoupled and quantitative indices are proposed to measure the impact of the system inherent characteristics on the initial-stage short-circuit current. The proposed decoupled short-circuit current calculation method and the quantitative indices are verified in a typical MMC-based MTDC transmission system under different system parameters and structures. The proposed quantitative indices can effectively evaluate the short-circuit current of the MMC-based MTDC system without calculating the short-circuit current, which indicates that they can provide guidance for the planning of DC system structure, determination of operational modes, and selection of system parameters from the perspective of short-circuit current.

Thyristor-Pair- and Damping-Submodule-Based Protection Against Valve-Side Single-Phase-to-Ground Faults in MMC-MTDC Systems

The single-phase-to-ground (SPG) fault at the valve side of the converter transformer is one of the most critical faults threatening the operation of bipolar half-bridge modular-multilevel-converter-based multi-terminal direct-current (HB-MMC MTDC) systems. Following a valve-side SPG fault, the upper-arm submodule (SM) capacitors in the faulty converter may suffer a serious overvoltage, and the grid-side fault currents may experience no zero-crossings. The existing protection methods generally rely on tripping the grid-side alternate-current circuit-breaker (ACCB), which cannot fully isolate the fault. The healthy poles and stations may encounter severe disturbance and be all blocked, which will lead to a power outage of the entire MTDC system. This paper proposes an ACCB-independent scheme based on thyristor-pairs and damping SMs to address these problems. Antiparallel thyristor-pairs are connected in series within the upper and lower arms in each phase. Serial damping SMs are connected within the lower arms to help the lower-arm thyristor pairs cut off the fault currents. Although the proposed additional devices inevitably cause conduction losses, they can effectively protect the faulty converter and healthy circuit in the MTDC system. Simulations in PSCAD/EMTDC have been conducted to verify the effectiveness of the proposed strategy.

Enhanced DC Voltage Regulation and Transient Response for Multi-Terminal VSC-HVDC System Using Direct Power Control

This paper presents a new control strategy for enhancing the dc voltage regulation and the transient response in the multi-terminal VSC-HVDC system by ensuring swift power-sharing among converters when encountering large disturbance such as converter loss or ac fault at one of the converter stations. The quick power sharing can be realized using direct power control (DPC) since it has a merit of fast power dynamic response. On the other hand, to limit the converter current during ac fault, a new coordination strategy between DPC and vector current control (VCC) is proposed. Furthermore, a new simplified model of the MTDC system is developed to study the performance of DPC in enhancing the dc voltage dynamics response analytically. The time-domain simulation of the detailed model of the four converter stations MTDC system verify that the proposed control strategy significantly reduces the overshoot in the transient dc voltage response following an outage or ac fault. As a result, together with the droop gain selection for the maximum permissible steady-state dc voltage deviation, the proposed control strategy can help to reduce the size of the dc chopper in the MTDC grid or even avoiding it. Moreover, the comparative analysis based on time-domain simulation depicts that the proposed simplified model has the same dc voltage response compared to the detailed model of the MTDC system except for a steady-state error of 0.47% and a little bit longer settling time (1.5 ms) at transient.

A Meshed MMC-Based MTDC System with Improved Voltage Margin Control for Robustness Towards Disturbances

A Meshed MMC-Based MTDC System with Improved Voltage Margin Control for Robustness Towards Disturbances