MTDC Grid 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.

A Decision Tree Based Ultra-high-speed Protection Scheme for Meshed MMC-MTDC Grids with Hybrid Lines

The reliable operation of modular multi-level converter based multi-terminal high voltage direct current (MMC-MTDC) grids requires high-speed and selective DC line protection. A single-ended protection technique is proposed for MTDC grids with offshore wind farms and hybrid lines that involve overhead lines and submarine cables in series. Positive and negative poles’ currents at one end of each line section are analyzed using discrete wavelet analysis (DWA). To classify fault type and identify fault zone, a fine decision tree is supplied with an energy index and the envelope slop of detail 1 coefficient (D1) obtained from DWA. Only 0.2 ms following the fault inception are needed for calculating the energy index and the envelope slope to perform relay functions. The approach was tested on a three-terminal two-poles ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document} 400 kV MMC-MTDC model. The simulation results validate the effectiveness of the suggested protection technique under various fault scenarios, even with up to 200 Ω fault resistance. The proposed technique is not only able to detect the faulty line, but also it identifies overhead line faults and submarine cable faults for hybrid type lines. Moreover, the proposed technique is not affected by wind power injection changes, AC faults, or data noise. The simulated model was produced in the MATLAB/Simulink platform.

Distributed Event-Triggered Fixed-Time Secondary Voltage Control and Automatic Power Sharing for MTDC Grids

Distributed Event-Triggered Fixed-Time Secondary Voltage Control and Automatic Power Sharing for MTDC Grids

Identifying suitable DC system models for an independent design of VSC controllers in MTDC grids

In AC-multi-terminal DC (MTDC) systems, if converter controllers are designed individually, the interactive system modes stemming from the dynamic couplings among the converters and the DC system components may result in the instability of the interconnected system (INTSYS). This paper presents a comparative system-level study to determine the suitable models for an individual design of converter controllers to prevent INTSYS instability. The goal is to identify suitable individual system (INDSYS) models that do not contain the internal dynamics of adjacent converters and only include the coupling dynamics corresponding to the interactive system modes among the converters. The use of these models for control design will result in the dynamic response of the INTSYS being close to that of the INDSYS models. Furthermore, such models can ensure INTSYS stability when converter controllers are designed individually without requiring the complete model of the INTSYS or internal dynamics of the adjacent converters. Both the master–slave and the droop control modes of operation are analyzed, and specific INDSYS models among those available in the literature are recommended for each operation mode. Time-domain simulations in PSCAD validate the accuracy of the INDSYS models. Furthermore, eigenvalue analysis along with the participation factor and sensitivity analyses for a large set of control parameters are employed to evaluate the efficacy of the recommended models.

Applicability Analysis of a Multi-Filar Meander R-SFCL and an RSC-DCCB in an MMC-MTDC Grid

modular multilevel converters (MMC)-based high voltage direct current (HVDC) topology is an effective solution to solve the power transmission demand between countries. However, the initial fault current can rise to several tens of kA within a few ms by capacitor discharge during a fault. This fault current can permanently damage the converter components and cables. Therefore, to increase the stability of the system, the protection of the DC system is very important. DC system protection methods include circuit breaker technologies and current limiting technologies. We have selected a circuit breaker with combined resonant current source (RCS) and a resistive superconducting fault current limiter (SFCL) as protection methods for the DC system. The RSC-DC circuit breaker has no conduction power losses by on-state voltage drop. Therefore, it can provide stable interruption compared to a solid-state DC circuit breaker. Since the resistive SFCL has a very fast response time, it can effectively limit the rapidly rising initial fault current. We wound the superconducting tape by the multi-filar meander method. This method can increase the length of the superconducting tape used and speed up the bubble diffusion during quenching. We modeled an MMC-HVDC grid, resistive SFCL, and RCS DC circuit breaker using PSCAD/EMTDC. In this paper, the interrupting and current limiting characteristics are discussed when the resistive SFCL and the RCS-DC circuit breaker proposed by us are connected to the MMC-based MTDC grid. In the simulation, the current limiting factor of resistive SFCL was up to about 58%. In addition, the combination of resistive SFCL and RCS-DC circuit breaker can shorten the interruption time when the fault occurs.

Permanent fault identification and location scheme for MTDC grid based on equivalent impedance phase characteristics and distributed parameter line

To ensure the reliability of the MTDC grid's recovery system during the reclosing stage, identifying permanent faults is crucial. Therefore, a permanent fault identification and location scheme is proposed based on equivalent impedance phase characteristics and distributed parameter lines. The equivalent impedances with distributed parameter lines in permanent and temporary faults are investigated. The proposed scheme for identifying a permanent fault is based on the phase characteristics of the equivalent impedances. The proposed scheme for permanent fault location is achieved by solving the fault location equation considering distributed parameter lines to determine the fault distance. Simulation results indicate that the proposed scheme is suitable for long-distance DC transmission lines and is less affected by the sampling rate, fault locations, fault resistance, and noise. The proposed scheme improves the reliability and sensitivity of permanent fault detection.

A Comprehensive Review of Multiport DC Circuit Breakers for MTdc Grid Protection

Voltage source converter-based multiterminal dc (MTdc) grids, including high- and medium- voltage level, have attracted attention in recent years and developed fast. To protect the MTdc grids selectively, the dc circuit breaker (DCCB) is identified as the best choice. However, due to the high unit price and the required large number of DCCBs in the future large-scale MTdc grids, the total investment cost is unacceptable. Besides, the large volume and weight also limits their applications. To solve these problems, a variety of multiport dc circuit breakers (MPCBs) are proposed, which achieve significantly reduced costs and footprints by component sharing and device integration. This paper first introduces the classifications and the basic principles of the existing MPCBs, furthermore summaries their technical requirements. Then, the economic and functional analysis of each MPCB is presented, and the performance comparisons among different topologies are elaborated. Finally, the challenges and future trends of the MPCBs are discussed and the findings are concluded.

Optimal power distribution with mitigation of DC voltage fluctuations using artificial intelligent droop control scheme for MTDC grid

Optimal power share contribution with minimum DC voltage deviations is highly recommended in voltage source converter multi-terminal DC (VSC-MTDC). Hence, in this study, an artificial neural network based intelligent adaptive droop control proposes to enhance MTDC closed-loop system dynamics in terms of optimal power share and small redistribution of terminal DC voltages with negligible oscillations even in the presence of AC grid load demand change, admittance matrix uncertainty, outage of terminals and integration of wind farms. This is achieved via three-layer neural network and its weights are adaptively updated using error-back propagation algorithm. The asymptotic convergence of proposed control is derived using the Lyapunov’s stability theorem. Thus, the proposed intelligent adaptive droop control strategy not only guarantees to improve power share contribution with minimum voltage but also improved power quality and service continuity of MTDC grid. The robustness of proposed intelligent adaptive droop control is demonstrated on five terminal multi-terminal DC grid via MATLAB®.

Improved Integrated Hybrid DC Circuit Breaker for MTDC Grid Protection

The current-limiting inductors are usually installed in the multi-terminal dc grid to limit the fault current rising speed. However, they will increase the energy dissipation pressure and fault isolation time of the dc circuit breaker, which puts forward higher requirements on the capacity of arresters in hybrid dc circuit breaker and the heat dissipation of grid equipment. Therefore, an improved integrated hybrid dc circuit breaker is proposed in this brief for applications in the multi-terminal dc grid. Compared with the fault isolation solution based on existing integrated hybrid dc circuit breakers, the proposed breaker has slightly higher cost of semiconductor switches but significantly reduced arrester capacity requirement and faster fault isolation speed by bypassing the inductance components. Extensive electromagnetic simulations and physical experiments verify the effectiveness of the proposed device.

Hierarchical Flexible Operation Approach on a VSC-MTDC Interconnected Hybrid Grid With a High Share of Renewable Power

Voltage source converter (VSC) based multi -terminal HVDC (VSC-MTDC) systems have emerged as flexible collector systems for multi-area interconnected AC/DC hybrid grids with high share of volatile renewables energy. However, it remains an open question as how to exploit the flexible and cooperative regulation of multiple VSC stations and enhance secure operation on the premise of information privacy protection of multiple independent dispatch areas. This paper proposed an Analytical Target Cascading (ATC)-based three-layer decentralized optimization approach on flexible operation of VSC-MTDC hybrid systems. The low layer is a chance-constrained stochastic optimal power flow (SOPF) problem for AC grids. To guard against the secure operation risk by a VSC's random unipolar atresia fault, a modified loss of load probability index (MLOLP) is proposed to improve the day-ahead generation-reserve scheme. The middle layer and the high layer are a multiperiod OPF problem for VSC stations and the MTDC grid, respectively, where a detailed power regulation model is built to fully exert the flexible cooperation of multiple VSC stations. Two case systems are tested to verify the effectiveness and superiority of the proposed flexible operation hierarchy.

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.

AI based controller optimization for VSC-MTDC grids

Electric power industry is continually adopting new techniques to improve the reliability and efficiency of the energy system and to cope with the increasing energy demand and the associated technical challenges. In recent years, the maturation of Artificial Intelligence (AI) led researchers to solve various problems in the power system by using AI techniques. Voltage Source Converter is the result of advancements in the field of power electronics and semiconductors technology, which holds a promising future for the realization of smart grid, renewable energy integration, and HVDC transmission system. Usually hit and trial method or the design engineer’s experience is used for the manual tuning of the PI controllers, which cannot yield superior performance. The process becomes more complicated when multiple grids are involved, such as in VSC-based MTDC grids. This research article use a deep learning optimization technique for the tuning of the VSC controllers, which resulted in quick settling time, better slew rate, less undershoot and low overshoot. The deep learning neural network is trained through the Particle Swarm Optimization (PSO) algorithm to produce the best possible tuned or optimally tuned parameters for the controllers. The optimal tuning of the controller will result in an overall better performance of the converter and the grid. A four-layered deep learning neural network and a three-terminal MTDC grid were designed and simulated in MATLAB/SIMULINK environment.

An Interactionless Duo Control Strategy for Bipolar Voltage-Source Converter in Renewables Integrated Multiterminal HVdc Grids

The dc voltage and frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ (PVF)$</tex-math></inline-formula> or modified dc voltage and frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ (PV^{2}F)$</tex-math></inline-formula> double droop control is commended for its ability to regulate both the dc voltage and the frequency in a decentralized approach. However, a convincing response is not achieved due to an interaction between the droop characteristics of dc voltage and frequency. This interaction affects the dc voltage and frequency support of the ac-system-surrounded multiterminal HVdc (AC-MTdc) grid. To overcome this effect, a Duo control strategy is proposed in this article, which takes advantage of a bipolar voltage-source converter topology in the MTdc grid. The virtue of the proposed control technique is emphasized by comparing it with the existing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ PV^{2}F$</tex-math></inline-formula> double droop control along with three case studies and two test systems. The validation of the interactionless Duo control strategy is carried out on a five-terminal CIGRE dc grid benchmark model integrated into a two-area power system (AC-MTdc grid-1) and a New England IEEE 39-bus system (AC-MTdc grid-2). These test systems are simulated in PSCAD/EMTdc software.

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.

Implementation and Validation of a Model Predictive Controller on a Lab-Scale Three-Terminal MTDC Grid

In this paper, a reliable methodology is proposed in order to implement and validate a Model Predictive Control (MPC) scheme on an actual Voltage Source Converter (VSC) integrated in a scale-down multi-terminal DC grid. The objective of the investigated MPC controller is to enable AC frequency support among two asynchronous AC areas through a High Voltage Direct Current (HVDC) grid, while considering physical constraints, such as maximum and minimum DC voltage. A systematic and accurate implementation strategy is proposed, based mainly on the Hardware In the Loop (HIL) and Power Hardware In the Loop (PHIL), leading to the real-life testing on VSC, controlled by a classical microcontroller. The technical problems during the implementation process, as well as the proposed solutions, are described in detail through this paper. This procedure is deemed valuable to bridge the gap between offline simulation and the actual implementation of such advanced control scheme on experimental test rig.

Threshold-free localized scheme for DC fault identification in multiterminal HVDC systems

In the future grid scenario, the continuous operation of large-capacity remote power transmission over an augmented DC network is imperative for implementing a reliable multi-terminal high voltage direct current (MT-HVDC) grid or, alternatively, an MTDC grid. A fast and selective DC fault identification method with improved reliability eventually enhances the operational continuity and flexibility of the MTDC grid. Based on the V/I ratio of a current limiting reactor, this paper proposes a new non-unit protection scheme for DC fault identification without a threshold value setting. The DC fault is precisely identified in a span of three sampling intervals by the V/I ratios of four sampling instants. The selectivity of the proposed scheme is verified in a four-terminal MTDC grid. Simulation results demonstrate that the effective detection, discrimination, and location of a DC fault strictly satisfies the speed requirement (<1ms) with a location error of less than 1%. Moreover, the relevant sensitivity analyses reveal that the proposed scheme remains invariant for a wide range of fault locations and resistances and perform efficiently, even with a lower sampling frequency. In addition, transient events apart from DC faults fail to trigger the protection, indicating the high robustness of the proposed scheme.

A Generalized sequential AC/DC power flow algorithm for VSC MTDC system with series interline DC current flow controller

In this paper, a generalized average model for various topologies of series interline power flow controller is developed and incorporated in AC/DC VSC MTDC power flow model. A new Newton Raphson based power flow formulation of AC/DC VSC MTDC system with series interline power flow controller in proposed.The proposed formulation accommodates the inclusion of capacitor voltage limits of Current Flow Controller (CFC), DC line current and duty cycle limits. Steady State feasibility of wind power evacuation is analyzed for different wind power penetration levels and CFC injected voltages. The impact of CFC injected voltages on rerouting and nullifying the power flows in MTDC grid are analyzed for different wind power penetration levels. Initialization of state variables for feasible steady state solution has been addressed. In order to ensure feasible solution a novel convergence criteria is proposed. The validity of the proposed AC/DC power flow formulation is demonstrated on 39 bus AC/DC system.

A Detailed Review of MMC Circuit Topologies and Modelling Issues

MMC is a promising technology for MTDC systems and would transform into the concept of Supergrids in the near future. The salient features of MMC are modularity, reduced dv/dt and di/dt stress on switches, voltage and power scalability, inherent fault blocking capacity, transformerless operation, and improved power quality. However, there are some technical issues and challenges to be critically analysed and addressed. There is room for development of novel and enhanced MMC based on SM configurations to enable higher efficiency, improved power quality, compactness, and DC fault blocking capacity. Moreover, development of efficient and accurate models is required for the studies of MTDC grids during steady-state and transient conditions. Literature review suggests a need for studying and comparing different MMC modelling approaches because no modelling technique can be best suited for all applications. The main contribution of this paper is to provide a comprehensive review of recent developments in MMC in terms of SM configurations. This paper also presents an in-depth review of systematic comparison of different models of MMC, which can enable appropriate selection of model based on target studies and desired accuracy and efficiency. Finally, the associated research challenges and future trends are presented.

Fault current calculation in MTDC grids considering MMC blocking

In this paper, a generic fault current calculation method based on coupled differential equations is proposed which is valid for both blocked and unblocked states of the MMC. The first part of the paper concerns the modeling aspects of MMCs in blocked and unblocked states, while the second part focusses on the representation of the DC grid. Overhead lines are modeled as lumped RL-elements, whereas cables are represented by an adapted π-model in order to consider the capacitor discharge. A simplified DC Circuit Breaker model is integrated into the set of coupled differential equations, which allows to estimate the energy absorption requirements during fault current suppression. The results of the method are compared to EMT simulation results and validated throughout different fault scenarios. The method enables the representation of temporary MMC blocking for a large timeframe of tens of milliseconds after fault occurrence and is convenient for the sizing of DC Circuit Breakers and DC reactors in MTDC grids.

Application of Resistance SFCL in MTDC Grid With Reclosing Protection Strategy

Resistive superconducting current limiter (RSFCL) is an effective technical means to solve the short-circuit current problem of DC transmission system. Aiming at the problem of large fault current and fast rising speed in multi terminal DC system (MTDC) with modular multilevel converters (MMC), a new protection strategy based on RSFCL is proposed to isolate the fault area quickly. Through analyzing the quench and recovery characteristics of RSFCL, the quench resistance meeting the reclosing requirements is determined, of which recovers to superconducting in 300 ms. The coordination principle and operation sequence of RSFCL, DC circuit breaker (DCCB) and MMC converter are analyzed for setting up the protection strategy. Taking the bipolar short circuit fault of overhead line in a three terminal flexible DC project as an example, the rationality of the proposed protection strategy is verified in PSCAD/EMTDC software. The simulation results show that the coordination strategy can help to quickly isolate the fault area, carry out fault ride through of non-fault station, accelerate the restart of fault pole converter, and reduce the outage time of MTDC grid.