Multi-terminal DC System Research Articles

Stability control and analysis of hydrogen production using a multi-terminal DC EV charging system with PV

In the shift towards sustainable energy, DC microgrids have emerged as efficient and reliable alternatives to traditional AC systems, particularly suited for low-voltage applications. The adoption of electric vehicles (EVs) in multi-terminal DC (MTDC) systems introduces challenges in maintaining voltage stability, particularly during master station failures. Our research introduces a decentralized control strategy with multi-mode switching to ensure stability under such conditions. We conduct a comprehensive stability analysis of MTDC systems heavily integrated with EVs, identifying the instability risks and proposing mitigation strategies. The effectiveness of our control approach is validated through simulation studies and a detailed state-space model. Additionally, we explore the integration of renewable energy sources and hydrogen production into MTDC systems for EV charging, which minimizes the need for a new power infrastructure and enhances renewable energy use. This integration optimally reduces renewable energy discard rates, thereby improving energy efficiency without compromising system performance. Our study provides practical insights and theoretical support for renewable energy-driven hydrogen production within DC microgrids. The proposed scheme's performance is validated through simulation studies using a state-space model.

The Study of Multi-Terminal DC Systems in an Offshore Wind Environment: A Focus on Cable Ripple Analysis

This paper studies the THD and AC losses on the DC cables of offshore wind farm-based multi-terminal HVDC systems when they extract and deliver power from and to more than one connection point. In the paper, the study of a full system PLECS + Simulink model with two branches, including a wind resource, a wind turbine, a Permanent Magnet Synchronous Generator (PMSG), a Pulse Width Modulation (PWM) rectifier, a Single Active Bridge (SAB) DC–DC converter, an Input Parallel Output Series (IPOS) DC–DC converter, HVDC cables, and a simplified onshore system, is presented. It focuses on the investigation of the output ripple content of multiple DC–DC converters on DC cables under different wind conditions with different voltage and power ratings. The importance of the study is providing a general understanding of the operation of the innovative offshore wind farm-based DC system, as well as the interaction between different DC–DC converters and their influence on cable ripple content under different situations.

Voltage margin control for offshore multi-use platform integration

This paper discusses a multiterminal direct current (MTDC) connection proposed for integration of offshore multi-use platforms into continental grids. Voltage source converters (VSC) were selected for their suitability for multiterminal dc systems and for their flexibility in control. A five terminal VSC-MTDC which includes offshore generation, storage, loads and ac connection, was modeled and simulated in DigSILENT Power Factory software. Voltage margin method has been used for reliable operation of the MTDC system without the need of fast communication. Simulation results show that the proposed system was able to maintain constant dc voltage operation during fluctuations in the generation and changes in the demand. Moreover, it was able to secure power supply to passive loads during loss of a dc voltage regulating terminal and to perform a dispatch scheme where it is possible to buy, sell or store energy attending to the price in the electricity market

Fault properties identification scheme for hybrid MTDC system based on LCC signal injection and distributed parameter line model

AbstractA fault properties identification scheme for a hybrid multi‐terminal DC (MTDC) system is proposed to improve the power supply reliability in the system recovery stage. Firstly, a current signal injection strategy based on line commutated converter (LCC) is proposed using the fault control capability. Secondly, the difference between the calculated value (CV) and the measured value (MV) of the current signal under permanent and temporary faults is analyzed for the distributed parameter line model. By introducing the dynamic time warping (DWT) algorithm to represent the difference between CV and MV, a fault properties identification scheme for permanent fault identification is constructed. Finally, the proposed scheme for permanent fault location with the distributed parameter model is obtained by introducing the trust region reflection (TRR) algorithm. The simulation results show that the proposed fault properties identification scheme has the function of permanent fault identification and location. The proposed scheme fully considers the effects of fault impedance, sampling rate, noise, and data loss. It can be applied to high voltage DC long‐distance transmission lines with LCC.

A Multi-Port DC Power Flow Controller Integrated With MMC Stations for Offshore Meshed Multi-Terminal HVDC Grids

Multi-terminal dc (MTDC) systems have the potential to facilitate the integration of offshore wind farms. A dc power flow controller (DPFC) is necessary for meshed multi-terminal high voltage dc (HVDC) grids to regulate the power flow in dc lines. However, DPFCs based on modular multilevel converters (MMCs) require two dc-ac conversion stages and an extra ac transformer, which increases the cost and loss for offshore platforms. This paper proposes a multi-port DPFC that is integrated internally into the offshore MMC station, eliminating the need for an extra transformer and reducing dc voltage ripple. Additionally, the full bridge submodules (FBSMs) in the DPFC can further reduce the number of FBSMs required in the MMC station for dc fault blocking. The proposed DPFC is composed only of cascaded FBSMs connected to the arms of the offshore MMC station, which can exchange power directly with the MMC valve to achieve power balance. By using the MMC station, the proposed DPFC requires a small kVA rating, approximately 1%-5% of the total system power rating. The performance of the proposed DPFC is validated through simulation of a hybrid system of the IEEE 39 bus and the Cigre HVDC benchmark, and also through down-scale experimental testing.

Impact of the Differences in VSC Average Model Parameters on the DC Voltage Critical Stability of an MTDC Power System

Voltage source converters (VSCs) are generally assumed to be identical when studying the DC voltage critical stability of a multi-terminal DC (MTDC) system, which in reality is not the case and can lead to erroneous results. In this study, the differences in VSC average model parameters, such as VSC control loops and operating states, were considered, and the mechanism of how these differences affect the DC voltage critical stability of an MTDC power system was clarified. In an MTDC power system with DC voltage droop control, it was found that these differences can be considered as two types of added dynamic impedances, which affect DC voltage critical stability by changing the construction of the DC network. Moreover, a method was developed to numerically determine the effects of these differences on DC voltage critical stability. It was verified that these effects are related to not only the dynamic characteristics of the added dynamic impedances but also the parameters and constructions of the DC network. By considering these differences, accuracy of DC voltage critical stability can be improved. Finally, an experimental platform of a four-terminal DC power system was established to validate the findings.

DC fault detection in hybrid MTDC systems using transient average of DC reactor voltage

The existence of various types of converters makes the fault characteristics of the hybrid multi-terminal DC (MTDC) system more complicated than the voltage source converter-based (VSC-based) MTDC system. A reduced line commutated converter (LCC) model for DC fault analysis is first proposed in this paper, where the LCC-based rectifier with control effects is regarded as an equivalent RLC circuit. By neglecting the resistive and capacitive components in the high-frequency domain, the original network analysis is significantly simplified to derive concise time-domain expressions of the initial DC fault current. On top of this, a single-terminal DC fault detection scheme using the transient average of DC reactor voltage is designed for the hybrid MTDC system. The thresholds for detecting faults on the LCC-connected and VSC-connected lines are calculated based on the derived fault current expressions, respectively. Simulation studies based on PSCAD/EMTDC have validated the proposed fault analysis model is accurate within the initial period after fault, while the proposed fault detection scheme for hybrid MTDC systems is fast, sensitive, and tolerant to high fault resistances.

Transient analysis method using high pass filter circuit in VSC interfaced multi-terminal DC system

Transient analysis method using high pass filter circuit in VSC interfaced multi-terminal DC system

Research on the Fault-Transient Characteristics of a DC Power System Considering the Cooperative Action of a Flexible Current-Limiting Device and a Circuit Breaker

As an effective carrier of a new energy collection, the DC power grid has low inertia and weak damping characteristics, making it essential to limit fault current and isolate the DC system. To quickly and effectively suppress fault current, a flexible current-limiting device (FCLD) is proposed, which can realize transient fault self-recovery without circuit breaker action and permanent and quick isolation of a fault. It improves the operational ability of the DC system under an asymmetric condition. First, a rectifier provides a set-slope current to each cascade inductor, so the voltage of the inductor can be clamped. Second, a controlled current source (CCS) is applied to generate inverse flux to prevent the inductor from magnetic saturation. The protection action time of the DC circuit breaker is reformulated. Finally, by considering the synergistic action of the current-limiting device, the circuit breaker, and the transient characteristics of the DC grid fault, the protection scheme of the multi-terminal flexible DC system can be formulated. To verify the validity of the proposed flexible current-limiting device, a multi-terminal flexible DC simulation platform is established, and the faults of DC lines are simulated and analyzed.

Fault Location Estimation in Voltage-Source-Converter-Based DC System: The $L$ Location

This article presents a new method for estimating the fault location in a voltage-source-converter-interfaced dc system. The proposed dc fault location estimation method evaluates the distance from the fault point to the dc line terminal by estimating the line inductance; hence, it is designated as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L$</tex-math></inline-formula> location. Since the fault impedance usually only includes resistance, the proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L$</tex-math></inline-formula> location is less affected by the variation of fault resistance. In addition, the ripple caused by the distributed capacitive discharge is excluded from the dc line current when the proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L$</tex-math></inline-formula> location is applied in the dc system with the distributed-parameter transmission line. The cubic curve-fitting method is used to reconstruct the current signal based on the least squares method. The effectiveness of the proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L$</tex-math></inline-formula> location is verified with an OPAL-RT-based multiterminal dc (MTDC) system and experimental point-to-point and MTDC systems. The comparison with the impedance estimation, the voltage-traveling-wave-based location, and the differential and directional protection substantiates the integral performance of the proposed method

Interaction Behavior and Stability Analysis of Low-Voltage Multi-Terminal DC System

The stability problem of low-voltage multi-terminal DC (MTDC) systems has drawn much interest. This paper focuses on the oscillation instability phenomenon of low-voltage MTDC systems and analyzes the interaction behavior between each voltage-source converter and the DC network oscillation mode. The typical structure and control strategy of a low-voltage MTDC system under master-slave control is first described, and its equivalent circuit is established. Then, an interaction behavior analytical model is proposed, and the source-network and network-load interaction characteristics are studied. Furthermore, a system stability analysis method is built based on the modularized modeling, which can solve the problem of how to address the complexity of high-order low-voltage MTDC systems. The transfer function of the whole system is conveniently established, revealing the system instability mechanism. The proposed method can provide effective stability assessment for low-voltage MTDC systems. Finally, the results are verified using electromagnetic transient simulation and experimentally validated using a low-voltage three-terminal DC system test platform with AC 0.4 kV/DC 800 V. This research can be flexibly extended to high-order systems for controller design and performance optimization.

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.

Effect of the Dynamics of the MTDC Power System on DC Voltage Oscillation Stability

Ignoring parts of the dynamics of a multiterminal DC (MTDC) power system can simplify its stability analysis but may miss some instability risks in the analyzed results. This study analyzed the stability of DC voltage oscillation by considering comprehensive dynamics of an MTDC system in a DC voltage time scale to reveal the stability mechanism of the DC voltage oscillation accurately. It demonstrated that a DC voltage oscillation loop is first formed by the DC voltage control of the voltage source converter (VSC). Subsequently, it is affected by the dynamics of the remaining DC system (RDCS) through two paths: (1) the equivalent conductance of the RDCS is equal to an additional damping added to the DC voltage oscillation loop, and (2) the equivalent susceptance forms a mirrored subsystem and results in an open-loop modal resonance, which always induces negative effects on DC voltage oscillation stability. These conclusions are also correct in parts of the MTDC system using DC voltage droop control. Moreover, a method was proposed to quickly determine the stable operating region of the RDCS. Finally, accuracy of the stability mechanism of the DC voltage oscillation and effectiveness of the proposed method are verified based on the SIMULINK platform.

A Fault Clearance and Restoration Approach for MMC-Based MTDC Grid

With the growth in continuous energy demand, high-voltage Multi-Terminal DC (MTDC) systems are technically and economically feasible to transmit bulk power and integrate additional energy sources. However, the high vulnerability of the MTDC systems to DC faults, especially pole-to-pole (P2P) faults, is technically challenging. The development of DC fault ride-through techniques such as DC circuit breakers is still challenging due to their high cost and complex operation. This paper presents the DC fault clearance and isolation method for an MMC-based MTDC grid without adopting the high-cost DC circuit breakers. Besides, a restoration sequence is proposed to re-energize the DC grid upon clearing the fault. An MMC-based four-terminal DC grid is implemented in a Control-Hardware-in-Loop (CHIL) environment based on Xilinx Virtex-7 FPGAs and Real-Time Digital Simulator (RTDS). The RTDS results show that the MTDC system satisfactorily rides through DC faults and can safely recover after DC faults.

Analysis of Interactive Behavior and Stability of Low-Voltage Multiterminal DC System Under Droop Control Modes

Low-voltage multiterminal dc system is one of the important forms in distribution network. Its stability issue has drawn worldwide attention for the characteristics of low inertia and weak damping. This article aims to analyze the interactive behavior between the voltage source converter (VSC) and the oscillation mode of the dc network. First, this article proposed a modeling method for dc voltage control loop considering the time-lag effect of inductor energy storage behavior on the ac side of the VSC. Based on this model, the interactive behavior between VSC and dc network, including the influence of droop coefficients and load power under dc voltage–ac power ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> ) droop control mode and under dc voltage–dc power ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> ) droop control mode, is studied. Then, the overall state-space model of the system is established. Furthermore, the stability of a low-voltage three-terminal dc system under different control modes and parameters is compared and analyzed in detail. Finally, the corresponding simulation and experimental verification validate the theoretical analysis. The research results show that under the same capacity and control parameters, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> mode can better improve the capability of power regulation and system stability. This research can provide an effective method for system control modes design.

Transient analysis in multi-terminal voltage source converter based direct current system: Median mode decomposition

Due to the capacitive discharging effect after the fault inception, the dc line current increases dramatically in the dc power system which is interfaced with the ac system through the voltage source converters (VSCs). Therefore, the fast response of the fault detection methods is important to interrupt the fault line with short delay. In recent years, dc fault detection methods based on the frequency-domain analysis have become popular, while their calculation burden is relatively high. In this study, the median mode decomposition (MMD) is introduced, which improves the traditional empirical mode decomposition (EMD). Accordingly, a new transient analysis method is developed with only computation in the time-domain. In contrast to the conventional EMD, the new MMD can peel off the intrinsic median mode function (IMMF) with less quantities of iterations. These characteristics are of significance in reducing the computation burden and accelerating the response speed of the detection algorithm in the actual use. Two multi-terminal dc (MTDC) systems are developed in PSCAD for the simulation verification, including a ring bus system and a radial bus system. In addition, a scale-down MTDC system is established for the experimental verification, which reveals sensitive response, high robustness against noise and 100 % reliability. From the comparison with four popular methods, the comprehensive performance of the proposed method has been underlined.

A new pilot protection scheme for modular multi‐level converter‐based HVDC system based on voltage matching factor

Concerning the incorrect operation of line protection in modular multi-level converter-based multi-terminal DC system, a pilot protection method based on voltage matching factor is proposed. First, the dynamic switching process of flexible DC converter sub-module is analyzed, and the expression of DC voltage of converter station considering the influence of alternating current (AC) system is derived. On this basis, the model of DC line voltage considering the influence of AC injection current is built, and the equivalent DC line voltage is calculated according to the voltage model. Then, by constructing the distortion factor which reflects the difference between the equivalent DC line voltage and actual DC line voltage, the protection criterion for the identification of internal and external faults is established. Finally, simulation tests on real time laboratory (RT-LAB) platform verify that the proposed scheme can correctly distinguish between internal and external faults in different fault cases, with fast operation speed and strong immunity to the fault resistance.

Hybrid Multiterminal HVDC System Based on Line-Commutated and Alternate Arm Converters

Challenges in long distance power transmission and large-scale integration of renewable energy systems drive the development of high-voltage direct current (HVDC) systems. HVDC systems can be configured as combinations of line-commutated converters (LCCs), voltage source converters (VSCs) and hybrid systems. However, each hybrid configuration poses its own challenges and requires unique control. Thus, it is necessary to explore other possible configurations, especially different configurations in more complex hybrid multiterminal DC (MTDC) systems and future dc grids. This paper develops the steps for integration of alternate arm converters (AACs) in hybrid MTDCs and demonstrates the operation of AACs in a hybrid MTDC system. The detailed system structure and control hierarchy are developed. The performance of the hybrid MTDC system under multiple operating scenarios including ac and dc faults are demonstrated, verifying the feasibility of an LCC and AAC-based hybrid MTDC system.

A Study of Protection Method for Hybrid Multiterminal UHVDC Lines Based on CEEMDAN–Teager Energy Operator

In the research, a protection scheme for hybrid multiterminal UHVDC lines based on the CEEMDAN and Teager energy operator is proposed. The fault direction criterion is proposed according to the polarity difference of the sudden variable of the mode component of the current line on both sides of the T-zone after the fault of the EHV multiterminal hybrid DC system. When the fault is located on the left or right side of the T-zone, CEEMDAN is used to decompose the mode component of the fault transient current and obtain the intrinsic mode function (IMF component) at different local characteristic time scales. Then, the Teager energy operator is used to calculate the instantaneous energy of the current high-frequency IMF1 component. Finally, the faults inside and outside the line are judged by comparing the maximum value of current high-frequency IMF1 instantaneous energy with the setting value. When the fault is located in the T-zone, it is determined as a fault outside the line zone. The protection scheme of hybrid multiterminal UHVDC lines is given. The simulation model of the Kunliulong hybrid multiterminal UHVDC line system is built in a PSCAD/EMTDC simulation platform, and the proposed protection method is verified. A large number of simulation results show that the protection scheme has certain anti-transition resistance ability and high reliability.

A New Design of MP-HDCCB Topology Based on Hybrid Switching Device

Since each branch of the multiterminal DC circuit system relies on the DC circuit breaker for breaking and fault isolation, the prohibitive cost and huge volume of the Hybrid DC Circuit Breaker (HCB) limit its development and broad application in multiterminal flexible DC systems. Multiport hybrid DC circuit breaker (MP-HDCCB) based on device and branch sharing reduces the configuration cost of the circuit breaker to a certain extent. In order to further reduce the cost of MP-HDCCB, a novel MP-HDCCB topology based on hybrid switching devices is proposed, adopting full controlled switching devices to achieve rapidity of breaking fault current, and using semi-controlled switching devices in series to withstand the transient interruption voltage (TIV), so as to reduce the construction cost and technical difficulty. In this paper, the working principle and fault breaking strategy of the topology are introduced in detail, then the parameters of the major circuit are analyzed theoretically, and the parameter design of each branch is given. In the end, the rationality and validity of the proposed topology is tested and verified by simulations and experimental tests.