Multi-terminal DC Transmission Research Articles

Frequency Stability Control Strategy for Voltage Source Converter-Based Multi-Terminal DC Transmission System

Frequency Stability Control Strategy for Voltage Source Converter-Based Multi-Terminal DC Transmission System

The Influence of Additional Virtual Synchronous Generator Technology in VSC-MTDC Systems with Wind Power on System Frequency

Introduction: A frequency control strategy is proposed based on additional virtual synchronous generator technology for voltage source converter-based multi-terminal high voltage direct current systems with wind power. Method: This strategy addresses the system's inertia reduction and frequency stability issues caused by integrating large amounts of wind power through multi-terminal DC transmission. Firstly, the virtual synchronous generator mathematical model is constructed based on the system structure. Secondly, for the problem of zero rotational inertia of voltage source converter in a flexible DC transmission system, based on the P-U droop control method of the converter station, additional virtual synchronous control generation technology is applied to simulate the P-f droop characteristics of the synchronous generator by adding virtual rotational inertia, so that the converter has the inertial response of synchronous generator to realize primary frequency regulation. Result: Finally, the simulation is verified on the PSCAD/ EMTDC platform with an example of a three-terminal parallel MTDC transmission system. Conclusion: the analyzed results demonstrate that the virtual synchronous generator control strategy is very valuable and useful for improving the frequency performance of the system.

Improved coordinated control strategy for VSC-MTDC system with DC voltage secondary regulation

Droop control is widely used in multi-terminal flexible DC (VSC-MTDC) transmission systems by virtue of the advantage of multi-station cooperative unbalanced power dissipation, however, the essence of the droop control strategy is to change the DC current to realize the unbalanced power dissipation, and the resulting DC voltage deviation will affect the normal operation of the system. Firstly, this paper theoretically analyses the working characteristics of the conventional droop control and proposes a control method to realize the quasi-differential-free regulation of DC voltage by translating the droop curve. Second, according to the power margin of the converter station, the feedforward compensation amount of each converter station is reasonably set to avoid the power impact on the converter station. Finally, for the problem that the actual value of DC voltage still deviates from the rated value, a control strategy containing secondary regulation of DC voltage is proposed to further restore the DC voltage to the initial value on the basis of ensuring the effect of power regulation, which improves the stability of the operation of the VSC-MTDC system. The final simulation results verify 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.

Adaptive integrated control strategy for MMC-MTDC transmission system considering dynamic frequency response and power sharing

Due to the lack of grid inertia caused by the increasing penetration of renewable energy sources, the ability of system to withstand disturbance is reduced. In attempt to address this challenge, an adaptive virtual inertial frequency control (AVIFC) strategy for multi-terminal dc (MMC-MTDC) transmission system considering dynamic frequency response is proposed. Based on the frequency variation characteristics in inertia response and primary response stages, the virtual inertia coefficient can be automatically adjusted. In the inertia response stage, considering the limitation of DC voltage, the virtual inertia coefficient adaptively decreases according to the frequency deviation to reduce the maximum frequency deviation. In the primary response stage, the virtual inertia coefficient decreases to 0 rapidly following the frequency deviation to speed up the frequency recovery. Furthermore, aiming at the problem of coordinated power sharing between converter stations of weak AC systems after disturbance, an adaptive droop control (ADC) strategy based on DC voltage deviation and converter station power margin is studied to realize the optimal distribution of unbalanced power so that DC voltage deviation is reduced, DC voltage regulation time is shortened and the stability of DC voltage is maintained at the same time. Finally, based on PSCAD / EMTDC simulation platform, the control model of a five-terminal MMC-MTDC system is established. Three working conditions, namely the load reduction, the load increment and the converter station out of operation, are selected to verify the effectiveness and feasibility of the proposed adaptive integrated control (AIC) strategy including AVIFC and ADC.

Hierarchical Adaptive Control of the Cyber–Physical Power System With MMC-MTDC for Transient Stability Improvement

Large-scale wind power integration into the power system has promoted the development of a multiterminal DC (MTDC) transmission grid with a modular multilevel converter (MMC). Basically, MTDC with MMC is a typical cyber–physical system with continuous coupling interactions between cyber assets and power systems. However, cyber events may introduce many internet-based vulnerabilities and even result in the loss of transient stability of the power system. Therefore, a voltage compensation-based two-level hierarchical adaptive control strategy is proposed in this article. At the higher level, a modified MMC output current reference calculation method is developed in the αβ framework to guarantee the transient stability of the power system, whereas a feedback adjustment method is proposed in the MMC control framework, at the bottom level, to contain the controller from deviating from its output reference while eliminating the impact of cyber communication delay on transient stability. The article shows that the proposed hierarchical control strategy can improve the transient stability of the power grid under the interference of three-phase ground faults in physical and communication delays in the cyber layer. Finally, the simulation results of the modified IEEE 9-bus test system with MMC–MTDC are used to demonstrate the effectiveness of the proposed scheme.

Multiterminal Hybrid DC Line Protection Based on Intrinsic Mode Energy Entropy

In view of the UHV multiterminal hybrid DC transmission system, the DC line protection with universal applicability, fast response speed, stability, and reliability is particularly important for its safe and economic operation. In this article, the boundary frequency characteristics of the UHV multiterminal hybrid DC transmission system are analyzed by considering the DC boundary of the rectifier side and inverter side, line frequency variation, and other factors. According to the fact that the boundary of the fault line has a strong attenuation effect on the high-frequency component of the transient current, a protection method based on the intrinsic mode energy entropy is proposed to distinguish the faults inside and outside. The criterion to initiate the protection is constructed by using the amplitude of the transient power; the fault direction criterion is constructed by using the positive and negative characteristics of the transient energy of the fault power detected by power direction elements on both sides of the T-zone; the fault pole selection criterion is constructed by using the ratio of low-frequency transient power changes of positive and negative poles. Finally, the Kun–Liu–Long line UHV three-terminal hybrid DC transmission system model is built on the PSCAD/EMTDC simulation platform, and a large number of simulation examples verify that the protection can operate reliably under different fault poles, fault positions, and transition resistances.

Capacitor voltage balance control strategy based on MMC-MTDC

The MMC is composed of a large number of sub-modules. The operating conditions of each sub-module are different, which will inevitably affect the capacitor voltage of each sub-module, resulting in the difference of the capacitor voltage of each sub-module. The problem of uneven voltage of the sub-module capacitor voltage of the MMC system is unavoidable, and the uneven voltage of the capacitor will greatly affect the stable operation of the MMC system. This paper studies the multi-terminal flexible DC transmission system (MMC-MTDC) based on modular multi-level converters. The multi-terminal MMC-MTDC system model is established through the MMC-MTDC converter station level and the MMC-MTDC system level controller, and then according to the shutdown process of MMC-MTDC system, several optimal control methods are proposed to realize energy feedback and sub-module discharge to achieve capacitor voltage balance. Finally, the effectiveness of the proposed method is verified by building a simulation model.

Hybrid MMC Redundancy Configuration Calculation Method Based on Heuristic Algorithm

Most of the existing redundant configuration methods of modular level converters (MMC) only consider the reliability of the MMC, and rarely consider the utilization of insulated gate bipolar transistors (IGBT). A heuristic algorithm-based calculation method for hybrid MMC redundancy configuration is proposed in order to better meet the engineering practicality. Firstly, the topology of the hybrid MMC is introduced, and the correlation between submodules is introduced into the calculation of its reliability. Secondly, the index of IGBT utilization is proposed to judge the economic advantage. The effects of correlation and service life on the reliability of the hybrid MMC and the IGBT utilization are analyzed. Then, heuristic factors are constructed with reliability and IGBT utilization, and the optimal redundancy configuration scheme of hybrid MMC is obtained by heuristic algorithm. Finally, based on the parameters of Kunliulong multi-terminal hybrid DC transmission project, the calculation is verified in Matlab and compared with the results of the difference method to illustrate the feasibility and superiority of the proposed redundancy configuration calculation method.

Modeling and Control Strategy for Multiterminal Flexible DC Distribution Network with Echelon Utilization Power Battery

With the integration of distributed renewable energy to the distribution network and the development of multiterminal flexible DC transmission technology, multiterminal flexible DC distribution network has broad application prospects. At the same time, with the rapid development of new energy vehicles, the echelon utilization of power battery has become a research hotspot. By analyzing the characteristics of flexible DC distribution network and echelon utilization battery, the structure and control strategy of modular multilevel converter (MMC), DC solid-state transformer, photovoltaic power generation, wind power generation, and echelon utilization battery energy storage system are established, respectively, in this paper. To achieve a DC network connection of various types of power supply and load, this paper proposes a starting method of multiterminal flexible DC distribution network and a cooperative control strategy of the wind-solar-storage system. A six-terminal ring-shape DC distribution network model is built in real-time digital simulation (RTDS) platform. The simulation results show that the modeling methods and control strategies of each component in the real-time simulation model meet the operation requirements of the multiterminal flexible DC distribution network, which provides a reference for the construction and research of the flexible DC distribution network.

Correction to: Adaptive coordinated control strategy for multi-terminal flexible DC transmission systems with deviation control

Correction to: Adaptive coordinated control strategy for multi-terminal flexible DC transmission systems with deviation control

Research on the Coordinated control Strategy of fault crossing capability of offshore wind power multi-terminal flexible DC system

To improve the ride-through capability (RTC) of the multi-terminal offshore wind power flexible DC system in response to faults, this paper is based on the basic structure and control strategy of the voltage source converter (VSC) station of the multi-terminal offshore wind power flexible DC transmission system (MTDC), and aims at the AC grid connected to the system with three-phase short circuits and wind turbine output changes According to the situation, the mechanism of voltage drop caused by the fault is analyzed, and the DC chopper energy bleeder circuit is connected to the DC side to cooperate with the comprehensive control scheme for fault ride-through of multi-terminal offshore wind power flexible DC collection system. Through theoretical analysis and building a simulation model, the design of the control strategy of the flexible collection system and the simulation study of fault ride-through is carried out on the Shandong Peninsula Beihai Wind Farm. The results show that: the VSC station controller of the offshore wind power multi-terminal system can make the multi-terminal system wind power distribution and consumption reasonable, and realize the stable operation of the system.

Solution to Fault of Multi-Terminal DC Transmission Systems Based on High Temperature Superconducting DC Cables

In this paper, we developed the small-signal state-space (SS) model of hybrid multi-terminal high-voltage direct-current (HVDC) systems and fault localization method in a failure situation. The multi-terminal HVDC (MTDC) system is composed of two wind farm side voltage-source converters (VSCs) and two grid side line-commutated converters (LCCs). To utilize relative advantages of the conventional line-commutated converter (LCC) and the voltage source converter (VSC) technologies, hybrid multi-terminal high-voltage direct-current (MTDC) technologies have been highlighted in recent years. For the models, grid side LCCs adopt distinct two control methods: master–slave control mode and voltage droop control mode. By utilizing root-locus analysis of the SS models for the hybrid MTDC system, we compare stability and responses of the target system according to control method. Furthermore, the proposed SS models are utilized in time-domain simulation to illustrate difference between master–slave control method and voltage droop control method. However, basic modeling method for hybrid MTDC system considering superconducting DC cables has not been proposed. In addition, when a failure occurs in MTDC system, conventional fault localization method cannot detect the fault location because the MTDC system is a complex form including a branch point. For coping with a failure situation, we propose a fault localization method for MTDC system including branch points. We model the MTDC system based on the actual experimental results and simulate a variety of failure scenarios. We propose the fault localization topology on a branch cable system using reflectometry method. Through the simulation results, we verify the performance of fault localization. In conclusion, guidelines to select control method in implementing hybrid MTDC systems for integrating offshore wind farms and to cope with failure method are provided in this paper.

Adaptive coordinated control strategy for multi-terminal flexible DC transmission systems with deviation control

Due to the advantages of multiple power supply points and multiple power receiving points, the multi-terminal direct current (MTDC) technology has gradually become the main trend in the future development of DC power grids. The coordinated control of a MTDC system is one of the key technologies to realize the stable operation of power systems. Droop control is an effective coordinated control method to maintain the power and voltage balance of a DC transmission system. However, improper selection of the droop coefficient leads to an imbalance of the power and voltage distribution. An improved coordinated control strategy based on an adaptive droop coefficient is proposed to realize a faster dynamic response and a more reasonable power distribution, which is applicable to all of the topologies of MTDC systems. The power and voltage deviation controls are added to prevent the output power and voltage from exceeding the range that a converter can bear. In addition, a lagging link is added in the current loop to compensate for the delay caused by the current loop and to improve the response speed of the system. Taking the MMC topology as an example, the dynamic characteristics of the proposed control strategy are analyzed, and a four-terminal MTDC simulation model based on the MATLAB/Simulink simulation platform has been established to verify the superiority of the proposed strategy under different disturbances.

A New Hybrid Dual Active Bridge Modular Multilevel Based DC–DC Converter for HVDC Networks

Multi-terminal high voltage DC transmission currently represents a leading technology in long-distance power transmission systems. Among the main technical challenges facing such technology, DC fault isolation, permitting different grounding schemes, providing interoperability, and high DC voltage stepping between different HVDC networks, and allowing high-speed power reversal without power interruption especially when connecting the pre-existing voltage source converters (VSC) and line commutated converters (LCC)-based HVDC networks. This paper introduces a new modular multilevel converter (MMC) based front-to-front DC-DC converter to interconnect two different types (LCC/VSC) of HVDC networks. The proposed topology comprises a voltage source MMC (VS-MMC) and a current source MMC (CS-MMC), while both are coupled via an AC link including the isolating transformer. The proposed topology can successfully provide an uninterruptible bi-directional power flow, high DC voltage stepping with a DC fault blocking capability, and low number of semiconductors due to the usage of only half-bridge SMs. The system design is provided with a detailed mathematical analysis. Furthermore, two active power control methodologies are proposed and compared. The first control technique is simpler and entails lower passive elements, while the second technique ensures a zero reactive power over the full range of active power flow. Furthermore, Losses analysis and comparison are provided between the two proposed control techniques. Finally, Control-Hardware-in-the-Loop (CHiL) test validation is employed to confirm the validity of the proposed system under healthy as well as different fault scenarios.

Influence of coupling characteristics on commutation failure of new hybrid HVDC transmission system

Hybrid cascaded multi-terminal DC transmission provides a more flexible transmission method, improves the voltage stability of the inverter AC system, and reduces the probability of commutation failure. The hybrid LCC-VSC at the receiving end of the DC system is a new type of topology.In order to study the influence of coupling characteristics between VSC and LCC on commutation failure in AC power grid at receiving end,firstly,a hybrid LCC-VSC simulation model is built on PSCAD to analyze the operation characteristics of the system during the fault and verify the reactive power support ability of VSC to LCC, and the improvement effect of VSC on LCC-HVDC under different reactive power control modes is compared and analyzed. In order to study the influence of AC/DC coupling on commutation failure of LCC, commutation failure immune factor (CFII) was introduced to study the influence of AC system strength on commutation failure of LCC. Finally, considering that the influence of AC/DC coupling on commutation failure is closely related to the degree of coupling, the electrical distance between LCC and VSC is further studied. The results show that the conclusion can provide theoretical guidance for the design and analysis of related projects.

Impact of control system on small-signal stability of MMC-based MTDC transmission system

This paper studies influence of control system dynamics on the small-signal stability of modular multilevel converter (MMC) based multi-terminal DC (MTDC) transmission system. The small signal model is derived based on the state–space model of an MMC-based MTDC. Eigenvalue analysis and participation factor analysis are applied to study the inherent oscillatory modes of MMC-based MTDC transmission systems. The impacts of control system parameters on small-signal stability of the system are investigated through sensitivity analysis. The study shows that the control systems strongly affected the small signal stability of the MMC-based MTDC and the d-axis proportional gains in d–q decoupling main controllers have great impact on system stability, which needs to be properly selected for stability enhancement.

Transient Voltage and Current Stresses Estimation of MMC-MTDC System via Discrete-Time Analysis

Transient voltage and current stresses are essential for the design and protection of multi-terminal DC (MTDC) transmission systems, especially under the short-term dynamics after severe grid faults. In this paper, we propose a discrete-time model for the modular multilevel converter based MTDC (MMC-MTDC) system considering the DC-side disturbances in the system. Based on the piecewise linear modeling, the state variables such as the transient voltage on the sub-module (SM), arm current, etc., can be calculated analytically for the critical short term (within 10 ms) after the severe DC fault while the system protection is yet to be activated. Parameter sensitivity analysis is also performed to give a better system design guideline. The estimation results are validated by detailed electromagnetic (EMT) circuit simulations and the scaled-down experimental laboratory prototype.

Research on Coordination Control Strategy of LCC-MMC Hybrid HVDC Transmission System under Fault

High voltage direct current based on modular multilevel converter (MMC-HVDC) has a good application prospect in engineering, but it is expensive and cannot handle DC faults and other disadvantages. So it is still not suitable for long-distance transmission. To solve these problems, this paper designs multi-terminal hybrid DC transmission system with a line commutated converter based on the rectifier side (LCC) and a modular multilevel converter based on the inverter side (MMC). Besides, for the asymmetric fault on the AC side of the system, the minimum trigger angle control strategy based on the rectifier side LCC and the maximum modulation ratio control strategy based on the inverter side MMC are proposed to improve the DC system transmission power and reduce the possibility of power transmission interruption. Finally, a model of hybrid DC transmission is built in PSCAD/EMTDC, the proposed strategy is verified its effectiveness.

Coordinated Flexible Damping Mechanism With Inertia Emulation Capability for MMC-MTDC Transmission Systems

The rapid growth in penetration levels of power-electronics-interfaced renewable power generation into grids has been reducing system kinetic inertia, which results in degraded frequency stability and oscillation damping. In an attempt to address this challenge, this article proposes a control scheme that realizes a coordinated flexible damping mechanism (CFDM) with inertia emulation capability for modular multilevel converter-based multiterminal dc (MMC-MTDC) transmission systems. The proposed CFDM scheme allows the MMC-MTDC system to autonomously provide an emulated inertial response with a flexible damping effect to ac systems in a similar fashion of synchronous generator, without the need for communication. To ensure the precision of the total provided damping power, the power losses of the converter stations and MTDC networks are also compensated in the algorithm. The CFDM controller parameters are optimized by the small-signal stability analysis. The effectiveness of the proposed CFDM scheme is demonstrated in a power system model simulated in MATLAB/Simulink, which consists of a three-terminal MMC-MTDC transmission system and three asynchronous ac girds. It is verified that the proposed CFDM scheme can effectively minimize grid frequency deviation and damp angular oscillation in grid events of load changes and faults.