Multi-terminal Direct Current Systems Research Articles

Frequency Support Control of Multi-Terminal Direct Current System Integrated Offshore Wind Farms Considering Direct Current Side Stability

The frequency stability of modern power systems is challenged due to widespread application of large-scale renewable energy systems, of which the offshore wind farm (OWF) is one of the dominant resources. The OWFs are usually integrated into the grid by multi-terminal direct current (MTDC) transmission systems, which makes the energy flow complicated and the frequency control design challenging. A frequency support control method of MTDC system integrated OWFs (referred to as the OWF-MTDC system) is proposed in this paper. First, the wind turbine generation system (WTGS) is controlled to reserve a certain amount of available power according to the real-time wind speed for more comprehensive frequency regulation. Then, the frequency support control of OWFs is designed, and they can release the rotor kinetic energy and reserved power to support the onshore grid frequency. In addition, the virtual inertia control of a modular multi-level converter (MMC) is designed, which can also provide frequency support in an emergency by use of the DC capacitor. To ensure that the frequency control of the OWF-MTDC system does not degrade the stability of the system, a detailed DC impedance model of the MMC-based MTDC systems is developed, considering the constant power control and DC voltage control. Based on the impedance model, the impact of the frequency control coefficients on the DC side stability of the MTDC system is analyzed. Simulation results validate the stability analysis and verify the proposed frequency control method, which can effectively provide frequency support to the onshore power grid.

Offshore MTDC Transmission Expansion for Renewable Energy Scale-up in Korean Power System: DC Highway.

In this study, we analyzed the impact of multi-terminal direct current (MTDC) system on the integration of renewable energy resources into the Korean power system. Due to the large-scale renewable energy plants planned to be integrated into the power system, line congestion is expected in the southern part of power system. Given the difficulty in constructing AC transmission lines due to social conflicts, we proposed an alternative solution using an offshore multi-terminal DC offshore transmission system. Firstly, we calculate the effective renewable energy plant generation capacity based on annual wind and solar radiation data. Next, we conduct PSS/E simulations to minimize future line congestion in the Korean power grid. The offshore terminal is designed to transfer the power generated in southern Korea and is verified using different terminal rating cases. The simulation result, including contingency analysis, demonstrate that transferring 80% of the generated renewable power achieves the best line flow condition. Therefore, the MTDC system is a possible candidate for integrating future renewable energy systems into the Korean power grid.

Distributed Cooperative Control of Offshore Wind Farms Integrated via MTDC System for Fast Frequency Support

This paper proposes a distributed cooperative control (DCC) scheme of offshore wind farms (OWFs) integrated via the multi-terminal direct current (MTDC) system for fast frequency support. The DCC scheme employs the consensus algorithm to share the frequency support burden among offshore wind turbines (WTs) suitably. The proposed DCC scheme can exploit the kinetic energy of all WTs adequately while ensuring the security by employing the consensus state index and changing droop control coefficients adaptively. After the frequency support, the asymptotic recovery scheme is proposed at the leader WT for smooth rotor speed re-storage, and other WTs will follow the leader to recover and reduce the second frequency drop. Besides, to realize the fast frequency support, the communication-free estimator is employed to estimate the onshore DC voltage using offshore local measured signals. Case studies are carried out on MATLAB and OPAL-RT real-time simulation platforms, respectively. Different control schemes are compared to elaborate the performance of the proposed DCC scheme considering the parameter uncertainty and noise disturbance.

An Improved Master-Slave Control Strategy for Automatic DC Voltage Control Under the Master Station Failure in MTDC System

The master-slave (MS) control is often used in multi-terminal direct current (MTDC) system due to its stable dc voltage control and precise power regulation. However, the MS switching, which aims to maintain dc voltage control under the master station failure, relies heavily on the high-speed communication between the power dispatch center and each station. Some methods in existing works are proposed to address the high communication dependence, at the cost of steady-state control performance. In this article, an improved MS control including five-state control process is proposed to realize the automatic MS switching without communication participation. Based on logic judgment, the multilayer dc voltage clamping mechanism is created to enhance the redundancy of the dc voltage control, and a reasonable dc voltage recovery process is included to eliminate the dc voltage deviation caused by the master station failure. Furthermore, the power and dc voltage hybrid controller are developed to improve the dynamic performance of each station during the MS switching. Simulation and experimental results demonstrate the effectiveness and advantages of proposed method.

Single-ended Fault Detection Scheme Using Support Vector Machine for Multi-terminal Direct Current Systems Based on Modular Multilevel Converter

Single-ended Fault Detection Scheme Using Support Vector Machine for Multi-terminal Direct Current Systems Based on Modular Multilevel Converter

Operational Reliability Evaluation of an MTDC-Based Hybrid AC–DC Power System Considering Conditional-Dependent Faults

The ever-growing applications of the multiterminal direct current (MTDC) systems have introduced interdependence in terms of operation conditions between ac and dc systems. When considering such a conditional dependence, the outage probabilities are affected by the previous fault events in hybrid ac–dc power systems and thus no longer independent, rendering the results from conventional reliability evaluation techniques to be inaccurate. This article provides a method to evaluate the operational reliability of the MTDC-based hybrid ac–dc power systems considering the conditional dependence between the ac and MTDC systems. First, a conditional-dependent fault analysis model is developed to describe the entire temporal fault process. In the proposed model, dynamic state models for components are built considering system parameters that affect their outage probabilities. A redispatch model for the hybrid ac–dc power system is then established to characterize the variation of operation status between different periods. After that, a framework to evaluate the operational reliability of a hybrid ac–dc power system considering conditional-dependent faults is formulated by combining the conditional fault analysis model and the Monte Carlo simulation technique. The impacts of conditional dependence on the reliability of hybrid ac–dc power systems are quantified by the modified reliability indices. Finally, the proposed method is validated by modified IEEE-RTS systems with different MTDC topologies and ac–dc coupling levels to demonstrate the impact of conditional-dependent faults.

Combined Operation Analysis of a Saturated Iron-Core Superconducting Fault Current Limiter and Circuit Breaker for an HVDC System Protection

Recently, in order to overcome the difficulties of interrupting fault currents in multi-terminal direct current systems (MTDC), studies combining a saturated iron-core type superconducting current limiter (SFCL) and a direct current circuit breaker (DCCB) have been conducted. However, the effect of inductance change of the SI-SFCL on the interrupting time of the DCCB during fault has not been studied yet. In this paper, the interrupting time delay caused by the dynamic behavior of the inductance change during the fault current blocking process of the SI-SFCL combined with a DCCB was analyzed through experiments and a new fault detection method considering this phenomenon was proposed. After designing and manufacturing the laboratory-scale SI-SFCL and DCCB, a fault current interrupting test was performed and the inductance change pattern of the SI-SFCL was analyzed. Based on the analysis results, a new fault detection technique was proposed to alleviate the interruption time delay that occurs when applying the combined protection system to a MTDC, and its effectiveness was verified through a simulation. These results will be useful for planning protection coordination strategies when introducing a SI-SFCL in combination with a DCCB in actual MTDC systems.

Reliability Assessment of a Multi-State HVDC System by Combining Markov and Matrix-Based Methods

The purpose of this paper is to highlight that, in order to assess the availability of different HVDC cable transmission systems, a more detailed characterization of the cable management significantly affects the availability estimation since the cable represents one of the most critical elements of such systems. The analyzed case study consists of a multi-terminal direct current system based on both line commutated converter and voltage source converter technologies in different configurations, whose availability is computed for different transmitted power capacities. For these analyses, the matrix-based reliability estimation method is exploited together with the Monte Carlo approach and the Markov state space one. This paper shows how reliability analysis requires a deep knowledge of the real installation conditions. The impact of these conditions on the reliability evaluation and the involved benefits are also presented.

Data-Driven Fault Detection and Classification for MTDC Systems by Integrating HCTSA and Softmax Regression

The requirement of fast fault isolation poses a great challenge to the safe operation of multi-terminal direct current (MTDC) systems. In order to make a better tradeoff between the speed and reliability of the protection scheme, it is imperative to mine more valuable information from fault transient signals. This paper puts forward a data-driven framework capable of digging out and synthesizing multi-dimensional features to achieve fast and reliable DC fault detection and classification in MTDC systems. Highly comparative time-series analysis (HCTSA) is first adopted to extract extensive features with clear physical interpretations from fault current waveforms, and a few features valuable to fault identification are then selected utilizing the greedy forward search. Based on the reduced features, a softmax regression classifier (SRC) is further proposed to calculate the probability of each fault category with a relatively minor on-line computational burden. Numerical simulations carried out in PSCAD/EMTDC have demonstrated the proposed approach is effective under different fault conditions, robust against noise corruptions as well as abnormal samplings, and replicable in various DC grids. In addition, comprehensive comparison studies with conventional derivative-based protection methods and some typical artificial intelligence based (AI-based) methods have been conducted. It is verified that the proposed method has the advantages of higher fault identification accuracy over conventional protections and shallow structure AI-based methods, better interpretability as well as lower on-line computing complexity over the deep architecture AI-based approaches.

Multi-objective optimal power flow considering the multi-terminal direct current

Introduction. In recent years, transmission systems comprise more direct current structures; their effects on alternating current power system may become significant and important. Also, multi-terminal direct current is favorable to the integration of large wind and solar power plants with a very beneficial ecological effect. The novelty of the proposed work consists in the effects of the aforementioned modern devices on transient stability, thus turn out to be an interesting research issue. In our view, they constitute a new challenge and an additional complexity for studying the dynamic behavior of modern electrical systems. Purpose. We sought a resolution to the problem of the transient stability constrained optimal power flow in the alternating current / direct current meshed networks. Convergence to security optimal power flow has been globally achieved. Methods. The solution of the problem was carried out in MATLAB environment, by an iterative combinatorial approach between optimized power flow computation and dynamic simulation. Results. A new transient stability constrained optimal power flow approach considering multi-terminal direct current systems can improve the transient stability after a contingency occurrence and operate the system economically within the system physical bounds. Practical value. The effectiveness and robustness of the proposed method is tested on the modified IEEE 14-bus test system with multi-objective optimization problem that reflect active power generation cost minimization and stability of the networks. It should be mentioned that active power losses are small in meshed networks relative to the standard network. The meshed networks led to a gain up to 46,214 % from the base case.

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.

Interactions between a half‐bridge MMC and a hybrid DC breaker during fault current interruption compared to a full‐bridge MMC

The large-scale integration of remote renewable energy resources into existing AC grids is one of the main drivers for the construction of multi-terminal direct current (MTDC) systems. Today, a few MTDC systems are already in execution stage or even in operation. However, with a growing number of extended MTDC systems, the probability of high-voltage DC (HVDC) insulation faults is likely to increase and therefore, reliable solutions for handling of such disturbances are crucial in order to maintain high availability. Since most HVDC converters are realised by modular multilevel converters (MMC) converters are based on half-bridge (HB) modules, they lack an inherent capability to control DC fault currents. Therefore, additional equipment like HVDC breakers needs to be installed as well. Among the various breaker types, the so-called ‘hybrid’ DC circuit breaker (DCCB) is considered most popular. This contribution provides a more holistic view on the overall system behaviour, represented AC grid, MMC HVDC converter, DC reactor and DCCB altogether. It discusses the physical interactions of these main components in MTDC systems. In particular, the influence of the electrical parameters of the hybrid DCCB and its required DC reactors on the MMC converter's internal current and voltage conditions is outlined.

Centralized Fuzzy Logic Based Optimization of PI Controllers for VSC Control in MTDC Network

Advancements in the field of power electronics led to global changes in the electrical energy generation, transmission, and distribution. The voltage source converter (VSC) based HVDC system is the future of the power system due to the advantages it offers in terms of renewable power generation, transmission, and integration. Currently, the two-terminal VSC–HVDC systems have been successfully commissioned. Multi-terminal VSC–HVDC system is the developing trend for higher power reliability, large scale integration, and smart operation. The performance of a VSC based multi-terminal direct current (MTDC) system greatly depends upon the tuning of the controller. Standard practice is to tune the PI controller using hit and trial method or based on the operator’s experience. However, the tuning process becomes complex when multiple grids are involved. Thus, for MTDC systems, the manual tuning of the controller does not yield the desired results. This paper presents a centralized fuzzy logic-based optimization technique for VSC control of the MTDC system to obtain the optimized parameters for the PI controllers. The optimized parameters ensure a better system performance in terms of fast settling time, better slew rate, minimum undershoot, and minimum overshoot response. The proposed technique is tested on a three-terminal MTDC network in SIMULINK / MATLAB.

Model Order Reduction of Multi-Terminal Direct-Current Grid Systems

This paper proposes a methodology to develop a linear low-order model of a multi-terminal direct-current (MTDC) system connected to practically-sized offshore wind farm systems. The method individually linearizes the model of wind farm systems and subsequently performs model order reduction (MOR) using iterative rational Krylov algorithm (IRKA). This is, therefore, a modular approach and expandable to any number of wind farm connections. On the other hand, the model order and non-linearity of other components are retained. Effectiveness of the proposed method is demonstrated on an MTDC system connected to two large-scale wind farm models by comparing the responses obtained from the full order model (FOM) and reduced order model (ROM). From the simulation studies, it can be seen that the ROM obtained from proposed method is able to maintain high degree accuracy of FOM responses at a much faster simulation time, and hence this can help power system operation planning. Various control functionalities and strategies, different operating conditions and faults of the MTDC system are also considered to investigate robustness of the proposed method.

Robust Current Sensorless Control of VSC-Based MTDC Transmissions for Integrating Wind Farms

This article presents an observer-based robust current sensorless control (RCSC) scheme for voltage-source-converter (VSC)-based multiterminal direct-current (MTDC) transmission system integrated with wind farms. Perturbation observer-based point of common coupling (PCC) voltage controller and dc-link voltage controller are designed for wind farm station (WFS) and grid-side station (GSS) of MTDC, respectively, for grid integration of multiple wind farms. Perturbation states are defined to describe the interactions of voltage control loops of nonlinear VSC model, and the feedback linearization control is realized based on the state estimations provided by perturbation observers to achieve the decoupled voltage control of MTDC. Neither current sensors nor parameters of MTDC are required in the RCSC scheme, thus achieving the robust sensor fault-tolerant operation of the MTDC system. Simulation and experimental studies are undertaken on a three-terminal MTDC transmission system integrated with two wind farms to verify the control performance of the RCSC scheme under the conditions of wind speed variations, sudden load changes, parameter variations, and current sensor faults.

MMC Based MTDC Grids: A Detailed Review on Issues and Challenges for Operation, Control and Protection Schemes

For decades high voltage alternating current (HVAC) was considered as most economical solution to transmit and deliver electric power. With the recent developments in power electronic devices, high voltage direct current (HVDC) system becomes most prominent technology. Multi-terminal direct current (MTDC) based system as a promising technology for future power system is the major focus area for researchers and industries these days. A number of MTDC systems have been implemented physically. The major motivation to construct such MTDC systems is the integration of large-scale offshore power sources such as wind turbines and solar systems. This article discusses the most critical challenges and issues related to operation, control and protection schemes for integration of modular multi-level converter (MMC) based MTDC systems. At first detailed literature survey has been presented to show the challenges for MMC based MTDC systems, then an analysis related to those challenges for operation, control and protection schemes for existing MMC based MTDC systems has been provided. Finally, a road map to tackle such challenges has been suggested.

A Subsynchronous Oscillation Suppression Method Based on Self-Adaptive Auto Disturbance Rejection Proportional Integral Control of Voltage Source Converter Based Multi-Terminal Direct Current System with Doubly-Fed Induction Generator-Based Wind Farm Access

A subsynchronous oscillation suppression strategy based on self-adaptive auto disturbance rejection proportional integral controller is proposed for doublyfed induction generator-based wind farm integrated into grid through voltage source converter based multi-terminal direct current. In this strategy, the nonlinear PI controller is constructed by fal function to replace the traditional linear PI controller, and then the tracking differentiator is used to arrange the appropriate transition process in combination with the idea of active disturbance rejection control, and the self-adaptive auto disturbance rejection proportional integral controller is designed. By applying the controller to the inner loop of the converter on the rotor side of the doubly-fed induction generator, the adaptability of the control parameters of the inner loop to the change of operating conditions of the system can be improved, and the dynamic performance of the system can be improved. The simulation results on PSCAD/EMTDC show that, compared with SSDC, when the wind speed is 7.5 m/s, 8.5 m/s and 9.5 m/s, the convergence time can be shortened by 0.2 s, 0.1 s and 0.25 s, respectively. When the number of grid-connected doubly-fed induction generator wind turbines is 200 and 220, the convergence time is shortened by 0.1s. When the self-adaptive auto disturbance rejection proportional integral controller and the multi-channel variable-parameter additional subsynchronous damping controller work together, the convergence time under the above three wind speeds are 2.7 s, 2.7 s and 2.3 s, respectively. When the number of grid-connected doubly-fed induction generator wind turbines is 200 and 220, the convergence time is 2.65 s and 2.75 s, respectively. It can be concluded that the self-adaptive auto disturbance rejection proportional integral controller can realize the effective suppression of the subsynchronous oscillation under different operating conditions of the wind farm via the comparison with the additional subsynchronous damping control of doubly-fed induction generator. Besides, subsynchronous oscillation will converge faster and the stability of the system can be enhanced when the self-adaptive auto disturbance rejection proportional integral controller and the multi-channel variable-parameter additional subsynchronous damping controller work together.

Simulation Study on Pole-to-Pole Short Circuit Faults of the Hybrid Multi-terminal DC Transmission System

The hybrid multi-terminal direct current (MTDC) system, with the economic superiority of LCCs and technical flexibility of VSCs, is one of the development directions of future DC transmission technology. This paper introduces the development of Hybrid-MTDC transmission projects as well as the related publications especially on fault control strategies. A ring hybrid three-terminal DC transmission system was built on PSCAD/EMTDC to simulate pole-to-pole short-circuit faults at different locations. The fault development process and fault characteristics on DC lines are analysed and summarized, which aims at providing the theoretical basis for the protection principles of hybrid MTDC systems.

Interarea Oscillation Damping Control Using High-Voltage DC Transmission: A Survey

High-voltage direct current (HVDC) transmission lines are increasingly being installed in power systems around the world, and this trend is expected to continue with advancements in power electronics technology. These advancements are also bringing multiterminal direct current (MTDC) systems closer to practical application. In addition, the continued deployment of phasor measurement units makes dynamic information about a large power system readily available for highly controllable components, such as HVDC lines. All these trends have increased the appeal of modulating HVDC lines and MTDC systems to provide grid services in addition to bulk power transfers. This paper provides a literature survey of HVDC and MTDC damping controllers for interarea oscillations in large interconnected power systems. The literature shows a progression from theoretical research to practical applications. There are already practical implementations of HVDC modulation for lines in point-to-point configuration, although the modulation of MTDC systems is still in the research stage. As a conclusion, this paper identifies and summarizes open questions that remain to be tackled by researchers and engineers.

Finite-time stabilisation of some power transmission systems

This paper presents the finite time stabilisation strategy of two problems: the first one is the control of the high voltage direct current based on voltage source converter, while the second is the control of the multi-terminal direct current transmission systems. Subject to finite-time control design strategy, a linear and nonlinear dynamic model are derived based on the state-space description. Furthermore, continuous or discontinuous finite-time feedbacks are proposed to ensure the tracking of the output variables and to enhance the stability of the studied high voltage direct current system. In addition, the proposed control strategy is extended for the multi-terminal direct current system. A comparative study between various approaches (Proportional-Integral control, continuous or discontinuous stabilising finite-time controllers and control by backstepping) is presented and shows that the finite-time continuous feedback gives an excellent transient response.