Multi-terminal VSC-HVDC Systems Research Articles

Selection of the Parameters of Reactive Power Compensation and Harmonic Filtering Devices for an HVDC Transmission System Based on Voltage Source Converters

This article describes a method for selecting the parameters of AC and DC filters for a multi-terminal HVDC transmission system based on voltage source converters (VSC-HVDC). The equivalent circuit model of a multi-terminal VSC-HVDC system with the parameters ±500 kV and 500 MW was developed in the Matlab Simulink software package and presented in d-q coordinate axes. The system voltage source converters in the model were controlled using a three-level pulse-width modulation (PWM) to reduce the level of current and voltage higher harmonic components and decrease the capacity of AC and DC filters. To select the most efficient filter for use as part of a multi-terminal VSC-HVDC system, high-pass filters (HPF) and low-pass filters (LPF) were considered.

Topology searching algorithm for multi-port hybrid circuit breakers based on graph theory

Due to the significant advantages of low cost, integrated multi-port hybrid DC circuit breakers (M-HCBs) with a reduced number of devices have attracted extensive attention for fault blocking in multi-terminal VSC-HVDC system. However, the current topology exploration by researchers is random, uncertain, and time-consuming. In order to provide more new cost-effective topologies, this paper innovatively proposes a topology searching algorithm for the IGBT-type M-HCB and uses the concept of ‘roadblock’ to simplify the adjacency matrix. It can be used to significantly save time spent on proposing a new M-HCB topology because viable topologies can be quickly and rigorously carried out from a large number of directed graphs using computers. Performance characteristics of all derived topologies are simultaneously obtained, and a comparison can be easily conducted to meet the needs of actual application scenarios. A three-port M-HCB example-specific application is given, along with some detailed output topologies that prove the validity and feasibility of the proposed method.

CNN–SVM Based Fault Detection, Classification and Location of Multi-terminal VSC–HVDC System

CNN–SVM Based Fault Detection, Classification and Location of Multi-terminal VSC–HVDC System

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

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

A novel control strategy based on a look-up table for optimal operation of MTDC systems in post-contingency conditions

Multi terminal VSC-HVDC systems are a promising solution to the problem of connecting offshore wind farms to AC grids. Optimal power sharing and appropriate control of DC-link voltages are essential and must be maintained during the operation of VSC-MTDC systems, particularly in post-contingency conditions. The traditional droop control methods cannot satisfy these requirements, and accordingly, this paper proposes a novel centralized control strategy based on a look-up table to ensure optimal power sharing and minimum DC voltage deviation immediately during post-contingency conditions by considering converter limits. It also reduces destructive effects (e.g., frequency deviation) on onshore AC grids and guarantees the stable operation of the entire MTDC system. The proposed look-up table is an array of data that relates operating conditions to optimal droop coefficients and is determined according to N-1 contingency analysis and a linearized system model. Stability constraints and contingencies such as wind power changes, converter outage, and DC line disconnection are considered in its formation procedure. Simulations performed on a 4-terminal VSC-MTDC system in the MATLAB-Simulink environment validate the effectiveness and superiority of the proposed control strategy.

Enhancing wind power transfer and protection of actual Egyptian 220 kV HVAC transmission system with multi-terminal VSC-HVDC system

Transmitting power using high-voltage direct current (HVDC) systems instead of high-voltage alternating current (HVAC) ones becomes desirable for integrating remote wind farms. Nowadays, Egypt planned to increase the wind power generation in the Red Sea zone from 1400 to 4000 MW by 2022. Unfortunately, the full capacity of the existing 220 kV HVAC transmission line connecting the current wind farms with the main grid is 1500 MW only. This paper aims first to pinpoint the challenges and drawbacks of the existing AC system, including limited capacity, and service quality. Then, it provides a comprehensive, technical and economic evaluation study of upgrading the existing 220 kV HVAC transmission system to a ± 160 kV HVDC system as compared with expanding the existed system to a conventional 500 kV HVAC transmission line. Finally, a thorough investigation study is presented for the proposed multi-terminal HVDC line in conjunction with its modified protection scheme based on a detailed simulation in MATLAB. Based on a variety of fault cases, an assessment of the performance and suitability of the proposed solutions is visualized. The results ensure the ability of the proposed HVDC system to be a suitable solution for integrating the wind farms in Egypt.

A Dynamic Power Management and Dedicated Control Strategy of a Flexible Multi-Terminal HVDC Grids for Offshore Wind Farms

Renewable energy systems are becoming more and more popular on the energy market and wind power has particularly shown a great potential in the field. Many wind farms able to produce thousands of MW are expected to be placed offshore away from the mainland. To this end, aiming to produce a significant amount of power and achieve a high performance while being economically competitive, appropriate transmission systems ought to be designed. This paper investigated a flexible Multi-terminal Direct current connection (MTDC), based on the voltage source converter (VSC) technology, connecting one or several offshore wind farms (OWFs) to one or different AC grids. The contribution of this article lies in the fact that it succeeded in setting up a balance between the produced power by the offshore wind farms and the received power by the different AC networks of the multi-terminal VSC-HVDC system, while checking grid constraints to prevent blackout. This was achieved by developing a global a supervision system based on a dynamic power flow management and dedicated control in order to fulfil an optimisation of the modulated architecture of the studied MTDC system. The multi-terminal VSC-HVDC system was simulated using MATLAB software and the provided results show the feasibility and effectiveness of the proposed control strategy.

Master-Slave Approach for a Multi-terminal VSC-HVDC Systems Connected Offshore Wind Farm

The world is facing today the global challenge of energy transition since countries need more and more energy to grow their economy on a planet where resources are limited and poorly distributed. The integration of renewable energies and especially offshore wind energy into high voltage direct current (VSC-HVDC) transmission systems demonstrates great flexibility and reliability. In this paper, a control strategy for a multi-terminal VSC-HVDC system based on Master-Slave approach is proposed to automatically share the real power variation and stabilize the DC bus voltage in presence of abnormal operating conditions.

A novel economic analysis on multiterminal VSC HVDC systems with wind farms by hierarchical small‐signal stability constrained OPF

In this paper, a novel economic analysis for multiterminal voltage‐source converter high‐voltage direct current (MTDC) systems with wind farms is developed. Both the reinforcement and operation methods of an MTDC system are determined hierarchically in the proposed approach. Based on these methods, not only wind power transmission but also the MTDC system is capable of contributing to the power transmission among alternating current (AC) grids in cases of low wind power production cases. As a consequence, the economic benefits produced by the MTDC system can be increased. This paper also focuses on the sensitivity analysis of small‐signal stability constraints on the developed approach. It is demonstrated that the system for wind power transmission without any MTDC systems is exposed to instability, which is solved by the proposed method. On top of that, the small‐signal stability improvement effect of the MTDC system is analyzed from the economic viewpoint. Under strict small‐signal stability constraints, the economic benefits obtained by the MTDC system increase. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

A Comprehensive Power Flow Approach for Multi-terminal VSC-HVDC System Considering Cross-regional Primary Frequency Responses

For the planning, operation and control of multiterminal voltage source converter (VSC) based high-voltage direct current (HVDC) (VSC-MTDC) systems, an accurate power flow formulation is a key starting point. Conventional power flow formulations assume the constant frequencies for all asynchronous AC systems. Therefore, a new feature about the complex coupling relations between AC frequencies, DC voltages and the exchanged power via VSC stations cannot be characterized if VSC-MTDC systems are required to provide cross-regional frequency responses. To address this issue, this paper proposes a comprehensive frequency-dependent power flow formulation. The proposed approach takes the frequencies of asynchronous AC systems as explicit variables, and investigates the novel bus models of the interlinking buses of VSC stations. The proposed approach accommodates different operation modes and frequency droop strategies of VSC stations, and considers the power losses of VSC stations. The effectiveness and generality of the developed approach are validated by a 6-terminal VSC-HVDC test system. The test system presents the characteristics of the coexistence of numerous VSC operation modes, the absence of slack buses in both AC and DC subsystems, and diversified grid configurations such as point-to-point integration of renewable energy sources and one AC system integrated with multiple VSC stations.

Distributionally Robust Chance-Constrained AC-OPF for Integrating Wind Energy Through Multi-Terminal VSC-HVDC

Using the multi-terminal VSC-HVdc (MTDC) system to collect far-distance offshore wind power into the main onshore ac grid has become one of the ideal transmission approaches for renewable energy integration. However, with traditional fixed dc droop control in MTDC system, the lack of the capability of regulating power flow might pose a great challenge to the system operation. Accordingly, a novel MTDC model has been established in this paper which establishes that the output power of onshore converters can be formulated as a linear function of the control variables, namely power initial set point of each onshore converter. Based on this model, considering the uncertainty from wind power forecast error, a distributionally robust chance-constrained AC-OPF model is proposed for the hybrid AC/MTDC system. Without any presumption on the probability distribution of uncertainty, this model ensures the secure operation by enforcing chance constraints under the worst-case probability distribution over an ambiguity set based on the Wasserstein-Moment metric. Aiming at obtaining a tractable chance-constraint reformulation, we first employ the partial linearization of ac power flow equations to yield a linear model which represents the system response under uncertainties. We then develop an efficient and scalable approach to derive the linear analytical reformulation of distributionally robust chance constraints. The performance of the proposed model is firstly tested on the 14-bus system for an illustrative purpose, and then on the 1354-bus system for demonstrating scalability.

Comparison of DC linear and non‐linear models for multi‐terminal VSC HVDC networks

The analysis and control design of multi-terminal VSC-HVDC systems requires a linearised model of these inherently non-linear systems. This paper develops such a model, from converter to multi-terminal system, discusses the key concepts and shows its use for some test cases. The linearised model is simulated in Matlab, and it is compared against the non-linear model results from an established DEM PSCAD model.

A simplified algorithm to solve optimal power flows in hybrid VSC-based AC/DC systems

High Voltage Direct Current systems based on Voltage Source Converters (VSC-HVDC) are increasingly being considered as a viable technology with advantages, above all when using underground or submarine cables, for bulk power transmission. In order to fully understand how VSC-HVDC systems can be best used within existing power systems, it is necessary to adapt conventional tools to carry out system-wide studies including this technology. Along this line, this paper proposes a simplified algorithm to solve optimal power flows (OPFs) in hybrid VSC-based Alternating Current/Direct Current (AC/DC) grids with multi-terminal VSC-HVDC systems. The proposed algorithm makes it possible to seamlessly extend a previous large-scale AC case to which several multi-terminal VSC-HVDC systems must be added. The proposed approach combines two ideas used previously in two different modelling approaches: each VSC is modelled as two generators with a coupling constraint; and DC grids are modelled as notional AC grids, since, in per unit, the equations for the former are a particular case of the latter with resistive lines and no reactive-power injections. In the proposed approach, the hybrid VSC-based AC/DC system is transformed into an equivalent only-AC system. Therefore, the OPF solution of the AC/DC system can be found with the same tool used for the previous AC problem and a simple extension of the original case.

Review of low voltage ride‐through technology of doubly‐fed induction generator

In recent years, with the increasing of wind power development, the demand for low voltage ride-through (LVRT) technology was proposed to ensure the safe and reliable operation of the power system. The key issue of LVRT strategy for doubly-fed induction generator (DFIG) varies according to the network synchronisation modes. Firstly, the existed LVRT technologies when DFIG is connected to AC network are surveyed. Then relevant literatures about LVRT are summarised and analysed in regard to power surplus of two-terminal system and DC voltage control of multi-terminal system, respectively, when the wind farm is integrated with VSC-high-voltage direct current (HVDC). In the end, the coordinated control strategy of multi-terminal VSC–HVDC system and LVRT technology for islanded system are prospected in this study.

Stability limitation and analytical evaluation of voltage droop controllers for VSC MTDC

This paper presents a systematic analysis of DC voltage stability of a multi-terminal VSC-HVDC (MTDC) system, with the emphasis on a comparative study of the most ubiquitous droop control configurations. The paper introduces a general framework for the analysis of various droop control configurations employed in MTDC systems. This framework is then used to compare leading droop control configurations in terms of their impact on the relative stability, performance and robustness of the overall MTDC system. A generalized analytical MTDC model that contains detailed models of AC and DC system components is derived. Limitations imposed by DC power flow, DC inductor, cable modeling and AC network impedance on DC system stability are identified. Classical and multivariable frequency response analysis and eigenvalue analysis are applied to open-loop and closed-loop models to compare the stability and robustness of five leading droop controllers, with the focus on feedback signal selection and controller parameterization. This paper also proposes an active stabilizing controller, which takes the form of a modified constant power control, to enhance the controllability and robustness of the DC voltage control.

Review of different fault detection methods and their impact on pre‐emptive VSC‐HVDC dc protection performance

Multi-terminal voltage-sourced converters (VSC) high-voltage direct current (HVDC) transmission system is expected to play a vital role in future power systems. Compared with ac power transmission, dc transmission is more vulnerable to faults due to low dc-side impedances and sensitive power electronics in the converters. Dc protection issues must be tackled before any multi-terminal VSC-HVDC grid can be built. The multi-terminal VSC-HVDC system is studied in detail using switching models for two-level converters, detailed equivalent models for the modular multi-level converters, detailed hybrid circuit breaker switching models and frequency-dependent phase models for dc cables. Using such high-fidelity system models, a systematic study of HVDC fault protection methodologies in more detail than previous studies is conducted. This is the first comprehensive study that includes pre-emptive circuit breaker operation. The results presented in this study underline the benefits of such a detailed treatment of the breaker, and of considering it as part of a fast power electronics system rather than isolated dc equipment. The study identifies the best existing fault detection method and tests it extensively. In order to further improve post-fault system recovery response, which is a key but often neglected part of previous studies, a novel bump-less transfer control has been implemented in the converters.

A Hybrid Current-Limiting Circuit for DC Line Fault in Multiterminal VSC-HVDC System

The high fault current of dc line is a major threat to multiterminal voltage-source-converter-based HVDC (VSC-HVDC) system. However, dc circuit breaker (DCCB) with large capacity and fast breaking speed is still under development. Therefore, fault current limitation is vital for the multiterminal VSC-HV DC system. This paper proposes a simple and easily applied hybrid current-limiting circuit (HCLC) at dc side, which consists of a current-limiting inductor (CLI) and an energy dissipation circuit (EDC) in parallel with the CLI. The CLI is designed to reduce the requirement for the DCCB's capacity and breaking speed. The EDC, which consists of thyristor-controlled resistors, is proposed to reduce the stress on energy absorption element (metal oxide arrester) in DCCB and to accelerate the fault current interruption. The design and discussion about the HCLC parameters are performed in detail. By employing the proposed HCLC, dc line fault in the multiterminal system can be isolated effectively with existing DCCB technology, and fast system restoration without power interruption of healthy part can be achieved. Numerous simulations with real-time digital simulator and comparisons with traditional schemes have demonstrated the promising performance of the proposed HCLC. The effectiveness of the HCLC's topology has also been verified by a simplified and scaled test circuit.

Transient-Voltage-Based Protection Scheme for DC Line Faults in the Multiterminal VSC-HVDC System

DC line faults are major issues for a multiterminal high-voltage direct current (HVDC) system based on voltage-source converter (VSC). The fault current increases quickly along with a large peak, and complete isolation of the faulted system is not a viable option. Therefore, protection with high selectivity and accuracy is essential. In this paper, a new protection scheme for dc line in multiterminal VSC-HVDC system is proposed, which consists of a main protection and a backup protection. Both the protection principles are based on the supplemental inductor placed at each end of the dc line. Fault identification can be achieved by calculating the ratio of the transient voltages (ROTV) at both sides of the inductor. The main protection is able to detect the fault quickly without communication, while the backup protection is a pilot method based on the ROTVs at both ends of dc line, which is employed to identify the high-resistance faults and offer a backup in case the former fails. Comparison with some previous protection methods shows that the performance of the proposed protection scheme is promising. Numerous simulation studies carried out in PSCAD/EMTDC and real-time digital simulator (RTDS) under various conditions have demonstrated that fault identification with high selectivity and strong robustness against fault resistance and disturbance can be achieved by employing the proposed protection scheme.

A dual-limiting power balance controller for multiterminal VSC-HVDC system

A dual-limiting power balance controller for multiterminal VSC-HVDC system

Active Power Control Strategies for Transient Stability Enhancement of AC/DC Grids With VSC-HVDC Multi-Terminal Systems

Multi-terminal High Voltage Direct Current (HVDC) using Voltage Source Converters (VSC-HVDC) is a promising technology which provides flexible control of active and reactive power and facilitates remote renewable energy integration, above all using long cables. This paper analyses an active power control strategy for multi-terminal VSC-HVDC systems tailored to enhance transient stability of hybrid AC/DC grids. The proposed strategy controls each VSC using frequency measurements of all terminals. Its performance is compared to a strategy in which each VSC is controlled using only local frequency measurements of the AC side, proving that the proposed strategy shows better performance, even taking into account reasonable communication delays. The paper also shows that the proposed strategy generally gives similar results to those obtained when each VSC is controlled using the speed of the centre of inertia (COI). The speed of the COI is a more comprehensive and richer figure than the one proposed in this paper but it is also much more complex to obtain. Simulation results with PSS/E of a test system have been used to illustrate the comparisons and the main contributions of the proposal.