Multi-terminal VSC-HVDC Research Articles

A D-T Hybrid DC FCL and Its Parameter Optimization Design Method for Application in Multiterminal VSC-HVDC Grids

A D-T Hybrid DC FCL and Its Parameter Optimization Design Method for Application in Multiterminal VSC-HVDC Grids

Adaptive Hierarchical Fractional-Order Sliding Mode Control Strategy for Multi-terminal VSC-HVDC System with Wind Farm

In this paper, an adaptive hierarchical fractional-order sliding mode controller (AHFSMC) for a multi-terminal Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) Integrated with a wind farm (WF) system is designed. Multi-terminal VSC-HVDC (MtVDC) connected to wind farms has received several attentions in the power sector because of its numerous benefits. The effectiveness of MtVDC, on the other hand, is dependent on the control scheme used. To achieve this goal, a hierarchical sliding mode control method by fractional-order calculus is used. The controller parameters are adjusted according to a suitable adaptation method to enhance the proposed controller's robustness compared to the system uncertainties coefficient. An appropriate Lyapunov-based approach is used to achieve the adaptation rule. This paper discusses a scheme to design additional controllers in MtVDC systems to damp electromechanical oscillations, one of several features of HVDC presently under active study. Numerical simulations validate the proposed control strategy's feasibility and efficiency. This novel approach is employed for the upgrading of system stability with the dynamic properties of the MtVDC in a variety of operative conditions.

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.

A Power Routing-Based Fault Detection Strategy for Multi-Terminal VSC-HVDC Grids

Fault detection is one of the most important research topics till today in the field of VSC-HVDC grids protection. In this subject, fast and reliable identification of faults is the most demanding issue, which still requires further study. In this respect, this paper proposes an index for fault detection, which is based on correlation between the powers of DC-link capacitor and the protected line. It is proved by an analytical time-domain transient expression that there is a direct correlation between these two powers under fault conditions. To implement this scheme, the routing of the discharged power of DC-link capacitor into the protected line is the key technique. To do so, the Pearson correlation factor is used in this paper, which efficiently captures the level and direction of the correlation between these two powers. The proposed method benefits from different salient features of very fast fault detection, accurate discrimination of faulted line from healthy lines, covering high-resistance faults, noise immunity, robustness against severe power reversal conditions, robustness against low sampling frequencies, not requiring extra sensor and communication infrastructure, etc. To evaluate the performance of the proposed approach, a complete set of fault scenarios are simulated on a meshed multi-terminal MMC-based VSC-HVDC network.

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

Computationally Effective Frequency Transient-Based Transmission Line Protection for Multiterminal VSC-HVdc

For the transmission line fault in multiterminal voltage source converter (VSC) based high-voltage direct current (HVdc), the frequency spectrum of internal fault is obviously higher compared with the external fault. In frequency transient-based protection, the fault is generally identified by the high-frequency component via the professional mathematical algorithm, such as Fourier and wavelet transforms. However, the burdensome computation would slow down the protection operation speed, and the details in the specific frequencies are far beyond the need for fault identification. In this article, a computationally effective protection is proposed without utilizing the complicated mathematical algorithm. Comprehensive index given from the supplemental inductor is developed, which shows the greater value for higher frequency spectrum, and requires fewer algebraic operations. The proposed protection based on the comprehensive index is with superior computational efficiency and maintains the advantage of frequency transient-based protection in fault identification accuracy. The protection performance is validated in four/three-terminal VSC-HVdc.

Wave front rising time based traveling wave protection for multi-terminal VSC-HVDC grids

To deal with the problem of high-resistance fault identification in multi-terminal VSC-HVDC grids, a novel wave front rising time based traveling wave protection is proposed in this paper. For such purpose, the traveling wave is firstly analyzed from its generation, propagation to refraction for the sake of discovering its influence factors. The wave front rising time is revealed to be approximately proportional to the fault distance while insensitive to the fault resistance and operation condition during line propagation. Moreover, it significantly increases when refraction occurs at the line terminal. Secondly, based on these characteristics, the novel wave front rising time based traveling wave protection as well as its analytical setting calculation is proposed. The proposed protection can work with higher-fault-resistance identification capability and faster-operating speed, compared with other protections. Besides, the efficiency of the proposed protection setting calculation is greatly improved since tedious electromagnetic transient simulations are avoided. Finally, the feasibility and robustness of the proposed protection are validated by a four-terminal VSC-HVDC grid on PSCAD/EMTDC platform.

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.

Short-term generation capacity expansion planning considering multi-terminal VSC[sbnd]HVDC links using a linear programming framework improved by shift factors

This paper describes the modelling of short-term generation capacity expansion planning (GCEP) for multi-terminal VSC-based HVDC transmission systems. The GCEP formulation is based on a mixed-integer linear programming (MILP) framework where transmission losses are considered by piecewise linearisation. As opposed to existing methods, the present GCEP formulation is based on shift factors which enable the representation of the hybrid AC/DC grids including VSC stations. This way new power generation investments can be efficiently determined at any of the VSC-connected AC systems. This new method lies in sharp contrast with the so-called classic GCEP method where the system nodal power balances are formulated using the voltage phase angles as decision variables thus increasing the model complexity. Indeed, to determine the GCEP investments, the present model aimed to multi-terminal HVDC links only uses AC/DC transmission line losses, power generations and projected generating power plants as decision variables. The new GCEP formulation is compared to the classic method using two compelling multi-terminal VSCHVDC test systems for validity and applicability purposes. Results show that the proposed formulation reduces the simulation running time by more than 80%, compared to the standard model, accompanied by acceptable accuracy of results since errors inferior to 5% are obtained.

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.

Review on Voltage Control of Multi Terminal Vsc-Hvdc Transmission System for Offshore Wind Power Plant

Review on Voltage Control of Multi Terminal Vsc-Hvdc Transmission System for Offshore Wind Power Plant

Simplified calculation method of threshold value for the non-unit transient-voltage based protection in multi-terminal VSC-HVDC grid

The non-unit protection based on transient voltage shows outstanding application prospect in the multi-terminal VSC-HVDC grid, due to the advantages including high operation speed, ability to withstand large transition resistance, et al. However, for this kind of protection, the threshold value is generally determined according to the simulation, because the theoretical calculation method is still lack of study. In this paper, the fault traveling wave propagation process in the multi-terminal VSC-HVDC grid is analyzed. And the calculation methods of the key parameters are deduced, such as the reflection coefficient, line-mode transition resistance and line-mode fault point voltage. Then the impact of the frequency-dependent line-parameter characteristics is discussed in detail. On this basis, the simplified calculation method of threshold value for the non-unit transient-voltage based protection is proposed. Compared with the simulation-based method, the computing load of the proposed method is reduced greatly. In addition, in contrast to the method developed for ROCOV-based dc protection, the rational fitting and inverse transform are not required in the proposed method, presenting easier implementation for engineering application. Finally, simulation cases prove that the proposed method calculates the fault transient voltage accurately. And the protection with the calculated threshold value distinguishes internal and external faults reliably.

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 Novel Distributed Supplementary Control of Multi-Terminal VSC-HVDC Grids for Rotor Angle Stability Enhancement of AC/DC Systems

This paper presents a distributed supplementary control for embedded Multi-Terminal VSC-HVDC (MTDC) grids to enhance the transient stability of the surrounding AC grid. Firstly, the need of supplementary controllers for embedded MTDC grids is demonstrated via the study of the Transient Energy Functions (TEF) of a simple hybrid AC/DC power system. Then, the proposed control structure is presented. This control aims to enhance the angle stability of the surrounding AC system via the modulation of the active power references of the converters. The objective of the control is to enhance AC transient stability while keeping the power balance of the DC grid even if converters reach their active power limits. The proposed control uses frequency, and angle measurements at the Point of Common Coupling (PCC) of the converters. The idea behind the proposed strategy is to match the control actions, and limits of each pair (i, j) of converters. If converter i modulates its power, converter j modulates the same amount of power in the opposite direction. The concept of virtual links is used in this paper to represent this matching. Finally, the proposed controller is tested on a modified version of the IEEE 39 bus system using EMT simulations.

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 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.

A VSC-based Model for Power Flow Assessment of Multi-terminal VSC-HVDC Transmission Systems

This paper puts forward a new practical voltage source converter (VSC) based AC-DC converter model suitable for conducting power flow assessment of multi-terminal VSC-based high-voltage direct current (VSC-MTDC) systems. The model uses an advanced method to handle the operational limits and control modes of VSCs into the power flow formulation. The new model is incorporated into a unified framework encompassing AC and DC power grids and is solved by using the Newton-Raphson method to enable quadratically convergent iterative solutions. The use of complementarity constraints, together with the Fischer-Burmeister function, is proposed to enable the seamless incorporation of operational control modes of VSC and automatic enforcement of any converter's operational limits that become violated during the iterative solution process. Thus, a dedicated process for checking limits is no longer required. Furthermore, all existing relationships between the VSC control laws and their operational limitsare considered directly during the solution of the power flow problem. The applicability of the new model is demonstrated with numerical examples using various multi-terminal AC-DC transmission networks, one of which is a utility-sized power system.

High Fault-Resistance Tolerable Traveling Wave Protection for Multi-Terminal VSC-HVDC

To deal with the high-resistance fault identification problem in multi-terminal VSC-HVDC, this paper proposes a high fault-resistance tolerable traveling wave protection. First, the non-unified characteristic of traveling wave front between internal and external faults is revealed, which makes the high-resistance fault identification difficult. Then, the traveling wave propagation regarding the reflection and refraction process at the remote end is investigated for the difference, and the distinctive current reduction phenomenon during the interval of two incident traveling waves under internal fault with resistance is discovered. Moreover, the current reduction phenomenon would become more significant with the increase of fault resistance. Based on this phenomenon, the high fault-resistance tolerable traveling wave protection is proposed, which could identify the high-resistance fault accurately with single-end measurement and is insensitive to fault resistance. The feasibility and robustness of the proposed protection are validated in a four-terminal VSC-HVDC.

On the Dynamic Modeling of Marine VSC-HVDC Power Grids Including Offshore Wind Farms

This article presents a new and comprehensive modeling framework to carry out system-wide dynamic studies of DFIG-based wind farms embedded in multi-terminal VSC-HVDC power grids. Contrary to existing, well-developed simulation frameworks for similar studies, using electromagnetic transient (EMT) solutions, this is an RMS-type formulation which maintains a high-degree of fidelity while enabling much faster steady-state and dynamic simulations than what it is possible to achieve with EMT simulators. The new RMS modeling framework includes AC/DC power grids of an arbitrary size, topology and number of offshore VSC-connected wind farms. A simulation tool with such a high degree of modeling versatility and numerical efficiency does not currently exist elsewhere. This has required the development, using first principles, of the RMS model of a DFIG with explicit representation of all the dynamic effects relevant for dynamic problems of the electromechanical type as opposed to the study of very fast EMT phenomena. All the control functions and parameters of the rotor-side converter, the grid-side converter and the DC link, are accounted for in the new DFIG model. The prowess of the new formulation is demonstrated using a six-terminal VSC-HVDC link with two VSC-connected 200-MW wind farms. The impact of the wind farms’ operation on both the DC grid and the AC grids is assessed. The fidelity of the output results of the new simulation tool is compared against those of the EMT-type model implemented in Simscape Electrical of Simulink. The article shows that they favorably compare with each other, with differences inferior to 3%. The computational efficiency of the new dynamic modeling framework for HVDC-connected wind farms is unassailable.