VSC-HVDC Link Research Articles

Islanded Operation and Control of Offshore Wind Farms Connected through a VSC-HVDC Link

A new control scheme for the operation of offshore wind farms connected through a VSC-HVDC link to the main onshore grid is presented in the paper. The control strategy has been developed to allow the islanded operation and black start of the wind farm as these requirements will probably be of great interest in the future due to the progressive increase of distributed generation. A complete model of the offshore wind farm with a VSC-HVDC link and its control scheme has been developed and analyzed through PSCAD simulations. Results show that the proposed control scheme is able to operate the plant under different steady-state and transient conditions.

P-f Stability of 2030 Nordic Power System due to Non-Synchronous Generation

The Nordic Power System is undergoing significant transformation and development in the coming years. A driving factor for this is the increased dependency on renewable energy sources and high voltage direct current transmission (HVDC). Various stability challenges due to the dynamics of reactive power - voltage (Q-V) balancing and active power - frequency (P-f) balancing seep into the system with such rise of non-synchronous generation (NSG). This paper investigates the projected impact of NSGs on the P-f dynamic performance of Nordic power system in 2030. As NSGs are systems with low inertia, power discrepancies lead to larger oscillations, higher Rate of Change of Frequency (RoCoF) and bigger maximum frequency deviation (MFD) values. To address these issues, Emulated Inertia (EI) control for wind turbines and Synthetic Inertia (SI) control for VSC-HVDC links are proposed in this paper. The simulation is carried out using DIgSILENT PowerFactory 2022 SP1 simulation software. The results obtained confirm the Effectiveness of the proposed EI and SI methods to enhance frequency response and mitigate deviations.

A Novel Simplified Modeling Approach for VSC-HVDC Links in Performance Analysis of Multi-Machine Systems

A Novel Simplified Modeling Approach for VSC-HVDC Links in Performance Analysis of Multi-Machine Systems

A Novel Approach for Steady State Calculations of VSC-HVDC Connected PMSG Based Offshore Wind Farms Integrated into Multi-Machine Systems

Offshore wind farms equipped with Direct Drive Permanent Magnet Synchronous Generators (DD-PMSG) are drawing increased attention due to their advantage over other variable speed technologies. VSC-HVDC links are considered the most suitable option for transferring power to the onshore system. The integration of VSC-HVDC connected DD-PMSG based offshore wind farms into multi-machine systems is explored in this paper. A novel approach for power flow and initial condition calculations is proposed to facilitate dynamic analysis of the system. For three cases of the most commonly specified quantities of the wind farm, efficient methods have been described. The cases comprise combinations of data like the total output of the wind farm, the number of wind turbines, the wind speed, or the output of individual wind turbine, which are frequently given in literature. This approach enables the user to build the dynamic model of the system in any basic graphical dynamic modeler and numerical computational software without requiring power system toolboxes or electromagnetic transient packages. The proposed methods are highly effective for studies focusing primarily on the dynamic aspects and controls of the system. Case studies and simulations are conducted to verify the proposed technique.

Secondary Frequency Control Considering Optimized Power Support From Virtual Power Plant Containing Aluminum Smelter Loads Through VSC-HVDC Link

The growing number of renewable energy replacing conventional generators results in a loss of the reserve for frequency control in power systems, while many industrial power grids often have excess energy supply due to abundant wind and solar energy resources. This paper proposes a secondary frequency control (SFC) strategy that allows industrial power grids to provide emergency high-voltage direct current (HVDC) power support (EDCPS) for emergency to a system requiring power support through a voltage source converter (VSC) HVDC link. An architecture including multiple model predictive control (MPC) controllers with periodic communication is designed to simultaneously obtain optimized EDCPS capacity and minimize adverse effects on the providing power support (PPS) system. Moreover, a model of a virtual power plant (VPP) containing aluminum smelter loads (ASLs) and a high penetration of wind power is established for the PPS system. The flexibility and controllability of the VPP are improved by the demand response of the ASLs. The uncertainty associated with wind power is considered by chance constraints. The effectiveness of the proposed strategy is verified by simulation results using the data of an actual industrial power grid in Inner Mongolia, China. The DC voltage of the VSCs and the DC in the potlines of the ASLs are also investigated in the simulation.

Review And Analysis On Hybrid Control Of Fault Ride Through And Transient Stability Of Vsc-Hvdc

A steady DC-Link Voltage, a CV voltage control and a reactive power control on the basis of the previous reviewers' analysis are the goals of the VSC-HVDC connecting controller. In order to achieve these objectives, the appropriate controller is required at both VSC-HVDC terminals. This paper presents a non-linear control approach to improve the VSC-HVDC relationship's control robustness. A sliding mode control (SMC) and an integrated proportional control (PI) were used for the configuration of a hybrid controller. The hybrid controller will provide mathematical simulation. This hybrid driver provides malfunction driving and the transient reliability of the VSC-HVDC link. This paper presents a non-linear approach to control to improve the robustness of the VSC-HVDC controller. The hybrid controller configuration is a control of the sliding mode (SMC) and an integrated proportional control (PI) for a hybrid controller design. Fault ride the stability of this hybrid control unit is provided through and transient VSC-HVDC links. MATLAB and Simulink simulate the hybrid controller and increases the performance of the controller. The simulation induces a symmetrical grid-side malfunction to maintain transient stability.

Innovative Unit Commitment Modeling Approach Considering Multi-infeed VSC-HVDC Links

This paper introduces an improved optimization model for the unit commitment (UC) problem of AC power systems considering multi-infeed VSC-HVDC links. The proposed formulation is based on the linear modeling of the AC network and point-to-point HVDC links, where power losses are properly considered by piecewise linearization of AC lines, DC links and VSC units. This UC model is featured by a mixed integer linear programming (MILP) framework, thus enabling effective calculations of the optimal hourly generation scheduling for multi-infeed VSC-HVDC power grids. These sophisticated power grid studies are associated with day-ahead electricity markets carried out by power system engineers at control centers. To demonstrate the applicability of the proposed MILP-based UC model, the IEEE RTS 24-bus test system is used to carry out two case studies. The first incorporates three point-to-point HVDC links where an in-depth analysis is carried by comparing its outcomes, with and without HVDC links. The second case study features two interconnected areas having three-infeed HVDC links and several tens of generation units. Both systems are analyzed for a 24-hr planning horizon. It is confirmed that the UC model described in this paper permits to study practical power grids with multi-infeed VSC-HVDC links.

Enhancement of the System Stability of DB-Controlled VSC Links Parallel Operated with a Weak Multi-Machine AC Power System

To allow the large-scale integration of renewable energy sources into the grid, VSC-HVDC is commonly utilized. However, with greater integration, several power system stability issues arise. In this study, we examined the small-signal rotor angle stability of a model of the Australian power system with embedded VSC-based HVDC links. A simplified model of the Australian power network was used to connect the potential geothermal resources in the region of Innamincka using a 1100 km HVDC link. We observed that the introduction of the new source of geothermal power generation had an adverse impact on the damping performance of the system. Therefore, two forms of stabilization are examined here: (i) generator power system stabilizers (PSS) fitted to the synchronous machines, which are used to convert geothermal energy to electrical power; and (ii) power oscillation damping controllers (PODs) fitted to the VSC-HVDC link. In the case of the PODs, two types of stabilizing input signals are considered: (i) local signals such as power flow in adjacent AC lines, and (ii) wide-area signals such as bus voltage angles at key nodes in the various regions of the system. It was concluded that the small-signal rotor angle stability of the interconnected AC/DC system can be greatly enhanced by employing the designed damping controllers.

Joint corrective optimization based on VSC-HVDC and distributed energy storage for power system security enhancement

The growing integration of flexible control devices like VSC-HVDC links and distributed energy storage (DES) devices into power systems introduces new challenges in addition to increasing the flexibility for power system operation. To explore the potential advantages of flexible controllable devices in power system operation, this paper proposes a joint corrective measure (JCM). Based on the VSC-HVDC and DES devices, post-contingency power flow violations in the hybrid AC/DC power system can be cured. The power sensitivity of adjusting line overloads with VSC-HVDC and DES is derived and calculated. Considering that not all corrective control variables are required to participate in subsequent optimization decision-making calculations, a dynamic search algorithm for the action threshold is designed to screen qualified control variables. Furthermore, to achieve an optimal tradeoff between the actions of the VSC-HVDC and DES, a corrective decision optimization modelling is performed with which the individual solutions to the line overloads after the contingencies are obtained. Finally, numerical tests on two modified hybrid AC/DC power systems are conducted to validate that the proposed JCM decision method can efficiently coordinate VSC-HVDC and DES, eliminate overload, and reduce decision-making time.

Modeling and Simulation of Variable Limits on Conditional Anti-Windup PI Controllers for VSC-Based Devices

This work focuses on variable limits of conditional anti-windup PI-controller described in the IEEE Std. 421.5-2016 applied in the current limiters of VSC-based applications. To overcome deadlock and chattering during numerical simulation of the Std. conditional anti-windup with variable limits, it proposes a method for software implementation based on Filippov theory. The robustness of the proposed implementation is studied through a VSC-HVDC link included in the WSCC 9-bus system and through STATCOM included in the Nordic-32 system. The case studies compare wind up, back calculation type anti-windup and heuristic methods applied in the conditional anti-windup with the proposed solution.

VSC-HVDC and Its Applications for Black Start Restoration Processes

System reliability is a measure of an electric grid system’s ability to deliver uninterrupted service at the proper voltage and frequency. This property of the electric system is commonly affected by critical processes, such as a total blackout. The electric system restoration is a complex process which consists of returning generators, transmission system elements, and restoring load following an outage of the electric system. However, the absence of a generator or unit of black start capabilities may worsen the duration and effects of blackouts, having severe consequences. Black start capability is important as it can reduce the interruption time, decrease the economic loss, and restart the power supply fast and efficiently. In recent years, several works have reported advances about the High Voltage Direct Current (HVDC) technology based on the Voltage-Source Converter (VSC) as an attractive and promising technology to increase black start capability. This paper is a review of the current studies of VSC-HVDC as black start power and discusses the advantages and limitations of recent methods. The major points addressed in this paper are as follows: the current theoretical approach of the black start process and the used HVDC technologies, the advantages of VSC-HVDC as black start power, a compressive review of the literature about the black start capabilities using VSC-HVDC technologies, and a description of the main methods recently used to provide an enhancement for restoration processes. Finally, this paper discusses new challenges and perspectives for VSC-HVDC links in order to provide an enhancement for restoration processes.

Optimal $\mathcal {H}_{\infty }$ Control Design for MMC-Based HVDC Links

The modular multilevel converter (MMC) has emerged as the preferred choice for voltage source converter (VSC)-based high voltage direct current (HVDC) systems due to its low losses, low harmonic distortion, modularity, and redundancy. These advantages come at the expense of a complex control system with the strong coupling among its control variables, which complicates the design procedure of both its individual control loops and the DC link controllers. In fact, the performance improvement of a control loop could lead to degraded performance of other loops. Hence, to deal with such a complex dynamic system, this article suggests adopting multivariable <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathcal {H}_{\infty }$</tex-math></inline-formula> optimal control techniques in order to ensure stability and optimized performance of a VSC-HVDC link. First, a full-order, centralized multi-input-multi-output (MIMO) controller is derived based on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathcal {H}_{\infty }$</tex-math></inline-formula> optimisation and used as a benchmark for the system performance level. Then, a fixed-structure, decentralized MIMO controller is synthesized to make a compromise between the optimal performance and feasibility of practical implementation. Finally, simulations are conducted to evaluate and compare the performance and robustness degradation due to the migration from a high-order, centralized controller towards a low-order, decentralized controller.

Analysis and validation of mathematical morphology filters for single-ended fault localization in VSC-HVDC links

Fast and accurate fault detection and localization are important topics in voltage source converter-based high-voltage direct current transmission (VSC-HVDC) grids in order to isolate the fault after the first milliseconds of occurrence. Common signal processing methods used for detecting fault traveling wave (TW) peaks like Fourier transform, S-transform and wavelet-based techniques transform the signal into the frequency domain, requiring complicated computations. In this paper, the application of mathematical morphology (MM)-based filters for detecting and locating faults in VSC-HVDC links is studied. MM-based methods analyze the signal in the time domain and detect the wave peaks accurately. Multiple MM-based filters resulted from basic MM operators are presented and used for TW-based fault study in VSC-HVDC grids. Several fault cases are applied to the CIGRE VSC-HVDC model in PSCAD, and the MM-based scripts are written in MATLAB. The impact of different sizes and types of the structuring elements on the accuracy of peak detection is analyzed. The results show the accuracy of MM filters for detecting and locating fault transient in VSC-HVDC links. The proposed method gives accurate results for both low-impedance and high-impedance faults.

Advanced automated emergency control strategy for embedded VSC-HVDC links

This paper proposes an advanced emergency control strategy for embedded VSC-HVDC links after AC disturbances, such as line tripping and generation loss. The control goals are to minimize the impact of the disturbance on the AC-network, which includes improving the long-term voltage stability with utilizing possible unloading capacity of parallel AC elements. The proposed algorithm is evaluated against other control strategies for embedded VSC-HVDC links by applying it to a simple demonstration network. The test system includes all necessary VSC converter dynamics, the VSC-HVDC link current and voltage limits and simple load recovery characteristics for an evaluation of the long term voltage stability. The simulation results are explained and the advantages of the proposed emergency control strategy are discussed.

A General Steady-State Voltage Stability Analysis for Hybrid Multi-Infeed HVDC Systems

As large-scale offshore wind farms are integrated into the power grid, a hybrid multi-infeed HVDC (HMIDC) system including both LCC-HVDC and VSC-HVDC links is facing a critical steady-state voltage stability problem which mainly stems from a weak AC system under high-level DC power injections. To overcome this challenge, this paper proposes a general steady-state voltage stability analysis to accurately measure the critical voltage stability point of HMIDC systems. In this paper, a typical HMIDC system is firstly modeled as a general dual-infeed system composed of an LCC-HVDC, a VSC-HVDC, and a virtual equivalent voltage source behind a virtual equivalent impedance at the AC side. Then a hybrid power sensitive factor (HPSF) is established to indicate the critical voltage stability point with the consideration of dynamic power-voltage characteristics of HVDC links, as well as operational changes in AC system (e.g. dynamics of excitation voltage control in generator and network topology change). Finally, a general voltage stability index, called hybrid generalized short circuit ratio (HGSCR), is further derived to effectively explain the critical voltage stability point of HMIDC systems. Moreover, multiple comparative studies with existing voltage stability indices are conducted in this paper to verify the effectiveness of the proposed general steady-state voltage stability analysis by using both PSCAD/EMTDC™ power flow and time-domain simulations.

Efficient method for the optimal economic operation problem in point-to-point VSC-HVDC connected AC grids based on Lagrange multipliers

This paper describes a method based on Lagrange multipliers for efficiently solving the economic dispatch in power systems including point-to-point VSC-HVDC links. The proposed formulation bases on linear models of the AC systems and HVDC links where power losses are properly considered through incremental transmission loss factors.This timely formulation preserves suitable outcome accuracy while showing a greater computational efficiency than what is achieved with classical, nonlinear OPF methods. Its main features are demonstrated using two compelling test cases: a system accommodating one point-to-point VSC-HVDC link and another comprising multi-infeed HVDC connected AC power grids. Results of this approach are compared with those calculated by the nonlinear interior point method. It is confirmed that both fundamentally different approaches are in good agreement since errors smaller than 0.5% were obtained for generation costs, whereas the computational time was reduced by more than 60% with the introduced method.To show the method applicability in realistic power systems, the 10-generator New England Test system incorporating a VSC-HVDC link is studied. The DC link effect on system operation is examined for a 24-hour period dispatch thus demonstrating its usefulness, unrivalled modelling versatility and numerical efficiency with respect to existing approaches.

Interaction Assessment and Stability Analysis of the MMC-Based VSC-HVDC Link

This paper investigates the dynamic behavior of a modular multi-level converter (MMC)-based HVDC link. An overall state-space model is developed to identify the system critical modes, considering the dynamics of the master MMC and slave MMC, their control systems, and the HVDC cable. Complementary to the state-space model, an impedance-based model is also derived to obtain the minimum phase margin (PM) of the system. In addition, a relative gain array (RGA) analysis is conducted to quantify the level of interactions among the control systems of master and slave MMCs and their impacts on stability. Finally, with the help of the results obtained from the system analysis (eigenvalue, phase margin, sensitivity, and RGA), the system dynamic performance is improved.

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.

A power system restoration method using voltage source converter–high-voltage direct current technology, aided by time-series neural network with firefly algorithm

Voltage source converter (VSC)–high-voltage direct current (HVDC) has become a new trend for achieving an efficient and reliable bulk power transmission. The main challenges in VSC-HVDC link are the “soft-start-up” or the restoration of power after the system blackout, with over voltage and less response delay. In this paper, VSC-HVDC link with firefly Levenberg–Marquardt-trained artificial neural network (ANN), termed as ANN-FLM, is proposed to restore the power quickly. The simulation is performed by creating blackout in the system through the introduction of a three-phase fault. The performance analysis is made on VSC-HVDC with ANN-FLM in existing proportional integral controller. The simulated results provide faster restoration and independent control of real and reactive powers. The proposed method minimizes the over voltage to 98% in maximum point, 84% in phase A, 61% in phase B and 82% in phase C, when simulated.

System congestion criteria for interconnection of WPP through VSC‐HVDC

The growth of utilisation of wind power plants (WPP) in power systems placed at long distance of the transmission power system has shown that the best connecting alternatives are by means of high-voltage direct current lines (HVDC) using voltage source converter (VSC) stations. Generally, the bus closest to the WPP and with the highest level of short circuit is selected as the best option to connect the VSC-HVDC link, mainly due to economic criteria. This paper proposes a methodology to select the best connection point in the AC system, taking advantage of the potential increase of transmission capability due to the reactive power controlled at the VSC converter stations; measured as the increment of the loadability margin computed by means of PV curves. Nevertheless, a trade-off between this benefit and the cost of overhead HVDC transmission line as consequence of the length differences among alternatives is required. This trade-off is computed as the cost of the equivalent STATCOM required to get the same loadability margin. Thus, a technical-economical comparison of connection alternatives allows the selection of the best option for connecting the VSC-HVDC line. The methodology has been applied to connect a WPP of 500 MW to the NETS.