Multi-terminal HVDC Grids Research Articles

Self‐energy device for HVDC breakers and its control strategy

In order to reduce the cost of energy devices for IGBT drive circuits of the hybrid HVDC breaker, a self-energy device for the HVDC breaker is proposed here, which connects to DC lines and takes power from DC lines directly. The device consists of three operating modes: the start-up charging mode, the steady-state charging mode, and the maintenance restart charging mode. In view of these modes, corresponding control strategies are proposed to ensure the stability of power supply to drive circuits, and the simulation model is built and studied. For multi-terminal HVDC systems or HVDC grids, several self-energy devices in different lines can be connected through a certain combination, which can get the DC current flow regulation function.

A non-unit line protection scheme for MMC-based multi-terminal HVDC grid

Multi-terminal flexible HVDC grids are required due to the high penetration of renewable energy. However, they are vulnerable to dc faults. To ensure their high reliability and continuous operation, fast and selective protection schemes are indispensable. This paper analyses the transient voltages in the case of dc line faults in modular multilevel converter (MMC)-based HVDC grid. Then, making use of the boundary effect of current-limiting reactors, a fast non-unit dc line protection scheme based on fault transient voltage is proposed. In addition, the faulty pole identification criterion is constructed based on fault currents. A MMC-based four-terminal HVDC grid is modelled using PSCAD/EMTDC, and extensive simulations are conducted to validate the effectiveness of the proposed scheme. Simulation results show that the proposed protection method not only can detect faults rapidly and accurately, but is also effective in detecting large-resistance faults.

Robust Predictive Sliding Mode Control for Multiterminal HVDC Grids

In this paper, a robust nonlinear control scheme is proposed for voltage source converter based multiterminal HVDC grids, based on the predictive sliding mode control (PSMC). This control is a hybrid of the multivariable nonlinear model predictive control and sliding mode control. The particle swarm optimization method is applied to obtain the optimal control parameters. This control fulfills the objectives of regulating terminal dc voltage, active and reactive power, improved dynamic performance, and superior robust control operation even with plant parameter changes and external measurement disturbances. The performance of the proposed PSMC scheme is compared with the optimal synergetic control and optimal PI. The results confirm the superior transient performance and robustness of the PSMC scheme.

Control Approach for the Multi-Terminal HVDC System for the Accurate Power Sharing

This paper presents a new control strategy for voltage source converter based Multi-Terminal High Voltage Direct Current (MTDC) systems. The proposed control approach ensures accurate power sharing between the droop-controlled converter stations. By communicating the power-sharing index between neighboring converters, the proposed approach achieves exact droop control operation independent of the DC system topology and line parameters. The pilot voltage droop based controller, which is an alternative communication-based approach for achieving precise power sharing, was used as a base case for comparison. Modal analysis is carried out to reveal the sensitivity of the system's eigenvalues to the changes in control parameters (e.g., power droop gain, proportional integral gains of the proposed controller) and the latencies in the communication. It is demonstrated that the proposed strategy remains in the stable operation even when excessive latencies are encountered in the communication. Nonlinear simulations are conducted in the MATLAB/Simulink environment in four- and five-terminal MTDC grids, validating the capability of the proposed controller to achieve the desirable performance under various operational conditions.

Droop Control for a Multi-Line Current Flow Controller in Meshed Multi-Terminal HVDC Grid Under Large DC Disturbances

The security, stability, and reliability of power supply are essential and must be guaranteed during operation of multi-terminal HVDC (MTDC) grids, particularly under disturbances, such as a line or converter loss, on the dc grid side. In this paper, a multi-line current flow controller (CFC) is expanded from the existing two-line CFC for meshed MTDC grids with two main contributions: a droop control scheme for a multi-line CFC, which is capable of coordinating dc line current/power sharing under large dc disturbances is proposed and a selection method for the droop gains is proposed. The multi-line CFC with the droop control scheme proposed can achieve full controllability of the currents among multiple dc lines and the security and continuity of power supply of the unfaulty dc lines can be guaranteed. The stability and dynamic performance of the droop controller can both be satisfied with the proposed selection method. The effectiveness of the proposed scheme and selection method is justified by RTDS simulations. The proposed scheme is simple and easy to implement. The proposed dynamic simulation-based selection method has the flexibility suiting both online and offline calculation tools and the consideration of system nonlinear characteristics and control limits/constraints.

Optimal placement and control variable setting of power flow controllers in multi-terminal HVDC grids for enhancing static security

This research proposes an approach to select an optimal place and control variable setting for recently proposed Power Flow Controller (PFC)s including series, cascaded, and interline PFCs in Multi-Terminal High-Voltage DC (MT-HVDC) grids based on sensitivity analysis technique to enhance static security. To do so, appropriate static power injection models of the PFCs are obtained. Accordingly, sensitivity relationships between the defined power performance index and each PFCs control variable are obtained. It is for the optimal PFCs placement purpose in order to utilize maximum capacities of the transmission grid. Optimal settings for the PFC as well as the controllable voltage source converters are computed in turn by applying sequential quadratic programming solver to the developed security-based DC optimal power-flow problem which includes several corrective constraints. The study scope is single contingencies affecting HVDC lines with the objective of eliminating the consequent overloaded HVDC lines and thereby enhancing the MT-HVDC grid security by controlling the power flows within the MT-HVDC grid. Static simulations are performed considering two generic four-terminal and eighth-terminal MT-HVDC test grids in order to demonstrate the proposed methods effectiveness and authenticate its robustness. The obtained results indicate that the MT-HVDC grid can remain secure under single HVDC line contingencies in most cases by implementing the proposed method.

A comprehensive power-flow model of multi-terminal PWM based VSC-HVDC systems with DC voltage droop control

This paper presents power-flow models of AC power systems integrated with voltage-sourced converter (VSC) based multi-terminal high voltage direct current (MTDC) grids. Pulse-width modulation (PWM) control is employed for the VSCs. In the proposed approach, the VSC modulation indices are considered as unknowns. For control of the MTDC grid, DC voltage droop control has been implemented. The proposed method includes both linear and nonlinear voltage droop characteristics. Converter losses are included in the proposed model. Depending upon the terminal end line active power specifications of the VSCs, two different voltage droop control models have been developed. The convergence properties in the proposed approach are found to be independent of the MTDC grid location, its topology, the MTDC control strategy employed and the operating point specification. Numerous case studies are carried out by applying different control modes to multiple topologies of MTDC grids embedded in the IEEE-300 bus test system. For all the case studies, the power-flow algorithms were implemented with MATLAB. In all occurrences, a mismatch error tolerance of 10−10 p.u. was selected. The results validate the proposed work.

Protection Systems for Multi-Terminal HVDC Grids

Protection Systems for Multi-Terminal HVDC Grids

Hybrid AC/DC Post-Contingency Power-Flow Algorithm Considering Control Interaction of Asynchronous Area

This paper presents an approach for the calculation and estimation of control interactions between asynchronous power systems coupled via multi-terminal high-voltage direct current grids using static analysis methods. This aims at possible application within online security assessment where, as of today, static analysis methods are still a common practice. Therefore, the postcontingency power flow is evaluated using an integrated approach for the solution of AC and DC systems in the power-flow algorithm. Thereby, equations for DC voltage control and AC frequency control of converter stations, generators, loads as well as wind turbines with frequency support are embedded into the extended system Jacobian. The results show that by using the proposed approach, the post-contingency power-flow situation can be accurately determined and the frequency changes in each subsystem can be sufficiently tracked. Using an application example of an offshore power system with frequency support by offshore wind turbines, all results have been validated against dynamic simulation, while simulation models and controllers are provided. It is shown that active power balancing controls of AC and DC systems could have a high impact on line loadings and need to be respected for security analyses.

Analysis of Faults in Multiterminal HVDC Grid for Definition of Test Requirements of HVDC Circuit Breakers

This paper provides a detailed analysis of the temporal development of fault currents in a multiterminal high-voltage direct-current (MT-HVDC) grid composed of a bipolar converter configuration. The sequence of events following the occurrence of a pole-to-ground fault is identified, divided into three distinct periods; namely, submodule capacitor discharge, arm current decay, and ac in-feed periods. The critical parameters that have a significant impact on the fault current in each period are discussed. The impacts of various parameters of the HVDC grid such as the size of the current limiting reactor, ac grid strength, as well as the location of the fault within the grid are studied through PSCAD/EMTDC simulation. Then, a fault current interruption process using models of various HVDC circuit breaker technologies and the resulting stresses are studied. Both serve as important inputs to define test procedures. It is found that the HVDC circuit breakers are subjected to not only dc current and voltage stresses but also energy stress. These stresses are translated into test requirements.

Power Flow Control in Multi-Terminal HVDC Grids Using a Serial-Parallel DC Power Flow Controller

Multi-terminal HVDC (MT-HVDC) grids have no capability of power flow control in a self-sufficient manner. To address this important issue, utilization of dc–dc high power and high-voltage converters is motivated. However, proposing suitable partial-rated dc–dc converters as well as their suitable modeling and control in both primary and secondary control layers as well as the stability analysis are the existing challenges that should be alleviated beforehand. This paper addresses the control of power flow problem through the application of a power converter with a different connection configuration, namely, serial parallel dc power flow controller (SPDC-PFC). The SPDC-PFC input is the transmission line voltage, and its output is transmission line current. Therefore, employing a full-power dc–dc converter is avoided as a merit. Additionally, in this paper, the common two-layer MT-HVDC grid control framework comprised of primary and secondary layers is efficiently modified in order to integrate the SPDC-PFC. A differential direct voltage versus active power droop control scheme is applied to the SPDC-PFC at the local control layer, guaranteeing dynamic stability, while an extended dc power-flow routine—integrating the SPDC-PFC—is developed at the secondary control layer to ensure the static stability of the entire MT-HVDC grid. The proposed control framework enables the SPDC-PFC to regulate the flow of current/power in the envisioned HVDC transmission line. From the static and dynamic simulation results conducted on the test CIGRE B4 MT-HVDC grid, successful operation of the proposed SPDC-PFC and control solutions are demonstrated by considering power flow control action. In more detail, the SPDC-PFC successfully regulates the compensated lines’ power to the desired reference both in static and dynamic simulations by introducing suitable compensation voltages. In addition, good dynamic performance under both SPDC-PFC power reference and wind power-infeed change is observed.

Multiport Hybrid HVDC Circuit Breaker

This paper proposes a new $\boldsymbol {n}$ -port hybrid dc circuit breaker for offshore multiterminal HVDC grid application. The $\boldsymbol {n}$ -port dc circuit breaker can substitute $\boldsymbol {n-1}$ hybrid dc circuit breakers at a dc bus with $\boldsymbol {n-1}$ adjacent dc transmission lines. The system-level behavior of the proposed multiport hybrid dc circuit breaker is similar to other hybrid dc circuit breakers. The operation principles of the proposed multiport dc circuit breaker are introduced, analyzed, and compared to the existing solution in this work. The components ratings are compared to the existing solution, and the functionality of the proposed device is verified by simulation.

Grid code compliant controllers for Multi-terminal HVDC grids aimed to integrate wind power: assessing their impact on the operational security of a real-world system

Exploiting multi-terminal High Voltage Direct Current (MTDC) grids for offshore wind park integration into continental bulk AC system raises several technical challenges, in particular the verification of the compliance of MTDC grids and offshore wind parks to the performance requirements (in terms of frequency, voltage support and fault ride through) which are being established by the grid codes, and the availability of general and customizable models to simulate their response. This work is aimed to provide a set of “open access” general models, able to fulfil the grid code requirements if suitably tuned, enabling the TSOs to perform their analyses without relying exclusively on vendor-specific models. In order to verify the realistic response of these models, an example of MTDC grid is simulated, connecting it to plausible operating scenarios of a real world AC system (the West Denmark grid) in target year 2020: some indications to properly tune control parameters are provided together with the results of several simulations aimed to check the fulfilment of grid code requirements in case of typical contingencies.

Coordinated Supervisory Control of Multi-Terminal HVDC Grids: A Model Predictive Control Approach

A coordinated supervisory control scheme for future multi-terminal high-voltage direct-current (HVDC) grids is proposed. The purpose is to supervise the grid and take appropriate actions to ensure power balance and prevent or remove voltage or current limit violations. First, using DC current and voltage measurements, the power references of the various Voltage Source Converters are updated according to participation factors. Next, the setpoints of the converters are smoothly adjusted to track those power references, while avoiding or correcting limit violations. The latter function resorts to model predictive control and a sensitivity model of the system. The efficiency of the proposed scheme has been tested through dynamic simulations of a five-terminal HVDC grid interconnecting two asynchronous AC areas and a wind farm.

Full-bridge MMC DC fault ride-through and STATCOM operation in multi-terminal HVDC grids

Abstract This paper investigates a control structure to enhance the DC fault ride-through capability of a full-bridge modular multilevel converter (MMC) station, while ensuring a stable controlled operation as a STATCOM during DC faults without the need for fault isolation. Taking advantage of the switching states of a full-bridge submodule, a DC current controller is proposed, which provides the DC voltage reference for the modulation when a DC fault is detected. By changing the outer controllers strategy from DC voltage or active power control to converter energy control during a fault, the decoupling of the converter operation from the DC side dynamics is realized. In this paper, the focus is on the control methodology at all times of operation and the evaluation of the STATCOM control during a fault. To this end, extensive simulations were performed on a three-terminal high voltage direct current (HVDC) grid in radial configuration and a pole-to-pole DC fault case was investigated. The results showed that the AC voltage and current were controlled within limits at all times, while the full-bridge MMC was able to provide reactive power support to the AC grid. Moreover, using the proposed control methodology, the transients at the operation transition points between STATCOM and inverter/rectifier operation were minimized and the stations were able to safely ride through the fault.

A Study on Protection Requirements for DC Faults in Multi-Terminal HVDC System

Recently, the construction of large-scale offshore wind farms and dissemination of new and renewable energy has encouraged a growing interest in DC systems. In particular, HVDC (high-voltage direct current) systems have been evaluated as the best alternative because of their power system interconnection and large-scale power transmission. Because HVDC systems offer many advantages such as a low insulation level, transmission losses, and improved power system stability, many studies on DC power systems are actively underway. As compared to the point-to-point HVDC system, which connects the electricity generated from geographically scattered renewable energy sources to an existing AC power system on a 1:1 basis, the MTDC (multi-terminal HVDC) system has garnered greater attention because of its ability to control energy flow by connecting several HVDC outputs to the HVDC. Because the VSC-HVDC, which operates as an MTDC, supplies not only active power, but also reactive power, it has been deemed the optimal solution to connect a renewable energy source to a system with no existing AC power supply, such as a wind-power generation system. An MTDC system, which possesses a structure that is more complex than that of the point-to-point HVDC system, should be equipped with a high-speed DC circuit breaker that can quickly separate a conversion station from the DC transmission line to which it is connected in the event of a fault or malfunction at the station. In addition, DC protection on the MTDC grid must be reliable and operate much faster than conventional AC protection to ensure that the converter sends a trip signal before it is damaged. In this paper, we propose a protection scheme for the DC line to effectively activate the DC circuit breaker, which is essential in an MTDC system; additionally, system modeling and simulation using the PSCAD/EMTDC were performed to verify the proposed algorithm.

Enhancing frequency stability by integrating non-conventional power sources through multi-terminal HVDC grid

The 2050 targets established by the EU will foster both larger penetration of renewable energy, especially wind power, and more cross-border interconnections. Moreover, this new framework requires the non-conventional power sources and power converter-based systems to be responsible for the duties traditionally carried out by conventional synchronous generators as frequency support. This paper presents how different power-electronic based technologies can provide frequency support individually and in a coordinated manner (with different priority given by the deadbands) ensuring a stable operation.The implemented scenarios examine challenging conditions, where the primary reserve of the interconnected conventional, renewable, and storage generation is fully utilized to tackle frequency incidents. This demonstrates how the joint regulation of the power electronic-based technologies enhances the frequency stability of the AC synchronous areas. The different control schemes and their interaction are investigated in Cigré DC grid benchmark adapted for frequency stability studies and implemented in Matlab/Simulink simulation tool. This modified grid includes 5-terminal HVDC grid with two offshore wind farms and three AC networks including battery and onshore wind farms.

Impact of Multi-terminal HVDC Grids on Enhancing Dynamic Power Transfer Capability

This paper proposes the exploitation of multiterminal HVDC grids to improve transfer capability in power systems. Multiterminal HVDC systems based on voltage source converters (VSC-MTDC) have been recognized as a promising alternative for the wind power integration. Under low wind scenarios, these grids originally dedicated for wind power transmission can be exploited as an additional interarea transmission path, providing extra dynamic security. The paper focuses on small-signal stability assessment, especially in poor damped oscillations associated with interarea modes. Simulations performed through a generic computational framework have shown that the high level of flexibility and controllability provided by voltage source converters can considerably improve the transfer capacity, while preserving adequate dynamic performance.

Impact of converter losses on the optimal power flow solution of hybrid networks based on VSC-MTDC

This paper studies the impact of voltage source converter (VSC) losses on the solution of the optimal power flow (OPF) of hybrid AC/DC systems with a multi-terminal configuration. The motivation of this analysis is that the expected development of high voltage direct current (HVDC) systems will entail an increase of the number of converter stations, and consequently, converter losses might not be negligible when compared with conventional transmission losses of AC and DC lines. Towards this end, an extended OPF model is proposed considering a combination of VSC based multi-terminal HVDC grids (VSC-MTDC) and AC systems. The OPF model represents converter losses according to the state-of-the art modeling where different expressions are used when the converter functions as an inverter or as a rectifier. Three simpler alternative approaches are also implemented and the obtained OPF solutions are assessed. In order to compare the OPF solutions, a comparison metric is proposed. Obtained results show that modeling the converter losses in a simplified way could lead to very different power flow solutions, especially for the DC branches.

A Novel Multiterminal VSC-HVdc Transmission Topology for Offshore Wind Farms

Multiterminal voltage source converter (VSC) topologies of high voltage direct current (HVdc) transmission systems (M-HVdc) for the integrating large offshore wind farms are scrutinized. A novel two wind farm with one onshore substation (2WF-1SS) topology is proposed. M-HVdc system standards are defined along with control schemes. HVdc switch gear and HVdc-link topologies are reviewed, considering the need of HVdc circuit breakers, dc cables, and extra offshore substations. Numerous M-HVdc-link topologies are analyzed and compared based on certain techno-economic criteria: length, number and ratings of HVdc circuits, cost, flexibility, stability, number of HVdc circuit breakers, and offshore substations. Operation of various topologies with respect to the designed control system is analyzed and compared through PSCAD/EMTDC simulations considering two different tests: permanent VSC disconnection and a dc line-to-line fault. Results show that 2WF-1SS is a promising topology for future multiterminal HVdc grids.