Multi-terminal HVDC Research Articles

Research on DC voltage droop strategy based on optimal DC power flow

A coordinated control based on the optimal DC power flow for MTDC is proposed to improve the transmission efficiency of multi-terminal HVDC, which adopts hierarchical structure including processing layer and implementation layer. The dcOPF model of optimal DC power flow is established in the solving layer. The minimum of the generation cost is set as the objective function, and the constraints include the line capacity, generator capacity, converter station capacity and power balance. The optimal solution is applied to the executing layer. When disturbs occur in some converter stations, new optimal solution will be re–calculated and updated to the executing layer and reduce the generation cost. The stability of the system will not be affected by communication fault. A four-terminal VSC-MTDC simulation model in the MATLAB/Simulink platform is built in the paper. The simulation results show that the proposed control strategy can minimize the generation cost and the control strategy has the advantages of simple structure and easy implementation.

A modified droop control structure for simultaneous power‐sharing and DC voltage oscillations damping in MT‐HVDC grids

With increasing energy demand, the use of renewable energy resources is increasing. Renewable energy sources require power electronic converters to get integrated into power grids. Multi-terminal high voltage direct current (MT-HVDC) systems are a promising solution for their grid integration. Using droop-based controller strategies in power converter stations of MT-HVDC grid, in addition, to providing power-sharing, can support the frequency of the connected AC grids. However, power changes lead to direct voltage deviations, thereby disrupting the stability of the MT-HVDC system. Here, a new control structure for the droop controller of the voltage source converter (VSC) controller is proposed which is named modified droop controller (M-Droop). This method, in addition to power-sharing, uses an appropriate control action to provide the required voltage stability. Here, analytical studies of the modified control strategy are reported. Moreover, through a developed MATLAB Simulink platform, its performance is verified in the events of transients, consisting of fault, variable load, and change in power generation.

Optimal economic-environmental dispatch in MT-HVDC systems via sine-cosine algorithm

This paper addresses the problem of optimal economic-environmental dispatch in Multi-Terminal High-Voltage Direct Current (MT-HVDC) networks using the Sine-Cosine Algorithm (SCA). This optimization methodology allows working with nonlinear non-convex large-scale optimization problems via sequential programming. The SCA works with an initial population and rules of advance based on the best current solution and sine and cosine functions that define the direction of the next solution. Three variants of the SCA are evaluated in a standard six-node MT-HVDC system considering a linear combination of the objective functions (i.e., greenhouse emissions and energy production costs). The main advantage of the proposed evolutionary approach lies in its pure algorithmic structure. Thus, it can be easily adapted to any continuous optimization problem. All numerical calculations are performed using MATLAB software.

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.

WITHDRAWN: Corrigendum to: “Research on DC Protection Strategy in Multi-Terminal Hybrid HVDC System” [Engineering 7 (2021) 1064–1075

WITHDRAWN: Corrigendum to: “Research on DC Protection Strategy in Multi-Terminal Hybrid HVDC System” [Engineering 7 (2021) 1064–1075

Fault Analysis and Traveling Wave Protection Based on Phase Characteristics for Hybrid Multiterminal HVDC Systems

By fully combining the advantages of line commutated converter (LCC) high-voltage direct current (HVDC) and modular multilevel converter (MMC)-HVDC systems, hybrid multiterminal HVDC (hybrid-MTDC) transmission technology has become a new technical means to overcome the shortcomings of conventional two-terminal HVDC systems. However, the conditions of high fault resistances and more sophisticated network topologies have brought considerable challenges to the existing traveling wave (TW) protection principles. In this article, a parallel hybrid-MTDC system is taken as the research object, and its mathematical analysis model is established. Based on the mathematical model, the TW characteristics that the phase of the high-frequency voltage TW during the occurrence of an internal and external fault approaches approximately 90° and 0°, respectively, are clarified. Therefore, considering the influences of reflection and refraction of TWs and line propagation, the phase-characteristics-based criterion is constructed. Using the proposed criterion, a protection scheme for hybrid-MTDC systems is proposed, which realizes the identification of line faults with high fault resistance. The proposed method is verified in detail with the electromagnetic transient simulation model using PSCAD/EMTDC, and the test results show that the proposed protection scheme is feasible and has high sensitivity, reliability, and adaptability to different operation modes.

A Smart Fault-Tackling Strategy Based on PFTE for AC Three-Phase-to-Ground Faults in the Multi-Terminal HVDC Wind Power Integration System: Further Foundings

This paper describes a smart fault tackling strategy based on power flow transfer entropy (PFTE) for AC three-phase-to-ground (TPG) faults in the multi-terminal HVDC (MTDC) wind power integration system. The fault characteristics and transient energy transfer of different positions and properties are analyzed. Then, a double integral discrimination method based on PFTE is proposed to further distinguish the fault property. Considering the power flow balance, an adaptive coordination strategy of wind farms and energy dissipation resistors is proposed to deal with different AC faults. Finally, a smart fault-tackling strategy based on PFTE for AC three-phase-to-ground (TPG) faults in the MTDC wind power integration system is proposed. Under the proposed smart fault-tackling strategy, the MTDC wind power integration system achieves uninterrupted operation during any AC TPG fault at the receiving end. The experiment results confirm the applicability of the proposed fault-tackling strategy.

Systematic and optimal design of droop-controlled MMCs in MT-HVDC networks

Multi-terminal High-Voltage Direct-Current (MT-HVDC) Voltage Source Converter (VSC) based grids are arising as an extension of point-to-point connections, aiming at boosting the integration of renewable energy resources through a more interconnected power system. This paper investigates and compares different energy-based control structures for Modular Multilevel Converters (MMC) operating in DC voltage droop control mode, identifying the most suitable ones in terms of stability. Then, a systematic, automatic and optimal control design based on H∞ for a generic and bidirectional MT-HVDC system, including multiple DC voltage droop controlled terminals, is developed, based on both steady-state and transient requirements under different contingencies.

Multi-Terminal DC Grid with Wind Power Injection

With the development of offshore wind generation, the interest in cross-country connections is also increasing, which requires models to study their complex static and dynamic behaviors. This paper presents the mathematical modeling of an offshore wind farm integrated into a cross-country HVDC network forming a multi-terminal high-voltage DC (MTDC) network. The voltage source converter models were added with the control of active power, reactive power, frequency, and DC link voltages at appropriate nodes in the MTDC, resembling a typical cross-country multi-terminal type of HVDC scenario. The mathematical model for the network together with the controllers were simulated in MATLABTM and experimentally verified using a real-time digital simulator hardware setup. The resulting static and dynamic responses from the hardware setup agreed well with those from simulations of the developed models.

Single terminal coordination-protection scheme for hybrid three-terminal DC line based on station terminal current characteristics

Grounding electrode is an essential component in hybrid multi-terminal HVDC transmission system, By connecting converter station to the bus bar as a main station, this work analyzes the fault current component characteristics of grounding electrode line under various fault conditions for a hybrid three terminal DC transmission system. It is concluded that under single pole line grounding fault, current at the measuring ends of grounding, fault and sound electrodes satisfies the Kirchhoff current theorem, while considering converter station as a node. However, for reverse fault of line protection, current at above mentioned electrodes doesn’t follow the Kirchhoff law. Under various DC line faults, fault line is selected according to the mutation direction of current at the back-side of bus bar. Essentially, the current obtained by grounding electrode’s measuring point is a zero mode current, hence, it can sense the fault current precisely and has directivity. A comprehensive single terminal coordination-protection scheme for DC lines in hybrid multi-terminal transmission system is constructed by using currents’ characteristics of station terminal. The proposed method does not rely on boundary elements, and has an ability to quickly identify internal and external faults, select the fault pole, and withstand high resistance. The developed algorithm for protection scheme is simple, requires low sampling rate, and ensures high reliability for protection scheme.

Decentralized Robust Frequency Regulation of Multi-terminal HVDC-linked Grids

This paper proposes a new strategy for optimal grid frequency regulation (FR) in an interconnected power system where regional ac grids and an offshore wind farm are linked via a multi-terminal high voltage direct-current (MTDC) network. In the proposed strategy, decentralized <i>H</i>_&#x221E; controllers are developed to coordinate the operations of synchronous generators and MTDC converters, thus achieving optimal power sharing of interconnected ac grids and minimizing frequency deviations in each grid. To develop the controllers, robust optimization problems are formulated and solved using a dynamic model of the hybrid MTDC-linked grids with model parameter uncertainty and decentralized control inputs and outputs. The model orders of the controllers are then reduced using a balanced truncation algorithm to eliminate unobservable and uncontrollable state variables while preserving their dominant response characteris- tics. Sensitivity and eigenvalue analyses are conducted focusing on the effects of grid measurements, parameter uncertainty levels, and communication time delays. Comparative case studies are also carried out to verify that the proposed strategy improves the effectiveness, stability, and robustness of real-time FR in MTDC-linked grids under various conditions.

Active Damping of VSC-MTDC Grid Equipped With Interline Power Flow Controller

To ensure the secure operation of a multiterminal high-voltage DC (MTDC) grid, power flow controllers (PFCs) are deployed to regulate current distribution among different transmission lines. Besides power flow management, PFCs are envisioned to provide a range of functionalities such as stability enhancement, oscillation damping, and ancillary services to the host MTDC grid. This paper extends the functionality of PFC to provide active damping of MTDC grid current oscillations caused by dc side resonance. Three simple and effective active compensators integrated with the control scheme of the PFC are proposed. A comprehensive small-signal model of the MTDC grid is developed. Eigenvalue and sensitivity analyses are conducted to evaluate the damping capability of the proposed compensators and assess their dynamic coupling with PFC control loops. Based on a detailed model of a five-terminal high-voltage dc grid, simulation results are provided to evaluate the performance and effectiveness of the proposed active compensators. The results showed the effectiveness of the proposed compensation schemes to increase the damping of the MTDC grid and enhance its dynamic response.

A Breaker Failure Detection Method Based on Nonlinear Parameter Estimation for Modular Hybrid DC Circuit Breakers

Modular hybrid dc circuit breakers (DCCBs) have been widely employed to protect multiterminal high-voltage dc (MT-HVdc) grids. However, breaker failures still pose a challenge to the protection of MT-HVdc grids, especially for a single module failure of breakers. To this end, a breaker failure detection method based on nonlinear parameter estimation is presented. This proposed method consists of three parts. The first is a signal preprocessing method that eliminates the effect of the RC circuit of the hybrid DCCB. The second is an offline optimal parameter acquisition method that has an improved numerical model of the metal–oxide varistor (MOV) and combines the Levenberg–Marquardt (LM) algorithm with the particle swarm optimization (PSO) algorithm. The third is an online breaker voltage error calculation that estimates the breaker voltage based on the improved numerical MOV model to detect the partial failure of the modular hybrid DCCB. The performance of the proposed algorithm is evaluated by both power system computer aided design (PSCAD)/electro-magnetic transient design and control (EMTDC) tests and field tests. The study results prove the effectiveness of the proposed method for the partial failure of the modular hybrid DCCB considering different faulty conditions and the aging of MOVs.

Non-Unit Line Protection Method for Multi-Terminal Hvdc System Based on Variational Mode Decomposition and Energy Entropy

Non-Unit Line Protection Method for Multi-Terminal Hvdc System Based on Variational Mode Decomposition and Energy Entropy

Technical Assessment of Hybrid HVDC Circuit Breaker Components under M-HVDC Faults

One of the technical challenges that needs to be addressed for the future of the multi-terminal high voltage direct current (M-HVDC) grid is DC fault isolation. In this regard, HVDC circuit breakers (DCCBs), particularly hybrid circuit breakers (H-DCCBs), are paramount. The H-DCCB, proposed by the ABB, has the potential to ensure a reliable and safer grid operation, mainly due to its millisecond-level current interruption capability and lower on-state losses as compared to electromechanical and solid-state based DCCBs. This paper aims to study and evaluate the operational parameters, e.g., electrical, and thermal stresses on the IGBT valves and energy absorbed by the surge arrestors within H-DCCB during different DC fault scenarios. A comprehensive set of modeling requirements matching with operational conditions are developed. A meshed four-terminal HVDC test bench consisting of twelve H-DCCBs is designed in PSCAD/EMTDC to study the impacts of the M-HVDC grid on the operational parameters of H-DCCB. Thus, the system under study is tested for different current interruption scenarios under a (i) low impedance fault current and (ii) high impedance fault current. Both grid-level and self-level protection strategies are implemented for each type of DC fault.

Voltage control of common capacitor and self-faults analysis of DC current flow controllers for meshed multi-terminal HVDC grids

In this paper, the control methodology of the common capacitor voltage and the DC current flow controller (DC CFC) under self-fault conditions are investigated with the following two contributions. (1) A selection method of the common capacitor voltage based on various specific control requirements with explicit control margins of the DC line currents is proposed with analytic derivations of two-line/three-line DC CFCs in the meshed three-terminal/four-terminal high-voltage direct current grids (HVDC), respectively. (2) The impact of different self-faults of the DC CFC on the control performance of the DC line currents is analyzed with a simplified topology with unidirectional current flow presented. Case studies are conducted in the time-domain simulation environment PSCAD/EMTDC and the results justify the effectiveness of the theoretical analysis. The proposed common capacitor voltage control method and self-fault analysis would be useful for practical applications of the DC CFC under both steady and transient states.

Global Optimal Stabilization of MT-HVDC Systems: Inverse Optimal Control Approach

The stabilization problem of multi-terminal high-voltage direct current (MT-HVDC) systems feeding constant power loads is addressed in this paper using an inverse optimal control (IOC). A hierarchical control structure using a convex optimization model in the secondary control stage and the IOC in the primary control stage is proposed to determine the set of references that allows the stabilization of the network under load variations. The main advantage of the IOC is that this control method ensures the closed-loop stability of the whole MT-HVDC system using a control Lyapunov function to determine the optimal control law. Numerical results in a reduced version of the CIGRE MT-HVDC system show the effectiveness of the IOC to stabilize the system under large disturbance scenarios, such as short-circuit events and topology changes. All the simulations are carried out in the MATLAB/Simulink environment.

Transient overvoltage characteristics in submarine cables of multi-terminal flexible HVDC transmission system

The overvoltage transient characteristics of submarine cables are one of the key technologies for submarine power transmission. This article takes the submarine cable of Zhoushan Multi-terminal Flexible HVDC Transmission Project as the research object. The MMC-HVDC simulation models at both ends of converter stations are built in PSCAD. The parameters and equivalent models of the flexible HVDC cables are determined, and the submarine DC cable operating overvoltage is calculated by using statistical methods. The simulation results show that the maximum overvoltage of the submarine cable, that is, the maximum overvoltage of the conductor to sheath, the sheath to the armour, and the armour to the ground are 400 kV, 7.27 kV and 0.45kV respectively. The maximum overvoltage when it occurs faults on grid-side or valve-side appears in the middle of the submarine cable. Maximum overvoltage of HVDC single pole grounding fault appears at the connection point of land cable and submarine cable. The simulation results and the discovery of features in this article provide reference in the design, selection, and testing of submarine power transmission equipment.

Efficient model-based DC fault detection and location scheme for multi-terminal HVDC systems with voltage source converter

Efficient model-based DC fault detection and location scheme for multi-terminal HVDC systems with voltage source converter

Equivalent Impedance Calculation Method for Control Stability Assessment in HVDC Grids

A major challenge in the development of multi-vendor HVDC networks are converter control interactions. While recent publications have reported interoperability issues such as persistent oscillations for first multi-vendor HVDC setups with AC-side coupling, multi-terminal HVDC networks are expected to face similar challenges. To investigate DC-side control interactions and mitigate possible interoperability issues, several methods based on the converters’ and DC network’s impedances have been proposed in literature. For DC network’s impedance modelling, most methods require detailed knowledge of all converters’ design and controls. However, in multi-vendor HVDC networks, converter control parameters are not expected to be shared due to proprietary reasons. Therefore, to facilitate impedance-based stability analyses in multi-vendor MTDC networks, methods that do not require the disclosure of the existing converter controls are needed. Here, detailed impedance measurements can be applied; however, they are time-consuming and require new measurement for a single configuration change. This paper proposes an equivalent impedance calculation method suitable for multi-vendor DC networks, which for available black-box models or converter impedance characteristics can be modularly applied for various network configurations, including different control settings and operating points, while significantly reducing the required time for obtaining an equivalent DC network impedance.