Multi-terminal Direct Current Transmission Research Articles

Dual-layer optimization planning for offshore wind farms considering power system security

The vigorous development of offshore wind farms (OWFs) has further promoted the utilization of wind energy. Owing to the demand for low-carbon environmental protection, the increase in the scale of OWF constructions has also had a significant impact on the power system, which cannot be ignored. Specifically, the uncertainty and volatility of wind-power generation should be considered in power system optimization planning schemes. Therefore, this study proposes a probabilistic power-flow analysis index construction method that employs the static-security region theory to optimize the OWFs–grid-connection topology, including the number, location, and capacity of the public connection points connected to the onshore power grid. This method can be used to calculate the probability indicators of power grid insecurity for OWFs–grid-connected systems with different network topologies. Furthermore, a dual-layer multi-objective programming model that considers the economics and stability for integrated planning of electrical collector and multiterminal direct current transmission systems is established. The proposed model comprises two inner-layer models and one outer-layer model. The inner-layer models are used to construct the collection and transmission topologies of the OWFs–grid-connected system that minimize the probability of grid insecurity and the planning cost of the outer-layer model. Finally, the effectiveness of the proposed OWFs planning model was verified using an improved IEEE 24-bus system that can be used for topology planning of electrical collector and transmission systems while ensuring grid stability.

Interaction Analysis of Multi-terminal Direct Current Transmission Systems Connected to Wind Farm: Determining the Optimal Range of Controller Coefficients

Interaction Analysis of Multi-terminal Direct Current Transmission Systems Connected to Wind Farm: Determining the Optimal Range of Controller Coefficients

Optimal Power Flow for Hybrid AC/DC Electrical Networks Configured With VSC-MTDC Transmission Lines and Renewable Energy Sources

This article presents the single objective optimal power flow (OPF) formulation incorporating both renewable energy sources, and voltage source converter-based multiterminal direct current transmission lines, simultaneously. To solve the formulated OPF problem, powerful metaheuristic optimization algorithms including adaptive guided differential evolution, marine predators algorithm, atom search optimization, stochastic fractal search (SFS), and fitness-distance balance-based SFS (FDB-SFS) are employed. The performance of the algorithms is tested for the minimization of fuel cost, pollutant emissions of thermal generators, voltage deviation, and active power loss in a modified IEEE 30-bus power network. The simulation results give that FDB-SFS achieved the best results on the fuel cost (786.5361 $/h), the fuel cost with valve point effect (815.6644 $/h), and the fuel cost with emission-carbon tax (820.5991 $/h). In addition, FDB-SFS reduced voltage deviation and active power loss values by 14.2587% and 6.7438% compared to SFS. The nonparametric Wilcoxon and Friedman statistical test results confirmed that FDB-SFS is an effective and robust algorithm that can be used in the optimization of the introduced OPF problem.

Robust damping control for VSC-MTDC system in offshore applications considering disturbance uncertainty

Multi-terminal direct current transmission based on voltage source converter (VSC-MTDC) has become an ideal solution for long distance offshore wind farm access systems. However, the integration of VSC-MTDC with offshore wind farms is vulnerable to various disturbances, which can lead to instability. To address this issue, this paper presents a robust damping control for VSC-MTDC in offshore applications that accounts for disturbance uncertainty. First, a small-signal VSC-MTDC model is developed considering the coupling characteristics of the cable, and the DC cable model is suitable for both unipolar and bipolar structures. Secondly, the key parameters affecting the oscillation mode are analyzed. On this basis, the paper proposes a robust damping strategy that adapts to the changing operating conditions of the system without impairing steady-state performance, while enhancing transient performance. Finally, the paper builds a VSC-MTDC with offshore wind farms on MATLAB/Simulink to validate the correctness and effectiveness of the proposed scheme. The simulation results demonstrate that the proposed control strategy exhibits better robustness and suppression ability under multiple types of disturbances compared to traditional additional damping control.

Offshore MTDC Transmission Expansion for Renewable Energy Scale-up in Korean Power System: DC Highway.

In this study, we analyzed the impact of multi-terminal direct current (MTDC) system on the integration of renewable energy resources into the Korean power system. Due to the large-scale renewable energy plants planned to be integrated into the power system, line congestion is expected in the southern part of power system. Given the difficulty in constructing AC transmission lines due to social conflicts, we proposed an alternative solution using an offshore multi-terminal DC offshore transmission system. Firstly, we calculate the effective renewable energy plant generation capacity based on annual wind and solar radiation data. Next, we conduct PSS/E simulations to minimize future line congestion in the Korean power grid. The offshore terminal is designed to transfer the power generated in southern Korea and is verified using different terminal rating cases. The simulation result, including contingency analysis, demonstrate that transferring 80% of the generated renewable power achieves the best line flow condition. Therefore, the MTDC system is a possible candidate for integrating future renewable energy systems into the Korean power grid.

Travelling wave fault location approach for hybrid LCC-MMC-MTDC transmission line based on frequency modification algorithm

Line-commutated-converter (LCC) based and modular-multilevel-converter (MMC) based hybrid high-voltage direct-current (HVDC) transmission technology is already applied in actual power grid, which has the ability to avoid commutation failure issue. Besides, aiming to acquire high transmission capacity, the multi-terminal direct-current (MTDC) approach is adopted. Although the travelling wave (TW) fault location approach is widely applied in actual power grid, its adaptability for hybrid MTDC topology and transmission distortion are not well considered. Therefore, a novel TW fault location approach for LCC-MMC-MTDC transmission line based on frequency modification algorithm is proposed in this paper. Firstly, the complete frequency modification algorithm suitable for MTDC transmission system is described. Secondly, the novel TW fault location approach and its implementation scheme are given. Eventually, via PSCAD/EMTDC, a series of case studies are illustrated based on typical ±400 kV hybrid LCC-MMC-MTDC simulation model, which implying high robustness against fault location and grounding resistance issues.

Travelling wave fault location for LCC–MMC–MTDC transmission lines based on frequency-dependent characteristic

The paper studies the fault location principles of multi-terminal direct-current (MTDC) hybrid transmission systems based on line-commutated converter (LCC) and modular multi-level converter (MMC). Two issues should be addressed: i) the travelling wave (TW) fault location principle adaptable for the hybrid multi-terminal transmission system; and ii) the algorithm of obtaining frequency-dependent characteristic of the TW signal. In the paper, a novel TW fault location principle adaptable for the LCC–MMC–MTDC hybrid transmission lines is developed based on the frequency-dependent characteristic of TW signal. Firstly, the frequency-dependent characteristic of HVDC transmission line is studied. Secondly, a novel TW fault location principle is proposed, comprised of TW arrival time detection, TW propagation velocity calculation, fault section identification and fault location method. A typical ±400kV LCC–MMC–MTDC hybrid system is constructed using PSCAD/EMTDC for case study to validate the accuracy of the proposed principle. Its robustness against fault location, fault resistance and fault type is well demonstrated.

MMC-MTDC transmission system with partially hybrid branches

This paper proposes a hybrid submodule modular multilevel converter (MMC) topology which is suitable for multi terminal direct current (MTDC) transmission systems. Each arm of the proposed MMC topology consists of a half-bridge submodule (HBSM) branch and two parallel full-bridge submodule (FBSM) branches. Comparing with the conventional MTDC transmission system, the proposed topology can selectively block the DC fault current and isolate the corresponding fault line without expensive DC circuit breakers (DCCBs). Thus, the influence range of the DC fault can be reduced and the reliability of the power supply can be improved as well. The corresponding modulation and voltage balancing strategies are developed for the proposed hybrid MMC topology. The feasibility of the proposed topology and control strategy is verified in the MATLAB/ Simulink simulation.

A Distributed Cooperative Control Based on Consensus Protocol for VSC-MTDC Systems

A distributed cooperative control (DCC) strategy based on the consensus protocol is proposed for providing frequency support through voltage source converter based multi-terminal direct current transmission (VSC-MTDC) systems. The DCC consists of a local P-V droop controller and a cooperative agent network for cooperation control. Each agent controller is composed of a frequency regulation subagent (FRA) for frequency support and a load ratio subagent (LRA) for balancing the power sharing amongst the stations connected to the same ac grid. Moreover, to reduce the impact of the communication link failure, a topology evaluating method is proposed to select the suitable topology of the communication network via graph theory. The small signal stability analysis and the steady-state analysis are performed to investigate the stability and the consensus performance of the VSC-MTDC systems with DCC. Compared with decentralized control, the DCC can simultaneously achieve better frequency support and power sharing without leading to a large dc voltage deviation. The effectiveness of the DCC is verified by simulations on a hybrid ac/dc grid with a 7-terminal VSC-MTDC in DIgSILENT/PowerFactory under the load step change event and communication failure event.

A Resonant Hybrid DC Circuit Breaker for Multi-Terminal HVDC Systems

High-voltage direct current (DC) transmission systems and multi-terminal direct current transmission systems are attracting attention for expanding the grid to promote introduction of renewable energy. Fault clearing in DC systems is difficult because there is no zero point of current. Hybrid circuit breakers are suitable for fault clearing in DC systems. Conventional hybrid circuit breakers have a hard-switching path that damages the switch. Hard switching damages the device and produces emissions due to harmonic noise. A novel resonant hybrid DC circuit breaker is proposed in this paper. The proposed circuit breaker reduces the damage to the switching device using soft switching due to the current zero point. The proposed circuit breaker is compared with conventional hybrid circuit breakers using numerical simulations. Interruption times and switching types of circuit breakers were compared. The simulation results of the fault clearing characteristics of the proposed breakers show that the proposed breakers have sufficient performance and are capable of stable reconnections in multi-terminal direct current transmission systems.

Robust Current Sensorless Control of VSC-Based MTDC Transmissions for Integrating Wind Farms

This article presents an observer-based robust current sensorless control (RCSC) scheme for voltage-source-converter (VSC)-based multiterminal direct-current (MTDC) transmission system integrated with wind farms. Perturbation observer-based point of common coupling (PCC) voltage controller and dc-link voltage controller are designed for wind farm station (WFS) and grid-side station (GSS) of MTDC, respectively, for grid integration of multiple wind farms. Perturbation states are defined to describe the interactions of voltage control loops of nonlinear VSC model, and the feedback linearization control is realized based on the state estimations provided by perturbation observers to achieve the decoupled voltage control of MTDC. Neither current sensors nor parameters of MTDC are required in the RCSC scheme, thus achieving the robust sensor fault-tolerant operation of the MTDC system. Simulation and experimental studies are undertaken on a three-terminal MTDC transmission system integrated with two wind farms to verify the control performance of the RCSC scheme under the conditions of wind speed variations, sudden load changes, parameter variations, and current sensor faults.

An IEGT Model for Analyzing the High Current Turn-off Characteristics in DC Circuit Breaker Applications

The direct current (DC) circuit breaker based on semiconductor power devices is one of the critical components in multiterminal flexible high voltage DC transmission technology. A press pack injection enhanced gate transistor (IEGT) can realize a low on-state voltage similar to a gate turn-off (GTO) thyristor while achieving better high current turn-off capability and wider safety operating area than an IGBT. Therefore, an IEGT is a promising candidate for high voltage DC circuit breaker applications. The fault current conducted by devices in DC circuit breaker applications is 4~6 times the device rated current, which brings great risks to reliable operation. Optimizing the device turn-off characteristics and then realizing reliable operation are indispensable parts of DC circuit breaker design. For analyzing the high current turn-off characteristics under the working conditions of a DC circuit breaker, an IEGT model is proposed in this paper. In the model, a simple method for predicting the static-state I-U characteristics is established. In addition, based on the differences in the gate structures of IEGTs and IGBTs and the influence of the gate structure on the nonlinear capacitances between device terminals, the nonlinear capacitances are greatly improved to better describe the turn-off transient process. Subsequently, the model is verified by experiments. The model is expected to be a convenient simulation tool for designing a high voltage DC circuit breaker with IEGTs.

Finite-time stabilisation of some power transmission systems

This paper presents the finite time stabilisation strategy of two problems: the first one is the control of the high voltage direct current based on voltage source converter, while the second is the control of the multi-terminal direct current transmission systems. Subject to finite-time control design strategy, a linear and nonlinear dynamic model are derived based on the state-space description. Furthermore, continuous or discontinuous finite-time feedbacks are proposed to ensure the tracking of the output variables and to enhance the stability of the studied high voltage direct current system. In addition, the proposed control strategy is extended for the multi-terminal direct current system. A comparative study between various approaches (Proportional-Integral control, continuous or discontinuous stabilising finite-time controllers and control by backstepping) is presented and shows that the finite-time continuous feedback gives an excellent transient response.

Small-Signal Stability Analysis of Interaction Modes in VSC MTDC Systems with Voltage Margin Control

Multi-terminal Direct Current Transmission (MTDC) is an emerging and promising technology for the transmission of electricity and the main initiator of the development of MTDC grids is offshore wind generation. However, prior to their construction, a thorough investigation of different aspects of their implementation and operation is required. In this research, an MTDC grid with voltage margin control consisting of voltage source converters (VSCs) and a high frequency cable model was implemented in Matlab/SIMULINK (R2015b, The MathWorks, Inc., Natick, MA, USA). Small-signal stability analysis was carried out to investigate the sensitivity of the grid’s interaction modes to the operating point, the structure of the grid, and the selection of the voltage controlling converter. Based on the findings of these analyses, a strategy for droop control method is proposed and demonstrated.

An Expert System for Real-Time Control of a Multi-Terminal Direct Current Transmission System

This paper presents a real-time expert system (RTES) for on-line monitoring and control of a Multi-terminal direct Current Transmission (MTDC) system. The expert system comprises of a two level black board architecture. This is implemented on a four node transputer-based multiprocessing system along with an intelligent high speed data acquisition system (IDAS). The dc converters considered in this paper are based on sinewave pulse width modulation (SPWM) technology. In this case the triggering strategy is more complex, but the resulting system control yields a number of advantages which cannot be realized otherwise. A novel firing pulse generator (FPG) hardware with suitable inter face to the IDAS makes it possible to perform this task in real-time.