High-voltage Dc Grids Research Articles

Distributed Voltage Regulation and Automatic Power Sharing in Multi-Terminal HVDC Grids

Conventional droop control methods for power-sharing in a multi-terminal high voltage DC (MTDC) grid lead to voltage deviation from the nominal value. Moreover, the power-sharing is inaccurate in the droop-controlled MTDC system. This paper proposes a secondary controller with a distributed architecture to compensate the voltage deviation and to achieve equal power sharing automatically among the converter stations. A distributed observer is used to estimate the average voltage of the converter stations. The proposed distributed controller and observer use local measurements of output powers and voltages, and only communicate with neighboring stations. Therefore, the requirement of the global information in the centralized secondary control schemes is eliminated, which reduces the communication requirement and improves the reliability of the MTDC grid. The case studies illustrate that the proposed distributed controller regulates the stations' average voltage and shares the power mismatch accurately. Impact of the communication time-delays and the control gain on stability of the MTDC grid are also investigated using the Lyapunov-KrasovskiiLMI condition. It is shown that reducing the control gain can stabilize the MTDC grid in the case of large communication time-delays.

A Method to Calculate Short-Circuit Faults in High-Voltage DC Grids

Accurate fault calculation is vital for the parameter design and protection research of high-voltage DC (HVDC) grids. This paper proposes a general method to calculate short-circuit faults in HVDC grids, and the method is based on the premise that the modular multilevel converter (MMC) blocking and the current interruption have not occurred. The MMC is based on the average value model, and the MMC current source can reflect the impacts of the AC system and MMC control mode. The fault calculations based on variable and constant MMC current sources are presented. The numerical inversion of the Laplace transform is used to obtain the voltages and currents in the time domain. The proposed method has the following merits: 1) it can accurately calculate voltages and currents while considering the fault-generated traveling wave effect and impacts of the AC system and MMC control mode; 2) it is more time efficient than the electromagnetic transient simulation; and 3) it is applicable to HVDC grids with different configurations, such as symmetrical bipolar and asymmetrical monopolar. The proposed method is validated by comparisons with previous calculation methods and PSCAD/EMTDC simulation.

Electric Contact Material Selection for Medium and High Voltage DC Circuit Breakers

Medium and high voltage DC grids are increasingly proposed for terrestrial, shipboard and aircraft power systems. However, multi terminal DC networks can only be realized with the development of a DC circuit breaker. Due to the difficulty in breaking DC currents, hybrid circuit breakers—which consist of a non-arcing fast mechanical disconnect switch (FMS) in parallel with a solid state switch—is the most promising solution. The stresses experienced by the electric contacts of FMS are different from those of AC and low voltage DC breakers and disconnect switches. The choice of contact material has a significant impact on the performance of the DC circuit breaker. The Ashby method is used to systematically identify the best suited contact materials by translating the requirements of FMS contacts into objectives and constraints and deriving material indices for each objective. Minimizing power loss, wear and overheating of contacts are identified as the key objectives. The results suggest that copper-based alloys and compounds are more suitable than silver based alloys and compounds and other contact materials.

Robust decentralized approach to interaction mitigation in VSC-HVDC grids through impedance minimization

An interconnection of independently designed voltage source converters (VSCs) and their control in high voltage DC grids can result in network-level detrimental interactions and even instability. In many cases, vendors are not willing to share proprietary information about their designs in a manner that allows the entire system to be designed considering the collective dynamics. Besides, such an endeavour is not realistic. In this paper, robust network-level global controllers are proposed to decouple interacting VSCs through an impedance shaping technique. Particularly, the global controllers are designed without any need to establish the internal controller structure of any VSC which is often proprietary. Rather, the global controllers rely on the externally measurable feedback impedance of each VSC in stand-alone and the rest of the system. It is demonstrated how robust convex optimization framework can be exploited to robustly shape global feedback impedances to obtain a decoupled network. The synthesized controllers are validated through nonlinear simulations of the physical model.

Protection of High-Voltage DC Grids Using Traveling-Wave Frequency Characteristics

Fast dc fault detection in high-voltage dc (HVdc) grids is paramount due to the possibility of attaining a severely high fault current within few milliseconds. This article proposes a new traveling-wave (TW) based protection scheme to detect and locate faults in HVdc grids. In contrast to the existing TW-based protection and fault location approaches, the proposed scheme, first, utilizes only the first locally measured TW after the inception of a fault, and, second, focuses on the frequency content and polarity of the TW, rather than its arrival time. The direction of the TW is identified based on the pattern of the current TW waveform. A frequency spectral ratio of the current TW is proposed to identify the fault zone and fault location with high accuracy. The proposed method ensures a detection speed of 50 $\mu$ s on overhead lines and 500 $\mu$ s on underground cables. A remote backup scheme is coordinated with the primary protection and operates within 1.05 ms, which meets the speed requirement of HVdc grids. The performance of the proposed scheme has been, first, assessed using the CIGRE B4 HVdc test grid under various conditions, and, second, compared to existing single- and two-terminal TW methods.

Damping HVDC Circuit Breaker With Current Commutation and Limiting Integrated

The fault current limiting in high-voltage dc (HVdc) grids is critical to reducing the breaking stress of dc circuit breaker and improving the breaking reliability. However, the existing HVdc circuit breakers rarely consider the rapid limiting of fault current in the breaking process. In this article, a novel HVdc circuit breaker integrating current commutation and damping is presented, which can realize fast current limiting and interruption while preserving large breaking capability and low cost. Simulation analysis of current commutation capability is carried out to determine the optimal parameters in the current damping module. Then, the fault current breaking performance of the proposed HVdc breaker is evaluated and compared with conventional hybrid HVdc breaker theoretically. Finally, a low-power prototype of the current damping dc breaker is built and tested to verify the validity of the proposed topology.

On Feasibility of Autonomous Frequency-Support Provision from Offshore HVDC Grids

Offshore multi-terminal high-voltage dc (HVDC) grids are emerging as a technical reliable and economical solution to transfer more offshore wind energy to inland power grids. It is also envisaged that the offshore HVDC grids pave the way for both offshore wind participation and sharing inland frequency reserves for efficient power systems’ frequency control. The frequency control mechanism in an HVDC grid can be either centralized or decentralized. An autonomous frequency control (AFC) is a decentralized control, which does not require communication links between dc grid terminals. In the AFC, the dc-link voltage is used as a medium to reflect the inland frequency change to other terminals. The AFC has some technical and non-technical challenges, especially when the dc grid is connected to more than one shore ac systems. Among challenges of the AFC are the deficiency in meeting grid code requirements, adverse reaction of converters, dc voltage variations, difficulties of offshore wind participation in power markets. This paper proposes a new methodology of frequency control which uses both centralized control and AFC simultaneously. The proposed methodology shall improve system security and mitigate the listed problems of the AFC. A four-terminal dc grid is described and used for analysis and demonstration of the proposed methodology.

An Auxiliary Fault Identification Strategy of Flexible HVDC Grid Based on Convolutional Neural Network With Branch Structures

Lightning disturbance may be misjudged as dc fault by the primary protection in the flexible high voltage dc (HVDC) grid. To solve this problem, an auxiliary fault identification strategy based on convolutional neural network with branch structures (BR-CNN) is proposed in this paper. In the proposed scheme, the voltage and current characteristic matrix is constructed as the input matrix of BR-CNN model and the output categories include positive pole-to-ground (PTG) fault and lightning disturbance. Voltage and current branches are constructed to extract high-level local features of input data layer by layer, and main branch is designed to realize the comprehensive utilization of voltage and current information. Through autonomous learning of the model, the nonlinear mapping relationship between input and output is constructed. The method only uses the single-terminal quantities, and can be used as an auxiliary criterion to improve the reliability of the primary protection. The test results verify the effectiveness of the method, and the recognition accuracy is better than the traditional classification models.

Short-circuit fault current-limiting characteristics of a resistive-type superconducting fault current limiter in DC grids

Resistive-type superconducting fault current limiters (R-SFCLs) can limit short-circuit fault current and help DC circuit breakers (DCCBs) to isolate short-circuit faults in medium voltage and high voltage DC (HVDC) systems. SFCLs are always used with circuit breakers to break short-circuit current in power systems. The interruption characteristics of the DCCBs influence the current-limiting characteristics of the SFCLs. However, there has been a lack of investigation into the current-limiting and interruption characteristics when R-SFCLs are combined with mechanical-type DCCBs (M-DCCBs) based on the self-excited oscillation interruption principle in medium voltage and HVDC systems. The objective of this paper is to obtain the short-circuit fault current-limiting characteristics of a designed R-SFCL combined with an M-DCCB in DC grids. This paper describes the design of a 10 kV DC R-SFCL which was tested in combination with an M-DCCB in a DC circuit in order to obtain the quenched resistance and the required current interruption characteristics. Besides, a modular multilevel converter-based multi-terminal direct current (MMC-MTDC) grid was used to investigate the current-limiting and interrupting characteristics of the R-SFCL and the M-DCCB in a realistic HVDC grid. The experimental results show that in the 10 kV DC circuit the R-SFCL combined with the M-DCCB limited a 10 kA DC to the peak value of 1416 A and then interrupted this limited current successfully. And unlike other methods of DC interruption, this did not create overvoltage after interruption. The highest voltage on the R-SFCL is the system voltage during interruption. This means that the R-SFCL can effectively delay the voltage drop in MMC-MTDC grids, and provide enough time for the M-DCCB to interrupt the limited current. Thus, a combination of R-SFCLs and M-DCCBs might be an effective and reliable method to interrupt short-circuit fault currents in medium voltage and HVDC grids.

A Reliable Protection Scheme for Fast DC Fault Clearance in a VSC-Based Meshed MTDC Grid

A multi-terminal high voltage DC (MTDC) grid, is an optimum and cost-effective transmission network to minimize the energy crisis worldwide largely. However, the core demand for a fast DC protection scheme with an extraordinary strict fault clearance time of a few milliseconds, is a key research gap in this network, holding back its development and scalability. To bridge this gap, this paper proposes a novel protection scheme for a meshed MTDC grid. The main goals of the scheme include accurate discrimination of a faulty line, rapid fault detection, fault location estimation, significant fault current reduction, and fully selective isolation of only the faulted line, while continuing normal power flow in the healthy grid zones. Reliability of the scheme for long and extra-long-distance power transmission is increased by aiding the differential protection and Type-D traveling wave (TW)-based algorithms utilizing the distributed optical current sensing technology with the other auxiliary methods and backup plans. These auxiliary methods include independent discrete wavelet transform (DWT), current derivative polarity principles with a minimum sample (short time) window, overcurrent relays, and AC circuit breakers (ACCBs). A faulty segment of a transmission line is accurately discriminated from the healthy ones by measuring a series of multi-point differential currents on it. A faulty line at a particular DC node is accurately discriminated using the differential protection by measuring the current flowing into or out of each line from each side at every node to obtain the algebraic sum. The current sum of a real-time local transient data and a delayed remote data is compared to a preset threshold level. DC fault current is significantly reduced below the breakable levels by coordinating bidirectional hybrid DC circuit breakers (HDCCBs) with the active and passive fault current limiters (FCLs) and the half bridge-VSC-based modular multilevel converters (MMCs). The proposed concepts are successfully verified by the simulation results under a variety of fault scenarios and are found to be accurate.

An Improved Non-Unit Traveling Wave Protection Method With Adaptive Threshold Value and Its Application in HVDC Grids

The performance of the traditional derivative-based non-unit traveling wave protection (TWP) in high voltage dc (HVDC) grids shows the problem of low-sensitivity when high-impedance faults occur. In this article, the characteristics of the fault initial traveling wave are analyzed first, two conclusions can be drawn: i) the distortion degree of the fault initial traveling wave is related to the fault location, ii) the amplitude of the fault initial traveling wave is related to the fault impedance. Then a zero-sequence current fitting method is proposed to obtain parameters respectively reflecting fault impedance and fault distance. When a fault occurs, a fault information extraction method based on the fitting parameters will give the estimation interval of the fault impedance, which provide reference for the adaptive threshold value selection of TWP. In this way, the influence of the fault impedance on TWP can be eliminated. A ±400 kV modular multilevel converter (MMC) HVDC grid is built in PSCAD/EMTDC. The verification of the proposed method considers different installation positions of the current limit inductor. The simulation results show that the proposed method can fast and correctly identify the dc line faults and effectively improves the protection sensitivity in detecting high-impedance faults.

Multiple-Modular High-Frequency DC Transformer With Parallel Clamping Switched Capacitor for Flexible MVDC and HVDC System Applications

The direct-current transformer (DCT) plays a key role in bus connection, power transmission, and electrical isolation in medium-voltage (MV) and high-voltage (HV) dc power grids. This article proposes a multiple-modular high-frequency DCT with a parallel clamping switched capacitor (CSC-DCT) for connecting dc buses. The proposed CSC-DCT is mainly composed of several modified dual-active bridges (DABs) where the CSC interfaces are employed to connect DABs. Each CSC is connected to the corresponding H-bridge of DAB in parallel; thus, the concentrated capacitors of the traditional DAB can be eliminated, and the capacitor voltages in CSC can be regulated. The CSC-DCT not only has the universal advantages of modular converters, such as flexible control, high switching frequency and power destiny, and easy installation and debugging, but also obtains direct power conversion, dc fault-handling capability, online redundant design, and capacitor voltage self-balance capability. All of the merits improve the reliability and power quality of dc power grids and simplify the design of the control system. In addition, CSC-DCT has the ideal ON-state current and soft-switching performance, which lead to lower switching current and high-frequency-link (HFL) current stress, as well as higher efficiency. The topology, modulation, control structure, voltage, and current characteristics and switching behaviors of CSC-DCT are investigated in detail, and the experiment results verify the correctness and effectiveness of the analysis and the proposed schemes.

Adaptive droop control strategy for autonomous power sharing and DC voltage control in wind farm‐MTDC grids

This study presents a novel adaptive droop control (ADC) strategy for power-sharing in a multi-terminal high-voltage DC grid while maintaining a desirable DC voltage level. The ADC scheme can share the power without burdening any power converters during one or more converter outages and huge power imbalance situations. To improve the effectiveness of the power sharing and DC voltage deviation control, the ADC is also implemented for wind farm converters. This is achieved through a derated mode operation of wind farms. The performance and effectiveness of the proposed control approach are evaluated in several case studies based on the specific type of converter outage under the various outage scenarios. The proposed method is validated on a five-terminal CIGRE B4 DC grid test system.

Impact of Primary Frequency Control of Offshore HVDC Grids on Interarea Modes of Power Systems

Offshore high-voltage DC (HVDC) grids are developing as a technically reliable and economical solution to transfer more offshore wind power to onshore power systems. It is also foreseen that the offshore HVDC grids pave the way for offshore wind participation in power systems’ balancing process through frequency support. The primary frequency control mechanism in an HVDC grid can be either centralized using communication links between HVDC terminals or decentralized by the simultaneous use of DC voltage and frequency droop controls. This paper investigates the impact of both types of primary frequency control of offshore HVDC grids on onshore power system dynamics. Parametric presentation of power systems’ electro-mechanical dynamics and HVDC controls is developed to analytically prove that the primary frequency control can improve the damping of interarea modes of onshore power systems. The key findings of the paper include showing that the simultaneous use of frequency and DC voltage droop controls on onshore converters results in an autonomous share of damping torque between onshore power systems even without any participation of offshore wind farms in the frequency control. It is also found that the resulting damping from the frequency control of offshore HVDC is not always reliable as it can be nullified by the power limits of HVDC converters or wind farms. Therefore, using power oscillation damping control in parallel with frequency control is suggested. The analytical findings are verified by simulations on a three-terminal offshore HVDC grid.

A Novel DCCB Reclosing Strategy for the Flexible HVDC Grid

The overhead line will be widely applied in the future flexible high-voltage dc (HVDC) grid. Correspondingly, effective reclosing strategy which can identify the fault property must be configured for the dc circuit breaker (DCCB) to improve the system power supply reliability. However, if the DCCB is reclosed directly, large second overcurrent may occur to damage the vulnerable devices in the system. Therefore, the inherent voltage characteristic difference between different dc faults (nonpermanent or permanent) is analyzed in the paper. And then a novel DCCB reclosing strategy for the flexible HVDC grid is proposed. Compared with the method which recloses DCCB directly, the proposed strategy can avoid the second damage under permanent fault condition, because it can identify the fault property without completely reclosing the DCCB (only the residual dc current breaker is reclosed before fault property identification). Moreover, compared with the typical sequential auto-reclosing strategy, the proposed strategy significantly reduces the additional control and design requirements on the DCCB. Finally, simulation cases based on the PSCAD/EMTDC are carried out to verify the correctness of the theoretical analysis and the superiorities of the proposed reclosing strategy.

The Enhanced DC Fault Current Calculation Method of MMC-HVDC Grid With FCLs

This paper introduces an enhanced dc fault current calculation method for modular multilevel converter-based high-voltage dc (HVdc) grid. The new method considers the fast switching of the fault current limiters (FCLs) and the nonlinear characteristics of the metal–oxide arresters (MOAs). The two peak-valley points of inflection in the dc fault current curves resulting from the insertion of the inductors and the energy dissipation in the MOAs are discussed. The two points are selected as important indicators in the design of the coordinated protection scheme and switching logics. Then, the impact of the total dc reactance distribution in the dc reactor and the FCL reactor, and the initial inductor current values on these two points are analyzed. Finally, the complete fault current calculation method including the action of dc circuit breakers is proposed. The method is validated by electromagnetic transient simulations and has shown good applicability on both the overhead line-based and cable-based HVdc grids.

Real-time boundary wavelet transform-based DC fault protection system for MTDC grids

In this paper, the real-time boundary wavelet transform (RT-BWT) method is proposed for dc fault protection in multi-terminal high voltage dc (MTDC) grids. The proposed method is developed and tested in a real-time simulation platform. A wide range of test scenarios have been adopted considering ac and dc fault conditions, including variations in fault resistance, fault event distance, dc reactor inductance, external fault events, and changes in topology. The results and statistical analysis show the effectiveness and performance of the RT-BWT method in providing fast, sensitive, selective, and robust dc fault protection, even without the need for communications.

Comprehensive study on fault‐ride through and voltage support by wind power generation in AC and DC transmission systems

This study exploits the impact of different low voltage ride-through (LVRT) methods and equipment on both the wind energy elements and the grid including wind turbine/farm ability to provide reactive compensation and maintain controllability during faults. The potential of using SFCL as an alternative LVRT equipment is preliminary studied. The study also exploits some severe scenarios that could face a multi-terminal high-voltage DC (HVDC) network. The influences of AC faults and control errors are examined. Results show limited deviations between the adopted LVRT methods. The wind turbine has to contribute to the stability of the AC collection grid of the wind farm, but it does not influence the grid, as both are decoupled through the multi-terminal HVDC grid. The implemented test systems and the examined events are developed in Matlab/Simulink and DIgSILENT.

A Thyristor Full-Bridge-Based DC Circuit Breaker

DC circuit breakers (DCCBs) are vital for the safe and continuous operation of large high-voltage dc grids. Insulated-gate bipolar transistor-based hybrid DCCBs (IHCBs) can interrupt currents quickly and have good operational characteristics. Thyristor-based hybrid DCCBs (THCBs) are superior to IHCBs in terms of interruption capacity because of the large surge current capacity of thyristors. Nevertheless, existing THCBs have limitations regarding one or more of the following characteristics: pre-activation, fast reclosing, quick interruption of small current and no requirement for an additional pre-charge source. This paper proposes a thyristor full-bridge-based dc circuit breaker (TFCB) with all favorable characteristics mentioned above. The TFCB contains a main conductor, a main breaker (MB), an auxiliary branch, and an energy absorber. After an interruption, the capacitor in the TFCB can restore its initial voltage quickly to guarantee that the TFCB is available for the next interruption. Quick interruption of small currents is achieved using the additional charge current provided by the auxiliary branch. Pre-activation is achieved using the conduction path in the MB. The parameter designs are analyzed to ensure that the TFCB reliably interrupts currents without damaging itself. The proposed TFCB is validated using prototype experiments and PSCAD/EMTDC simulations.

A Self-Balanced Bidirectional Medium-/High-Voltage Hybrid Modular DC–DC Converter With Low-Voltage Common DC-Link and Sequential Charging/Discharging of Submodules Capacitors

This paper proposes a medium-/high-voltage high-power hybrid modular dc–dc converter to interconnect two different dc-voltage levels in medium-/high-voltage dc grids. The converter consists of half-bridge submodules (SMs) rated at a moderate voltage level, i.e., series connection of semiconductor devices is avoided while connecting two high dc-voltage levels. The proposed architecture provides self-balancing operation, i.e., unlike conventional modular converters, no need for capacitor voltage measurement, which enhances the system reliability and simplicity. Sequential charging/discharging of SMs capacitors is adopted in the proposed approach to ensure a self-balanced operation. The proposed architecture can provide a high conversion ratio as well as power transfer in both directions, i.e., bidirectional dc–dc converter. Detailed illustration of the proposed approach operational concept, design, and overall control system is presented. Simulation and experimental results are presented to show the viability of the proposed configuration and validate the claims.