Multi-terminal High-voltage DC Grid Research Articles

A Multi-Port DC Power Flow Controller Integrated With MMC Stations for Offshore Meshed Multi-Terminal HVDC Grids

Multi-terminal dc (MTDC) systems have the potential to facilitate the integration of offshore wind farms. A dc power flow controller (DPFC) is necessary for meshed multi-terminal high voltage dc (HVDC) grids to regulate the power flow in dc lines. However, DPFCs based on modular multilevel converters (MMCs) require two dc-ac conversion stages and an extra ac transformer, which increases the cost and loss for offshore platforms. This paper proposes a multi-port DPFC that is integrated internally into the offshore MMC station, eliminating the need for an extra transformer and reducing dc voltage ripple. Additionally, the full bridge submodules (FBSMs) in the DPFC can further reduce the number of FBSMs required in the MMC station for dc fault blocking. The proposed DPFC is composed only of cascaded FBSMs connected to the arms of the offshore MMC station, which can exchange power directly with the MMC valve to achieve power balance. By using the MMC station, the proposed DPFC requires a small kVA rating, approximately 1%-5% of the total system power rating. The performance of the proposed DPFC is validated through simulation of a hybrid system of the IEEE 39 bus and the Cigre HVDC benchmark, and also through down-scale experimental testing.

Traveling Wave-Based Primary Protection and Fault Localization Scheme for MTDC Grid Considering IEC 61869-9 Measurement Standard

Traveling wave (TW) based protection and fault localization requires high sampling frequency measurements (500 kHz - 1 MHz) and are non-compliant to the IEC 61869-9 measurement standards, which allows maximum sampling frequency of 96 kHz for DC application. This work proposes a novel low sampling frequency (50 kHz) TW based primary protection and real-time DC fault localization technique for multi terminal high voltage DC (MTDC) grid which is compliant with the aforementioned measurement standard. In the proposed MTDC protection scheme, a TW arrival time (TWAT) is estimated under low sampling frequency measurement using cascaded combination of modified mathematical morphology gradient (MMG), sobel operator and linear regression tool. Subsequently, the TWAT information at both DC link terminals is communicated via dedicated communication channel compliant to IEC-61850-9-2 standard. And if the TWAT difference between DC terminals is less than the TW propagation threshold for the protected DC line, then a DC trip signal is issued to the associated hybrid DC circuit breaker (HDCCB). For fast power system restoration, the same TWAT information is also used for real-time fault localization in the faulted DC line, which is also compliant with IEC 61869-9 measurement standards. Finally, the practical feasibility of the proposed algorithm is demonstrated through control hardware-in-loop testing with real-time digital simulation setup and TI TMS320F28379d digital signal processor card.

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.

Fast protection strategy for DC transmission lines of MMC-based MT-HVDC grid

Multi-terminal high voltage DC (MT-HVDC) grid has broad application prospects in connecting different energy sources, asynchronous interconnection of power grids, remote load power supply, and other fields. At present, the key technologies that affect the development of MT-HVDC transmission system include swift fault identification and location in the DC line and its rapid isolation. Traditional fault monitoring relies on line communication, which cannot guarantee the rapidity and reliability of protection; moreover, it may even cause device damage. A fault identification scheme based on a single-terminal transient is presented in this paper. This scheme calculates the line inductance by using the rise rate of fault current at the initial stage of the fault, and determines the occurrence of the fault by comparing the observed line inductance with the set value, which lays a foundation for calculating the location of the fault point using distance protection. A simulation model on the PSCAD/EMTDC platform is built; the simulation example verifies that the proposed scheme can identify faults under dissimilar conditions while maintaining a low error level on the premise of no communication lines so as to meet the protection requirements of the MT-HVDC grid.

A pilot line protection for MT-HVDC grids using similarity of traveling waveforms

Protection of dc line faults is a major challenge for the operation of multi-terminal high-voltage dc (MT-HVDC) grids. To avoid blocking converter and system shutdown, ultra-fast protection is required to detect and isolate line fault with the application of hybrid dc circuit breaker. This paper proposes a pilot protection scheme based on improved Hausdorff distance to detect the overhead line fault for MT-HVDC grid. Compared with literature, this protection method owns higher withstanding ability of noise interference, start-up delay and data anomaly. Moreover, the fault segment can be correctly identified as independent of fault location, and protection sensitivity is maintained even under internal ground fault with high impedance of up to 1000 Ω. The issue of dead zone fault recognition is considered and solved in this paper with reasonable sacrifice in the sampling frequency requirement. Based on Zhangbei HVDC grid in China, a four-terminal dc grid model is developed in PSCAD/EMTDC environment for fault simulation, and the data processing is performed in MATLAB platform. The feasibility and effectiveness of this protection scheme are verified by a simulation study that considers different scenarios.

Technical Assessment on Self-Charging Mechanical HVDC Circuit Breaker

High-voltage DC (HVDC) circuit breaker is the necessary equipment for multiterminal HVDC grids, which is a prospective power transmission method in the future. Therefore, the research about HVDC interruption ushered in an explosive development in recent years. The mechanical DC circuit breaker based on the precharged capacitor for current commutation and interruption is considered to be one of the most promising HVDC interruption solution. However, the commutation capacitor requires a complicated precharging circuit, especially in high-voltage applications, which brings a great challenge to the promotion of mechanical interruption solution. In this article, a novel self-charging mechanical HVDC circuit breaker is proposed. It utilizes the induced voltage generated by part of current limiting reactor to adaptively charge the commutation capacitor under different fault current conditions without extra auxiliary charging circuit. By investigating the influence of capacitance and inductance on the charging time and current commutation process, the optimized parameters are determined. Then, the breaking performance of proposed HVDC breaker under different rising rates of fault current is evaluated in a two-terminal HVDC system. Finally, a low-power prototype is developed to verify the feasibility of the proposed topology and an improved scheme for rated current breaking is discussed.

Adjustable Wind Farm Frequency Support Through Multi-Terminal HVDC Grids

In the future power systems, a large number of offshore wind farms will be connected to the AC grids through high voltage DC (HVDC) and multi-terminal DC (MTDC) grids. As wind power penetration level increases, complex grid codes and regulations will be imposed on wind turbines for frequency support. To follow any grid code and requirement for frequency support, two important features should be included in the wind turbine frequency support: i) It should be able to adjust the maximum additional power that the wind turbine temporarily provides for frequency support; ii) It should be capable of adjusting the time interval in which the wind turbine provides additional temporary power. The first feature is mainly important for reducing rate of change of frequency (RoCoF) and improving the frequency nadir while the second one is mainly important for fast frequency recovery from its nadir and improving the second frequency drop. This paper indicates that the conventional method cannot offer both of the two aforementioned features. To address this issue, two approaches are proposed for frequency support by wind turbines. The first one uses P-ω <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> and P-f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">WF</sub> droops in each wind turbine controller, where P, ω <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> , and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">WF</sub> represent the wind turbine power, rotor speed, and wind farm frequency. The second method employs P-· ω <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> and P-f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">WF</sub> droops in each wind turbine controller. Performance and effectiveness of the proposed methods are evaluated by time-domain simulation studies on an MTDC grid in the PSCAD/EMTDC software environment.

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 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.

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.

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.

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.

Towards robust OPF solution strategy for the future AC/DC grids: case of VSC‐HVDC‐connected offshore wind farms

This study jointly addresses two major challenges in power system operations: (i) sustained growth of intermittent offshore wind farms (OWFs) connected to AC grid via multi-terminal voltage source converter (VSC)-based high-voltage DC (HVDC) grid that brings new challenges to the power system operation, and (ii) dealing with non-linearity of the AC power flow equations with the multi-terminal VSC-based HVDC grid model. To overcome these challenges, firstly, to deal with the uncertainties caused by the high penetration of the intermittent OWFs, this study introduces a robust optimisation approach. The proposed framework is computationally efficient and does not require the probability density function of the wind speed. The proposed decision-making framework finds the optimal decision variables in a way that they remain robust against the set of uncertainties. Secondly, the mathematical representation of the full AC optimal power flow (OPF) problem, with the added modelling of multi-terminal VSC-based HVDC grid in a day-ahead scheduling problem, is a mixed-integer non-linear programming (MINLP) optimisation problem, which is computationally burdensome for large-scale systems. Accordingly, this paper proposes a computationally efficient method for adjustment of solutions set points, which is also compatible with existing customary solvers with minimal modification efforts.

Review of the development of multi-terminal HVDC and DC power grid

Traditional power equipment, power-grid structures, and operation technology are becoming increasingly powerless with the large-scale renewable energy access to the grid. Thus, we must adopt new technologies, new equipment, and new grid structure to satisfy future requirements in energy patterns. Accordingly, the multiterminal direct current (MTDC) transmission system is receiving increasing attention. This paper starts with a brief description of current developments in MTDC worldwide. The MTDC project, which has been placed into practical operation, is introduced by the Italian–Corsica–Sardinian three-terminal high-voltage DC (HVDC) project. We then describe the basic characteristics and regulations of multiterminal DC transmission. The current mainstream of several control methods are described. In the third chapter, the key to the development of MTDC system or hardware and software technology that restricts the development of multiterminal DC transmission is discussed. This chapter focuses on the comparison of double-ended HVDC and multiterminal HVDC in most aspects and subsequently elaborates the key and difficult point of MTDC development. Finally, this paper summarizes the prospect of a DC power grid. In a few decades, China can build a strong cross-strait AC-DC hybrid power grid.

Rapid Faults Detection for Controlling Multi-terminal High Voltage DC Grids under AC Grid Contingencies

To control power flow for integration of distributed energy onto urban power grids, rapid and accurate detection of the amplitude, phase-angle, and frequency offset of the grid voltage's positive and negative sequence components especially under grid fault conditions are more significant. This paper presents a new faults detection method that is capable of tracking signal deviations on the grid-voltage accurately and rapidly even in the case that bus-voltage contains high order harmonics and random noises. The experimental results verify the validity of the proposed method under various grid-fault conditions.

Market Integration Scheme of a Multi-Terminal HVDC Grid in the North Seas

The development of a multi-terminal (MT) high voltage DC (HVDC) grid based on voltage source converter (VSC) technology has been envisaged as a key development for harnessing the vast offshore wind production potential of the North Seas. In this paper, market integration of a centrally dispatched MT HVDC Grid based on droop control is examined. Particular emphasis is given on the management of onshore imbalance volumes due to offshore wind power forecast errors. The economic importance of the control choices of the operator of such an active transmission grid is highlighted, and regulatory implications are briefly discussed. The main contribution of the paper is the coherent development of a droop-controlled MT HVDC grid scheme that integrates optimal power flow (OPF) dispatch, and imbalance volume management.