High Voltage Direct Current Grids Research Articles

Getting Ready for Multi-Vendor and Multi-Terminal HVDC Technology

Interoperable multi-vendor High-Voltage Direct-Current (HVDC) grids are a key enabler for the integration of renewable energy (in particular offshore wind) and its transmission over longer distances to consumers. However, most HVDC systems today are single-vendor and point-to-point. Various technical and non-technical aspects need to be considered, for example, (real-time) testing, legal aspects (intellectual property and regulation), and the multi-vendor interoperability process. This paper presents findings from the READY4DC project, which is a larger and open European effort involving diverse stakeholders, including HVDC manufacturers, transmission system operators, wind developers, academia, and research institutes. It summarizes key technical recommendations, emphasizing comprehensive interaction studies and the development of a structured legal framework to facilitate the development and operation of a multi-vendor, multi-terminal HVDC grid. The READY4DC project highlights the need for increased harmonization, transparent communication among stakeholders, and future-oriented research to ensure the robustness and interoperability of interconnected grids. Collaborative efforts are key for addressing technical complexities and advancing the deployment of multi-vendor multi-terminal HVDC technology.

A DC fault current fast-computing method of MMC-HVDC grid with short circuit protection equipment

The multi-terminal modular multi-level converter-based high voltage direct current (MMC-HVDC) grid with short circuit protection equipment (SCPE) is so complex that it is difficult to estimate its fault current and analyze the performance of SCPE by conventional time-domain numerical calculation method, it meets three big obstacles. This paper has made significant progress in overcoming these obstacles. 1). By applying the modern electrical circuit theory, a systematic formulation of the differential equation set for fault current calculation is developed to avoid a lot of complex and cumbersome matrix manual calculations. 2). A novel Y-Delta transformation in the s-domain is proposed to develop an eliminating virtual node approach for a complex MMC-HVDC grid, including the ring, radial, and hybrid topologies. 3). It is difficult to solve the equivalent circuit of MMC-HVDC grid with SCPE since SCPE is a time-variable-nonlinear circuit. A canonical voltage source model of SCPE is established to transform the time-variable-nonlinear circuit into a piecewise linear circuit. Based on the three significant progresses, a DC fault current fast-computing method of MMC-HVDC grid with SCPE is put forward to deal with all kinds of MMC-HVDC grids with several kinds of SCPEs. Then, the performance of several kinds of SCPE is analyzed and compared by this method. Consequently, the proposed DC fault current fast-computing method is a new powerful tool to estimate the fault current of MMC-HVDC grid and analyze the performance of SCPE.

Fault location in high-voltage direct-current grids based on panoramic voltage features and vision transformers

This paper presents a novel fault location method designed for high-voltage direct-current (HVDC) grids equipped with quick-action protections and circuit breakers. The method relies on a very short data window of single-ended voltage signals, which allows the acquisition of necessary measurements before the ultra-fast fault-isolation stage. Through fault analysis of the simulated four-terminal HVDC grid, it is observed that the fault distance induces a unique transient response in the voltage signal. However, due to the complex mathematical representation of loop components and the multiple fault factors involved, a data-driven strategy is adopted for fault distance estimation. Based on this strategy, four signal processing techniques, including Gramian angular field, recurrence plot, Markov transition field, and continuous wavelet transform, are used to extract panoramic voltage features. Additionally, four advanced lightweight variants of vision transformers (ViT), including MobileViT, EfficientFormer, Efficient-Model, and EdgeViT, are considered for constructing the regression model. The optimal combination of panoramic voltage features and regression models for configuring the locator is determined by statistical evaluation. The results of semi-physical experiments based on real-time digital simulators demonstrate that the proposed method, first, can accurately estimate the fault distance, second, possesses robustness against high transition resistance, low sampling frequency, and strong noise interference, and third, is practically feasible.

Non-unit protection method for MMC-HVDC grids based on selective drop rate of voltage traveling waves

Fast, sensitive, and selective protection principles are one of the major challenges in the feasibility of modular multi-terminal (MMC) high voltage direct current (HVDC) grids. Rate of change of voltage (ROCOV) and transient-based solutions are the traditional and widely accepted protection principles. Despite the speed and practicality of these solutions, they generally suffer from sensitivity and selectivity issues, particularly when dealing with high-resistance faults and low-size current limiting inductors (CLIs). To improve upon these methods, this paper proposes a new primary protection method that utilizes a selective drop rate of fault-generated voltage traveling waves (TW) to detect internal DC line faults. This is achieved by a comprehensive analysis of the line-mode fault-generated voltage (LFGV) under various internal and external fault scenarios. As the key fault characteristics, the proposed method exploits the minimum points of initial LFGV and the corresponding time to form the basis of the proposed protection method. The effectiveness of this approach is evaluated using a four-terminal MMC-HVDC grid in PSCAD/EMTDC. Compared to ROCOV and transient-based solutions, the proposed method identifies internal faults up to 1250 Ω with fast response, while maintaining its practicality and independence to CLI size.

Stability and control of VSC-based HVDC systems: A systematic review

The technological development in the area of power electronics has paved the way for the construction of High Voltage Direct Current (HVDC) systems. The utilization of HVDC grids alongside conventional High Voltage Alternating Current (HVAC) grids poses several challenges, especially, from stability and control points of view. Indeed, moving towards such systems in the context of conventional Alternating Current (AC) power systems cannot be possible without ensuring the overall stability of hybrid HVAC/HVDC grids. This paper analyzes different aspects of the stability of Voltage Source Converter (VSC)-based HVDC grids and presents various methods of improving stability based on a systematic and comprehensive review. In addition, this paper provides a concise classification of various control methods to improve the operation of such grids and the advantages of each method.

Low-Frequency and Sub-Synchronous Power Oscillation Suppression and Analysis of Hybrid Line Commutated Converter for HVDC Grid

Building high-proportion renewable energy grid requires economical and reliable High Voltage Direct Current (HVDC) converters. Line Commutated Converter (LCC) is widely used because of its long transmission distance, low power loss, low manufacturing cost, and large transmission capacity, which modular multilevel converter (MMC) cannot achieve, but has commutation failure (CF) problems. Low-frequency and sub-synchronous power oscillation caused by CF will affect the safety of bulk power systems. This paper theoretically analyzes the power oscillation during CF and CF recovery, and then proposes a novel hybrid line commutated converter (H-LCC) based on integrated gate commutated thyristor (IGCT), which can effectively mitigate CF and suppress power oscillation. The principle of H-LCC suppressing power oscillation is analyzed. On this basis, the performance of LCC, MMC and H-LCC under the same fault conditions is compared by a simulation study, which proves that H-LCC can improve the security and stability of high-proportion renewable energy power systems.

A Decision Tree Based Ultra-high-speed Protection Scheme for Meshed MMC-MTDC Grids with Hybrid Lines

The reliable operation of modular multi-level converter based multi-terminal high voltage direct current (MMC-MTDC) grids requires high-speed and selective DC line protection. A single-ended protection technique is proposed for MTDC grids with offshore wind farms and hybrid lines that involve overhead lines and submarine cables in series. Positive and negative poles’ currents at one end of each line section are analyzed using discrete wavelet analysis (DWA). To classify fault type and identify fault zone, a fine decision tree is supplied with an energy index and the envelope slop of detail 1 coefficient (D1) obtained from DWA. Only 0.2 ms following the fault inception are needed for calculating the energy index and the envelope slope to perform relay functions. The approach was tested on a three-terminal two-poles ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document} 400 kV MMC-MTDC model. The simulation results validate the effectiveness of the suggested protection technique under various fault scenarios, even with up to 200 Ω fault resistance. The proposed technique is not only able to detect the faulty line, but also it identifies overhead line faults and submarine cable faults for hybrid type lines. Moreover, the proposed technique is not affected by wind power injection changes, AC faults, or data noise. The simulated model was produced in the MATLAB/Simulink platform.

AI & Physics-Based Bad Command/Data Detection in Large Power Electronics Systems: Multi-Port Autonomous Reconfigurable Solar Power Plant (MARS)

Detection of bad data from measurement sensors and bad commands from control centers need to be carried out to avoid instabilities within large power electronics systems. Towards the same, in this paper, model and data driven methods are proposed to identify anomalies in measured data and commands received by large power electronics systems. The large power electronics system considered in this paper is a multi-port autonomous reconfigurable solar power plant (MARS), which consists of photovoltaic (PV) and energy storage systems (ESSs) that connect to high-voltage direct current (HVdc) system and transmission ac power grid. The proposed algorithms in the MARS power plant to detect bad data from measurements and bad commands from control centers are evaluated in simulations and hardware-in-the-loop (HIL) tests. It has been observed that the proposed algorithms are able to detect bad measurements and commands in all the use cases evaluated.

DC protection coordination for multi‐terminal HB‐MMC‐based HVDC grids

AbstractFast progress of High Voltage Direct Current (HVDC) technology has been considerable over the past decades. Emerging Modular Multilevel Converter (MMC) provides power systems with promising solutions to integrate large‐scale renewable energy sources, as well as multiple asynchronous power grids. Consequently, implementation of a multi‐terminal HVDC transmission system has been easier using MMC converters. However, protection of multi‐terminal MMC‐based HVDC grids under DC fault conditions is a challenge for the industry. This study sheds light on concept of system‐level DC protection coordination for multi‐terminal HVDC grids. Although, there are different protection methods for HVDC transmission systems including multi‐terminal ones, there are no well‐defined protection coordination strategies for transmission links protection and belonging HVDC converters under DC fault conditions. The proposed protection coordination prevents unnecessary tripping of MMCs in the multi‐area HVDC grids and supports the system with minimum effect on healthy parts. For the first time, different HVDC Converter Tripping (HCT) profiles are presented based on a fully selective protection strategy and their characteristics are discussed comprehensively. Simulation results demonstrate the effectiveness of the proposed strategy for multi‐terminal MMC‐based HVDC grids. The proposed coordination can be applied to DC grid code requirements of meshed HVDC grids in the future.

Performance analysis of voltage source converter based high voltage direct current line under small control perturbations

High voltage direct current (HVDC) systems provide important advantages; among them the ability to transmit enormous amounts of electrical power over great distances at low cost. As a result, planners of power systems consider it as a viable choice for power transmission and interconnection of asynchronous networks. Depending on HVDC grids, continental/super grids have been recently constructed to promote global economic development. The study described in the paper focuses on the behavior of a voltage sourced converter (VSC) based HVDC transmission system comprising three arms-neutral point clamped (NPC) converters interconnecting two asynchronous alternating current (AC)networks. In addition, the system components, and the vector control strategy of active/reactive powers and direct current (DC)bus voltage are simulated in MATLAB/Simulink under varying situations by adjusting the controller’s settings. The study records and analyzes AC/DC voltages and active/reactive powers at two converter stations undervarying power and voltage conditions. The results of the study provide key performance indicators, such as settling time (tsett), steady state error (SSE), overshot/undershoot (OS%/US%), and correlation factor (CF), which demonstrate the robustness of thesystem’s control.

Asymmetric DC-DC Converter for HVDC Heterogeneous Interconnections

Dc-dc converters are an important element for the future development of high voltage direct current (HVDC) grids. This paper presents a new dc-dc converter for the interconnection between different grid topologies. In particular, one elementary converter can interconnect an asymmetric monopole with a symmetric monopole. The use of two elementary converters allows the interconnection between a bipole and a symmetric monopole keeping the inherent redundancy of the bipole in case of a fault on that line. The converter is analyzed and sized for a proposed a case study. The converter is simulated in a Simulink-Matlab environment. The results prove the adequate performance of the control strategy during the power exchanges between the lines. A fault on the bipole is simulated showing the uninterrupted power transfer between the healthy pole and the symmetric monopole line bipole redundancy).

A new multi-port interline DC power flow controller for multi-terminal HVDC grids

In Multi-Terminal High-Voltage Direct Current (MT-HVDC) Grids, the current of Direct Current (DC) lines are naturally dispersed depending on the DC line resistance and the difference in the voltage at various DC terminals. These characteristics can have direct impacts on the proper operation and stability of MT-HVDC systems and affect the DC power flow in such grids. Using multi-port Interline DC Power Flow Controllers (IPFCs) is an effective solution to improve the power flow controllability and reliability of MT-HVDC grids and also control the current of each DC line. This article aims at presenting a multi-port IPFC to be utilized in series with DC lines in High-Voltage Direct Current (HVDC) grids, providing bidirectional current flow with fewer power switches for each output port. Furthermore, the proposed multi-port IPFC benefits from lower Alternating Current (AC) voltage ripple in the case of increasing the number of the output ports and reducing total power losses in the multi-port IPFC. The operating principle, circuit features, modulation technique, and control scheme of the proposed multi-port IPFC are thoroughly presented. Through dynamic simulations, the performance of the proposed multi-port IPFC is validated using the CIGRÉ B4.58 DC Grid Test System. Finally, real-time Control-Hardware-in-the-Loop (C-HIL) testing under different operating conditions is performed to demonstrate the applicability and effectiveness of the proposed multi-port IPFC.

Direct current power system stabilizers for HVDC grids: Current status

AbstractA power system stabilizer (PSS) is a control system integrated into the control structure of specific generation units within AC grids. It monitors current, voltage, and machine shaft speed. Analysing these variables, the PSS generates appropriate control signals to the voltage regulator unit, aiming to damp system oscillations. With the advancement of high‐voltage direct current (HVDC) overlaid high‐voltage alternative current (HVAC) grids, it is anticipated that direct current power system stabilizers (DC‐PSS) will be developed to perform a similar role as their AC counterparts. DC‐PSS will be responsible for monitoring and controlling DC voltage levels, ensuring stable operations. This paper focuses on DC‐PSS in HVDC grids, designed to ensure stable operation and mitigate voltage fluctuations. Unlike conventional AC power systems, HVDC includes only DC voltage and power. The input signal for DC‐PSS is the variations in DC voltage, and the output signal is proportional to the power changes at the specific bus where the DC‐PSS is installed, aiming to minimize DC voltage oscillations. These characteristics pose significant challenges in DC‐PSS. The paper addresses the challenges and highlights issues such as inertia and low‐frequency oscillations associated with DC‐PSS. Various control methods are presented and a comparison is made among these methods.

LoRaWAN-based IoT protocol for three levels central protection scheme in MT-HVDC networks with ANFIS-PSO restoration algorithm

A new three-level adaptive central protection scheme has been introduced for multi-terminal high voltage direct current (MT-HVDC) transmission grids. The primary protection is based on a differential relay, while two backup protection levels are proposed: a local dv/dt under-voltage relay and a centralized overcurrent relay at the AC grid side. The coordination of these three protection levels is managed based on a convenient coordination time. The communication between the central protection unit and the Fault Isolation Devices (FIDs) is established using the LoRaWAN-based Internet of Things (IoT) protocol. In case of fault clearance triggered by the third protection level, the faulty line is detected and isolated using the adaptive neuro-fuzzy inference system (ANFIS) algorithm. The restoration process of the healthy lines focuses on minimizing load shedding and line power loss. The protection scheme proposed in this article has been implemented and tested under various fault conditions using the MATLAB/Simulink® environment. The results demonstrate the effectiveness and efficiency of the proposed protection scheme in detecting and isolating faulty lines in MT-HVDC transmission grids, along with a rapid restoration technique.

An Adaptable Fault Current Derivative Calculation Method for Derivative Relays in HVDC Grids

This paper presents an adaptable method for fault current derivative calculation in high voltage direct current (HVDC) grids composed of modular multilevel converters (MMCs). The proposed method can be used for current derivative calculation under different fault scenarios including pole-to-pole, pole-to-ground, and pole-to-metallic. The proposed method is adaptable as it can provide accurate fault current derivatives in various grid topologies such as symmetric monopole, asymmetric monopole with ground or metallic return, and bipole with ground or metallic return, as well as grids with different types of converters with and without fault-blocking capability including full-bridge MMCs (FB-MMCs), half-bridge MMCs (HB-MMCs), or a mix of HB- and FB-MMCs. The paper also demonstrates how the proposed current derivative calculation method can be used to form a derivative relay, which is fast, selective, computationally efficient, and insensitive to fault resistance. Furthermore, using the proposed current derivative calculation method, all relay settings are analytically calculated instead of being obtained through time-consuming simulation studies. Simulation results for various fault scenarios, grid topologies, and converter configurations show that the calculation method is accurate and the presented relaying algorithm can detect various faults within 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> s, even when the fault resistance is as high as 500 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Upomega$</tex-math></inline-formula> .

A multiport hybrid DC circuit breaker with high economic efficiency and adaptive reclosing capability

In a high voltage direct current (HVDC) grid, hybrid DC circuit breakers (HCBs) are one of the effective solutions to interrupt fault currents. The conventional two-port HCB features high economic costs since the expensive insulated gate bipolar transistors (IGBTs) are needed. Each line in the multiterminal HVDC grid must be equipped with the HCB, which is extremely costly to implement. In addition, the likelihood of temporary faults is considerably raised by the fact that the HVDC grid transfers power across overhead lines (OHLs). By reclosing operations after temporary faults, the HVDC grid can resume normal operation. However, it is vital to discern between temporary faults and permanent faults before reclosing operation to prevent secondary shocks to the HVDC grid caused by blindly reclosing on permanent faults. For the problems of high cost and blind reclosing of HCBs, this article proposes a multiport HCB (MHCB) with high economy and adaptive reclosing capability. All ports of the MHCB share a single main breaker (MB) for fault isolation. The MHCB utilizes the capacitor voltage to lower the operating voltage of the metal oxide varistor (MOV), which reduces the number of IGBTs connected together at both ends of the MOV. With the above design, the expense of the MHCB is significantly decreased. Additionally, the MHCB achieves adaptive reclosing by using the difference of current amplitudes in the capacitor branch under permanent and temporary faults.

A non-unit line protection method for MMC-HVDC grids based on the curvatures of backward traveling waves

The existing protection techniques for high-voltage direct-current (HVDC) grids suffer from several shortcomings such as high sampling frequency, poor robustness, and reliance on simulation for threshold setting. To solve these problems, this paper proposes a non-unit protection method for modular multilevel converter (MMC)-based HVDC grids using the curvatures of backward traveling waves. To this end, the propagation characteristics of traveling waves and the boundary characteristics of DC lines are first studied, then the analytical expressions of backward traveling waves are derived. Moreover, the curvatures of backward traveling waves are analyzed. On this basis, a non-unit protection method is proposed, including zone selection, disturbance identification, and pole selection. At last, with a protection platform and a real-time digital simulator (RTDS) platform of the MMC-HVDC grid, the accuracy and the robustness of the proposed protection method are verified. The results show that the protection method can correctly identify faults with different distances and resistance in 1 ms and has strong robustness against transition resistance, sampling frequency, boundary value, noise, system topology, and line parameters.

From pole-to-ground fault current return paths in a meshed HVDC network to a grounding modelling simplification for protection studies

Meshed bipolar High Voltage Direct Current grids are considered as one of the preferable solutions for integration of renewable energy sources and increasing the security of power systems on a continental scale. In this context, several fault current studies are proposed in the literature, considering different grounding methods for modular multilevel converter neutral points. But these studies often focus on fault current paths to the fault location, and none of them analyzes the return paths of fault current from the fault location. This article deals with fault currents return paths in case of pole-to-ground fault in a grounding configuration using surge arresters in all stations except one, which is solidly grounded. The influence of this solidly grounded point location on the return paths of fault currents is evaluated. With these results, a modelling simplification is considered for HVDC protection studies. Specifically, the discussion investigates whether all MMC neutral points can be solidly grounded.

HVDC grids stability enhancement through the integration of battery energy storage systems

AbstractIn high voltage direct current (HVDC) grids, the reference values of power controllers are normally adjusted based on the DC‐link voltage as the DC voltage is regulated by the droop controllers. As a matter of fact, except for the DC slack bus, there is a trade‐off between power‐sharing and voltage regulation of all the remaining HVDC buses. The main issue with this control strategy is that in case of contingencies, significant voltage and power oscillations throughout HVDC grids can be expected. To address such issues, the integration of a battery energy storage system into HVDC grids through a multi‐port DC/DC power converter is investigated in this paper. The DC/DC converter used in this paper consists of three ports: (1) two ports are connected in cascade with the intended DC transmission line, formerly named cascaded power flow controller, and (2) the third port is for the integration of the battery energy storage system to form a multi‐functional device. Therefore, the proposed framework can potentially achieve small‐signal stability enhancement and regulate/adjust the power throughout the HVDC grids.

Application of Time-Frequency Analysis with Selection of Suitable Mother Wavelets for Transient States of HVDC Transmission Lines

Recent developments in power semiconductor devices have made High Voltage Direct Current (HVDC) transmission systems the most crucial technology for future power systems. Today several HVDC projects have been commissioned around the world. The primary concern for such an advanced HVDC grid is to follow standard grid codes to maintain the safety and stability of the electrical equipment during transient and dynamic circumstances. Although the HVDC system has been recognized as the most prominent technology to transmit electric power, some issues must be addressed, such as selective and quick detection of faults. For selective fault detection, it is essential to look into the several fault detection strategies that fortify the HVDC system with time-frequency signal processing techniques. This research uses a time frequency-based method to identify the different HVDC transmission line faults. The most suitable mother wavelet for various transient states is selected by analyzing fault voltage and current signals based on variance, standard deviation, and range parameters. An experimental setup of a thyristor-based HVDC transmission system has been developed in the MATLAB/Simulink environment, and multiple fault scenarios have been analyzed. The simulation outcomes reveal that the most effective mother wavelet for detecting DC line, symmetrical and unsymmetrical faults occurs at the 6th level of fault signal decomposition. By examining current and voltage signals, our work has determined the best mother wavelet for various faults. Using more than one mother wavelet to detect faults is more reliable than using a single mother wavelet to detect all faults. Additionally, it has been noted that each fault's voltage and current signals have two distinct mother wavelets.