Multi-terminal HVDC Grids Research Articles

Fast communication-aided protection scheme for half-bridge MMC-based Multi-terminal HVDC grids

The protection of half-bridge modular multilevel converter-based (MMC) HVDC grids demands balancing selectivity, sensitivity, and robustness while ensuring fast operation. However, existing single-ended protection schemes often struggle with balancing selectivity and sensitivity, while double-ended schemes face challenges in achieving both speed and sensitivity. This study investigates the theoretical characteristics of first fault current traveling waves (FFCTWs) as they propagate along the faulted and non-faulted lines within a grid. Theoretical analysis reveals unique mode-1 FFCTW characteristics at faulted line ends over that of non-faulted ones, overcoming the shortcomings in methods based on the similarity of FFCTWs in ring grids. Protection criteria implementation utilizes directional morphological features of mode-1 currents at the line ends and of local poles’ currents for fault discrimination and classification, respectively. The proposed method avoids certain conditions like strong boundary features or line parameters to be applicable and can operate effectively even under measurement anomalies. To validate the proposed protection scheme, a meshed MMCHVDC grid is modeled using the PSCAD software. Simulation studies verify the selectivity and sensitivity of the scheme in case of various types of low and high-impedance faults, as well as robustness against non-fault operating conditions. Moreover, the applicability of the proposed method is assessed as primary protection and in comparison to state-of-the-art protection methods.

An Integrated Multiport Circuit Breaker With Current Flow Controlling Capability for Meshed Multiterminal HVDC Grids

An Integrated Multiport Circuit Breaker With Current Flow Controlling Capability for Meshed Multiterminal HVDC Grids

Fast Contingency Analysis and Control for Overload Mitigation in Integrated High Voltage AC and Multi-Terminal HVDC Grids

Fast Contingency Analysis and Control for Overload Mitigation in Integrated High Voltage AC and Multi-Terminal HVDC Grids

Optimal control scheme for multi-terminal high-voltage direct current system to improve robust grid frequency regulations

This paper proposes an optimal control scheme for robust frequency regulation (FR) in power networks that utilize multi-terminal high-voltage direct current (MT-HVDC) systems to interconnect AC grids. The proposed scheme involves the development of a Linear Quadratic Gaussian (LQG) controller to minimize FR in each grid. A state space model of the MT-HVDC grids is derived to design the controller, considering the DC-link voltage and active power controls at the rectifier and inverter, respectively. Additionally, the model incorporates inertia emulation (IE) and droop controllers. Based on the derived state-space model, an LQG controller is designed to achieve optimal frequency control. The stability of the proposed scheme is analyzed through sensitivity and eigenvalue analysis, with a focus on the influence of grid parameters and control gains. Comparative case studies demonstrate that the proposed scheme outperforms conventional strategies in terms of improving the effectiveness and robustness of real-time FR in MT-HVDC grids. The proposed control scheme involves an LQG controller that considers various factors and demonstrates superior performance compared to conventional methods, as supported by comparative case studies.

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.

New analysis of VSC-based modular multilevel DC-DC converter with low interfacing inductor for hybrid LCC/VSC HVDC network interconnections

The integration of multiterminal hybrid HVDC grids connecting LCC- and VSC-based networks faces several technical challenges such as DC fault isolation, ensuring multi-vendor interoperability, managing high DC voltage levels, and facilitating high-speed power reversal without interruptions. The two-stage DC-DC converter emerges as a key solution to address these challenges. By implementing the modular multilevel converter (MMC) structure, the converter's basic topology includes half-bridge sub-modules on the VSC side and full-bridge sub-modules on the LCC side. However, while this topology has been discussed in the literature, its connection to an LCC-based network with controlled current magnitude lacks detailed analysis regarding operational challenges, control strategies under various scenarios, and design considerations. This paper fills this gap by providing comprehensive mathematical analysis, design insights, and control strategies for the modular DC-DC converter to regulate DC voltage on the LCC-HVDC side. Additionally, the proposed control scheme minimizes the interfacing inductor between the two bridges, ensuring uninterrupted power flow during reversal and effective handling of DC faults. Validation through Control-Hardware-in-the-Loop testing across diverse operational and fault scenarios, along with a comparative analysis of different converters, further strengthens the findings.© 2017 Elsevier Inc. All rights reserved.

Headroom-based Frequency and DC Voltage Control for Large Disturbances in Multi-Terminal HVDC (MTDC) Grids

Headroom-based Frequency and DC Voltage Control for Large Disturbances in Multi-Terminal HVDC (MTDC) Grids

New Functions of VSC Based HVDC Transmission Systems that Can Effectively Support the Future South-East European Grid with Low Inertia and Large Amount of Res Generation

Abstract The energy mix in South-East Europe is nowadays mainly composed by hydro and fossil fuels generation almost equally distributed in the share of electricity generation. Wind and solar PV generation play a very marginal role playing cumulatively less than 7% of the total generation, while fossil fuels exceed 52% of the total generation. A Multi-Terminal HVDC grid between the different countries in South-East Europe may provide various operational and socio-economic gains. In this paper are presented some grid specific challenges and explored some new functions of HVDC transmission systems based on VSC technology that can effectively support the grid with large amount of RES. The paper aims to explore how the VSC converters can improve the system’s voltage stability and provide the necessary frequency control when the inertia in the grid is very low and how the HVDC transmission system can accomplish transferring the primary and secondary reserves solving the rising problems related to dispatchability and inertia response in RES based power grids.

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.

Design consideration for frequency containment reserve provisions by a multi‐terminal HVDC system

AbstractFrequency control is considered to be an important function of multi‐terminal HVDC (MTDC) systems. Sharing frequency containment reserves (FCRs) among interconnected AC systems using HVDC technology is particularly attractive in the European context. However, coordination of the contribution between systems with different sizes, quality requirements, and dynamic characteristics requires harmonized rules and well‐defined control strategies. If the controllers are not properly designed, they can lead to disproportional supports, noncompliance with the existing regulation framework, or even degradation of the frequency quality. This paper presents a general analytical methodology for assessing the compliance of MTDC grid control solutions to the essential requirements of the FCR framework, and then examines three different implementation solutions as illustrative examples to validate the method. Through this process, the interaction mechanisms behind each solution are analytically clarified. The performance analyses are then confirmed using load‐frequency models of CE, Nordic, and GB systems coupled with a three‐terminal DC system. It is revealed that, while certain control solutions result in insufficient performance with respect to the desired FCR provision, the newly proposed distributed solution for each pair of converters is the most compliant with the existing framework thanks to the highest degrees of freedom.

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 systematic review of Intelligent Fault-Tolerant Protection Scheme for Multi-terminal HVDC Grids


 
 
 Due to the advancement of power electronics devices and control techniques, the modular multilevel converter (MMC) has become the most attractive converter for multiterminal direct current (MTDC) grids thanks to its most relevant features, such as modularity and scalability. Despite their advantages, conventional MMCs face a major challenge with: i) fault-tolerant operation strategy; i) energy losses in conversion; iii) lack of DC fault handling capability. This paper provides a systematic review to identify the gaps in the literature about Intelligent Fault-Tolerant Protection Schemes for multi-terminal HVDC grids. Through the bibliometric analysis, it was possible to identify topics still to be developed within the four main clusters (Offshore wind farms, Wind turbines, Voltage Source Converters, and Wind power). The research topic opens three research paths: the first is the analysis of failures in HVDC (High Voltage Direct Current) grid equipment by the FDD (Fault Detection and Diagnosis) method; the second is failure analysis by the IFDD (Inverse Fault Detection and Diagnosis) method and the third is the possibility of interconnecting the different energy generation zones with different frequencies.
 
 

Multisegmented Intelligent Solution for MT-HVDC Grid Protection

Fault detection continues to be a relevant and ongoing topic in multiterminal High Voltage Direct Current (MT-HVDC) grid protection. In MT-HVDC grids, however, high DC-fault currents result from a failure of a complex protective threshold in traditional protection schemes, making Voltage Source Converter (VSC) vulnerable to such potent transient currents. In this innovative single-ended DC protection scheme, multiple time window segments are used to consider the effects of the transient period across limiting inductors at each end of the link. Multiple segments of 0–0.8, 0.8–1.5, and 1.5–3.0 ms reduce relay failure and improve the sensitivity to high fault impedance while requiring minimal computational effort. It employs feature extraction tools such as Stationary Wavelet Transform and Random Search (RS)-based Artificial Neural Networks (ANNs) for detecting transmission line faults within DC networks. Its goal is to improve the accuracy and reliability of protective relays as a result of various fault events. Simulations showed that the proposed algorithms could effectively identify any input data segment and detect DC transmission faults up to 500 ohms. Accuracy for the first segment is 100% for fault impedance up to 200 ohms, whereas the second and third segments show 100% accuracy for high impedance faults up to 400 ohms. In addition, they maintain 100% stability even under external disturbances.

A failure detection method based on multivariate variational mode decomposition and skewness for modular DC circuit breakers

Modular DC circuit breakers (MDCCBs) are widely used to protect multi-terminal HVDC (MT-HVDC) girds. However, the metal-oxide varistor (MOV) failure detection of MDCCBs still poses a challenge for the reliability of MT-HVDC grids. To this end, this paper analyzes the faulty features of the MDCCB and proposes a failure detection method that uses a multivariate variational mode decomposition (MVMD) and a differential skewness. The proposed method is as follows. First, MVMD decomposes the differential current, which is obtained by the current of the MDCCB energy absorption branch, into its time–frequency components, and then the time–frequency component that has the highest frequency is selected as the feature component. Second, a differential skewness is constructed by the feature component, and the differential skewness is employed to detect the MOV failure of the MDCCB. Finally, the performance of the proposed method is evaluated by a four-terminal symmetric bipolar HVDC system with MDCCBs. The simulation experiment results show that the proposed method can correctly detect the MOV failures of the MDCCB in different faulty conditions, even in the weak failure condition with noise influence.

Assessment and Mitigation of DC Breaker Impacts on VSC-MTDC Grid Equipped With Power Flow Controller

DC circuit breakers are required to interrupt dc faults in multi-terminal HVDC (MTDC) grids. Consequently, their current limiting inductors can substantially impact the dynamic performance and stability of MTDC grids. However, the literature does not address the assessment and mitigation of dc breaker inductance impacts on the stability of an MTDC grid equipped with a power flow controller (PFC). To fill in this gap, this paper evaluates the effects of dc breaker inductance on the stability of a PFC-equipped MTDC grid. Furthermore, the paper expands the capability of the PFC via the development of a PFC-based damping controller to improve the dynamic performance of the MTDC grid and enhance its damping. First, a comprehensive small-signal model of the PFC-equipped MTDC grid is developed. Then, eigenvalue and frequency response analyses are employed to assess the dynamic performance and stability of the MTDC grid, considering the effects of breakers inductances, PFC and converter station control parameters, and network parameter uncertainty. Finally, time-domain simulations and hardware-in-the-loop real-time simulations are carried out to evaluate the performance of the proposed damping controller and verify the theoretical analysis.

Power Flow Study of MT-HVDC Grid Compensated by Multiport Interline DC Power Flow Controller

DC power flow controllers (DCPFCs) are emerging devices to control power flow in voltage source converter (VSC)-based multi-terminal HVDC (MT-HVDC) grids. In this paper, a novel Newton-Raphson (NR)-based DC power flow solver (DCPFS) is proposed to solve the DC power flow problem (DCPFP) by using a novel multiport interline DC power flow controller (MIDCPFC), where physical and control state variables of the whole system (MIDCPFC and MT-HVDC grid) are modified simultaneously to achieve predetermined control objectives. The static model (SM) and the power injection model (PIM) of the considered MIDCPFC have been derived and their equations are embedded within the proposed DCPFS. Since there are no fictitious buses in the proposed DCPFS, the original conductance matrix of the system and its symmetry are preserved, and only minor modifications are needed on the original system's Jacobin matrix. It is very straightforward to implement the proposed DCPFS as the voltage of intermediate capacitor of the MIDCPFC is treated as an independent variable, and thus, an external process to control its value is not needed. In this study, comprehensive models have been proposed to model losses of the MIDCPFC and VSCs for the first time; and the shunt conductance of HVDC lines have also been considered. Finally, a modified 15-bus MT-HVDC grid is proposed for verification purposes. The obtained results verify the accuracy and efficacy of the proposed concepts, models, and formulations of this study.

A Novel Control Technique for Enhancing the Operation of MTDC Grids

This paper develops a novel control approach for the droop-controlled Voltage Source Converters (VSC) of Multi-Terminal High Voltage Direct Current (MTDC) systems. The frequency consensus controller is shown to assist in damping the inter-area oscillations and providing enhanced mutual frequency support. Such features, however, might be achieved at the expense of overloading some of the VSCs interfaced to the synchronously connected ac grids. Thus, a new power-sharing control loop, based on the proposed power deviation ratio (PDR) index, is developed to enhance the distribution of active power mismatches between those VSCs. The developed PDR loop, which regulates the ratios of the mismatched power-sharing by considering both the scheduled power injections and the available capacities of the VSCs, enhances the mutual frequency support capability between ac areas of the MTDC system. Furthermore, a newly proposed equidistant voltage control (EVC) loop of the proposed controller regulates the dc system’s voltages such that they are equally far from upper and lower voltage limits. This technique increases the safety margin in voltage regulation during events that cause dc system’s voltage profile variation. The comparative advantage of the proposed controller is verified through modal and participation factor analysis and through comprehensive time-domain simulations.

Protection of Multi-Terminal HVDC Grids: A Comprehensive Review

Multi-terminal HVDC grids facilitate the integration of various renewable resources from distant locations; in addition, they enhance the reliability and stability of the grid. Protection is one of the major obstacles in realizing reliable and secure multi-terminal HVDC grids. This paper presents a comprehensive review of the existing protection schemes for multi-terminal HVDC grids. First, DC fault current stages are demonstrated; in addition, fault analysis studies and the existing fault current calculation methods are reviewed. Then, HVDC grid protection requirements including multi-vendor interoperability conditions are extensively discussed. Furthermore, primary protection algorithms are classified into single- and double-ended schemes, and a detailed comparison between each category is presented such that the distinctive algorithms from each group are highlighted. Moreover, the recent DC reclosing schemes are reviewed highlighting their role in enhancing grid stability and ensuring supply continuity. Finally, available standards for HVDC protection systems alongside their design considerations and procedures are thoroughly outlined. This paper focuses on the recently proposed methods to design reliable protection schemes for multi-terminal HVDC grids and highlights the main advantages and disadvantages associated with them; thus, it offers a beneficial guide for researchers in the HVDC protection field.

An Interactionless Duo Control Strategy for Bipolar Voltage-Source Converter in Renewables Integrated Multiterminal HVdc Grids

The dc voltage and frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ (PVF)$</tex-math></inline-formula> or modified dc voltage and frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ (PV^{2}F)$</tex-math></inline-formula> double droop control is commended for its ability to regulate both the dc voltage and the frequency in a decentralized approach. However, a convincing response is not achieved due to an interaction between the droop characteristics of dc voltage and frequency. This interaction affects the dc voltage and frequency support of the ac-system-surrounded multiterminal HVdc (AC-MTdc) grid. To overcome this effect, a Duo control strategy is proposed in this article, which takes advantage of a bipolar voltage-source converter topology in the MTdc grid. The virtue of the proposed control technique is emphasized by comparing it with the existing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ PV^{2}F$</tex-math></inline-formula> double droop control along with three case studies and two test systems. The validation of the interactionless Duo control strategy is carried out on a five-terminal CIGRE dc grid benchmark model integrated into a two-area power system (AC-MTdc grid-1) and a New England IEEE 39-bus system (AC-MTdc grid-2). These test systems are simulated in PSCAD/EMTdc software.

Modelling of Resistive Type Superconducting Fault Current Limiter for HVDC Grids

The protection of high voltage direct current (HVDC) grids is a challenge considering that the protection system must detect, locate, and interrupt large fault currents in a few milliseconds. Resistive type superconducting fault current limiters (R-SFCL) can help solve that difficult task, reducing the extremely demanding ratings of HVDC circuit breakers. This paper presents different approaches to model R-SFCLs in order to analyze their suitability for assessing the performance of HVDC grid protection, including the step model, the exponential model, the RQ model, and the magneto-thermal model. In the first instance, the R-SFCL models are evaluated in a test grid to analyze their parameterisation and select the most adequate model for the study of HVDC grids. The RQ model is finally chosen for its simplicity but closer behavior to the magneto thermal model in terms of fault resistance dependency and resistance evolution curve. Then, the performance of an RQ type R-SFCL model in conjunction with a mechanical circuit breaker is evaluated in a multiterminal HVDC grid with different fault cases. This way, fault currents are greatly decreased as well as circuit breaker requirements. Hence, the R-SFCL under study enables a reliable protection of the HVDC grid.