Multi-terminal HVDC System Research Articles

A Non-unit transient travelling wave protection scheme for multi-terminal HVDC system

The rapid and accurate dc line protection scheme is of great value in isolating dc line faults and improving the reliability of multi-terminal HVDC systems. Nowadays, the traveling wave (TW) protection is widely used as the primary protection in HVDC transmission systems. However, the traditional TW protection has the problems of poor resistance to high impedance faults and the setting value depends on simulation. For the hybrid three-terminal LCC/MMC HVDC system, this paper provides a fast non-unit protection scheme. Firstly, the time-domain expressions of the backward line-mode voltage TW after different line faults occur are derived respectively. Based on the expressions, a non-unit protection scheme depending on the difference value of the line-mode backward TW is proposed. Finally, the scheme is verified by simulation. The method only needs single-ended data to realize the rapid dc line protection without converter station communication which has a clear theoretical calculation method in setting values and a strong robustness to high-resistance fault.

An Improved Adaptive Coordination Control of Wind Integrated Multi-Terminal HVdc System

The increasing penetration of the renewables and integration of the power electronic devices leads to lower system inertia, which is changeling the system stability of a multiterminal HVdc (MTdc) system. This article presents an improved adaptive predictive control with the multiobjective targets coordinating the key parameters that the dc voltage, ac frequency and power-sharing among the terminals in MTdc system. Specifically, we contribute two main points to the relevant literature, with the purpose of distinguishing our study from existing ones. First, the proposed method is based on minimal information exchange by only considering neighboring terminals. Second, the adaptive control is achieved by setting a weighted fitness function to adaptively tune the weights with the effective integration of the trust-region and particle swarm optimization. A four-terminal HVdc system built within the IEEE 30-bus ac system is used as the study case to validate the robustness and efficiency of the proposed method. In the case study regarding the multiobjective fitness function, the proposed approach benefits in suppressing the voltage deviation, providing frequency support and establishing an automatically updated power equilibrium leveraging by the adaptive parameters.

Wavelet multiscale principal components and traveling waves enabled machine learning technique for protection of MT-HVDC systems

The interconnection of renewable energy sources to conventional power grids is widely recognized and promoted by the deployment of high voltage direct current (HVDC) technology. One of the biggest challenges associated with the HVDC systems is the rapid rate of rising DC fault currents, resulting in the immediate collapse of the converter stations. Conventional schemes of AC protection and low-speed DC protection are not worthy and mature because of the abrupt rising characteristics of DC currents. Therefore, protection techniques are being developed for reliable bulk power transfer and interconnection of unsynchronized grids via DC links. Timely identification, classification, and determination of location are the most fundamental characteristics of any protection scheme. This research paper proposes a machine learning-based protection method that establishes a rapid and reliable solution addressing the drawbacks of existing protection schemes. A multi-terminal high voltage direct current (MT-HVDC) test system is designed in Matlab/Simulink. Pole-to-ground and pole-to-pole faults are applied at different fault locations to assess the validity of the proposed algorithm. For this, DC voltages and currents are measured and analyzed. Power spectral analysis of traveling waves and Wavelet multiscale principal components analysis are employed for the extraction of featured data to classify and locate the DC faults. This reduced dimensional featured data is utilized for training and testing the support vector machine learning algorithm to analyze the accuracy of the proposed protection technique. Simulation results confirm the accuracy of rapid identification, classification, and location of DC faults.

Decoupled Control for Double-T Dc-Dc MMC Topology for MT-HVdc/MVdc Grids

This paper proposes a decoupled control of a dc-dc modular multilevel converter (MMC) based on a double-T topology intended for multi-terminal high voltage direct current (MT-HVdc) transmission systems or emerging distribution systems operating in medium voltage direct current (MVdc). The aim of the proposed control strategy is to obtain an input current with reduced harmonic content and to eliminate the output ac common-mode voltage, which is not allowed in MT-HVdc systems. The control strategy consists of injecting two circulating ac currents and two dc currents that allow the energy balance between the arms of the converter and the general energy balance of the topology. The dc and ac currents are decoupled and allow control over load variations and reference changes in the dc-links. The proposed topology is mathematically modeled and the control method is then derived. Simulation results are presented to validate the proposed system.

Adaptive Control Strategy for Frequency Support From MTDC Connected Offshore Wind Power Plants

Grid integration of large-scale power electronic interfaced renewable energy sources leads to a low inertia system, thus resulting in a high rate of change of frequency (RoCoF) and frequency deviations in the power system following a credible contingency. In this article, a realistic adaptive communication-independent control strategy for effective inertial and primary frequency support from multiterminal HVDC (MTDC) connected offshore wind farms (OWFs) has been proposed. The proposed strategy can identify and distinguish the grid under disturbance among the other healthy grids in the MTDC system by an adaptive droop frequency support (ADFS) scheme at each receiving end converter (REC). On the OWF side, the control system of wind turbine generators has been designed to coordinate with the ADFS scheme to comply with the grid code regulations (GCRs) for inertial and primary frequency support. The proposed ADFS control scheme ensures that while providing frequency support to the grid under disturbance, frequency deviation limits of the healthy neighboring grids are not breached. Moreover, the proposed strategy complies adequately with the maximum permissible limits of HVDC voltage deviation. Furthermore, the proposed strategy is dependent on only local measurements and can be easily customized to comply with any applicable GCR. The reliability and effectiveness of the proposed robust control strategy have been demonstrated on three power systems interconnected through the MTDC system to two OWFs.

Artificial Intelligence-based Control Techniques for HVDC Systems

The electrical energy industry depends, among other things, on the ability of networks to deal with uncertainties from several directions. Smart-grid systems in high-voltage direct current (HVDC) networks, being an application of artificial intelligence (AI), are a reliable way to achieve this goal as they solve complex problems in power system engineering using AI algorithms. Due to their distinctive characteristics, they are usually effective approaches for optimization problems. They have been successfully applied to HVDC systems. This paper presents a number of issues in HVDC transmission systems. It reviews AI applications such as HVDC transmission system controllers and power flow control within DC grids in multi-terminal HVDC systems. Advancements in HVDC systems enable better performance under varying conditions to obtain the optimal dynamic response in practical settings. However, they also pose difficulties in mathematical modeling as they are non-linear and complex. ANN-based controllers have replaced traditional PI controllers in the rectifier of the HVDC link. Moreover, the combination of ANN and fuzzy logic has proven to be a powerful strategy for controlling excessively non-linear loads. Future research can focus on developing AI algorithms for an advanced control scheme for UPFC devices. Also, there is a need for a comprehensive analysis of power fluctuations or steady-state errors that can be eliminated by the quick response of this control scheme. This survey was informed by the need to develop adaptive AI controllers to enhance the performance of HVDC systems based on their promising results in the control of power systems. Doi: 10.28991/ESJ-2023-07-02-024 Full Text: PDF

Protection scheme for multi-terminal HVDC system with superconducting cables based on artificial intelligence algorithms

This paper presents the development of a novel data-driven fault detection and classification scheme for DC faults in multi-terminal HVDC transmission system which incorporates superconducting cables and modular multi-level converters. As the deployment of superconducting cables for bulk power transmission from remote renewable generation is progressively increasing in the future energy grids, many fault-related challenges have been raised (i.e., fault detection, protection sensitivity/stability). In this context, the applications of Artificial Intelligence techniques have started to be considered as a powerful tool for the development of robust fault management solutions. The proposed artificial intelligence-based method utilizes local current and voltage measurements to detect and classify all types of faults on the DC cables and DC buses, without the requirement of measurements exchange among different DC substations. The performance of the proposed scheme has been assessed through detailed transient simulation analysis and the results confirmed its effectiveness against a wide range of fault conditions (i.e., various fault types, fault locations and fault resistances). Furthermore, the feasibility of the developed scheme for real-time implementation has been validated using real-time software in the loop testing. The results revealed that the proposed algorithm can correctly, and within a very short period of time (i.e. less than 2 ms) detect and classify the faults within the protected zone and concurrently remain stable during external faults. Additionally, the generalization capability of the algorithm has been verified against influencing factors such as the addition of noise, highlighting the robustness of the presented scheme.

Steady-state power distribution in VSC-based MTDC systems and dc grids under mixed P/V and I/V droop control

This paper proposes a steady-state power distribution derivation method for voltage source converter (VSC)-based multi-terminal HVDC (MTDC) systems and dc grids under mixed power/voltage (P/V) and current/voltage (I/V) droop control. P/V and I/V droop control are two commonly used control schemes, which can be combined to achieve co-regulation of powers & currents in MTDC systems and dc grids. The proposed method can be used to estimate the power distributions under different scenarios for MTDC systems and dc grids based on VSCs with mixed P/V and I/V droop control. After determining the initial operating point based on an estimation-correction algorithm, redistributed power due to power disturbances, current changes or converter outages is analyzed in detail considering converter overload. An excess power reduction strategy is further proposed to avoid violation of power limits after converter outage. The accuracy of the proposed method is validated through multiple scenarios in a modular multilevel converter (MMC)-based four-terminal dc grid. The comparison between the proposed method and other approaches in the current literature further demonstrates the advantages of proposed power distribution derivation method. • Determination of initial operating point for mixed P/V and I/V droop control by proposed estimation-correction algorithm. • Derivation of power redistribution after dc power/ current reference changes and converter outages. • Consideration of possible converter overloads caused by different system disturbances.

Secondary frequency control strategy considering DoS attacks for MTDC system

The utilization of communication networks brings benefits to frequency support strategies for multi-terminal high voltage direct current (MTDC) systems, yet leads to the systems facing cyber security risks. In particular, the denial of service (DoS) attacks will cause loss of measured signals and make the MTDC system unstable. This paper is concerned with the secondary frequency control problem defending DoS attacks in MTDC systems. Focusing on energy-limited and periodic DoS attacks, a Bernoulli process model is firstly adopted, and the control objective considering DoS attacks is built. Thereafter a switching scheme of the attacked station from droop control to secondary frequency regulation is devised for frequency support of MTDC systems. Two strategies respectively using the reference value and the data of the previous time interval are then proposed to compensate for the lost data due to DoS attacks. Four scenarios are simulated on a four-terminal MTDC system to verify the effectiveness of the two proposed solutions. The results show that, if the system is in equilibrium states during DoS attacks, both of the two proposed strategies are effective. Whereas the solution using the data of the previous time interval has better performances in the subsequent regulation process after load disturbance. • A switching control mechanism is devised for restoring the frequency of MTDC systems. • DoS attack will seriously affect the control process of MTDC systems. • Strategy(1) can be utilized when the system is in operating phase of steady state. • Strategy(2) has a better performance in the operating state when the system changes.

A protection method for multi-terminal HVDC system based on fuzzy approach

This paper presents a Fuzzy Inference System (FIS)-based method to detect a fault, identify the faulty section, and recognize the faulty pole in the Voltage Source Converter (VSC)-based Multi-Terminal High Voltage Direct Current (MT-HVDC) system. The method uses only rectifier end measurements of voltage and current signals. To achieve complete protection of the MT-HVDC system, three separate frameworks of FIS modules have been developed. The FIS-1 detects the presence of fault in either AC or DC segments. The FIS-2 identifies the fault section and eventually, the FIS-3 recognizes the faulty pole. The proposed method provides a rapid fault detection and it does not require any communication media as it is based on the rectifier-end measurements only. The efficacy of the implemented FIS-based method is evaluated in an MT-HVDC system simulated in MATLAB environment.Important highlights of this method are:-•FIS-based method is deployed using simple if-then rules, therefore very easy to implement.•Effortless means to exploit one end measurements of MTDC system in an environment accustomed to power engineer.•Proposed method provides a rapid fault detection and it does not require any communication link.

Small-Signal Stability of DC Current Flow Controller Integrated Meshed Multi-Terminal HVDC System

In this paper, the small-signal stability of a DC current flow controller (CFC) integrated meshed multi-terminal HVDC (MTDC) system is investigated with unique contributions: (1) The dynamics of a multi-line CFC, interconnected voltage- source converters (VSCs) and the DC transmission lines are considered in the small-signal modelling. (2) The switching effect of the capacitive components in the π-type DC transmission lines is identified. The comparative studies of modelling the DC transmission lines using π-type/T-type equivalent circuit are conducted. (3) The impact of the CFC on the AC side of the MTDC system is taken into account with analysis of the critical short-circuit ratio (CSCR) through root-locus analysis and participation factors. The relationship of the SCR with VSC outer-loop control and power transfer is analyzed. (4) The influence of the CFC control system/structural parameters on the small-signal stability of the entire system is investigated. The relationship of the CFC integration with the VSC outer-loop control is analyzed. The adjustment of single/multiple parameters for enhancing the system stability and a sequential parameter tuning approach for multiple parameters are explained. The effectiveness of the proposed method applied in different MTDC systems with CFC is verified. It would be useful for the configuration and parametric design of the CFC integrated meshed MTDC systems.

Stability and Sensitivity Analysis of Multi-Vendor, Multi-Terminal HVDC Systems

Multi-terminal HVDC projects are increasingly developed in recent years to enhance the flexibility of energy transmission. Differing from the point-to-point HVDC systems, it is more challenging to design the multi-terminal HVDC system and ensure stable interoperability among converter systems, especially when the converters are manufactured from different vendors. Although impedance-based stability analysis allows for analyzing such systems based on black-box models, it is still difficult to utilize those black-box models to identify the root cause of potential instability. To tackle this challenge, this paper proposes a multi-level sensitivity analysis approach using frequency-domain sensitivity functions based on the impedance-based stability criterion. The proposed method differs from the classical sensitivity analysis based on state-space or transfer-function models, as it is purely based on black-box impedance models. Case studies on a four-terminal HVDC system are carried out for stability and sensitivity analysis based on the impedance measurement in PSCAD, through which the most sensitive HVDC station can be identified. The proposed theory and the analyzed results are finally validated by electromagnetic transient simulations.

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.

An Adaptive Wireless Power Sharing Control for Multiterminal HVDC

Power sharing among multiterminal high voltage direct current terminals (MT-HVDC) is mainly developed based on a priority or sequential manners, which uses to prevent the problem of overloading due to a predefined controller coefficient. Furthermore, fixed power sharing control also suffers from an inability to identify power availability at a rectification station. There is a need for a controller that ensures an efficient power sharing among the MT-HVDC terminals, prevents the possibility of overloading, and utilizes the available power sharing. A new adaptive wireless control for active power sharing among multiterminal (MT-HVDC) systems, including power availability and power management policy, is proposed in this paper. The proposed control strategy solves these issues and, this proposed controller strategy is a generic method that can be applied for unlimited number of converter stations. The rational of this proposed controller is to increase the system reliability by avoiding the necessity of fast communication links. The test system in this paper consists of four converter stations based on three phase-two AC voltage levels. The proposed control strategy for a multiterminal HVDC system is conducted in the power systems computer aided design/electromagnetic transient design and control (PSCAD/EMTDC) simulation environment. The simulation results significantly show the flexibility and usefulness of the proposed power sharing control provided by the new adaptive wireless method.

General Power Flow Calculation for Multi-Terminal HVDC System Based on Sensitivity Analysis and Extended AC Grid

The power flow calculation of the multiterminal high voltage direct current (MTDC) system is essential for planning sustainable energy sources and power flow analysis for the MTDC system. However, the traditional unified methods require a large system of non-linear equations leading to low calculational efficiency. Also, for a large DC grid, there is a concern about the convergence of sequential methods. This paper proposes a general power flow calculation method for voltage source converter (VSC) based MTDC systems. Based on an extended topology of an AC grid, a generalized calculation model of the MTDC power flow is proposed. Then, a novel sensitivity analysis-based power flow (SAPF) method is proposed, in which the state variables of the extended AC grid are calculated via sensitivity analysis. With a smaller system of equations, the proposed SAPF method has less computational burden than the traditional unified methods and causes no convergence problem compared to sequential methods. To further improve the calculational efficiency, the sensitivity-based variable updating is adopted to accelerate the iterative process. By comparing with the existing methods in calculating power flow for different MTDC systems, including large systems with multiple AC/DC grids, the effectiveness and scalability of the proposed methods are verified.

Multiple objective optimization based on particle swarm algorithm for MMC-MTDC system

Multi-terminal high voltage DC (MTDC) network is an effective technology to integrate large-scale offshore wind energy sources into conventional AC grids and improve the stability and flexibility of the power system. In this paper, firstly, an analytical model of a general applicable MTDC system integrated with several isolated AC grids is established. Then, an improved AC-DC power flow algorithm is used to eliminate the additional DC slack bus or droop bus iteration (SBI/DBI) step of the conventional AC-DC sequential power flow. A multi-objective optimal power flow (MOPF) algorithm is proposed to minimize two optimization targets, i.e., overall active power loss and generation costs of the system. To increase the degree of freedom, adaptive droop control is used in the proposed optimization algorithm in which the voltage references and droop coefficients of the modular multilevel converters (MMCs) are control variables. A multiple objective particle swarm optimization (MOPSO) method is applied to solve the MOPF problem and achieve the Pareto front. A technique for order of preference by similarity to ideal solution (TOPSIS) is incorporated in the decision analysis section and helps the decision maker to identify the best compromise solution.

An ANN-based protection technique for MTDC systems with multiple configurations

A unified protection technique for different configurations of Multi-Terminal high voltage Direct Current (MTDC) systems is presented in this paper. The technique is based on using positive and negative poles’ currents of one end for each line. The spectral energy of two distinct frequency bands captured from fault currents, together with the fault current initial slope, are used to supply a feed forward Artificial Neural Network (ANN) for identifying fault zone and classifying fault type. Different configurations of MTDC systems are used to train and test the proposed ANN including radial system, meshed system without Current Limiting Reactors (CLRs), and meshed system with CLRs. Different fault types, fault locations, fault inception times, and fault resistances are used to train and test the proposed ANN. The proposed algorithm showed an excellent performance in the identification of the fault zone and the classification of the fault type for various MTDC configurations. Moreover, the algorithm does not respond to non- DC fault abnormal conditions.

Advanced insulating materials contributing to “carbon neutrality”: Opportunities, issues and challenges

Advanced insulating materials contributing to “carbon neutrality”: Opportunities, issues and challenges

A Combined Hierarchical and Autonomous DC Grid Control for Proportional Power Sharing With Minimized Voltage Variation and Transmission Loss

This paper presents a combined control scheme that consists of two complementary control strategies, namely hierarchical control and autonomous control to handle contingencies in a multi-terminal high voltage direct current (MTDC) system. The proposed hierarchical control considers three optimal targets, i.e., equal or proportional power sharing, minimizations of transmission loss and total DC voltage variation. The primary control layer implements the droop control for voltage source converters (VSCs). While in the secondary control layer, voltage reference setpoints of the VSCs in droop control scheme are calculated by the proposed optimal power flow method with the equal or proportional power sharing as equality constraints. As the hierarchical control relies on communication system, the autonomous control for power sharing can be activated during communication system malfunctioning or rapid power variation caused by renewable energy sources. The combined control strategy can greatly increase the reliability of the MTDC system. The simulation results of a five-terminal MTDC network conducted in the MATLAB/Simulink software verify the performance of the proposed combined control strategy.

High voltage direct current transmission cables to help decarbonisation in Europe: Recent achievements and issues

High Voltage Direct Current (HVDC) underground and submarine cables constitute an essential technology for the long-distance transmission of renewable electrical power with low losses, thereby fostering decarbonisation. In the past 2 decades, all this yielded a quasi-exponential rise of HVDC cable systems installed worldwide, but has also made the working conditions of HVDC cables harsher. In this paper, the contribution of HVDC cables to decarbonisation is tackled, with particular reference to the situation in Europe, and a few simple calculations showing in general terms the quantitative contribution provided by HVDC cable systems to decarbonisation are given. Then, the major issues towards long-lasting and reliable HVDC cable systems are summarised briefly, focussing—for the sake of brevity—on the influence of the main HVDC cable technologies, of cable laying environment and of duty service. Some hints are also provided at the chances given by multiterminal HVDC systems.