Multi-terminal HVDC System Research Articles

An economic-environmental energy management system design for MT-HVDC networks via a semi-definite programming approximation with robust analysis

This research deals with the economic-environmental dispatch problem (EEDP) in multi-terminal high-voltage direct-current (MT-HVDC) systems by proposing a convex approximation based on semi-definite programming (SDP). The exact formulation of the EEDP corresponds to a non-convex, nonlinear programming problem due to the presence of a nonlinear quadratic constraint regarding the products between voltage variables. The thermal plants' economic and objective functions are modeled using typical quadratic functions. The SDP approach allows reaching a convex approximation that ensures the global optimal solution for each objective function independently or the construction of the optimal Pareto front via the weighting-factor optimization methodology. The proposed SDP approach also considers uncertainties in the demand and in the available power of renewable sources, which makes it robust. The main contribution of this research is a multi-period analysis that includes large-scale renewable generation sources and a robust analysis regarding demand and variations in renewable generation. Numerical results in two MT-HVDC systems demonstrate the effectiveness of the proposed SDP approach when compared to combinatorial optimization algorithms. All numerical simulations were carried out using the CVX convex disciplined tool, with the help of the SEDUMI and SDPT solvers, in the MATLAB programming environment.

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.

Robust economic–environmental dispatch problem for BESS and PV generators in MT-HVDC systems: A MI-MPC approach

The research addresses the energy management problem (EMP) in multi-terminal high-voltage direct current (MT-HVDC) systems using a mixed-integer model predictive control (MI-MPC). The MT-HVDC system consists of conventional thermal generation plants (fossil sources), renewable generation sources (solar photovoltaic plants), and battery energy storage systems (BESS). This combination creates a complex nonlinear EMP, as it introduces the power losses of the transmission networks through B-coefficients. The main contributions of this research can be summarized in three elements: (i) the formulation of a measurement- or simulation-based approach to determine B−coefficients in MT-HVDC systems for calculating power losses; (ii) the convexification of the EMP model through MI-MPC approaches, allowing to minimize economic and environmental objective functions; and (iii) the inclusion of uncertainties through the robust convex method. Numerical validations in the 11-bus grid, including the linear transportation model for the MT-HVDC system, confirm the multi-objective nature of the EMP when considering economic and environmental indices. In addition, the effect of uncertainties on generation and demand causes significant variations in the optimal Pareto front when compared to the deterministic scenario.

A Novel Direct Current Circuit Breaker with a Gradually Increasing Counter-Current

A reliable and cost-effective mechanical direct current circuit breaker (DCCB) is a promising solution for DC interruption. However, the typical mechanical DCCB has difficulty in interrupting a rated current, because the high oscillating current superimposed on the rated current generates a steep current slope at current zero-crossing (CZC) points, which makes it difficult for the vacuum interrupter to extinguish the arc. The objective of this paper is to present a novel DCCB topology with a gradually increasing counter-current. It utilizes a full-controlled converter, a semi-controlled full bridge, and an LC oscillation branch to generate a gradually increasing counter-current, which is superimposed on any fault current and generates a smooth current slope at CZC points. The proposed DCCB topology is modeled with PSCAD, and the current slope and the initial transient interruption voltage (ITIV) at CZC are analyzed and compared with the typical mechanical DCCB. The results indicate that the current slope at CZC decreases by 57–84% in full-range current interruptions, and the ITIV can be reduced by the same extent. Additionally, the performance of the proposed DCCB is evaluated in a four-terminal HVDC system. A cost and performance comparison is conducted among the main topologies. The obtained results show that the proposed DCCB is a reliable solution for the multi-terminal HVDC system.

The Study of Multi-Terminal DC Systems in an Offshore Wind Environment: A Focus on Cable Ripple Analysis

This paper studies the THD and AC losses on the DC cables of offshore wind farm-based multi-terminal HVDC systems when they extract and deliver power from and to more than one connection point. In the paper, the study of a full system PLECS + Simulink model with two branches, including a wind resource, a wind turbine, a Permanent Magnet Synchronous Generator (PMSG), a Pulse Width Modulation (PWM) rectifier, a Single Active Bridge (SAB) DC–DC converter, an Input Parallel Output Series (IPOS) DC–DC converter, HVDC cables, and a simplified onshore system, is presented. It focuses on the investigation of the output ripple content of multiple DC–DC converters on DC cables under different wind conditions with different voltage and power ratings. The importance of the study is providing a general understanding of the operation of the innovative offshore wind farm-based DC system, as well as the interaction between different DC–DC converters and their influence on cable ripple content under different situations.

The Influence of Additional Virtual Synchronous Generator Technology in VSC-MTDC Systems with Wind Power on System Frequency

Introduction: A frequency control strategy is proposed based on additional virtual synchronous generator technology for voltage source converter-based multi-terminal high voltage direct current systems with wind power. Method: This strategy addresses the system's inertia reduction and frequency stability issues caused by integrating large amounts of wind power through multi-terminal DC transmission. Firstly, the virtual synchronous generator mathematical model is constructed based on the system structure. Secondly, for the problem of zero rotational inertia of voltage source converter in a flexible DC transmission system, based on the P-U droop control method of the converter station, additional virtual synchronous control generation technology is applied to simulate the P-f droop characteristics of the synchronous generator by adding virtual rotational inertia, so that the converter has the inertial response of synchronous generator to realize primary frequency regulation. Result: Finally, the simulation is verified on the PSCAD/ EMTDC platform with an example of a three-terminal parallel MTDC transmission system. Conclusion: the analyzed results demonstrate that the virtual synchronous generator control strategy is very valuable and useful for improving the frequency performance of the system.

Power Flow Solution on Multi-Terminal HVDC Systems: Supergrid Case

High Voltage Direct Current (HVDC) systems offer distinct advantages for the integration of offshore wind farms to inland grid system. HVDC transmission system based on Voltage Source Converter (VSC) enables multi-terminal use HVDC for the integration of large-scale wind power in the North Sea. That network requires a special formulation for power flow analysis as opposed to the conventional method employed on AC networks. This paper presents a sequential AC/DC power flow algorithm, which is proposed for the analysis of multi-terminal VSC HVDC (VSC-MTDC) systems. This sequential power flow method can be implemented easily in an existing AC power flow package and is very flexible when compared with unified methods. Gauss-Siedel is used to solve DC power balance equations, as it offers two keys advantages: very fast and simple computational implementation, and errors do not accumulate during the calculation. The algorithm is tested using the WSCC 3-machine, 9-bus system with a 3-terminal MTDC network and the results are compared with those obtained from DIgSILENT PowerFactoryTM demonstrating the validity of the proposed algorithm. As an aggregate value, a representative test case of the projected scheme for the phase I of the Supergrid project on the North Sea is presented. The proposed approach presented in this paper is used to calculate DC power flows for some scenarios.

Enhanced Fault Localization in Multi-Terminal HVDC Systems Using Improved Gaussian Process Regression

Enhanced Fault Localization in Multi-Terminal HVDC Systems Using Improved Gaussian Process Regression

Power Sharing Control Trends, Challenges, and Solutions in Multi-Terminal HVDC Systems: A Comprehensive Survey

Power Sharing Control Trends, Challenges, and Solutions in Multi-Terminal HVDC Systems: A Comprehensive Survey

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.

Efficient fault detection, localization, and isolation in MT-HVDC systems based on distance protection and LoRaWAN communication

Multiterminal High-Voltage Direct Current (MT-HVDC) systems offer numerous benefits compared to conventional alternating current (AC) power systems, including higher power density and improved efficiency. However, the need for adequate protection schemes for HVDC systems remains a significant obstacle to their widespread adoption. Much attention has been given to developing HVDC protection methods to address this. Moreover, the protection of MT-HVDC systems presents a challenge due to bidirectional power flow, dynamic system characteristics, and fault current characteristics that cannot be addressed using conventional methods. This paper represents a centralized protection unit based on a distance protection scheme that involves a two-stage relay process. The first stage involves fault detection by measuring voltage and current to obtain the system impedance, which is then compared to the reach point. The second stage involves identifying the fault location by selecting the correct faulty zone. This technique provides both main and backup protection. A central control unit supports the approach presented in this paper to communicate relays and update their settings. The LoRaWAN communication protocol is employed, as it provides more excellent coverage than other standardized communication technologies and can cover long distances. The proposed method is studied under different scenarios, including system contingency, simultaneous faults, fault resistances, locations, and types. The results of this technique provide the effectiveness of the proposed method. The fault can be cleared within 1.32–1.8 ms.

Analysis of Secondary Controller on MTDC Link with Solar PV Integration for Inter-Area Power Oscillation Damping

Integration of renewable energy sources is important in limiting the continuous environmental degradation and emissions caused by energy generation from fossil fuels and thus becoming a better alternative for a large-scale power mix. However, an adequate analysis of the interaction with the alternating current (AC) network during network disturbance, especially during inter-area power (IAP) oscillations is needed. Insufficient damping of oscillations can significantly impact the reliability and effective operation of a whole power system. Therefore, this paper focuses on the stability of the modified Kundur two-area four-machine (MKTAFM) system. A robust secondary controller is proposed and implemented on a line commutated converter (LCC)-based multi-terminal high voltage direct current (MTDC) system. The solution consists of a local generator controller and the LCC MTDC (LMTDC) system, voltage-dependent current order limiter, and extinction angle controller. The proposed robust controller is designed for the LMTDC systems to further dampen the inter-area power oscillations. Three operational scenarios were implemented in this study, which are the local generator controller and double circuits AC line, local generator controller with LMTDC controllers, and local generator controller with LMTDC controllers and secondary controller. The simulation result carried out on PSCAD/EMTDC recorded better damping of the inter-area power oscillation with LMTDC. A considerable improvement of 100% damping of the IAP oscillations was observed when a secondary controller was implemented on the LMTDC.

Operation Characteristics of a Mechanical DC Circuit Breaker With a Resistive Superconducting Element for High-Reliability HVDC Applications

The issue of DC transmission and distribution systems is increasing due to the increase in DC supply and demand. The fu-ture MTDC (Multi-Terminal HVDC) system is becoming a big issue beyond the general PTP (Point to Point) HVDC system. We live in an era where the super grid is gradually expanding. However, the protection device is required to protect the grid conversion equip-ment from fault currents during line transients to progress from the HVDC stage to the MTDC stage in PTP. The DC fault current, about ten times higher than AC, occurs at a very high speed because of the lack of zero-crossings. We propose a resistive-mechanical DC circuit breaker with a resistive superconducting element is a com-bined model of a resistance-type superconducting element and a me-chanical DC circuit breaker. In detail, it limits the initial fault cur-rent by quenching the resistive superconducting element. Also, it creates an artificial zero-crossing through the LC divergence oscil-lation circuit of the mechanical DC circuit breaker to perform safe fault protection. In this paper, the fault protection of the resistive-mechanical DC circuit breaker with a resistive superconducting el-ement was tested to confirm the current limiting rate of the initial fault current, the artificial zero-crossing, and the total cutoff time. As a result, the initial fault current was limited by about 17.3% by quenching the resistive-superconducting element.

Neuro-Fuzzy Based High-Voltage DC Model to Optimize Frequency Stability of an Offshore Wind Farm

Lack of synchronization between high voltage DC systems linking offshore wind farms and the onshore grid is a natural consequence owing to the stochastic nature of wind energy. The poor synchronization results in increased system disturbances, grid contingencies, power loss, and frequency instability. Emphasizing frequency stability analysis, this research investigates a dynamic coordination control technique for a Double Fed Induction Generator (DFIG) consisting of OWFs integrated with a hybrid multi-terminal HVDC (MTDC) system. Line commutated converters (LCC) and voltage source converters (VSC) are used in the suggested control method in order to ensure frequency stability. The adaptive neuro-fuzzy inference approach is used to accurately predict wind speed in order to further improve frequency stability. The proposed HVDC system can integrate multiple distributed OWFs with the onshore grid system, and the control strategy is designed based on this concept. In order to ensure the transient stability of the HVDC system, the DFIG-based OWF is regulated by a rotor side controller (RSC) and a grid side controller (GSC) at the grid side using a STATCOM. The devised HVDC (MTDC) is simulated in MATLAB/SIMULINK, and the performance is evaluated in terms of different parameters, such as frequency, wind power, rotor and stator side current, torque, speed, and power. Experimental results are compared to a conventional optimal power flow (OPF) model to validate the performance.

Non-unit protection method for multi-terminal MMC HVDC systems based on fault severity analysis and coordinated relay operation

Non-unit protection method for multi-terminal MMC HVDC systems based on fault severity analysis and coordinated relay operation

A novel voltage projection component-based protection scheme for hybrid multi-terminal HVDC transmission lines

Combining the technical advantages of line commutated converter (LCC) and modular multilevel converter (MMC), hybrid multi-terminal HVDC (Hybrid-MTDC) system provides larger capacity, more economical and flexible transmission mode, which has become an important development trend of dc technology in the future. However, high fault resistance, complex topology and large transmission network pose great challenges to the protection of Hybrid-MTDC transmission lines. In this paper, the variation trajectories of internal and external fault voltage vectors with fault resistance are elaborated, and a projection axis through the origin is constructed on the vertical direction of the external metallic fault voltage vector. Combining the voltage amplitude and phase characteristics, the projection component amplifies the characteristic difference between internal and external high-resistance faults. On this basis, a voltage projection component-based protection scheme for Hybrid-MTDC transmission lines is proposed, which can sensitively and reliably identify the line faults under high fault resistance. Finally, the proposed method is thoroughly validated in the PSCAD/EMTDC simulation model of Kun-Liu-Long hybrid three-terminal HVDC system, and the results demonstrate that the proposed protection scheme is practicable, highly sensitive and reliable.

A coordinated control scheme for VSC-MTDC system with multiple renewable energy sites based on voltage margin and adaptive drooping control

This paper designs a novel coordinated control scheme to improve the control performance of voltage source converters (VSC) based multi-terminal high voltage direct current (MTDC) transmission network by merging the benefits of adaptive drooping and voltage margin controllers. In this proposed scheme, the droop coefficient is used to manage the DC voltage and maintain power sharing by shifting the control modes of the rectifiers and inverters in the presence of considerable power disturbances. The droop coefficient is planned based on the stability limit of DC voltage. The performance of the designed control scheme is compared to that of existing coordinated control and voltage margin droop control approaches. A ±400 kV five-terminal VSC-MTDC network is developed in PSCAD to verify the planned control method. The simulation results indicate that the proposed control scheme can achieve good performance for controlling the DC voltage and maintaining active power in the MTDC system.

Improved droop control strategy for an MMC-MTDC connected to offshore wind farms with dynamic correction of the actual operating point

To solve the problems of overload of converter stations and to reduce operating loss of multi-terminal High-Voltage DC (MTDC) grids based on modular multilevel converters (MMC), an improved droop control strategy is proposed in this paper. The novelty of the suggested strategy is that two droop coefficients aimed at reducing system loss and saving power margin are non-linear weighted combined to realize multi-objective optimized operation in the full operating voltage range. First, a power allocation mechanism model of an MTDC system is built to show the influence of the droop coefficient on the DC voltage under the droop control strategy. Then, the droop coefficient under minimum operating loss is calculated and the adaptive droop coefficient is introduced to save enough power margin when the system meets large unbalanced power. In addition, a self-coordinating weight factor with a low-voltage acceleration factor is employed so that MMC can correct the operating point dynamically. Finally, a derived five-terminal MMC-MTDC is built in PSCAD/EMTDC and the effectiveness and feasibility of the proposed control method is validated by simulation.

Sequential model predictive control for five-level MMC-based multi-terminal HVDC systems

Sequential model predictive control for five-level MMC-based multi-terminal HVDC systems

Fault identification scheme for protection and adaptive reclosing in a hybrid multi-terminal HVDC system

A fault identification scheme for protection and adaptive reclosing is proposed for a hybrid multi-terminal HVDC system to increase the reliability of fault isolation and reclosing. By analyzing the "zero passing" characteristic of current at the local end during the converter capacitor discharge stage, the fault identification scheme is proposed. The distributed parameter-based fault location equation, which incorporates fault distance and fault impedance, is developed with the injection signal and the distributed parameter model during the adaptive reclosing stage. The fault distance is determined using a trust region reflection algorithm to identify the permanent fault, and a fault identification scheme for adaptive reclosing is developed. Simulation results show that the proposed scheme is suitable for long-distance transmission lines with strong anti-fault impedance and anti-interference performance. Also, it is less affected by communication delay and DC boundary strength than existing methods.