Multi-terminal HVDC Research Articles

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.

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.

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.

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.

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.

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.

Stability Analysis and Control Design for Multiterminal HVDC Network With Power Flow Controller

The power flow control in high-voltage direct current (HVDC) grids can be improved by employing the interline dc power flow controller (IDCPFC). This article presents an improved control scheme using IDCPFC to balance power flow in the dc networks. In this work, a differential-mode transformer (DMT) is used in combination with a bidirectional buck converter to form a variable DMT. In comparison to the existing methods, the proposed system is scalable to higher voltages and powers, which makes it suitable for high-voltage dc transmission systems. Analysis of a small-signal model of the proposed method and design of compensator for improvement of transient behavior is performed. A simulation is performed with independent feed-forward and feedback control to show the effectiveness of the proposed method. The proposed method is also validated through a 600-W experimental prototype. The experimental results show that the current flow between lines is controlled for unbalance load conditions.

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.

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.

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.

A Review on Multi-Terminal High Voltage Direct Current Networks for Wind Power Integration

With the growing pressure to substitute fossil fuel-based generation, Renewable Energy Sources (RES) have become one of the main solutions from the power sector in the fight against climate change. Offshore wind farms, for example, are an interesting alternative to increase renewable power production, but they represent a challenge when being interconnected to the grid, since new installations are being pushed further off the coast due to noise and visual pollution restrictions. In this context, Multi-Terminal High Voltage Direct Current (MT-HVDC) networks are the most preferred technology for this purpose and for onshore grid reinforcements. They also enable the delivery of power from the shore to offshore Oil and Gas (O&G) production platforms, which can help lower the emissions in the transition away from fossil fuels. In this work, we review relevant aspects of the operation and control of MT-HVDC networks for wind power integration. The review approaches topics such as the main characteristics of MT-HVDC projects under discussion/commissioned around the world, rising challenges in the control and the operation of MT-HVDC networks and the modeling and the control of the Modular Multilevel Converter (MMC) stations. To illustrate the challenges on designing the control system of a MT-HVDC network and to corroborate the technical discussions, a simulation of a three-terminal MT-HVDC network integrating wind power generation and offshore O&G production units to the onshore grid is performed in Matlab’s Simscape Electrical toolbox. The results highlight the main differences between two alternatives to design the control system for an MT-HVDC network.

Modeling of Multi Terminal VSC-HVDC links using port-Hamiltonian Systems

In recent years, different strategies have been used to model HVDC systems. A usual strategy is to look for the models simplification (or linearization) to do a small-signal analysis. However, VSC-HVDC multiterminal links have intrinsically non-linear characteristics that need to be considered. Port-Hamiltonian (pH) systems provide a modeling methodology that allows incorporating the elements of the HVDC system in a modular way while explicitly providing the energy function, internal interconnection, and dissipation, which is helpful for analysis and control. This paper presents the modeling of a multiterminal HVDC network commonly used in the Electrical Power Systems literature from the pH perspective. It aims to use the pH structure to incorporate elements into the system while preserving its properties. Numerical simulations are carried out to show a comparison between the common model and the pH representation and it is shown that the latter reproduces the same dynamics of the former without loss of information but gaining in simplicity and structure.

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.

A control strategy for improving power system resilience in N-k contingency

With the global climate change, the frequency of extreme weather is getting higher and higher, and the threat to the safe operation of the power system is gradually increasing, which is likely to cause large-scale power outages and then result in N-k contingencies. Meanwhile, the Modular Multi-level Converter (MMC) based Multi-Terminal High Voltage Direct Current (MTDC) power system has been confirmed that can provide the possibility for the network interconnection between regional power systems and various renewable energy resources to boost supply reliability and economy. To enhance the resilience of the power grid, a generation rescheduling scheme by optimal emergency control that considers the risk-based dynamic security constraint and reactive power constraints is proposed. Based on the transient stability criterion of the rotor angle speed of the center of inertia (COI), the active power is adjusted to improve the transient stability of the system. The usefulness of the proposed control strategy is highlighted on a 10-machine-39-bus hybrid power system built on MATMTDC, a MATLAB-based open-source software. The obtained results demonstrate that the state optimization control strategy is capable of enhancing the resilience of hybrid power systems and improving the transient stability when suffering N-k contingencies.

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.

Adaptive drooping control scheme for VSC-MTDC system with multiple renewable energy sites based on variable droop constant

The generated power from multiple renewable energy sites can be collected and integrated into the conventional alternating current grid by using a multi-terminal high voltage direct current (MTDC) system based on the voltage source converter (VSC). Following the converter outage or large disturbance, this system with a fixed drooping control scheme suffers overloading of the power converter, resulting in the imbalance of active power-sharing and large deviation of direct current voltages. To improve control performance, this paper proposes a novel adaptive drooping control approach according to the variable droop constant to keep balance active power-sharing and manage the alternation of DC voltage in huge power disturbances. The designed scheme is implemented on the both rectifier side and inverter side converters. The variable droop constant is evaluated according to the converter headroom with DC voltage deviations. The performance of the planned control method is compared with the fixed and variable droop control techniques. A 400 kV four-terminal VSC-MTDC test network is developed to verify the effectiveness of the designed control method. The proposed control technique, based on the PSCAD simulation results, can achieve good performance in the rectifier and inverter converter outage and faults operating scenarios.

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.

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

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