Four-terminal HVDC Research Articles

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.

A Fast and Robust Local-Based Protection Algorithm Based on the High-Frequency Transient for HVDC Grid

With the expansion of HVDC grids, short-circuit faults and their protection issues are among the main challenges of these networks, which should be given more attention. This study presents an investigation of the DC side fault current in HVDC grids during the transient period. Also, a fast and robust protection algorithm is introduced that is essential for secure and appropriate HVDC grids operation. The Short-Time Fourier Transform (STFT) is applied to extract the frequency components of the fault current, and the time features of these components are utilized as the basis of the proposed algorithm. A four-terminal cable-based HVDC grid with bipolar configuration is modeled via EMTP, and a wide range of simulation scenarios are applied for various fault locations, fault impedances, and operation points. Furthermore, the algorithm thresholds are determined with the explicit procedure. Simulation results indicate that the proposed method is fast, selective, sensitive, and robust against AC-side fault, operating condition variations, fault distance, and fault direction. Also, it has an appropriate performance during high impedance faults.

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.

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.

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.

Nonunit Distance Protection Algorithm for Multiterminal MMC-HVdc Systems Using DC Capacitor Resonance Frequency

High-voltage dc (HVdc) transmission has been largely used to interconnect asynchronous systems and integrate renewable energy resources into electrical grids. However, the high short-circuit currents and low-tolerance of power electronics equipment impose new challenges for these systems’ protection. To address these challenges, a new distance protection algorithm for HVdc grids with modular multilevel converters (MMCs) is proposed in this article. The proposed algorithm identifies the faulty line/cable using the resonance frequency of a dc capacitor installed in each terminal. The technique was tested in a four-terminal HVdc grid with five cables and ten circuit breakers. The algorithm was highly selective for faults in the lines and provided fast identification, in less than 1 ms, without communication amongst terminals. The algorithm was tested in hardware under high-noise conditions and provided reliable results.

Topology and control of a novel hybrid HVDC circuit breaker with self-powered capability

Power supply for IGBT drive circuit of HVDC circuit breaker is a great challenge in practical engineering. At present, most of the HVDC circuit breakers rely on external power sources. This paper proposes a self-powered hybrid HVDC circuit breaker. The HVDC circuit breaker consists of an ultra-fast mechanical disconnector (UFD), a load commutation switch (LCS) and a main circuit breaker (MB). The LCS and the MB are formed by series connection of enhanced half-bridge sub-modules (EHBSMs). By this design, the energy stored in the EHBSM capacitor can be used for IGBT triggering. Thus, the self-powered function of the HVDC circuit breaker is realized. Three operation modes of the HVDC circuit breaker, including the star-up mode, the steady-state operation mode and the DC fault handling mode are analysed in detail. Finally, a four-terminal HVDC model is constructed in PSCAD simulation software. The simulation results verify the feasibility and effectiveness of the self-powered HVDC circuit breaker.

A global fault current limiting strategy for the MMC-HVDC grid with a reduced DC reactor

The large current limiting reactor (CLR) and DC circuit breaker (DCCB) are required in the half-bridge modular multilevel converter (HB-MMC) project to isolate the DC fault. To limit the fault current and reduce the CLR, a global virtual impedance-based fault current limiting (VIFCL) control strategy is proposed in this paper. This strategy includes two parts: the first part is a simplified adaptive fault current limiting control for limiting the DC fault current by reducing the arm modulating voltage of the MMC adaptively. The second part is a virtual impedance control for limiting the AC currents and the arm currents of the MMCs by mapping the virtual impedance to the reactive power control. The effectiveness of the proposed VIFCL control strategy is verified in the PSCAD/EMTDC using a four-terminal HVDC grid. The proposed global VIFCL control can limit the interrupted currents of the DCCBs and reduce the CLR. The proposed strategy is beneficial for the MMCs to ride through the DC faults safely without a significant influence on the power recovery of the HVDC system.

Technical Assessment of Hybrid HVDC Circuit Breaker Components under M-HVDC Faults

One of the technical challenges that needs to be addressed for the future of the multi-terminal high voltage direct current (M-HVDC) grid is DC fault isolation. In this regard, HVDC circuit breakers (DCCBs), particularly hybrid circuit breakers (H-DCCBs), are paramount. The H-DCCB, proposed by the ABB, has the potential to ensure a reliable and safer grid operation, mainly due to its millisecond-level current interruption capability and lower on-state losses as compared to electromechanical and solid-state based DCCBs. This paper aims to study and evaluate the operational parameters, e.g., electrical, and thermal stresses on the IGBT valves and energy absorbed by the surge arrestors within H-DCCB during different DC fault scenarios. A comprehensive set of modeling requirements matching with operational conditions are developed. A meshed four-terminal HVDC test bench consisting of twelve H-DCCBs is designed in PSCAD/EMTDC to study the impacts of the M-HVDC grid on the operational parameters of H-DCCB. Thus, the system under study is tested for different current interruption scenarios under a (i) low impedance fault current and (ii) high impedance fault current. Both grid-level and self-level protection strategies are implemented for each type of DC fault.

Circuit Breaker Failure Protection Strategy for HVDC Grids

HVDC grids demand the fast and reliable operation of the protection system. The failure of any protection element should initialize a backup protection almost immediately in order to assure the system’s stability. This paper proposes a novel backup strategy that covers the failure of the primary protection including the malfunctioning of the HVDC circuit breaker. Only local voltage measurements are employed in the proposed backup protection and the voltage derivative is calculated at both sides of the limiting inductor. Consequently, the speed and reliability of the protection system are enhanced, since no communication channel is needed. This paper contains a thorough specification of the proposed protection strategy. This strategy is validated in a four-terminal HVDC grid with various fault case scenarios, including high-resistance fault cases. The operation of the backup protection is reliable and remarkably fast.

Autonomous Synchronizing and Frequency Response Control of Multi-terminal DC Systems With Wind Farm Integration

Recent analyses have shown that the grid-integration of offshore wind farms through MTDC systems has brought low inertia and small-signal stability issues, in which the dynamics of phase-locked-loop (PLL) play a crucial role. To address this issue, this article proposes a control strategy for the multi-terminal VSCs aiming at PLL-less synchronization and autonomous frequency response of the MTDC system. One of the significant features of the proposed control is that the deviation of the grid frequency can be instantaneously reflected on the deviation of the DC voltage without ancillary control. Based on this feature, a fast inertia response and primary frequency regulation among wind farms and AC systems interconnected by the MTDC system can be achieved. A small-signal model is established to evaluate the overall system stability using the proposed control. Finally, comparative studies of this proposed control with the conventional PLL-based vector control are conducted in PSCAD/EMTDC based on a practical MTDC system in China, the Zhangbei four-terminal HVDC transmission system. The analysis shows that the proposed control exhibits advantages in weak grid operation and autonomous frequency response.

Centralized Locating Strategy of Fault Current Limiters in MMC-Based Multiterminal HVdc Grid

In this article, a centralized locating strategy of fault current limiter (FCL) in high-voltage direct current (HVdc) grid is proposed and analyzed. By employing FCLs at the output sides of modular-multilevel-converters (MMCs), the proposed centralized locating strategy could reduce the number of FCLs in multiterminal high-voltage direct current (MTdc) grid. A typical application scenario based on Zhangbei–Beijing four-terminal HVdc grid is introduced in this article. Mathematical model of the proposed strategy is built and numerical solutions are calculated. Then, the simulation model is built in PSCAD/EMTDC and different locating strategies of FCL are studied. Simulation results verify the rationality of theoretical analysis. Also, calculation and simulation results demonstrate that the proposed centralized strategy has similar current-limiting performance with the conventional distributed FCL locating strategy. When the proposed strategy is applied, the MMCs could be protected from overcurrents. What is more, the reliability and reconnection of partial system after faults are ensured. The healthy part of MTdc grid could maintain continuous operation and transmit active power as normal. When applying different FCL locating strategies, the total investment of FCLs is calculated and compared. Calculation results prove that the proposed centralized locating strategy can effectively reduce the investment of FCLs compared with the conventional distributed strategy and is more suitable for MTdc grid.

An Adaptive Fault Current Limiting Control for MMC and Its Application in DC Grid

This paper proposes an adaptive fault current limiting control (AFCLC) for modular multilevel converters (MMC). Without introducing extra current limiting devices, this control scheme enables fast fault current suppression during DC faults. The AFCLC will be triggered automatically once DC faults occur. By adaptively reducing the output DC voltages of MMCs, the fault current can be suppressed. Compared with the existing current limiting methods, the proposed AFCLC has a better performance on fault current limiting, since it only depends on the real-time operating condition and no fault detection delay is imposed. Firstly, the principle of the proposed AFCLC together with the mathematical analysis is disclosed. Then, the sensitivity analysis of the impact of key control parameters on the current limiting effect is investigated. Finally, the effectiveness of AFCLC is demonstrated in a four-terminal HVDC grid test model. The simulation results show that the proposed AFCLC can reduce the interrupted current and energy absorption of a DCCB from 10.39 kA and 38.24 MJ to 4.62 kA and 8.32 MJ, respectively. The simulation results also prove that the AFCLC will not affect the accuracy of the DC fault detection algorithms under DC faults.

Bridge-Type Integrated Hybrid DC Circuit Breakers

The inclusion of a large number of controllable semiconductor devices in conventional hybrid dc circuit breakers (HCBs) may significantly increase the cost of an HVdc grid protection scheme. In an attempt to reduce this cost, this paper proposes the use of two novel topologies of bridge-type integrated HCBs (BT-ICBs). The two configurations are examined in detail, their operation sequences are established, and a detailed parametric analysis is conducted. The total number of controllable semiconductor devices in a BT-ICB is assessed with the aid of selectivity studies, and a comparison is made when conventional HCB and other ICB topologies are considered. It is shown that the proposed configurations employ 50% to more than 70% less controllable devices compared with conventional HCBs. The proposed BT-ICB topologies are tested in PSCAD/EMTDC using a four-terminal HVdc grid. The simulation results demonstrate the capability and effectiveness of the proposed solutions to isolate different types of dc faults at either a dc line, a converter terminal, or a dc bus.

Modeling of VSCs Considering Input and Output Active Power Dynamics for Multi-Terminal HVDC Interaction Analysis in DC Voltage Control Timescale

Multi-terminal high-voltage direct current (MTDC) systems based on voltage source converters (VSCs) are increasingly utilized. In such a case, dynamic interactions among different VSCs of each terminal become one of the leading problems for the ac/dc interconnected system. This paper presents a frequency-domain small-signal model of MTDC-linked VSCs in dc voltage control (DVC) timescale based on the motion equation concept. Control schemes, including two typical dc voltage-droop control, phase-locked loop, and vector control in DVC timescale are taken into account adequately. By considering the dynamics of both input and output active powers across the dc capacitor, the proposed model aims to describe the external characteristics of MTDC-linked VSCs independent of grid conditions, which makes it easy to extend in various types of MTDC systems. Interaction analysis of a typical four-terminal HVdc system was conducted to validate the feasibility of the presented model.

A High-Performance and Economical Multiport Hybrid Direct Current Circuit Breaker

Hybrid dc circuit breaker (HCB) is an effective device for interrupting dc fault currents in dc grids. Conventional two-port HCB requires too many insulated gate bipolar transistors (IGBTs) in series in its main breaker, resulting in considerable costs. Recently proposed multiport hybrid dc circuit breakers (multiport HCBs, and the number of ports is greater than or equal to 2) interrupt fault currents on multiple lines using only one shared main breaker, significantly reducing the implementation costs but losing some fault handling capabilities. To overcome these obstacles, we propose a novel high-performance economical multiport hybrid dc circuit breaker (MP-HCB) in this article. The topology and its control method are proposed. Due to the designed ring-connection structure, the proposed MP-HCB can avoid dc bus fault, and thus is more reliable. The electrical stresses of the breaker are theoretically analyzed, and the parameters are determined to make the MP-HCB capable of multiple-line faults. Compared with the typical two-port HCB and the existing multiport HCBs, the proposed MP-HCB can achieve full fault current interruption capability with lower costs. To protect a dc node connecting with three lines, the cost of the semiconductors of the proposed MP-HCB is only 1/3 of the typical two-port HCB and 1/2 of the existing multiport HCB. Finally, the performances of the proposed MP-HCB are verified via a four-terminal HVdc system in power systems computer aided design/ electromagnetic transients including DC (PSCAD/EMTDC).

Dynamics and Stability of Meshed Multiterminal HVDC Networks

This paper investigates the existence of an equilibrium point in multiterminal HVdc grids, assesses its uniqueness, and defines conditions to ensure its stability. An offshore multiterminal HVdc system including two wind farms is selected as an application test case. At first, a generalized dynamic model of the network is proposed, using multigraph theory. Such model captures the frequency dependence of transmission lines and cables; it is nonlinear due to the constant power behavior of the converter terminals using droop regulation, and presents a suitable degree of simplifications of the modular multilevel converters, under given conditions, to allow system level studies over potentially large networks. Based on this model, the existence and uniqueness of the equilibrium point are demonstrated by returning the analysis to a load-flow problem and using the Banach fixed point theorem. Additionally, the stability of the equilibrium is analyzed by obtaining a Lyapunov function by the Krasovskii's theorem. Computational results obtained for the selected four-terminal HVdc grid corroborate the requirement for the existence and stability of the equilibrium point.

Protection of transmission lines in multi-terminal HVDC grids using travelling waves morphological gradient

This paper presents a new method for the protection of transmission lines in voltage source converter based multi-terminal HVDC grids. The fault generated travelling waves at the faulted line ends are filtered by series inductors. The filtered voltage waveform processed by morphological gradient accompanied by fault voltage drop is employed for fast single-end protection of HVDC lines. The proposed method is stable against external DC and AC faults, as well as faults at opposite pole and the breaker opening transients from the adjacent lines. Moreover, it does not require communications between the line ends, it is capable to detect and discriminate relatively high resistance faults, and uses relatively low sampling rate. The effectiveness of the proposed protection method is validated by simulation study on a four-terminal HVDC grid including modular multilevel voltage source converters.

An Intelligent Implementation of Fuzzy Search Based Nonlinear Droop Control Scheme for Multi-Terminal DC Transmission Systems

Multi-Terminal DC (MTDC) transmission systems typically operate under active power and DC voltage ( $P$ - $U_{DC}$ ) droop control for active power dispatch, and its design is greatly influenced by DC network characteristics. Hence, this paper firstly analyzes $P$ - $U_{DC}$ droop characteristics when implemented on Grid-Side Voltage-Source Converters (GSVSCs) of a MTDC network, in which variations of DC cables’ resistances with temperature are taken into account. The analysis establishes that the actual droop characteristics of GSVSCs are nonlinear, different from the widely assumption of linear relationships. This finding is the fundamental for the design of the accurate power dispatch for GSVSCs. Therefore, an improved Fuzzy Search based Droop Control (FSDC) scheme for MTDC systems is proposed, which performs real-time search to capture the accurate droop curves of GSVSCs over full range of permissible DC voltage and active power. The FSDC is implemented without the assumption of the prior knowledge of DC network parameters which are subject to variations and uncertainty, and instead the accurate droop curves are periodically obtained and updated. The technical viability of the FSDC and its superiority over the conventional linear droop scheme under the variations of DC cable resistances and power flow variations is verified by a four-terminal HVDC model established in MATLAB/Simulink.

Fast single‐end line protection method for the meshed multi‐terminal HVDC grid

In view of the short-circuit fault characteristics, the protection technique of the dc lines limits the development of the modular multi-level converter (MMC)-based HVDC grid in a large extent. This study derives the calculation formulas of the transient voltages of internal and external faults and proposes a single-end protection scheme based on the transition period (TP) from the abrupt changing point to the first extreme point of the fault voltage. Due to the current-limiting reactors, the TP of the internal fault is much shorter than that of the external fault, which is used to discriminate internal fault from external fault. A four-terminal symmetrical bipolar HVDC grid based on MMC is modelled using PSCAD/EMTDC, and simulation results demonstrate the validity of the new proposed dc transmission line protection method.