Multi-terminal DC Grid Research Articles

A Thyristor-Based Multiline Hybrid DCCB in Protection Coordination With MMC in Multiterminal DC Grids

A Thyristor-Based Multiline Hybrid DCCB in Protection Coordination With MMC in Multiterminal DC Grids

Small signal analysis of DC voltage control based on a virtual resistance of DC/DC converter integrated in a multiterminal DC grid

AbstractThe future multi‐terminal direct‐current (MTDC) grid will require the interconnection of point‐to‐point high‐voltage (HV) DC links with different specifications such as DC voltage level, system grounding configuration and HVDC technology. To adapt these differences, it is obligatory for DC/DC converters to interconnect HVDC links. Additionally, they are capable of providing supplementary functionalities as they are highly controllable devices. In this article, a primary virtual resistance DC voltage controller associated with DC/DC converter is proposed for managing DC grid voltages of the interconnected HVDC grids, increasing the reliability of the system. The commonly known topology, Front‐to‐Front Modular Multilevel Converter (F2F‐MMC) is adopted for DC/DC converter. Time‐domain simulations are performed using EMTP software for validating the controller behaviour under power disturbances and large events of loss of one converter in a MMC‐based MTDC system. The converters are modelled using reduced order modelling (ROM) methodology. Apart from this, dynamic studies have been carried out using a linear state space model for small‐signal stability analysis of a HVDC system integrating DC/DC converter with a virtual resistance DC voltage controller. The results are examined through parametric sensitivity analysis.

Mitigating SSO in an Actual DFIG-MTDC System: Field Implementation and Tests

Recently, a new type of subsynchronous oscillation (SSO) event was observed in the doubly fed induction generator (DFIG)-based wind farm connected to a multi-terminal dc (MTDC) grid. It has been established that converter-based devices can interact with each other and other grid components. Generally, it is difficult to replicate an SSO event and devise a mitigation scheme due to the complex and black-box nature of converter-based devices. This paper demonstrates the field implementation of a model-independent method to mitigate the SSO using additional band-rejection filters in the DFIG's converter controller. The proposed method is unique in the sense that the complete design and implementation were carried out without detailed models or prior EMT studies. Field results illustrate that the risk of SSO can be significantly reduced by supplementing the DFIGs with properly tuned band-rejection filters, which reshape the DFIGs' impedance characteristics at the desired frequency. The test results obtained under various operating conditions further confirm the effectiveness of the mitigation scheme. The implementation and field tests of the filter-based mitigation scheme in an actual system can provide a practical reference for solving similar SSO issues in other large-scale systems without going through detailed EMT studies.

Integrated Equipment with Functions of Current Flow Control and Fault Isolation for Multiterminal DC Grids

Integrated Equipment with Functions of Current Flow Control and Fault Isolation for Multiterminal DC Grids

A Fault Detection Algorithm Based on Artificial Neural Network Threshold Selection in Multi-Terminal DC Grids

The DC fault is one of the critical challenges in the High Voltage Multi-Terminal Direct Current (HV-MTDC) grid. The fixed value threshold used in some implementations may be affected by different grid conditions and thus causes errors in the fault detection procedure. This paper offers a two-stage fault detection algorithm based on the integrated Artificial Neural Network (ANN) and Discrete Wavelet Transform (DWT) theory. The main innovations of the proposed method are: 1) Increasing the reliability of the fault detection operation by solving the fixed value threshold problems; 2) Offering high robustness against the wide range of fault resistances; 3) Providing bus protection capability and 4) Setting relays locally, and avoiding the requirement to high-speed communications links. Also, it offers DC fault detection in a short period of time. The performance of the proposed protection scheme has been ascertained via simulations in the MATLAB/Simulink environment. The results prove the robustness of the proposed method against noise disturbance, high resistance fault, and grid parameters variations.

Fault protection scheme of flexible DC grid based on transient power instantaneous energy difference

The DC line protection is the key to the stable operation of the multi-terminal flexible DC grid, and the current limiting reactor (CLR) as the boundary condition provides great convenience for the design of the flexible DC grid fault protection scheme. Using the boundary condition characteristics of the CLR, the paper proposes a fault protection method for flexible DC grid based on transient power instantaneous energy (IE) difference. First, the equivalent circuit of the DC line of the flexible DC grid under different faults is established. On this basis, the conclusion that the transient power amplitude of internal fault is much larger than the external is deduced from the full-power in the time-domain. The Teager energy operator is used to calculate the maximum IE of the transient power, and then the internal and external faults are identified. Secondly, according to the difference of the transient power amplitude of the CLR on the fault pole and the healthy pole, the ratio of the IE of the positive and negative CLR transient power is used to construct the fault pole selection. The scheme is simple and easy to implement, and does not need to consider the synchronization problem of data at both sides. Finally, the simulation results show that the proposed method can better satisfy the speediness and selectivity requirements of DC grid protection. The scheme can accurately detect faults under different working conditions, and has strong fault resistances capability.

A Comprehensive Review of Multiport DC Circuit Breakers for MTdc Grid Protection

Voltage source converter-based multiterminal dc (MTdc) grids, including high- and medium- voltage level, have attracted attention in recent years and developed fast. To protect the MTdc grids selectively, the dc circuit breaker (DCCB) is identified as the best choice. However, due to the high unit price and the required large number of DCCBs in the future large-scale MTdc grids, the total investment cost is unacceptable. Besides, the large volume and weight also limits their applications. To solve these problems, a variety of multiport dc circuit breakers (MPCBs) are proposed, which achieve significantly reduced costs and footprints by component sharing and device integration. This paper first introduces the classifications and the basic principles of the existing MPCBs, furthermore summaries their technical requirements. Then, the economic and functional analysis of each MPCB is presented, and the performance comparisons among different topologies are elaborated. Finally, the challenges and future trends of the MPCBs are discussed and the findings are concluded.

Optimal power distribution with mitigation of DC voltage fluctuations using artificial intelligent droop control scheme for MTDC grid

Optimal power share contribution with minimum DC voltage deviations is highly recommended in voltage source converter multi-terminal DC (VSC-MTDC). Hence, in this study, an artificial neural network based intelligent adaptive droop control proposes to enhance MTDC closed-loop system dynamics in terms of optimal power share and small redistribution of terminal DC voltages with negligible oscillations even in the presence of AC grid load demand change, admittance matrix uncertainty, outage of terminals and integration of wind farms. This is achieved via three-layer neural network and its weights are adaptively updated using error-back propagation algorithm. The asymptotic convergence of proposed control is derived using the Lyapunov’s stability theorem. Thus, the proposed intelligent adaptive droop control strategy not only guarantees to improve power share contribution with minimum voltage but also improved power quality and service continuity of MTDC grid. The robustness of proposed intelligent adaptive droop control is demonstrated on five terminal multi-terminal DC grid via MATLAB®.

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.

Multiport Current-Limiting Hybrid DC Circuit Breaker for MTdc Grids

The coordination configuration of current-limiting inductors (CLIs) and hybrid dc circuit breakers (HCBs) is currently the most commonly used fault handling method to ensure fault ride-through of multi-terminal dc (MTdc) grids after the dc-side fault occurs. However, the high cost of HCB limits its large-scale application. In addition, the installation of the large CLIs will adversely affect the HCB performance, such as extending the fault current interruption time and increasing the energy dissipation pressure of the arrester. To solve these problems, a multiport current-limiting hybrid dc circuit breaker (MCL-HCB) is proposed in this paper. Compared with the typical HCBs, the MCL-HCB can achieve full selective protection of all connected dc lines with significantly reduced investment costs. Besides, it can suppress the rising speed of the fault current by embedded CLIs, while eliminating their negative impacts on the fault current interruption. The feasibility and superiority of the proposed MCL-HCB are verified by simulations in the PSCAD/EMTDC software and experiments in the scaled-down experimental setup. Besides, the performance comparison between the MCL-HCB and existing solutions is elaborated.

A Multiport Current Flow Controller for Meshed Multiterminal DC Grids

In the meshed multiterminal dc (MTDC) grid, the individual line currents cannot be controlled precisely without additional equipment. To address this issue, the current flow controllers (CFCs) that can regulate the line current and avoid line overloads are adopted. A new multiport current flow controller topology with independent regulation capability has been proposed in this article. This CFC topology is composed of the main circuit and auxiliary circuit, which are connected via the coupled capacitors. The proposed CFC topology can regulate one or more line currents in the MTDC grids and can realize line currents regulation independently. Furthermore, it can be applied in medium voltage or high voltage dc systems due to the modular structure. The power losses of the proposed CFC are minimized because the auxiliary circuit transmits small power and operates in resonant mode. Meanwhile, the operation principle and the control strategy of the proposed CFC topology are illustrated in this article. The comparison with other CFC topologies previously proposed proves the advantages of the proposed topology. Simulations performed in MATLAB/Simulink and experiments based on a down-scaled prototype are completed to verify the operation principle and control strategy.

Single-ended DC line protection scheme based on first peak time of current-limiting reactor voltage

Fast and sensitive DC line protection is an important guarantee for the security of the flexible DC grid. However, the reliability and sensitivity of the DC line protection are greatly affected by the high fault resistance. To overcome this problem, this paper proposes a novel single-ended line protection scheme for multi-terminal flexible DC grid. Firstly, the propagation characteristics of traveling wave after the internal and external faults are analyzed. Furthermore, a single-ended protection criterion based on the first peak time of current-limiting reactor transient voltage is proposed to distinguish the internal and forward external faults, which is independent of simulation. Then, based on the change characteristics of fault component current, a fault direction criterion and fault pole identification criterion are constructed to identify the backward fault and fault pole, respectively. Finally, extensive simulations and experiments show that the proposed protection scheme can identify the fault quickly and sensitively even for the internal remote-end fault with high fault resistance.

GriDConv – control, design, and experimental verification of a lab-scale high-voltage DC-DC converter

The advent of the Modular Multilevel Converter technology allows building multi-terminal high-voltage DC grids. In order to interconnect DC grids with different voltage levels, highly efficient and compact high-voltage DC-DC (HVDC) converters are required. This paper presents the control and design of a transformerless high-voltage DC-DC converter, which has been designed and built within the framework of the research project GriDConv at the Graz University of Technology. The converter basically consists of high-voltage half-bridges in combination with a series of submodules with full-bridge technology. A dedicated control strategy for the converter is developed, which allows advantageous soft switching of the high-voltage half-bridges, which considerably reduces switching losses of the half-bridges. In addition, a balancing strategy for the individual cell voltages as well as a control of total cell voltage are presented. The developed control strategies are discussed in detail and, in the end, verified through experiments on a constructed 50,text{k}text{W} lab-scale hardware demonstrator.

Adaptive Reclosing Scheme Based on Traveling Wave Injection For Multi-Terminal dc Grids

The hybrid dc circuit breaker (HCB) has the advantages of fast action speed and low operating loss, which is an ideal method for fault isolation of multi-terminal dc grids. For multi-terminal dc grids that transmit power through overhead lines, HCBs are required to have reclosing capability due to the high fault probability and the fact that most of the faults are temporary faults. To avoid the secondary fault strike and equipment damage that may be caused by the reclosing of the HCB when the permanent fault occurs, an adaptive reclosing scheme based on traveling wave injection is proposed in this paper. The scheme injects traveling wave signal into the fault dc line through the additionally configured auxiliary discharge branch in the HCB, and then uses the reflection characteristic of the traveling wave signal on the dc line to identify temporary and permanent faults, to be able to realize fast reclosing when the temporary fault occurs and reliably avoid reclosing after the permanent fault occurs. The test results in the simulation model of the four-terminal dc grid show that the proposed adaptive reclosing scheme can quickly and reliably identify temporary and permanent faults, greatly shorten the power outage time of temporary faults. In addition, it has the advantages of easiness to implement, high reliability, robustness to high-resistance fault and no dead zone, etc.

Traveling Wave-Based Primary Protection and Fault Localization Scheme for MTDC Grid Considering IEC 61869-9 Measurement Standard

Traveling wave (TW) based protection and fault localization requires high sampling frequency measurements (500 kHz - 1 MHz) and are non-compliant to the IEC 61869-9 measurement standards, which allows maximum sampling frequency of 96 kHz for DC application. This work proposes a novel low sampling frequency (50 kHz) TW based primary protection and real-time DC fault localization technique for multi terminal high voltage DC (MTDC) grid which is compliant with the aforementioned measurement standard. In the proposed MTDC protection scheme, a TW arrival time (TWAT) is estimated under low sampling frequency measurement using cascaded combination of modified mathematical morphology gradient (MMG), sobel operator and linear regression tool. Subsequently, the TWAT information at both DC link terminals is communicated via dedicated communication channel compliant to IEC-61850-9-2 standard. And if the TWAT difference between DC terminals is less than the TW propagation threshold for the protected DC line, then a DC trip signal is issued to the associated hybrid DC circuit breaker (HDCCB). For fast power system restoration, the same TWAT information is also used for real-time fault localization in the faulted DC line, which is also compliant with IEC 61869-9 measurement standards. Finally, the practical feasibility of the proposed algorithm is demonstrated through control hardware-in-loop testing with real-time digital simulation setup and TI TMS320F28379d digital signal processor card.

Multiport Hybrid DC Circuit Breaker With Current Flow Control for MTDC Grids

The multiterminal dc (MTDC) grid plays an important role in large-scale power transmission and renewable energy integration owing to its excellent performance. However, there are still some problems existing in the MTDC grid, such as difficult fault current interruption and current flow control, which impedes its development. Currently, the mentioned two problems can be separately solved by hybrid dc circuit breaker (HCB) and current flow controller (CFC). However, when configured separately, they are expensive and have large footprints. Therefore, this article proposes a multiport hybrid dc circuit breaker (MHCB) with current flow control capability. The HCBs near the same dc bus are first integrated into a single MHCB and then novel CFCs are embedded into the MHCB to replace the load commutation switches. Compared with the existing MHCBs, the proposed topology shows advantages in the aspects of implementation cost, power loss, and operation speed. The operation principle is analyzed and the basis for parameter design is given in detail. The effectiveness of the proposed topology is verified by extensive simulations and experiments. Besides, the proposed MHCB is compared with the existing solutions.

Improved Integrated Hybrid DC Circuit Breaker for MTDC Grid Protection

The current-limiting inductors are usually installed in the multi-terminal dc grid to limit the fault current rising speed. However, they will increase the energy dissipation pressure and fault isolation time of the dc circuit breaker, which puts forward higher requirements on the capacity of arresters in hybrid dc circuit breaker and the heat dissipation of grid equipment. Therefore, an improved integrated hybrid dc circuit breaker is proposed in this brief for applications in the multi-terminal dc grid. Compared with the fault isolation solution based on existing integrated hybrid dc circuit breakers, the proposed breaker has slightly higher cost of semiconductor switches but significantly reduced arrester capacity requirement and faster fault isolation speed by bypassing the inductance components. Extensive electromagnetic simulations and physical experiments verify the effectiveness of the proposed device.

A multi‐port hybrid circuit breaker integrated with power flow control ability

Power flow controllers and DC circuit breakers are compulsory components in a multi-terminal DC grid with overhead lines. They are both composed of similar power electronic devices and thus can be reasonably combined together to realize cost savings. This paper proposes a multi-port circuit breaker integrated with a multi-port power flow controller (M-PFCHCB). The topology structure, operation principles, control strategy and parameter configuration are illustrated in detail. The main breakers are composed of unidirectional power electronic switches, and the selection branch is based on two diode strings, which can significantly save the investment of the breaker. The proposed M-PFCHCB is able to deal with the fault isolation and the ultra-fast disconnector failure with high performance. Besides, the power flow of multiple lines can be simultaneously regulated by the M-PFCHCB without any external power supply. The internal power balance is achieved by controlling the AC circulating current inside the M-PFCHCB. The simulations conducted on PSCAD in a five-terminal DC grid with eight DC lines verify the feasibility of the proposed M-PFCHCB. The technical and economic analysis performed proves that the proposed M-PFCHCB can achieve the comprehensive functions of fault isolation and power flow control with low construction cost.

Stability improvement of large-scale power systems including offshore wind farms and MTDC grid aiming at compensation of time delay in sending robust damping signals

The main aim of this paper is to design a bi-level damping controller for compensating local and inter-area oscillating modes in the power system. Regarding the need for compensation of time delays in measuring the wide-area signals and transmitting them to the damping controllers, a method is required that can appropriately obtain the controllers’ gains in the presence of signal delays. A robust H∞-based criterion is employed in this paper to design a bi-level damping controller. In the proposed design, to reduce conservatism of linear matrix inequality (LMI), an auxiliary function is defined in Lyapunov function. In this function, by including triple adder, reinforced variables, and cross-terms, the conservatism reduction of LMI and preparation of appropriate gains for controllers using state-feedback is fulfilled. The second purpose is to apply the proposed approach on a large-scale power system in the presence of wind power units where transmission of their output power is carried out by multi-terminal DC (MTDC) grid. Regarding the large scale of the power system and control converters in each of the equipment, geometric measurements based on observability and controllability are utilized to acquire accurate damping signals for the input of each controller. Simulations are obtained in different uncertainty scenarios. The results clearly demonstrate robustness of the conducted approach.

A Fault Clearance and Restoration Approach for MMC-Based MTDC Grid

With the growth in continuous energy demand, high-voltage Multi-Terminal DC (MTDC) systems are technically and economically feasible to transmit bulk power and integrate additional energy sources. However, the high vulnerability of the MTDC systems to DC faults, especially pole-to-pole (P2P) faults, is technically challenging. The development of DC fault ride-through techniques such as DC circuit breakers is still challenging due to their high cost and complex operation. This paper presents the DC fault clearance and isolation method for an MMC-based MTDC grid without adopting the high-cost DC circuit breakers. Besides, a restoration sequence is proposed to re-energize the DC grid upon clearing the fault. An MMC-based four-terminal DC grid is implemented in a Control-Hardware-in-Loop (CHIL) environment based on Xilinx Virtex-7 FPGAs and Real-Time Digital Simulator (RTDS). The RTDS results show that the MTDC system satisfactorily rides through DC faults and can safely recover after DC faults.