Multi-terminal DC Networks Research Articles

Enhancing Dynamic Stability Performance of Hybrid AC/DC Power Grids Using a New Robust Controller

This research investigates the dynamic performance of a multi-terminal DC (MTDC) network linked to a wind farm side converter. The suggested approach is based on loop shaping, mutual quality decomposition, and an effective H controller architecture (MQDM). The transfer function was built as the wind power plant shifted from the Master-Slave control approach to the conventional feedback control method. The singular value curves of the original open-loop system and the suggested solution are compared in order to derive the weighting functions of the robust H controller. The control gain is then calculated by utilizing the MQDM approach to get the system's transfer function from the state space. By using the single value order reduction approach and frequency response analysis to generate a robust controller's gain, the suggested methodology was put into practice. The suggested method for controlling the DC grid voltage on the MTDC converter on the wind farm side has now been implemented in the MATLAB environment. Applying various uncertainties to the DC grid, wind turbines, and AC grid using simulation data demonstrates how reliable the suggested approach is.

A Multiport Embedded DC Power Flow Controller for Meshed DC Distribution Grids

Ring and meshed dc distribution grids have attracted increasing interest lately. Their multipoint and multiterminal structure facilitates flexible operational control and reliable power supply across the network. However, the dc grids implemented with existing power converters lack sufficient control freedoms for proper power flow adjustment; therefore, it is necessary to develop additional dc power flow controllers (DCPFCs) to be added to these grids for improved power flow control capability. Specific to multiterminal dc networks built with modular multilevel converters (MMCs), this article proposes an embedded DCPFC (e-DCPFC) directly integrated in the MMC topology. Unlike previously proposed solutions, the e-DCPFC configuration is derived from the conventional MMC and does not need any isolation transformers or external power supply. Its other main features include modular construction, convenience for multiport extension and bidirectional power flow adjustment capability. In this article, the topology, working principle, and the coordinated control strategy for e-DCPFC and its host MMC are elaborated. Experimental results from a lab prototype verify the proposed topology and its performance under typical working conditions.

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.

Multi-Terminal DC Grid with Wind Power Injection

With the development of offshore wind generation, the interest in cross-country connections is also increasing, which requires models to study their complex static and dynamic behaviors. This paper presents the mathematical modeling of an offshore wind farm integrated into a cross-country HVDC network forming a multi-terminal high-voltage DC (MTDC) network. The voltage source converter models were added with the control of active power, reactive power, frequency, and DC link voltages at appropriate nodes in the MTDC, resembling a typical cross-country multi-terminal type of HVDC scenario. The mathematical model for the network together with the controllers were simulated in MATLABTM and experimentally verified using a real-time digital simulator hardware setup. The resulting static and dynamic responses from the hardware setup agreed well with those from simulations of the developed models.

A graph theoretic approach to ensure a scalable performance measure in Multi-Agent Systems with MIMO agents

In this paper we consider issues of scalable performance measures in multi-agent systems, that is, performance measures that are independent of the size of the network. We include recent results for finding such performance measures and algorithms for a distributed design process, such that networks are able to reach a desired performance measure given constraints. Further, we can show a graph theoretic approach for guaranteeing a bound on the state deviations stemming from external disturbances. We can also show that this may be verified using local information only, thus yielding a fully scalable method. We exemplify this on a fictitious multi-terminal DC network.

Protection of MTDC Network Using a Resistive Type Superconducting Fault Current Limiter

Multi-terminal DC network (MTDC) offers great potential for long distance huge power delivery with multi-direction power transmission capability. However, the key obstacle in a realization of MTDC is the lack of existing commercial protection device can withstand the DC fault that rises rapidly and surge tenfold within several milliseconds over the whole system. The new technology called Superconducting Fault Current Limiter (SFCL) could bring a solution to the main bottleneck of the MTDC networks. In this work, an electro-thermal model of resistive type SFCL in series with a hybrid DC circuit breaker is proposed to protect a five terminal MTDC network. The numerical analysis carried out using (EMTP-RV®) software, and the simulation results show how effectively the SFCL can reduce the fault current and increase the breaking capability. Moreover, system stability is remarkably improved.

MMC control optimization approach to facilitate DC-side interoperability in MTDC networks

HVDC systems are expected to evolve from today's dominating point-to-point towards multi-terminal DC (MTDC) systems. Considering the European market structure, these MTDC networks are envisaged to be multi-vendor based. Due to proprietary reasons, the controls and tuning of the converters are not expected to be shared. Recent studies have shown interoperability based instabilities in multi-vendor based MTDC networks, which might lead to a shutdown of the network. To facilitate the envisioned development towards MTDC networks, the control interoperability between different converters in case of network extensions has to be ensured without having access to existing converters controls. This paper therefore presents an enhanced control tuning approach for Modular Multilevel Converters which are being connected to an existing DC network to ensure control interoperability. The existing network is seen as a black-box and only information accessible at the terminals of the new converter station is used. Using a frequency-sweep on the DC side, the equivalent MMC and network impedances at the terminals are derived. The proposed control optimization method incorporates the information gained from the impedance analysis and thereby ensures control stability. An example based on a three-terminal HVDC network demonstrates the validity of the concept.

The Effects of Filter Capacitors on Cable Ripple at Different Sections of the Wind Farm Based Multi-Terminal DC System

In most DC power systems, power electronic devices can introduce ripple content into the DC grid, where large input ripple currents on the DC link can have a negative influence. Under these circumstances, DC side filters play an important role in the reduction of ripple content. In this paper, based on a full detailed closed loop model of the entire offshore wind farm multi-terminal DC network, the effect of filter capacitors connected at different sections of the system on the limitation of the AC ripple content, particularly in the DC cables, is studied. In contrast to other work on HVDC for offshore wind, where simplified or equivalent circuits are mainly used while concentrating on the power and control system, this study can be regarded as the specific study of filter capacitors operating within a detailed system model. Utilising the advantages of PLECS, which is a highly effective tool in the simulation of power electronic circuits, no small-signal or simplified equivalent models are used in the system, and the entire study is based on detailed and accurate models of the semiconductor elements and transformers, which help to provide more realistic simulation results and a better understanding of the system. Another novel point in this paper is a new concept, relative losses, which is proposed to simplify extensively the calculation of losses in this research. Finally, the size of the filter capacitors at different sections of the system under different situations is suggested.

Supervised control of hybrid AC-DC grids for power balance restoration

In this paper, the flexibility of hybrid AC-DC distribution networks is exploited to coordinate multiple Distributed Energy Resources (DERs) with the aim of promptly restoring unexpected power imbalances caused by intermittent Renewable Energy Sources (RESs) and loads. Given the potential large-scale nature of the problem, the AC distribution network is decomposed into non-overlapping areas named clusters, equipped with MicroGrids (MGs) and non-dispatchable units, and interconnected also by the DC network. Each cluster is endowed with a Model Predictive Controller designed to compensate the local active power variability by requesting balancing services to the local MGs. A supervisory layer is designed and activated to optimally transfer power through the controllable DC links guaranteeing enough operative margins to each cluster. The designed architecture is tested on a benchmark grid composed of the IEEE 37-bus and 13-bus systems, connected by a multi-terminal DC network. The reported numerical results witness the effectiveness of the proposed approach.

Fault Simulations in a Multiterminal High Voltage DC Network with Modular Multilevel Converters Using Full-Bridge Submodules

The concept of high-voltage DC transmission using a multiterminal configuration is presently a central topic of research and investment due to rekindled interest in renewable energy resource integration. Moreover, great attention is given to fault analysis, which leads to the necessity of developing proper tools that enable proficient dynamic simulations. This paper leverages models and control system design techniques and demonstrates their appropriateness for scenarios in which faults are applied. Furthermore, this paper relies on full-bridge submodule topologies in order to underline the increase in resilience that such a configuration brings to the multiterminal DC network, after an unexpected disturbance. Therefore, strong focus is given to fault response, considering that converters use a full-bridge topology and that overhead power lines connect the terminals.

Grid Code-Dependent Frequency Control Optimization in Multi-Terminal DC Networks

The increasing deployment of wind power is reducing inertia in power systems. High-voltage direct current (HVDC) technology can help to improve the stability of AC areas in which a frequency response is required. Moreover, multi-terminal DC (MTDC) networks can be optimized to distribute active power to several AC areas by droop control setting schemes that adjust converter control parameters. To this end, in this paper, particle swarm optimization (PSO) is used to improve the primary frequency response in AC areas considering several grid limitations and constraints. The frequency control uses an optimization process that minimizes the frequency nadir and the settling time in the primary frequency response. Secondly, another layer is proposed for the redistribution of active power among several AC areas, if required, without reserving wind power capacity. This method takes advantage of the MTDC topology and considers the grid code limitations at the same time. Two scenarios are defined to provide grid code-compliant frequency control.

Optimal power flow in multi‐terminal HVDC networks with embedded high‐power DC–DC converters for voltage matching and flexible DC transmission

High-power DC–DC converters are integrated into multi-terminal high-voltage DC (MTDC) networks, typically for voltage matching or flexible DC transmission. This study addresses droop-controlled radial and mesh MTDC networks with transmission loss minimisation considering the impact of integrating a high-power DC–DC converter. DC voltage droop control can achieve minimum power loss in radial MTDC transmission network by controlling the power-sharing among the receiving nodes inversely proportional to their transmission line resistances. Updating the droop characteristics by an optimisation algorithm guarantees optimal power flow (OPF) during renewable source power generation fluctuation. Meanwhile, the mesh network DC voltage references can be updated by the OPF for transmission loss minimisation. This study presents the impact of incorporating a high-power DC–DC converter at different system access lines of a radial MTDC network while targeting efficient power transfer and DC transmission loss minimisation. The effect of introducing a high-power DC–DC converter on the droop characteristic settings during steady-state operation in a radial MTDC network is investigated. Moreover, the impact of a high-power DC–DC converter in mesh MTDC network with OPF operation is explored. To validate the presented concepts, the CIGRE B4 MTDC network is used as a benchmark system with a metaheuristic optimisation technique.

Electric Contact Material Selection for Medium and High Voltage DC Circuit Breakers

Medium and high voltage DC grids are increasingly proposed for terrestrial, shipboard and aircraft power systems. However, multi terminal DC networks can only be realized with the development of a DC circuit breaker. Due to the difficulty in breaking DC currents, hybrid circuit breakers—which consist of a non-arcing fast mechanical disconnect switch (FMS) in parallel with a solid state switch—is the most promising solution. The stresses experienced by the electric contacts of FMS are different from those of AC and low voltage DC breakers and disconnect switches. The choice of contact material has a significant impact on the performance of the DC circuit breaker. The Ashby method is used to systematically identify the best suited contact materials by translating the requirements of FMS contacts into objectives and constraints and deriving material indices for each objective. Minimizing power loss, wear and overheating of contacts are identified as the key objectives. The results suggest that copper-based alloys and compounds are more suitable than silver based alloys and compounds and other contact materials.

Backstepping Technique-Based Customer-End Voltage Control Strategy of DC Distribution Network

This article develops a robust control strategy for voltage regulation of customer-end ac loads, where the low-voltage multiterminal dc network has been used for power distribution purposes. The control law for load-end dc–ac converter is synthesized by Lyapunov’s direct method using the backstepping design process. The resultant control action is simply obtained by a linear combination of states and reference set-points and does not add extra dynamics into the system avoiding complexity. The proposed method does not require load current measurement. Simulation studies in MATLAB/Simulink show better dynamic performance compared with conventional PI controllers. The experimental results from a laboratory-scale prototype demonstrate the robustness of the proposed controller against variations in type and the amount of load profile and dc voltage.

Impact of forced and unforced system parameter variations on network stability and system economics of radial MTDC networks

This paper focuses on investigating the impact of system parameters uncertainty on voltage droop-controlled Multi-Terminal High-Voltage DC (MTDC) networks. The voltage droop typically controls the power-sharing among the receiving end-nodes for minimum transmission power loss. The hierarchical control layers are used to update the droop constants to cope with the system power/parameter variations. In this paper, a study is presented elucidating the effect of unforced system parameter variations, as transmission line resistance uncertainty and the power injection variation, on the droop characteristic settings for radial MTDC networks, from system economics and network steady-state and dynamic stability perspectives. Moreover, forced system parameter variations, such as those due to cyber-attacks, are addressed in this paper. Cyber threat with a malicious attack on the communication system in the hierarchical control layers is considered particularly during power variations to investigate the impact of falsifying the DC voltage droop characteristics. Towards generalization, two different droop-controlled radial MTDC networks, with optimal droop gains adjusted for minimum transmission loss, are employed for explicating and assessing the presented concept.

Comprehensive study on fault‐ride through and voltage support by wind power generation in AC and DC transmission systems

This study exploits the impact of different low voltage ride-through (LVRT) methods and equipment on both the wind energy elements and the grid including wind turbine/farm ability to provide reactive compensation and maintain controllability during faults. The potential of using SFCL as an alternative LVRT equipment is preliminary studied. The study also exploits some severe scenarios that could face a multi-terminal high-voltage DC (HVDC) network. The influences of AC faults and control errors are examined. Results show limited deviations between the adopted LVRT methods. The wind turbine has to contribute to the stability of the AC collection grid of the wind farm, but it does not influence the grid, as both are decoupled through the multi-terminal HVDC grid. The implemented test systems and the examined events are developed in Matlab/Simulink and DIgSILENT.

A Continuation Power Flow Model of Multi-Area AC/DC Interconnected Bulk Systems Incorporating Voltage Source Converter-Based Multi-Terminal DC Networks and Its Decoupling Algorithm

Existing continuation power flow (CPF) models mainly focus on the regional independent systems, which are not suitable for multi-area AC/DC interconnected systems because the market trading behaviors and security control for power allocation of tie-lines are ignored. This study presents a novel CPF model and its decoupling algorithm for multi-area AC/DC interconnected systems incorporating a voltage source converter (VSC)-based multi-terminal direct current (MTDC) network. This CPF model includes the following unique features: (1) In view of the bilateral power trading contracts among regional subsystems, the nonlinear constraint equations of directional trading active power via interface are derived, and the multi-balancing machine strategy is introduced to realize the active power balance of each subsystem. (2) An accurate simulation method for the security control behaviors of the power allocation in tie-lines is proposed, which includes a specific selection strategy for automatic generation control units and a generation re-dispatch strategy. These two strategies work together to prevent the serious overload in tie-lines during load growth and improve the voltage stability margin of the interconnected bulk systems. (3) The switching characteristic of reactive power control behaviors of VSC stations is simulated in the CPF calculation. In the end, a novel decoupling CPF algorithm based on bi-directional iteration is presented to realize the decomposition and coordination calculation. This decoupling algorithm preserves the precision and convergence of integrated CPF algorithms, and it has an apparent advantage on the calculation speed. Furthermore, this decoupling algorithm also can easily reflects the effects of the control mode changes of VSC stations to the voltage stability margin of AC system. Case studies and comparative analysis on the IEEE two-area RTS-96 system indicate the effectiveness and validity of the proposed CPF model and corresponding decoupling algorithm.

DC Fault Control and High-Speed Switch Design for an HVDC Network Protection Based on Fault-Blocking Converters

Future multi-terminal dc networks are envisioned for the large-scale integration of renewable energy sources into today's power systems. An essential aspect for the reliable operation of these systems is the fast and selective dc-side fault handling and separation of faulted lines. The objective of this paper is the development and evaluation of a multi-terminal dc protection strategy based on fault-blocking converters (e.g., full-bridge based modular multi-level converters) and high-speed switches. Both a suitable fault control and a new high-speed switch design for a fast separation of faulted dc lines are elaborated. The novel concept is intended to reduce the requirements on the high-speed switches compared to dc circuit breakers and to ensure a fast restoration of the active power transmission capability, as well as the dc voltage. The fault-clearing strategy is analyzed using electromagnetic transient (EMT) simulations under variation of the fault location, type, and resistance. The investigations are carried out in a cable-based multi-terminal HVdc system with full-bridge modular multi-level converters in symmetrical monopole configuration.

Open-Loop Modal Coupling Analysis for a Multi-Input Multi-Output Interconnected MTDC/AC Power System

This paper examines the small-signal stability of an integrated multiterminal dc (MTDC)/ac power system. The examination is based on a closed-loop multi-input multi-output interconnected model of the MTDC/ac power system, which comprises an MTDC subsystem and an ac subsystem. The focus of the examination is the condition of open-loop modal coupling when an open-loop oscillation mode of the MTDC subsystem is close to an open-loop oscillation mode of the ac subsystem on the complex plane. Analysis in the paper indicates that when the open-loop modal coupling happens, the multivariable ac–dc dynamic interactions between the MTDC and ac subsystems degrade the small-signal stability of the MTDC/ac power system. Estimation on the degree of stability degradation is derived. Hence, the paper reveals the mechanism about why the integration of the MTDC network may bring about the small-signal instability risk. In addition, a method of open-loop modal analysis to detect the risk is proposed. Example multimachine power systems with an MTDC network are presented to validate the analysis and conclusions made in the paper. Application of the proposed open-loop modal coupling analysis for parameter tuning of converter control systems of the MTDC network is demonstrated.

Stability Evaluation on the Droop Controller Parameters of Multi-Terminal DC Transmission Systems Using Small-Signal Model

For a complex multi-terminal DC (MTDC) transmission system, controller parameter settings exert tremendous influence on the system dynamics and transient stability. The traditional droop control scheme, which aims to locally control the converter's power by regulating its DC voltage according to the pre-defined droop slopes, does not normally employ communications for coordination with other terminals. The droop scheme is designed for power sharing purpose and its controller gains' impact on the MTDC network voltage is usually ignored. Moreover, the droop controller parameters are selected without considering various operating states of the MTDC terminals. This paper proposes a droop controller parameters select the criterion to cater to MTDC network voltage stability. To do so, a detailed small-signal model for a four-terminal MTDC system candidate with typical network topology, considering each converter station's electrical characteristics and cascaded control loops, is established. In order to maintain the fidelity for DC network modeling, the model also takes account of the DC network electrical dynamics and losses. A time-domain simulation model is built in Matlab Simulink to verify the correctness of the small-signal model.