Multi-terminal DC Grid Research Articles

Analytical Calculation of Transient Short-Circuit Currents for MMC-Based MTDC Grids

DC short-circuit current calculation is necessary to design and protect modular multilevel converter-based (MMC-based) multiterminal dc (MTDC) grids. In this article, a general and analytical short-circuit current calculation method is proposed for bipolar MTDC grids. For pole-to-pole (P2P) faults, the superposed fault equivalent network of the MTDC grid is first developed in the frequency-domain. Then, a decoupling method based on the network inductance ratio is proposed, which simplifies the high-order network into the superposition of several independent two-order <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> circuits. As a result, the short-circuit current can be expressed as the sum of the currents of these <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> circuits. For pole-to-ground (P2G) faults, the phase-domain dc line impedance considering the coupling effects is determined based on its modal-domain parameters, so that the P2G fault current can be calculated using the method of the P2P fault. The proposed method’s effectiveness is verified through simulations and experiments, and the calculation error is classified and analyzed. Compared with the existing methods, the proposed method is more accessible, more accurate, and available for hand-solution.

Bridge-Type Multiport Fault Current Limiter for Applications in MTdc Grids

Since the dc-side fault in the multiterminal dc grid propagates extremely fast, the fault current limiter (FCL) is essential to achieve fault ride-through of the healthy network. Current-limiting inductors (CLIs) are currently the most commonly used FCLs in projects that have been built, are under construction, or are planned. The CLIs can be designed based on the system requirements, such as converter fault ride-through and dc circuit breaker (DCCB) interrupting capacity. Unfortunately, the installation of CLIs has negative impacts on the system transient response and operational stability, the fault isolation speed and energy dissipation of DCCBs. To solve these problems, an economical bridge-type multiport fault current limiter (BT-MFCL) is proposed in this article, which can be installed at the dc bus with multiple dc lines connected and can cooperate with the existing DCCBs. The BT-MFCL is composed of a CLI on each port and a shared energy dissipation circuit (EDC). The fault current can be limited by the embedded CLIs in the BT-MFCL without a time delay, while the EDC can eliminate the negative influences of CLIs. Extensive simulations and experiments have verified the effectiveness of the BT-MFCL. Besides, the performance comparison of the BT-MFCL with existing solutions is elaborated.

Implementation and Validation of a Model Predictive Controller on a Lab-Scale Three-Terminal MTDC Grid

In this paper, a reliable methodology is proposed in order to implement and validate a Model Predictive Control (MPC) scheme on an actual Voltage Source Converter (VSC) integrated in a scale-down multi-terminal DC grid. The objective of the investigated MPC controller is to enable AC frequency support among two asynchronous AC areas through a High Voltage Direct Current (HVDC) grid, while considering physical constraints, such as maximum and minimum DC voltage. A systematic and accurate implementation strategy is proposed, based mainly on the Hardware In the Loop (HIL) and Power Hardware In the Loop (PHIL), leading to the real-life testing on VSC, controlled by a classical microcontroller. The technical problems during the implementation process, as well as the proposed solutions, are described in detail through this paper. This procedure is deemed valuable to bridge the gap between offline simulation and the actual implementation of such advanced control scheme on experimental test rig.

Online distributed design for control cost reduction

In this paper we consider the problem of reducing control costs in Multi-Agent Systems in a distributed fashion, while maintaining or seeking a certain level of global performance. The problem is formulated to minimize the controller gains locally, while respecting a global bound on the state deviations due to effects of external disturbances We show that with the chosen performance measure this can be reformulated as a convex optimization problem. However, due to the limited knowledge of the agents and structural changes in the networks, currently existing algorithms for solving distributed optimization problems cannot be used as is. We therefore derive an algorithm that is capable of solving the problem, while also respecting limitations caused by the aforementioned issues. We end the paper with an example where the problem is solved in a Multi-Terminal DC-grid.

Multiport Hybrid DC Circuit Breaker With Reduced Fault Isolation Time and Soft Reclosing Capability

The hybrid dc circuit breaker is one of the most promising dc circuit breakers (DCCBs). However, its cost is relatively high, and the completion of its fault isolation is significantly delayed because of the large current limiting inductors configured in the multiterminal dc grid. Besides, due to the utilization of overhead lines, the probability of temporary faults is much increased, which means there is a great possibility that the power supply can be rapidly restored by reclosing DCCBs. However, the temporary faults must be distinguished from the permanent faults before the reclosing operation to avoid damaging the fragile power electronic equipment. Therefore, considering the above issues, a novel multiport hybrid dc circuit breaker (MHCB) is proposed in this article. The MHCB mainly has three advantages: the reduced implementation cost by sharing the expensive main breaker among multiple lines, the reduced fault isolation time by bypassing the current limiting inductor during energy dissipation, and the ability to reclose softly. Extensive simulations and experiments verify the effectiveness of the device. In addition, the comparison of the new device and existed solutions is presented.

Transient characteristic analysis for large-scale power system with MMC-MTDC

Modular multi-level converter (MMC)-based multi-terminal dc (MTDC) grid (abbreviated as MMC-MTDC) is promising in the integration and transmission of bulk renewable energy. This paper analyzes the transient characteristic of large-scale power systems with the MMC-MTDC. First, the transient stability-electromagnetic transient (TS-EMT) model of the system is established. The corresponding simulation flowchart is also presented, where the MMC-MTDC grid dynamics are calculated by EMT simulation and the ac grid is simulated by the TS program. Second, based on the mathematical model of the MMC-MTDC grid, the transient characteristics during various faults are analyzed. Third, it analyzes the influences of the controller parameters on the transient characteristics. Test results on the North-China power system with Zhangbei MTDC verifies the correctness of the transient characteristic analyses.

Review, Classification and Loss Comparison of Modular Multilevel Converter Submodules for HVDC Applications

The circuit topology of a submodule (SM) in an modular multilevel converter (MMC) defines many of the functionalities of the complete power electronics conversion system and the specific applications that a specific MMC configuration can support. Most prominent among all applications for the MMC is its use in high-voltage direct current (HVDC) transmission systems and multiterminal dc grids. The aim of the paper is to provide a comprehensive review and classification of the many different SM circuit topologies that have been proposed for the MMC up to date. Using an 800-MVA, point-to-point MMC-based HVDC transmission system as a benchmark, the presented analysis identifies the limitations and drawbacks of certain SM configurations that limit their broader adoption as MMC SMs. A hybrid model of an MMC arm and appropriate implementations of voltage-balancing algorithms are used for detailed loss comparison of all SMs and to quantify differences among multiple SMs. The review also provides a comprehensive benchmark among all SM configurations, broad recommendations for the benefits and limitations of different SM topologies which can be further expanded based on the requirements of a specific application, and identifies future opportunities.

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.

Investigation of Subsynchronous Oscillation in a DFIG-based Wind Power Plant Connected to MTDC Grid

This paper investigates a new type of subsynchronous oscillation (SSO) event observed in a real-world doubly fed induction generator (DFIG)-based wind power plant (WPP) connected to a multiterminal dc (MTDC) grid in Northern China. The system under study is unique and, to date, is the world&#x0027;s largest four-terminal DC transmission network, which connects far-away wind and photovoltaic power plants to regional load centers. For the first time, this paper utilized field-measured data to analyze the characteristics and reveal the underlying mechanism of the SSO in doubly fed induction generators (DFIGs) connected to HVDC grids. The analysis is carried out using the impedance network model established from the impedances obtained from field recorded data. The investigations on the field data concluded that the SSO is essentially caused by the subsynchronous interaction between the DFIGs and the MTDC grid without the presence of a physical series&#x002F;shunt compensation or weak AC grid. Instead, it is found that the SSO is triggered due to the converter-grid resonance where the MTDC grid presented itself as a virtual capacitor and the DFIGs showed negative resistance at the subsynchronous frequency range.

Active Damping of VSC-MTDC Grid Equipped With Interline Power Flow Controller

To ensure the secure operation of a multiterminal high-voltage DC (MTDC) grid, power flow controllers (PFCs) are deployed to regulate current distribution among different transmission lines. Besides power flow management, PFCs are envisioned to provide a range of functionalities such as stability enhancement, oscillation damping, and ancillary services to the host MTDC grid. This paper extends the functionality of PFC to provide active damping of MTDC grid current oscillations caused by dc side resonance. Three simple and effective active compensators integrated with the control scheme of the PFC are proposed. A comprehensive small-signal model of the MTDC grid is developed. Eigenvalue and sensitivity analyses are conducted to evaluate the damping capability of the proposed compensators and assess their dynamic coupling with PFC control loops. Based on a detailed model of a five-terminal high-voltage dc grid, simulation results are provided to evaluate the performance and effectiveness of the proposed active compensators. The results showed the effectiveness of the proposed compensation schemes to increase the damping of the MTDC grid and enhance its dynamic response.

Parallel-in-time-and-space electromagnetic transient simulation of multi-terminal DC grids with device-level switch modelling

Parallel-in-time-and-space electromagnetic transient simulation of multi-terminal DC grids with device-level switch modelling

DC grid controller for optimized operation of voltage source converter based multi-terminal HVDC networks

Generally, multi-terminal dc grids provide full control over the dc powers that converter terminals exchange with their respective host ac grids. Nevertheless, the same level of controllability does not exist over the power flow in the individual dc lines of a highly meshed HVDC network, and this increases the risk of overloading the dc cables that present lower resistances. To address the highlighted shortcoming, this paper presents a generic dc grid controller that uses nonlinear constrained optimization to optimize the performance of multi-terminal HVDC networks for any desirable operational objective, within system physical constraints such as dc cable and converter thermal limits, and minimum and maximum dc voltage limits. The presented dc grid controller performs online optimization at regular intervals to dynamically estimate and update the set-points of the converter terminals as system operating conditions vary. The technical viability of the proposed controller is assessed using a generic seven-terminal HVDC network that uses voltage sourced modular multilevel converters. Simulations from the scenarios that prioritize dc grid power loss or operational cost minimization show that the presented dc grid controller exhibits good performance during normal operation as converter terminals set-points vary, and during dc grid reconfiguration following simulated successive outages of the dc cables. It has been shown that the performance of the proposed DC grid control is not affected by the parameter variation such as changes of resistance with temperature.

Parallel‐in‐time‐and‐space electromagnetic transient simulation of multi‐terminal DC grids with device‐level switch modelling

The electromagnetic transient (EMT) simulation of multi-terminal DC (MTDC) grids requires a detailed device-level modular multilevel converter (MMC) model, which can have thousands of state variables and complex internal structures. The fast device-level insulated gate bipolar transistor (IGBT) transient requires a very small time-step, making the computational overhead prohibitive. Based on the analysis of the parallel-in-time (PiT) implementation of detailed modelled MMCs, this paper proposes a task-based hybrid PiT algorithm to achieve high parallel efficiency and speed-up of MMC with device-level modelling. Moreover, a transmission line model(TLM)-based parallel-in-time-and-space (PiT+PiS) method is proposed to connect PiT grids to conventional or other PiT grids and exploit the maximum parallelism. Simulation results show greater than 30 × speed-up and 60% parallel efficiency on a 48 cores computer for the hybrid PiT method in a 201-level three-phase MMC test case, and 20 × speed-up in the transient simulation of CIGRÉ B4 DC grid test system for the PiT+PiS method.

Planning a HVDC South American Pacific Electric Interconnection Based on MTDC

The continuous development of DC technology and its increasing implementation in projects, mainly in Europe, North America, and Asia, have created the need for methodologies to plan the DC interconnections between countries. However, the planning process should not be developed in a local context but rather, in the long run, thus enabling the inclusion of regional electrical systems. This paper presents two main methodologies. The first one aims to design local Multi-Terminal DC grids (MTDC) at the national level, using the graph theory. The second methodology presented is the integration of local MTDC grids that can later be extended to regional, continental, or subcontinental grids. The methodology is applied in the planning of regional electrical integration of the Pacific basin countries.

Implementation of DC voltage controllers on enhancing the stability of multi-terminal DC grids

Because of the increasing penetration of intermittent green energy resources like offshore wind farms, solar photovoltaic, the multi-terminal DC grid using VSC technology is considered a promising solution for interconnecting these future energies. To improve the stability of the multi-terminal direct current (MTDC) network, DC voltage control strategies based on voltage margin and voltage droop technique have been developed and investigated in this article. These two control strategies are implemented in the proposed model, a ±400 kV meshed multi-terminal MTDC network based on VSC technology with four terminals during the outage converter. The simulation results include the comparison and analysis of both techniques under the outage converter equipped with constant DC voltage control, then the outage converter equipped with constant active power control. The simulation results confirm that the DC voltage droop technique has a better dynamic performance of power sharing and DC voltage regulation.

Fast protection strategy for DC transmission lines of MMC-based MT-HVDC grid

Multi-terminal high voltage DC (MT-HVDC) grid has broad application prospects in connecting different energy sources, asynchronous interconnection of power grids, remote load power supply, and other fields. At present, the key technologies that affect the development of MT-HVDC transmission system include swift fault identification and location in the DC line and its rapid isolation. Traditional fault monitoring relies on line communication, which cannot guarantee the rapidity and reliability of protection; moreover, it may even cause device damage. A fault identification scheme based on a single-terminal transient is presented in this paper. This scheme calculates the line inductance by using the rise rate of fault current at the initial stage of the fault, and determines the occurrence of the fault by comparing the observed line inductance with the set value, which lays a foundation for calculating the location of the fault point using distance protection. A simulation model on the PSCAD/EMTDC platform is built; the simulation example verifies that the proposed scheme can identify faults under dissimilar conditions while maintaining a low error level on the premise of no communication lines so as to meet the protection requirements of the MT-HVDC grid.

Single-end fault identification scheme for multi-terminal DC grid based on amplitude similarity of injection signal

Effective fault identification scheme plays an important role in the fault isolation stage and system recovery stage of multi-terminal DC grid. Therefore, an active detection based single-end DC fault identification scheme is proposed to improve the reliability and sensitivity of fault identification. The control strategy is suggested of the generation of sinusoidal signals at different frequencies by the hybrid modular multilevel converters. Based on the blocking effect of the fault branch on the injection signal, by introducing the improved correlation discrimination algorithm, a single-end fault identification scheme is proposed by using the amplitude difference between the measured and calculated values of injection signals. The proposed scheme can be used for fault isolation and adaptive system recovery. Intensive simulation studies prove the improved sensitivity, and reliability of the proposed scheme for the full length of transmission line without relying on DC boundary. Moreover, the scheme can identify the permanent fault during the system recovery stage.

Interoperability of modular VSC topologies in Multi-converter multiterminal DC systems

The ongoing transformation of power systems driven by the need for more interconnected networks will eventually lead multiterminal dc (MTDC) grids to become more commonplace in future electricity networks. Addressing interoperability challenges associated with multiple converter topologies for future MTDC implementations is a paramount requirement. This paper introduces a multi-converter MTDC (MC-MTDC) system combining both state-of-the-art and emerging modular voltage source converter (VSC) topologies. The developed MC-MTDC system, (i) demonstrates operation of the modular multilevel converter (MMC) and the alternate arm converter (AAC) in different MTDC configurations, and (ii) sets the basis for further development of MTDC configurations with diverse modular VSC topologies. The developed MC-MTDC system will also facilitate accurate testing and validation of multi-converter interoperability using real-time electromagnetic transient (EMT) models. The design parameters of the AAC-based terminals are determined ensuring compliance with existing MMC-based dc power systems. Detailed analysis and a complete set of results based on real-time implementation for different operating scenarios including steady-state, transient, and contingencies, verify multi-converter interoperability and associated control functions in different MTDC configurations.

Topology-Cognizant Optimal Power Flow in Multi-Terminal DC Grids

In this paper, we propose a topology-cognizant optimal power flow (OPF) paradigm with additional safety constraints for multi-terminal direct current (MTDC) grids. The resulting formulation is concerned with the optimization of controller set-points, i.e., voltage and power levels at each bus, and the switching status of transmission lines that collectively referred to as grid topology. A pair of additional safety constraints are integrated into the problem formulation to prevent voltage violations caused by power fluctuations in between two controller set-point updates. Searching for a grid topology that offers more efficient operation leads to a mixed-integer nonlinear program (MINLP) which is computationally challenging due to: i) Non-convex power flow equations, (ii) Non-convex converter loss equations, and iii) Binary variables accounting for the operational status of transmission lines. Non-convexities of power flows and converter loss equations are tackled by means of a mixed-integer second-order cone programming (MISOCP) relaxation, while the optimal switching status of transmission lines are determined via a branch-and-bound search. Numerical results for the modified IEEE 14, 30, and 57-bus systems are used to verify the merits of the proposed method. Furthermore, this method is experimentally validated using the CIGRE B4 DC grid benchmark in a real-time hardware-in-the-loop platform.

Flexible controls to reduce DC voltage deviations in multi‐terminal DC grids

Various disturbances in voltage source converter based high-voltage, multi-terminal DC grids will significantly cause DC voltage deviations when the classical voltage droop control is adopted. The unsatisfactory voltage deviations weaken the voltage regulation ability and increase the risk of instability. Two flexible controls at the top of the classical voltage droop control to reduce voltage deviations are proposed, and the power–voltage (P–V) curves of the classical voltage droop control are translated co-ordinately. Firstly, a global DC voltage oscillation damping control designed based on the Lyapunov theory is proposed for transient voltage deviations. Secondly, a constant weighted voltage control is designed for reducing steady weighted voltage deviations. The steady active power injections keep almost unchanged under proposed controls, consistent with those under only the classical voltage droop control. The stability influence and the power flow impact are carried out to evaluate the proposed controls theoretically. Tests are conducted on a multi-terminal DC grid with five-terminal voltage source converters. Simulation results show the proposed controls' effectiveness and superiority under various fault conditions, including both the outage of a constant power voltage source converter and a droop voltage source converter.