Multi-terminal DC System Research Articles

A Modular Isolated Topology for Instantaneous Reactive Power Compensation

This paper presents a novel topology for instantaneous reactive power compensation. The topology is derived from the Dynamic-Current or Dyna-C, which is a patented power converter capable of transferring energy for two or multiterminal dc, single- and/or multiphase ac systems. The proposed topology, named dynamic VAr compensator (DVC), has a current source power conversion stage and can provide instantaneous leading or lagging reactive power. The proposed DVC has three individual bridges coupled through a three-winding high-frequency transformer. It has a modular structure that allows it to be series stacked for medium-voltage applications. In addition, the high-frequency Galvanic isolation link provides interphase fault isolation among the three phases. The circuit eliminates the use of electrolytic capacitors for bulk energy storage, which usually lowers life and reliability. Operating principles, theoretical analysis, and design of the proposed topology are described. The converter functionality is validated through simulation as well as experimental results.

Inertia Estimation of Power Grid with VSC-MTDC System

Abstract The increase of renewable energy generations and HVDC links with low or zero inertia will affect the dynamic behavior of power systems and stability margins. The inertia characteristics of power system is a critical factor for the analysis of the system dynamic stability when subjected to disturbances. To meet the increasing integration level of the renewable energy resources in the future, this paper proposes an inertia estimation algorithm to analyze both the regional and global inertia characteristics of power systems. It is found that the inertia estimation results are highly related to several factors, such as the grid structure, the inertia time constant of synchronous generators and the grid-interface location of the electronics, which also reveal the impact of a multi-terminal DC (MTDC) system connection. The effectiveness of the proposed method is verified and demonstrated for three networks by the simulations in the DigSILENT/PowerFactory environment.

Improved Voltage Control Strategy for Multi-terminal Flexible DC System

This paper presents an improved Voltage Droop control strategy, which updates the reference of DC link voltage by adding the difference between reference and actual voltage, which also adopts a dynamic P-V droop coefficient to improve the DC voltage level. By applying this method, we can eliminate the steady-state deviation of DC link voltage caused by ignoring the power loss in converter. In this paper, in order to demonstrate the reduction of deviation of DC link voltage, an improved control strategy is implemented for a three-terminal flexible DC power system modelled in PSCAD/EMTDC.

Fault-ride Through Control Strategy of Multi-terminal High Voltage DC Systems

Abstract A coordinate control strategy is proposed to secure fault-ride through (FRT) for voltage source converter (VSC) based multi-terminal high voltage dc transmissions (MTDC) integrating the offshore wind farms (OWFs). The short circuit fault in the ac grid obviously reduces the voltage at the connection point which, in turn, block the power output of MTDC systems instantaneously. The resulting power imbalance in MTDC system correspondingly charge the DC capacitors. In the absence of suitable countermeasures, this would lead to increased dc voltage, which would further lead to potential equipment damage in the installation. Therefore, for the OWF-integrated MTDC systems, keep ing the dc voltage within a reasonable value is the key to achieving the fault-ride through. The grid side converters (GSCs), wind farm side converters (WFCs), and doubly fed induction generators (DFIGs) will operate under the fault operation mode when there is a three-phase symmetrical fault. An improved reactive power compensation method is applied to GSCs’ controllers to enhance the stability of the ac grid. The controllers of WFCs reduce the ac voltage amplitude of OWFs according to the dc voltage signal, which correspondingly decreases the power transmitted to the MTDC system. As a result, the increase in dc voltage is restrained. Since the degree of voltage decline for WFCs is limited by the over-current capability of the power electronics, an active power reduction control of DFIG is used to complement with this method. Different simulation scenarios are presented by DIgSILENT PowerFactory to verify the effectiveness of the proposed control strategy.

Capacitive discharge based transient analysis with fault detection methodology in DC system

Multi-terminal DC (MTDC) system based on voltage source converter (VSC) promises a new opportunity for bulk power transmission but it is still met with dire protection issues. This paper presents a transient analysis based on DC-link capacitive discharge which can be leveraged to design a fault detection method. Under certain critical fault conditions, the fault-induced transient can be quite weak for fault detection to work properly. However, the proposed method is minimally influenced by the fault location and resistance. The fault detection time can be kept within 1 ms, which is well-suited to the requirement of the DC circuit breaker. Another important feature of this method is the ability to distinctly recognize the bus fault and external faults, such as AC fault and sudden blocking of gate signal. Besides that, it only requires the local measurement at the terminal to perform necessary computations. The fault detection is tested on a simulation model developed in PSCAD/EMTDC and experimental model, from which different fault types and conditions are investigated. For fair evaluation, the performance of this method on MTDC system is compared with di/dt method.

VSG-based adaptive droop control for frequency and active power regulation in the MTDC system

In this paper, a VSG (virtual synchronous generator)-based method with adaptive active power and DC voltage droop is proposed for the control of VSC stations in the multi-terminal DC (MTDC) system. This control strategy can improve the inertial level of the AC networks and attenuate the rate of change of frequency when a disturbance occurs. In addition, the droop control of the active power and DC voltage is implemented to make the AC networks share the unbalanced power in the MTDC. The droop coefficients are adaptively adjusted depending on the frequency margin of every AC network, which makes the allocation of unbalanced power among AC networks more reasonable from the frequency variation perspective. The control strategy is evaluated in the scenarios of sudden load change and wind turbine tripping, and the results are presented to demonstrate its effectiveness.

Power Flow Features and Balancing in MTDC Integrated Offshore Wind Farms

This paper proposes a novel control strategy for converter stations of multi-terminal DC (MTDC) grid system. It optimizes the offshore wind farms power generation dispatched to the separate onshore AC systems while minimizing their impacts on AC grids. Based on the MTDC power flow analysis, the minimal control dimensions of MTDC converters are firstly proposed. The impact of DC resistances on power dispatching among MTDC grid is then fully discussed by analyzing the structure of inverse Jacobian matrix of MTDC grid power flow equation and the power dispatch law for each onshore AC grid is deduced subsequently. Finally, an optimization strategy that seeks to re-dispatch the power to each onshore converter according to the system inertia of connected AC grid is proposed. Case study using a typical five-terminal MTDC system shows that the proposed strategy can effectively mitigate wind power fluctuations by approximately equalizing the induced disturbances to the system frequency of each AC grid.

Research and application on multi‐terminal and DC grids based on VSC‐HVDC technology in China

Voltage source converter (VSC)-based high-voltage direct current (HVDC) and multi-terminal (MT)/DC grid technologies are the new HVDC transmission technologies after ultra-high voltage alternative current (UHVAC) and UHVDC transmission technologies which have been widely used in China. The application of the new technologies has resulted in a rapid increase in the number of schemes in construction and planning worldwide. This has been stimulated by the greater level of functionality available from the VSC technology, which makes it suitable for a wide variety of applications. These include the integration of off-shore wind farms, embedded links within AC networks and interconnectors, especially where the AC networks are relatively ‘weak’. VSC technology has renewed interest in MT DC systems, which may ultimately lead to wide area DC grids. This study outlines the research and application on MT and DC grids in China with respect to VSC-HVDC key technologies and DC grid key technologies based on the presentation given in the International Workshop on Next Generation Power Equipment held on 23 September 2016 in Xian, China. The briefing details of the VSC-HVDC projects constructed and to be constructed in China are summarised in this study.

The Model and Parameters Based on the Operation Mode of a 500kV Multi-terminal Flexible DC Power Grid

The operating conditions of 500kV Zhangjiakou-Beijing Demonstration Project at all operation modes were studied regarding a multi-terminal flexible DC system with DC switches and a true bipolar wiring method. Based on the actual AC-DC hybrid method, this paper concluded 34 kinds of N-1 operation modes for keeping the system stable, simulated the multi-level converter valve (MMC) bank by using RTDS small-step elements, adopted an ideal voltage balancing method for the treatment of sub-units and established a MCC system simulating structure in compliance with the site. The operation modes of the system were simulated by selecting the parameters which are consistent with the line, using such parameters as the actual configurations and parameters of key elements such as the converter transformer, filter, bridge arm reactor, converter, etc. adopted in the project, and establishing a flexible DC control system with a layered structure. According to the result of the simulation, the corresponding modes of the system were provided and references were given for the feasibility study and implementation of the project.

Coordinated Control of Wind Farms and MTDC Grids for System Frequency Support

This paper proposes a coordinated control strategy to provide system frequency support for separated main grids from offshore wind farms (WFs) that are integrated through multi-terminal DC (MTDC) system. The control strategy consists of two control loops including the MTDC frequency regulation loop (FRL) and the automatic WF power regulation loop (AWFPRL). In events of AC main grid disturbances, the former can re-dispatch MTDC grid power flow to support system frequency, while the latter can adjust wind power output accordingly through the variable frequency control of WF-connected converters. Inertia energy stored in wind turbines can be utilized to stabilize system frequency via MTDC grids. Moreover, wind power fluctuations can also be smoothed by the proposed control. An expression of frequency response coefficient is calculated, which is used to quantify the contribution of frequency support from offshore WF-connected MTDC system. Case studies have been conducted to demonstrate the effectiveness of the proposed control strategy using a four-terminal DC grid.

A Novel Cloud-Based Platform for Implementation of Oblivious Power Routing for Clusters of Microgrids

There has been an increasing demand for connectivity of the clusters of microgrids to increase their flexibility and security. This paper presents a framework for implementation, simulation, and evaluation of a novel power routing algorithm for clusters of microgrids. The presumed cluster is composed of multiple direct current (dc) microgrids connected together through multi-terminal dc system in a meshed network. In this structure, the energy is redirected from the microgrid with excessive power generation capacity to the microgrid which has power shortage to supply its internal loads. The key contribution of this paper is that each microgrid in the cluster is unaware of the current state and other flows of the cluster. In this approach, the optimal power flow problem is solved for the system while managing congestion and mitigating power losses. The proposed methodology works for both radial and non-radial networks regardless of the network topology, scale, and number of microgrids involved in the cluster. Therefore, it is also well suited for large-scale optimal power routing problems that will emerge in the future clusters of microgrids. The effectiveness of the proposed algorithm is verified by MATLAB simulation. We also present a comprehensive cloud-based platform for further implementation of the proposed algorithm on the OPAL-RT real-time digital simulation system. The communication paths between the microgrids and the cloud environment can be emulated by OMNeT++.

Active distribution power system with multi‐terminal DC links

A fast power restoration operational scheme and relevant stabilising control is proposed for active distribution power systems with multi-terminal DC network in replacement of the conventional normal open switches. A nine-feeder benchmark distribution power system is established with a four-terminal medium power DC system injected. The proposed power restoration scheme is based on the coordination among distributed control among relays, load switches, voltage source converters and autonomous operation of multi-terminal DC system. A DC stabiliser is proposed with virtual impedance method to damp out potential oscillation caused by constant power load terminals. The proposed system and controls are validated by frequency domain state-space model and time-domain case study with Matlab/Simulink.

Coordinated frequency control from offshore wind power plants connected to multi terminal DC system considering wind speed variation

A coordinated fast primary frequency control scheme from offshore wind power plants (OWPPs) integrated to a three terminal high voltage DC (HVDC) system is proposed in this study. The impact of wind speed variation on the OWPP active power output and thus on the AC grid frequency and DC grid voltage is analysed. The removal of active power support from OWPP after the frequency control action may result in second frequency (and DC voltage) dips. Three different methods to mitigate these secondary effects are proposed, such as, (i) Varying the droop gains of the HVDC converter (ii) Releasing the active power support from OWPP with a ramp rate limiter and (iii) An alternative method for the wind turbine overloading considering rotor speed. The effectiveness of the proposed control scheme is demonstrated on a wind power plant integrated into a three terminal HVDC system developed in DIgSILIENT PowerFactory. The results show that the proposed coordinated frequency control method performs effectively at different wind speeds and minimises the secondary effects on frequency and DC voltage.

Decentralized Current Sharing Accuracy for Multi-terminal High Voltage DC System Based on Droop Control

In this article, a decentralized droop control scheme is proposed based on traditional current-voltage type droop control. This approach not only can eliminate the deviation of DC output voltage, it can also adjust the sharing accuracy of DC output current in multi-terminal high voltage DC systems. The averaging DC output voltage and current of two neighboring converters are employed to reduce the complexity of the communication networks, which include both radial and mesh configurations. In order to demonstrate the stability of the proposed method, the Pi-section line model of DC cable is used to simplify the model of multi-terminal high voltage DC networks, and the impact of varying DC cable impedance parameters and low bandwidth communication delay time are analyzed in detail. Finally, simulation results derived from a multi-terminal high voltage DC system is implemented in MATLAB/SIMULI-NK to verify the proposed approach.

Multi‐terminal HVDC grids with inertia mimicry capability

The high-voltage multi-terminal dc (MTDC) systems are foreseen to experience an important development in the next years. Currently, they have appeared to be a prevailing technical and economical solution for harvesting offshore wind energy. In this study, inertia mimicry capability is added to a voltage-source converter-HVDC grid-side station in an MTDC grid connected to a weak ac grid, which can have low inertia or even operate as an islanded grid. The presented inertia mimicry control is integrated in the generalised voltage droop strategy implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid to provide higher flexibility, and thus controllability to the network. Besides, complete control framework from the operational point of view is developed to integrate the low-level control of the converter stations in the supervisory control centre of the MTDC grid. A scaled laboratory test results considering the international council on large electric systems (CIGRE) B4 MTDC grid demonstrate the good performance of the converter station when it is connected to a weak islanded ac grid.

Power flow and transmission loss analysis of modular multi‐level converter based multi‐terminal high‐voltage DC systems

Interconnected voltage-source converter based multi-terminal high-voltage DC (HVDC) systems provide better system redundancy, higher flexibility and capability of exchanging power between multiple areas. Recent developments in modular multi-level converter technology makes multi-terminal HVDC (MTDC) transmission more promising than before. This study provides an in-depth analysis of the power flow and transmission loss for MTDC systems with different grid topologies. A voltage–power droop control based algorithm is applied to solve the power flow problems. The main contribution of this study is the novel way of determining and calculating the transmission line losses for different MTDC network topology configurations. Three MTDC system topologies are investigated. Simulated case studies are used to observe the system power flow and transmission losses for all three topologies.

A New Control Structure for Multiterminal DC Grids to Damp Interarea Oscillations

This paper analyzes the control structure of the multiterminal dc (MTDC) system to damp ac system interarea oscillations through active power modulation. A new control structure is presented that maximizes the relative controllability without the need for communication among the dc terminals. In point-to-point high-voltage dc (HVDC) transmission, the active power modulation of the two terminals occurs in opposite directions. In this case, the control direction is given and only needs to be phase compensated to align for maximal damping. In the case of MTDC systems, the control direction interrelates with the active power modulation share of the dc terminals and the relative controllability depends on this. The new control structure eliminates the need for communication between the dc terminals by performing dc voltage feedback loop shaping. This makes it possible to modulate the power in one terminal and let the other terminals react on the dc voltage change. Through loop shaping, where the feedback gain varies in the frequency plane compared to the regular droop design, the control direction is aligned with the direction of highest relative controllability. Loop shaping takes place without influencing the high frequency or steady-state gain. Simulations in the Nordic32 test system show the validity of the proposed controller and its structure.

Thévenin Equivalent of Voltage-Source Converters for DC Fault Studies

This paper presents a simple modulation index-dependent Thevenin equivalent circuit model of the voltage-source converters (VSCs) and the associated anti-parallel diode rectifier. The model derivation uses orthogonal transformation and a transformer analogy to transfer impedances to the dc side of the converter. The paper proposes a switched equivalent representation of a three-phase diode rectifier formed due to the antiparallel diodes when VSC switching is inhibited. The proposed model is implemented in Alternate Transients Program–Electromagnetic Transients Program and simulation results are compared with the switched model. The proposed model is shown to under predict the fault currents for pole-ground (PG) faults and to provide a nearly exact match for pole-pole faults. The rectifier equivalent provides increased accuracy in the transient region with the inclusion of the source inductance. A multiterminal dc (MTDC) system model is used to study effects of aggregation which gives increased accuracy of the equivalent circuit. It is shown that the MTDC system simulation with the proposed Thevenin equivalent is about 3 times faster than full-switched circuit simulation for a time step of 5 $\mu$ s. The MTDC system simulation with the Thevenin equivalent circuit is still accurate and 126 times faster when the time step is increased to 750 $\mu$ s.

Interaction between the voltage‐droop and the frequency‐droop control for multi‐terminal HVDC systems

Multi-terminal DC systems must provide a high level of reliability and may have to support the interconnected AC grids. This can be achieved by providing AC/DC converters with a dual controller combining both the voltage-droop and the frequency-droop control techniques. In this article, the coupling between those droops is theoretically investigated and quantified, and a proposed corrected value of the frequency-droop parameter allows the system to comply with the TSO's requirements with regards to the frequency support of the AC grids despite the unintended interaction of the two droops. A dynamic frequency-droop parameter is then proposed to also cope with the controller limits. Finally, an electromagnetic transient study of a five-terminal HVDC grid is performed to validate the theoretical results.

Operation and control of an insulated gate bipolar transistor‐based current controlling device for power flow applications in multi‐terminal high‐voltage direct current grids

One of the main problems that need to be solved to allow the realisation of multi-terminal high-voltage direct current (HVDC) transmission systems is the absence of a practical power flow control method. Industry leaders and researchers have proposed a few methods of power flow control based on either the control of converter station or the connection of new power electronic equipment to the grid. This study presents the operation and control of a three-port insulated gate bipolar transistor-based current flow control (CFC) device suitable for multi-terminal HVDC systems. Key features and functionalities of the proposed controller including the balancing of cable currents, limiting the magnitude of cable current and current nulling are demonstrated. The three-port CFC was simulated using power system computer aided design (PSCAD)/electromagnetic transient and direct current (EMTDC), network simulation software to evaluate its steady state and dynamic performance. Furthermore, low-power prototypes are implemented for a two and three-ports CFC to experimentally validate their different functionalities. Simulation and experimental studies explore the fast dynamic response and the results show that the CFC studied may have a significant role to play in the control of power flows in multi-terminal high-voltage DC systems.