Multi-terminal HVDC Grids Research Articles

Test Circuits for HVDC Circuit Breakers

High voltage direct current circuit breakers (HVDC CBs) are necessary for reliable and safe operation of future multi-terminal meshed HVDC grids. So far no commercially mature products of such equipment exist. A few industrial concepts as a result of advances in power electronics have been proposed and prototypes have been built. However, performances of these concepts have never been demonstrated under realistic operation conditions. Since these tests require construction of high power DC sources incurring significant investment costs, alternative test circuits providing equivalent stresses must be considered. In this paper various test methods and circuits used for testing HVDC CBs are investigated and their performance is evaluated against the stresses in a conceptual future HVDC grid. A novel method of testing an HVDC CB using existing installations in a high power ac test laboratory is proposed and its prospective fault current and voltage is experimentally demonstrated. It is concluded that adequate test circuits need to maintain the supply voltage not only up to the current zero in the main interrupter unit of the HVDC CB, but also during the entire energy dissipation phase of the interruption process.

Modelling of a VSC-based multi-terminal HVDC network for dynamic stability analysis

PurposeSelf-commuted voltage source converter (VSC) can significantly extend the flexibility and operability of an HVDC system and be used to implement the concept of multi-terminal HVDC (MTDC) grid. To take full advantage of MTDC systems, its overall behaviour must be characterized in quasi static and dynamic states. Based on the numerous literatures, a dedicated two-level VSC model and its local controllers and DC grid voltage regulators are developed for this purpose. Furthermore, the requirement of the system to guarantee all the physical constrains must be well assessed and concrete demonstrations must be provided by numerical simulations.Design/methodology/approachFirst, a two-level VSC model and its local controllers and DC grid voltage regulators are developed. Then, DC cable models are investigated and their characteristics are assessed in the frequency domain. Those developed models are combined to form a three-terminal HVDC grid system on Matlab/Simulink platform. To analyze the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed.FindingsTo analyze the stability of this electrical system, the dynamics of the system against variations of power dispatch are observed. The differences in the DC grid voltage dynamics and the power flow of the converter stations coming from the embedded primary controls are analysed, and the technical requirements for both cases are assessed.Originality/valueIn this paper, the dynamic stability of an MTDC system has been analysed and assessed through an adequate simulation model, including its control scheme and the cable models. The interest of the improved PI model for cables is highlighted.

Power‐dependent droop‐based control strategy for multi‐terminal HVDC transmission grids

The concept of voltage source converter based multi-terminal high-voltage direct current (MTDC) grids, represents both challenges and opportunities for the future of large power transfer and integration of renewable energy sources. For this kind of grids, the control aspect is of great importance, with voltage-droop based methods considered as one of the most attractive solutions. All existing strategies are designed to maintain the level of voltage in the MTDC grid constant during unexpected events, thus sacrificing the power flow. The aim of this study is to propose a new droop controller structure that maintains the dc-grid voltage close to the nominal value and at the same time tries to preserve the power flow in the dc grid, following events such as faults or disconnection of stations. The control scheme is presented and simulations are carried out in a four- and five-terminal MTDC grid, proving the validity of the concept.

Distributed Frequency Control Through MTDC Transmission Systems

In this paper, we propose distributed dynamic controllers for sharing both frequency containment and restoration reserves of asynchronous ac systems connected through a multi-terminal HVDC (MTDC) grid. The communication structure of the controller is distributed in the sense that only local and neighboring state information is needed, rather than the complete state. We derive sufficient stability conditions, which guarantee that the ac frequencies converge to the nominal frequency. Simultaneously, a global quadratic power generation cost function is minimized. The proposed controller also regulates the voltages of the MTDC grid, asymptotically minimizing a quadratic cost function of the deviations from the nominal dc voltages. The results are valid for distributed cable models of the HVDC grid (e.g., $\pi$ -links), as well as ac systems of arbitrary number of synchronous machines, each modeled by the swing equation. We also propose a decentralized communication-free version of the controller. The proposed controllers are tested on a high-order dynamic model of a power system consisting of asynchronous ac grids, modeled as IEEE 14 bus networks, connected through a six-terminal HVDC grid. The performance of the controller is successfully evaluated through simulation.

Investigation on Economic and Reliable Operation of Meshed MTDC/AC Grid as Impacted by Offshore Wind Farms

The influence of complementarity balancing connected grid and control of offshore wind power generation on the economic and reliable operation of hybrid multiterminal HVDC (MTDC) and AC power system is investigated in the paper. The variances of distribution curve of both transmission loss of AC/DC system and voltage of critical nodes assessed by cumulant-based method are used to demonstrate how the different complementarity conditions, generation balancing strategies, MTDC grid topologies, and control schemes affect power system operation. In this paper, a study case of 16-generator 68-bus power system connected with three offshore wind farms is presented. The variance indices of transmission loss and voltage magnitude are calculated and then compared under different complementarity levels, active power balancing strategies, HVDC transmission systems, and controls, respectively. The analysis reveals that higher complementarity and multi-machine balancing strategy can effectively reduce system operational cost and enhance operational reliability. Compared to the point-to-point topology, the control pattern and parameter setting of multi-terminal topology are two key factors to determine its impact on system operation.

A non-communication based protection algorithm for multi-terminal HVDC grids

This paper introduces a non-communication based protection algorithm for Multi-Terminal High Voltage Direct Current (MTHVDC) transmission lines. The algorithm relies on using electrical current data at one end for identification of the fault zone without communication between relays. The concept is to use the fault generated transients to differentiate between internal and external faults. Shunt capacitors with predetermined values are installed at the boundary between each two HVDC line sections to assist, together with the busbar stray capacitance, in the classification of internal faults. The high frequency transients will reach the relaying point without any attenuation if the fault is internal; nevertheless, the high frequency transients will be highly attenuated if the fault is external due to the stray capacitance and the installed shunt capacitor at the connection busbar. Discrete Wavelet Transform (DWT) is used to analyze the fault current signals. MTHVDC system modeling and relay design are carried out in the MATLAB/SIMULINK environment. The results of the proposed algorithm were highly reliable regarding different types of faults with different fault resistances and at various locations.

Unidirectional Protection Strategy for Multi-terminal HVDC Grids

Protection issue is identified as the main drawback of emerging multi-terminal HVDC grids. Multi-terminal HVDC grid demands fast short circuit fault current interruption. Fast DC circuit breakers as a promising solution can be implemented as either bidirectional or unidirectional devices. In addition to less implementation cost, the unidirectional DC circuit breakers have less power losses as compared to the bidirectional devices. A protection strategy for multi-terminal HVDC grid based on unidirectional breaking devices is discussed and assessed in this paper. The performance of unidirectional protection strategy is examined under different fault scenarios in a fourterminal MMC-HVDC grid model. Furthermore, the impacts of unidirectional protection strategy on power converters and also current interruption and surge arrester ratings of the DC circuit breakers are discussed.

Application study of superconducting fault current limiters in meshed HVDC grids protected by fast protection relays

The complexity of DC side short circuit fault current interruption has been identified as a major drawback of realization of multi-terminal HVDC grids. Although application of superconducting fault current limiters (SFCLs) in protection of HVDC systems based on slow protective schemes has been studied in the literature, its application in meshed HVDC systems based on modern and fast protective schemes has not been investigated, yet. This paper scrutinizes the SFCL application in the meshed HVDC grids protected by the fast DC protective schemes and fast hybrid DC circuit breakers. The purpose of SFCL application is to reduce the current interruption requirement of the HVDC circuit breaker and size of its elements. This research analyzes the requirements of suitable SFCL considering the fault operation behaviors of local protective relays and the hybrid DC circuit breakers. The results show that the appropriated SFCL can reduce fault current peak, size of current limiting inductor and also fault identification time. The application of SFCL not only reduces the size of surge arrester in the hybrid DC circuit breaker, but also can significantly decrease the current stress on the hybrid DC circuit breaker.

Small-signal Stability Analysis and Control System Design of a Meshed Multi-terminal High-Voltage Direct Current Grid with a Current Flow Controller

A DC current flow controller can provide branch current control in a meshed multi-terminal high-voltage direct current (HVDC) grid. However, the introduction of a DC current flow controller may affect the stability of the multi-terminal HVDC. Hence, the dynamic characteristics of the multi-terminal HVDC with the DC current flow controller should be investigated. This article focuses on small-signal stability analysis of a current flow controller-equipped meshed three-terminal HVDC system. A small-signal model for the multi-terminal HVDC with the DC current flow controller is established. Based on the stability analysis of the small-signal model, a control system is designed for the DC current flow controller, fulfilling the system stability requirement. Finally, non-linear dynamic simulations on the real-time digital simulator are conducted, and simulation results are compared with a theoretical model to validate the proposed controller for the DC current flow controller. In addition, dynamic impacts of the DC current flow controller on the meshed multi-terminal HVDC grid are discussed.

Evaluation of an enhanced power dispatch control scheme for multi-terminal HVDC grids using Monte-Carlo simulation

This paper presents a new multi-objective optimization technique for multi-terminal DC (MTDC) networks to enhance power dispatch control under stochastic operating conditions. The proposed method utilizes the DC voltage droop control concept to achieve distributed DC voltage control of the MTDC grid, while a centralized power dispatch controller monitors system operation and updates the voltage droop settings of the onshore converters in order for them to comply with scheduled power set-points dispatched by the transmission system operator, maintaining at the same time low system losses. The overall optimization problem is formulated and its effectiveness is assessed applying the Monte-Carlo simulation method. A five-terminal MTDC grid is selected as a study-case system, which corresponds to the planning for the future interconnection of Aegean Sea islands with significant wind potential to the mainland grid.

Guest Editorial Special Section on HVDC Systems for Large Offshore Wind Power Plants

The papers in this special section focus on HVDC systems for large offshore wind power plants. These plants have experienced an important growth in the last decade and are becoming a key energy source worldwide. Offshore wind presents clear advantages due to high and stable wind speeds offshore, enabling the use of larger wind turbines than those commonly used on inland wind power plants. However, transmitting power to load centres also brings a number of technical challenges. Depending on the distance to the shore and the power rating, AC or DC technology may be the preferable option for integrating offshore wind power to onshore grids. There are a number of issues which have to be considered requiring substantial research and development. For DC connections, (Voltage Source Converter) VSC-HVDC technology has shown advantages over (Line Commutated Converter) LCC-HVDC. The development of multiterminal HVDC grids though brings further challenges related to system operation, control, and protection and coordination between HVDC converters, wind power plants, and the onshore system. The development of power electronic switches has reached a point where the size, rating, and the system efficiency can meet the needs of commercial HVDC. Wind power plants connected to HVDC transmission system require specific control and protection coordination between HVDC converters and wind turbines.

An Improved Corrective Security Constrained OPF for Meshed AC/DC Grids With Multi-Terminal VSC-HVDC

This paper presents an improved corrective security constrained optimal power flow (CSCOPF) for a meshed AC/DC power transmission network with voltage source converter based multi-terminal HVDC (VSC-MTDC) grids. The security constrains of N-1 contingencies for both AC and DC networks are fully considered in the CSCOPF problem. The corrective actions employed in this paper include not only the traditional long-term rescheduling control actions of the AC system, but also the short term post-contingency fast corrective control of multi-terminal VSC-HVDC. A hybrid method is developed to solve the improved CSCOPF problem and the effectiveness of the proposed method is evaluated and demonstrated in this paper by two example meshed AC/DC power systems.

Distributed MPC for frequency regulation in multi-terminal HVDC grids

Multi-Terminal high voltage Direct Current (MTDC) transmission lines enable radial or meshed DC grid configurations to be used in electrical power networks, and in turn allow for significant flexibility in the development of future DC power networks. In this paper distributed MPC is proposed for providing Automatic Generation Control (AGC) in Alternating Current (AC) areas connected to MTDC grids. Additionally, a novel modal analysis technique is derived for the distributed MPC algorithm, which in turn can be used to determine the convergence and stability properties of the closed-loop system.

Risk-Based Security and Control Framework for Power System Operation Under Significant Amounts of HVDC-connected Wind Power Generation

In the context of power system operation, probabilistic techniques can provide a deeper insight into security aspects compared to deterministic approaches, by quantitatively considering power system uncertainties and contingency impact. Moreover, they can be helpful to identify and suggest operators the most adequate control actions to reduce the operational risk. This paper proposes an original framework for risk-based assessment and control of operational security in High-Voltage Alternating Current (HVAC) power systems connected to Multi-Terminal High-Voltage Direct Current (MTDC) networks like those envisaged for the integration of future, large off-shore wind farms. After the presentation of a security assessment methodology based on the concept of risk, the paper investigates three possible preventive control strategies to reduce the risk of high current by exploiting control resources available in the system (specifically, generating units and power injections from MTDC grids). Results of the methodology applied to a test system and to a realistic power system (adapted from the Italian transmission network) are presented and discussed.

A DC current flow controller for meshed modular multilevel converter multiterminal HVDC grids

This paper proposes the design of a novel DC current flow controller (CFC) and evaluates the control performance of balancing and regulating the DC branch currents using the DC CFC in a meshed multi-terminal HVDC (MTDC) grid. The DC CFC consists of two identical full bridge DC-DC converters with the capacitors of the two converters being connected in parallel. The scalability of the DC CFC is easily achievable due to the identical bridge converter topology; the cost of this DC CFC is also relatively low due to its simple physical structure and low voltage ratings. The control performance of the DC CFC is tested on a meshed 3-terminal (3-T) HVDC grid, which is based on modular multilevel converters (MMC). The DC branch current control in the meshed MTDC grid is achieved using the proposed control strategy of the DC CFC, and is verified through case studies on the real-time digital simulator (RTDS).

Droop setting design for multi-terminal HVDC grids considering voltage deviation impacts

In multi-terminal high voltage direct current (HVDC) grids, the widely deployed droop control strategies will cause a non-uniform voltage deviation on the power flow, which is determined by the network topology and droop settings. This voltage deviation results in an inconsistent power flow pattern when the dispatch references are changed, which could be detrimental to the operation and seamless integration of HVDC grids. In this paper, a novel droop setting design method is proposed to address this problem for a more precise power dispatch. The effects of voltage deviations on the power sharing accuracy and transmission loss are analysed. This paper shows that there is a trade-off between minimizing the voltage deviation, ensuring a proper power delivery and reducing the total transmission loss in the droop setting design. The efficacy of the proposed method is confirmed by simulation studies.

Hierarchical power control of multiterminal HVDC grids

This article introduces a hierarchical power control structure for Multi-terminal High Voltage Direct Current (MT-HVDC) systems. The presented hierarchy is similar to the control structure used in classical AC transmission systems and is divided in primary, secondary and tertiary control actions. The voltage control in the MT-HVDC scheme acts in a way similar to the primary control action of generators in AC systems, while the secondary control action is performed by an outer power control loop. The design of the individual controllers and the interaction between these control loops is discussed in detail. Furthermore, the operational characteristics are described and the main operating points are identified. Scenarios including a power reference change and a grid side converter disconnection have been simulated in order to test and verify the proposed method. The main contribution of the paper is the development of a control methodology, providing a separation of the different control actions in different time domains, similar to what is already in use (and therefore known) within traditional power systems.

Coordinated Frequency Control through MTDC Transmission Systems

In this paper we propose a distributed dynamic controller for sharing frequency control reserves of asynchronous AC systems connected through a multi-terminal HVDC (MTDC) grid. We derive sufficient stability conditions, which guarantee that the frequencies of the AC systems converge to the nominal frequency. Simultaneously, the global quadratic cost of power generation is minimized, resulting in an optimal distribution of generation control reserves. The proposed controller also regulates the voltages of the MTDC grid, asymptotically minimizing a quadratic cost function of the deviations from the nominal voltages. The proposed controller is tested on a high-order dynamic model of a power system consisting of asynchronous AC grids, modelled as IEEE 14 bus networks, connected through a six-terminal HVDC grid. The performance of the controller is successfully evaluated through simulation.

Interior point algorithm‐based power flow optimisation of a combined AC and DC multi‐terminal grid

The high cost of power electronic equipment, lower reliability and poor power handling capacity of the semiconductor devices had stalled the deployment of systems based on DC (multi-terminal direct current system (MTDC)) networks. The introduction of voltage source converters (VSCs) for transmission has renewed the interest in the development of large interconnected grids based on both alternate current (AC) and DC transmission networks. Such a grid platform also realises the added advantage of integrating the renewable energy sources into the grid. Thus a grid based on DC MTDC network is a possible solution to improve energy security and check the increasing supply demand gap. An optimal power solution for combined AC and DC grids obtained by the solution of the interior point algorithm is proposed in this study. Multi-terminal HVDC grids lie at the heart of various suggested transmission capacity increases. A significant difference is observed when MTDC grids are solved for power flows in place of conventional AC grids. This study deals with the power flow problem of a combined MTDC and an AC grid. The AC side is modelled with the full power flow equations and the VSCs are modelled using a connecting line, two generators and an AC node. The VSC and the DC losses are also considered. The optimisation focuses on several different goals. Three different scenarios are presented in an arbitrary grid network with ten AC nodes and five converter stations.

Modelling and Analysis of CIGRE HVDC Offshore Multi-terminal Benchmark Grid

HVDC power transmission systems are being commissioned in the North Sea to link inland power grids with large offshore wind power farms. The growing number of large wind farms is driving the need to design more reliable high power transportation systems, leading to the conception of multi-terminal HVDC grids. These future grids will be purely formed by power electronics converters, the control of which will have to be designed to ensure proper performance and robust operation. This paper presents the analysis of the benchmark system proposed by the CIGRE working group SC B4, focusing on the proper selection of the components models and parameters. The small signal model of the system is first formulated. Then the bandwidth of the different control loops is obtained and the fidelity of the HVDC line model is selected accordingly. Finally, the performance of the benchmark system is studied by means of small signal analysis and compared numerical time series simulation of the non-linear model.