Multi-terminal HVDC Networks Research Articles

A Novel Model of HVDC Hybrid-Type Superconducting Circuit Breaker and Its Performance Analysis for Limiting and Breaking DC Fault Currents

The key obstacle in integrating high-voltage direct current (HVDC) point-to-point networks into meshed multiterminal HVDC networks (MTDC) is the absence of dc circuit breakers (DCCBs), which can timely and reliably isolate the faulty HVDC network from the MTDC. In this paper, a novel hybrid-type superconducting DCCB model (SDCCB) is proposed. The SDCCB has a superconducting fault current limiter (SFCL) located in the main line, to limit the fault current until the final trip signal to the SDCCB is given. After the trip signal, insulated-gate bipolar transistor (IGBT) switches located in the main line will commutate the fault current into a parallel line, where dc current is forced to zero by combination of IGBTs and surge arresters. DC fault current behavior in MTDC and fundamental requirements of DCCB for MTDC were described, followed by an explanation of the working principles of the SDCCB. To prove the viability of the SDCCB, a simulation analysis demonstrating SDCCB current interruption performance was done for changing the intensity of dc fault current. It was observed that the passive current limiting by SFCL caused significant reduction in fault current interruption stress for SDCCB. Furthermore, fundamental design requirements for SFCL, including the effect of SFCL quenching impedance on SFCL voltage rating and energy dissipation capacity, were investigated. Finally, advantages and limitations of the SDCCB were highlighted.

Fault Current Interruption in Multiterminal HVDC Networks

In a multiterminal HVDC network, dc circuit breakers (CBs) are required at each end of a line to selectively isolate a fault. Several CB concepts have been implemented in PSCAD, which differ significantly in their structure and performance. This paper describes the interaction of the different HVDC CB topologies with a meshed four-terminal network and assesses their performance in terms of maximum currents and voltages. The tradeoff between network parameters and CB requirements is analyzed.

Feasibility analysis of a novel hybrid-type superconducting circuit breaker in multi-terminal HVDC networks

Voltage source converter-based HVDC systems (VSC-HVDC) are a better alternative than conventional thyristor-based HVDC systems, especially for developing multi-terminal HVDC systems (MTDC). However, one of the key obstacles in developing MTDC is the absence of an adequate protection system that can quickly detect faults, locate the faulty line and trip the HVDC circuit breakers (DCCBs) to interrupt the DC fault current. In this paper, a novel hybrid-type superconducting circuit breaker (SDCCB) is proposed and feasibility analyses of its application in MTDC are presented. The SDCCB has a superconducting fault current limiter (SFCL) located in the main current path to limit fault currents until the final trip signal is received. After the trip signal the IGBT located in the main line commutates the current into a parallel line where DC current is forced to zero by the combination of IGBTs and surge arresters. Fault simulations for three-, four- and five-terminal MTDC were performed and SDCCB performance was evaluated in these MTDC. Passive current limitation by SFCL caused a significant reduction of fault current interruption stress in the SDCCB. It was observed that the DC current could change direction in MTDC after a fault and the SDCCB was modified to break the DC current in both the forward and reverse directions. The simulation results suggest that the proposed SDCCB could successfully suppress the DC fault current, cause a timely interruption, and isolate the faulty HVDC line in MTDC.

Coordinated direct current matching control strategy for multi-terminal DC transmission systems with integrated wind farms

A new direct current matching control (DCMC) scheme is proposed in this paper. The scheme is ideally suited for the integration of a large number of wind farms with AC grid systems via a multi-terminal HVDC (MTDC) network incorporating several grid-side converters. The proposed DCMC, which matches, in a near-instantaneous fashion, the cumulative injected DC currents from all wind farms with the total of the output DC currents to the AC grids (via inverters) by communicating real-time data between all terminals, is an improvement upon and potential replacement for conventional DC voltage droop and master–slave control strategies. Through the utilization of a wide-area supervisory control and data acquisition (WA-SCADA) system, the proposed DCMC aims to enhance MTDC network voltage stability and facilitate flexible power dispatch to the supplied AC grids, while maximizing the total amount of generated wind power and offering more flexibility in terms of the ability for wind farms to independently control and maximize their outputs without any requirement for output to be constrained. A six-terminal MTDC system connecting three wind farms to three independent mainland AC grids is used to validate the proposed DCMC and compare its performance with conventional control strategies, three simulation studies are carried out to test and verify the DCMC.

Generic inertia emulation controller for multi‐terminal voltage‐source‐converter high voltage direct current systems

A generic inertia emulation controller (INEC) scheme for multi-terminal voltage-source-converter (VSC)-based high voltage direct current (HVDC) systems is proposed in this study. The INEC can be incorporated in any grid-side VSC station, allowing the multi-terminal HVDC (MTDC) terminal to contribute an inertial response to connected AC systems during system disturbances, in a fashion similar to synchronous generators. The DC-link capacitors within the MTDC are utilised by the INEC scheme to exchange stored energy with the AC system by varying the overall DC voltage level of the MTDC network within a safe and pre-defined range. A theoretical treatment of the INEC algorithm and its implementation and integration within a conventional VSC control system are presented, and the impact on the total DC capacitance required within the MTDC network to ensure that DC voltages vary within an acceptable range is discussed. The proposed INEC scheme is validated using a MATLAB/SIMULINK model under various changes in demand and AC network faults. The model incorporates a multi-machine AC power system connected to a MTDC transmission system with multiple converter-interfaced nodes. The effectiveness of the INEC in damping post-fault oscillations and in enhancing AC grid frequency stability is also investigated.

Multiterminal HVDC Networks—What is the Preferred Topology?

This paper investigates the influence of dc network topology with respect to two important aspects: the steady-state losses and the transient fault currents. For this, optimal power flow in a combined ac-dc multiterminal HVDC cable system based on voltage-source converters and the development of prospective fault currents during a pole-to-ground fault is simulated. The results prove that an evaluation of grid topologies must include both of the aforementioned aspects and that no network topology can optimize both at the same time.

Bilinear and nonlinear control algorithms for a DC/DC converter for Multi-Terminal HVDC networks

This paper presents the modeling and control of a multilevel DC/DC bidirectional converter suitable for medium voltage and medium power applications, with a special interest in renewable applications. The multilevel converter is based on Dual Active Bridge (DAB) and its average dynamic model. Two different control strategies are addressed in this paper in order to achieve the overall control of the DC/DC converter: a bilinear systems' control based on quadratic feedback control, and a nonlinear control based in Lyapunov theory. System's controllability and stability are studied, in particular by the analysis of zero dynamics. The performance of both controllers are illustrated by computer simulations, and the concluding remarks analyze theirs characteristics and draw some comparisons.

Probabilistic Analysis of Small-Signal Rotor Angle/Voltage Stability of Large-Scale AC/DC Power Systems as Affected by Grid-Connected Offshore Wind Generation

This paper investigates the probabilistic small-signal rotor angle/voltage stability of an AC/DC power system to which offshore wind generation is connected through a multi-terminal HVDC (MTDC) network. The paper applies an analytical method based on multi-point linearization to improve the accuracy of assessment of system probabilistic stability. It proposes to examine the probabilistic small-signal voltage stability of the AC/DC power system as the voltage stability is always a concern of HVDC network. The participation factor is then employed to further analyze this phenomenon and give effective remedial measures. In the paper, an example of 16-machine AC/DC power system to which three offshore wind farms is connected through aN MTDC network is presented. The proposed method in the paper is compared with the conventional Gram-Charlier expansion-based method and the nonlinear Monte Carlo simulation.

Power system stability enhancement by WAMS-based supplementary control of multi-terminal HVDC networks

This paper deals with supplementary control of a MTDC network designed for the stability enhancement of a AC power system. The proposed control is a WAMS-based control modulating the real and reactive power at the terminals of the DC network. Relevant control formulas have been derived for a linear multi-machine system model with the application of the direct Lyapunov method. Validity and robustness of the proposed control has been verified by computer simulation for a multi-machine test system using a nonlinear model and detailed modeling of power system components. The proposed control is robust and insensitive to changes in the network configuration and loading conditions in the AC power system. In the case, when more of the MTDC networks and/or the HVDC links are used in one interconnected power system the proposed stabilizing control produces additive damping i.e. each controlled network element contributes to the positive damping. Some practical aspects have also been discussed. The proposed WAMS-based stabilizing control of the MTDC network is innovative by both its main concept and the derivation of control formulas using the direct Lyapunov method.

North Sea Offshore Modelling Schemes with VSC-HVDC Technology: Control and Dynamic Performance Assessment

The present thinking and trend for connection of large offshore wind farms, dispersed over wide areas, is to use multi-terminal HVDC networks rather than point-to-point DC transmission systems. The aim behind this approach is to improve the security of supply and minimise the loss of generation during scheduled maintenance or unexpected disturbances in any part of the power network. This paper describes various models of multi-terminal HVDC networks connecting offshore wind farms to a number of mainland AC grids which have been developed in MATLAB-Simulink with the main objective of facilitating numerous studies such as steady-state power flow, optimal power dispatch analysis, transient stability, and provision of ancillaries.

Description and Comparison of DC Voltage Control Strategies for Offshore MTDC Networks: Steady-State and Fault Analysis

Estimates are that circa 40 GW of offshore wind power capacity is going to be installed throughout Europe by the end of this decade. In this scenario, a pan-European offshore grid network is needed in order to efficiently integrate large amounts of offshore wind into the different European countries’ transmission networks. In this paper, the dynamic model of a multi-terminal HVDC (MTDC) transmission system composed of voltage-source converters is presented. Afterwards, the dynamic models are used to compare four different methods for controlling the DC voltage inside MTDC networks, viz.: droop control, ratio control, priority control and voltage margin method. Lastly, a case study is performed in a four-node MTDC network and the different control strategies are compared during steady-state and an onshore three-phase fnult.

Provision of Inertial and Primary Frequency Control Services Using Offshore Multiterminal HVDC Networks

This paper addresses the problem of providing frequency control services, including inertia emulation and primary frequency control, from offshore wind farms connected through a multiterminal HVDC network. The proposed strategy consists of a cascading control mechanism based on dc voltage regulation at the onshore converters and frequency regulation at the offshore converters. The control scheme involves only local measurements and actions avoiding security and reliability issues of control structures based on remote information. The effectiveness of the proposed strategy is illustrated in a test system that consists of two nonsynchronous areas linked by a multiterminal HVDC grid where two offshore wind farms are also connected.

A Probabilistic Risk Assessment and Control methodology for HVAC electrical grids connected to multiterminal HVDC networks

AbstractProbabilistic techniques are raising more and more interest in power systems operation and planning, due to the fact that they allow to quantitatively consider power system uncertainties and contingency impact, combined into risk indices. This paper presents a probabilistic technique to assess and control the operational risk of High-Voltage Alternating Current (HVAC) power systems connected to Multi-Terminal High-Voltage Direct Current (MT HVDC) networks like those envisaged for the integration of future, large off-shore wind farms. The proposed method minimizes the operational risk by re-dispatching the active power outputs of conventional generators and by shifting the power injections from the HVDC network, taking into account the operating limits of the HVDC network (in particular, cable current limits). Results of the methodology applied to an IEEE test system are presented and discussed, and some conclusions are drawn.

A Multiterminal HVDC Load-Flow with Flexible Control Specifications

This paper presents a sequential ac-dc load-flow method for parallel multiterminal de networks. P.u.-expressed de equations are treated separately from ac equations and an interface is used to check and adjust their mutual convergence. A dc converter acts like a load on its ac bus; when it is a rectifier P and Q power are consumed, and when it is an inverter Q is consumed and P is a negative load, or a generation. This method distinguishes itself by its ability to handle a large variety of converter controls, by its capability to modify established controls to respect angle, transformer tap, voltage and current limits at the converters. The interface between the ac and de equations is designed to increase ac-dc convergence possibilities. Standard characteristics such as fixed tap and discrete tap-steps at the converter transformer are also handled. Numerical examples which demonstrate several features of the described method are presented. The Institut de Recherche d'Hydro-Qu??bec (IREQ) has been active for many years in the area of de simulator studies. The IREQ de simulator can simulate simple back-to-back links up to complex multiterminal HVDC networks imbedded into ac networks. The complexity of the networks studied on the de simulator has increased greatly these last years. The requirement therefore, for effective set-up of the simulator studies, is that it is necessary to have a digital ac-dc load-flow computer program which is flexible and can effectively handle arbitrary configurations of multiterminal HVDC systems.

Homogeneous, nonlinear, load-flow algorithm for HV-AC and compound HV-ACDC systems

A general load-flow algorithm of the Jacobi or Gauss-Seidel type is introduced, which is constructed quite differently from the many algorithms described in the extensive literature on this subject. The algorithm is well suited for parallel processing at whatever level of parellelism. Nonlinear relationships for injections such as composite loads, nonlinear transmission ‘lines’ such as TCUL and phase-shifting transformers, AC/DC converters and multiterminal DC components are handled homogeneously by the basic algorithm without modifications. Constraints on voltage, current and power can be included without involving the complexities of nonlinear programming. A homogeneous model for multiterminal HV-DC networks is introduced which allows such networks to be treated as quite regular AC components.

Digital Computer Simulation of the Dynamic Behaviour of Multi-Terminal HVDC-Networks

This paper presents the results of the simulation studies on a digital computer of the dynamic behaviour of multiterminal high voltage direct current (H.V.D.C.) network equiped with a multi-level control system. A new algorithm for the numerical solution of the differential equations of variable configuration multiterminal D.C. networks in transient conditions, is formulated. Detailed representation of modern converters system of control in its function as a “constant current control” and as a “constant power control” is carried out. Also the simulation of the centralized system of control, for the balance of the reference values of all the converter stations is realized in detail. Finally the parameters of the converters systems of control are choosen to obtain the optimal system performance.