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Power sharing control and wind power curtailing for offshore multi-terminal VSC-HVDC transmission
Abstract
Worldwide, many countries spend billions of dollars on the development of renewable energy sources, especially wind generation, to counter the effects of global warming and in response to other environmental concerns. Given the increasing number of remotely located large power offshore wind farms, power sharing control and voltage regulation are significant challenges in the development of large multi-terminal voltage source converter high-voltage direct current (MT VSC-HVDC) transmission grids. Additionally, a wind-power curtailment algorithm is needed in cases of high windpower generation during low power demand from onshore AC networks. This work presents a power sharing control and wind power curtailment algorithm. This algorithm is based on selecting the optimal droop parameters of the power receiving converters and the reference power output for the offshore wind farms to share the offshore wind energy to the MT HVDC network among different AC grids based on desired shares. These desired power shares are defined to fulfill the active power requirements of the connected systems to achieve their objectives.
... For the multiterminal-HVDC network in offshore windfarms, power sharing is also a key probelm. Reference [29] proposed a supervisory control method for multiterminal-HVDC offshore windfarms systems. It designed the sharing of the active power generated from offshore windfarms among onshore AC grids under normal operation conditions. ...
High-voltage direct current (HVDC) transmission systems are a promising solution for long distances power transmission offshore wind farms. In order to satisfy the reliability requirements of receiving-end grid and system, the topology, operation and control of HVDC transmission systems for offshore wind farms should be paid more attention. Thus, the aim of this manuscript is to offer a comprehensive summary of existing topology, operation and control methods applied to HVDC transmission system for offshore wind farms. Special attention is provided to the ac grid fault through control methods, droop control methods, power sharing rules and specific requirements of HVDC system planning, model, design and investment. The results are important for understanding the operation of VSC-HVDC in offshore wind farms.
The Multi-Zonal Medium Voltage DC (MVDC) Shipboard Power System (SPS) architecture, proposed by the U.S. Navy for their future combatant system, consists of several voltage source converters (VSCs). The proposed architecture is tightly-coupled, power-limited and its performance needs to be evaluated for security, reliability, and survivability. Following system damage or a fault, the current flow pattern in the DC network may change, which may result in the failure of VSCs due to overvoltage developed across them in certain operating conditions. For a given MVDC system, DC voltage reference setting for one of the VSCs operating in the voltage regulator mode, and the optimal power reference settings of the remaining VSCs in the power dispatcher mode have to be pre-determined. These settings and control modes of VSCs are needed to maintain the DC voltage within desired margins (usually 5% around the nominal DC voltage), both in “pre-fault” and “post-fault outage” conditions. The problem has been formulated as an optimization problem with three different objective functions. Computational intelligence techniques have been applied for solving the optimization problem. These include the genetic algorithm (GA) and biogeography based optimization (BBO) methods. The results have been compared with a conventional Lagrange multiplier based method.
This paper presents a methodology to study the impact of different DC voltage droop settings on the AC system stability. When several Voltage Source Converters (VSC) in a DC system jointly control the DC voltage by means of droop control, an outage of one converter results in an abrupt change of the active power set-points of several other converters. This relative power change is related to the relative droop settings of the other converters. In this paper, the influence of the power sharing on the AC system stability is discussed by means of a singular value decomposition.
An optimal power flow based control scheme is proposed in order to minimize the losses in multi-terminal HVDC systems for large offshore wind power plants. The scheme optimizes system power flows while delivering the generated wind power to the terrestrial AC grid (or grids). The proposed scheme is compared to droop control schemes in two case studies in order to illustrate the power loss reduction achieved. Dynamic simulations have been provided in order to show the dynamic behavior of both the controller and the robust operation for wind speed changes and when a communication fault occurs.
Estimates are that circa 40GW 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 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 control. Lastly, a case study is performed in a four-node MTDC network and the different control strategies are compared.
The effect of load models on distributed generation (DG) planning in distribution system is investigated in this work. It is shown that load models can significantly affect the DG planning. Normally a constant power (real and reactive) load model is assumed in most of the studies. Such assumptions may lead to inconsistent and misleading results about deferral values, loss reduction, payback period, and other subsequent calculations. It has been demonstrated that DG planning based on such assumptions would not be effective after implementation. It is shown that load models can significantly affect the optimal location and sizing of DG resources in distribution systems. A comparative study of real and reactive power loss, real and reactive power intake at the main substation and MVA support provided by installing DG resources for different type of loads models has been performed.
The paper addresses the control of multi-terminal high voltage direct current (HVDC) networks with voltage source converters (VSCs). A general model is presented in order to describe the behavior of the multi-terminal VSC-HVDC networks in any possible operating condition. Based on this model, we propose a systematic design procedure for computing a decentralized static output feedback (SOF), the so-called droop control, with the aim of controlling the DC voltages. The design procedure is reduced to solving a convex optimization problem with linear matrix inequalities (LMIs). A four-terminal VSC-HVDC grid is use to illustrated the application of the proposed procedure.
This paper analyzes selected issues concerning operational strategies for multiterminal dc offshore power systems (OPSs). In the first section, the main challenges regarding the operation of such an OPS are discussed, and a proposal is given for a new observer-based management system (OBMS) used for operation coordination using references from the energy market. In the second section, different operational scenarios for the OPS are investigated in order to present the OBMS application. For this purpose, a test system that considers the expected European future wind power development is introduced. Then, using reference power flows from the hourly market, the influence of the OBMS optimization process on the power flows aiming at minimum deviations from the reference energy market signals will be presented. Some exemplary test simulations are performed and recommendations are given.
Although HVDC transmission systems have been available since mid-1950s, almost all installations worldwide are point-to-point systems. In the past, the lower reliability and higher costs of power electronic converters, together with complex controls and need for fast telecommunication links, may have prevented the construction of multiterminal DC (MTDC) networks. The introduction of voltage-source converters for transmission purposes has renewed the interest in the development of supergrids for integration of remote renewable sources, such as offshore wind. The main focus of the present work is on the control and operation of MTDC networks for integration of offshore wind energy systems. After a brief introduction, this paper proposes a classification of MTDC networks. The most utilized control structures for VSC-HVDC are presented, since it is currently recognized as the best candidate for the development of supergrids, followed by a discussion of the merits and shortcomings of available DC voltage control methods. Subsequently, a novel control strategy-with distributed slack nodes-is proposed by means of a DC optimal power flow. The distributed voltage control (DVC) strategy is numerically illustrated by loss minimization in an MTDC network. Finally, dynamic simulations are performed to demonstrate the benefits of the DVC strategy.
This paper discusses the impact of dc transmission voltage drops on the distribution of dc grid balancing power when dc voltage droop control is applied. DC line voltage drops in a multiterminal VSC-HVDC (MTDC) system result in nonuniform variations of dc bus voltages when changes in dc grid power flow occur. This in turn affects the distribution of instantaneous balancing power in a MTDC that uses dc voltage droop control. The values of dc voltage droop constants determine the degree of impact that dc voltage drops will have on the sharing of balancing power in the dc grid. In this paper, an analytical expression for estimating the distribution of balancing power which accounts for dc line voltage drops is derived. A five-terminal MTDC was modelled in PSCAD for demonstrating the effects of dc line voltage drops as well as for validating the proposed analytical expression which estimates balancing power distribution.
Power generation through wind is expected to play a major role in the world's future energy portfolio. Nevertheless, wind power integration remains a challenging research area due to the special characteristics of wind power generation. Specifically, offshore wind has received significant attention worldwide due to the vast generation potential available. The electrical infrastructure of offshore wind farms is thus of significant importance. The multi-terminal HVDC solution represents a preferable solution and has shown promise in solving wind farm interconnection problems. Droop control techniques have been proposed as a means to regulate the DC voltage and facilitate the automatic coordination between different converters without the need for fast communication between units. Different methodologies have been developed to select the droop gains to satisfy the system performance specifications. In this work, a control design methodology is proposed for power sharing among the multi-terminal HVDC feeders providing that the power transmission efficiency is optimized. A simulation study on a 400-kV/1000-MW four-terminal HVDC transmission topology is conducted to ensure the validity of the proposed methodology.
This paper analyzes the influence of the converter droop settings and the dc grid network topology on the power sharing in a dc grid based on voltage source converter high voltage direct current technology. The paper presents an analytical tool to study the effect of the droop control settings on the steady-state voltage deviations and power sharing after a converter outage, thereby accounting for dc grid behavior. Furthermore, an optimization algorithm is developed, taking into account two conflicting optimization criteria. The simulation results show that, when selecting appropriate values for the converter gains, a tradeoff has to be made between the power sharing and the maximum allowable dc voltage deviation after an outage.
Multi-terminal HVDC transmission technology using voltage source converters is proposed for integrating large offshore wind farms with transmission grids. Different DC voltage control and power dispatch strategies are proposed to demonstrate the flexibility and capability of such a transmission system in integrating large-scale variable wind generation. Various options for ensuring satisfactory ride through onshore grid faults are discussed. PSCAD/EMTDC simulations on a four-terminal high voltage direct current (HVDC) system with two offshore and two onshore converter stations during wind speed and power variations, and control mode switching are presented to show the robust performance and ability of the proposed system. Further studies during severe fault on one of the connected onshore AC networks are provided to validate the fault ride through capability of the multi-terminal HVDC system.
Operation and Control of Multiterminal HVDC Transmission for Offshore Wind Farms
- J Liang
- T Jing
- S Member
- O Gomis-Bellmunt
Methodology for Droop Control Dynamic Analysis of Multiterminal VSC-HVDC Grids for Offshore Wind Farms
- E Prieto-Araujo
- F D Bianchi
- A Junyent-Ferr
- S Member
Droop control for loss minimization in HVDC multi-terminal transmission systems for large offshore wind farms
- M Arags-Pealba
- A Egea-Lvarez
- S G Arellano
- O Gomis-Bellmunt