A Novel Distributed Direct-Voltage Control Strategy for Grid Integration of Offshore Wind Energy Systems Through MTDC Network
TL;DR: A classification of MTDC networks is proposed and 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.
Abstract: 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.
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TL;DR: In this article, an overview of the state of the art in dc microgrid protection and grounding is provided, which discusses both design of practical protective devices and their integration into overall protection systems.
Abstract: DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out.
964 citations
TL;DR: In this paper, a quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels.
Abstract: DC fault protection is one challenge impeding the development of multi-terminal DC grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and DC fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode DC component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a DC to DC transformer where two converters operating in the proposed mode are coupled by an AC transformer for voltage matching and galvanic isolation. The proposed DC transformer is shown to be suitable for high-voltage high-power applications due to the low switching frequency, high efficiency, modularity, and reliability. The DC transformer facilitates DC voltage regulation and near instant isolation of DC faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.
311 citations
Cites background from "A Novel Distributed Direct-Voltage ..."
...commands throughout the network can be exchanged between individual dc busses and a control center, which determines optimal load flow scenarios similar to ac systems [13]....
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TL;DR: In this paper, the problem posed by complex, nonlinear controllers for power system load flows employing multi-terminal voltage source converter (VSC) HVDC systems is addressed.
Abstract: This paper addresses the problem posed by complex, nonlinear controllers for power system load flows employing multi-terminal voltage source converter (VSC) HVDC systems. More realistic dc grid control strategies can thus be carefully considered in power flow analysis of ac/dc grids. Power flow methods for multi-terminal VSC-HVDC (MTDC) systems are analyzed for different types of dc voltage control techniques and the weaknesses of present methods are addressed. As distributed voltage control is likely to be adopted by practical dc grids, a new generalized algorithm is proposed to solve the power flow problems with various nonlinear voltage droops, and the method to incorporate this algorithm with ac power flow models is also developed. With five sets of voltage characteristics implemented, the proposed scheme is applied to a five-terminal test system and shows satisfactory performance. For a range of wind power variations and converter outages, post-contingency behaviors of the system under the five control scenarios are examined. The impact of these controls on the power flow solutions is assessed.
251 citations
Cites background or methods from "A Novel Distributed Direct-Voltage ..."
...Research on power flow of dc network is provided in [7], [12]–[14]....
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...Basic dc power flow analysis has been applied in [13] and [14] to represent the dc transmission losses and to optimize the voltage references....
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TL;DR: In this article, the authors present the current status of the offshore wind industry and identify trends in Offshore Wind Projects (OWPs) This was accomplished via a thorough analysis of the key characteristics of the commissioned and under-construction European OWPs, including country, installed capacity, number of turbines, water depth, project area, distance to shore, transmission technology and investment cost.
Abstract: The aim of this paper is to present the current status of the offshore wind industry and to identify trends in Offshore Wind Projects (OWPs) This was accomplished via a thorough analysis of the key characteristics – commissioning country, installed capacity, number of turbines, water depth, project area, distance to shore, transmission technology and investment cost – of the commissioned and under construction European OWPs Furthermore, the current status of the several countries outside of Europe was also investigated The analysis revealed that the European offshore wind power grew on average 361% yearly since 2001 Currently, there are 7748 MW installed and 3198 MW under construction distributed among 76 OWPs situated in European waters These projects are spread among ten countries, with the highest share of offshore projects belonging to the northern European countries The UK has 46% of the total installed European offshore wind capacity with 26 projects, Germany ranks second with 16, while Denmark is third with 13 projects These countries constitute 88% of the European offshore capacity The analysis also showed that, although the installed capacity of the OWPs is growing, the projects׳ area is not increasing at the same pace due to the release of turbines with higher rated capacities which allow projects to increase their power nameplate without proportionally increasing the number of turbines The average distance to shore and the water depth are both increasing throughout the years Although the average investment cost per project is rising with the higher distances to shore and water depths, the multi-GW plans of the northern European and Asian countries indicate that the industry will continue to grow The European Union targets of having 40 GW of offshore wind capacity deployed by 2020 in Europe and 150 GW by 2030 may represent plausible scenarios since the required growth is below the European
173 citations
TL;DR: In this article, the authors proposed an effective dc voltage and power sharing control structure for multiterminal dc (MTDC) grids based on an optimal power flow (OPF) procedure and voltage-droop control implemented in the different hierarchical layers.
Abstract: This paper proposes an effective dc voltage and power-sharing control structure for multiterminal dc (MTDC) grids based on an optimal power flow (OPF) procedure and voltage-droop control implemented in the different hierarchical layers. In the proposed approach, an OPF algorithm is executed at the secondary control level of the MTDC grid to find the optimal reference values for the dc voltages and active power of the voltage-regulating converters. Then, at the primary control level, the voltage-droop characteristics of the voltage-regulating converters are tuned based upon the OPF results. In this control structure, the optimally-tuned voltage-droop controllers lead to the optimal operation of the MTDC grid. In case of variation in load or generation of the grid, a new stable operating point is achieved based on the voltage-droop characteristics. Then by executing a new OPF, the voltage-droop characteristics are returned for optimal operation of the MTDC grid after the load or generation variations. This paper also considers the integration of frequency support loop in the proposed control framework in case of connection of weak ac grids. The simulations performed on a study case inspired by the CIGRE B4 dc grid test system demonstrate the efficient grid performance under the proposed control strategy.
153 citations
References
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TL;DR: In this paper, input-admittance expressions for a voltage-source converter are derived and it is seen how the admittance can be shaped in order to get a positive real part in the desired frequency regions by adjusting the controller parameters.
Abstract: A controlled power electronic converter can cause local instabilities when interacting with other dynamic subsystems in a power system. Oscillations at a certain frequency cannot, however, build up if the converter differential input admittance has a positive conductance (real part) at that frequency, since power is then dissipated. In this paper, input-admittance expressions for a voltage-source converter are derived. It is seen how the admittance can be shaped in order to get a positive real part in the desired frequency regions by adjusting the controller parameters.
927 citations
"A Novel Distributed Direct-Voltage ..." refers background in this paper
...In the latter case, the closed-loop dynamics of the dc-system voltage controller would be nonlinear and dependent on the operating point [23]....
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...a controllable-voltage source connected to an ac transmission network by means of a series reactor [22], [23]....
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...d- and q-axes and reduces the effect of the grid-side voltage variations on the converter current [23], [25]....
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TL;DR: In this article, a power-synchronization control method for grid-connected voltage-source converters (VSCs) is proposed, which utilizes the internal synchronization mechanism in ac systems, in principle, similar to the operation of a synchronous machine.
Abstract: In this paper, a novel control method of grid-connected voltage-source converters (VSCs) is proposed. The method can be generally applied for all grid-connected VSCs but may be of most importance in high-voltage dc (HVDC) applications. Different from the previous control methods, the proposed method utilizes the internal synchronization mechanism in ac systems, in principle, similar to the operation of a synchronous machine. By using this type of power-synchronization control, the VSC avoids the instability caused by a standard phase-locked loop in a weak AC-system connection. Moreover, a VSC terminal can give the weak ac system strong voltage support, just like a normal synchronous machine does. The control method is verified by both analytical models and time simulations.
836 citations
"A Novel Distributed Direct-Voltage ..." refers background in this paper
...Recently, another control strategy has been proposed, known as power-synchronization control, which augments the maximum transferable power through the VSC link [26], [27]....
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TL;DR: It is concluded that the conventional droop method cannot achieve efficient power sharing for the case of a system with complex impedance condition, and a novel droop controller that considers the impact of complex impedance is proposed.
Abstract: This paper investigates the characteristics of the active and reactive power sharing in a parallel inverters system under different system impedance conditions. The analyses conclude that the conventional droop method cannot achieve efficient power sharing for the case of a system with complex impedance condition. To achieve the proper power balance and minimize the circulating current in the different impedance situations, a novel droop controller that considers the impact of complex impedance is proposed in this paper. This controller can simplify the coupled active and reactive power relationships, which are caused by the complex impedance in the parallel system. In addition, a virtual complex impedance loop is included in the proposed controller to minimize the fundamental and harmonic circulating current that flows in the parallel system. Compared to the other methods, the proposed controller can achieve accurate power sharing, offers efficient dynamic performance, and is more adaptive to different line impedance situations. Simulation and experimental results are presented to prove the validity and the improvements achieved by the proposed controller.
779 citations
"A Novel Distributed Direct-Voltage ..." refers background in this paper
...This stability issue then limits the ratio range that can be used to share power among the terminals [39]....
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...However, the main downside of droop control is the fact that power-flow control is limited [39]....
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TL;DR: In this paper, the benefits and drawbacks of grid connecting offshore wind farms through a dc link were evaluated for a 100MW wind farm and cases of larger wind farms (200 and 500 MW) were presented.
Abstract: This paper presents a technical and economic analysis to evaluate the benefits and drawbacks of grid connecting offshore wind farms through a dc link. A first case, concerning a 100-MW wind farm, is thoroughly investigated and cases of larger wind farms (200 and 500 MW) are presented. Three different transmission solutions are compared: 150-kV ac, 400-kV ac, and high-voltage dc based on voltage sourced converters (VSC-HVDC). After a brief overview of the features of these connection solutions, the related operational aspects are evaluated. An economic assessment compares the dc connection option to the ac alternatives, taking into account the investment, operation, and maintenance costs, and the negative valorization of losses and energy not supplied. Economic assessment includes sensitivity analyses of parameters, which could impact the 100-MW wind farm: distance, component costs, dc converter reliability, and dc converter losses
730 citations
TL;DR: In this paper, cable faults in VSC-based dc networks are analyzed in detail with the identification and definition of the most serious stages of the fault that need to be avoided and a fault location method is proposed.
Abstract: The application of high-power voltage-source converters (VSCs) to multiterminal dc networks is attracting research interest. The development of VSC-based dc networks is constrained by the lack of operational experience, the immaturity of appropriate protective devices, and the lack of appropriate fault analysis techniques. VSCs are vulnerable to dc-cable short-circuit and ground faults due to the high discharge current from the dc-link capacitance. However, faults occurring along the interconnecting dc cables are most likely to threaten system operation. In this paper, cable faults in VSC-based dc networks are analyzed in detail with the identification and definition of the most serious stages of the fault that need to be avoided. A fault location method is proposed because this is a prerequisite for an effective design of a fault protection scheme. It is demonstrated that it is relatively easy to evaluate the distance to a short-circuit fault using voltage reference comparison. For the more difficult challenge of locating ground faults, a method of estimating both the ground resistance and the distance to the fault is proposed by analyzing the initial stage of the fault transient. Analysis of the proposed method is provided and is based on simulation results, with a range of fault resistances, distances, and operational conditions considered.
665 citations
"A Novel Distributed Direct-Voltage ..." refers background in this paper
...necessary for clearing faults in the dc line or stations [21]....
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...terruption in the complete system, while in a parallel system, only the faulted converter would have a service disruption [21]....
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