Adaptive Droop Control for Effective Power Sharing in Multi-Terminal DC (MTDC) Grids
TL;DR: In this article, a scheme for adapting the droop coefficients to share the burden according to the available headroom of each converter station is proposed, and the advantage of this adaptive (variable) droop scheme for autonomous power sharing is established through transient simulations on an MTDC grid with four bipolar converters and DC cable network with metallic return.
Abstract: Following a converter outage in a Multi-Terminal DC (MTDC) grid, it is critical that the healthy converter stations share the power mismatch/burden in a desirable way. A fixed value of power-voltage droop in the DC link voltage control loops can ensure proper distribution according to the converter ratings. Here a scheme for adapting the droop coefficients to share the burden according to the available headroom of each converter station is proposed. Advantage of this adaptive (variable) droop scheme for autonomous power sharing is established through transient simulations on an MTDC grid with four bipolar converters and DC cable network with metallic return. Results for both rectifier and inverter outages under two different scenarios are presented. Post-contingency steady-state operating points obtained from transient simulation are shown to be consistent with those derived analytically. Impact of varying droop coefficients on the stability of the MTDC grid is established. An averaged model in Matlab/SIMULINK which has been validated against detailed switched model in EMTDC/PSCAD is used for the stability and modal analysis.
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TL;DR: In this article, a distributed adaptive droop mechanism is proposed for secondary/primary control of dc microgrids, where the conventional secondary control that adjusts the voltage set point for the local droop mechanisms is replaced by a voltage regulator.
Abstract: A distributed-adaptive droop mechanism is proposed for secondary/primary control of dc microgrids. The conventional secondary control that adjusts the voltage set point for the local droop mechanism is replaced by a voltage regulator. A current regulator is also added to fine-tune the droop coefficient for different loading conditions. The voltage regulator uses an observer that processes neighbors' data to estimate the average voltage across the microgrid. This estimation is further used to generate a voltage correction term to adjust the local voltage set point. The current regulator compares the local per-unit current of each converter with the neighbors' on a communication graph and, accordingly, provides an impedance correction term. This term is then used to update the droop coefficient and synchronize per-unit currents or, equivalently, provide proportional load sharing. The proposed controller precisely accounts for the transmission/distribution line impedances. The controller on each converter exchanges data with only its neighbor converters on a sparse communication graph spanned across the microgrid. Global dynamic model of the microgrid is derived with the proposed controller engaged. A low-voltage dc microgrid prototype is used to verify the controller performance, link-failure resiliency, and the plug-and-play capability.
372 citations
Cites background from "Adaptive Droop Control for Effectiv..."
...Constant droop is commonly used for power reference tracking and load sharing in grid-connected and islanded modes, respectively [26], [27]....
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TL;DR: This paper presents a distributed hierarchical control framework to ensure reliable operation of dc microgrid (MG) clusters and an adaptive droop method is proposed for this level, which determines droop coefficients according to the state-of-charge of batteries automatically.
Abstract: This paper presents a distributed hierarchical control framework to ensure reliable operation of dc microgrid (MG) clusters. In this hierarchy, primary control is used to regulate the common bus voltage inside each MG locally. An adaptive droop method is proposed for this level, which determines droop coefficients according to the state-of-charge (SOC) of batteries automatically. A small-signal model is developed to investigate effects of the system parameters, constant power loads, as well as line impedance between the MGs on stability of these systems. In the secondary level, a distributed consensus-based voltage regulator is introduced to eliminate the average voltage deviation over the MGs. This distributed averaging method allows the power flow control between the MGs to be achieved at the same time, as it can be accomplished only at the cost of having voltage deviation inside the system. Another distributed policy is employed then to regulate the power flow among the MGs according to their local SOCs. The proposed distributed controllers on each MG communicate with only the neighbor MGs through a communication infrastructure. Finally, the developed small-signal model is expanded for MG clusters with all the proposed control loops. The effectiveness of the proposed hierarchical scheme is verified through detailed hardware-in-the-loop simulations.
332 citations
Cites methods from "Adaptive Droop Control for Effectiv..."
...An adaptive droop scheme is proposed for multiterminal dc grids in [8] to share the load according to the available headroom of converters....
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...In this regard, several adaptive droop methods have been presented recently [8]–[12]....
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TL;DR: In this paper, the authors discuss the extension of electromechanical stability models of voltage source converter high voltage direct current (VSC HVDC) to multi-terminal (MTDC) systems.
Abstract: This paper discusses the extension of electromechanical stability models of voltage source converter high voltage direct current (VSC HVDC) to multi-terminal (MTDC) systems. The paper introduces a control model with a cascaded DC voltage control at every converter that allows a two-terminal VSC HVDC system to cope with converter outages. When extended to an MTDC system, the model naturally evolves into a master-slave set-up with converters taking over the DC voltage control in case the DC voltage controlling converter fails. It is shown that the model can be used to include a voltage droop control to share the power imbalance after a contingency in the DC system amongst the converters in the system. Finally, the paper discusses two possible model reductions, in line with the assumptions made in transient stability modeling. The control algorithms and VSC HVDC systems have been implemented using both MatDyn, an open source MATLAB transient stability program, as well as the commercial power system simulation package EUROSTAG.
233 citations
Cites methods from "Adaptive Droop Control for Effectiv..."
...The droop values can be optimized taking into account the DC system dynamics, as done in [8] or both the AC and DC system dynamics, as in [6]....
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...2279268 used to study alternative outer controller structures [3]–[5], and optimized control settings [6]–[8] as well as dynamic interaction with the AC system [9], [10] and system frequency support [11], [12]....
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...The two main control methods are voltage margin control [5], [16] and DC voltage droop control [6]–[8], [10], [17]....
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...In [6], an alternative approach has been proposed to use an adaptive droop control based on a common voltage feedback signal instead of a local voltage measurement, thereby requiring communication....
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TL;DR: In this paper, a generalized voltage droop (GVD) control strategy for dc voltage control and power sharing in voltage source converter (VSC)-based multiterminal dc (MTDC) grids is proposed.
Abstract: This paper proposes a generalized voltage droop (GVD) control strategy for dc voltage control and power sharing in voltage source converter (VSC)-based multiterminal dc (MTDC) grids The proposed GVD control is implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid, and constitutes an alternative to the conventional voltage droop characteristics of voltage-regulating VSC stations, providing higher flexibility and, thus, controllability to these networks As a difference with other methods, the proposed GVD control strategy can be operated in three different control modes, including conventional voltage droop control, fixed active power control, and fixed dc voltage control, by adjusting the GVD characteristics of the voltage-regulating converters Such adjustment is carried out in the secondary layer of the hierarchical control structure The proposed strategy improves the control and power-sharing capabilities of the conventional voltage droop, and enhances its maneuverability The simulation results, obtained by employing a CIGRE B4 dc grid test system, demonstrate the efficiency of the proposed approach and its flexibility in active power sharing and power control as well as voltage control In these analysis, it will be also shown how the transitions between the operating modes of the GVD control does not give rise to active power oscillations in the MTDC grids
221 citations
Additional excerpts
...The voltage droop control, as the most commonly employed strategy for DC voltage control in the MTDC grids, several converters contribute to the DC voltage control of the grid [17]....
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TL;DR: In this article, 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 was analyzed.
Abstract: 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.
174 citations
Cites methods from "Adaptive Droop Control for Effectiv..."
...As an alternative, a common voltage feedback signal can be used as proposed in [19] and used in [13]....
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...In [13], the droop coefficients were made adaptive to account for the available headroom of the different converters, using a common voltage feedback signal [19]....
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References
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Book•
01 Jan 1994
TL;DR: In this article, the authors present a model for the power system stability problem in modern power systems based on Synchronous Machine Theory and Modelling, and a model representation of the synchronous machine representation in stability studies.
Abstract: Part I: Characteristics of Modern Power Systems. Introduction to the Power System Stability Problem. Part II: Synchronous Machine Theory and Modelling. Synchronous Machine Parameters. Synchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems. High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Machine Representation in Stability Studies. AC Transmission. Power System Loads. Excitation in Stability Studies. Prime Mover and Energy Supply Systems, High-Voltage Direct-Current Transmission. Control of Active Power and Reactive Power. Part III: Small Signal Stability. Transient Stability. Voltage Stability. Subsynchronous Oscillations. Mid-Term and Long-Term Stability. Methods of Improving System Stability.
13,467 citations
"Adaptive Droop Control for Effectiv..." refers background in this paper
...It is a measure of the relative participation of the th state variable in the th mode and vice versa [15]....
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01 Jun 1990
TL;DR: In this paper, a more general averaging procedure that encompasses state-space averaging and that is potentially applicable to a much broader class of circuits and systems is presented, including resonant type converters.
Abstract: A more general averaging procedure that encompasses state-space averaging and that is potentially applicable to a much broader class of circuits and systems is presented. Examples of its application in resonant and PWM power convertors are presented. The technique is shown to be effective on a number of examples. including resonant type converters. The approach offers refinements to the theory of state-space averaging, permitting a framework for analysis and design when small ripple conditions do not hold. The method may find applications in simulation and design since it is considerably easier to simulate an averaged model than a switched model. >
1,144 citations
17 Sep 1991
TL;DR: The advanced static VAr compensator (ASVC) as mentioned in this paper is based on the principle that a self-commutating static inverter can be connected between three-phase AC power lines and an energy storage device, such as an inductor or capacitor, and controlled to draw mainly reactive current from the lines.
Abstract: The advanced static VAr compensator (ASVC) is based on the principle that a self-commutating static inverter can be connected between three-phase AC power lines and an energy-storage device, such as an inductor or capacitor, and controlled to draw mainly reactive current from the lines. This capability is analogous to that of the rotating synchronous condenser and it can be used in a similar way for the dynamic compensation of power transmission systems, providing voltage support, increased transient stability, and improved damping. The authors present a simplified mathematical model of the ASVC that has made it possible to derive the transfer functions needed for control system synthesis. The resulting control system designs are briefly outlined and further analysis is presented to show the behaviour of the ASVC when the line voltage is unbalanced or distorted. The analysis is based on a vectorial transformation of variables, first described by R.H. Park (1928) for AC machine analysis, and later, using complex numbers, by W.V. Lyon (1954) in the theory of instantaneous symmetrical components.
1,039 citations
01 Jul 1993
TL;DR: In this paper, two fundamentally different types of invertor can be used for this purpose, one providing control of output voltage magnitude and phase angle, and the other having only phase angle control.
Abstract: The advanced static Var compensator (now widely known as the static condenser or STATCON) uses a high power self-commutating inverter to draw reactive current from a transmission line. Two fundamentally different types of invertor can be used for this purpose, one providing control of output voltage magnitude and phase angle, and the other having only phase angle control. For each of these types, the governing equations are derived, and frequency domain analysis is used to obtain the relevant transfer functions for control system synthesis. Further analysis is provided to determine the response of the STATCON to negative sequence and harmonic voltage components on the transmission line. The results are illustrated with measured waveforms obtained from a scaled analogue model of an 80 MVAr STATCON.<
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974 citations
"Adaptive Droop Control for Effectiv..." refers methods in this paper
...Decoupled current control strategy [8] as shown in Fig....
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...We have followed a similar approach as in [8] for modeling the converters which is described here briefly....
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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
"Adaptive Droop Control for Effectiv..." refers methods in this paper
...Squared values of half of the reference and measured DC link voltages are normally used for the voltage control loops [13]....
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