Search

This paper develops a data-driven strategy for identification and voltage control for DC-DC power converters. The proposed strategy does not require a pre-defined standard model of the power converters and only relies on power converter measurement data, including sampled output voltage and the duty ratio to identify a valid dynamic model for them over their operating regime. To derive the power converter model from the measurements, a local model network (LMN) is used, which is able to describe converter dynamics through some locally active linear sub-models, individually responsible for representing a particular operating regime of the power converters. Later, a local linear controller is established considering the identified LMN to generate the control signal (i.e., duty ratio) for the power converters. Simulation results for a stand-alone boost converter as well as a bidirectional converter in a test DC microgrid demonstrate merit and satisfactory performance of the proposed data-driven identification and control strategy. Moreover, comparisons to a conventional proportional-integral (PI) controllers demonstrate the merits of the proposed approach.

This paper develops a data-driven strategy for identification and voltage control for DC-DC power converters. The proposed strategy does not require a pre-defined standard model of the power converters and only relies on power converter measurement data, including sampled output voltage and the duty ratio to identify a valid dynamic model for them over their operating regime. To derive the power converter model from the measurements, a local model network (LMN) is used, which is able to describe converter dynamics through some locally active linear sub-models, individually responsible for representing a particular operating regime of the power converters. Later, a local linear controller is established considering the identified LMN to generate the control signal (i.e., duty ratio) for the power converters. Simulation results for a stand-alone boost converter as well as a bidirectional converter in a test DC microgrid demonstrate merit and satisfactory performance of the proposed data-driven identification and control strategy. Moreover, comparisons to a conventional proportional-integral (PI) controllers demonstrate the merits of the proposed approach., Intelligent Electrical Power Grids

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., Peer Reviewed, Postprint (author's final draft)

The electric networks of the future will make an extensive use of DC grids. Therefore, the control of Multi-terminal DC (MTDC) grids is a key issue, which is gathering the attention of the industry and the research community. In this regard, this paper proposes a grid control strategy for voltage-source converter (VSC)-based MTDC networks, based on the use of the particle swarm optimization (PSO) technique. In the proposed approach, the controllers of the power converters belonging to the MTDC grid are acting based on the concept of vector control, in which the AC currents and voltages are transformed into a rotating direct-quadrature (dq) reference frame for controlling of the active and reactive powers as well as the DC and AC voltages. Since the VSCs are nonlinear plants in nature, the classical approaches for tuning of the control system, which are usually based on the approximate linear model of the plants, do not lead to optimal results. As an alternative, in this paper an efficient PSO algorithm is used for tuning optimally the parameters of the controllers in the MTDC grid. In addition, the voltage droop control scheme is utilized to ensure the active power balance within the MTDC grid. The simulation results, obtained through a detailed model of a four-terminal DC grid, demonstrate the effi-ciency of the proposed approach. Finally a comparison with PI controllers which have been conventionally tuned also confirmed the favorable performance of the proposed PSO-tuned controllers., Peer Reviewed, Postprint (author's final draft)

This paper proposes an intelligent control scheme for DC voltage regulationin a DC micro-grid integrating photovoltaic (PV) generation, energy storage and electric loads. The maximum power generation of the PV panel is followed using the incremental conductance (IC) maximum power point tracking (MPPT) algorithm while a high-performance local linear controller (LLC)is developed for the DC voltage control in the micro-grid.The LLC, as a data-driven control strategy, controls the bidirectional converter located between the battery and the DC micro-grid to regulate the micro-grid's voltage at the specified reference value.In order to validate the controller different simulations including variations in the load and in the solar irradiance are performed. Finally a comparison with PI based controller demonstrates the advantages of the proposed control technique for regulating theDC micro-grid's voltage., Peer Reviewed, Postprint (published version)

In this paper, DC-link voltage control in DC microgrids with photovoltaic (PV) generation and battery, is addressed based on an intelligent approach. The proposed strategy is based on the modeling of the power interface, i.e. power electronic converter, located between the PV array, battery and DC bus, by use of measurement data. For this purpose, a local model network (LMN) is developed to model the converter and then a local linear control (LLC) strategy is designed based on the LMN. Simulation results on a DC microgrid demonstrates efficacy of the proposed approach., Peer Reviewed, Postprint (published version)

This paper develops a flexible DC transmission system (FDCTS) to power control and load flow control within the multi-terminal (MTDC) grids. The FDCTS include static power electronics-based elements for electric energy transmission inside a DC grid. Considering the current advancements in the field of power electronics, this paper proposes a cascaded power flow controller (CPFC) as the first element of the FDTCS for providing power control in MTDC grids. It is shown through dynamic simulations that the CPFC is able to control power flowing through a particular DC transmission line, and thus enhancing controllability in the MTDC grids., Peer Reviewed, Postprint (published version)

This paper develops a hybrid power flow controller (HPFC) to take one step forward for building future flexible DC transmission system (FDCTS). The idea for proposing HPFC is inspired by the successful operation of flexible AC transmission systems (FACTS) devices, the FDCTS includes static power electronics-based elements to provide voltage regulation, power control and load flow control in the multi-terminal DC (MTDC) grids. The HPFC, as one of the devices of the FDCTS, has a hybrid connection scheme (i.e. series-parallel) to the grid and allows power flow control through DC transmission lines by imposing a series voltage to the controlled DC line. In this work, in order to demonstrate the eligibilities of the HPFC, the performance of this device is included in the DC power flow formulation for the power flow control purposes. Finally, it will be shown through dynamic simulations that the proposed HPFC is able to control the power flow through a particular DC transmission line., Peer Reviewed, Postprint (published version)

The current route of achieving the ultimate plan for flawless operation and control of the multi-terminal DC (MTDC) grids can be significantly accelerated by learning from the vast and valuable experiences gained from the operation of the AC power grids for more than a century. This paper introduces concept of flexible DC transmission system (FDCTS), inspired by the successful operation of flexible AC transmission systems (FACTS), to provide voltage regulation, power control and load flow control within MTDC grids. Considering the current advancements in the field of power electronics, this paper recognizes DC-DC converters as the first element of the FDTCS for providing voltage and power control in MTDC grids. By use of DC-DC converters, this paper developes two elements of the FDCTS, namely the cascaded power flow controller (CPFC) and hybrid power flow controller (HPFC). In this paper, to demonstrate the eligibility of the CPFC and HPFC to play the role of an FDCTS, they are included in the DC power flow formulation for DC voltage regulation and power flow control purposes., Peer Reviewed, Postprint (published version)

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., Peer Reviewed, Postprint (published version)

This paper proposes a generalized voltage droop (GVD) control strategy for control and power sharing in voltage source converter (VSC)-based multi-terminal DC (MTDC) grids. In the proposed approach, the conventional voltage droop characteristics of voltage-regulating VSC stations are replaced by the GVD characteristics. 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 proper adjustment of the GVD characteristics of the voltage-regulating converters. The proposed strategy improves the control and power-sharing capabilities of the conventional voltage droop, and enhances its maneuverability. Simulation results, obtained by detailed modeling in a four-terminal high voltage DC grid demonstrated the efficacy of the proposed approach and its flexibility in active power sharing. Moreover, based on the obtained results, switching between operating mode of the GVD control approach does not results in oscillations of the active powers flowing inside the MTDC grids., Postprint (published version)

Peer Reviewed, Postprint (published version)

This paper proposes a novel control strategy in multi-terminal DC (MTDC) grids with offshore wind generation based on the idea of generalized voltage droop (GVD). In the proposed strategy, the GVD characteristics are assigned to the voltage-regulating converter station, enhancing their control maneuverability and enabling them to perform necessary DC voltage control and power sharing tasks. By adjusting the coefficients of the corresponding GVD characteristic, a converter station can be operated in three operating modes, namely (1) fixed active power control mode, (2) conventional voltage droop control mode or (3) fixed DC voltage control mode. Results of simulations on a test four-terminal DC grid including two offshore wind farm and two AC grids show the capabilities of the proposed control strategy. Moreover, simulation results confirmed stable operation of the GVD control in transition from one operating mode to another., Postprint (published version)

This paper addresses the important issue of Multi-Terminal DC (MTDC) grid control strategy based on the particle swarm optimization (PSO) technique. The MTDC grid is controlled by the concept of vector control, in which the AC currents and voltages are transformed into the rotating directquadrature (d-q) reference quantities which are subsequently used for decoupled control of the active and reactive powers as well as the DC and AC voltages. The paper employs an efficient PSO algorithm for optimal tuning of the controllers’ parameters in an MTDC grid. In addition, voltage droop control scheme is utilized to ensure the balance of active power within the MTDC grid. Simulation results that has obtained through detailed modeling of a four-terminal DC grid, demonstrate the efficiency of the proposed approach. Comparison to controllers optimized by genetic algorithm (GA) also confirmed the favorable performance of the proposed PSO-tuned controllers., Postprint (published version)

This paper presents a hierarchical control structure for Multi terminal HVDC grids. The hierarchical control structure is inspired by the three-layer control structure of largescale AC power systems and it is divided into primary, secondary and tertiary control. This structure permits to create an easy interaction between power dispatch and the lower controllers. Moreover, a generalized voltage droop (GVD) characteristic is also proposed which facilitates control issues inside the Multi terminal HVDC grids. In the proposed control structure, the coefficients of the GVD characteristics are set based on the solution of a DC power flow algorithm, executed in the secondary control level., Postprint (published version)

This paper proposes a novel optimized fuzzy logic controller (OFLC) for current-mode control (CMC)-based voltage regulation in DC-DC converters. The implementation of the proposed fuzzy logic controller (FLC) does not depend on the mathematical model of the converter and only requires knowledge about its operation modes. The fast and efficient algorithm of particle swarm optimization (PSO) has been employed to enhance the performance of the FLC by fine tuning its membership functions. The reference current for the current control loop has been obtained in the outer voltage loop by measuring load current and output voltage and calculating the load resistive impedance. The proposed OFLC has been used in four different case studies for evaluating its performance under different conditions of input voltage and load variations as well as variations in the system parameters, e.g. inductance and capacitance of the converter circuit. The results of simulations and comparison to a proportional-integral (PI) controller demonstrates the fast and robust performance of the proposed controller, able to effectively regulate the output voltage in presence of high perturbations in the input voltage and high variation of the converter components nominal values, Postprint (published version)

With the rapid proliferation of the DC distribution systems, special attentions are paid to the photovoltaic (PV) generations. This paper addresses the problem of maximum power point tracking (MPPT) for PV systems using a local neuro fuzzy (LNF) network and steepest descent (SD) optimization algorithm. The proposed approach, termed LNF + SD, first identifies a valid an accurate model for the PV system using the LNF network and through measurement data. Then the identified PV model is used for MPPT by SD optimization algorithm. The salient modeling abilities of the proposed LNF network results in a reliable and dependable PV model which takes voltage, temperature and insolation variations into account. The proposed approach is evaluated using several identification and MPPT simulations. The simulation results showed the accuracy of the LNF network in modeling of PV systems. Furthermore, simulations carried out for assessment of the MPPT performance during insolation transients demonstrated the high efficiency of the proposed LNF + SD approach for MPPT applications. Performance of the proposed method MPPT, while the PV array was supplying loads through DC-DC converters was also analyzed. Comparisons to the perturb-and-observe (P&O) and fuzzy logic methods revealed the superiority of the proposed approach, Postprint (published version)