Tommaso Caldognetto

Microgrids Operation Based on Master–Slave Cooperative Control

Low-voltage microgrids can be seen as the basic tiles of the smart grid patchwork owing to their capability to efficiently manage the distributed energy resources (DERs) in aggregate form. They can support the grid in terms of demand response, power quality, ride through capability, and at the same time, they can ensure electrical continuity to the loads, even in case of grid failure. This paper describes a simple and effective approach to manage microgrids by synergistic control of the power electronic interfaces acting therein, i.e., the utility interface (UI), installed at the point of common coupling with the utility and the energy gateways (EGs), interfacing the DERs with the distribution grid. The proposed master-slave control uses the UI as control master for the EGs. In grid-connected operation, the UI performs as a grid-supporting unit and dispatches active and reactive power references to the EGs so as to improve energy efficiency and power quality; in islanded operation, the UI performs as a grid-forming voltage source and ensures the power balance by exploiting every power source and energy storage unit available in the microgrid. This paper discusses the theoretical background, architecture, and algorithms of the proposed master-slave control and demonstrates the resulting microgrid performance by means of simulation and experimental results.

Power-Based Control of Low-Voltage Microgrids

This paper presents a simple and robust control technique for distributed energy resources (DERs) in microgrids. The technique utilizes the full potential of DERs during grid-connected and islanded operating modes. In grid-connected mode, the control pursues quasi-optimum operation of the microgrid so as to reduce the distribution losses and voltage deviations while fully exploiting renewable energy sources. In islanded mode, it effectively manages any available energy source, including storage devices, to ensure a safe and smooth autonomous operation of the microgrid. In addition, prompt adaptation to variations of the generated and absorbed power is ensured in each operating condition. The proposed control can be implemented by an Information and Communication Technology architecture, which is inherently flexible and scalable, allows plug-and-play integration of distributed energy sources, and does not involve time-critical communications.

Rapid Prototyping of Digital Controllers for Microgrid Inverters

A rapid prototyping methodology for digital controllers is presented in this paper. Its main application is in the development, debugging, and test of microgrid inverter controllers. To fulfill the application requirements, these systems are characterized by complex multilayer architectures, extending from pulse width modulation (PWM) and current control loops up to global optimization and high level communication functions. The complexity and the wide variability of the different layer implementations make digital control mandatory. However, developing so complex digital controllers on conventional hardware platforms, like digital signal processors (DSPs) or even FPGAs, is not the most practical choice. This paper shows how multiplatform control devices, where software configurable DSP functions and programmable logic circuits are efficiently combined, represent the optimal solution for this field of application. Furthermore, this paper proposes hardware-in-the-loop real-time simulation as an effective means of developing and debugging complex hardware and software codesigned controllers. A case study is presented and used to illustrate the different design and test phases, from initial concept and numerical simulation to final experimental verification.

Dead-Beat Current Controller for Voltage-Source Converters With Improved Large-Signal Response

A digital dead-beat current controller for voltage source converters is presented in this paper. The control structure is specified in a digital hardware description language, synthesized, and deployed on a field-programmable gate array chip. By updating, with negligible computation delay, the duty cycle twice in a switching period, the reference current error is nulled in half a modulation period, so that the controllers small-signal bandwidth is maximized. In addition, due to a simple transient detection circuit, the large-signal response delay is reduced to a small fraction of the modulation period, which is determined by the chosen current signal oversampling rate. The controller can effectively support different voltage-source inverter applications, such as active filters, uninterruptible power supplies, microgrid distributed energy resource controllers, and dc-dc converter applications, including interface converters for renewable energy sources, laboratory battery chargers, and electronic welding machines.

Microgrids operation based on master-slave cooperative control

Low-voltage microgrids can be seen as the basic tiles of the smart grid patchwork owing to their capability to efficiently manage the distributed energy resources (DERs) in aggregate form. They can support the grid in terms of demand response, power quality, ride through capability, and at the same time, they can ensure electrical continuity to the loads, even in case of grid failure. This paper describes a simple and effective approach to manage microgrids by synergistic control of the power electronic interfaces acting therein, i.e., the utility interface (UI), installed at the point of common coupling with the utility and the energy gateways (EGs), interfacing the DERs with the distribution grid. The proposed master-slave control uses the UI as control master for the EGs. In grid-connected operation, the UI performs as a grid-supporting unit and dispatches active and reactive power references to the EGs so as to improve energy efficiency and power quality; in islanded operation, the UI performs as a grid-forming voltage source and ensures the power balance by exploiting every power source and energy storage unit available in the microgrid. This paper discusses the theoretical background, architecture, and algorithms of the proposed master-slave control and demonstrates the resulting microgrid performance by means of simulation and experimental results.

Centralized Control of Distributed Single-Phase Inverters Arbitrarily Connected to Three-Phase Four-Wire Microgrids

This paper proposes an effective technique to control the power flow among different phases of a three-phase four-wire distribution power system by means of single-phase converters arbitrarily connected among the phases. The aim is to enhance the power quality at the point-of-common-coupling of a microgrid, improve voltage profile through the lines, and reduce the overall distribution losses. The technique is based on a master/slave organization where the distributed single-phase converters act as slave units driven by a centralized master controller. Active, reactive, and unbalance power terms are processed by the master controller and shared proportionally among distributed energy resources to achieve the compensation target at the point-of-common-coupling. The proposed control technique is evaluated in simulation considering the model of a real urban power distribution grid under non-sinusoidal and asymmetrical voltage conditions. The main results, concerning both steady-state and transient conditions, are finally reported and discussed.

Improving microgrid performance by cooperative control of distributed energy sources

This paper proposes a simple and effective approach to control the distributed energy resources (DERs) in a low-voltage microgrid, to increase energy efficiency and hosting capacity of the microgrid itself, with the only requirement of narrow-band communication among distributed loads and generators. More specifically, the control aims at taking full advantage of the power control capability allowed by distributed renewable sources and energy storage units, to optimize energy efficiency and voltage stability. The resulting distributed control architecture is scalable, ensures prompt response to power transients, and provides proper power sharing even if some DERs saturate their power capacity.

Selective compensation of reactive, unbalance, and distortion power in smart grids by synergistic control of distributed switching power interfaces

Smart grids feature distributed energy resources interfaced to the grid by means of switching power converters. Synergistic control of these units can considerably increase the distribution efficiency and power quality by limiting the unbalance, reactive, and harmonic currents flowing through distribution lines. This can be achieved by a distributed control and communication architecture which implements suitable control strategies. This paper deals with a control approach, based on conservative power commands, which allows selective elimination of the main causes affecting the power quality, i.e., load asymmetry, harmonics, and useless reactive power flow. As a result, distribution efficiency improves, voltage asymmetry and distortion diminishes, and the distribution infrastructure is better exploited, thus allowing an increase of the hosting capacity of the grid.

A Nonlinear Wide-Bandwidth Digital Current Controller for DC–DC and DC–AC Converters

A fully digital, nonlinear, wide-bandwidth current controller for dc-ac and dc-dc voltage source converters is presented in this paper. Exploiting oversampling, the controller mimics an analog hysteresis current controller, but it does not employ analog comparators, digital-to-analog converters, or any other analog signal pre- or postprocessing circuitry. Indeed, it fully virtualizes the hysteresis controllers operation and, based only on a nonlinear, efficient current error processing algorithm, drives the power converter at almost constant switching frequency. Overall, it offers the same excellent dynamic performance of the analog hysteresis controller and, at the same time, solves most of the related problems. Because the current error sample processing algorithm is inherently parallel in structure, the controller is suited for VHDL synthesis and field-programmable gate-array implementation, which guarantees flexibility and low cost, together with minimum computation and signal conversion delays. Its intended application areas include active filters, uninterruptible power supplies, microgrid distributed energy resource controllers, laboratory battery testers, and welding machines.

Online wideband identification of single-phase AC power grid impedances using an existing grid-tied power electronic inverter

This paper presents an original implementation, in an embedded controller, of a recently proposed online wideband single-phase AC system impedance identification technique and its validation via a Hardware In the Loop (HIL) real-time simulation setup. The technique uses an existing grid-tied inverter for estimation of wide bandwidth single-phase AC grid impedance in addition to performing its original power conversion function. A small-signal Pseudo Random Binary Sequence (PRBS), a digital approximation of white noise, can be super-imposed on the inverter switching commands. This injected white noise signal is wide bandwidth in nature, and, using appropriate cross correlation techniques, the wideband AC grid impedance can be measured. In smart grid applications, online wideband monitoring of AC power grid impedances is a key enabler of a set of capabilities, such as health monitoring, active filter retuning, and adaptive control of inverters. To evaluate the effectiveness of the online wide bandwidth AC power impedance identification technique in all of these smart grid applications, the proposed HIL setup aims at the numerical evaluation of the identification technique performance as well as the evaluation of all practical implementation issues.