D. Roiu

Bio: D. Roiu is an academic researcher from Polytechnic University of Turin. The author has contributed to research in topics: AC power & Ripple. The author has an hindex of 8, co-authored 14 publications receiving 532 citations.

Enhanced power quality control strategy for single-phase inverters in distributed generation systems

Abstract: Power electronic converters are commonly used for interfacing distributed generation systems to the electrical power network. This paper deals with a single-phase inverter for distributed generation systems requiring power quality features, such as harmonic and reactive power compensation for grid-connected operation. The idea is to integrate the DG unit functions with shunt active power filter capabilities. With the proposed approach, the inverter controls the active power flow from the renewable energy source to the grid and also performs the non-linear load current harmonic compensation keeping the grid current almost sinusoidal. The control scheme employs a current reference generator based on Sinusoidal Signal Integrator (SSI) and Instantaneous Reactive Power (IRP) theory together with a repetitive current controller. Experimental results obtained on a 4 kVA inverter prototype demonstrate the feasibility of the proposed solution.

New Stationary Frame Control Scheme for Three-Phase PWM Rectifiers Under Unbalanced Voltage Dips Conditions

Abstract: A new stationary frame control scheme for three-phase pulsewidth-modulation (PWM) rectifiers operating under unbalanced voltage dips conditions is proposed in this paper The proposed control scheme regulates the instantaneous active power at the converter poles to minimize the harmonics of the input currents and the output voltage ripple This paper's novelty is the development of a new current-reference generator implemented directly in stationary reference frame This allows using proportional sinusoidal signal integrator (P-SSI) controllers for simultaneous compensation of both positive and negative current sequence components No phase-locked loop (PLL) strategies and coordinate transformations are needed for the proposed current-reference generator Experimental results are presented for a 20-kV A alternative current (ac)/direct current (dc) converter prototype to demonstrate the effectiveness of the proposed control scheme A comparison with two other existing control techniques is also performed Fast dynamic performance with small dc-link voltage ripple and input sinusoidal currents are obtained with this control scheme, even under severe voltage dips operating conditions

New Stationary Frame Control Scheme for Three Phase PWM Rectifiers under Unbalanced Voltage Dips Conditions

Abstract: A new stationary frame control scheme for three-phase pulse width modulation (PWM) rectifiers operating under unbalanced voltage dips conditions is proposed in this paper. The proposed control scheme regulates the instantaneous active power at the converter poles to minimize the harmonics of the input currents and the output voltage. The paper novelty is the development of a new current-reference calculation implemented directly in stationary reference frame. This allows using proportional - sinusoidal signal integrators (P-SSI) controllers for simultaneous compensation of both positive and negative current sequence components. No phase locked loop (PLL) strategies and coordinate transformations are needed. A comparison with two other existing control techniques is also performed. Experimental results are presented for a 20 kVA AC/DC converter prototype to demonstrate the effectiveness of the proposed control scheme. Fast dynamic performance with small DC-link voltage ripple and input sinusoidal currents are obtained with this control scheme even under severe voltage dips operating conditions.

Analysis of current controllers for active power filters using selective harmonic compensation schemes

Abstract: This paper presents a frequency response analysis of current controllers based on selective harmonic compensation schemes for three-phase voltage–source shunt Active Power Filters (APFs). To perform this analysis, the time delay introduced by the inverter and the sampling is taken into account in the current controller closed loop transfer function. Doing this, the differences among the schemes are pointed out, demonstrating the importance and effectiveness of the delay compensation to maintain the whole system stability. In addition, the performance of each current controller is evaluated by means of the total harmonic distortion of the mains line current and time response for fast load variations. The main goal is to identify the most suitable solution in terms of current controllers based on selective harmonic compensation for an industrial implementation. All the considered current control schemes have been tested on the same digital platform. Experimental results are presented for a 25-kVA shunt APF prototype compensating a nonlinear inductive load. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Frequency-domain analysis of resonant current controllers for active power conditioners

Abstract: This paper performs a frequency-domain analysis of three different resonant current controllers for active power conditioners. The analysis is focused on the closed loop frequency response and Nyquist plots for all current controllers of interest. The system stability when the active power conditioner attempts compensating high order harmonics is investigated. To perform this analysis, the time delay introduced by the inverter and the sampling is taken into account in the current controller closed loop transfer function. This procedure demonstrates the importance and effectiveness of the delay compensation to maintain the whole system stability. Experimental results obtained for a 25 kVA shunt active power conditioner prototype are presented to confirm the theoretical analysis.

Emerging Power Quality Challenges Due to Integration of Renewable Energy Sources

Abstract: Renewable energy becomes a key contributor to our modern society, but their integration to power grid poses significant technical challenges. Power quality is an important aspect of renewable energy integration. The major power quality concerns are: 1) Voltage and frequency fluctuations, which are caused by noncontrollable variability of renewable energy resources. The intermittent nature of renewable energy resources due to ever-changing weather conditions leads to voltage and frequency fluctuations at the interconnected power grid. 2) Harmonics, which are introduced by power electronic devices utilized in renewable energy generation. When penetration level of renewable energy is high, the influence of harmonics could be significant. In this paper, an extensive literature review is conducted on emerging power quality challenges due to renewable energy integration. This paper consists of two sections: 1) Power quality problem definition. Wind turbines and solar photovoltaic systems and their power quality issues are summarized. 2) Existing approaches to improve power quality. Various methods are reviewed, and the control-technology-based power quality improvement is the major focus of this paper. The future research directions for emerging power quality challenges for renewable energy integration are recommended.

An Islanding Microgrid Power Sharing Approach Using Enhanced Virtual Impedance Control Scheme

Abstract: In order to address the load sharing problem in islanding microgrids, this paper proposes an enhanced distributed generation (DG) unit virtual impedance control approach. The proposed method can realize accurate regulation of DG unit equivalent impedance at both fundamental and selected harmonic frequencies. In contrast to conventional virtual impedance control methods, where only a line current feed-forward term is added to the DG voltage reference, the proposed virtual impedance at fundamental and harmonic frequencies is regulated using DG line current and point of common coupling (PCC) voltage feed-forward terms, respectively. With this modification, the impacts of mismatched physical feeder impedances are compensated. Thus, better reactive and harmonic power sharing can be realized. Additionally, this paper also demonstrates that PCC harmonic voltages can be mitigated by reducing the magnitude of DG unit equivalent harmonic impedance. Finally, in order to alleviate the computing load at DG unit local controller, this paper further exploits the band-pass capability of conventionally resonant controllers. With the implementation of proposed resonant controller, accurate power sharing and PCC harmonic voltage compensation are achieved without using any fundamental and harmonic components extractions. Experimental results from a scaled single-phase microgrid prototype are provided to validate the feasibility of the proposed virtual impedance control approach.

Optimal planning of distributed generation systems in distribution system: A review

Abstract: This paper attempts to present the state of art of research work carried out on the optimal planning of distributed generation (DG) systems under different aspects. There are number of important issues to be considered while carrying out studies related to the planning and operational aspects of DG. The planning of the electric system with the presence of DG requires the definition of several factors, such as: the best technology to be used, the number and the capacity of the units, the best location, the type of network connection, etc. The impact of DG in system operating characteristics, such as electric losses, voltage profile, stability and reliability needs to be appropriately evaluated. For that reason, the use of an optimization method capable of indicating the best solution for a given distribution network can be very useful for the system planning engineer, when dealing with the increase of DG penetration that is happening nowadays. The selection of the best places for installation and the preferable size of the DG units in large distribution systems is a complex combinatorial optimization problem. This paper aims at providing a review of the relevant aspects related to DG and its impact that DG might have on the operation of distributed networks. This paper covers the review of basics of DG, DG definition, current status of DG technologies, potential advantages and disadvantages, review for optimal placement of DG systems, optimizations techniques/methodologies used in optimal planning of DG in distribution systems. An attempt has been made to judge that which methodologies/techniques are suitable for optimal placement of DG systems based on the available literature and detail comparison(s) of each one.

Analysis and Design of Resonant Current Controllers for Voltage-Source Converters by Means of Nyquist Diagrams and Sensitivity Function

Abstract: The following two types of resonant controllers are mainly employed to obtain high performance in voltage-source converters: 1) proportional + resonant (PR) and 2) vector proportional + integral (VPI). The analysis and design of PR controllers is usually performed by Bode diagrams and phase-margin criterion. However, this approach presents some limitations when resonant frequencies are higher than the crossover frequency defined by the proportional gain. This condition occurs in selective harmonic control and applications with high reference frequency with respect to the switching frequency, e.g., high-power converters with a low switching frequency. In such cases, additional 0-dB crossings (phase margins) appear; therefore, the usual methods for simple systems are no longer valid. In addition, VPI controllers always present multiple 0-dB crossings in their frequency response. In this paper, the proximity to the instability of PR and VPI controllers is evaluated and optimized through Nyquist diagrams. A systematic method is proposed to obtain the highest stability and avoidance of closed-loop anomalous peaks: it is achieved by the minimization of the inverse of the Nyquist trajectory distance to the critical point, i.e., the sensitivity function. Finally, several experimental tests, including an active power filter that operates at a low switching frequency and compensates harmonics up to the Nyquist frequency, validate the theoretical approach.

Passivity-Based Controller Design of Grid-Connected VSCs for Prevention of Electrical Resonance Instability

Abstract: The time delay in the current control loop of a grid-connected voltage-source converter (VSC) may cause destabilization of electrical resonances in the grid or in the VSC's input filter. Instability is prevented if the input admittance of the VSC can be made passive. This paper presents an analytical controller design method for obtaining passivity. The method is equally applicable to single- and three-phase systems, i.e., in the latter case, for both stationary- and synchronous-frame control. Simulations and experiments verify the theoretical results.