AC Current Regulation Research Articles

Analysis of Stationary- and Synchronous-Reference Frames for Three-Phase Three-Wire Grid-Connected Converter AC Current Regulators

The current state of the art shows that unbalance and distortion on the voltage waveforms at the terminals of a grid-connected inverter disturb its output currents. This paper compares AC linear current regulators for three-phase three-wire voltage source converters with three different reference frames, namely: (1) natural (abc), (2) orthogonal stationary (αβ), and (3) orthogonal synchronous (dq). The quantitative comparison analysis is based on mathematical models of grid disturbances using the impedance-based analysis, the computational effort assessment, as well as the steady-state and transient performance evaluation based on experimental results. The control scheme devised in the dq-frame has the highest computational effort and inferior performance under negative-sequence voltage disturbances, whereas it shows superior performance under positive-sequence voltages among the reference frames evaluated. In contrast, the stationary natural frame abc has the lowest computational effort due to its straightforward implementation, with similar results in terms of steady-state and transient behavior. The αβ-frame is an intermediate solution in terms of computational cost.

Guidelines for generalised quantitative frequency domain design of optimised multi‐resonant AC current regulators for grid‐connected inverters

This study establishes a set of frequency-domain phase margin-oriented quantitative design guidelines for deriving coefficients of optimised multi-resonant AC current regulators, consisting of series-connected terms. Such an arrangement allows straightforward extension of existing design guidelines for developing coefficients of optimised single-resonant AC current regulators. Practical restrictions imposed by, relatively, low switching frequency are also considered, yielding corresponding design guidelines generalisation.

Voltage Modulation and Current Control of Boost Inverters for Stand-Alone or Grid-Tied Operation

Boost inverters can step up dc voltage and carry out dc-to-ac conversion by means of a differential output across two boost converters. Although the differential output is beneficial to reject the common-mode noise, the inconsistency in circuit parameters between the two converters may cause an inevitable dc component in ac current. In addition, since the voltage gain of the boost converter is in nonlinear correlation with the duty ratio, the control bandwidth required for input state and output state would disparate and make the control design difficult. To overcome the nonlinearity of the boost inverter, a passivity-based voltage modulator was presented to take the place of the modulator derived from the ideal voltage gain. While the input state (inductor current) is stabilized by the proposed modulator according to the desired output voltage and ac current, the remaining problem is the control of the dc-biased sinusoidal output voltage to meet the required ac current. The integration of a dc-component compensator, a proportional-resonant controller, and a voltage drop compensator were presented for achieving ac current regulation with a satisfactory response. To further refine the power efficiency of the boost inverter, the lower limit of the dc bias of the converter output voltage was considered. Experimental results show the effectiveness of the proposed approaches.

Optimized Design of Multiresonant AC Current Regulators for Single-Phase Grid-Connected Photovoltaic Inverters

In this brief, extension of known rules for designing optimized single-resonant ac current regulators to multiresonant current regulators parameter selection for single-phase grid-connected photovoltaic inverters is proposed. The proposed generalization is made possible by utilizing multiplied rather than summed resonant terms within the controller. A set of phase-margin oriented design guidelines is established, explicitly defining each controller parameter to corresponding figure of merit. Simulations and matching experimental results are shown to verify the proposed approach.

A Signal Conditioning Antiwindup Approach for Digital Stationary Frame Current Regulators

AC current regulators are susceptible to controller windup, i.e., the condition in which the controller dynamic states ramp the controller output up to very large values when the commanded reference cannot be achieved because of inverter saturation. Recovering from saturation can take considerable time once the linear mode has been restored, which degrades the current regulator's transient performance and can also impact on its stability. This paper proposes an improved formulation using the concept of signal conditioning, where windup is avoided by feeding the controller states entirely from the constrained inverter pulsewidth modulation command. Unlike previous approaches, this technique has the benefit of a simplified realization, since the controller transfer functions do not require algebraic reformulation. This makes it particularly suitable for multiple cascaded resonators used in harmonic compensation systems. The technique has been validated using simulation and experimental investigations for a grid-connected inverter.

Generalised optimised design of single‐resonant AC current regulators

This Letter generalises frequency-domain oriented design guidelines for optimised single-resonant three-parameter AC current regulators for grid-connected converters. Unlike in previously established set of design guidelines, possible practical restrictions imposed by relatively low-switching frequency are taken into account in the proposed methodology. In addition, resonant term bandwidth is selected according to the desired robustness to fundamental frequency variations. As a result, a closed set of three non-linear equations is created. The solution maximises the loop gain crossover frequency for a given phase margin while respecting frequency robustness and resonant gain restrictions, set a priori.

Current control based power management strategy for distributed power generation system

The integration of renewable energy sources (RES) based distributed generation (DG) systems into electric grid has many challenges such as synchronization, control, power management (PM) and power quality problems. This paper proposes a references current generator (RCG) based PM strategy to control three phase inverter and manage power flow among the DG energy sources, electric grid and load demand under balanced and unbalanced grid conditions. The amplitudes of active and reactive power oscillations are also eliminated and controlled through only one flexible control parameter (FCP) under grid faults and harmonic distortions. Compared with previous similar studies, one of the important contributions is capable to inject maximum active power and minimum reactive power into electric grid and load at inverter power capacity under grid faults Another contribution is to extract the positive and negative sequence (PNS) voltage and current components with the improved fast and robust dual adaptive filters based phase locked loop (DAF-PLL). Fractional order proportional integral (FOPI) is selected as an attractive solution for AC current regulation to exhibit fast transient response and to achieve zero steady-state errors as compared with conventional current regulation controllers in synchronous or stationary frames.

A flexible control strategy with overcurrent limitation in distributed generation systems

Distributed Generation (DG) systems are typically interfaced with distribution lines by modern power converter devices, which their controllers and dynamic behaviours are significantly influenced by unbalanced grid faults. The active and reactive power control with positive-negative sequences (PNS) is one of fundamental of power converter control under grid fault conditions. This paper proposes a reference current generator (RCG) based flexible power control strategy to enable regulation of active and reactive power with minimizing active and reactive power oscillations. Current limitation control is embedded into the RCG in order to keep maximum current injection in safety limitation for overcurrent protection under grid faults and harmonic distortions. The proposed control strategy has been also accomplished maximum active power and minimum reactive power transfer capability to electric grid. The analytical expression of active and reactive power oscillations depending on flexible control parameters are comprehensively investigated as theoretically and examined with simulations. Fractional order proportional integral (FOPI) controller is preferred to minimise steady state error of AC current regulation and provide faster processing time instead of conventional PI and proportional resonant (PR) controllers. An important contribution for similar previous studies is that PNS voltage and current components are separated by dual average filter based phase locked loop (DAPLL) which is firstly proposed in this paper. The performance of proposed controller is compared with multiple complex-coefficients filter (MCCF-PLL) based controller. Theoretical analysis and simulation results verify the correctness and effectiveness of the proposed solution.

High-Performance Current Regulation for Low-Pulse-Ratio Inverters

AC current regulation for low-pulse-ratio inverters is particularly challenging because the controller bandwidth is often comparable to the target fundamental frequency and this constrains the transient response. This paper presents an improved current regulation strategy to address this issue, using state feedback concepts to deterministically compensate for pulsewidth modulation (PWM) transport delay so that the controller gains can be significantly increased. Controller design principles are presented using discrete linear-quadratic-regulator theory, and an elegant strategy is applied to make the system more robust to PWM saturation. Experimental results for both 50-Hz and 400-Hz systems confirm the theoretical concepts and demonstrate the superior transient performance of the proposed strategy.

High Performance Stationary Frame AC Current Regulation Incorporating Transport Delay Compensation

This paper presents an enhanced stationary frame PI current regulator that uses feedback of the controller/modulator transport delay to significantly improve both the controller performance and its robustness to variations in load parameters. An improved anti-windup saturation strategy is then proposed to further enhance the system performance. Matching simulation and experimental results demonstrate the controller’s superior transient and tracking performance compared to a standard PI implementation.

Investigation of LED Light Output Performance Characteristics Under Different Alternating Current Regulation Modes

LED performance characteristics under dc regulation modes have been studied extensively. But limited literatures are dedicated to studies of LED performance characteristics under different ac regulation modes. With direct ac powering LEDs for lighting application being available and more attractive in efficiency due to the elimination of ac-dc converter, investigation of the influence of different ac current regulation modes on LED performance characteristics becomes important and essential. This paper presents an experimental investigation of different current regulation modes on LED light output with different LED connection configurations. It is found certain current regulation mode under which a higher light output with the same absolute average current can be obtained. The result is due to facts that the absolute average current is the measurement of electrons and holes recombination rate inside the LED, and second-order nonlinear effect and thermal issue lead to the different light output for different ac regulation modes.

Optimized Design of Stationary Frame Three Phase AC Current Regulators

Current regulation plays an important role in modern power electronic AC conversion systems. The most direct strategy to regulate such currents is to use a simple closed loop proportional-integral (PI) regulator, which has no theoretical stability limits as the proportional and integral gains are increased, since it is only a second order system. However, pulsewidth modulation (PWM) transport and controller sampling delays limit the gain values that can be achieved in practical systems. Taking these limitations into account, this paper presents an analytical method to determine the best possible gains that can be achieved for any class of practical linear AC current controller. The analysis shows that the maximum possible proportional gain is determined by the plant series inductance, the DC bus voltage and the transport and sampling delays, while the maximum possible integral gain is determined primarily by the transport and sampling delays. The work is applicable to stationary frame PI regulators, stationary frame controllers with back electromotive force compensation, stationary frame P+ resonant (PR) controllers, and synchronous d- q frame controllers, since they all have identical proportional and integral gains that must be optimized for any particular application.

From Adaptive PBC to PI Nested Control for STATCOM

Abstract This paper presents an application of the dissipativity-based approach for the dynamic control of reactive power compensators based on PWM Voltage Sourced Inverter (VSI), such as STATCOM's (Static Synchronous Compensators). To achieve ac current and dc voltage regulation, we propose a control law obtained following the passivity-based controller (PBC) design procedure known as nested-loop PBC . Moreover, adaptive laws are introduced to consider the uncertainties in some parameters and to avoid steady-state errors. We show that the nonlinear control law thus obtained can be interpreted as a nested connection of two proportional-integral algorithms. The inner PI control loop is devoted to the control of the current amplitude, and the outer PI loop regulates the output voltage in the capacitor of the STATCOM system. Simulation results are included to demonstrate the performance of the proposed controller.