Single-phase Grid-connected Converters Research Articles

A Simple Approach to Reject DC Offset for Single-Phase Synchronous Reference Frame PLL in Grid-Tied Converters

Synchronous reference frame phase-locked loop (SRF-PLL) is widely used for grid voltage synchronization in single-phase grid-connected power converters. However, in the actual situation, dc offset component may be introduced in the input of the PLL, due to the transient fault of the power grid, sampling and measurement error, or A/D signal processing. A simple yet effective approach with additional all-pass filter based dc rejecter is presented for SRF-PLL, in this paper. Thereby, correct estimation and undesirable periodic ripple free can be achieved in SRF-PLL, when the input signal contains dc offset. The second order generalized integrator based PLL (SOGI-PLL) is first introduced, followed by the analysis on influence of the input dc offsets in SRF-PLL. The structure of enhanced-SOGI (ESOGI) with its analysis of dc offset rejection effects and performance have been then formulated in detail. Finally, experimental results are presented to demonstrate the effectiveness of the proposed ESOGI based PLL.

All-Pass-Filter-Based PLL for Single-Phase Grid-Connected Converters Under Distorted Grid Conditions

All-pass-filter-based single-phase phase-locked loop (APF-PLL) is a widely used grid synchronization method in grid-connected power converters. However, the performance of APF-PLL degrades under distorted grid conditions and the estimated phase and frequency will contain low-frequency oscillation terms, which is not conducive to practical application. To deal with this problem, this paper proposes two advanced APF-PLLs, which have good ability to suppress harmonics and dc offset interference without sacrificing the dynamic response of the system. In the first one, a band-pass filter (BPF) is used as the prefilter of APF-PLL to process the input voltage signal, and in the second one, the APF-PLL input signal is processed by using a harmonic decoupling network (HDN) and multiple prefilters. In addition, their parameter design procedure is also described in detail and is easy to follow. The experimental results presented in this paper finally verify the superiority and effectiveness of the two proposed methods.

Robust Control Strategy Design for Single-Phase Grid-Connected Converters Under System Perturbations

In single-phase grid-connected converters, unexpected perturbations and variations of system parameters can deteriorate the performance of grid current and dc-link voltage control and even jeopardize the stability and operation of loads. In this paper, a robust control strategy for the single-phase grid-connected converter operating under multiple system perturbations is proposed. An adaptive quasi-proportional-resonant (AQ-PR) controller for the grid current is adopted in combination with an equivalent inductance identification algorithm, which assures that the parameters of the AQ-PR controller can be online calibrated to attain accurate current regulation under ac-side inductance uncertainties. A super-twisting sliding mode (ST-SM) controller for the dc-link voltage is proposed to enhance system behavior under both internal and external disturbances. The convergence of the controller is ensured by utilizing Lyapunov theory. Experimental prototype and rigorous tests are presented to validate the feasibility and robustness of the proposed approach under various perturbations.

Eigenvalue-Based Harmonic Instability Analysis of Electrical Railway Vehicle-Network System

Dynamical movement of electrical rail vehicle will change the topology of the vehicle-network system. Small-signal state-space modeling of the vehicle-network system is proposed in this paper to study the harmonic instability of electrical railway. This state-space model takes into account the complete structure and multiconductor nature of the network, and the single-phase grid-connected converters of the vehicle. With the modular models of system and by flexibly combining the submodules, the vehicle-network state-space model is constructed, and the change of the vehicle positions can be realized. The eigenvalue-based method is used to analyze the harmonic instability. Influences of converter controller parameters, sampling frequency, and particularly, the vehicle positions on the high-frequency harmonic modes are analyzed. Participation factors of state variables are calculated and compared. Besides, catenary node voltages under high-frequency harmonic modes are analyzed according to the mode shape to find the most affected nodes. Finally, the theoretical analysis results are verified by nonlinear time-domain simulations and measured waveforms.

An Adaptive PI Controller Design for DC-Link Voltage Control of Single-Phase Grid-Connected Converters

Conventionally, standard proportional and integral (PI) controllers with constant PI gains are commonly used for the dc-link voltage control of single-phase grid-connected converters (GCCs). For such controllers, the selection of the PI gains will lead to a tradeoff between two control objectives: 1) the reduction of the dc-link voltage fluctuations caused by random swings of the active power drawn by the single-phase GCC; and 2) the reduction of the grid current harmonics mainly caused by the 2f oscillation of the active power in single-phase applications. To solve this tradeoff, this paper presents a systematic approach for the design of an adaptive PI controller for the dc-link voltage control of single-phase GCCs. The proposed design approach is simple and it provides a convenient method to properly determine the adaptive PI controller parameters. Representative simulation and experimental results are presented and discussed in order to show the effectiveness of the proposed dc-link voltage controller.

A Four-Quadrant Single-Phase Grid-Connected Converter With Only Two High-Frequency Switches

This paper presents a new four-quadrant (4Q) boost-type converter with the use of active virtual ground (AVG) technology. The presented topology can step up the ac grid voltage to a regulated dc voltage under a stable bidirectional current flow and support the power transmission in either real power or reactive power delivery. With the use of the proposed modulation method, only two high-frequency switches are required through the 4Q operation. Under a full-bridge converter structure, an efficient system can be guaranteed. Also, benefiting from the AVG technology, an LCL filter is formed at the system input over the 4Q operation. Both magnitude of leakage current and grid current ripple are also minimized to a small value. Thus, a high-efficiency and low-noise 4Q converter is guaranteed. The presented topology is successfully implemented on a 750 VA prototype and the performance is experimentally verified on it, which shows good agreement with the theoretical knowledge.

Enhanced orthogonal signal generator for a single‐phase grid‐connected converter

This study presents a new orthogonal signal generation (OSG) technique for the control system of a single-phase grid-connected voltage-source converter (VSC). The existing methods mostly suffer from delay, rely on the system parameters, or require multiple inputs. Among them, the second-order generalised integrator (SOGI) excludes the drawbacks of the other methods and shows better steady-state and dynamic response. The proposed OSG method here includes the advantages of the SOGI technique while demonstrating superior dynamic response and higher disturbance rejection capability. It is structurally simple without adding complexity to the control system. The proposed method is implemented in a DQ -frame current controller and its feasibility and reliability are verified through mathematical analysis, simulation, and experimental results.

Voltage Harmonic Suppression by Means of Grid-Connected Converters Using only Local Measurements

A single-phase grid-connected converter is considered in this paper in the presence of harmonic problems introduced non-linear loads. In order to compensate the harmonics caused by the loads, a local voltage support scheme is proposed. This is an added feature because its implementation is in parallel with a conventional current control method. Distinctively, the measurements of the grid or load current are not needed since the scheme is based on only local measurements. On top of a fundamental part for desired power injection, the converter output current comprises a harmonic part for compensation. Thus, the grid current harmonic distortion is minimized and the enhancement of the local voltage quality is achieved. A comprehensive model analysis indicates that the proposed strategy can help to attenuate harmonics of the local voltage without compromising on the quality of the fundamental current injection. Experimental results validate the effectiveness of the proposed control scheme. Moreover, the impact of grid frequency estimation error on the control strategy’s performance is quantified theoretically and experimentally.

Dual-Comparison One-Cycle Control for Single-Phase Bidirectional Power Converters

Dual-comparison one-cycle control for the bidirectional power flow in single-phase grid-connected converters is proposed in this paper. Advantages of dual-comparison one-cycle control when compared to conventional one-cycle control is that it does not give any steady-state dc offset, no light-load instability, and implements unipolar pulsewidth modulation without having to sense the grid voltage. Addition of a fictitious current term, which is generated from the gating signal of one of the active devices, using a bandpass filter, enables the bidirectional power flow. The quality factor of the bandpass filter is shown to impact the total harmonic distortion in current, fundamental displacement angle, and steady-state dc offset. The optimal value of a quality factor, obtained with aid of simulations, gives a superior performance. The current drawn or injected into the grid is seen to lag the grid voltage. The reason for a nonunity displacement power factor is analyzed and a compensation to achieve a unity displacement power factor is also proposed in this paper. Detailed simulation and experimental studies are carried out to validate the proposed control and the high performance that can be obtained.

Analysis and Mitigation of Dead-Time Harmonics in the Single-Phase Full-Bridge PWM Converter With Repetitive Controllers

In order to prevent the power switching devices (e.g., an insulated-gate bipolar transistor, IGBT) from shoot-through in voltage-source converters during a switching period, the dead time is added either in the hardware driver circuits of the IGBTs or implemented in software in pulse width modulation (PWM) schemes. Both solutions will contribute to a degradation of the injected current quality. As a consequence, the harmonics induced by the dead time (referred to as “dead-time harmonics” hereafter) have to be compensated in order to achieve a satisfactory current quality, as required by standards. In this paper, the emission mechanism of dead-time harmonics in single-phase PWM inverters is, thus, presented considering the modulation schemes in detail. More importantly, a repetitive controller has been adopted to eliminate the dead-time effect in single-phase grid-connected PWM converters. The repetitive controller has been plugged into a proportional-resonant-based fundamental-frequency current controller so as to mitigate the dead-time harmonics and also to maintain the control of the fundamental-frequency grid current in terms of dynamics. Simulations and experiments are provided, which confirm that the repetitive controller can effectively compensate the dead-time harmonics and other low-order distortions, and also, it is a simple method without hardware modifications.

Undersampling Control for a DC–DC Boost and Forward Dual Active Bridge for a Single-Phase Grid-Connected PV Converter

Digital control methods and digital controllers are widely adopted in power electronics applications due to technical and economic advantages against its analog counterparts. Switched-mode power converters with switching frequencies of hundreds of kilohertz to megahertz impose a challenge such as less sampling time in which control algorithm calculations must be realized in digital implementations. Software-based controllers are designed and implemented in code lines executed in general-purpose DSP, being the most preferred digital implementation. Although high-frequency applications with more complex algorithm calculations restrict the maximum allowable sampling frequency, that can be even below the converter switching frequency. The study, analysis and design in such special condition is a main contribution of the paper. Without loss of generality, a software-based digital design under processing speed limitation is developed for two DC–DC power converters that compose a single-phase grid-connected PV converter: (1) a boost converter and (2) a unidirectional power flow DAB topology.

Noninvasive Technique for DC-Link Capacitance Estimation in Single-Phase Inverters

With increased penetration of renewable sources, single-phase grid-connected converters are finding increased applications. Condition monitoring of dc-link capacitor is required for preventive maintenance, which improves the reliability of the overall system. Significant literature on condition monitoring techniques for capacitor is applicable to three-phase converter or three-stage ac/dc/ac converters. Recent publications on health monitoring in single-phase converters require either configuring/operating converter in a specific mode or quantify aging of aluminum electrolytic capacitor (AEC) in variables other than equivalent series resistance and capacitance of AEC. This letter proposes an online noninvasive technique to estimate the capacitance of dc link, which is a better indicator for health assessment of the AEC. Estimation of capacitance is done using the available measurement for the converter control and is implemented in the same digital controller used for control of converter. Simulation and experimental verification are carried out on a prototype of single-phase grid-connected system.

Grid Voltage Estimation and Current Control of a Single-Phase Grid-Connected Converter Without Grid Voltage Sensor

This paper investigates the possibility of estimating the grid voltage and controlling a single-phase grid-connected converter without a dedicated grid voltage sensor. The main focus is given to current-controlled architectures which need phase-locked loops (PLL) for their operation. The grid voltage estimation method is proposed based on the active and reactive power delivered by the converter to the grid. In addition to the estimation of the grid voltage, the proposed method also generates the unit vectors and frequency information similar to a conventional PLL structure. The influence of various parameters like the dead time, equivalent loss resistance, and the filter inductance on the proposed estimation method is studied in details. Experimental results validate the feasibility of the proposed scheme.

Single-Phase PLLs: A Review of Recent Advances

Single-phase phase-locked loops (PLLs) are popular for the synchronization and control of single-phase grid-connected converters. They are also widely used for monitoring and diagnostic purposes in the power and energy areas. In recent years, a large number of single-phase PLLs with different structures and properties have been proposed in the literature. The main aim of this paper is to provide a review of these PLLs. To this end, the single-phase PLLs are first classified into two major categories: 1) power-based PLLs and 2) quadrature signal generation-based PLLs. The members of each category are then described and their pros and cons are discussed. This paper provides a deep insight into characteristics of different single-phase PLLs, and therefore, can be considered as a reference for researchers and engineers.

Performance Improvement for Two-Stage Single-Phase Grid-Connected Converters Using a Fast DC Bus Control Scheme and a Novel Synchronous Frame Current Controller

Two-stage single-phase grid-connected converters are widely used in renewable energy applications. Due to the presence of a second harmonic ripple across the DC bus voltage, it is very challenging to design the DC bus voltage control scheme in single-phase grid-connected inverters. The DC bus voltage controller must filter the ripple and balance a tradeoff between low harmonic distortion and high bandwidth. This paper presents a fast DC bus voltage controller, which uses a second order digital finite impulse response (FIR) notch filter in conjunction with input power feedforward scheme to ensure the steady-state and dynamic performance. To gain the input power without extra hardware, a Kalman filter is incorporated to estimate the DC bus input current. At the same time, a modulation compensation strategy is implemented to eliminate the nonlinearity of the grid current control loop, which is caused by the DC bus voltage ripple. Moreover, a novel synchronous frame current controller for single-phase systems is also introduced, and its equivalent model in stationary frame has been derived. Simulation and experimental results are provided to verify the effective of the proposed control scheme.

A Frequency-Fixed SOGI-Based PLL for Single-Phase Grid-Connected Converters

Second-order generalized integrator (SOGI) based phase-locked loops (PLLs) are widely used for grid synchronization in single-phase grid-connected power converters. Previously, the estimated frequency of the PLL stage is fed back to the front-end SOGI block to make SOGI-PLLs frequency-adaptive, which increases the implementation complexity, and makes the tuning sensitive, thus reducing stability margins. Alternatively, a frequency-fixed SOGI-based PLL (briefly called FFSOGI-PLL) is proposed to ensure stability and simple implementation in this letter. It is commonly known that the in-phase and quadrature-phase signals generated by the frequency-fixed SOGI are of different amplitudes in the presence of frequency drifts, which causes second-harmonic ripples in the estimated parameters of the PLL loop. To deal with this issue, a simple yet effective method is developed in FFSOGI-PLL. The standard SOGI-PLL is first introduced, followed by the working principle and small-signal model of FFSOGI-PLL. The FFSOGI-PLL is then compared with the SOGI-PLL in terms of stability and transient performance. Finally, experimental results are presented to demonstrate the effectiveness of FFSOGI-PLL.

A Magnetic Integrated LLCL Filter for Grid-Connected Voltage-Source Converters

High-order passive filters, such as the well-known LCL filters, are normally employed in the grid-connected power conversion systems to effectively attenuate the switching frequency harmonic introduced by the modulation of power converters. Although much more compact than the conventional single-inductor L filters, such passive filters are still bulky and expensive when compared with their active counterparts, e.g., the semiconductor switches. In order to improve the system power density and reduce its cost, the magnetic integration technique has been widely adopted so that the discrete inductors of passive filters are replaced by the integrated inductor, resulting in smaller magnetic cores and, therefore, the decreased volumes of passive filters. For conventional magnetic integrated LCL filters, the converter-side inductor and the grid-side inductor are integrated together and their coupling coefficient is intentionally minimized. In this letter, this coupling coefficient is fully utilized and properly designed, and the resulting coupling effect is equivalent to inserting an additional inductor into the filter capacitor branch loop. The integrated inductor and the filter capacitor can form an integrated LLCL filter, which exhibits the advantages of both the LLCL filter and magnetic integration, e.g., enhanced harmonic attenuation, reduced filter inductances, and system volume without adding the extra trap inductor. Finally, experimental results obtained from a single-phase grid-connected voltage-source converter interfaced by the proposed integrated filter are presented to validate its effectiveness.

Novel Integrated CM Inductor for Single-Phase Power Converters With Reduced EMI

Ground leakage current and electromagnetic interference noise are important challenges during design of grid-connected power converters. Use of common-mode (CM) choke helps to mitigate these issues. A common practice is to use a separate CM filter and a differential mode LCL filter. In this paper, an integrated CM inductor (ICMI) is proposed in which the LCL filter boost inductor and the CM choke are combined into a single component. This results in a reduction in the number of magnetic components required to build the power converter. Steps involving design of the ICMI are provided. The proposed ICMI is fabricated in the laboratory. Experimental test are carried out on a 5-kW single-phase grid-connected power converter to validate the effectiveness of the proposed ICMI. The results show that the use of proposed ICMI along with an added ground capacitor in the power circuit as a CM filter can reduce the electromagnetic interference noise level up to 30 dB.

Cascade-Free Model Predictive Control for Single-Phase Grid-Connected Power Converters

In a conventional finite control set model predictive control (FCS-MPC) formulation, active and reactive power control loops rely on the predictive controller, whereas the dc-bus voltage is usually governed by a PI-based control loop. This originates from fact that the dynamic equations for describing the predictions of these variables are heavily coupled. In this paper, a cascade-free FCS-MPC for single-phase grid-connected power converters is presented. The proposed control algorithm is formulated in terms of established dynamic references design, which was originally proposed to directly govern active and reactive power, and dc-voltage in three-phase power converters. In this paper, the dynamic reference design concept is extended to control single-phase grid-connected power converters. The proposed control algorithm does not use instantaneous ac-power calculations; instead, it directly formulates the optimal control problem on the grid-current in the original stationary reference frame. The experimental results obtained with a single-phase grid-connected neutral point clamped (NPC) converter confirm a successful design, where system constraints, e.g., maximum power and weighted switching frequency, are easily taken into account.

DC Offset Rejection Improvement in Single-Phase SOGI-PLL Algorithms: Methods Review and Experimental Evaluation

DC offset in the input of phase-locked loops (PLLs) is a challenging problem since it will result in fundamental frequency oscillations in the estimated phase and frequency. In this paper, a comprehensive analysis and performance evaluation of several advanced second-order generalized integrator (SOGI)-based PLL methods in enhancing the dc offset rejection capability for single-phase grid-connected power converters is presented. These methods include the cascade SOGI, modified SOGI, $\alpha \beta $ -frame delayed signal cancellation (DSC), complex coefficient filter, in-loop dq-frame DSC, notch filter, and moving average filter-based SOGI-PLL. Main characteristics and design aspects of these methods are presented. Main performance indexes, such as the setting time, frequency or phase errors are defined and these methods are systematically compared under various scenarios with both numerical and experimental results.