Single-phase Grid-connected Converters Research Articles

Advanced Single-Phase PLL-Based Transfer Delay Operators: A Comprehensive Review and Optimal Loop Filter Design

In recent years, several research works have addressed and developed the phase-locked loop (PLL) in single-phase grid-connected converters with different structures and properties. Each has merits and demerits, such as a complex structure, high computational burden, and slow transient response. This paper aims to comprehensively review advanced single-phase PLLs based on transport delay operators to realize signal orthogonality. A deep insight into the PLLs’ small-signal modeling, main characteristics, stability analysis, and loop filter design are provided in this paper. The main advantages and drawbacks are explained for each type of PLL in terms of different performance indexes, such as settling time, estimation error, and ripples in the estimated grid information. This paper also aims to provide optimal tuning and design of the loop filter gains from the large-signal model point of view, including all the nonlinearities, adopting the stochastic optimization method. All simulations are implemented using the MATLAB/Simulink 2018b environment to validate all theoretical analyses of this paper. The sampling and nominal frequencies are set to be 100 kHz and 50 Hz throughout all the simulation studies.

Generalized Envelope-Based Modeling of Single-Phase Grid-Connected Power Converters

Generalized Envelope-Based Modeling of Single-Phase Grid-Connected Power Converters

Simple solution of DC-offset rejection based phase-locked loop for single-phase grid-connected converters

Distributed Generators (DG) systems based on Renewable Energy Sources (RES) such as hydro, wind, and solar power plants have been spread widely due to their lower cost and the advanced capability of connecting them with the grid. The power generated from the DG must be shaped to be interfaced with the grid employing power electronics converters. The grid-connected power electronics converters must be synchronized with the grid (i.e., the same fundamental component of the grid frequency, phase, amplitude, and sequence). Synchronization techniques are employed to achieve accurate and fast grid synchronization between the converter and the grid. The existence of (DC-offset) in the input of Phase Locked Loop (PLL) caused synchronization problems as it causes oscillations in the estimated fundamental grid phase, frequency, and amplitude. In addition, the closed-loop system stability can be affected. This work proposes a simple technique for grid synchronization based on PLL with a phase angle correction. The proposed method was developed using Transfer Delay (TD) and Delay Signal Cancelation (DSC) operators; then, the small single model and stability analysis was employed. Several scenarios were developed to compare the proposed method with previous methods using MATLAB/Simulink tool. The scenarios involve introducing phase jumps, DC offsets, and amplitude changes to the grid voltage. Additionally, the grid frequency was also changed. The results show that the proposed PLL can solved the DC-offset problem using any delay time and fully synchronized with the grid. Moreover, the proposed PLL has the fastest dynamic response and shortest synchronization time over the other methods from literature.

Power Dissipation Measurements at Power Pulsation Compensators for Single-Phase Grid-Connected Converters

Power Dissipation Measurements at Power Pulsation Compensators for Single-Phase Grid-Connected Converters

Design of Dual-Notch-Filter-Based Controllers for Enhancing the Dynamic Response of Universal Single-Phase Grid-Connected Power Converters

Trade-off between transient response and grid-side current quality is a well-known issue of single-phase mains-connected power converters. A dual-loop control structure (usually based on PI or type-II controllers) is typically employed in such systems to regulate the DC link voltage to a constant reference (in order to maintain power balance) while forcing the grid-side current to have a specific shape (in order to comply with power quality requirements). Introducing notch term/s (tuned to certain multiple/s of the mains base frequency) into one of the loops allows either for the improvement of the dynamic performance without worsening the total harmonic distortion of grid-side current or for the enhancement of the current quality without impairing the dynamic response. Since the maximum tolerable value of total harmonic distortion is typically imposed by a certain power quality standard, it is desirable to enhance the transient response of the power converter system as much as possible while keeping the total harmonic distortion at the maximum allowed value. However, universal off-grid operating power conversion systems must support both 50 Hz and 60 Hz mains; consequently, tuning the notch term/s to 50 Hz multiple/s would not be sufficient for a 60 Hz mains operation and vice-versa. Consequently, this work examines the possibility of introducing a dual-notch term into the control loop in order to cover both above-mentioned base frequencies. It is demonstrated that under typical base frequency uncertainty values, the performances of dual-notch terms are nearly decoupled so that the term tuned to a 50 Hz frequency (and optionally to its multiples) has nearly no influence at a 60 Hz mains operation and vice-versa. Consequently, the methodology allows for the improvement of the dynamics of universal grid-connected power converters without total harmonic distortion (THD) deterioration. A stability analysis of the proposed control structure is carried out and quantitative design guidelines, allowing for the attainment of an optimized dynamic response for a given maximum tolerable total harmonic distortion, minimum allowed phase margin and a certain base frequency uncertainty, are established. It is shown that a DC link voltage loop bandwidth of 52 Hz may be attained while keeping the grid-side current THD below 5%. Simulations and experimental results support well the proposed design methodology.

Fast and robust DC link voltage control of single-phase grid-connected converters supporting 50 Hz and 60 Hz mains with universal magnitudes range

It is well-known that dynamics of single-phase grid-connected power converters employing PI or type-II DC link voltage controllers is limited by the maximum tolerable value of utility-side current total harmonic distortion (THD). Moreover, universal DC link voltage controller must be capable of supporting both 50 Hz and 60 Hz utilities and universal range of magnitudes (85Vrms–265Vrms). As a result, DC link crossover frequency value (and hence corresponding settling time) may vary significantly in case grid voltage magnitude variations are not compensated in the controller. In order to cope with the above-mentioned deficiencies, the paper suggests a design methodology for a novel dual-frequency-tuned utility magnitude insensitive plug-in disturbance observer (DOB), supporting universal utility frequencies and magnitudes so that the transient response of a grid-connected converter is improved without altering the existing PI or type-II DC link voltage controller coefficients. These characteristics are attained by robust modification of DC link voltage loop plant to possess notch-like properties around two frequencies of interest and treating the grid voltage magnitude value as non-measurable uncertainty translated into input disturbance and subsequently cancelled. Analytical design guidelines for obtaining PI + DOB controller coefficients are provided for desired values of utility-side current THD and DC link voltage loop phase margin (PM). Simulations and experimental results are provided to validate the feasibility of proposed design methodology.

An Improved Q-Axis Current Control to Mitigate the Low-Frequency Oscillation in a Single-Phase Grid-Connected Converter System

An electric railway system is a typical single-phase grid-connected converter system, and the low-frequency oscillation (LFO) phenomenon in electric railway systems has been widely reported around the world. Previous research has indicated that the LFO is a small-signal instability issue caused by impedance mismatching between the traction network system and electric trains. Therefore, this paper proposes an improved q-axis current control method to reshape the train’s impedance. The proposed method can be implemented easily by relating a reverse q-axis reactive current directly to the reference of the q-axis current under the dq current decoupled control. Moreover, considering the additional q-axis reactive current control, a small-signal impedance model of a train–network system is built. The impedance-based analysis results indicate that the proposed q-axis reactive current feedback control can increase the magnitude of the train’s impedance, which is beneficial to enhancing the system’s stability. Finally, this paper employs experimental results to verify the effectiveness of the proposed method.

Real-Time Sensor Fault Identification and Remediation for Single-Phase Grid-Connected Converters Using Hybrid Observers With Unknown Input Adaptation

Unexpected faults that occur in the sensors of the single-phase grid-connected converter (SGC) may deteriorate the control performance ofthe grid current and dc-link voltage and pose risks to the connected distributed generation system. This article proposes an unknown input sliding mode observer (UI-SMO) based sensor fault identification and remediation strategy. Therein, an UI-SMO is established that integrates the observations of the grid current and dc-link voltage with the reconstruction of the unknown input (i.e., load condition of the SGC). Such an observer-based analytical redundancy generates the residuals that indicate the abnormalities of the sensors for fault identification. An adaptive fault-tolerant control (AFTC) strategy for the SGC is developed, where an automated control system reconfiguration with unknown input adaptation is seamlessly implemented by substituting the data reported by the faulty sensors with their observed values. Extensive simulation and experimental studies validate the effectiveness of the proposed UI-SMO-based sensor fault identification and AFTC strategy of the SGC.

Fast Bus Voltage Control of Single-Phase Grid-Connected Converter With Unified Harmonic Mitigation

This paper presents a comprehensive analysis of the harmonic sources of the single-phase grid-connected voltage source converter (VSC), which leads to an alternative approach for DC bus voltage control of the VSC under grid voltage distortion with a significant switching dead time. A selective current harmonic controller with the zero-reference structure plays a vital role in rejecting the harmonic components in the grid reference current created by the bus voltage control loop and the harmonic components in the grid voltage and in the VSC caused by the switching dead time. Therefore, the bus voltage control can adopt a conventional proportional-integral regulator tuned at a fast bandwidth. The proposed control scheme implemented in the multiple unbalanced synchronous reference frames was validated with a bidirectional 2-kVA VSC under grid voltage distortion and a significant dead time. Furthermore, the proposed control scheme exhibited the transient response and grid current quality superior to the conventional bus voltage control methods with a notch filter and a low-pass filter. The proposed control scheme has inherent frequency adaption.

Decoupled Magnetic Integration of Symmetrical LCL Filter With a Common-Mode Inductor for Single-Phase Grid-Connected Converters

High-frequency power electronic devices in grid-connected converters excited vast switching harmonics and electromagnetic interference (EMI) noises. Conventional <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -type harmonic filters can well attenuate switching harmonics, but have a poor EMI suppression capability, especially for common-mode (CM) noise. Symmetrical filter structure and an additional EMI filter are generally needed to effectively suppress EMI in power converters, but this will inevitably increase the size and cost of the output filter. In order to tackle this issue without sacrificing the filter performance, this article proposes a decoupled magnetic integration of symmetrical <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filters with a CM inductor for single-phase grid-connected converters. The harmonic inductors of a symmetrical <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter and a CM inductor are integrated on the side-limbs and middle-limb of an EE-type magnetic core, respectively. Due to the decoupled magnetic structure and the reasonable assignment of the windings, the magnetic coupling between converter-side inductors and grid-side inductors can be greatly canceled to improve the performance of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filters on the basis of the integration of CM inductor to suppress CM EMI. The detailed design guidance of the proposed integration method is presented. A single-phase decoupled magnetic integration filter is fabricated and tested, and the experimental results verify the effectiveness and feasibility of the proposed method.

DC offset rejection in a frequency-fixed second-order generalized integrator-based phase-locked loop for single-phase grid-connected applications

Fast and accurate monitoring of the phase, amplitude, and frequency of the grid voltage is essential for single-phase grid-connected converters. The presence of DC offset in the grid voltage is detrimental to not only grid synchronization but also the closed-loop stability of the grid-connected converters. In this paper, a new synchronization method to mitigate the effect of DC offset is presented using arbitrarily delayed signal cancelation (ADSC) in a second-order generalized integrator (SOGI) phase-locked loop (PLL). A frequency-fixed SOGI-based PLL (FFSOGI-PLL) is adopted to ensure better stability and to reduce the complexity compared with other SOGI-based PLLs. A small-signal model of the proposed PLL is derived for the systematic design of proportional-integral (PI) controller gains. The effects of frequency variation and ADSC on the proposed PLL are considered, and correction methods are adopted to accurately estimate grid information. The simulation results are presented, along with comparisons to other single-phase PLLs in terms of settling time, peak frequency, and phase error to validate the proposed PLL. The dynamic performance of the proposed PLL is also experimentally validated. Overall, the proposed PLL has the fastest transient response and better dynamic performance than the other PLLs for almost all performance indices, offering an improved solution for precise grid synchronization in single-phase applications.

Design of a Digital Voltage Controller in Single-Phase Grid-Connected Renewable Converters

Design of a Digital Voltage Controller in Single-Phase Grid-Connected Renewable Converters

Fast Grid Voltage Synchronization Using Modified FLL in Single-Phase Grid-Connected Power Systems

One of the critical components for the efficient operation of single-phase grid-connected converters is the synchronization unit. This paper presents a fast and adaptive phase-locked loop (PLL) structure that ameliorates the dynamic response of the estimated frequency and amplitude for grid-connected single-phase power systems. The second-order generalized integrator (SOGI) with a novel frequency-locked loop (FLL) is utilized which contains a DC offset rejection loop. The proposed method not only eliminates the transient response of the estimated frequency which is produced by FLL in grid voltage phase angle jumps, but also improves the PLL dynamic characteristics. The whole system has been simulated in MATLAB Simulink environment where a very small settling time for the estimated frequency of the FLL has been achieved. Therefore, it will improve the whole dynamic parameters of the system. Based on the simulation results, the settling time for the estimated frequency and amplitude are 22 ms and 10 ms, respectively.

Phase-locked loop with DC[sbnd]offset removal for single-phase grid-connected converters

DC-offset in the input of the phase-locked loop (PLL) is an emerging problem that causes oscillations in the estimated fundamental grid phase, frequency, and voltage amplitude. The DC-offset rejection in grid synchronization is a difficult task due to its low-frequency nature. This paper proposes a method to remove the DC-offset in the single-phase grid synchronization utilizing delay signal cancellation (DSC) and a variable-length time delay (VLTD) based PLL. The small-signal model, stability analysis, and controller gains selection are discussed. The proposed PLL is compared with other single-phase PLLs in terms of the phase settling time, the phase percent maximum overshoot, and the peak of the estimated frequency, to show its advantages.

Development of Frequency Fixed Sliding Discrete Fourier Transform Filter Based Single-Phase Phase-Locked Loop

The single-phase phase-locked loop (PLL) is essential for the stable operation and control of single-phase grid-connected converters. However, in practical applications, the grid voltage is usually affected by harmonics and dc offset, which will cause errors in the output of the PLL. Therefore, using sliding discrete Fourier transform filter (SDFT) as a prefilter, this paper proposes an improved single-phase synchronous reference frame PLL with a fixed sampling frequency, which can accurately and quickly obtain the grid parameters under distorted grid conditions. Most importantly, the paper theoretically analyzes phase and amplitude errors generated by SDFT for the first time and a quantitative compensation method is proposed, which is straightforward to be used by a reader. Finally, the proposed PLL is compared with other PLLs through simulation and experiments. The experimental results show the effectiveness and practicability of the proposed method.

Modeling and Control of a Single-Phase Grid-Connected Inverter with LCL Filter

The increasing penetration of renewable energy sources is pushing low-voltage electrical grids to become predominantly power electronic-based. Consequently, the design and operation of the related grid-connected converters must be achieved under proper manner, in order to maintain stability and support reliable operation of the entire power system. Thus, this work presents the modeling and control of a single-phase grid-connected multifunctional converter, which operates as a current-controlled voltage source inverter using an LCL-type output filter. An active damping approach is employed for attenuation of oscillations occurring from interactions between the grid and the inductances and capacitance of the LCL filter. Additionally, the voltage and current control loops of the inverter are addressed in this paper, along with the design of their respective controllers, considering that the grid impedance is unknown. Experimental results demonstrate stable operation of the inverter and show its satisfactory behavior under steady state conditions, proving its capability to offer ancillary services such as active filtering.

Harmonic-Domain SISO Equivalent Impedance Modeling and Stability Analysis of a Single-Phase Grid-Connected VSC

This article presents a harmonic-domain single-input single-output (SISO) equivalent modeling technique for the impedance modeling and stability analysis of a single-phase grid-connected voltage-source converter (VSC). The basis is a conversion technique that transforms a harmonic transfer function (HTF)-based model into a SISO equivalent model while preserving all the information of frequency couplings. The proposed SISO modeling concept is useful for understanding the meaning and consequence of SISO impedance measurement of an interconnected system with frequency couplings, which further enables a simpler impedance measurement and impedance-based analysis. Applications of this method for the VSC model reduction and stability characteristic analyses are presented. From these results, useful conclusions regarding the accuracy of three types of reduced-order VSC impedance models and the stability effects of the VSC control with and without compensation for dc voltage variation are obtained. The presented examples of applications demonstrate how the proposed SISO modeling technique facilitates a simpler and efficient impedance-based analysis. Finally, experimental results verify the validity of the proposed VSC-SISO admittance and corresponding analyses.

Generalized Design Approach of a Family of Grid-Connected Converters Based on Active Virtual Ground Technique for Single-Phase AC Microgrid Applications

The paper presents the state-of-the-art technique Active Virtual Ground (AVG) in the design of single-phase grid-connected converters which fully covers the entire applications on AC microgrids. Based on the three main components of the microgrid, the presented topologies belonging to the AVG series are divided into three different groups, which are associated with power consumers, renewable energy, and energy storage systems. The whole series of the single-phase converters are also based on the latest AVG technique. Therefore, in each application, a corresponding single-stage AVG converter can be found, which has high efficiency, low leakage current, and continuous grid current. The topology advantages and the working principle of the AVG converter family are demonstrated through the circuit evaluation and are experimentally verified in a set of 650 to 800 W prototypes which shows a good agreement on between both of the experimental results and theoretical finding.

Two-Sample PLL With Harmonic Filtering Capability Applicable to Single-Phase Grid-Connected Converters

The two-sample phase locked loop (2S PLL) in single-phase digitally controlled grid-connected power converters provide synchronization with a minimal computational burden. However, the distortion of the grid voltage deteriorates the performance of the 2S quadrature signal generator. To solve this issue, this article introduces a harmonic filtering (HF) structure based on observers of the input voltage for the fundamental and selected harmonics. The stability and sensitivity of the 2S PLL with HF is analyzed. In comparison with second-order generalized integrator (SOGI)-based HF, the observers provide a narrower bandpass, and the subsequent deterioration of the response time is compensated by adapting the filter gains dynamically. The results obtained, both in simulation and experimentally, validate the proposal and compare its performance with other widely adopted PLLs providing harmonic rejection capability. The computational burden is analyzed and in the case of the proposals depends on the number of observers and the use or not of the adaptive strategy based on steepest descent.

A Current Decoupling Control Scheme for LCL-Type Single-Phase Grid-Connected Converter

For better performance, different current control schemes are used in LCL-type grid-connected converters. However, these schemes are mostly used in three-phase grid-connected converters and multiple current sensors are needed. To address these issues, a current decoupling control scheme for single-phase grid-connected converters in rotating reference frame is proposed in the paper. Because an extended state observer is adopted in the control scheme, only one current sensor is needed and the complicated coupling relationships between the $d$ axis components and $q$ axis components can be eliminated completely. Moreover, to improve the dynamic and steady-state performance of converters, a state feedback closed-loop system structure and nonlinear controllers are adopted. The current decoupling control scheme is validated in an LCL-type single-phase grid-connected converter. The experimental results show that a fast-dynamic response under load change and voltage sag can be achieved besides preferable static performances with the proposed control scheme.