Conventional Synchronous Reference Frame Research Articles

Analysis of optimized phase locked loop for grid synchronization of solar PV system under grid disturbances

AbstractIn this work, an optimal phase locked loop (PLL) is employed to estimate the fundamental grid frequency and phase angle under grid disturbances, which largely reduces the frequency and phase error. Conventional synchronous reference frame (SRF) offers considerable results for balanced grid conditions but has poor performance under grid disturbances with improper phase detection. In this regard, gravitational search algorithm (GSA)‐based optimization is utilized to optimally tune the proportional and integral (PI) gain values of conventional SRF PLL, which ensures the proper tracking of fundamental phase angle and frequency even during abnormal grid conditions, that is, voltage unbalance, frequency variations, high‐frequency harmonics, and phase variations. Further, integral square error (ISE) criteria, settling time, and overshoot are considered as objective functions to obtain the optimal value of current gain, present in the control loop of a grid‐tied solar photovoltaic (PV) system. The supremacy of the proposed work, compared to SRF and optimal SRF PLL, is tested with MATLAB®/Simulink studies and verified by dSPACE‐1104‐based hardware‐in‐loop (HIL) results for real‐time validation.

A Robust Forward Compensated Type-2 PLL Without DC Phase Error During Frequency Ramp for Embedded Magnetic Encoder

To achieve high performance vector control of permanent magnet synchronous machine (PMSM), high precision rotor angle is essential. Compared to traditionally used angle feedback device, e.g., encoder and resolver, the embedded magnetic encoder (EME) achieves high precision angle estimation without extra space occupation by installing several linear Hall sensors into PMSM cavity to monitor the flux leakage and rotor angle. The conventional synchronous reference frame (SRF) phase-locked loop (PLL) can extract phase and frequency from three or two Hall signals simultaneously. Although the type-2 SRF-PLL shows good robust performance against parameter variation, phase error emerges when input frequency changes. More specifically, a DC phase error appears in type-2 SRF-PLL during speed ramp stage due to insufficient order. Increase of the loop order removes the DC phase error but leads to instability risk. In this paper, a novel forward compensation (FC) module is attached on type-2 SRF-PLL to achieve phase error-free when frequency changes, where the instability risk is avoided. The effectiveness of proposed FC module is analytically and experimentally proved, where the phase error elimination, noise immunity and dynamic response are evaluated.

Performance Improvement of Weak Grid-Connected Wind Energy System Using FLSRF-Controlled DSTATCOM

Wind energy (WE) is emerging as a green and clean energy source. The power quality (PQ) significantly deteriorates when WE sources are connected to the weak ac grid. This is due to the variable wind speed, the impedance of weak grids, and nonlinear loads. This article proposes a methodology to improve the PQ in the weak grid with WE penetration using additional distribution static compensator (DSTATCOM). Fuzzy logic (FL) controller is implemented in the conventional synchronous reference frame (SRF) scheme to minimize the oscillations in the voltage signals. Also, a feedforward block of the WE source is incorporated in the FLSRF control scheme for adapting changes associated with the wind speed. The proposed control scheme accommodates the changes related to the grid’s strength, wind speed, load currents, and dc-link voltage dynamics to estimate the switching signals for the DSTATCOM. The MATLAB simulation and experimental studies established the effectiveness of the proposed method for enhancing the PQ performance of weak grid-connected WE sources in the presence of nonlinear loads.

Implementation and performance comparison of harmonic mitigation schemes for three-phase grid-connected voltage-source converter under grid voltage distortion: HiL and experimental validation

This paper presents an implementation methodology for a current harmonic compensation technique of three-phase grid-connected voltage-source converter (VSC), which was applied under the grid voltage distortion and frequency variation. The proportional–integral plus multi-resonant controllers (PIMR) were used to mitigate the high-order harmonic current. The harmonic compensators were used in the synchronous reference frame at harmonic orders 6th and 12th for suppression of the grid voltage harmonic orders 5th, 7th, 11th, and 13th with a total harmonic distortion of 4.69%. The PIMR controller was transformed to the discrete-time domain and implemented on a TMS320F28379D digital signal processor. The PIMR controller was compared with the conventional multiple synchronous reference frame (PIMSR) controller. The two control schemes were validated with a hardware-in-the-loop (HiL) and with a 5 kW three-phase LCL-filtered grid-connected VSC. The PIMR and PIMSR controllers exhibited excellent harmonic rejection on the grid current with a total harmonic distortion close to 1%, which complied with the IEEE 1547 standard. The grid current can track the reference command with a fast dynamic response. Moreover, the PIMR controller consumed a computational time of 1.625μs, 35% of the PIMSR controller.

Control of distribution static compensator with active damped inductor‐capacitor‐inductor filter using dual loop αβ‐frame current controllers for power quality improvement

AbstractA distribution system is connected with many non‐linear loads, and generated harmonies are mitigated by using distribution static compensator (DSTATCOM) which is coupled at the point of common coupling through a voltage source inverter (VSI) and filter circuit. High‐frequency switching ripples are generated along with the output of the VSI during pulse width modulation switching. Inductor‐capacitor‐inductor (LCL) filter is used to suppress the high frequency switching noises. Still, the primary concern about the LCL filter is generation of resonance peak at the resonance frequency. The conventional synchronous reference frame current controller with LCL filter has a complex coupling between and axis; hence decoupling is very difficult. This research article presents a new technique based on active damped dual loop ‐frame current controllers to control the DSTATCOM with LCL filter for achieving improved load compensation. The dual loop controller is enhanced by using capacitor current in the inside loop and grid current at the outside loop with proportional resonant (PR) regulator parallel with harmonic compensator (HC). The proposed method effectively dampens the resonance peak under stationary reference frame The PR controller offers unlimited gain at the fundamental frequency and HC can offer more gain at the specific harmonic frequencies. Hence, characteristics of the PR and HC controllers used together to achieve minimum steady‐state error and the problematic decoupling are ignored. The complete DSTATCOM model with proposed control scheme is discussed with PR and (PR + HC) controllers separately under balanced, unbalanced source voltage. Stability analysis is carried out through Bode and pole‐zero plots to judge the performance of the proposed current controllers. The obtained results, including total harmonic distortion of the source current, are compared with conventional passive damped ‐frame current controller.

Least Mean Fourth Theory-based Dynamic Voltage Restorer for the Mitigation of Voltage Sag and Harmonics

In the present paper, Dynamic Voltage Restorer (DVR) controller using the Least Mean Fourth (LMF) algorithm is presented. The proposed DVR is used for protecting various voltage-sensitive loads from voltage-related power quality issues (i.e. balanced voltage sag, unbalanced voltage sag and voltage harmonic distortion, balanced and unbalanced swell, etc.) in a distribution system. The active and reactive power components of distorted source voltage are extracted using an adaptive LMF-based control algorithm for the calculation of reference load voltage for the voltage source converter (VSC) of DVR. The control algorithm doesn’t require a phase-locked loop (PLL) and thus is less complex, robust and easy to implement. The adopted control algorithm for DVR is verified in MATLAB by simulation studies. The adaptive LMF-controlled DVR produces excellent dynamics response as compared to conventional adaptive Least Mean Square (LMS) and Synchronous reference frame theory (SRFT)-controlled DVR. The convergence rate of the source voltage fundamental active weight component is higher for the proposed adaptive LMF-controlled DVR. Various power quality issues such as balanced sag, unbalanced sag, balanced and unbalanced swell and a combination of these power quality issues are mitigated with the help of the LMF-controlled DVR. The simulation results are verified through real-time experimentation using an OPAL-RT under an RT lab environment.

Phase locked-loop with decaying DC transient removal for three-phase grids

Frequency and phase of the power grid, which are critical for reliable control and protection of grid-tied devices, are generally detected by the closed-loop phase locked-loop (PLL). In highly inductive high-voltage transmission systems, decaying DC (DDC) components with large amplitude can be easily introduced by load disturbances and/or grid abnormalities, leading to severe performance degradation of the PLL during the transient. Focusing on this issue, in this paper, modifications to the conventional synchronous reference frame (SRF)-PLL have been made to address the short-term disturbances including the DDC component, and the system operation is divided into the normal state and the DDC-transient state. The SRF-PLL is only adopted for the normal state where the DDC component is negligible. In the presence of a significant DDC component, as well as disturbances including negative-/zero-sequence components and harmonics, the weak effectiveness of the conventional SRF-PLL is proved, and an efficient DDC component extraction method, with a detection time of 0.5 grid cycle, is introduced for the three-phase system. The real-time amplitude and phase of the positive-sequence component can be efficiently extracted via the proposed scheme, by exploiting the transient signal properties in the dq-frame and assuming a constant grid frequency during the short transient. Finally, a proper design of switching logic has been proposed to allow for the fast and precise transition between the normal and the DDC-transient state, thereby ensuring high steady-state accuracy as well as short-term DDC transient immunity. Hardware-in-the-loop based experiments have been used to verify the effectiveness of the proposed PLL technique.

Comparative Study of Symmetrical Controlled Grid-Connected Inverters

The asymmetrical dynamics of the conventional synchronous reference frame phase-locked loop (PLL) enhance the complexity of the system and endanger the stability of the grid-connected voltage source converters. In this article, the admittance modeling of three different symmetrical control strategies, i.e., voltage oriented control with symmetrical PLL, voltage modulated direct power control, and proportional–resonant control, is proposed and uniformed based on the same power tracking property. The voltage feedforward and decoupling of the controllers are both considered. Moreover, due to the symmetrical structure of the three control strategies, their admittance models can directly be modeled under <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha \beta$</tex-math></inline-formula> reference frame as single-input single-output systems. In addition, to reveal the similarity between different control methods and emphasize their differences, the admittance models are constructed by using the same framework. The admittance modeling is verified using frequency-scan, and the control performance is tested using MATLAB/Simulink Simscape Power System and experimental prototype. Finally, the stability criterion based on the proposed admittance modeling is verified by simulation results carried out in real-time simulation platform RTLAB from Opal-RT.

Improved dynamic performance of permanent magnet synchronous generator based grid connected wind energy system

ABSTRACT This comprehensive work investigates the reactive power control capability of a dynamic voltage restorer (DVR) for a permanent magnet synchronous generator (PMSG)-based grid-connected wind energy system (WES). Series compensation scheme-based DVR is employed to mitigate three-phase faults by injecting regulated voltage at the point of common coupling (PCC) bus through a series injection transformer. An improved hysteresis controller, a multi-loop-based controller consists of a hybrid system of the conventional PI controller, and feed-forward with feedback loop-based voltage-controlled DVR is the novelty of this article. DVR-modified hysteresis control highlights how grid-connected WES might increase their stability and dynamic performance. The suggested approach operates quickly, smoothly, and consistently, and it is designed and implemented in Matlab-Simulink software, and the performance of the proposed controller is validated with a comparative analysis between conventional synchronous reference frame controller DVRs. As a result, the total performance can demonstrate the efficiency of the recommended strategy. The suggested system produces very low THD values, 1.04% for grid-connected WES, when compared to the other approach. The instructive subject of this work, the clear choice of control method and electric circuit, ensures that the optimal fact of DVR topology meets the required power quality. This study also provides useful information to researchers in this sector.

Adaptive Impedance-Conditioned Phase-Locked Loop for the VSC Converter Connected to Weak Grid

In this paper, an adaptive version of the impedance-conditioned phase-locked loop (IC-PLL), namely the adaptive IC-PLL (AIC-PLL), is proposed. The IC-PLL has recently been proposed to address the issue of synchronisation with a weak AC grid by supplementing the conventional synchronous reference frame phase-locked loop (SRF-PLL) with an additional virtual impedance term. The resulting IC-PLL aims to synchronise the converter to a remote and stronger point in the grid, hence increasing the upper bound on the achievable power transfer achieved by the VSC converter connected to the weak grid. However, the issue of the variable grid strength imposes another challenge in the operation of the IC-PLL. This is because the IC-PLL requires impedance estimation methods to estimate the value of the virtual impedance part. In AIC-PLL, the virtual impedance part is estimated by appending another dynamic loop in the exciting IC-PLL. In this method, an additional closed loop is involved so that the values of the virtual inductance and resistance are internally estimated and adapted. Hence, the VSC converter becomes effectively viable for the case of the grid strength variable, where the estimation of the grid impedance becomes unnecessary. The results show that the converter that relies on AIC-PLL has the ability to transfer power that is approximately equal to the theoretical maximum power while maintaining satisfactory dynamic performance.

Harmonics Mitigation of a Solar PV-Fuel Cell Based Microgrid System using a Shunt Active Power Filter

The increased penetration of distributed energy resources is inspiring the entire design of conventional electrical power systems. A Microgrid (MG) includes distributed generation, loads, energy storage, and a control system capable of operating in grid-connected mode and/or island mode. The power quality (PQ) issue is one of the main technical challenges in an MG power system. To improve PQ, it is necessary to analyze the harmonic distortion of the system. Moreover, harmonic distortion in MG networks has significantly reduced PQ, affecting the stability of the system. The shunt active power filter (SAPF) has been extensively used to diminish the current harmonics and verified as being the best solution. Hence, in this paper, the impact of PQ issues in an adopted standalone MG system (comprising solar and fuel cell based renewable energy sources) is investigated in the presence of SAPF. The SAPF is realized using a conventional synchronous reference frame (SRF) technique for current generation with a pulse-width modulation voltage source inverter technique to generate pulses for the inverter along with a PI controller to regulate the DC-link capacitor voltage. The proposed model is developed in MATLAB/SIMULINK and the results validate the superiority of the proposed technique over others in terms of harmonic elimination.

Modelling and vector control of dual three‐phase PMSM with one‐phase open

This study proposes a generic mathematical modelling and decoupling fault-tolerant vector control for dual three-phase permanent magnet synchronous machine (PMSM) with one phase open based on the conventional dual three-phase voltage source inverters, accounting for the mutual coupling between two sets of three-phase windings and the second harmonic inductance. When the dual three-phase PMSM has one phase open, the permanent flux-linkages are asymmetric and there are second harmonic components in the conventional synchronous reference frame (dq-frame). Based on the proposed mathematical modelling, both permanent magnet flux-linkages and currents become DC values in the dq-frame, and therefore, the conventional proportional integral (PI) controller can be used to regulate the dq-axis currents. Then, a decoupling fault-tolerant vector control with/without a dedicated feed-forward compensation is proposed to validate the correctness of the proposed mathematical modelling. Experimental results on a prototype dual three-phase PMSM with one phase open show that the second harmonic dq-axis currents can be well suppressed simply by the conventional PI controller and dedicated feed-forward compensation. It also shows that the decoupling fault-tolerant control based on the proposed modelling and control method has excellent dynamic performance, which is equivalent to the vector space decomposition control for the healthy machine.

Performance Improvement of a Distributed Generation System Using Unified Power Quality Conditioner

This paper deals with performance improvement of a distributed generation (DG) system using unified power quality conditioner (UPQC). Despite of the several benefits of DG like excellent energy supply, reducing expansion of power distribution system, environmental friendly and so on, there are several challenges existing due to the integration of DG with the grid or operating it in standalone mode. Power quality (PQ) issue is one of the main technical challenges in DG power system. Moreover, harmonic and voltage sag compensation in a DG networks has significantly reduced PQ, which affects the stability of the system. Hence, in this paper, impact of PQ issues in an adopted standalone DG system (comprising of solar, wind and fuel cell based renewable energy sources) is investigated in the presence of UPQC. The realization of UPQC is carried out using conventional synchronous reference frame (SRF) and modified SRF (MSRF) techniques for reference current generation with pulse width modulation voltage source inverter technique for generation of pulses for the inverter along with a PI controller for regulation of DC-link capacitor voltage. The voltage sag compensation with current and voltage harmonics are estimated at different source (ideal and non-ideal) conditions with different control scheme like SRF and MSRF technique. The simulation model of conventional SRF and MSRF technique based UPQC is developed under MATLAB/SIMULINKR environment. The result obtained validates the superiority of proposed technique over others in terms of harmonics elimination and sag compensation.

A novel phase-locked loop applied on frequency fluctuation of EAST power supply

The excellent performance of synchronous phase locked technology is the basis of improving the power quality of Experimental Advanced Superconducting Tokamak (EAST) power system. The conventional synchronous reference frame phase-locked loop (SRF-PLL) with low pass filter (LPF) has a poor filter capacity for the 2nd harmonic component as the fundamental frequency fluctuation due to the high-power, high-parameter operation of the EAST. To improve accuracy of the phase estimation, adaptive notch filter (ANF) will be introduced into the LSRF-PLL to reject the disturbance of 2nd harmonic component, which can adaptively track the fluctuation of fundamental frequency with least mean square (LMS) algorithm. A novel composite algorithm of LPF and ANF SRF-PLL (LASRF-PLL) is presented to suppress the disturbance of the frequency fluctuation and harmonic pollution. At the last, the performance of the proposed LASRF-PLL is finally validated by simulation results of MATLAB software.

A Bus Clamping PWM-Based Improved Control of Grid Tied PV Inverter with LCL Filter Under Varying Grid Frequency Condition

ABSTRACT This paper presents a modified Synchronous Reference Frame Phase locked loop (MSRFPLL) for better synchronizing the output of the inverter with the grid. The proposed PLL is able to track the changes in the grid frequency in a faster way as compared to the conventional synchronous reference frame PLL (SRF-PLL). In order to switch the inverter, an advanced bus clamping based space vector PWM is being used which is novel for photovoltaic application. This PWM gives better DC bus utilization as compared to the conventional Sine PWM technique. To keep the THD within a permissible limit of 5% in accordance with the IEEE-519 & 1547 standard LCL filter with passive damping is implemented. DSPACE-1104 DAC card has been used for better interfacing of the experimental set up with MATLAB. The overall system is also being simulated in MATLAB/Simulink environment and the simulated results have been validated with the experimental results.

Virtual charge‐based synchronisation and feedback linearisation‐based current control of a three‐phase grid‐connected inverter without grid voltage sensors

A conventional synchronous (rotating) reference frame control requires small-signal models and introduces cross-coupling terms. In this study, a virtual charge-based scheme for grid synchronisation with only current measurements has been proposed for a three-phase grid-connected inverter with a first-order filter. The methodology described in this study curbs the dependency of the inverter on the information of the grid voltage to generate unit vectors from the phase-locked loop. This kind of estimation is especially important during grid voltage sensor failures or a distant point common coupling or weak grid conditions. In the proposed estimation scheme, no noisy differentiation is involved and only the nominal values of the interfacing filter parameters are utilised, making the implementation easier in digital microcontrollers. Also, a current control architecture based on feedback linearisation concept has been explored and embedded with the proposed grid voltage estimation technique, and the corresponding performance has been evaluated via simulation and experimental studies.

Fully decoupled fixed-time cascade control of three-phase voltage source converters with LC output filter

This paper presents a fully decoupled fixed-time cascade control system of three-phase voltage source converters (VSCs) with LC output filter. The proposed controller has a cascade structure and it is designed in the synchronous reference frame d-q. The outer loop is for voltage control and the inner loop is for current control. A fixed-time controller is developed for voltage regulation and reference tracking before the desired pre-fixed time despite the unknown disturbances. The current controller uses PI controllers for current regulation and feedforward terms to decouple the direct and quadrature of voltage and current control loops. Unlike the conventional PI-based synchronous reference frame VSC control, the proposed approach provides better control performance with smaller and smoother control signal and enables the d-q components of voltage and current to be fully decoupled. A comparative simulation study with the conventional control system is provided to highlight the effectiveness of the proposed approach.

Finite-Time Disturbance-Observer-Based Integral Terminal Sliding Mode Controller for Three-Phase Synchronous Rectifier

This article is concerned with the design of a finite-time disturbance-observer-based integral terminal sliding mode controller for the effective performance of three-phase synchronous rectifiers. The proposed control technique is developed based on the conventional synchronous reference frame model of the three-phase grid-connected converter, and the system dynamics is described in terms of a time-varying non-linear state equation. The variation of DC-load is considered as a disturbance. Therefore, a combination of a fast disturbance observer and an integral terminal sliding mode controller is utilized to produce the reference value of the direct axis for the current control loop. In this research, by employing Lyapunov stability theorem in the theoretical analysis and by numerical simulations, it is confirmed that the proposed closed-loop system is stable and the states converge to desired values in finite time even in the presence of load disturbance and control input saturation. The integral terminal sliding mode controller is utilized to maintain a robust performance along with a faster response of the converter. In order to demonstrate the performance ability of the proposed control scheme under real condition, an AC power source, impregnated with low order harmonics, is assumed. A real-time laboratory setup of the synchronous rectifier has been developed successfully, and the effective performance of the proposed control technique is fully proven.

Design and implementation of Legendre‐based neural network controller in grid‐connected PV systems

This study presents the development of Legendre-based functional neural network algorithm for shunt compensation in photovoltaic (PV)-based grid-connected system. The controller is developed for improving power quality (PQ) and the compensator is controlled to work in current control mode. It injects the requisite compensating current depending on the nature of the load current. The compensator is also interfaced with PV source and the controller design incorporates its contribution too. Some of the PQ problems studied include curtailment of harmonics, providing necessary reactive power, power factor improvement and so on. Results under distorted grid, varying solar irradiation and variety of loads have been presented. The proposed algorithm is designed using non-linear functional Legendre expansion of load current and has not been used for compensation or PQ problem alleviation till date. Both simulation and experimental results verify that the proposed algorithm performs far better than the adaptive popular backpropagation multilayer perceptron neural network, recurrent neural network and non-adaptive conventional synchronous reference frame theory based techniques.

Recursive Implementation of Multiple Delayed Signal Cancellation Operators and Their Applications in Prefiltered and In-Loop Filtered PLLs Under Adverse Grid Conditions

Phase-locked loops (PLLs) are widely utilized for grid synchronization and control of power electronic converters. More recently, both in-loop-filtered- and prefiltered-based PLLs have become more popular due to their prompt and accurate estimations of grid voltage quantities. In the past, an advanced filter viz., multiple delayed signal cancellation (MDSC) operators, has been successfully investigated in both $\alpha \beta$ -frame and $dq$ -frame for extracting fundamental-frequency positive-sequence grid voltage quantities by using conventional synchronous reference frame PLL. These operators can provide more flexibility to configure the delay time and undesired harmonics in comparison with the existing cascaded delayed signal cancellation operators. In this paper, the recursive-form structures of MDSC operators are analyzed, which are derived from the direct-form MDSC realizations. The recursive-form structures of MDSC operators are simple in implementation when compared to direct-form realizations. With the recursive-form MDSC implementations, both prefiltered MDSC-PLL and in-loop filtered MDSC-PLL will be investigated in detail. The design guidelines of these recursive-form-based MDSC-PLLs are presented. Comparison studies of the proposed PLLs and the validation of their dynamic performances are done by experimental verifications.