Second-order Generalized Integrator Frequency-locked Loop Research Articles

SOGI-FLL error-and-hold algorithm for improving the response in face of voltage sags and swells with a small computational burden

The Second Order Generalized Integrator–Frequency Locked Loop (SOGI-FLL) is a widely used and popular adaptive filter for estimating grid voltage parameters with minimal computational burden. However, it is vulnerable to voltage sag and swell faults, especially voltage sags that can significantly distort the estimated frequency. In this paper, we propose an error-and-hold algorithm for the SOGI-FLL that can quickly detect faults and hold the estimated frequency during these perturbations. The algorithm uses the absolute value of the SOGI's error, its average, and the average of the FLL's estimated frequency to operate. It reduces induced errors in the SOGI-FLL's quadrature outputs, improves the FLL's transient response, holds the estimated frequency, and restores the phase to its previous value before the fault. The proposed algorithm is a straightforward and low computational burden algorithm that can be executed on a low-cost processor. We validate the effectiveness of the proposed error-and-hold algorithm through simulations and experimental results.

Research on an Adaptive Compound Control Strategy of a Hybrid Compensation System

This paper investigates the parallel harmonic resonance problem for hybrid compensation systems, consisting of active power filters and thyristor-switched capacitors, and proposes an adaptive composite control strategy for solving the parallel harmonic resonance problem that may arise in practical applications of hybrid compensation systems. In practice, a hybrid compensation system can effectively solve harmonic and reactive power problems, but the equivalent reactance of the thyristor-switched capacitor and the supply line may form a parallel resonant circuit, which may generate parallel harmonic resonance when excited by a harmonic source at the non-linear load side, affecting the quality and stable operation of the system. The adaptive composite control strategy employs a second-order generalized integrator-frequency-locked loop (SOGI-FLL) to extract the harmonic voltage at the point of common coupling (PCC) and generate an adaptive damping current command using an adaptive algorithm, which adaptively adjusts the parameters of the resonance suppression controller through harmonic content limitation. Matlab/Simulink simulations show that the method effectively achieves harmonic resonance suppression of the power supply system under complex operating conditions, thus ensuring the stable operation and power quality of the power supply system. Therefore, the proposed control strategy is feasible and effective.

Encoderless Control of PMa-SynRG Based-Stator Current Vector for Wind Generation Systems

In this paper, a novel encoderless control of permanent magnet assisted synchronous reluctance generator (PMa-SynRG) based-stator current vector for renewable energy systems is presented. For the encoderless control, the PMa-SynRG rotor position angle along with rotor angular speed are estimated ultimately from the generator's stator currents, as the stator inductance is only requisite nevertheless the stator resistance. Indeed, this encoderless control technique employs one current control loop for regulating the magnitude of the stator current instead of employing two current control loops. For eliminating the current measurement noise and enhancing the phase angle tracking performance, a modified second-order generalized integrator frequency-locked loop (MSOGI-FLL) technique is employed and compared with the standard SOGI-FLL under different voltage/frequency conditions. The proposed technique has a simple computational implementation while accomplishing a rapid along with precise estimation performance of the PMa-SynRG rotor position angle and speed. The performance is validated using simulations and real-time hardware-in-the-loop Typhoon HIL implementation.

A Novel Dead-Time Elimination Method for Voltage Source Multilevel Converters

Voltage source multilevel converters (MLCs) are widely applied in recent years in a wide range of industrial applications. However, time delay inserted in switching signals to prevent short through can cause the dead-time effects, including output waveform distortion and voltage drop. In many kinds of MLCs, the freewheeling paths during dead time can also produce unexpected output voltage levels, which can cross a few consecutive output levels, introducing large <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dt</i> . Typical examples of those MLCs are 5L-active neutral-point-clamped, 7L-hybrid-clamped, 4L-nested neutral-point-clamped (NNPC) converter, etc. To solve this problem, a generic dead-time elimination (DTE) method is proposed. This method splits the complementary switches and replaces them with controllable switches coupled with uncontrollable diodes, decomposing the phase-leg of the converter into two basic switching cells, thus obviating the need for dead time. To address the zero-current clamping effect inherent to the DTE method, a second-order generalized integrator frequency-locked loop based current detector is also developed. The proposed DTE method is suitable for various MLCs. It cannot only effectively eliminate the unexpected levels, but also significantly reduce the output distortion and regains the output rms value. An application example of 4L-NNPC converter is given. The effectiveness of the proposed method is verified by both simulation and experiment.

Power Quality Analysis by H-Bridge DSTATCOM Control by Icosθ and ESRF SOGI-FLL Methods for Different Industrial Loads

This work develops the analysis of power quality by the H-bridge Static Distribution Compensator (DSTATCOM) as well as its control techniques in different industry-based loading conditions. The function of DSTATCOM is to diminish power quality problems arising due to commercial as well as industrial loads. For reference current extraction, the novel Icosθ and proposed enhanced SRF SOGI-FLL (synchronous reference frame second-order generalized integrator frequency-locked loop) controller have been adopted in the H-bridge DSTATCOM. The Icosθ controller’s performance is dependent on the in-phase and quadrature-phase angle, which changes accordingly as load changes, whereas the proposed enhanced SRF SOGI-FLL controller works in synchronization with the grid with an inverter. The two control techniques were compared in terms of balancing, power factor improvement, DC-link voltage control, and harmonic minimization. The harmonics minimization of the proposed controller has been validated by IEEE 519 standards. The extracted reference currents are fed to the hysteresis current controller for the generation of pulses toward the inverter switches of DSTATCOM. The DSTATCOM system along with control algorithms have been tested on various loading conditions, i.e., voltage source- and current source-based non-linear loads, induction heating-based loads, and electric arc furnace. The complete DSTATCOM systems were implemented and executed in the MATLAB/Simulink platform and then power quality improvement features were investigated.

Enhanced Control of Three-Phase Grid-Connected Renewables with Fault Ride-Through Capability under Voltage Sags

The uninterruptible operation of grid-connected renewables under the occurrence of grid voltage sags is addressed in this work. This is achieved due to the incorporation of an enhanced control algorithm of a renewable source. The low-voltage ride-through algorithm was developed in accordance to the voltage profile introduced by IEC 61400-21 regarding grid voltage sags. To guarantee continuous operation of the renewable agent during voltage sags, not only instantaneous reactive power but also instantaneous active power under moderate voltage sags was injected to the utility grid fulfilling grid code requirements. A dual second-order generalized integrator frequency-locked loop synchronization algorithm was used to estimate the system’s frequency, together with the positive and the negative sequences of the three-phase utility grid voltages when unbalanced sags occurred. The current control was made in a stationary reference frame by using proportional-resonant regulators, and a DC voltage source was used to emulate the primary energy from any type of renewable system. The validation of the proposed control algorithm was conducted for a three-phase grid-connected renewable system with an apparent power of 500 kVA. The results from several experimental tests demonstrated the proper behavior of the enhanced algorithm.

SOGI-FLL Grid Frequency Monitoring with an Error-Based Algorithm for a Better Response in Face of Voltage Sag and Swell Faults

The SOGI-FLL (second-order generalized-integrator frequency-locked-loop) is a well-known and simple adaptive filter that allows estimation of the parameters of the grid voltage with a small computational burden. However, this structure has shown to be sensitive to the events of voltage sags and swell faults, especially to voltage sags that deeply distort the estimated frequency. In this paper an algorithm is proposed to face the fault that modifies the SOGI-FLLs gains in order to achieve a better transient response with a reduced perturbation in the estimated frequency. The algorithm uses the SOGI’s instantaneous and absolute error to detect the fault and change the SOGI-FLL gains during the fault. Moreover, the average of the absolute error is used for returning to normal operating conditions. The average value is obtained by means of a single low pass filter (LPF). The approach is easy to implement and represents a low computational burden for being implemented into a digital processor. The performance is evaluated by using simulations and real-time Typhoon Hardware in the Loop (HIL) results.

Frequency Locked Loop in Estimating Active, Reactive and Apparent Powers

In this paper, a new power calculation method has been presented. This method is based on a second-order generalized integrator frequency locked loop (SOGIFLL) and has enhanced features over classical methods for power calculation widely used in industry. The FLLs have a wide variety of applications such as power converters, grid synchronization, sensorless flux estimation, and control of motor drives. The nature of the FLL allows for it to be a potentially perfect calculation method for power calculation. The obtained results showcase the effectiveness of the proposed power calculation method.

Improved harmonic detection method based on ip-iq theory

Based on the traditional ip-iq harmonic detection theory, the accuracy of harmonic detection is easily affected by the phase-locked loop (PLL) output phase error, and the single low-pass filter (LPF) detection accuracy and filtering effect cannot be simultaneously. In this paper, an improved harmonic detection method based on the second-order generalized integrator-frequency locked loop (SOGI-FLL) technique is proposed to generate sine and cosine signals with the same frequency as the grid voltage; The traditional low-pass filter and average filter are used in series to improve the response speed and accuracy. Through theoretical analysis of the improved harmonic detection method and simulation in MATLAB environment, the theory and simulation results prove the effectiveness of the improved method.

Three-phase phase-locked loop algorithm and application to a static synchronous compensator

This work presents a phase-locked loop (PLL) algorithm for the detection of positive-sequence and negative-sequence grid voltages in three-phase systems. Since it employs an adaptive filter (AF), it is possible to track the fundamental component of the grid voltages accurately even in the presence of harmonics, subharmonics, and interharmonics. This aspect is of major concern for practical applications, especially because some popular algorithms present limited performance in such scenarios, e.g., the decoupled double synchronous reference frame PLL (DDSRF-PLL) and double second-order generalized integrator frequency-locked loop (DSOGI-FLL). A thorough comparison among the proposed method and other similar structures is presented in terms of steady-state and dynamic behaviors to show that the introduced PLL presents high immunity to power quality disturbances. Unfortunately, a possible drawback lies in the slow dynamic response due to the AF. In order to verify the applicability of the proposed approach, it is also incorporated to the control structure of a grid-connected power converter operating as a static synchronous compensator (STATCOM). This converter aims at regulating the ac voltage at the point of common coupling (PCC) and power factor correction. A detailed mathematical analysis is presented and experimental results are also discussed to validate the theoretical assumptions.

Frequency-Tracking Algorithm Based on SOGI-FLL for Wireless Power Transfer System to Operate ZPA Region

The wireless power transfer (WPT) system has attracted attention for energy transmission without physical contact. However, a WPT system has low coupling condition because of a big air gap between transmitter and receiver coils. The low coupling condition has a high leakage inductance. To overcome this problem, we design a proposed system for WPT using series-series (S-S) topology of one resonant circuit. To obtain the higher efficiency power conversion of the WPT system, it has to operate the resonant frequency in the zero phase angle (ZPA) point even under mutual coefficient and load variation. Therefore, we propose the resonant frequency tracking algorithm to operate ZPA point based on the second order generalized integrator-frequency locked loop (SOGI-FLL) method. This proposed frequency-tracking algorithm can estimate ZPA point by changing switching frequency. We can reduce the switching loss with this proposed algorithm and improve the low conversion efficiency of the WPT system. The performance of the proposed frequency-tracking algorithm is automatically verified through various coupling coefficients and the load variation.

An Improved Adaptive Control Strategy in Grid-Tied PV System With Active Power Filter for Power Quality Enhancement

This article investigates the power quality enhancement of a grid-tied photovoltaic (PV) distribution system by employing a fuzzy logic proportional-integrator-derivative multiple complex coefficient filter multiple second-order generalized integrator frequency-locked loop (FLPID-MCCF-MSOGI-FLL) hybrid control scheme based shunt active power filter. The MSOGI-FLL reference current generation strategy is implemented to mitigate the current harmonics by extracting the fundamental constituents (FCs) from the nonlinear load currents, whereas an MCCF is employed to separate the FC from the distorted grid voltages and eliminates the voltage harmonics during extremely polluted grid voltage condition. The main objective of using FLPID is to maintain the stable power between dc and ac sides by regulating the dc-link voltage constant under transient conditions. To track the maximum power from the PV panel under varying environmental condition, the particle swarm optimization based perturb and observe technique is used in this article. The comparative analysis is analyzed to check the effectiveness of the proposed hybrid control scheme with existing and adaptive control techniques in respect of power quality, better dc offset rejection, better FC and frequency extraction, and grid synchronization. The system with the proposed control scheme is simulated on MATLAB/Simulink and validated in a real-time field-programmable gate array platform under different test scenarios. In conclusion, the harmonic content of grid currents and voltages is found well within the IEEE-519 standard limits.

New SOGI-FLL Grid Frequency Monitoring with a Finite State Machine Approach for Better Response in the Face of Voltage Sag and Swell Faults

The SOGI-FLL (Second-Order Generalized Integrator–Frequency-Locked Loop) is a well-known and simple adaptive filter that allows for estimating the parameters of grid voltage with a small computational burden. However, the SOGI-FLL has been shown to be especially sensitive to voltage sags and voltage swells, which deeply distort the estimated parameters, especially the frequency. This problem can be alleviated by simply using a saturation block at the Frequency-Locked Loop (FLL) output to limit the impact of distortion on the estimated frequency. Improving upon this straightforward approach, in this paper we propose the use of a finite state machine (FSM) for the definition of the different states of the SOGI-FLL frequency response during a voltage sag or swell fault. The FSM approach allows for applying different gains during the fault, enhancing the SOGI-FLL transient response. The performance of the FSM-based SOGI-FLL is evaluated by using simulation results, which show a better and faster response to these kinds of faults.

An Implementation of Solar PV Array Based Multifunctional EV Charger

In this article, an implementation of solar photovoltaic (PV) array powered grid-connected residential electric vehicle (EV) charger is presented, which caters the need of an EV, household loads, and the grid. The charger is enabled to operate autonomously using a PV array for providing an uninterruptible charging and power to household loads. However, in the absence of the PV array or insufficient PV array generation, the grid-connected mode of operation is presented. Moreover, the charger is supported with the synchronization and seamless mode switching control, so that the charger automatically connects/disconnects from the grid without disturbing the EV charging and household supply. The charger is also enabled with the vehicle-to-grid active/reactive power support to the grid and vehicle-to-home power transfer for supporting the local loads in an islanded condition. The charger is also controlled to operate as an active power filter for achieving the unity power factor operation and total harmonic distortion of the grid current within 5%. Moreover, for achieving energy management, a dc-link voltage regulation based energy management strategy is used and a sliding mode control is used for regulating the dc-link voltage. For satisfactory operation under distorted voltage condition, a second-order generalized integrator frequency-locked loop with dc offset rejection is used to generate the sinusoidal reference grid current. The charger is designed for a single-phase 230-V, 50-Hz grid and it is experimentally validated in the laboratory.

A Combined DPWM Based on DSOGI-FLL for Switching-Loss Reduction and Dead-Time Compensation

High efficiency and high output power quality of converters are always two goals pursued in power electronic systems. In this paper, the conventional discontinuous pulse width modulation (DPWM) for switching-loss reduction and a modified time-based dead-time compensation for reducing output current harmonics are investigated. And a combined DPWM for switching-loss reduction and dead-time compensation is proposed, where the amplitude adjustment and the zero-sequence injection in modulation waves are carried out separately in different current intervals related to polarities of three-phase currents. Moreover, in order to detect accurate polarities of the three-phase currents with ripples and harmonics, the second-order generalized integrator frequency-locked loop (DSOGI-FLL) is adopted and inserted into the combined DPWM. Finally, a combined DPWM based on DSOGI-FLL is proposed for switching-loss reduction and dead-time compensation. The comparison of experimental results with several PWM schemes verifies that, the proposed PWM scheme can both effectively reduce the switching loss and compensate the dead-time, simultaneously enhancing the efficiency and the output power quality of the converter.

Simplified Second-Order Generalized Integrator - Frequency-Locked Loop

Second-Order Generalized Integrator -- Frequency-Locked Loop (SOGI-FLL) is a popular technique available in the grid synchronization literature. This technique uses gain normalization in the frequency locked-loop. This increases the computational complexity. In this paper, we propose an alternative implementation to reduce the computational complexity of the SOGI-FLL. The proposed implementation modifies mainly the frequency locked-loop part and requires normalized voltage measurement. dSPACE 1104 board-based hardware implementation shows that the proposed implementation executes 20% faster than the standard implementation. This could be very beneficial for high switching frequency application e.g. >= 1MHz. In addition to the nominal frequency case, multi-resonant implementation is also proposed to tackle grid harmonics using a~simpler harmonic decoupling network. Small signal dynamical modeling and tuning are performed for both implementations. Dynamical equivalence is also established between the two implementations. Experimental comparative analysis demonstrates similar or better performance (depending on test scenarios) with respect to the standard implementation of the SOGI-FLL.

Enhanced structure of second-order generalized integrator frequency-locked loop suitable for DC-offset rejection in single-phase systems

For the proper operation of a microgrid, an accurate estimation of the grid parameters such as phase angle, operating frequency and amplitude under various disturbances is of prime interest. In this paper, an enhanced second-order generalized integrator-based frequency-locked loop (ESOGI-FLL) intended for DC-offset rejection in both grid-connected and islanded operation of a single-phase voltage source inverter (VSI) is presented. The main contribution in this work is the ability of the proposed scheme to remove the overall uninspected distortion in the orthogonal component and oscillations that affect the estimated frequency and phase caused mainly by the inherent DC-offset in the input voltage. Unlike the most popular PLLs dealing with DC-offset rejection, the proposed scheme has a great advantage in term of structure complexity and computational time. Numerical simulations as well as practical assessments are carried out to highlight the effectiveness and robustness of the proposed scheme in term of parameters estimation of highly shifted input voltage. In addition, the proposed scheme is tested against corrupted input voltage such as frequency change, phase jump and voltage sags. The paper also reports a detailed comparative study with conventional SOGI-FLL and third-order generalized integrator based FLL. The obtained results have shown best performances of the proposed ESOGI-FLL in term of DC-offset rejection in the orthogonal component, ripples elimination in the estimated frequency and less computational time.

Comparison of SOGI-FLL with SOGI-PLL for Single-Phase Grid-Connected Inverters

Phase locked loop (PLL) is a feedback system that has a vital role in the grid-connected inverter systems. The primary function of PLL is to obtain various grid information, such as magnitude, frequency, and phase. That used as the basis for the synchronization process. The primary purpose of this paper is to model and compare the performance SOGI-FLL (Second Order Generalized Integrator - Frequency Locked Loop) With SOGI-PLL, which is a relatively standard PLL. Besides that, this paper also aims to investigate the effect of FLL feedback gain on synchronization performance. Based on the simulation results, the performance of SOGI-FLL in its steady state is superior to that of SOGI-PLL especially for the condition of a single phase power systems that change in frequency.

A Study on Stability Control of Grid Connected DC Distribution System Based on Second Order Generalized Integrator-Frequency Locked Loop (SOGI-FLL)

This paper studies a second order generalized integrator-frequency locked loop (SOGI-FLL) control scheme applicable for 3-phase alternating current/direct current (AC/DC) pulse width modulation (PWM) converters used in DC distribution systems. The 3-phase AC/DC PWM converter is the most important power conversion system of DC distribution, since it can boost 380 Vrms 3-phase line-to-line AC voltage to 700 Vdc DC output with various DC load devices and grid voltages. The direct-quadrature (d-q) transformation, positive sequence voltage extraction, proportional integral (PI) voltage/current control, and phase locked loop (PLL) are necessary to control the 3-phase AC/DC PWM converter. Besides, a digital filter, such as low pass filter and all pass filter, are essential in the conventional synchronous reference frame-phase locked loop (SRF-PLL) method to eliminate the low order harmonics of input. However, they limit the bandwidth of the controller, which directly affects the output voltage and load of 3-phase AC/DC PWM converter when sever voltage fluctuation, such as sag, swell, etc. occurred in the grid. On the other hand, the proposed control method using SOGI-FLL is able to do phase angle detection, positive sequence voltage extraction, and harmonic filtering without additional digital filters, so that more stable and fast transient control is achieved in the DC distribution system. To verify the improvement of the characteristics in the unbalanced voltage and frequency fluctuation of the grid, a simulation and experiment are implemented with 50 kW 3-phase AC/DC PWM converter used in DC distribution.

A DSOGI-FLL-Based Dead-Time Elimination PWM for Three-Phase Power Converters

The dead-time elimination pulsewidth modulation (PWM) enables the drive pulses of the upper and the lower switching devices to alternate according to the current polarity, thus abandoning the dead time and essentially avoiding the dead-time effect in power converters. However, the current zero crossing will be distorted by the current jump generated by the drive-pulse alternation, and the current zero-crossing distortion will be further intensified if errors exist in the detected current polarity. Thanks to the noise-attenuation and frequency-adaptability characteristics of the double second-order generalized integrator frequency-locked loop (DSOGI-FLL), it can obtain accurate current polarities even in harmonic and unbalanced conditions. A DSOGI-FLL-based dead-time elimination PWM is, therefore, proposed in this paper, and several improvements are made to minimize the current zero-crossing distortion. An underlap period is added when alternating the upper and lower drive pulses to smooth the current jump at zero crossing, and a delay compensation term is inserted in the DSOGI-FLL to compensate both the current measurement delay and the control delay. The effectiveness of the proposed DSOGI-FLL-based dead-time elimination PWM has been validated by experiments, respectively, in the unbalanced-current, power-change, and frequency-variation conditions with an RL load, as well as in a grid-connected converter.