Fundamental Positive Sequence Research Articles

Power quality enhancement of three-phase solar photovoltaic system-based generalized integrator controlled UPQC

Abstract As distributed generation becomes increasingly prevalent, providing reliable and stable electrical energy to the grid is of paramount importance. Therefore, precise monitoring of grid voltage is necessary in order to synchronize compensating current and voltage with the grid. The Dual Second-Order Generalized Integrator (DSOGI) can extract the fundamental positive sequence component from three-phase grid signals, enabling accurate determination of grid phase and current signals for rapid and precise grid compensation. This paper presents a control algorithm for extracting and compensating UPQC reference signals based on the Second-Order Generalized Integrator (SOGI). The improved photovoltaic-fed Unified Power Quality Conditioner System (PV-UPQC) is verified and analyzed under conditions of unbalanced grid voltage, current harmonics, as well as voltage sags and swells. It can be demonstrated that the enhanced system exhibits superior adaptability to the grid.

A Modified Control Strategy for Three-Phase Four-Switch Active Power Filters Based on Fundamental Positive Sequence Extraction

Three-phase four-switch active power filters (APFs) have attracted attention due to their low amount of semiconductors and low cost. The traditional control strategy of three-phase four-switch APFs usually includes phase-locked loops (PLLs) and rotating coordinate transformation for harmonic detection, resulting in complicated calculations and increased computation. In this paper, a modified control strategy for three-phase four-switch APFs based on fundamental positive sequence extraction is proposed, eliminating PLLs and rotating coordinate transformation with trigonometric calculations. Harmonic extraction is based on the fundamental positive sequence extraction method, while non-locked phase loop coordinate transformation is proposed to eliminate trigonometric calculations. Quasi-PR control is adopted for current tracking, and DC voltage control is designed to suppress voltage imbalance between the two split capacitors on the DC side. The space vector pulse width modulation (SVPWM) method is modified for a reduced-switch APF topology. The proposed control strategy guarantees excellent harmonic compensation: harmonic currents are significantly suppressed when the APFs are working, the THD of the source current decreases to 3.86%, the bus voltage fluctuation on the DC side is small, the voltage remains stable, and the computational complexity is reduced. Finally, a simulation and an experimental hardware platform are established to validate the feasibility and performance of the proposed control strategy. The experimental results show that it has good performance in suppressing harmonics and improving power quality.

Improved Reference Current Extraction Method for Electric Aircraft Active Power Filter Application

This paper proposes a robust reference current extraction method useful for active power filters (APFs) connected to the More Electric Aircraft (MEA) grid for addressing power quality issues. Hence, it is essential to investigate and develop better APF harmonic compensating control algorithms that can eliminate the impact of harmonics emerging from the nonlinear loads connected to the MEA grid. The proposed method envisages a half-cycle moving window discrete Fourier transform filter approach to rapidly extract the fundamental positive sequence (FPS) from a polluted three-phase grid voltage. Thus, allowing the harmonic cancelation method to generate desired clean reference current which is in phase with FPS component of the grid voltage. This reference current is used in conjunction with the hysteresis current controller to generate appropriate gate pulses for the current-controlled voltage source APF. The employment of the digital filter in the control structure of the APF ensures improved power quality for the MEA grid as compared to traditional filtering algorithms. The numerical and experimental results demonstrate that the proposed control solution helps to reduce the total harmonic distortion under abnormal grid disturbances, thus, indicating the potential benefits of the proposed work for addressing power quality issues in MEA applications.

Neural Network Quadrature Signal Generator-Based Virtual Flux Estimation for Predictive Control of PWM Converters

In this paper, a frequency-adaptive neural network-based virtual flux (FANN-VF) estimator is developed for sensorless control of a pulse-width modulation converter under unbalanced and distorted grid conditions. This method exploits two parallel FANNs configured as quadrature signal generators. The VF fundamental positive and negative sequence components (PNSCs) are inherently separated in the estimator without employing any cascaded filters. A frequency-locked loop is introduced to accurately track the frequency variations. The estimated VF PNSCs are directly exploited to compute the power references in VF-based predictive direct power control scheme. The developed strategy ensures constant active power and sinusoidal current waveforms under nonideal grid conditions. Simulation and experimental tests are performed to verify the effectiveness of the proposed method.

Research on a New Synchronous Phase-Locked Method for Renewable Energy Grid-connected Converters

Background:: The phase-locked loop (PLL) is widely used to estimate synchronization information such as magnitude and phase angle of the grid voltage, which is essential for gridconnected power generation from distributed renewable energy sources. However, the harmonics in the grid often affect the accurate extraction of fundamental positive sequence, resulting in phase deviations. Methods:: A dual extended novel third-order generalized integrator (DENTOGI) is proposed and placed into the control outer loop of the PLL. This approach can be applied in the condition of the grid with DC offset voltage (DCOV). Additionally, the PLL is proposed by introducing moving average filter (MAF) and eliminating the proportional controller of the integration link to suppress high-frequency harmonics and speed up the response. Results:: A comparative experiment using Matlab/Simulink is conducted to evaluate the proposed PLL against the conventional PLLs. The experimental results confirm that the proposed PLL exhibits superior filtering performance for DC offset and harmonic components when compared to the conventional PLL. Additionally, the proposed PLL demonstrates better dynamic adjustment capabilities. Conclusion:: The main contribution of this paper is to propose a phase-locked method that can eliminate DCOV and harmonic components, which can be applied in non-ideal power grid conditions. The proposed PLL based on DENTOGI and MAF can lock the grid phase stably without errors under the grid fault involving DCOV and harmonic disturbance, it also exhibits the characteristics of fast response.

Control strategy for three-phase four-switch active power filter based on positive sequence extraction in stationary coordinate fundamental reference frame

Traditional three-phase four-switch active power filter often uses the method of phase-locked loop (PLL) and low-pass filter for harmonic detection, which seriously affects the detection accuracy. In order to improve the detection accuracy, a lock-free three-phase four-switch active power filter based on fundamental positive sequence extraction is proposed. The harmonic detection adopts the fundamental positive sequence extraction method, and the rotation coordinate axis transformation adopts the phase-locked loop (PLL) free method. Compared with the traditional three-phase four-switch active power filter, this method does not require a phase-locked loop and a low-pass filter, and does not need complex coordinate transformation[2].The control method is simpler. Finally, simulation results prove the effectiveness of the control method.

Analysis and modelling of a phase‐locked loop based on a novel cascade structure of SOGI

AbstractIn order to attenuate the oscillations on the estimated frequency of a phase‐locked loop (PLL), the effective suppression of low‐order harmonics in grid voltage is a crucial issue in three‐phase grid‐connected inverters, which use the standard PLL based on a double second‐order generalized integrator (DSOGI‐PLL). Analysis shows that the SOGI module exerts a stronger filtering effect in generating the quadrature signal than the in‐phase signal. In this context, two novel cascaded SOGI module filtering structures have been developed, based on which the PLLs are proposed. In comparison to the previously proposed method for addressing the problem of input voltage harmonics, the proposed method has stronger filtering ability and can flexibly select the levels of SOGI module cascading based on the degree of grid voltage distortion to achieve a balance between system dynamic performance and filtering performance. The proposed PLLs' approximation small signal model is established, and the method for designing the system control parameters is also provided. In the case of frequency steps, phase angle jumps, harmonic injection, and unbalanced voltage sag, Matlab modelling and experimental results demonstrate that the proposed PLLs can efficiently extract the frequency and phase of the fundamental positive sequence component and behave better in steady‐state and dynamic performance.

Frequency-Locked Loop Based New Automatic Phase-Shift Method for Active Islanding Detection of Three-Phase MicroGrid

This paper proposes an active Islanding Detection Method (IDM) integrating Frequency Locked-Loop (FLL) for the detection of voltage phase and frequency at the point of common coupling (PCC) and a modified Auto Phase Shift (APS) method for three-phase Distributed Energy Resource (DER) serving a local RLC load. The FLL is composed of a double second-order generalized integrator (DSOGI) to attenuate the harmonics and pass the fundamental positive sequence components in a frequency adaptive manner. Different from traditional APS methods, a new variable frequency term with an exponential factor is injected into the phase shift algorithm to reduce the region of Non-Detection Zone (NDZ). This modified approach enables the decentralized power generation system to detect the islanding phenomenon more quickly. Simulations and experimental tests conducted on a DG system on OPALRT (OP5600) and hardware test-bed demonstrate the behaviour of the proposed model within IEEE Standards. Additionally, a comparative evaluation was performed on the seamless transition of a microgrid (MG) from Grid-connected mode to Islanded mode and vice versa, using two different MGs, each having a single voltage source converter (VSC) and a local load but with control circuits incorporating DSOGI-FLL and ROGI-FLL respectively. Hardware testing results indicated that the control circuit based on ROGI-FLL demonstrated superior performance.

A Novel Control Scheme Using UAPF in an Integrated PV Grid-Tied System

This article investigates the photovoltaic fed universal active power filter (PV-UAPF) system for enhancing power quality indices. A novel comb filter together with second-order generalized integrator (CFSOGI) based instantaneous power balance theory with power angle (IPBTPA) control scheme is implemented. Under distorted or adverse grid conditions, the CFSOGI effectively extracts the phase angle, frequency, and fundamental positive and negative sequence components (FPNSC) to overcome the limitations of adaptive notch filters. Here, a comb filter constitutes a cluster/group of zeros at multiple harmonic frequencies to achieve notch peaks thereby providing superior harmonic rejection ability. Whereas, IPBTPA generates the reference signals to operate the system at dual compensation strategy with or without solar PV. The power angle is used to extract the desired load voltages under grid voltage fluctuations thereby reducing the burden on the shunt converter. Moreover, the hysteresis controller generates the switching pulses based on the calculation of reference and load powers which improves the dynamic performance by maintaining system power balance and effective DC voltage regulation. Besides, reactive power compensation, suppression of harmonic currents and mitigation of grid voltage fluctuations is also carried out to strengthen the system further. The system is also tested in grid voltage interruption for islanding mode to realize a microgrid system. The overall system performance is also validated using simulation and in a real-time experimental set-up, and results are demonstrated under various conditions.

Mixed third-fourth order generalised integrator based PLL for grid integration of solar photovoltaic systems

ABSTRACT Increasing energy demand and the need for reduction in carbon emissions has necessitated the grid integration of solar photovoltaic (PV) systems, which are being widely employed in the tropical region. In a single-stage PV system, a voltage source inverter (VSI) is used to interface the PV panels with the grid. For the control and grid synchronisation of this VSI, an instantaneous phase angle of the fundamental positive sequence component (FPS) of grid voltage needs to be determined. Generalised integrator-based phase-locked loops (PLLs) are widely employed for this determination. This paper proposes a new orthogonal signal generator (OSG) for a generalised integrator-based PLL, known as mixed third-fourth order generalised integrator-based PLL (MTFOGI-PLL). The proposed MTFOGI offers fast dynamic response, better filtering and dc-offset rejection capability. The reported PLL is implemented as a part of the voltage-oriented control of the grid-tied PV system. The MTFOGI-PLL based operation of the grid-tied PV system is analysed under ideal and non-ideal grid conditions. From the results, it is observed that the proposed PLL accurately extracts the FPS voltages under different operating conditions resulting in the functioning of the grid-tied PV system as per the grid codes.

EV charging station with cascaded low-pass filtering scheme-based control of unified power quality conditioner

Abstract This paper proposes a cascaded low-pass filter (CLPF) scheme for the control of a unified power quality conditioner (UPQC) installed for enhancing the power quality of an electric vehicle (EV) charging station. With the incorporation of UPQC, the EV charging station draws sinusoidal currents at unity power factor and the supply voltage is maintained at the nominal value at the charger input. In the CLPF scheme, the α–β components of the load current are individually processed through a cascade connection of two LPFs to determine the corresponding fundamental orthogonal components. Based on thus determined components, the instantaneous and peak value and phase angle of the fundamental positive sequence component (FPSC) of the load current are computed. Similarly, the corresponding quantities related to the load and supply voltages are also computed with the CLPF scheme. With the computations related to the load current and supply voltage, the unit voltage templates (UVTs), power factor and fundamental active component (FAC) of the load current are calculated. For the control of shunt compensation, reference currents are generated based on the FAC of the load current and UVTs. Alternately, the control of series compensation is employed with the help of UVTs and peak amplitude of the FPSC of the load voltage. The performance of CLPF scheme-based extraction is compared with that of the earlier reported schemes through simulation and experimental studies. The performance comparison reveals a faster dynamic and more accurate steady-state response with the proposed scheme. The performance analysis of the proposed CLPF scheme-based control of a UPQC deployed at the EV charging station for different operating conditions demonstrates station operation with requisite reactive power compensation and mitigation of voltage sag/swell, and prevention of propagation of harmonic and unbalanced currents into the grid. This results in increased reliability of charger operation, energy savings and increased efficiency of the distribution network.

New Algorithm for 2-terminal Transmission Line Fault Location Integrating Voltage Phasor Feature and Phase Angle Jump Checking

The low precision and instability of fault location results of power system transmission lines is a major problem that affects the safe operation of power systems, which needs to be solved more effectively. With the increasing coverage of the synchronic Phasor Measurement Unit (PMU) in power systems, it has laid a good foundation for the research and application of high-precision fault location method based on 2-termnal synchronous measurement of phasor. Based on this, a new high-precision fault location algorithm integrating voltage phasor feature and phase-angle jump checking is proposed in this paper. Firstly, the synchronous fundamental positive sequence voltage and current phasors of the fault line are extracted; and then all the roots(including real and virtual roots) are calculated by using the condition that the fundamental positive sequence voltage phasor at the fault point is equal; finally the unique real root represents the actual fault point can be determined by using the feature that the check function of phase-angle will step-jump when crossing the fault point from left to right, and then the purpose of high precision fault location is realized. This algorithm has the characteristics of strong versatility, high precision and immunity to transition resistance. The results of simulation and example show the feasibility and effectiveness of the fault location algorithm the fault location algorithm proposed in this paper.

A Fixed-Frame Positive-Sequence Extractor: An Alternative to the MAF-Based Park-Filter

Park-filters based on moving average filters (MAF) are a popular technique to extract the fundamental positive sequence component with zero-phase-shift from electrical three-phase signals, which is used to build references for power converters' controllers, as well as to help in grid synchronization tasks. Their implementation is, however, computationally burdensome, as it requires mapping the signals from the <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> -fixed reference frame coordinates to the synchronous frame coordinates, where they are processed using the moving average filter, and the result is finally transformed back to <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> -fixed reference frame coordinates. To overcome this issue, the present paper proposes and experimentally tests an alternative filter that directly operates on the <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> -fixed reference frame, thus avoiding the Park transformations. This considerably reduces the computational burden to almost one fifth. The proposed filter comprises a comb-filter cascaded with a second order harmonic oscillator. The proposed filter, referred to as CF-SOHO, guarantees the extraction of the fundamental positive sequence component with unitary gain and without phase-shift from unbalanced and harmonic-distorted signals, i.e., all harmonics (including DC components) contained in the incoming signals are fully suppressed. It can also cope with typical electrical stationary and dynamic disturbances.

T-Shaped Transmission Line Fault Location Based on Phase-Angle Jump燙hecking

In order to effectively solve the dead-zone and low-precision of T-shaped transmission line fault location, a new T-shaped transmission line fault location algorithm based on phase-angle jump checking is proposed in this paper. Firstly, the 3-terminal synchronous fundamental positive sequence voltage and current phasors are extracted and substituted into the fault branch distance function to realize the selection of fault branch when the fault occurs; Secondly, use the condition of the fundamental positive sequence voltage phasor at the fault point is equal to calculate all roots (including real root and virtual roots); Finally, the phase-angle jump check function is used for checking calculation, and then the only real root can be determined as the actual fault distance, thereby achieving the purpose of high-precision fault location. MATLAB simulation results show that the proposed new algorithm is feasible and effective with high fault location accuracy and good versatility.

Improvement Strategy for Voltage Quality of Microgrid Based on Master-Slave Control

Due to the nonlinear and unbalanced load of micro-grid, the output voltage quality on the main inverter side is easily affected in the micro-grid based on Master-Slave control.So, this paper put up a strategy that compensating the harmonic current and negative-sequence current by subordinate inverter. This strategy makes the current that flow through main inverter only have fundamental positive sequence component, so as to achieve the aim of improving the quality of the output voltage. Finally, the simulation model is built in the PSCAD simulation environment, which shows the effectiveness of the strategy.

Grid integrated solar energy transfer system with a two‐layer complex coefficient filter‐based control

In this paper, a novel control strategy for grid-integrated solar energy transfer system (SETS) is proposed which enhances the grid power quality along with active power injection. Some of the conventional grid synchronization controls face problems due to imperfect estimation of fundamental active load current during both grid-side and load-side disturbances. The proposed two-layer multiple-complex coefficient filter control improves the performance of SETS by accurately estimating the fundamental active load current at adverse operating conditions. One layer extracts the fundamental positive sequence of the sensed three-phase grid voltages and derives the unit in-phase voltage templates for grid currents reference generation to achieve unity power factor operation. The second layer evaluates the average active fundamental component of the load currents, and finally derives the reference amplitude of three-phase grid currents. The two-stage SETS consists of a quadratic boost DC-DC converter to extract the maximum solar array power forming an interface between PV array and inverter. It also provides high voltage gain and decreases the length of the PV array string while maintaining a low DC-link voltage ripple, extending the service life of the SETS. The effect of a nonlinear unbalanced load is nullified and therefore the grid current's waveform quality is maintained in compliance with the IEEE-519 standard during the dynamic operating conditions. The SETS provides sinusoidal, balanced, and steady grid currents at unity power factor withstanding adverse operating conditions such as distorted grid voltages, varying irradiance and varying unbalanced nonlinear load currents. Due to its flexibility to operate in such adverse conditions, the proposed control based SETS is suitable for grid-connected applications. Simulations are carried out in MATLAB/Simulink and results validate the performance of the SETS. The hardware prototype of the proposed SETS is developed and rigorously tested under various operating conditions to validate the proposed claims.

Development and application of measurement data pre-processing tools for parameter identification considering q-axis voltage

At present, when model parameter identification is carried out, measurement data from phase measurement units or fault recorders are generally used directly. These two types of devices can directly provide the fundamental positive sequence quantities required for parameter identification, but cannot output the dq components. If these measurement data can be fully utilized for parameter identification, it is very beneficial to improve the model accuracy. In this paper, according to the engineering needs of load model parameter identification, the extraction method and variation law of dq components are studied, and the data pre-processing tool is developed and put into use.

A voltage unbalance compensation strategy based on virtual impedance for DGs in island microgrid without sequence component separation

The increasing application of unbalanced loads may cause power quality issues in microgrid. In this paper, a voltage unbalance compensation strategy for distributed generators (DGs) in island microgrid is proposed to compensate the unbalanced bus voltage and improve the distribution effect of load current. The control system of the DGs mainly consists of active and reactive power droop controllers, a virtual impedance loop, voltage controllers. As the proposed voltage controller has two well decoupled control branches to independently control fundamental positive and negative sequence voltages, sequence component detection is not necessary for the implementation of virtual impedance. Then the positive sequence virtual impedance can be introduced to improve the power distribution effect of the DGs, and the negative sequence virtual impedance is introduced to compensate the voltage drop of the negative sequence current on the line impedance and to reduce the negative sequence circulating current. The design approach of the control system is discussed in detail. Finally, the validity and effectiveness of the proposed method are verified by simulation and experimental results.

A Novel Phase-Locked Loop for Mitigating the Subsequent Commutation Failures of LCC-HVDC Systems

The AC fault ride-through and support capability of line commutated converter based high voltage direct current (LCC-HVDC) have significant influence on the safe operation of power system. The ability of phase locked loop (PLL) to synchronize with the voltage phase under AC faults would affect the recovery performance of LCC-HVDC systems. This paper analyzes the influence of the conventional PLL on the subsequent commutation failure (SCF) of LCC-HVDC systems and points out that the conventional PLL has some problems such as slow dynamic response, significant frequency fluctuation, and coupling of phase and frequency detection under fault conditions. A novel PLL is proposed to address the problems. Cascade delayed signal cancellation operators are combined with a mathematical filter, forming a hybrid pre-filter to extract fundamental positive sequence voltage for the PLL. To reduce the fluctuation of PLL's frequency during AC faults, the estimated frequency is fixed when the fault is detected by a fault detector. As a consequence, the decoupling between phase detection and frequency detection is realized, and the proposed PLL's dynamic response performance is enhanced. Simulation results show that the proposed PLL can provide accurate phase reference for the DC control system and reduce the probability of SCFs of LCC-HVDC system.

Control of UPQC based on steady state linear Kalman filter for compensation of power quality problems

A frequency lock loop (FLL) based steady state linear Kalman filter (SSLKF) for unified power quality conditioner (UPQC) control in three-phase systems is introduced. The SSLKF provides a highly accurate and fast estimation of grid frequency and the fundamental components (FCs) of the input signals. The Kalman filter is designed using an optimized filtering technique and intrinsic adaptive bandwidth architecture, and is easily integrated into a multiple model system. Therefore, the Kalman state estimator is fast and simple. The fundamental positive sequence components (FPSCs) of the grid voltages in a UPQC system are estimated via these SSLKF-FLL based filters. The estimation of reference signals for a UPQC controller is based on these FPSCs. Therefore, both active filters of a UPQC can perform one and more functions towards improving power quality in a distribution network. In addition to the SSLKF-FLL based algorithm, a bat optimization algorithm (based on the echolocation phenomenon of bats) is implemented to estimate the value of the proportional integral (PI) controller gains. The bat algorithm has a tendency to automatically zoom into a region where a promising alternative solution occurs, preventing the solution from becoming trapped in a local minima. The complete three-phase UPQC is simulated in the Matlab/Simulink platform and the hardware is tested under various power quality problems.