Grid-tied Converters Research Articles

Passivity Enhancement of Grid-Tied Converters by Series LC-Filtered Active Damper

The series LC -filtered active damper can be used for stabilizing a grid converter tied to the nonideal grid. Its operation principle is to mimic a damping resistance at the resonance frequencies appearing in the grid. However, the selection of the damping resistance has not been fully analyzed in the literature. Its effect with parasitic capacitance present in the grid has also usually been ignored, even though it may bring new challenges to the active damper. To address these issues, passivity is applied to study the grid converter stability before the understanding gained is used for formulating a damping resistance selection method. The method formulated can further be improved by admittance shaping so that system stability can always be ensured even when considering grid parasitic capacitance and control imperfection. Experimental results obtained have verified the expectations, and, hence, the suitability of the presented methods for the series LC -filtered active damper.

DC-Link Control Filtering Options for Torque Ripple Reduction in Low-Power Wind Turbines

Small wind energy conversion systems (WECSs) are becoming an attractive option for distributed energy generation. WECSs use permanent-magnet synchronous generators (PMSGs) directly coupled to the wind turbine and connected to the grid through a single-phase grid-tie converter. The loading produced on the dc link is characterized by large ripple currents at twice the grid frequency. These ripple currents are reflected through the dc bus into the PMSG, causing increased heating and ripple torque. In this paper, the PMSG inverter is used to control the dc-link voltage. In order to avoid reflecting the ripple currents into the PMSG, the feedback dc-link voltage is passed through a filter. The Butterworth filters, notch filters, antiresonant filter (ARF) and moving average filter (MAF) are considered. For a fair comparison, formulas are provided to tune the filter parameters so that dc-link voltage control will achieve the selected bandwidth. The different filtering options produce different levels of torque ripple reduction. The notch filter, ARF, and MAF obtain the best results and there is a tradeoff between the filter implementation complexity, bandwidth, overshoot, and the torque ripple reduction. Simulations and experiments using a 2.5-kW PMSG turbine generator validate the proposals.

Flexible active compensation based on load conformity factors applied to non‐sinusoidal and asymmetrical voltage conditions

This study proposes a flexible active power filter (APF) controller operating selectively to satisfy a set of desired load performance indices defined at the source side. The definition of such indices, and of the corresponding current references, is based on the orthogonal instantaneous current decomposition and conformity factors provided by the conservative power theory. This flexible approach can be applied to single- or three-phase APFs or other grid-tied converters, as those interfacing distributed generators in smart grids. The current controller is based on a modified hybrid P-type iterative learning controller which has shown good steady-state and dynamic performances. To validate the proposed approach, a three-phase four-wire APF connected to a non-linear and unbalanced load has been considered. Experimental results have been generated under ideal and non-ideal voltage sources, showing the effectiveness of the proposed flexible compensation scheme, even for weak grid scenarios.

Enhanced Predictive Current Control of Three-Phase Grid-Tied Reversible Converters with Improved Switching Patterns

A predictive current control strategy can realize flexible regulation of three-phase grid-tied converters based on system behaviour prediction and cost function minimization. However, when the predictive current control strategy with conventional switching patterns is adopted, the predicted duration time for voltage vectors turns out to be negative in some cases, especially under the conditions of bidirectional power flows and transient situations, leading to system performance deteriorations. This paper aims to clarify the real reason for this phenomenon under bidirectional power flows, i.e., rectifier mode and inverter mode, and, furthermore, seeks to propose effective solutions. A detailed analysis of instantaneous current variations under different conditions was conducted. An enhanced predictive current control strategy with improved switching patterns was then proposed. An experimental platform was built based on a commercial converter produced by Danfoss, and moreover, relative experiments were carried out, confirming the superiority of the proposed scheme.

A Method for Identification of the Equivalent Inductance and Resistance in the Plant Model of Current-Controlled Grid-Tied Converters

Precise knowledge of the plant time constant $L/R$ is essential to perform a thorough analysis and design of the current control loop in voltage source converters (VSCs). From the perspective of the current controller dynamics in the low-frequency range, such plant time constant is also suitable for most cases in which an LCL filter is used. As the loop behavior can be significantly influenced by the VSC working conditions, the effects associated to converter losses should be included in the model, through an equivalent series resistance. In addition, the plant inductance may also present important uncertainties with respect to the value of the VSC L/LCL interface filter measured at rated conditions. Thus, in this paper, a method is presented to estimate both parameters of the plant time constant, i.e., the equivalent inductance and resistance in the plant model of current-controlled VSCs. The proposed technique is based on the evaluation of the closed-loop transient responses of both axes of the synchronous reference frame when a proportional-integral current controller is implemented. The method gives a set of resistance and inductance values that should be employed for a rigorous design of the current controllers. Experimental results validate the approach.

A Voltage Detection Method for the Voltage Ride-Through Operation of Renewable Energy Generation Systems Under Grid Voltage Distortion Conditions

This paper proposes a new voltage detection method to assist the low-voltage ride-through operation of grid-tied converters. In particular, it consists of one synchronous reference frame phase-locked-loop (SRF-PLL) and two voltage detection modules. The fast voltage detection module tracks maximum points or zero points of three-phase grid voltage waveforms to calculate the phase voltage amplitude. The accurate voltage detection module operated in parallel assumes an additional cascaded delayed signal cancellation block to filter out the possible low-order voltage harmonics and then employs the same operational principle as the fast voltage detection module to precisely derive the phase voltage amplitude. Coupling with the SRF-PLL, where the grid voltage amplitude before voltage sag or surge can be precisely calculated, the voltage variation ratios can then be derived in sequence. A progressive logic algorithm is designed to determine the converter control commands by fully considering the different voltage variation ratios derived at different time points. Doing so, the proposed method can assist the grid-tied converter to satisfy the grid voltage ride-through operation requirements under various grid voltage distortion conditions. Finally, validity and accuracy of the proposed method were verified by simulation and experimental results.

Experimental evaluation of DC charging architecture for fully-electrified low-power two-wheeler

This paper is aimed to present an experimental criterion that allows researchers and designers to evaluate the performance of DC micro-grids dedicated to charging operations of full EV. The laboratory methodology is explained through tests on a demonstrator of power architecture, specifically designed as simplified case study of a DC charging station for fully-electrified low-power two-wheeler, such as: electric scooters and bikes. This experimental prototype is composed by a 20kW AC/DC bidirectional grid-tied converter, which realizes the DC conversion stage, and two DC/DC power converters, interconnected with the micro-grid through a DC bus. The power architecture provides on one hand the charging operations of electric two-wheeler battery packs and on the other hand the integration of the micro-grid with an ES buffer, which has the main function of supporting the main grid while an electric vehicle is on charge. The performance of the considered architecture is characterized and analyzed in different operative conditions, through a specific management of the energy fluxes. The laboratory tests evaluate efficiency, charging times and impact on the main grid, with specific reference to the DC charging operations of electric scooters. The obtained experimental results show the advantages of adopting the DC buffer architecture, compared with an AC commercial battery charger, considered as reference in this work. Finally, the numerical data of this paper, related to the single power components and to the experimental results evaluated for the whole demonstrator, effectively support the lack of knowledge in the literature about charging stations for EVs. In fact, these pieces of information, based on experimental tests, are expected to support the building of simulation models and the identification of the best energy management and control strategies to be adopted in a smart-grid scenario, characterized by distributed generation systems including renewable energy sources.

Electric Vehicle Charging Station Using a Neutral Point Clamped Converter With Bipolar DC Bus

This paper proposes a novel architecture for plug-in electric vehicles (PEVs) dc charging station at the megawatt level, through the use of a grid-tied neutral point clamped (NPC) converter. The proposed bipolar dc structure reduces the step-down effort on the dc-dc fast chargers. In addition, this paper proposes a balancing mechanism that allows handling any difference on the dc loads while keeping the midpoint voltage accurately regulated. By formally defining the unbalance operation limit, the proposed control scheme is able to provide complementary balancing capabilities by the use of an additional NPC leg acting as a bidirectional dc-dc stage, simulating the minimal load condition and allowing the modulator to keep the control on the dc voltages under any load scenario. The proposed solution enables fast charging for PEVs concentrating several charging units into a central grid-tied converter. In this paper, simulation and experimental results are presented to validate the proposed charging station architecture.

A Technique to Estimate the Equivalent Loss Resistance of Grid-Tied Converters for Current Control Analysis and Design

Rigorous analysis and design of the current control loop in voltage source converters (VSCs) requires an accurate modeling. The loop behavior can be significantly influenced by the VSC working conditions. To consider such effect, converter losses should be included in the model, which can be done by means of an equivalent series resistance. This paper proposes a method to identify the VSC equivalent loss resistance for the proper tuning of the current control loop. It is based on analysis of the closed-loop transient response provided by a synchronous proportional-integral current controller, according to the internal model principle. The method gives a set of loss resistance values linked to working conditions, which can be used to improve the tuning of the current controllers, either by online adaptation of the controller gains or by open-loop adaptive adjustment of them according to prestored data. The developed identification procedure is tested in the laboratory at different specifications of power level and switching frequency.

DC Microgrid Operational Method for Enhanced Service Reliability Using DC Bus Signaling

This paper proposes a DC microgrid operational strategy and control method for improved service reliability. The objective is to supply power to as many non-critical loads as possible, while providing an uninterrupted power supply to critical loads. The DC bus signaling method, in which DC voltage is an information carrier, is employed to implement the operational strategy in a decentralized manner. During grid-connected operation, a grid-tied converter balances the power of the microgrid by controlling the DC voltage. All loads are connected to the microgrid, and operate normally. During islanded operation, distributed generators (DGs), a backup generator, or an energy storage system balances the power. However, some non-critical loads may be disconnected from the microgrid to ensure the uninterrupted power supply to critical loads. For enhanced service reliability, disconnected loads can be automatically reconnected if certain conditions are satisfied. Control rules are proposed for all devices, and detailed microgrid operational modes and transition conditions are then discussed. Additionally, methods to determine control parameter settings are proposed. PSCAD/EMTDC simulation results demonstrate the performance and effectiveness of the proposed operational strategy and control method.

A Self-commissioning Notch Filter for Active Damping in a Three-Phase LCL -Filter-Based Grid-Tie Converter

LCL-filters are used to mitigate the harmonic current content in grid converters. The LCL-filter resonance must be damped in order to avoid stability problems in the current control. Active damping avoids resistors at the expense of increased control complexity. Large grid impedance variations can challenge the LCL-filter stability. Active damping by using a notch filter on the reference voltage for the modulator is simple to implement and does not require additional sensors. With the notch frequency tuned for the resonant frequency the voltage reference does not contain any component susceptible of exciting the LCL-filter. However, the notch filter tuning requires considerable design effort and the variations in the resonance frequency limit the LCL-filter robustness. This paper proposes a simple tuning procedure for the notch filter that results in proper robustness. In order to cope with the grid inductance variations it is proposed to estimate the resonance frequency by means of Fourier analysis. The Goertzel algorithm, instead of the FFT, is used to reduce the calculation and memory requirements. Thus, the proposed self-commissioning notch filter results robust and consumes little computational resources. Finally, the analysis is validated with both simulation and experiments.

Robust Predictive Control of Grid-Tied Converters Based on Direct Power Control

This study focuses on the control of instantaneous complex power of a grid-connected voltage-source inverter (VSI). In this study, a new space-vector-modulation-based direct power control approach is proposed: the robust predictive direct power control (RP-DPC). The proposed predictive control algorithm ensures that both, instantaneous real and imaginary powers, track the reference with high speed and accuracy reducing steady-state errors. In order to reduce the total harmonic distortion (THD) in the output currents, a fundamental frequency positive sequence detector is used in conjunction with a prediction grid voltage observer. Comparative simulations and experimental results of a $10$ -kW three-phase grid-connected VSI showing the steady-state and transient performance of the proposed RP-DPC are given. A low THD and balanced output currents are maintained even under severe voltage unbalance conditions.

A New $LCL$-Filter With In-Series Parallel Resonant Circuit for Single-Phase Grid-Tied Inverter

When designing a higher order output power filter for a transformerless grid-tied converter using pulsewidth modulation (PWM), the requirements on the leakage current, the electromagnetic interference (EMI) noise, and the harmonics of the grid-injected current should be addressed. For an LCL-filter-based single-phase grid-tied full-bridge inverter system, it is possible to decrease the total inductance as well as the size and the cost, if the harmonic currents around the switching frequency can be fully suppressed. In this paper, the grid-injected current harmonics and the conducted EMI noise are investigated for the conventional LCL-filter-based system. Based on this, a modified LCL-filter topology using an extra parallel LrCr resonant circuit is proposed to reduce the total inductance value, without increasing the capacitive reactive power. The validity is verified through the experiments on a 500-W 110-V/50-Hz prototype.

Control Scheme for a Single-Phase Grid-Tied Voltage Source Converter With Reduced Number of Sensors

This paper proposes a grid voltage sensorless controller for single-phase voltage source converters. The controller is composed of an observer and a current control scheme. The proposed observer replaces the grid voltage measurement and is used to generate the current references and to compensate the perturbation effect of the actual grid voltage on the coupling inductor. This allows us to implement a simple proportional resonant current control algorithm. The proposed observer also estimates the grid frequency, allowing us to immunize the controller to grid frequency variations.

Coupled inductor filter with frequency notching characteristics for grid interface

A novel design of a power grid interface using a coupled inductor filter with a frequency notching characteristic is presented in this paper. Traditional grid-tie power converters are connected to the supply grid via an L filter, an LC filter or an LCL filter, respectively, to reduce the harmonics caused by the switching. The efficiency of the grid interface can be improved by reducing the switching frequency and the size of the filtering inductor. To reduce the size of the filtering inductor and retaining the similar attenuation characteristics for the switching ripples, a coupled inductor filter is used to replace the conventional filter. To illustrate the capability of the proposed approach, the design of a coupled inductor notch filter to attenuate the switching ripples is provided. A simple controller has also been derived for the grid interface to act as an active power filter such that a power factor correction and a harmonics reduction can both be achieved. The proposed coupled inductor filter could be applied to any applications requiring a power grid interface. The experimental results are included to demonstrate the effectiveness of the proposed filter.

A Modified LLCL Filter With the Reduced Conducted EMI Noise

For a transformerless grid-tied converter using pulse width modulation, the harmonics of grid-injected current, the leakage current, and the electromagnetic interference (EMI) noise are three important issues during designing of the output filter. In this paper, the common mode and the differential mode EMI noises are investigated for the LCL- and LLCL-filter-based single-phase full-bridge grid-tied inverters. Based on this, a modified LLCL-filter topology is proposed to provide enough attenuation on the conducted EMI noise as well as to reduce the dc-side leakage current. The parameter design method of the filter is also developed. The comparative analysis and discussion on four filter cases (the conventional LCL filter, the conventional LLCL filter, the modified LCL filter, and the modified LLCL filter) are carried out and verified through simulations and experiments on a 0.5-kW, 110 V/50 Hz single-phase full-bridge grid-tied inverter prototype.

High-Efficiency Grid-Tied Power Converter for Battery Energy Storage

This letter proposes a new grid-tied power converter for battery energy storage. The operation of the proposed system was verified through experimental works with a 9- kW hardware prototype. The proposed system can have relatively higher efficiency and smaller size than the existing system.

Power quality investigation of a solar PV transformer-less grid-connected system fed DVR

This paper presents a single stage transformer-less grid-connected solar photovoltaic (PV) system with an active and reactive power control. In the absence of active input power, the grid-tied voltage source converter (VSC) is operated in a reactive power generation mode, which powers the control circuitry, and maintains a regulated DC voltage to the VSC. A data-based maximum power point tracking (MPPT) control scheme which performs power quality control at a maximum power by reducing the total harmonic distortion (THD) in grid injected current as per IEEE-519/1547 standards is implemented. A proportional-integral (PI) controller based dynamic voltage restorer (DVR) control scheme is implemented which controls the grid side converter during single-phase to ground fault. The analysis includes the grid current THD along with the corresponding variation of the active and reactive power during the fault condition. The MPPT tracks the actual variable DC link voltage while deriving the maximum power from the solar PV array, and maintains the DC link voltage constant by changing the modulation index of the VSC. Simulation results using Matlab/Simulink are presented to demonstrate the feasibility and validations of the proposed novel MPPT and DVR control systems under different environmental conditions.

A Three-Level Common-Mode Voltage Eliminated Inverter With Single DC Supply Using Flying Capacitor Inverter and Cascaded H-Bridge

A three-level common-mode voltage eliminated inverter with single dc supply using flying capacitor inverter and cascaded H-bridge has been proposed in this paper. The three phase space vector polygon formed by this configuration and the polygon formed by the common-mode eliminated states have been discussed. The entire system is simulated in Simulink and the results are experimentally verified. This system has an advantage that if one of devices in the H-bridge fails, the system can still be operated as a normal three-level inverter at full power. This inverter has many other advantages like use of single dc supply, making it possible for a back-to-back grid-tied converter application, improved reliability, etc.

Power quality improvement of solar photovoltaic transformer-less grid-connected system with maximum power point tracking control

This paper presents a transformer-less single-stage grid-connected solar photovoltaic (PV) system with active reactive power control. In the absence of active input power, grid-tied voltage source converter (VSC) is operated in the reactive power generation mode, which powers control circuitry and maintains regulated DC voltage. Control scheme has been implemented so that the grid-connected converter continuously serves local load. A data-based maximum power point tracking (MPPT) has been implemented at maximum power which performs power quality control by reducing total harmonic distortion (THD) in grid-injected current under varying environmental conditions. Standards (IEEE-519/1547) stipulates that current with THD greater than 5% cannot be injected into the grid by any distributed generation (DG) source. MPPT tracks actual variable DC link voltage while deriving maximum power from PV array and maintains DC link voltage constant by changing the converter modulation index. Simulation results with the PV model and MPPT technique validations demonstrate effectiveness of the proposed system.