Two-stage Photovoltaic Inverter Research Articles

Analysis and Design of Zero-Voltage-Switching Multiphase AC/DC Converters

In this article, the generic analysis and design of zero-voltage-switching (ZVS) multiphase ac/dc converters are proposed. With the edge aligned pulsewidth modulation (PWM) (EA-PWM) scheme, all switches of ZVS multiphase ac/dc converters can realize ZVS operation. Besides, the switching frequency is fixed and the auxiliary switch only operates once in each switching period. Its operation stages in a switching cycle are analyzed and a generic ZVS condition is derived. Based on the proposed theory, a ZVS two-stage three-phase photovoltaic (PV) inverter is investigated, which is regarded as a five-phase converter. Its ZVS condition under different working conditions is discussed. Finally, a 10-kW prototype with 150 kHz switching frequency of the ZVS two-stage three-phase PV inverter is built and the experimental results are given to verify the analysis. In addition, the extension of the ZVS multiphase converter in different applications is introduced.

Voltage Control and Power-Shortage Mode Switch of PV Inverter in the Islanded Microgrid Without Other Energy Sources

The growing installation capacity of distributed photovoltaic inverter needs change of the operation method in the modern power grid. Recently, to better integrate into the grid, the active power control is investigated in photovoltaic inverter. Meanwhile, without energy storage, the photovoltaic inverter can use voltage control method to further support the grid voltage and frequency. Due to the stochastic photovoltaic power, the mode switch is needed when the maximum power still cannot meet the grid demands. The operation of PV inverter in this power shortage mode has not been sufficiently studied. Therefore, this paper investigates the mode switch method for voltage-controlled two-stage photovoltaic inverter. The power conversion principle of synchronous generator is realized in the inverter and the dc/dc converter maintains the dc-side voltage. The mode switch method between normal operation and power-shortage state is proposed. With this method, the PV inverter can real-time adjust the output power of photovoltaic battery and match the load demand without support from other energy source even under PV output power's intense change.

DC/DC Stage Contribution to Bus Voltage in 1000- and 1500-V Grid-Connected Solar Inverters

Modern architectures of transformerless, three-phase-grid-connected photovoltaic (PV) inverter for 1000- and 1500-V commercial/residential applications are analyzed and compared from the point of view of the energy harvested during one year period, system efficiency, and power density. Scenarios for different ac grid voltage levels and inverters topologies are analyzed based on one- and two-stage solutions. The architectures with and without included dc/dc stage are compared from the point of view of the optimized power density. In the case of the two-stage PV inverter architecture, the mini-boost converter concept, presented in previous publications, is extended and the standard boost topology that employs 1200-V rated IGBTs and diodes is replaced by a multilevel, hybrid power converter topology that exhibits high efficiency due to the partial power processing and new classes of 900- and 650-V SiC and GaN devices employed. The inverter’s output filter design space (DS) has been researched from the point of view of the dc bus voltage variation. The cases are analyzed for fully controlled bus voltage at its minimum value and when allowed to vary in the full MPP range. Optimization results of the output filter DSs for the minimum and maximum operating dc bus voltage are compared in terms of the occupied volume in order to investigate whether it is possible to obtain more compact inverter stage by better control of dc bus voltage. The results show that in the case of controlled low dc bus voltage level, it is feasible to design a filter that occupies up to 50% less volume than the filter designed for the full MPP voltage variation. Even more, this can be achieved by maintaining the system efficiency. The simulation and experimental results complement the analysis of the 1000- and 1500-V grid-connected PV inverter systems.

Impedance characteristics investigation and oscillation stability analysis for two-stage PV inverter under weak grid condition

In order to obtain impedance characteristics of the photovoltaic (PV) inverter and reveal potential stability issues of the PV inverter connected to a weak grid, a complete impedance model of the two-stage PV inverter is established in this paper. The model includes the PV arrays, front-end Boost converter, and rear-end inverter with output LCL filter. The impedance modeling of the PV inverter is derived at different interfaces of DC-link and point of common coupling (PCC), where the DC impedance characteristic at DC-link and d-q axis impedance feature at AC side are both investigated. It is worth noting that the DC output impedance of front-end circuit behaves as a first-order low-pass filter, and it is mainly influenced by the DC-link capacitor. Moreover, a significant feature is found that a reduced integral gain in the outer voltage loop may cause a negative resistance and capacitive impedance in d-axis impedance component. In addition, based on the impedance analysis method, the oscillation stability issues of the grid-connected PV inverter in case of a weak grid are evaluated by different divisions of source-load subsystems. It is demonstrated that the increase of the PLL bandwidth and the decrease of the integral gain of the outer voltage loop will both lead to the oscillation of the PV inverter. Moreover, the increase of the output capacity will deteriorate the stability margin, resulting in the high frequency oscillation of the system. Finally, the impedance model and the stability analysis are verified by the simulation results implemented in PSCAD/EMTDC platform.

Reliability-based trade-off analysis of reactive power capability in PV inverters under different sizing ratio

Due to the intermittent characteristic of solar irradiance, photovoltaic (PV) inverters usually operate below rated power conditions. In this scenario, commercial PV inverters can be used to provide ancillary services, such as reactive power compensation. On the other hand, over-sizing PV inverters by increasing the number of installed PV panels can increase the generated energy during low irradiance conditions. Despite the advantages, both approaches can increase the PV inverter wear-out, leading to early failures. The trade-off between reactive power compensation and lifetime consumption under different inverter sizing ratios (ISR) was not previously addressed in the literature. Hence, this paper proposes to evaluate the system-level reliability of a single-phase two-stage PV inverter performing reactive power compensation. The analysis is carried out from three different mission profiles (Aalborg, Goiânia, and Izaña). The thermal loading on the inverter components is translated to lifetime prediction. The results from Goiânia show that the PV inverter lifetime is reduced from 22.6 to 6.2 years when compensating reactive power. In addition, this reduction is observed in all three regions, going from 40.8 to 12.4 years and 10 to 4.4 years, for Aalborg and Izaña, respectively. To improve the lifetime, a reactive power capability factor QR is applied to the maximum reactive power condition. The unreliability map is presented as a tool to match the inverter sizing ratio with reactive power capability in agreement with lifetime requirements. An economic analysis based on Levelized Cost of Electricity (LCOE) showed that the reactive power compensation increases the costs of the system installed in Goiânia with ISR = 100 % by 46.6 % in comparison with the conventional operation. These additional costs can be reduced if the reactive power is partially compensated.

An Improved Lyapunov Function Based Control Strategy for Multitasking Distributed PV Inverter

The ELF (Extended Lyapunov Function) based control approach for single phase two-stage multitasking PV (photovoltaic) inverter is demonstrated in this paper. The introduced PV inverter can operate under the uncertainty of non-linear loads and PV characteristic beyond giving the closed feedback system stability analysis. The two stage PV inverter is introduced, in initial step a boost circuit is used to harnessing of crest power at PV panel side. Here time ratio of the boost circuit is planed with adaptive INC (Incremental Conductance Method). In the secondary step, the control of grid connected VSI (Voltage Source Inverter) is managed by ELF (Extended Lyapunov Function) strategy. To get best dynamic response, difference of PV feed forward to the loss component of DC bus capacitor. Other side, control of VSI and maintain stability of PV inverter, the ELF technique is reduces error in-between input and output, with there is no disturbance of equilibrium point in PV inverter. The PV inverter performance observed at changing of insolation and load variations using MATLAB /simulation. Various simulation results are presented to validate the harmonics compensation strategy.

Simulation Models For Three-Phase Grid Connected PV Inverters Enabling Current Limitation Under Unbalanced Faults

Normally unbalanced grid voltage dips may lead to unbalanced non-sinusoidal current injections, DC-link voltage oscillations, and active and/or reactive power oscillations with twice the grid fundamental frequency in three-phase grid-connected Photovoltaic (PV) systems. Double grid frequency oscillations at the DC-link of the conventional two-stage PV inverters can further deteriorate the DC-link capacitor, which is one of the most important limiting components in the system. Proper control of these converters may efficiently address this problem. In such solutions, Current Reference Calculation (CRC) is one of the most important issues that should be coped with the reliable operation of grid-connected converters under unbalanced grid faults. Therefore, this paper proposes and simulates CRC methods and presents the results in order to improve the quality of the grid-connected PV system under unbalanced grid voltage fault.

A Single-Stage Buck-Boost Three-Level Neutral-Point-Clamped Inverter with Two Input Sources for the Grid-Tied Photovoltaic Power Generation

This paper proposes a novel single-stage buck-boost three-Level neutral-point-clamped (NPC) inverter with two independent dc sources coupled for the grid-tied photovoltaic (PV) application, which can effectively solve the unbalanced operational conditions generally appearing between two independent PV sources. The proposed control scheme can simultaneously guarantee the maximum power point (MPP) operation of both PV sources and maintain the output waveform quality. Compared to the traditional two-stage PV inverter, the proposed NPC inverter could reduce the PV array voltage requirement and the voltage rating of dc-link capacitors; also it shows advantages in operational efficiency. MATLAB simulations and experimental results are presented to examine the performance of the proposed three-level NPC inverter.

Virtual Inertia-Based Control Strategy of Two-Stage Photovoltaic Inverters for Frequency Support in Islanded Micro-Grid

For an islanded micro-grid with a high penetration of photovoltaic (PV) power generators, the low inertia reserve and the maximum peak power tracking control may increase the difficulty of maintaining the system’s supply–demand balance, and cause frequency instability, especially when the available generation is excessive. This will require changes in the way the PV inverter is controlled. In this paper, a virtual inertia frequency control (VIFC) strategy is proposed to let the two-stage PV inverters emulate inertia and support the system frequency with a timely response (e.g., inertia response), and the required power for inertia emulation is obtained from both the DC-link capacitor and the PV reserved energy. As the rate of the system frequency change can be reduced with the inertia increase, the proposed method can mitigate the frequency contingency event before the superior-level coordination control is enabled for the frequency restoration. The simulation results demonstrate the effectiveness of the proposed method.

A Voltage-Independent Islanding Detection Method and Low-Voltage Ride Through of a Two-Stage PV Inverter

This paper presents an islanding detection method for a two-stage photovoltaic (PV) inverter. The novelty in this method is that it does not rely on instantaneous value of the grid voltage for identifying the islanded condition. Instead, the islanding condition is detected by the saturation of the proportional integral (PI) controller of the outer voltage control loop. This makes the proposed detection algorithm more reliable compared with other voltage-based detection algorithms, as it is immune to malfunctions caused by the sudden load changes or other transients. In addition, a simple but accurate implementation strategy for low-voltage ride through (LVRT) operation is also proposed and integrated with the anti-islanding operation. Both these methods are placed inside the overall control structure that can perform seamless transition between grid-connected and standalone modes of operation. The simulation results demonstrate the effectiveness of the proposed anti-islanding and LVRT techniques. Finally, hardware results on a 2 KW laboratory prototype are presented for experimental verification of the proposed anti-islanding and LVRT schemes.

Adaptive DC-Link Voltage Control of Two-Stage Photovoltaic Inverter During Low Voltage Ride-Through Operation

This paper proposes an adaptive dc-link voltage control method for the two-stage photovoltaic inverter during the low voltage ride-through (LVRT) operation period. The dc-link voltage will be controlled to follow the change of grid voltage during the LVRT operation to maintain the high modulation ratio so that the high frequency harmonics injected into the grid can be attenuated significantly. Besides, when suffering the asymmetrical grid faults, the proposed control method could to some extent attenuate the double-line-frequency dc-link voltage ripple to keep the dc-link voltage in the safe operational range by shifting the double-line-frequency power ripple to the front-end dc input source, which can be achieved by intentionally fluctuating the dc input power or employing a bidirectional dc-dc converter depending on the voltage drop ratio and the input power level. The theoretical findings were verified by MATLAB simulations and the constructed experimental prototype.

Closed-Form Solution of Time-Varying Model and Its Applications for Output Current Harmonics in Two-Stage PV Inverter

The single-phase photovoltaic (PV) inverter needs significant capacitance to buffer the double-line frequency power pulsation at ac port. The two-stage inverter allows the designer to choose the dc-link voltage and the capacitor size flexibly. With the reduced capacitance, the lifetime of the dc-link capacitor can be prolonged by replacing the electrolytic capacitors with film capacitors. However, the capacitance deduction results in high double-line frequency voltage ripple on dc-link, which increases a series of odd harmonics in the output current. This paper hence analyzes the harmonics caused by the voltage ripple in an inverter with feedback control. The inverter is modeled as a time-varying system by considering the dc-link voltage ripple. A closed-form solution is derived to calculate the amplitude of the ripple-caused harmonics. This analysis helps the designer to understand the effect of the dc-link voltage ripple on current harmonics, evaluate effectiveness of existing approaches, and stimulate new ideas and solutions. The study also derived the theoretical limit to select dc-link capacitance and sampling rate of current reference without violating the grid-tied regulations in power quality. The analysis is verified both by simulation and experimental evaluation.