DC bus voltage equalization in single-phase split-capacitor three-level neutral-point-clamped half-bridge inverters for PV applications

This paper proposes a new valve performance test circuit for MMC based HVDC power station. It is necessary for the valve test circuit to satisfy the standard in IEC-62501. The standard requires the valve current resembles the actual arm current of the HVDC system in operation. The arm current contains fundamental ac component, the second harmonic component, and dc component. Conventional valve test circuits provide only fundamental ac component, therefore, they are not suitable for the valve test with actual arm current. Furthermore, the voltage magnitude of the power source for the valve test should be equal to the total sum of the submodule voltages consisting the valve. The proposed test circuit provides not only fundamental ac component but also the second harmonic and dc components, and the voltage magnitude of the power source is considerably reduced compared to that of the conventional methods. Simulation and experimental results show the usefulness of the proposed scheme.

This paper proposed a control method for output and circulating currents of modular multilevel converter (MMC). The output and circulating current are controlled with the help of arm currents, which contain DC, fundamental frequency, and double frequency components. The arm current is transformed into a stationary reference frame (SRF) to isolate the DC and AC components. The AC component is controlled with a conventional proportional resonant (PR) controller, while the DC component is controlled by a proportional controller. The effective control of the upper arm and lower arm ultimately controls the output current so that it delivers the required power to the grid and circulating current in such a way that the second harmonic component is completely vanished leaving behind only the DC component. Comparative results of leg-level control based on PR controller are included in the paper to show the effectiveness of the proposed control scheme. A three-phase, five-level MMC is developed in MATLAB/Simulink to verify the effectiveness of the proposed control method.

This paper proposes a novel scheme for the current controller for the grid-side converter (GSC) of permanent-magnet synchronous generator (PMSG) wind turbines to eliminate the high-order harmonics in the grid currents under grid voltage disturbances. The voltage unbalance and harmonics in three-phase systems cause grid current distortions. In order to mitigate the input current distortions, multi-loop current controllers are applied, where the positive-sequence component is regulated by proportional-integral (PI) controllers, and the negative-sequence and high-order harmonic components are regulated by proportional-resonance (PR) controllers. For extracting the positive/negative-sequence and harmonic components of the grid voltages and currents without a phase delay or magnitude reduction, composite observers are applied, which give faster and more precise estimation results. In addition, an active damping method using PR controllers to damp the grid current component of the resonant frequency is employed to improve the operating stability of VSCs with inductor-capacitor-inductor (LCL) filters. The validity of the proposed method is verified by simulation and experimental results.

Grid integration improvement for single-phase inverters of small wind turbines under distorted voltage conditions

This study presents model development and validation of the photovoltaic (PV) power using the real test data. The major contributions of this research are in two‐fold: First, the western electricity coordinating council (WECC) PV power plant model is discussed through comparison with tested data from one commercial PV inverter in China. The WECC PV model is modified and the improvement can provide more holistic view on the PV power plant dynamics. Second, the hybrid data simulation method is applied to validate the validity of the generic PV power model based on the real‐test data. The model simulation outputs are compared with the real measurements under typical scenarios. The research results presented in the study propose the application range for the generic PV power model for the electromechanic related stability analysis. Furthermore, the study also proposes the model improvement suggestions to increase the model credibility in simulating large system stability dynamics.

A modified direct power control (DPC) strategy for three-level neutral point clamped (NPC) converters under unbalanced and harmonically distorted grid voltage conditions is proposed in the paper. The modified (DPC) strategy is implemented in the stationary reference frame without the necessity of either the space vector modulation or the synchronous speed d-q transformation. A compensation method is adopted in the conventional DPC to eliminate the negative and harmonic sequence components of grid current for the purpose of obtaining the symmetrical and sinusoidal grid current. Finally, simulation results based on matlab validate the availability of the proposed DPC strategy.

A novel method is proposed to compensate the three-phase unbalanced currents of power grid under three-phase unbalanced load for a two-stage photovoltaic (PV) power system without the augmentation of active power filter (APF). The PV power system is composed of an interleaved DC/DC converter and a three-level neutral-point clamped (NPC) inverter. Moreover, in the proposed method, dq0-axis compensation currents are obtained through low pass filters (LPFs) to compensate the three-phase unbalanced currents of power grid. Furthermore, to improve the control performance of the DC bus voltage of the PV power system under unbalanced load variation condition, an online trained compensatory neural fuzzy network with an asymmetric membership function (CFNN-AMF) is proposed to replace the traditional proportional-integral (PI) controller for the DC bus voltage control. In the proposed CFNN-AMF, the compensatory parameter to integrate pessimistic and optimistic operations of fuzzy systems is embedded in the CFNN. In addition, the dimensions of the Gaussian membership functions are directly extended to AMFs. Additionally, the proposed controllers of the PV power system are implemented by two control platforms using floating-point digital signal processor (DSP). Finally, excellent compensation performance for the three-phase currents of power grid under three-phase unbalanced load can be achieved from the experimental results.

Power demand is rising worldwide, forcing the search for an alternative source to solve the imminent power crisis, which is expected to be accomplished through the deployment of grid synchronized electric vehicles (EV). Wind and solar energy are incorporated into this integration to those concerns. One smart grid innovation that permits energy exchange between the EV and the grid is vehicle to grid (V2G) technology. For the purpose of charging EVs, this research proposes employing hybrid energy. Here, the neighboring household’s linear and non linear demands are powered by the excess electricity from the photovoltaics (PV) panel, wind turbine, and AC generator that is utilized to charge the EV battery. Because of its dependability and independence from the real system model, a proportional integral (PI) controller is used to maximize the DC voltage extracted from the PV panel. The improved DC voltage overall is a result of the SEPIC converter’s improved DC output. Concurrently, an AC generator is incorporated into the system to supply extra power to the grid. An artificial neural network (ANN) controller is used to convert the power generated by the PV panel’s Sepic conversion process into an improved output that is then converted to AC using a 3 phase Voltage Source Inverter (VSI). Harmonic components in the VSI output are corrected with an LC filter to ensure grid compatibility and safe current injection into the grid. This paper discusses the usage of a bidirectional converter to adjust the EV battery’s voltage. MATLAB 2021a / Simulink software is used to simulate and validate the proposed approach.

A direct-display plasma density and temperature meter was developed using a floating double-probe method. A small ac voltage was superposed on the dc voltage, and the third harmonic component of ac current was detected. The phase reversal of the third harmonic component was observed at Vdc = V* = 1.30 kTe, and an experiment was carried out to fix the dc voltage automatically at this voltage V* detecting the phase reversal of the third harmonic component. The plasma density and electron temperature were obtained directly from the dc voltage V* and the dc current corresponding to the voltage V*.

Corona-generated audible noise is induced by the collisions between space charges and air molecules. It has been proven that there is a close correlation between audible noise and corona current from DC corona discharge. Analysis on the correlation between audible noise and corona current can promote the cognition of the generation mechanism of corona discharge. In this paper, time-domain waveforms of AC corona-generated audible noise and corona current are measured simultaneously. The one-to-one relationship between sound pressure pulses and corona current pulses can be found and is used to remove the interferences from background noise. After the interferences are removed, the linear correlated relationships between sound pressure pulse amplitude and corona current pulse amplitude are obtained through statistical analysis. Besides, frequency components at the harmonics of power frequency (50 Hz) can be found both in the frequency spectrums of audible noise and corona current through frequency analysis. Furthermore, the self-correlation relationships between harmonic components below 400 Hz with the 50 Hz component are analyzed for audible noise and corona current and corresponding empirical formulas are proposed to calculate the harmonic components based on the 50 Hz component. Finally, based on the AC corona discharge process and generation mechanism of audible noise and corona current, the correlation between audible noise and corona current in time domain and frequency domain are interpreted qualitatively. Besides, with the aid of analytical expressions of periodic square waves, sound pressure pulses, and corona current pulses, the modulation effects from the AC voltage on the pulse trains are used to interpret the generation of the harmonic components of audible noise and corona current.

In a weak grid environment, at renewable energy integration, the harmonics and unbalance issues are very frequent due to nonlinear loads. In this regard, achieving unbalance compensation involves two control targets, which are harmonics compensation and then unbalance compensation. Under nonlinear loads, in a weak grid environment, voltage compensation or current compensation alone not sufficient to improve power quality. To overcome the aforementioned drawback, a flexible compensation strategy is proposed for a current-controlled inverter connected to an unbalanced-distorted weak grid under nonlinear load conditions. The proposed compensation strategy uses harmonic components of nonlinear load current, and harmonic components of voltage are the feedback variables. This compensation strategy performs both voltage and nonlinear current compensation simultaneously with seamless interaction between voltage harmonic components and current harmonics components. The proposed compensation strategy achieves unbalance compensation and harmonics compensation simultaneously in a weak grid environment without stability issues. Further, to improve the performance of the proposed compensation strategy, individual phase compensation of the current controllers is included, and the effectiveness and superiority of the proposed strategy are validated through various test cases.

In this work, a mathematical model of a three-phase nonlinear transformer is suggested. The model enables simulating the transformer operation with allowance for its nonlinearity and covers needs of the relay protection. Our model has been developed on the basis of a mathematical model with phase coordinates, where differential equations are composed by the Kirchhoff’s phase-voltage law. Based on this model, we first compose a mathematical model for simulating steady-state operation modes of a transformer, taking into account the asymmetry and nonlinearity of its ferromagnetic core. In this model, the initial values of inductances and mutual inductances of loops are determined from the main phase inductance calculated by the experimentally found no-load current, and their current values are determined from the currents in windings and the magnetic fluxes in legs of the transformer core. The magnetic fluxes are calculated by the nodal-pair method. This improved mathematical model is verified through a comparison between the calculated harmonic components of the phase currents and the experimental results. The harmonic components are calculated with the use of Fourier expansion of the calculated phase currents. Their experimental values are determined with a spectrum analyzer. The calculated and experimental harmonic components of the currents of phase A during no-load and rated-load operation of the transformer are tabulated. The comparison of these results shows that the mathematical model of a three-phase transformer we suggest makes it possible to simulate currents in transformer windings under steady-state operation modes with accuracy acceptable for relay protection.

This paper proposes a novel control algorithm to enhance the fault ride-through (FRT) capability of a photovoltaic (PV) system. In this method, the overcurrent of the grid-tied inverter is suppressed to a preset value using model current predictive control (MCPC) algorithm, and its DC-link overvoltage is removed using a non-maximum power point tracker (non-MPPT) algorithm. Therefore, the inverter overcurrent and its DC-link overvoltage problems can be placed under decoupling control using a DC/AC converter controller and DC/DC converter controller, respectively. Using eight switching modes for the three-phase inverter, the MCPC of DC/AC converter controller establishes a value function between the current reference value and output current value. Then, by introducing the switching mode (corresponding to the minimum value function) into the DC/AC converter controller, if the current reference is set at the rated value, the inverter’s output current can be inhibited to the rated value under fault conditions. Moreover, a balanced three-phase rated current can always be obtained under either a symmetric or asymmetric fault. However, together with the MCPC, a DC-link overvoltage will appear. To remove this DC-link overvoltage quickly, based on the grid system voltage sag level, the non-MPPT algorithm is used to calculate the adjusted power for PV arrays. Then, based on the calculated power, the amended duty cycle can be obtained and immediately introduced into the DC/DC converter controller to tune the PV arrays’ output power, which significantly decreases the power imbalance between the inverter’s AC and DC sides, thus inhibiting the DC-link overvoltage. In addition, to inhibit DC-link voltage fluctuations further, DC-link voltage feedforward compensation is introduced into the DC/DC converter controller. In particular, under an asymmetric fault condition, with the help of the voltage feedforward compensation, the second harmonic frequency components of the DC-link voltage can also be removed. Finally, based on the theoretical derivation and simulation results, it can be proved that the major problems troubling a PV system’s FRT technologies can be resolved by the proposed method, especially the problem related to the harmonic components of the DC-link voltage.

Introduction: The wide application of photovoltaic (PV) generation has practical significance for reducing fossil energy dependence and environmental pollution. To smooth the fluctuation of PV’s output power and eliminate the coupled double-line frequency ripple in two-stage grid-connected PV systems, it is indispensable to find an effective solution. This study aims to propose a control scheme based on supercapacitor energy storage and extended state optimal tracking control method to solve the problem of disturbance and oscillation fluctuations in the DC-bus voltage in the two-stage distributed grid-connected PV systems. Methods: Firstly, the dynamic model of the SC subsystem is established. Secondly, a mathematical model including all the unstable states of reference input, step, and double line frequency disturbance signals is built. Then, the extended state space model is constructed based on these two models. Furthermore, the optimal state feedback controller is designed by using the optimal tracking strategy. Results: In the case of the DC-bus voltage control test, when the PV output power fluctuates, the SC makes a rapid response and compensates for the power deficiency. After such a dynamic adjustment process, the DC-bus voltage stabilizes at the reference value of 400 V, which achieves the accurate regulation of the DC-bus voltage. The maximum absolute error is 1 V, the maximum relative error is 0.25%, the maximum overshoot is 0.75%, and the settling time is less than 10 ms. In the case of the gird-connected operation test, the dynamic responses of the DC-bus voltage and its error also demonstrate the excellent performance of accurate tracking and suppression of multiple fluctuations. Accordingly, the MPPT control functions well. For instance, taking 1.5 s, the PV output power drops suddenly, and the active power demand increases at the same time, which makes the power difference reach the maximum. Then, the SC promptly increases its output current to provide power support. At this time point, the maximum overshoot of the voltage tracking is 4.5%, and the maximum absolute error is less than 1 V. Discussion: This study provides a novel idea and approach to address the problem of suppressing the multiple fluctuations coupled to the DC-bus voltage in the two-stage grid-connected PV system via the single-phase DC/AC inverter. However, the results are limited to simulations. Hence, based on the work of this article, further studies should be conducted in applying the method in real operation systems and adapting it to deal with the multiple fluctuations suppression of the grid-- connected PV systems under asymmetrical grid conditions. Conclusion: The proposed control scheme can not only effectively restrain the fluctuations in the PV output power and load demand but also suppress the double line frequency ripple so that the DC-bus voltage can always be accurately regulated to its reference value, which also guarantees the reliable realization of the PV’s MPPT control. Additionally, the dynamic and static performances of the system, such as overshoot, rapidity, and tracking error, are satisfactory

Selecting inter-phase circulation, up and lower bridge arm current, and AC output voltage wave of the inverter as research objects, this paper analysed harmonic components in Modular Multilevel Converter (MMC) in Nearest Level Modulation (NLM) when MMC is used as inverter. With theoretical deduction, this paper points out that the up and lower bridge arm current mainly contains DC current component, fundamental component and second harmonic component, inter-phase circulation contains only even harmonic, AC output voltage wave contains only odd harmonic components. Finally, the derivation results are simulated and verified by 11-level PWM converter model (See Fig.1 and Fig.2) based on Matlab/Simulink.