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In order to improve the robustness of fault diagnosis method based on model in the modular multilevel converter, an improved predictive model of the circulating current is established. A correction method is proposed to compensate for the uncertainties of the parameters and the measurement errors, and the disturbances are eliminated by using multi-step prediction method. The fault detection method...
To improve the reliability of modular multilevel converter (MMC), the fault submodules must be isolated rapidly. In this paper, a new fast fault diagnosis method is proposed containing two steps: fault detecting and fault locating. Fault detection step finds failures by improved predictive model which detects the fault by comparing the difference between the predictive value and the measured one of...
Current-controlled LCL-filtered inverters are usually paralleled together for interconnecting the renewable energy resources into the grid. When the inverter output impedance is not well designed, the harmonic circulating currents out of the control bandwidth may be easily aroused. To reveal this resonant phenomenon and to enhance the stability of the parallel system, this paper builds the parallel...
Silicon carbide (SiC) devices attract widespread attention of scholars because of their superior characteristics. However, the interaction between the upper and lower devices during the switching process in the bridge circuit will affect the safe operation of SiC devices. To suppress the crosstalk spurious voltage, this paper proposes a gate driver circuit based on magnetic coupling for SiC devices...
To improve the reliability of photovoltaic (PV) energy storage (PVES) system, the output voltage robust PI controller design criteria and method are proposed in this paper. First, the boost converter with an input capacitor is taken as an example, and its small signal model including the dynamic equivalent circuit of solar cell is built up. Based on this, the stable condition of the PVES system in...
The impedance-based stability criterion is usually used to evaluate the performance of a current-controlled inverter in the weak grid with considerable grid impedance at the point of common coupling (PCC). The stability of the real inverter-grid system is the same as the virtual system where the ratio of the grid impedance and the inverter output impedance is the open-loop transfer function. However,...
The model predictive control (MPC) predicts all the possible behaviors and selects an optimal switching state which minimizes the predefined cost function at the next step, and is emerging as a powerful control method in power electronics. However, for a single phase T-NPC inverter with the conventional MPC method, only three different voltage vectors can be generated, which significantly impairs...
For grid-connected inverters, the feedforward of the grid voltage at the point of common coupling (PCC) highly suppresses the grid current harmonics caused by the grid distortion; however, when the PCC grid is weak with non-negligible grid impedance, the feedforward of the PCC voltage arouses serious current harmonics or even instability. In order to maintain the stability in the weak grid case, the...
For LCL-filtered grid-connected inverters, classical control methods such as the proportional grid voltage feedforward, the harmonic quasi resonant (HQR) control etc., are widely utilized to achieve low current distortion. However, the system stability is easily challenged by the grid impedance and the increased number of the paralleled inverters. Therefore, this paper orients to guarantee the harmonic...
According to the control requirements of modular power used in energy storage systems, a parallel control strategy for bi-directional DC-DC converter is proposed and applied to the bi-directional DC-DC converter combined by current-fed half bridge and voltage-fed half bridge. While converters operate in parallel, the master-slave control based on digital communications can solve the problem of unequal...
Real-time sampling and feedback can effectively reduce the sampling delay and improve the robustness of the active damping control for grid-connected LCL-filtered inverter. However, the practicability of this approach would be limited by the aliasing problem of sampling. In order to solve the aliasing problem, a systematic analysis is done to figure out the relation between the sampling instant of...
Three-level (TL) technique can be used to reduce the voltage stress of switches to half and effectively decrease the filter's volume and weight. However, few literatures make further discussions on the construction methods for TL converters. Therefore, a simple construction method for TL DC-DC converters based on switch-diode-capacitor (SDC) network was proposed in this paper. This method was applicable...
From the perspective of output current control stability, this study investigates the stable operating area of photovoltaic (PV) cells feeding the DC - DC converter in output current regulation (OCR) mode. In this study, the Buck interface converter adding an additional input capacitor is taken as an example, and its small-signal model including the dynamic equivalent circuit of PV cells is deduced...
Grid-connected inverters with the proportional grid voltage feedforward usually achieve satisfied output impedance, which contributes to the low order harmonic elimination. However, a potential positive feedback link is introduced, which decreases the phase margin significantly or even trigger instability when inverters are connected to utility with the non-negligible grid impedance. To maintain the...
Non-ideal grid conditions include the voltage imbalance, distortion and frequency variation of three-phase grid voltage. In addition to positive-sequence components, three-phase grid contains negative-sequence, zero-sequence and harmonic components under non-ideal grid conditions. First, an oscillating signal of twice the grid frequency on the d-axis was generated by the negative sequence, which caused...
Dynamic Voltage Corrector (DVC) with short-circuit fault isolation is proposed to improve the power quality and reliability of the power grid. The device which is composed of a bidirectional AC-DC half-bridge converter and a neutral point clamped (NPC) three-level half-bridge converter is able to correct wide range voltage sag and swelling. The topology can achieve energy feedback, so the load short-circuit...
Harmonic Quasi Resonant (HQR) control is widely adopted to suppress the harmonic currents for LCL-filtered inverters in stiff grid. However, inverters with HQR control may observe instability when the point of the common coupling (PCC) behaves uncertain impedance or the HQR control frequency is out of the bandwidth of the system control loop. Besides, it could not effectively eliminate the harmonics...
The dynamic sag corrector is used at the sensitive loads terminal in order to avoid the grid voltage sag. This paper focuses on both the correction of grid voltage swell/sag in large range and the quality of compensated voltage. A wide correction range dynamic voltage corrector is proposed for solving above problems with small cost. It possesses dual functions of both the wide range voltage correction...
For grid-connected LCL-filtered inverters, recent applications often observe a weak grid at the point of common coupling (PCC) with non-negligible grid impedance. In this case, the previous control methods would perform poor or even cause the inverter protection. Especially when the feedforward of PCC voltage is designed to suppress grid current harmonics, phase and gain margins will be largely reduced...
For grid-connected inverter, an LCL-type filter is widely used to suppress switching harmonics, and the capacitor current feedback is used to suppress the LCL resonance. In the published literatures, the relation among LCL parameters, capacitor current feedback and resonance damping was discussed; however, their relation with the low-order current distortion was not seriously considered. Considering...
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