In this paper a new methodology is presented to calculate the average power quickly and accurately for the single-phase paralleled inverters intended to be applied in a droop-control microgrid system. Most existing droop control systems utilize a simple first-order filter to calculate the active and reactive power. The added filter can smooth the calculated results but it tends to hurt the system dynamic response due to a low cut-off frequency that is intended to get enough ripple attenuation. This low frequency pole in the low-pass filter could also introduce system instability under certain load conditions. A new filter calculation methodology is thus proposed to achieve better ripple attenuation with faster response by combining a low-pass filter, a delay, and summation blocks. Both simulation and experiment are used to verify the effectiveness of the proposed power calculation methodology.I.
Due to the absence of communication needs and great reliability, the droop-control technique is a great choice for controlling of inverters that are subjected to load sharing or to work in an islanded mode. On the other hand, current-controlled inverters are often used in grid-connected systems due to their fast response to power the implementation of maximum power point tracking (MPPT) algorithms to maximize the power extracted from these systems. However, the application of such algorithms in gridconnected droop-controlled systems is hampered by differences in the dynamic responses of the respective techniques. In this context, this study presents the development of a strategy that enables a push-pull converter controlled by MPPT and a low-power plug and play grid-connected inverter governed by droop control to operate stably even under variations in solar radiation. The goal is achieved based on the following two approaches: designing the dclink capacitor properly and using a control loop in order to adapt the droop curves in accordance with the available input power. Theoretical analysis and experimental results have proven the viability of the approach.
Due to the absence of communication needs and great reliability, the droop-control technique is a great choice for controlling of inverters that are subjected to load sharing or to work in an islanded mode. On the other hand, current-controlled inverters are often used in grid-connected systems due to their fast response to power the implementation of maximum power point tracking (MPPT) algorithms to maximize the power extracted from these systems. However, the application of such algorithms in gridconnected droop-controlled systems is hampered by differences in the dynamic responses of the respective techniques. In this context, this study presents the development of a strategy that enables a push-pull converter controlled by MPPT and a low-power plug and play grid-connected inverter governed by droop control to operate stably even under variations in solar radiation. The goal is achieved based on the following two approaches: designing the dclink capacitor properly and using a control loop in order to adapt the droop curves in accordance with the available input power. Theoretical analysis and experimental results have proven the viability of the approach.
Modeling of a complete photovoltaic (PV) inverter system poses a significant challenge because it involves different time-scaled dynamics including filter elements, fast current loop, voltage loop, maximum power point tracking (MPPT) loop, and response of the energy source. This paper intends to utilize the state-space modeling approach for stability analysis for a gridtie PV inverter system by eigenvalue evaluation with different irradiation levels, PV output voltages, voltage-loop gains and current-loop gains. The stability analysis can not only show more insight of the two-loop interaction with current-loop gain variation, but also show the stability trend due to the change of both the source voltage and the source impedance with the input PV voltage change, which are quite difficult to detect with conventional single-variable transfer function analysis. The controller based on state-space modeling and analysis has been designed and implemented. Experimental results are used to verify the stability trend predicted in the model.
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