Abstract
This paper investigates active damping of LCL filter resonance in grid-connected inverters with only inverter current feedback control, since it only needs to sample one current to realize both current control and inverter protection. The traditional single-loop inverter current control (SLICC) can damp the LCL filter resonance actively. However, if the control delay is considered in digital control, the system stability will depend on the ratio of the LCL resonance frequency f res to the sampling frequency f s , and the valid damping region is only up to f s / 6 . Considering that the design region of the LCL resonance frequency f res is up to f s / 2 , the system can easily become unstable due to the LCL resonance frequency shifting. Thus, this paper proposes an improved active damping method based on SLICC, including the asymmetric regular sampling method and delay compensation method. The improved sampling method minimizes the control delay without introducing a switching ripple, and the delay compensation method further compensates for the delay effect. With a proper parameter design, the upper limit of the valid damping region is extended up to f s / 2 , which can cover all the possible resonance frequencies, and it has inherent robustness against grid-impedance variation. Finally, a few simulations in MATLAB/SIMULINK and experiments based on a 6 kW prototype are performed to verify the theoretical analysis.
Highlights
The renewable energy distributed generation technique is attracting more and more attention, to deal with the increasing concern of the exhaustion of fossil fuels and the environmental pollution
As the grid-connected inverter is controlled by the pulse width modulation (PWM), the output current contains an amount of switching harmonics. e LCL filter is widely used due to its better attenuation capability of the switching harmonics and cost advantage compared with L filter
The traditional active damping techniques can be classified into two modes based on whether an additional sensor is needed, except for the essential sensors of the control current and grid voltage: the single-loop and the multiloop feedback control. e multiloop feedback control methods introduce additional damping terms into the current control loop by sensing additional variables, such as capacitor current [3–6] or capacitor voltage [7]
Summary
The renewable energy distributed generation technique is attracting more and more attention, to deal with the increasing concern of the exhaustion of fossil fuels and the environmental pollution. In order to increase the damping, an active damping inner loop control is introduced It is implemented by extracting the high-frequency components of the inverter current or grid current to simulate the capacitor current [13–15], and the first-order High Pass Filter (HPF) is the most popular filter. By inserting a digital filter in the forward path, the frequency characteristics around the LCL resonance frequency can be changed Such a solution relies on the accuk6rate filter parameters; the variation of grid inductance may greatly affect the system stability. Is method extends the valid damping region up to fs/2, improving the system stability margin greatly without increasing the control algorithm complexity and system cost It has inherent robustness against grid-impedance variation.
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