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In the conventional variable speed drive, the dc-link including the dc-choke and electrolytic capacitor is bulky with limited lifetime. The slim dc-link drive is thus proposed for the lower cost and longer lifetime by using the film capacitor to replace the electrolytic capacitor and dc-choke. However, such drive system suffers from the potential instability issue, due to the negative incremental...
This paper deals with the problem of LCL filter resonance in grid connected inverter control. The system equations are reformulated to allow the application of the active disturbance rejection control (ADRC). The resonance, assumed unknown, is treated as a disturbance, then estimated and mitigated. By using this new robust control, a high level of performance is achieved with a minimum complexity...
Active damping of LCL-filter resonance based on single-state feedback control is widely used with voltage source converters. Its robustness against grid impedance variation has always been a major concern with its controller design. To deal with this issue, this paper begins by developing a generalized impedance-based model of grid current control with feedback-type active damping. Then, a controller...
This paper investigates the stability regions of current control for LCL-filtered grid-connected converters, where no active or passive damping is required to stabilize the closed-loop control system. It is already identified in the literature that if the LCL resonance frequency is placed within 1/6 to 1/2 of the system sampling frequency, the grid current control can be directly used without damping...
This paper proposes a novel LCL-filter design method and its current control for grid-connected converters. With the proposed design method, it is possible to set the resonance frequency of the LCL-filter to be higher than the Nyquist frequency, i.e. half of the system sampling frequency, and this observation is so far not discussed in the literature. In this case, very cost-effective LCL-filter design...
The proportional capacitor-current feedback active damping method has been widely used to suppress the LCL-filter resonance. However, the time delay in the damping control loop may lead to non-minimum phase or even unstable responses when the resonance frequency varies in a wide range. To improve the robustness of damping, this paper proposes an improved damping controller with the capacitor current...
The capacitor voltage of an LCL-filter can also be used for active damping, if it is fed back for synchronization. By this way, an extra current sensor can be avoided. Compared with the existing active damping techniques designed with capacitor current feedback, the capacitor voltage feedback active damping would however demand a noise-sensitive derivative term. A digital realization of this derivative...
This paper proposes a unified grid current control for grid-interactive distributed generation inverters. In the approach, the grid-side current, instead of inverter-side current, is controlled as an inner loop, while the filter capacitor voltage is indirectly regulated through a virtual admittance in the outer loop. It, therefore, provides several superior features over traditional control schemes:...
The single-phase differential mode buck inverter is recently introduced with a differential mode for power transfer and a common mode for actively decoupling the second-order power oscillation. However, it is limited to islanded applications with an LC filter. This paper addresses the stability and control of a grid-connected differential mode buck inverter with an LCL filter. A generalized small-signal...
The capacitor voltage feedback active damping control is an attractive way to suppress LCL-filter resonance especially for the systems where the capacitor voltage is used for grid synchronization, since no extra sensors are added. The derivative is the core of the capacitor voltage feedback active damping control. However, in digital systems, the discrete implementation of the derivative suffers from...
The LLCL-filter is emerging into grid-connected converters due to a smaller size compared to the conventional the LCL-filter. Similar to LCL-filter, the high order resonance introduces potential instability to the overall system which should be properly damped either passively or actively. In this paper, different feedback coefficients like the proportional, derivative, integral, high pass and low...
Compared with LCL filter, an LLCL-filter is characterized with smaller size and lower cost for grid-connected inverters. But this high order filter may also have resonant problem which will affect the system stability. Many methods can be used to alleviate the resonant problem including active damping, passive damping or even without damping if it is designed conservative. In this paper, an overview...
This paper addresses the interaction between harmonic resonant controllers and active damping of LCL resonance in voltage source converters. A virtual series R-C damper in parallel with the filter capacitor is proposed with the capacitor current feedback loop. The phase lag resulting from the digital computation and modulation delays is thus compensated by the virtual capacitor. The frequency region...
A higher order passive power filter (LLCL-filter) for the grid-tied inverter is becoming attractive for the industrial applications due to the possibility to reduce the cost of the copper and the magnetic material. To avoid the well-known stability problems of the LLCL-filter it is requested to use either passive or active damping methods. This paper analyzes the stability when damping is required...
LCL-filters are used to mitigate the harmonic current content in grid converters. The LCL-filter resonance must be damped in order to avoid stability problems in the current control. Active damping avoids resistors at the expense of increased control complexity. Large grid impedance variations can challenge the LCL-filter stability. Active damping by using a notch filter on the reference voltage for...
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