Control framework and method for improving adaptive capacity of LCL type grid-connected inverter to weak power grid

Abstract

The invention discloses a control architecture and method for improving the adaptive capacity of an LCL type grid-connected inverter to a weak power grid. The control architecture comprises an LCL type grid-connected inverter main control loop, a virtual impedance branch, a power grid voltage full feed-forward branch and a grid-side inductive voltage differential feedback active damping branch. The difference value between the reference current and the network access current passes through a current loop controller to form a main control loop; the network-entering current passes through the virtual impedance branch to obtain a power grid impedance voltage, and the power grid impedance voltage is subtracted from the common coupling point voltage to obtain a power grid voltage; the power grid voltage forms full feed-forward of the power grid voltage through a proportional feed-forward term and a secondary differential term; the filter capacitor voltage and the power grid voltage are subtracted to obtain the voltage on the grid-side inductor and the power grid impedance, and active damping is formed through differential feedback. The adaptive capacity of the LCL type grid-connected inverter to a weak power grid is improved, that is, when the power grid contains large impedance and rich background harmonic waves, it can still be guaranteed that the inverter outputs high-quality grid-entering current, and the method has important practical value.

Description

technical field [0001] The invention relates to the related field of grid-connected inverter control, in particular to a control framework and method for improving the adaptability of an LCL-type grid-connected inverter to a weak power grid. Background technique [0002] Long-distance transmission lines, more isolation transformers, a large number of distributed power generation equipment and various nonlinear equipment are connected to the public coupling point (Point of Common Coupling, PCC), so that the power grid can be equivalent to an inductive power grid viewed from the PCC. A weak network in which the impedance is connected in series with a voltage source rich in background harmonics. Compared with the strong power grid, the weak power grid has the following two characteristics: (1) The impedance of the power grid cannot be ignored and changes with the operation mode of the power grid; (2) The power grid contains rich background harmonics. [0003] When modeling the...

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Problems solved by technology

The former uses the high open-loop gain of the resonant controller to achieve harmonic suppression, but the power grid usually contains multiple harmonics, so this method needs to be connected to multiple resonant controllers, which increases the computational burden of the system; the latter uses the equivalent transformation of the control block diagram, Eliminate the influence of the grid voltage by adding a feed-forward branch

The essence of both is to increase the output impedance of the inverter to improve its anti-interference ability, but due to the existence of the grid impedance, on the one hand, the cut-off frequency of the control system will decrease with the increase of the grid impedance. When the cut-off frequency is lower than the resonance adjustment The system will be unstable when the resonant frequency of the transformer is low; on the other hand, the feedforward PCC voltage will introduce the grid impedance voltage to form a positive feedback, reducing the system stability margin. When the grid impedance is large, the system will be unstable, and different forms of active damping are used. The form of the grid voltage full feedforward is also different. Taking the conventional filter capacitor current proportional feedback active damping as an example, the grid voltage full feedforward contains three branches, namely the proportional branch, the primary micro branch, and the secondary Micro-branches increase the complexity of system control

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