Abstract
This paper presents an improvement of the capacitive emulation (CE) method to reduce the line current distortion caused by grid-tied LCL-filtered converters. In these cases, the grid voltage is applied to the LCL’s capacitor, which generates a distorted capacitor current that pollutes the line. The CE method consists in controlling the converter currents instead of the grid currents, while the converter generates a copy of the distorted capacitor current, so that both the copy and the distorted currents cancel each other in the grid. Therefore, we can say that the converter emulates a negative capacitance connected to the grid, while at the same time delivers its active and reactive powers at the fundamental frequency. The CE method is achieved by adding an estimation of the distorted capacitor current to the converter current reference. However, an effective capacitive emulation requires a current control capable of accurately tracking all harmonics added to the current reference. In this sense, this paper proposes the use of a new integral predictive current control (IPCC), a dead-beat type of control that ensures a constant closed-loop group delay in a wide bandwidth. Unlike a PI control where the closed-loop delay varies with the frequency of the tracked harmonic, the constant control delay of the IPCC can be effectively compensated with a buffer-based advanced current reference. The effectiveness of the proposed CE method with IPCC control to reduce the total harmonic distortion (THD) of the line currents has been proved experimentally on a 10 kVA transformerless grid-connected three-phase inverter.
Highlights
Two different current control strategies are used for grid-connected converters withLCL filter [1,2,3,4]: to sense and control the converter currents, i.e., converter current feedback (CCF), or to sense and control the grid currents directly, i.e., grid current feedback (GCF).The main drawback of the CCF scheme is that, even if the controlled converter currents are achieved sinusoidal, the line currents are distorted by the LCL’s capacitor currents
The GCF has a higher loop gain resonance peak compared to the CCF [3,9], which must be damped by means of active damping (AD)
This paper combines the capacitive emulation (CE) technique with a new integral predictive current control (IPCC), which together significantly reduces the harmonic distortion of the line currents when using LCL filters and converter current feedback (CCF)
Summary
Two different current control strategies are used for grid-connected converters with. The converter emulates a negative capacitance connected to the grid while delivering the active and reactive powers at the fundamental frequency It was proven in [12] that the CE method can compensate current harmonics up to the L2 -C resonance (where L2 and C are defined in Figure 1) giving better results than the GCF techniques in terms of THDi. The CE method consists on: 2. The delay compensation in [12] was not fully effective for all harmonics, as the PI control used did not exhibit a constant group delay in closed-loop for all frequencies To overcome this drawback, present paper proposes the use of a new predictive current control to implement the CE method, which features a constant group delay (a constant number of samples) in closed-loop. The cost function minimisation of the FS-MPC does not guarantee a constant closed-loop group delay
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