Reduction of Computation Delay for Improving Stability and Control Performance of LCL-Type Grid-Connected Inverters
As illustrated in Chap. 8 , in the digitally controlled LCL-type grid-connected inverters, proportional feedback of the capacitor current is equivalent to a frequency-dependent virtual impedance connected in parallel with the filter capacitor due to the
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Reduction of Computation Delay for Improving Stability and Control Performance of LCL-Type Grid-Connected Inverters
Abstract As illustrated in Chap. 8, in the digitally controlled LCL-type grid-connected inverters, proportional feedback of the capacitor current is equivalent to a frequency-dependent virtual impedance connected in parallel with the filter capacitor due to the control delay including the computation and pulse-width modulation (PWM) delays. This virtual impedance leads to the change of the LCL filter resonance frequency. At the frequencies higher than one-sixth of the sampling frequency (fs/6), the virtual impedance contains a negative resistor component. So, if the actual resonance frequency is higher than fs/6, a pair of open-loop right-half-plane (RHP) poles are generated. As a result, the LCL-type grid-connected inverter is easier to be unstable if the resonance frequency is moved closer to fs/6 due to the variation of grid impedance. Meanwhile, the computation and PWM delays also reduce the control bandwidth greatly and thus impose a severe limitation on the low-frequency gains. Therefore, it is desirable to reduce the control delay so as to improve the stability and the control performance of the grid-connected inverter. In this chapter, the influence of the control delay on the LCL-type grid-connected inverter is firstly analyzed. Then, the real-time sampling method [1] and real-time computation method with dual sampling modes [2] are proposed to reduce or even remove the computation delay. Finally, the experimental results from a 6-kW prototype verify the effectiveness of the proposed methods.
Keywords Grid-connected inverter LCL filter Active damping trol Open-loop unstable poles Grid impedance
Digital con-
As illustrated in Chap. 8, in the digitally controlled LCL-type grid-connected inverters, proportional feedback of the capacitor current is equivalent to a frequency-dependent virtual impedance connected in parallel with the filter capacitor due to the control delay including the computation and pulse-width modulation (PWM) delays. This virtual impedance leads to the change of the LCL filter resonance frequency. At the frequencies higher than one-sixth of the sampling frequency (fs/6), the virtual impedance contains a negative resistor component. So, © Springer Nature Singapore Pte Ltd. and Science Press 2018 X. Ruan et al., Control Techniques for LCL-Type Grid-Connected Inverters, CPSS Power Electronics Series, DOI 10.1007/978-981-10-4277-5_9
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Reduction of Computation Delay for Improving Stability …
if the actual resonance frequency is higher than fs/6, a pair of open-loop right-half-plane (RHP) poles are generated. As a result, the LCL-type grid-connected inverter is easier to be unstable if the resonance frequency is moved closer to fs/6 due to the variation of grid impedance. Meanwhile, the computation and PWM delays also reduce the control bandwidth greatly and thus impose a severe limitation on the low-frequency gains. Therefore, it is desirable to reduce t
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