In this application note, PED-Board is used to fully control a 4-leg inverter in off-grid generating applications as depicted in Figure 1. Output voltage regulation is accomplished by the well-known Repetitive Control (RC) structure with can be straightforwardly implemented on the FPGA thanks also to the LabVIEW environment. Measures are acquired by the PED-Board using the application specific Adapter Board which was designed on purpose. Output currents are also measured to provide overcurrent protection. Overvoltage and overcurrent trip can be configured from the front panel of the RealTime (RT) target.

RC_PI_SystemSchemeFigure 1. Block scheme of a typical application for a stand-alone 4-leg VSI.

Implemented control scheme is summarized in Figure 2 where the classical RC form can be easy recognized. As stabilizing controller, the PI type has been selected for its simple implementation being able also to provide the necessary attenuation of the RC high frequency effects. PI controller discretization is performed by ZOH on the RT target. After, discretized controller coefficients are sent to the FPGA.

RC_PI_SchemeFullFigure 2. Repetitive control scheme and its detailed implementation.

With reference to RC implementation, it can be intended as simple delay lines which can be realized by the available LabVIEW feedback node. In order to strongly reduce the arithmetical calculation errors, the NI single precision floating point toolkit has been used. Achievable result is shown in Figure 3 when the inverter is loaded by a 3ph diode bridge rectifier. It can be recognizing the near-zero THD of the output voltages even when a strongly non-linear load has to be fed. Comparable behavior is obtained when single phase non-linear load is used. Depicted results could be achieved thanks to the high precision measurement chain of the PED-Board, jointly with the floating point algorithm implementation on the NI-SoM.

RC_PI_3ph14kW_20Adiv_3phDiode_workedFigure 3. Achieved output voltages when a non-linear load is fed (3 phase diode bridge rectifier).

Single precision floating point (SGL) implementation of the Repetitive Control has been made possible thanks to the NI SGL toolkit. It can be downloaded directly from the download section.

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[1]   Costa-Castello, R.; Nebot, J.; Grino, R., “Demonstration of the internal model principle by digital repetitive control of an educational laboratory plant,” in Education, IEEE Transactions on, vol.48, no.1, pp.73-80, Feb. 2005.
[2]   Lidozzi, A.; Chao Ji; Solero, L.; Zanchetta, P.; Crescimbini, F., “Resonant–Repetitive Combined Control for Stand-Alone Power Supply Units,” in Industry Applications, IEEE Transactions on, vol.51, no.6, pp.4653-4663, Nov.-Dec. 2015.
[3]   Tianqi Liu; Danwei Wang, “Parallel Structure Fractional Repetitive Control for PWM Inverters,” in Industrial Electronics, IEEE Transactions on, vol.62, no.8, pp.5045-5054, Aug. 2015.
[4]   Rohouma, W.; Zanchetta, P.; Wheeler, P.W.; Empringham, L., “A Four-Leg Matrix Converter Ground Power Unit With Repetitive Voltage Control,” in Industrial Electronics, IEEE Transactions on , vol.62, no.4, pp.2032-2040, April 2015.

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