Calculation of maximum power in a utility-interactive photovoltaic-generating system by using PWM current-source inverter

Abstract

In a utility-interactive photovoltaic (PV)-generating system, a PWM inverter is used for the connection between the PV arrays and the utility. The inverter is controlled to keep the output power at the maximum point for the PV. Although the PV power can be easily found by using the measured PV voltage and PV current, the installation of two detectors in the system is indispensable to obtain the exact PV power. In system operation with the maximum power that is fed into the utility, the PV does not always operate at the maximum point of PV output power because of power losses in the system. It has been reported that the use of the measured ac voltage or ac current on the inverter output side can exclude system losses from consideration and achieve system operation with maximum utility power. In this manner, however, since the ac output waveforms of the PWM inverter consist of many pulse trains, a filter circuit may be necessary for the detector to obtain the exact values of the ac output. This paper proposes a method of calculation of the utility power in the PV-generating system by using a PWM-current source inverter. In this system, only the PV current is measured, and it is employed for power calculations in combination with the control parameter of the PWM inverter. Since the proposed approach monitors the utility power instead of the PV power, control that does not need to take system losses into consideration can be achieved. This paper presents a method of calculation of the power for a single-phase–inverter PV system and gives basic experimental characteristics. An application of the proposed method to a three-phase PWM inverter–PV system is also described, and experimental results are given. © 1998 Scripta Technica, Electr Eng Jpn, 125(1): 55–64, 1998