High efficiency photovoltaic grid connected flyback microinverter / Radin Za’im Radin Umar

Radin Za’im , Radin Umar (2023) High efficiency photovoltaic grid connected flyback microinverter / Radin Za’im Radin Umar. PhD thesis, Universiti Malaya.

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Abstract

In the first part of this thesis, a new competitive cost, non-invasive and isolated current sensing method for the photovoltaic flyback microinverter is presented. This is done by using the flyback transformer itself as a current sensor via the introduction of a single turn third winding to the flyback transformer. The integration of the open circuit voltage of the third winding’s using a newly proposed ground-clamped-integrator allows for the sensing of the magnetizing current. Controlling the magnetizing current solves the control complexity problem with continuous conduction mode (CCM). The triangular linearity of magnetizing current allows hysteresis control to be employed, resulting in straighforward CCM control like that of the boundary conduction mode and discontinuous conduction mode. An experimental grid connected microinverter prototype is built. The prototype achieves the performances of 0.9988 power factor, 1.9% grid current THD, 99% static MPPT efficiency and MPPT dynamic efficiency of 98.50%. This is achieved under condition of full PV power of 210W, peak power point input of PV voltage of 41.3V, and rms output grid voltage of 240V, and maximum power point tracking range of 35V-43V. In the middle part of the thesis, a new method is introduced to reduce the high voltage stress experienced by secondary gallium nitride (GaN) transistor in high gain application (high step up). The technique is coined as the leakage bypass. It is demonstrated that the proposed method provides 2.3 times decrease of percentage of overshoot to the stress voltage. The leakage bypass is relevent because commercially available GaN is only restricted to 650V of absolute maximum voltage rating. The technique allows the secondary GaN to adhere to this restriction. The final part of this thesis presents the development of a very high efficiency flyback microinverter. The developed microinverter incorporates various techniques separately known to provide reduced losses on the flyback inverter and combine them into one working microinverter prototype. Additionally, a new active snubber technique is proposed to provide for better recovery of the parasitic primary switch’s output capacitance charge which would otherwise be lost as electromagnetic oscillation. Gallium Nitride (GaN) is also utilized in the design and a new leakage bypass circuitry is proposed to allow for safe operation by limiting the peak transient voltage to within the limit of the GaN technology (650V). As a result, a high full system weighted efficiency of 96.67% and 97.02% full system peak efficiency is achieved. A comparison between the developed hardware and commercial microinverter was performed, and it is shown that the resulting efficiency performance is comparable to commercial microinverter.

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