This doctoral thesis focuses on single phase inverters for individual photovoltaic modules, also known as microinverters. The input of a microinverter is supplied by the photovoltaic module which is a source of constant DC power. The microinverter can supply a local load connected to its output terminals or inject the current into the utility grid. Alternating power is required at the converter's output in both cases. Due to the difference between the instantaneous input and output power, a decoupling capacitor must be incorporated within the microinverter's circuit. The voltage level and the required capacitance of the decoupling capacitor strongly influence the total size, production cost and lifetime of the converter. Hence, the selection of the decoupling capacitor represents one of the main design challenges. The total size of the microinverter also depends on the operating switching frequency, the increase of which allows for a reduction in the size of passive components within the circuit. Nonetheless, operation with a higher switching frequency leads to increased switching losses in semiconductor devices and consequently lower power conversion efficiency. An overview of existing microinverter structures and their operating principles is provided within this thesis. A two stage microinverter with DC-link provides among the best properties in terms of the size and lifetime of the decoupling capacitor, but suffers from high switching losses when operating at higher switching frequencies. In order to overcome this drawback, a new microinverter is proposed based on a two stage microinverter with DC-link. Its main advantage is in soft switching of the utilised transistors and diodes, which allows the operation at higher switching frequencies. Instead of a DC-link, the proposed converter uses a high frequency alternating link (HFAC-link) in combination with an active decoupling circuit. In order to meet the specific operating requirements a pulse-density modulation technique is utilised. Due to a complex control strategy and high switching frequency (100 kHz) the control is implemented within a field programmable gate array (FPGA). Experimental results confirm proper operation of the proposed microinverter.
D.09 Tutoring for postgraduate students
COBISS.SI-ID: 282658048The work comprehensively evaluates the effects of reactive power generation in photovoltaic power plants. It presents technical and economic aspects of the reactive power generation for both, the photovoltaic power plant and the electricity network. In order to ensure adequate voltage profiles in the electricity network, the reactive power generation in photovoltaic power plants should be applied first whilst the reduction of their active power generation power is an emergency measure. The reactive power generation in photovoltaic power plants can reduce electricity network losses. The reactive power generation and active power curtailment in photovoltaic power plants, combined with net-metering, enables an increase in annual net energy supply self-sufficiency in electricity networks without significant investments.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 19235606An assessment of technical benefits achieved by a local control system in low voltage distribution networks with photovoltaic systems (PVS) is performed in this work. According to current requirements, PVS must be able to generate reactive power up to cos(fi) = 0,8, considering conditions at the point of common coupling. Reactive power generated that way insignificantly influences the voltage profile. It does not cover network needs for reactive power, but it influences the network losses. In the proposed local control system the smart meters provide data concentrator with instantaneous measured data, which are used to determine the references for reactive power generation, considering network voltage profile and losses. The references are sent to PVS. Technical aspects of proposed local control are evaluated for three existing networks with PVS.
F.18 Transfer of new know-how to direct users (seminars, fora, conferences)
COBISS.SI-ID: 18749462