A More Efficient Current Source Inverter with Series Connected AC Capacitors for Photovoltaic and Fuel Cell Applications Christian Klumpner, Photong Chonlatee, Patrick Wheeler Department of Electrical and Electronic Engineering, University of Nottingham, UK Abstract Renewable energy sources such as photovoltaics (PVs) or fuel cells (FCs) are not fitted for direct power grid connection because they deliver DC voltage and current. In addition, they also have a high internal resistance/nonlinear V/I dependence, which is why a power electronic interface is always needed. This paper presents the implementation of a three-phase power electronic interface for PV/FCs that uses a single conversion stage approach based on a current source inverter (CSI) topol- ogy with series connected capacitors, that would need only six reverse blocking IGBTs and due to the possibility to reduce the AC voltage at high load, would reduce the size of magnetics and the losses compared to a standard CSI. 1. Introduction In order to increase the utilization of renewable energy sources such as wind, photovoltaics (PV) or fuel cells (FC) that use hydrogen generated from renewable energy, more research is needed to constantly decrease their specific costs ($/kW installed), an important part being not only the capturing of the renewable energy cheaply, but also interfacing it to the power grid in an effi- ciently and cost effective way. There are a few alternative power converter to- pologies available to connect a PV/FC to the AC power grid [1]-[3]. It could use a two-stage ar- rangement by using a DC/DC converter (with or without isolation to boost the low voltage typically delivered by the PV/FC to a higher level suitable for a typical DC/AC PWM inverter. If the DC/DC converter is not galvanically isolated, additional precaution when designing the DC/DC inverter should be considered in order to comply with the safety regulations and to contain any potential EMI [3]. In case the DC/DC converter provides galvanic isolation by means of a high frequency transformer, all the latter issues are automatically solved. In addition, the high frequency trans- former is small and operates typically with very high efficiency (99%) and because it allows the adaptation of the semiconductor voltage/current levels, it will require an installed power in semi- conductors close to the power processed which means it will not be very expensive. A more ad- vanced DC/AC inverter technology may involve the modulation of the high frequency voltage, re- quiring a more efficient unfolding inverter. Another solution is to use a DC/AC inverter that converts the DC power delivered by the PV/FC into AC voltage at the supply frequency which is stepped up at the grid level by a low frequency transformer, making this solution the simplest technologically, but due to the large size of a 50/60 Hz transformer, the bulkiest/heaviest. The last solution consists of using a single stage DC/AC converter, which seems the simplest but has several drawbacks: if a Voltage Source In- verter (VSI) which is the most popular grid side interface [3]-[4], is used, a higher voltage level exceeding the peak line-to-line grid voltage level will be necessary on the DC-side (PV/FC) to pro- vide proper operation. In case of a 415 Vrms line voltage, this means that the PV/FCs have to be connected in series to deliver voltage in excess of 585V, which raises serious safety issues [3]. On the other hand, an important requirement is that the current drawn from the PV/FC terminals to have a low ripple, which would require addi- tional DC-side filtering. But these two restrictions make the current source inverter (CSI) [5]-[9] the ideal choice. In addition, the CSI has the capabil- ity to boost the voltage from the DC side to the AC side which means that a lower DC-voltage would be needed, solving partly the safety is- sues. However, this option to reduce too much the DC-side voltage is not economical as the higher the DC/AC voltage transfer ratio is, the higher the installed power in the semiconductors would be and so the cost. Actually, the smallest voltage transfer ratio that a current source in- verter can achieve whilst still providing sinusoidal grid currents is 1.154. brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by Nottingham ePrints