A Current Source Inverter with Series Connected AC Capacitors for Photovoltaic Application with Grid Fault Ride Through Capability C. Photong C. Klumpner P. Wheeler Department of Electrical and Electronic Engineering, Faculty of Engineering, University of Nottingham University Park, Nottingham, NG7 2RD, United Kingdom eexcp2@nottingham.ac.uk christian.klumpner@nottingham.ac.uk pat.wheeler@nottingham.ac.uk Abstract- The current source inverter (CSI) is preferred to interface a renewable source such photovoltaics (PV) to the AC power grid because it can provide smooth current source in the DC side, which fits to the PV behaviour and also steps-up the voltage from DC side to AC side, which allows smaller and safer DC voltage levels to be used. However, the CSI has the problem that during an AC grid fault, it cannot operate properly. In this paper, a single-stage power conversion approach based on a CSI inverter with series capacitors is proposed, proven to provide improved efficiency and smaller DC inductor size and the capability to ride through AC grid faults with full reactive power injection support when compared to a standard CSI. I. INTRODUCTION In order to feed the DC power generated by a renewable energy source such a Photovoltaic (PV) into the AC power network, a DC/AC power inverter is needed. However, besides providing a high conversion efficiency, the inverter should meet also two additional requirements: the ability to adapt to the change in the DC voltage generated by the renewable energy source and the capability of ride through grid faults such as deep voltage sags and power interruptions, whilst providing reactive power support to the grid. The first requirement is a consequence of the basic V-I characteristics of a PV illustrated in Fig. 1, that makes that the higher power to be delivered when the voltage across its terminals is lower than the maximum (no-load) voltage. The ratio between maximum power voltage and no load voltage for a PV is typically within a range of 0.7-0.8. If the PV load resistance is varied from infinite to zero, the generated power level initially increases according to the increase in the load current that also causes a fall in the PV terminal voltage: zero power at no-load which is where the terminal voltage is maximum (OP1), then smaller and higher power at light load (OP2) and higher load (OP3) and finally reaching the maximum power point MPP (OP4). After reaching the MPP, the power generated reverses its trend (OP5 and OP6) and even becomes zero again when short-circuiting the PV terminals (OP7). This complex V-I-P behavior may be accommodated by a few alternative power converter topologies analyzed in [1]- [5]. The first solution may use a nonisolated DC/DC converter to boost the low voltage at DC side (PV) to a higher sufficient level for a DC/AC inverter. Since the DC/DC converter does not provide galvanical isolation, additional precaution should be used to ensure the safety regulation and to withstand any potential EMI [3]. Second solution is to add galvanic isolation by using a high switching frequency transformer in the DC/DC converter. As it has small size, very high efficiency (99%) and the possibility to then match to the standard semiconductor voltage/current ratings, it enabled a wide spread of this solution. The third solution consists of using a DC/AC inverter directly at the PV/FC end to convert the DC power into AC at low voltage level before stepping up at the grid voltage level using a line frequency transformer. This solution uses the simplest technology but a large 50/60Hz transformer size makes it the heaviest/bulkiest. The last solution is to use a single stage DC/AC inverter, which would use either a Voltage Source Inverter (VSI) or a Current Source Inverter (CSI). If the chosen topology is a VSI, a higher voltage level exceeding the peak line-to-line grid voltage level is always needed on the DC side (PV) to provide proper operation. Since at no load, the DC link voltage further increases, this can raise serious insulation and safety issues [3]. In contrast, a CSI has the capability to boost the voltage from the DC side to the AC side which means that a lower DC voltage can be used, compared to a VSI. Moreover, the current drawn from the PV terminals should have a low level of ripple, which would require DC-link current filtering (use of an inductor), a requirement that again points at the CSI as the ideal choice [4]. One drawback of the CSI is that if the input AC voltage level reduces (e.g. during a voltage sag), it makes the CSI unable to operate properly. It may be possible to enable 0 0.5 1 1.5 0 0.5 1 1.5 PV Terminal Voltage (p.u.) Power Current (p.u.) OP1 OP2 OP7 OP3 OP4 OP5 OP6 0 0.5 1 1.5 0 0.5 1 1.5 PV Terminal Voltage (p.u.) Power Current (p.u.) OP1 OP2 OP7 OP3 OP4 OP5 OP6 Fig. 1. Typical characteristic curves of a PV