Modeling and Control of a Master-Slave PV Inverter With N-Paralleled Inverters and Three-Phase Three-Limb Inductors

In order to maximize the profitability of big photovoltaic (PV) plants, high-power PV inverters of more than 500 kW are becoming attractive. The master-slave (MS) inverter is one of the most interesting architectures. Usually, it is composed of N-paralleled three-phase inverters connected to the medium voltage grid through a transformer with N secondaries on the low-voltage side. This type of transformer is up to 40% more expensive than a single secondary one. This paper analyses the case in which the N inverters are connected to a single secondary transformer so that the N inverters share the ac and dc sides. Furthermore, three-phase three-limb inductors are used, which are around 25% cheaper than single-phase equivalents. Although the use of this type of transformer and inductors is highly attractive in economic terms, it does generate two types of couplings between the different inverters, compromising the system stability. This paper presents a new control technique that achieves a stable system, invariant with N, and independent of the type of inductors and transformer used. Stability of the system is guaranteed in any situation and it can be implemented with a low computational cost. The simulations and the practical results obtained with an Ingecon Sun 625HE-MS confirm the correct functioning of this control technique.