Conversores CC–CC bidirecionais para regulação série de tensão aplicados em sistemas fotovoltaicos

Abstract

This work provides some conceptual and practical contributions about the use of DC–DC power converters connected as step-up/down series voltage regulators in string or multi-string architectures of grid-tie photovoltaic systems, which allow to carry out the partial power processing. In practical terms, the basic implementation restrictions and also the requirements for operation with bidirectional active power flow required to develop topologies of this type are defined. From a conceptual point of view, it is determined that the series connection is a necessary condition, but only a thorough analysis in terms of the non-active power processed can define the requirements that allow characterizing which series regulators are capable of performing partial power processing. For this, it is proposed a new application to the well-known Fryze power theory, which is used to develop a mathematical procedure that allows to evaluate both active and non-active power processing in DC–DC power converters. Through a comparative analysis between two series regulators based on H-bridge converters, it is verified that partial power processing can be achieved by adjusting the turns ratio of the transformers used in these two topologies. Thus, the main advantages of using series regulators to replace traditional step-up/down power converters are demonstrated. Based on the voltage regulation range definition and both European and Californian efficiencies, it is verified that the average value of the weighted total non-active power is higher under the cascade-connected non-inverting buck–boost operation (European: 793.18 var; Californian: 955.19 var), being lower under the series-connected full-bridge/push–pull converter (European: 518.14 var; Californian: 623.89 var) and the series-connected half-bridge/push–pull converter operation (European: 458.68 var; Californian: 552.39 var), thus this latter is the evaluated topology that performs the least power processing. In order to validate the approaches, two 2202 W prototypes were designed and submitted to numerous laboratory tests. Regarding the non-active power processed by the circuit elements of the DC–DC converters, all values obtained experimentally were consistent with the numerical and circuit simulations results. In terms of performance, the series-connected halfbridge/ push–pull prototype without snubbers resulted in weighted average global efficiencies of approximately 98.38% (European) and 98.95% (Californian). Under the same operation conditions, the cascade-connected non-inverting buck–boost prototype resulted in weighted average efficiencies of approximately 98.70% (European) and 98.73% (Californian).