# Energy Efficiency Improvement in Reconfigurable Photovoltaic Systems: An Evaluation of Team Systems
## 3 GCPV Systems with Adaptive Sizing Ratio
### MIX Systems  
The company Fronius introduced the term MIX (Master Inverter X-change) in 2003. It is used to define the reconfigurable operation principle of some series of grid-connected PV inverters [39]. The companies Emerson Electric [40], ABB [41], and Vacon [42] use the term Multimaster to describe a very similar reconfigurable operation principle applied to central inverters.  
This kind of system is formed by a single PVG (as an energy-caption subsystem) and one energy-processing subsystem, where the energy-processing subsystem comprised several parallel-connected inverters. One inverter assumes the role of leader, and the remaining inverters are considered follower devices.  
Under low-isolation conditions, the PVG output power is handled by the master inverter, thus allowing for higher low-load efficiency. When isolation is higher, two or more inverters are operating in parallel connection [43; 44]. The variation of the energy-processing subsystem rated power is achieved thanks to the variation of the number of inverters operating in parallel. The number of inverters connected in parallel to the PVG will depend on the energy generated by the PVG at any moment.  
Figure 4 shows an example of a GCPV system based on the MIX or Multimaster concept and formed by a set of three inverters that can operate in parallel connection.  
The configuration shown in Figure 4 corresponds to the configuration used in periods of low energy production and has the highest \(R_{S}\) value. When the energy production of the PVG increases and exceeds a certain threshold, it should be changed to the configuration shown in Figure 4, which has a lower \(R_{S}\) value than the previous situation. Figure 4 shows the configuration to be used during periods of greatest energy production. It has the lowest \(R_{S}\) value of the three possible configurations.  
When considering this kind of GCPV system, several key aspects must be addressed:  
Figure 4: MIX system based on three inverters to set the energy-processing subsystem. (**a**) Configuration used during periods of low energy production with a higher sizing ratio. (**b**) Configuration after exceeding a certain power threshold, with a lower sizing ratio than the previous configuration. (**c**) Configuration used during periods of greatest energy production. It has the lowest sizing ratio of the three possible configurations.  
* The maximum number of inverters used to configure the energy-processing subsystem.
* The rated power of the used inverters (\(P_{INV1}\), \(P_{INV2}\), \(\ldots\)).
* The optimum number of inverters that must be connected in parallel at any time.
* The moment in which a new inverter of the energy-processing subsystem must be connected or disconnected.  
The implementation of MIX systems implies the utilization of grid-connected inverters with the possibility of operation in parallel connection, and these inverters are not available in the market in open platforms. As presented previously, different manufacturers offer products based on this operation mode, implementing their proprietary algorithms to ensure the correct sharing of current (and power) between the inverters operating in parallel. Consequently, it is practically impossible to perform tests for the comparison of MIX systems and systems based on central inverters exactly in the same conditions.  
For this reason, a further description of these reconfigurable systems is not included in this work. Interested readers can consult reference [38] for a more exhaustive modeling of these systems.