The rooftop solar system can be a standalone system as well as grid tied system. In the grid tied system also there can be a number of schemes depending upon the reliability of supply the consumer needs. Wherever the battery is not envisaged, the solar system can be directly connected to consumer AC bus and the total energy of the solar system will be supplied to consumer/grid depending upon the requirement of the consumer. The scheme for grid tied rooftop solar system is shown below.
1. Switch S (built in Inverter) will open out automatically in grid failure and closes on its restoration after a time delay.
2. Tariff for operators of solar roof top devices shall be based on feed tariff fixed by Regulator, both on solar
power consumed by operator and the solar power fed into the grid i.e SE
3. CI=Consumer Import, GI/GE=Grid Import & Export, SE/SI=Solar export and Import, SW –Manual Lockable switch for grid maintenance
In case where battery is also envisaged, the scheme of connection for solar PV system will depend upon the way the battery is charged. There are two possible ways of charging battery. First one is where there is AC coupling i.e. first the DC produced by solar is converted into AC and battery is connected through a charger which converts AC into DC. This arrangement is shown in below configuration.
In this arrangement as long as grid is available the solar system, consumer, and battery system will be tied up with grid. In case of grid failure solar system, battery system and consumer load would be disconnected form grid and solar will be connected to battery and AC loads through another route. This arrangement has been finalized with the premise that normally consumer and battery system will take supply from gird. This scheme is envisaged in Europe where feed in tariff is employed. However the only difference proposed here is regarding the positioning of the solar meter(SM). In the scheme in vogue in Europe for feed in tariff the SM is towards the grid side and does not record the energy drawn by the consumer /battery during grid failure. In the suggested scheme as per above configuration, during grid failure also it measures the solar generation including the energy drawn by battery and the consumer load.
In the second arrangement, solar system directly charges the battery through charge controller i.e. DC coupling (below Configuration). In this arrangement the AC system is not involved to charge the battery and battery will always be connected to solar system. In case, battery is discharged due to any reason whatsoever solar system will first charge the battery and the excess generation from the solar will be
fed into grid. This scheme would not reflect the true gross generation produced by the solar PV system.