The overall rating of the PV installation will be about 100 kWp, requiring total commensurate power hardware rating. For roof-top installation, with single-phase units, the basic power module will be rated at 5 kVA. The power module incorporates the charge controller and the inverter functions. This will be procured directly from an established vendor in the Indian market and the procured product must allow complete re-configurability of its control. Installation of the power modules will be entirely carried out by the external agency as per the design prescribed by the research group. Each module-pair will be connected to a small PV array on the roof-top of one building. Several such roof-top installations will directly feed to the micro-grid. To include the storage capacity, this will require an additional power module connected to it. On the micro-grid, ac outputs of these power modules will be connected in parallel and are required to share the total load, subject to availability of local generation. However, parallel connection of ac stages is technically involved due to the small impedance between adjacent ac stages and deviation in the associated parameters. This requires active equalization of the inter-stage impedances which is to be ensured by the control algorithm.
The control algorithm is required to perform several critical tasks, failing any of which will cause loss of plant availability. The first task is to extract sine-wave output from the rectangular voltage generated by the inverter. This is a specific application, which is best realized in a limited size with a resonant network. However, this also introduces unwanted resonance which needs to be damped by controller action. Due to a number of inverter units in parallel, a small mismatch in the output voltages or internal impedances among the units cause a significant level of circulating current, which could be catastrophic for the units which use semiconductor switches with very low heat capacity. The standard approach is to use the master-slave arrangement where the inverter designated as master decides the common ac bus voltage and all other inverters operate in the current control mode. The drawback of this scheme is that any problem with the master inverter causes total shutdown. The approach to be followed is a voltage controlled scheme which regulates its own current based on feedback of local variables. Remote information includes the number of operating units and the total load current as well as a time-stamped reference template for the ac bus voltage, all of which are not in real-time. For this, availability of the Ethernet backbone has been assumed, which will be anyway necessary for the load-source communication.
During synchronized operation, i.e. when the smart grid is synchronized to the utility grid, each ac stage is not influenced by the operation of other stages in parallel. The output voltage and frequency is solely decided by the distribution system. When the distribution system is de- energized or otherwise made unavailable, the entire system should revert to the islanding mode without any glitch in the bus voltage or frequency. Islanding is physically ensured through controlled switchgear which isolates the utility grid from the local islanded grid. Availability of fast switchgear of adequate rating is assumed and its design selection, procurement and installation do not form any part of this activity.
When the utility grid is re-energized, this event is sensed and the common ac bus voltage is slowly brought up to match that of the prevailing grid conditions, before synchronizing with the utility by closing of the fast switchgear. The state of the main grid is reported by the voltage and current transducers and the controller decides the rate at which the ac bus voltage and phase is matched with that of the utility. One fail-safe option is to switch off the loads during re-synchronization interval but the aim will be to achieve this changeover without load disconnection. Various options to ensure maximum up-time for loads will be investigated and the promising ones tested in the actual installation.