A main component of the laboratory is a microgrid that comprises a PV generator, a small Wind Turbine, battery energy storage, controllable loads and a controlled interconnection to the local LV grid. The battery unit, the PV generator and the Wind Turbine are connected to the AC grid via fast-acting DC/AC power converters. The converters are suitably controlled to permit the operation of the system either interconnected to the LV network (grid-tied), or in stand-alone (island) mode, with a seamless transfer from the one mode to the other. A schematic diagram of the microgrid system is depicted in the figure below, along with a photo of the actual installation.
The central component of the microgrid system is the battery inverter, which regulates the voltage and frequency when the system operates in island mode, taking over the control of active and reactive power. The battery unit power electronics interface, consists of a Cuk DC/DC converter and a voltage source PWM inverter, both bi-directional, permitting thus charging and discharging of the batteries. The DC/DC converter provides the constant 380 V DC voltage to the DC/AC converter input. The HF transformer, operating at 16.6 kHz, provides electrical isolation between the battery bank and the grid. The four-quadrant DC/AC converter comprises a single phase IGBT bridge, output filters and a grid-connection inductor.
In more details, the microgrid features:
- PV generator: Modules : 10 series connected - Single crystal Si, 110W, 12 V per module.Inverter: SMA/Sunny Boy/1100 W
- Small Wind Turbine: Generator: 3 phase axial flux PMSG, 850W. Inverter: SMA/Windy Boy/ 1700W. The Wind Turbine was fully constructed by students of NTUA.
- Batteries: Cells: Lead-acid, vented type, 30 cells, 2 V, 250/370 Ah.Inverter: SMA/Sunny Island/4.5 kVA, bi-directional, suitable for grid-connected and islanded operation.
- Grid: Connection to local building distribution (lab switchboard). MCB for protection – Contactor for control
- Loads: Controllable resistive-inductive load, switchable capacitive. Lighting (incandescent, CFL).Small motor and other available appliances
The 3-phase microgrid with main elements 3 single-phase battery inverters Sunny Island 2012, is being extended with the connection of new energy sources.
A single-phase inverter Danfoss DLX 2.0 is connected to the 3-phase microgrid. The inverter is dc-supplied with photovoltaic modules of 2.1KW from the parking area.