We have designed and constructed an experimental system based on a 500 kW dc having a solar-pannel unit, a wind turbine generator unit, an electric power storage unit, fuel cell, diesel generator power-leveling unit, and an ac-grid connected inverter unit. The electrical system is assumed to be radial with several feeders and a collection of loads. The radial system is connected to the distribution system through a separation device, usually a static switch, called point of common coupling (PCC). Each feeder has circuit breaker and power ?ow controller. For energy storage and to provide effective power back up we have Batteries and Diesel generator.
Starting with solar PV the output of the Solar PV is passed through a controlled current source & a capacitor is connected across it which is used for varying the output voltage. The voltage due to charging and discharging of that capacitor is the solar output voltage. The constant DC voltage is fed to the input terminals of boost converter. The firing of the IGBT is done by producing gate pulses from the MPPT block. The MPP is obtained by tracking the current and voltage of the output with changing temperature and irradiation, and thereby varying the duty cycle so as to operate at maximum power point. There are basically two techniques used in this project: P&O and IC method. The gate pulses produced are given to the switch of boost converter. According its functioning, an increased solar output is obtained at the end of boost converter stage. The next stage is the Inverter & filter stage. The dc output is fed to the terminals of a 3-phase VSI, which operates in 1800 conduction mode and produces a stepped waveform in the line voltage. The inverter used is a 2 level inverter as it creates two levels of voltage in the line voltage output. The stepped waveform output is then passed through a LC filter that produces the required sinusoidal output. The values of L & C were chosen by hit & trial method by comparing the THD of output sinusoidal wave. The sinusoidal output was stepped up through a transformer of required turn’s ratio to the grid voltage level. The grid in this project is modeled as a 3-phase source with appropriate rating. The VSC control block is used to produce the firing pulses for the inverter so that the PV is synchronized with grid. The grid voltage, current ; DC link voltage is given as input to the controller. The frequency of the grid (phase angle) is obtained using the PLL block.
In wind generator, we have used induction type wind turbine the output of which is AC output. This AC output is passed through a 3 phase capacitor load. This compensates for the effect of inductive reactance causing lagging current and thereby improving power factor and bringing it closer to unity.3 phase transformer connected thereafter is a step up transformer that steps up the output voltage of the turbine to the grid voltage. The filter connected across helps to eliminate the harmonics. The values of L ; C were chosen by hit ; trial method by comparing the THD of output sinusoidal wave.
The fuel cell used in the micro grid system is further connected to a dc-dc converter which is a boost converter followed by a VSC inverter that provides the ac output at desired frequency. A Phase Locked Loop (PLL) was also designed to synchronize the inverter with the grid voltage. Finally L filter was connected between the inverter and the grid to fulfill the standard specifications of current harmonics injected to the grid. The step up transformer is then used for stepping up of voltage to the required grid voltage level.
In order to operate the microgrid in grid connected mode or off-grid mode a control logic circuit is designed in MATLAB/SIMULINK. In the on-grid system, when power output from the renewable greater than the load power then the excess power is transported to the grid. In the off-grid systems when power output from the renewable sources greater than the load power, batteries (energy storage systems) operate and excess energy is stored in it. When the renewable output power is less than the load, diesel generator and fuel cell are used to cover this shortage.