This paper proposes a novel 3- stand-alone solar PV system configuration that uses high gain, high efficiency (96%) dc-dc converters both in the forward power stage as well as the bidirectional battery interface. The high voltage gain converters enable the use of low voltage PV and battery sources. This results in minimization of partial shading and parasitic capacitance effects on the PV source. Series connection of a large number of battery modules is obviated, preventing the overcharging and deep discharging issues that reduce the battery life. Also, the proposed configuration facilitates “required power tracking (RPT)” of the PV source as per the load requirements eliminating the use of expensive and ‘difficult to manage’ dump loads. High performance inverter operation is achieved through abc to dq reference frame transformation, which helps in generating precise information about the load’s active power component for RPT, regulation of ac output voltage and minimization of control complexity. Inverter output voltage is regulated by controlling the modulation index of sinusoidal pulse width modulation, resulting in a stable and reliable system operation. The active power demand is controlled by regulating the dc link voltage. All the analytical, simulation and experimental results of this work are presented.
- Power conversion
- Pulse width modulation converters
- Power conditioning, Inverters
- Three-phase electric power
- Power control
- Photovoltaic cells
- Energy conversion
- Solar power generation
- High gain DC-DC Converter
Fig. 1. Simplified block diagram of a two stage stand alone PV system
EXPECTED SIMULATION RESULTS:
Fig. 2. Simulation results of the proposed system during the sequence of events considered
Fig.3 Dynamic response of the dc link: (a) An effective load (Reff) connected across the dc link; (b) Response to step change in effective load (200W to 400W); (c) Response to step change in reference dc link voltage, * Vdc from 250V to 400V.
This paper has described and implemented a novel 3- solar PV inverter system for stand-alone applications. Considering that high PV side voltage leads to several drawbacks, a low voltage PV source is used in the system. The limitation of low voltage PV source is overcome by using a special high voltage gain front end dc-dc converter capable of operating at high efficiency and MPPT. The proposed scheme is particularly conducive to long battery life by as it ensures no battery overcharge or deep discharge. For this purpose, the conventional MPPT scheme is replaced by RPT, which ensures only the required power is tracked from the PV source. This prevents the drawing of excess power from the PV source and the use and management of expensive ‘dump’ loads. Not only the main power stage, but the battery interfacing bi-directional stage also supports high voltage gain with high efficiency. Due to the use of special high gain, high efficiency converters in the power stage, the overall efficiency of the system is 94%. Preliminary investigations have yielded encouraging results. The capacity of the proposed control strategy can be enhanced for high power operation by interfacing other renewable sources (fuel cell stack, wind etc.) to the dc link of the proposed system without significantly altering the control strategy. In spite of the good performance of the proposed system, as verified through several simulation and experimental results, there are some limitations too, as listed below:
- The high gain, high efficiency dc-dc converters used in the proposed system may be difficult to design for high power levels.
- In the proposed system, battery is interfaced with the high voltage (400V) dc link requiring a high voltage gain, high efficiency dc-dc converter. Battery interfacing to the low voltage (40V) dc bus should be explored.
- The proposed system uses a large number of sensors, which may increase the cost and complexity. All these issues are being currently investigated and the findings will be reported in a future paper.
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