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martes, 4 de diciembre de 2012

Advanced Digital Isolation Technologies Boost Solar Power Inverter Reliability

Date Posted: October 02, 2012 11:32 AM
Author: Don Alfano

    Today’s photovoltaic (PV) power systems offer a sustainable alternative to fossil-fueled power plants, providing lower long-term operating costs, modular scalability, higher efficiency, and a smaller carbon footprint. These power systems use PV panels to convert sunlight into dc voltage and solar power inverters to transform the dc voltage into ac voltage suitable for a commercial grid.

    In addition to performing this essential dc-ac conversion, the solar inverter provides grid-disconnect capabilities to prevent the PV system from powering a disconnected utility. An inverter remaining online during grid disconnect or delivering power through an unreliable connection can cause the PV system to back-feed local utility transformers, creating thousands of volts at the utility pole and endangering both the system and utility workers. Upon reconnect, the inverter cannot deliver power until it detects rated utility voltage and frequency over a five-minute period.

    The inverter compensates for environmental conditions that affect power output. PV-panel output voltage and current are susceptible to variations in temperature and light intensity per cell unit area (called “irradiance”). The cell output voltage is inversely proportional to cell temperature, and cell current is directly proportional to irradiance. Wide variations in these parameters cause the optimum inverter voltage/current operating point to move significantly.
    The inverter addresses this issue by using closed-loop control to maintain operation at the maximum power point when the product of voltage and current is at its highest value. The inverter also provides manual and automatic input/output disconnect for service operations, EMI/RFI conducted and radiated suppression, and ground fault interruption. Encased in a ruggedized package, the inverter is designed to remain in full-power operation for more than 25 years.
    The single-phase PV inverter uses a digital power controller and isolated high-side/low-side gate drivers to pulse-width modulate a high-wattage, full-bridge power stage (Fig. 1). Using feedback from the local ac mains and the power stage, the controller modulates the MOSFETs to synthesize a discrete-time, grid-synchronized 60-Hz high-voltage ac switching pattern. This discrete switching pattern is then filtered to remove high-frequency noise components and delivered to the grid through an isolation transformer.

    A PV inverter’s hardware design is full of tradeoffs that can be problematic if the designer makes the wrong choice. For example, PV systems are expected to operate reliably and at full rated output for at least 25 years, and yet they must be competitively priced, forcing the designer to make tough cost/reliability tradeoffs.

    The high efficiency requirements of inverters often require the use of more expensive gate drivers, power switches, and magnetic components. Twenty-five-year system life in an outdoor environment increases the cost of inverter system packaging due to the higher costs of advanced insulation and potting compounds. These considerations and the cost-competitive nature of the inverter market combine to create difficult decisions for the designer.

    The PV inverter’s exposure to extreme heat and cold for 25 years should cause the designer to take a major pause when considering inverter components. For example, electrolytic capacitors that filter out ripple, as well as optocouplers that provide galvanic isolation, have no chance of “going the distance” in lifetime longevity. Electrolytic capacitors dry out and fail, and the optocoupler’s LED brightness gradually fades in the face of excessive heat and input current. Workarounds for these delicate components include replacing electrolytic capacitors with costly film capacitors and substituting modern CMOS isolators for the optocouplers.



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