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Balancing safety and cost-effectiveness in solar power inverter installations

July 11, 2011 | Bernard Richard, Claude Gudel and Stephane Rollier | 222902987
Bernard Richard, Claude Gudel and Stephane Rollier of LEM examine how to balance safety and cost-effectiveness issues that relates to solar power inverter installations.
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In response to rising fossil fuel costs and environmental concerns, installation rates of solar photovoltaic panels, are rising rapidly. A further factor encouraging PV deployment has been incentives in the form of favorable feed-in tariffs to national grids; almost 99% (Reference 1) of the energy produced by solar installations is grid-connected via an inverter. All PV installations demand accurate measurement of generated current, both for efficient control of the inverter, and protection purposes.

Grid-connected inverters may, or may not include a transformer to provide galvanic isolation: transformer-less configurations incur a higher risk of leakage to earth. Four main inverter designs are commonly encountered. Two designs use a transformer (at low or high frequency) and two designs are transformer-less; with or without a DC chopper or step-up converter. For cost but also size, efficiency, weight reasons, transformers are less favoured for new designs.

MPPT control, inverter control and protection
For each different topology, instantaneous DC current and voltage output of the PV panel must be measured, to establish the MPP (Maximum Power Point) at which the maximum output power can be extracted from the solar panel. Current measurement is also needed as an input to the control loop of the inverter, and to ensure protection against short circuit or overload. Open-loop and closed-loop Hall effect technologies are used for the current and voltage transducers.

DC current injection measurement
In transformer-less designs and in high-frequency transformer configurations, the DC current that an inverter is permitted to inject into the grid must be limited to a maximum value of between 10 mA and 1 A, according to different standards that apply in different countries (relevant standards include IEC 61727, IEEE 1547, UL 1741, and VDE 0126-1). This requires transducers with very high accuracy (better than 1%) and very low offset and gain drifts; an ideal technology is the closed loop Fluxgate transducer (Fig 1).
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