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Issue #211 February 2008
Intelligent Energy Solutions
Solar-Powering the Circuit Cellar
Part 3: Wiring & Electronics
by Steve Ciarcia
Start | Back to the Solar Panels | Wiring and Inverters | It's All About Architecture | Maximum Power Point Tracking | So, Does It Work? | Conservation | Soaking Up Some Photons | Sources & PDF
IT’S ALL ABOUT ARCHITECTURE
The Xantrex GT inverter architecture is specifically designed for grid-tied PV systems (see Figure 3). It uses a two-stage power conversion topology to convert DC current from the PV array into AC current directly consumed by the house or net-metered back into the utility grid. A transformer isolated DC-to-DC converter stage converts the PV array voltage (anywhere within 235 to 550 VDC for the GT5.0 or 200 to 550 VDC for the GT3.3) and converts it to a fixed internal bus voltage of around 400 VDC. The DC-to-DC converter runs at 15 kHz and uses a high-frequency switched-mode power conversion technique similar to electronic power supplies. It also exploits the use of a relatively small, high-frequency isolation transformer and efficient PWM voltage regulation.
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| Figure 3—Xantrex GT inverter architecture. |
The internal DC bus has a substantial amount of energy stored in electrolytic capacitors. This is needed to decouple the power flow between the two power converter stages. The output stage delivers power in a “lumpy” fashion because the power in a single-phase AC circuit varies as sin2 (Wt) (i.e., from zero to maximum at 120 Hz in a 60-Hz circuit). However, the power drawn from the PV array must be continuous to maximize the energy harvested from the array (i.e., no 120-Hz ripple). The DC bus capacitors provide the necessary filtering (or energy storage) to ensure that the input DC-to-DC converter can operate with minimal ripple.
The output DC-to-AC inverter stage is an H-bridge configuration using IGBTs, similar to the bridge circuits used in DC servo motor controllers. It operates as a pulse- width-modulated switched-mode power converter with a 20-kHz switching frequency. The pulse-width modulator is controlled by an inner feedback loop that operates the converter as a controlled AC current source. The inverter controller establishes a sinusoidal current reference that is in phase with the utility voltage and the feedback loop controls the inverter output current to be a low-distortion (less than 5% THD) sinusoid in phase with the utility voltage. An output LC filter (low pass) removes the harmonics present in the 20-kHz switching. As you might expect, the net result is an output current that complies with utility standards (derived from IEEE standard 1547) for power quality (harmonic distortion and DC offset) and FCC Class B EMI limits for conducted emissions.
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