Control Intelligence Improves Renewable Energy Efficiency

Sep 1, 2007 12:00 PM
By Arefeen Mohammed, C2000 Applications Engineer, Texas Instruments, Dallas

The blocks of the essential inverter circuit are shown in the top portion of Fig. 2. (The bottom portion will be discussed later.) First, dc-dc conversion raises or lowers the incoming voltage, adjusting its output for greatest efficiency. After some additional voltage buffering, MOSFETs in a bridge use a switching frequency, usually between 18 kHz to 20 kHz, to convert the dc to an ac voltage. Finally, a low-pass filter smooths the switched ac to a sinusoidal signal for use in generating a grid-frequency ac output. (Fig. 2 does not show the dc-dc conversion and regulation that are required for battery charging.)

Transformers and Protection

Because the source input is usually not high enough, the system can either step up the voltage with a transformer on the ac side or boost it in the dc-dc conversion stage. Just as an ac transformer inherently provides galvanic isolation, so does a phase-shifted full-bridge dc-dc converter with zero-voltage switching, thus making the latter equivalently a transformer. Fig. 3 shows a commonly used dc-ac circuit with a transformer for single-phase inversion, based on an H-bridge configuration controlled by four pulse-width modulated signals.

On one hand, transformers add weight, bulk and cost, and they also cause a reduction in efficiency of about 2%. On the other hand, they increase circuit protection and human safety by isolating the two sides of the circuit electrically, preventing a dc fault from flowing to the ac side and an ac leakage current from developing a potential issue between the PV panels and ground. The design may include a residual current protection device (RCD) that monitors the currents of all phases and then trips the relay if the current exceeds a certain value. Because of the risk of current leakage, RCDs are especially important for safety in transformerless systems.

Protection of the system mandates inclusion of a relay to protect the conversion and charging circuitry against voltage surges and spikes on the grid. In addition, if a power line is damaged or the utility has to shut it down, the inverter needs to stop feeding out electricity to the utility. A “nonislanding” inverter senses that the line has been de-energized, is undervoltage or overvoltage, or has a significant disturbance for whatever reason. When this happens, the inverter automatically disconnects from the utility grid, thereby not becoming an electricity generating “island.”

Maximizing Charging Power

The efficiency of battery charging depends on the input voltage, which can be highly variable, depending on wind conditions for a turbine or on the season, cloud cover and time of day for PV panels. Battery conditions vary, too, depending on the charge state, so sometimes it may be necessary to adjust the voltage and current ratio to increase the total power delivered and speed charging.

Maximum power output to the battery occurs when the product of voltage and current is at its peak, the MPP. MPPT is designed to determine this point and adjust the dc-dc voltage conversion to maximize the charging output. MPPT can increase the overall efficiency of a solar-power system by one-third or more during winter months, and its effect on other types of systems can be significant, too. Fig. 4 shows how the determination of MPP can vary with different conditions.

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