Inverters lighten up

An electric vehicle is like a competitive runner in one regard: Every pound it must carry is a pound that forces it to consume more energy. No surprise, then, that there is a concerted effort in industry today toward coming up with EV components and systems that are as light as possible.

The trend toward light components has even affected inverters. Many traditional inverter designs incorporate magnetic components such as transformers and line reactors that can weigh tens or even hundreds of pounds. Thus there is an incentive to create inverter designs that don’t need to use these heavy and bulky magnetics.

Solar inverters rated at 30 kW in place at the University of Texas at San Antonio.

Researchers have devised several such inverters that eliminate the need for large energy storage components. One in particular that has transitioned from the research lab to commercial use has its first commercial application not in EVs but in connecting solar arrays to the grid. Today six 30 kW inverters installed at the University of Texas at San Antonio convert up to ± 450 V from solar arrays into three-phase ac at 480 V that feeds onto the power grid. The topology will also be used for other applications including wind converters, bidirectional battery / vehicle chargers, and variable frequency drives for induction motors. Each 30 kW solar inverter weighs in at just 94 lb compared with conventional solar inverters with traditional magnetics that can be 1,200 lb or even more.

This solar inverter uses a topology that is a compact alternative to existing methods. It also eliminates the need for a bulky and lossy transformer. It is called a buck-boost, indirect-transfer, pulsed current-sourced topology. It is unlike ordinary inverters which normally use an input stage to charge a capacitor -- often called a dc link -- that serves as a power source for an output stage that creates an ac waveform. Instead, the new design uses a pure ac link formed by an inductor-capacitor pair between the inverter input and output. The link charges from the input(s) and discharges into the output phases via a modulation scheme that results in high efficiency, unity power factor, and low harmonics.

The input side of the converter uses four reverse-blocking switches such as a series diode/IGBT. Two bidirectional switches are in each leg of the converter output. There is an ac link between the input and output formed by a low-reactive-rating inductor/capcitor pair. In a completely indirect energy transfer operation, the dc input from the PV panels charges the link inductor. This inductive energy then discharges to the output phases.

It is useful to compare this new kind of inverter with traditional techniques. Most commercial inverters for photovoltaic applications include a transformer and several sections of power conversion. Without transformer isolation, there is a galvanic connection of grid and PV-generator and a leakage current may flow through the capacitance between PV-generator and ground. The magnitude of this capacitance depends on environmental influences. For example, it will be large when the PV-generator is covered with salty fog forming a conductive path to the grounded metallic frame of the PV modules.