Full Integrity Commutation Technique

When powering a load with a renewable energy source while ensuring grid backup, an alternative to a grid-tie inverter is a power source commutator. The downside to this approach is a degradation in the power quality due to an interruption or distortion of the sine wave as a result of the commutation. What is proposed herein is a method of maintaining full integrity of a dual AC power source by implementing a sophisticated commutation between a power inverter and the electrical grid.

The general operation of the commutator system is straight forward: Once the battery pack contains enough electrical charge, the inverter is activated and the commutator is automatically switched to “battery” mode, allowing the inverter to power 100% of the AC output load. Once the battery pack's charge becomes too low to supply energy to the inverter, the inverter is deactivated and the commutator is automatically switched to “grid” mode, allowing the entire AC output load to be powered by the electrical grid (mains) instead. The automatic commutation between inverter and grid is executed without interruption or distortion of the sine wave.

In order to create this commutator, two solid-state relays are configured as a single selector switch. This selector switch and the inverter are controlled by a special firmware technique that’s responsible for maintaining the integrity of the waveform. When the commutator is switched from “grid” mode to “battery” mode, the controller waits for the grid's instantaneous voltage to cross the 0V level on its way towards the positive side of the wave. Once this happens the commutator is switched and the inverter is activated, starting at the 0V level and continuing towards the positive side of the wave. On the other hand, when the commutator is switched from “battery” mode to “grid” mode, a more complex method is used. The controller commands the inverter to execute two “frequency bursts” that are essentially very slight changes in frequency from the nominal frequency value. One frequency burst is a slightly higher frequency value than the nominal, and the other is a slightly lower frequency value than the nominal. Both higher and lower frequencies are however within the grid's frequency tolerance range. During the frequency bursts, the controller waits for the inverter sine wave and the grid sine wave to synchronize. The controller knows that the waves are synchronized when they are both at the 0V crossing on their way towards the positive side of the wave. At this point the commutator is switched and the inverter is deactivated.