What is a Grid-tie Inverter?

Grid-tie inverter is one which can be tied up with electrical power grid. It converts direct current to alternating current in a form suitable for injection into the electrical power grid. That means it generates electricity in the same voltage, frequency and phase of the power grid. Grid-tie inverters can be used for connection to the grid from local power generators like solar panels. They must accurately match the voltage, frequency, and phase of the grid sine wave alternating current waveform. Phase angle should be typically within one degree of the grid waveform.

Internal circuitry of the grid-tie inverter senses the grid waveforms and outputs an alternating current corresponding to that. When the grid-tie inverter is properly configured, it enables the building to use an alternate power system such as solar panels without extensive rewiring and without storage batteries. When there is deficit in production, power is drawn from the grid while the excess production if any is transferred to the grid.

Different types of grid-tie inverters are those with low frequency transformer coupling, high frequency types and even transformerless types. High frequency transformer types use dual conversion instead of single stage conversion to finally match the grid power parameters. The image from Wikipedia shown here is a transformer coupled grid-tie inverter. Transformerless inverters need galvanic isolation between DC side and AC side. Otherwise dangerous DC voltage can reach the grid under fault conditions. This mandates additional safety measures to be implemented for preventing this.

Grid-tie inverters disconnect quickly from the grid and shuts down by design if the grid power is down. This precaution is essential to prevent electricity from the inverter harming persons repairing the power grid. So if you are using a grid-tie inverter alone at home, you cannot use solar power when the grid is down. Back up off-grid storage systems with grid isolation is needed for use in such situations.

Now what is galvanic isolation? It is isolating functional sections of electrical systems to prevent current flow between those. There is no direct path of conduction between sections which have galvanic isolation. Most commonly used method of galvanic isolation is a transformer. That is why transformerless grid-tie inverters need additional methods of galvanic isolation. Capacitors for blocking DC, opto-isolators, Hall effect sensors and relays are some of the other methods used for galvanic isolation. Medical equipment coming in direct electrical contact with persons, like cardiac monitors and ECG machines need galvanic isolation when operated on mains supply.