Feeding Solar Power Back To The Grid

Feeding Solar Power back into the electricity network is the job of your inverter. We have talked about sizing your inverter and how the location of your solar power panels can effect the costs of installing a solar power system. One of the concerns we have come across is “Impedance”; this is also an important factor to keep in mind when building/designing a solar power system.

It is important to remember that, in a solar power system, optimising the size of each component is extremely important if you want to maximise the economic and technical efficiencies of the system. In order to understand how to do so, it is essential to understand the concept of “electrical impedance”.

What is electrical impedance and why is it important in my solar power system?

As we have discussed in this previous blog entry, solar power system arrays produce Direct Current (DC) electricity, which is then converted to Alternating Current (AC) electricity via an inverter, to then be either fed into the electricity grid in a grid-connected solar system, or used to power your appliances directly. Electrical Impedance is defined on the UNSW Faculty of Physics website as the ratio of voltage to current (also discussed in this blog entry)–it is essentially ‘resistance’ in a circuit, but specifically for AC electricity. This means, in short, that impedance is what inhibits current flow between two points in an AC electrical circuit.

So why does impedance matter in a solar power system?

When the first set of electrons that are pushed through the wires, connecting your solar panel to the inverter, they are in DC form and must be converted to AC  in order for them to reach the grid. After leaving the inverter, which converts DC to AC, the electrons are now ready to be transported in to the grid, however there are some restrictions here that will require a bit more explaining.

The transport of  electricity around a network usually takes three forms: large, medium and small power distribution and transmission, as can be seen in the image at the top of this article. The reason for the three different sizes is to minimize the losses that occur during the transportation of electricity over long distances. The other reason for the different sizes is based on the estimate of how much power will be consumed by that region, for example some power lines to properties in the outback are sized at 330kVA where as some are sized at 110kVA or even less (read here for an explanation of what kVA is, but it is essentially the same thing as Watts, but specifically for DC electricity). The size of these power lines is pre-determined by the network companies like Country Energy or Energy Australia, and if you build a solar power system and your output power does not match the voltage and phase of the power line that it is connected to, there will be an impedance mismatch which will result in losses. (Note: This is an example/reference for rural properties and their larger scale systems and not urban properties or complexes in metropolitan areas.)

Impedance matching is an important concept because impedance acts as resistance to the power that you are transmitting back to the network. Essentially, it reduces the actual amount of power received by the network and thus reduces the efficiency of your solar power system.

Another way of thinking about the grid and the network is like a circulation system of blood vessels, arteries, veins and capillaries, and just as you can’t feed an artery into a capillary, you can’t feed a large solar power system into a point towards the end of the grid without certain adjustments to your system, in this case your inverter.

Therefore, when considering the size of your inverter, it is important to determine not only the most appropriate size of the system based on your requirements, but also the best size based on what the network is able to accept from you at the connecting point of your meter.

Written by Prateek Chourdia

MEngSc – Photovoltaics and Solar Energy, UNSW

Solar Energy Analyst

Solar Choice

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Definition of Impedance from UNSW Faculty of Physics Website

Previous Solar Choice Blogs: Inverters for stand-alone power systems : Stand-alone solar systems: how to minimise cost : DC vs AC and why this important in understanding solar power