Renewables improving the grid

Grid voltage corrected infographic

An innovative trial is using smart solar inverters on homes, both on their own and combined with batteries, to improve grid stability. By Lance Turner.

Many Australians are all too aware how unstable the electricity grid can be at times, especially under large loads, such as when everyone gets home and cranks up the air conditioner on a hot day. Other factors that can affect local grid stability can include large numbers of distributed generation sources (such as home PV systems) in a small area, long grid distribution lines, and old, poorly maintained or undersized grid equipment such as transformers and cables.

The result can be a number of problems (see Figure 1), including low or excessive grid voltage, low or high grid frequency or poor power factor (a mismatch of the voltage and current waveforms).

While upgrading grid equipment is one possible solution, it’s not the only answer. Long feeder lines experience both increases and decreases in voltage along the line due to the natural impedance (like resistance) of the cables—homes a long way down the feeder can see an ohm or more of impedance between the substation and the home.

At times of light load (energy consumption) but high PV generation, such as the middle of a sunny weekday, the feeder may see a steadily increasing grid voltage along its length; for each ohm of impedance along the feeder line, every amp flowing into the grid raises the voltage on the grid by 1 volt. For example, each 5 kW solar system can be adding 20 amps into the grid, or an increase of up to 20 volts above the other end of the feeder line. In the evening when solar generation is almost zero but demand is high, this same grid impedance causes the voltage to sag. Thus, the voltages along the feeder, especially towards the far end, can vary widely (see Figure 2).

A good example of how extensive the problem can be is in Figure 3, which shows the high and low grid excursion events (where the grid voltage tends towards the allowable limits) for a selected substation over a two-year period.
Although this can be mitigated by an increase in cable size to lower resistance and installing transformers with a higher capacity, such upgrades are expensive and can never eliminate voltage variation caused by system impedances. So, other, smarter options are now being considered.

Read the full article in ReNew 142.