I was on a Brazilian navy ship. I had been employed to design some noise-reduction measures but, as soon as I was on board, I was hustled down to the main electrical control room by the ship’s engineer. He could speak no English and I no Portuguese.
The ship had three diesel generators running and all connected in parallel. The engineer pointed out that, while each generator was supplying an equal proportion of the electrical load, the current supplied by each generator differed, as indicated on the instruments. Okay, I said, your problem is that you have to trim the voltage regulator of the alternators. Or I would have said this if I could speak Portuguese. So, I tentatively opened the door of the electrical control board of one generator and, since fate is kind, saw a small knob marked ‘voltage trim’. I swiftly adjusted the voltage trims of all the generators. Lo! The current outputs became all the same. The thing to grasp is that, when generators are in parallel, adjusting the voltage of one generator will not alter the voltage of the generator – it will just adjust the current. This is due to the magnetic circuits of the generators.
Now solar arrays generally consist of solar cells that are connected in parallel and series and supply direct current, which is fed to an inverter which produces alternating current, which can be connected to the grid. Very often, these inverters are set to supply power at a maximum power point and, in doing so, can cause overvoltage on the grid.
When this happens, the inverter trips and resets and will reconnect after about one minute or less. However, if this happens to a group of homes with solar power, then the sudden loss of generation can cause the grid supply to be the sole source of supply. If the grid supply is not significantly larger than the connected solar supply, then the grid is overloaded and finally trips on overcurrent.
Another effect is that most grids are designed so that, at the most distant connection, a few kilometres away, the voltage is at least 205 V, which, at the nearest connection, the transformer output, requires a voltage of 240 V. Now if there are a whole lot of solar panels boosting the grid voltage at various points, then overvoltages will occur.
Naturally, this will not happen at night. So, the thing to do is to have the transformer output on sunny days set to about 215 V and change this to 240 V at night, when all the solar is off line. This is very impractical for urban power supplies. It is all unnecessary as well. The solar panel can supply power quite adequately to normal grid voltages. It would take a lifetime to explain to the average solar installer how this would work. According to ABC Australia, high voltages on solar-fed grids is an issue.
There is another issue here in South Africa and, in fact, in Cape Town, my current city of residence. It is absolutely delicious. So I was visiting somebody who is a militant environmentalist. She had found out that we disagreed on many points relating to conservation and she told me that she was “off the grid”. I said, wow, oh really? I asked to see her solar panels on the roof. By my estimate, 1 200 W of panels. I asked to see the storage batteries. Three of them, each rated at 102 amp-hours. The numbers just did not add up. So I asked how she could be off the grid. Oh, she said, light-emitting diode lights. Gas cooker. Solar hot water. Now this house is a double storey, with a guest section. But how? Then I found that, in fact, the municipal prepaid meter in her board was bypassed. So a con artist had relieved her of R40 000, installed some batteries and a solar panel or two, bypassed the meter and, hey, off grid! Now that’s a good renewable business.