Can a very large power system be operated with only distributed solar generation?
Last week I delivered training to TenneT TSO staff and we discussed, amongst others, the consequences of the growth of the variable renewable share. The following question came up: “Is it possible to run a large (size of Europe or US) electric power delivery system based on millions of small scale distributed solar generation?” Assuming that the installed renewable generation capacity is sufficient high and some form of storage is employed to ride through critical periods with little generation the answer to this question is YES from an energy perspective, as is confirmed in many studies. It must be noted however that these studies do not take reactive power into account, assume that there are no transmission constraints and do not talk about the decrease of the grid intertia (related to frequency stability).
If more generation takes place at distribution level (low and medium voltage) and even replaces central (traditional) generation at higher voltage levels, reactive power becomes more important because it is directly coupled with local voltage control and reactive power is difficult to transmit across longer distances. Therefore one of the questions is where does the reactive power come from in such a scenario? Most of it has to come from the generators themselves, the grid connected inverters must be able to absorb and generate sufficient reactive power to maintain the voltages throughout the power system. Because reactive power transmission is difficult, possibly also a number of reactive power sources e.g. in the form of (controllable) capacitors or static VAR compensators have to be placed at strategic locations in the grid. When the grid connected solar inverters and the reactive power control units cannot produce the reactive power needed by the system, the voltage sags and ultimately collapses and disables the flow of power. In case of short circuits in the grid the reactive power demand becomes temporary very high, this is in fact one of the reasons why present-day (larger) grid connected renewable sources are required to deliver reactive power for a certain amount of time (Fault Ride Through capability). In addition to the above mentioned measures the reactive power demand at all customer loads needs to be carefully compensated at all time in order to minimize reactive power transmission.
In summary: for a small scale renewable generation dominated future one of the requirements is that the (local) reactive power balance has to be carefully maintained under all normal and fault conditions to ensure a stable operation with adequate voltage control. Of course island power systems can be used to test this, as well as the low-inertia issue. Translating the island outcome to a very large power system with transmission constraints is however not straightforward.