Short circuit current and protection in the Smart Grid
Last week I was with a colleague in Amman, Jordan to deliver a workshop on Smart Grid for the TSO NEPCO.
The workshop was very well attended -- some 50 people were present – and one of the topics raised was the problem of increased short circuit current in the power grid, as most of the Jordanian generation is conventional. For the future, there are plans and possibilities to increase the share of renewables, notably wind and solar power.
The discussion moved towards the use of current limiting devices, such as reactors and (superconducting) fault current limiters and other solutions like busbar splitting. But then the question was asked, what would happen in the future if more and more inverter connected generation would be connected to the grid, especially in the medium- and low voltage? When thinking about this, it obvious that short circuit current is not going to increase; certainly not in smart distribution grids because inverters limit the short circuit current to slightly above nominal current. Thinking a bit further this means that conventional protection, which is based on short circuit current and power plow in one direction (coming from the feeder transformer), is not going to work anymore. A typical fault in an inverter-rich distribution grid, probably with even the feeder transformer replaced by an electronic equivalent, hardly gives any change in the current but is only observable as "no voltage" at the fault location. This implies that the protection system has to be changed drastically for such a distribution grid to locate and isolate the fault.
This brings me to the answer to the broader question that was also raised during this workshop: "Peter, how will the Smart Grid be different from today's grid?" The answer I gave: It will probably look more or less the same as today's grid with the same components, transmission lines, cables, transformers, switchgear, etc., and some (relatively) new components based on power electronics and DC. But the main difference will be the power flow steering, control and protection and consequently the need for more sensors and metering.