The strange case of the disappearing short circuit current
In my last blog, I discussed some issues related to whether or not a very large power system can be operated with only distributed solar generation? While I was speaking with some customers last week a similar question came up: “Is it possible to energize the entire interconnected power delivery system from the bottom up, with millions and millions of small (renewable) generators that are sized at a few kVA?” This led to a discussion about one of the key design criteria for power systems, which states that equipment must be able to withstand the short circuit current caused by a failure during the time needed for the protection system to detect and react. Due to the increase in electricity use, it seems obvious that the short circuit capability in high- and medium-voltage transmission systems needs to increase well beyond the standard 25kA and 64 kA practices. It is true that this increase is observed when more conventional generation is added. This increase of short circuit current directs towards solutions like the introduction of current limiting devices such as reactors and (superconducting) fault current limiters and other solutions such as busbar splitting. But what happens if the added new generation is based on variable renewables like wind and solar and possibly even replaces conventional generation?
The inverter-connected renewable generation would be predominantly connected to the medium- and low-voltage grid. When thinking about this, it obvious that the short circuit current is not going to increase, certainly not in smart grids because inverters limit the short circuit current to somewhat, e.g. 1.2 to 2 times above nominal current, depending on the control algorithm used. Thinking a bit further, it implies that conventional protection, which is based on a high short circuit current and power flow in one direction (coming from the feeder transformer), is not going to work anymore. A typical fault in a future inverter-rich distribution grid, possibly even equipped with an electronic equivalent of the feeder transformer, hardly gives any change in the current. This implies that the fundamentals of the protection system have to change drastically for such power systems to be able to detect and isolate the fault—in other words, fuses won’t work anymore.
For more information about this topic take a look at a paper my colleague Cornelis Plet wrote recently.
To summarize: an increasing share of inverter connected renewables combined with power electronics (enhanced) transformers in the distribution system leads to a significant change of short circuit current behavior in magnitude and direction. This development poses big challenges for protecting the future power system and requires careful thinking about how to achieve the transition without compromising availability and reliability.