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Short circuit current and protection

Last week I was in Amman, Jordan with a colleague to host a workshop on Smart Grids for the TSO NEPCO. The workshop was very well attended and sparked many conversations about the issue with the increase of short circuit current in the power grid, as most of the Jordanian generation is conventional. In the future there are plans and possibilities to increase the share of renewables, notably wind and solar power.

The discussion was focused around the use of current limiting devices as reactors and (superconducting) fault current limiters, as well as other solutions, i.e., busbar splitting. One attendee asked me what would happen if more inverter connected generation was connected to the grid in the future (especially in the MV and LV). It’s clear that short circuit current is not going to increase—certainly not in smart distribution grids, as 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), are 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 must drastically change for a distribution grid to locate and isolate the fault.

I was also asked how the future smart grid be different from today’s grid. It will likely look the same as today’s grid with the same components, transmission lines, cables, transformers, switchgear, and (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.

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