Current Source in Short circuit analysis

[surgeArrester]

Lately, i am having doubts in simulation of short circuits regarding PV plants.

PV will be connected to inverters and then to a step up transformers then maybe another step up transformers until we can met the kV level at the connection point.

Upon simulation most inverters are modelled as 1.0/Isc as impedance with a voltage source of 1.0pu.. I have tried this in etap, neplan, and others.

But according to a book, by Tleiss, it is said said that inverters are modelled as current source rather than voltage sources. It is said that applying the 1.0/Isc will only be applicable if the fault is on the inverters ac side terminals.

Can anyone give an idea on if it is possible to model current sources on etap or an software aside from digsilent?


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[BabyBear]

I think when you use a PV component in etap or some other software, the current source model is already inside the component. When the etap engine, recognizes it, it prepares the Ybus matrix or any other algorithm for calculation of short-circuits. In pscad this is different. Three current source components should be build to create an average model of a PV. There is also a detailed PV model which consists of several components. You may want to check NREL document in relation to PV modeling in PSCAD.

[surgeArrester]

Hmm.. Thanks.. I tried etap but it models inverter as a voltage source. I tried to make a series of faults and the contribution from the inverter changes. It is not supposed to change if we considered it as a constant current source. Manufacturer's declaration on the inverter i have used stated that their inverter is able to give full load current during faults constantly. That's why i am looking for a constant current source.

In digsilent, i was able to find a constant current source, however, i always cannot prove by hand on simple systems how digsilent calculate faults. On etap or neplan, i was able to check it by hand on simple systems.

I need to check by hand so that i could verifying if i am simulating it correctly...

I have check some NREL docs and they said model it as constant source..

I would have some difficulty if i am to model it in pscad since it is a big network..


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[BabyBear]

I saw a document yesterday about short-circuit analysis for PV. It says that current source may be used for modeling purposes although there is no yet accepted model for inverters when dealing with short-circuit calculations.

[surgeArrester]

I agree to that, I think I have also seen a similar document.

But I cannot let go of the manufacturer's declaration regarding the inverter's fault response. Although there are inverters that during voltage dip (during faults) it will automatically disconnect. But in their declaration (through their testing) they said that the inverter will still provide full load current as its steady state response constantly.

By the way, I have why digsilent provides different answer. I will using digsilent for this simulation.

Thanks for the response.

[vlady34]

I work in the area of protection and readings according to short-circuit currents that provide the plants are negligible and are taken as zero. So, they are protected only by voltage. In DIgSILENT, the model provided by manufacturers and specifications are additionally admitted the fault current of the panels is usually a percentage of the load current is entered. Actually it is not the investor who provides fault current, plus bring these various protections that are added to the models that the manufacturer provides.

[surgeArrester]

I also agree to that, even if i model the inverters as current sources, only that to step up twice the value will be small or nothing compared to the fault contributed by the grid.

Also, since the current sources are modelled with an angle of zero and added to the fault with a lagging angle, it will decrease the fault.

Thanks for the response. We are using SMA 900CP XT. I also agree that they are to be protected by voltage elements.

The only downside on my part is that i don't have any recorded actual fault readings or comtrade files from the PV plants.


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[vlady34]

Equally, I have no fault data because of the plants we have installed has had no internal flaws.

[surgeArrester]

I have read from iec 60909-2016 (only in a presentation) they have some clauses for modelling inverters..


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[surgeArrester]

I dont know if any one have those files


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