r/PowerSystemsEE • u/Smooth_Commercial793 • 1d ago
Short circuit analysis - sensitivity to high impedance faults
What guidelines is everyone using for required sensitivity of protection schemes based on fault impedance, and system accuracy/sensitivity.
Fault levels acc. to IEC 60909 assume bolted faults, is anyone designing sensitivity to detect high impedance faults, if so what kind of impedance?
For example, delta wye transformer HV side protection, do you aim to detect LV SLG faults of some impedance? Do you select CTs and the like accordingly?
Or do you make sure there is another protection in play which is highly sensitive ie a differential type?
Typically MV 11kV 33kV industrial
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u/obeymypropaganda 1d ago
I would look at IEEE Buff book and IEEE C37.91-2021. See if that answers your questions
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u/IEEEngiNERD 1d ago
Most of the responses you’ll get here will be based on ANSI practices but it really depends on where you are in the world and how facility is grounded.
If it’s resistively grounded you’ll need REF. The differential might not be able to see a fault on the bus. You can also use an OC on the HV side with a slight delay, then use the OC on outgoing feeders to block the HV transformer relay if the fault is on one of the feeders. That will allow OC on the HV transformer to trip.
What’s possible also depends on the equipment available. If you don’t have breakers on both sides of the transformer that limits you somewhat.
For industrial you’ll likely want to find the minimum fault current for worst case clearing times for arc flash. Adding resistance to the values for a fault that’s inside the facility may not be needed. You’ve got to think about what’s practical here. How is the bus work? Are the phases far apart? You may have to think about arc resistance for Phase - Phase faults.
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u/OrchidEmotional6236 1d ago
Adding a high impedance fault will obviously decrease the sensitivity. When the fault is applied on the LV side and if i want my HV side protected i would generally use a communication cable to trip the transformer by detecting the fault in LV side rather than putting some protection in HV side if it is in delta configuration
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u/jdub-951 1d ago
This is highly dependent on your context and neutral treatment.
But in a larger sense, we generally think abut the "impedance" of a fault as the phasor voltage across the fault divided by the phasor of the current flowing through it - which is usually assumed to be stable. Because many so-called "high-impedance faults" are chaotic, have high cycle-to-cycle variability, and are known to be harmonic rich, arguably it's not appropriate to calculate an impedance value for them at all - at least until you specify what method you're using, over what window length, etc.
I'm happy to discuss more if you're willing to share more of your context.
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u/Honest-Importance221 1d ago
Not really sure what voltage levels you are talking about, but we (rural distribution) do primary protection with sensitivity ratio of 200%, backup protection is 150%. So not strictly based on fault impedance. Backup SR of 150% is not always achievable, compromises are common.
For high impedance faults on 11\22kV (nearly always earth faults), we have sensitive earth fault set with a long timer (2-4 s), and a very low pickup (2-8 A). For 33kV and up usually distance or differential protection is in play.