Transformer Overcurrent Protection - What to Consider When Setting Protection Relays

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In this video we discuss how to develop relay settings for overcurrent protection of power transformers.
We go over what to consider when selecting the pickups and time dials of instantaneous and inverse-time overcurrent elements as well as how to make sure the power transformer is protected against both overload and fault conditions.
This video is excerpted from Romero Engineering Company's Power System Protection: Transformer Protection online course. Check out the full course at:
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[@hossamismail2515]

Informative video and so thanks for sharing knowledge please when we need to make 58% shift curve during coordination process.Some office Consultants did curve shift and sometimes no need to shift although same transformer protection scheme Thanks

[@nolantopey9246]

Great video! Quick question: For the Instantaneous pickup current setting of 3614.4A how do you know that it will pick up for internal TX faults or TX HS faults?

[@romeroengineering]

Hi Nolan. You could verify this by looking at the fault current for a fault on the high side bushings of the transformer. You can simulate this using a short circuit software like ETAP, and should see that your instantaneous overcurrent element picks up with plenty of margin. The fault current magnitude will decrease the further down the transformer the fault is. Eventually, for a fault on the low side of the transformer, the fault current should be low enough that your instantaneous element does not pick up.

[@mathai123a]

Can you make a video for two phase fault also

[@romeroengineering]

Thanks, we'll consider it for a future video

[@maryamd99]

Another well-explained video from you! Thanks for making this. I have a question. Is there a formula or calculation method that we can use to provide coordination between two OC relays without the help of software? Let me give you an example. This is the setting of our 51 protection at 125% of FL:
Pick-up: 90.21A (Sec: 0.18A with 200:1 CT), TD.2 U4 Curve.
I am asked to revise it to provide enough coordination with the upstream OC phase relay of:
PU = 400A, TD.1 U4 Curve.
How should I do it if I don't have access to ETAP?

[@romeroengineering]

Hi Maryam, good question. The instruction manual for each relay has an equation for each curve type. You could plot this using a software like excel, or just calculate the trip time at a specific fault current by hand. You can do this for both relays and make sure the coordination time is at least 0.3 seconds between the two.
For example, for the SEL U4 curve the equation is:
Tp = TD*(0.0352+5.67/(M^2-1)), where Tp is the trip time in seconds, TD is the time dial, and M is the multiples of pickup (i.e., fault current divided by pickup).

[@maryamd99]

@@romeroengineering Thank you

[@tuna6423]

Hi Romero,
I am wondering at 15:21
The pickup of 51 Relay, it is now cross to thermal damage curve of transformer
I am wondering why we select curve lower mechanical damage curve of transformer while relay pickup curve is upper thermal damage curve of transformer consider OK.

[@DeepakKumar-nv8mx]

Sir,i want to know the transformer HT Breaker control drawings with practically seeing it on control panel! Like breaker closing circuit, close permissive, tripping circuit!

[@raafeterguez]

Thank you for sharing your knowledge, i have a small question , how to calculate Max High Side current fault without software

[@romeroengineering]

Hi Raafet. I have a full course where I cover fault current calculations by hand using the symmetrical components technique. We show step by step how to calculate fault currents by hand for all types of faults: three-phase, single-line-to-ground, line-line, and double-line-to-ground. You can view more details here:
www.romeroengineering.co/courses-symmetrical-components-for-power-system-protection-engineers

[@patricklaakak9458]

How connect and protect two transformers with a single breaker relay please?

[@romeroengineering]

Hi Patrick, we can go over your project to come up with a solution during a one-on-one session, you can sign up here: www.romeroengineering.co/offers/ZEo7VkVz/checkout

[@alzakitalha1378]

hello brother, do u have any course covers ETAP for power systems protection engineers

[@romeroengineering]

Hi Alzaki, we are actually currently working on an ETAP course for protection engineers, which will be released in the next month or so.
You can get access to that course once released, and all other courses on our platform here:
www.romeroengineering.co/offers/FRPvBQDG/checkout

[@aatirehrarsiddiqui8894]

Any chance you'd try this sort of thing in SKM PTW?

[@romeroengineering]

Hi Aatir, thanks for the recommendation, we'll consider it for a future video

[@ARzik-ei9ct]

To avoid that both curves cross each other just updated scc To the relay and the curves are going to be limited to the prospective fault current. It provides better presentation.

[@romeroengineering]

Hi Rzik, agreed, you can limit the display of the overcurrent curves to the available fault current level. However, I generally do not like to do this because, if the fault current level changes in the future (increases), if your curves miscoordinated at higher current levels, it would be possible that the curves are not coordinated anymore at the higher fault current level. So, in general, I like to display the entire curve and check for coordination regardless of the fault current level.

[@ARzik-ei9ct]

@@romeroengineering I like your approach, really helpful

[@ARzik-ei9ct]

Thanks for sharing, how did u choose 1.5 as factor for setting 51H?

[@romeroengineering]

You would need to pick a margin above the FLA. 50% above, i.e., setting your 51 element at 1.5xFLA provides good margin.

[@ARzik-ei9ct]

@@romeroengineering thanks