Electronics > Projects, Designs, and Technical Stuff
Power Transformer Inrush Current Limiter
Titan4285:
Greetings,
Currently I am in senior design for EE at my university and have been tasked with coming up with a method to limit inrush current to a 1.5 MW transformer for the purpose of protecting expensive fuses. Our group through online sources, faculty, and sponsor have decided to go with a design that will try to limit inrush current by means a series resistor and controlled relay (one thoughts were to make this work for one phase a duplicate the scheme for the other 3 phases). I have read numerous papers describing how this technique has been employed in the past but I want to ask if anyone has any helpful resources or practical experience with this current limiting scheme that they would be able to send our way.
Sylvi:
Hi
Inrush limit circuits vary from what you describe - series-R switched out by relay - to simple thermistors, to complex active circuitry which could be thyristor-based. However, most of the examples you see are at power levels <20kVA. Any circuit can be scaled up or down power-wise, voltage-wise and current-wise, but the power levels you are dealing with require hydraulic relays and other fun stuff. I doubt a bank of thyristos would be feasible although what is available to high-power industry is not known to us civilians :)
You may not be able to improve upon what has been done traditionally? Good luck!
exe:
May be this will be helpful: http://sound.whsites.net/articles/soft-start.htm .
Here was my initial attempt: https://www.eevblog.com/forum/projects/softstart-circuit-with-relay-please-critique/ . Since then I redesigned the circuit, but the thread may be helpful to identify some concerns with such circuits.
Then I re-iterated the project, now it looks almost like this: https://www.eevblog.com/forum/beginners/is-this-softstart-circuit-safe-(clearance-creepage-and-slots)/ .
The interesting thing is, I started adding a lot of features to the project, but at some point took a step back and removed many things.
If a cap needed across the switch is unclear for me. I decided not to have one as I presume secondaries have enough capacitance to absorb inductive kickback. But my tranny is much smaller than yours.
schmitt trigger:
Many moons ago, we were faced with a similar situation and a 200 KVA load fed from 480V 3phase.
What we did is to build our own resistors. Calculated the required resistance, went to a nichrome wire chart and selected a particular gauge which could withstand the maximum inrush current, and purchased the required lengths of wire. To build the resistor, we used household bricks as the wire formers. One for each phase.
And with a simple PLC, we controlled the activation of the of the power contactor which would short out these power resistors.
In operation, the wire would glow red-hot for a couple of seconds, but that was perfectly fine.
For your power level, you may want to add multiple resistor stages in series, and each would be shorted sequentially.
exe:
--- Quote from: schmitt trigger on March 06, 2019, 06:15:33 pm ---Many moons ago, we were faced with a similar situation and a 200 KVA load fed from 480V 3phase.
--- End quote ---
Did you have any problems with switching the load off?
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