Power Transformer Inrush Current Limiter - Page 1
Forum section: electronics / projects

author: Titan4285
author_rank: Newbie
timestamp: March 06, 2019, 05:05:08 pm
content: 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.

author: Sylvi
author_rank: Regular Contributor
timestamp: March 06, 2019, 05:54:56 pm
content: HiInrush 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 civiliansYou may not be able to improve upon what has been done traditionally? Good luck!

author: exe
author_rank: Unknown
timestamp: March 06, 2019, 06:07:22 pm
content: 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 06, 2019, 06:15:33 pm
content: 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.

author: exe
author_rank: Unknown
timestamp: March 06, 2019, 06:29:28 pm
content: Quote from: schmitt trigger on March 06, 2019, 06:15:33 pmDid you have any problems with switching the load off?

author: exe
author_rank: Unknown
timestamp: March 06, 2019, 06:51:01 pm
content: Huh, I just realized it's not 1.5KW, but 1.5MW. Forget what I said before about my circuit

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 06, 2019, 06:58:05 pm
content: Related to the inrush limiting resistors? No problems.

author: jmelson
author_rank: Super Contributor
timestamp: March 06, 2019, 07:59:16 pm
content: At this power level, I think resistors are not going to be practical. What you need is a set of inductors. It may be possible to build air-core inductors that also have just a bit of resistance to help. I believe I have seen such things in substations on occasion.Jon

author: Benta
author_rank: Super Contributor
timestamp: March 06, 2019, 08:56:33 pm
content: The key to limiting transformer inrush current is to prevent it happening.Why does it happen? Because applying voltage at an unfortunate point on the sinusoidal supply voltage drives the transformer into saturation.Switching on the transformer at the voltage zero-crossing point is the worst case scenario, as the core is not yet magnetized. This means that the B-field will increase to double of what it's supposed to be, as it's not starting from -B, but rather from 0.Energizing the transformer at close to peak AC voltage will avoid this (the exact cut-in angle needs to be analyzed), as it lets the transformer build up a symmetrical B-field, avoiding saturation.In my opinion, hefty disc-type SCRs can handle this at your power level, with appropriate control circuits. Relays are not precise enough, timing-wise.Further analysis about starting the transformer under load (if necessary) is definitely necessary.And dV/dt capacitors and dI/dt inductors are indicated.

author: jmelson
author_rank: Super Contributor
timestamp: March 06, 2019, 11:28:39 pm
content: Quote from: Benta on March 06, 2019, 08:56:33 pmYes, technically, you are on the right track. But, this is a 1.5 MVA transformer, which sounds to me like medium-voltage supply, at least, if not high voltage.In the power industry, MV can be up to 45 KV. A set of 45 KV SCRs are going to be pretty big. You also have to be VERY sure to never let this thing rectify the supply, or it could shut down a whole feeder, blacking out a small region.Jon

author: dave3533
author_rank: Contributor
timestamp: March 07, 2019, 01:25:12 am
content: I always thought the Motorola App Note AN1542 would be fun to try...not at the power levels you're looking at though.

author: jbb
author_rank: Super Contributor
timestamp: March 07, 2019, 08:12:56 am
content: I haven’t gone that big, but in power systems there’s a lot to be said for big dumb designs.Are you above 1kV? If so, everything will get more expensive...Some options:- A Point On Wave breaker system can be quite effective (calculates and closes relay on a per-phase basis) if you can get them- Big series resistors can be very effective, but bypass switchgear can be expensive (especially for medium voltage). Series resistors can also be made from bulk cast iron or stainless steel sheets cut into meander patterns (with nice wide tabs at the ends to boot the wires on). Remember fuses or thermal protection so they don’t catch fire if the bypass switch doesn’t close.- Can you inject magnetising current through the secondary (or tertiary) winding? (Can be very helpful if primary is medium voltage)

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 07, 2019, 04:03:49 pm
content: What Benta is saying is the optimal solution, but requires to precisely determine the firing angle such that the applied flux is completely opposite from remanence.

author: coppercone2
author_rank: Super Contributor
timestamp: March 07, 2019, 05:57:11 pm
content: https://en.m.wikipedia.org/wiki/Current_limiting_reactor

author: splin
author_rank: Frequent Contributor
timestamp: March 07, 2019, 06:48:19 pm
content: If you have another source of power from another transformer, or one that can be synchronised to the mains, how about partially magnetizing the core via a secondary winding - just enough to limit the inrush to the required level for a short time when switching on?No danger of rectifying the incoming feed and low voltage, hopefully cheaper switches/relays could be used. Or even discharge a capacitor into a secondary at just the right time? Plenty of current available for the short time required. You can use electrolytics to keep the cost down and use semiconuctor switches to match polarity of the i/c supply at point of switch on.Synchronisation might be tricky if the main switches are mechanical, but triggering by the detection of primary voltage being applied, or even secondary voltage, might be quick enough given sufficient inductance in the primary windings and supply wiring (and the secondary's inductance will be rather less than the primary's).

author: Benta
author_rank: Super Contributor
timestamp: March 08, 2019, 12:31:17 am
content: jbb brought up a great point:A tertiary winding on the transformer.Letting the tertiary winding build up the magnetic field at much lower power than the main primary (using maximum voltage switching with SCRs, as I suggested earlier), cutting in the main primary supply at a later point is easy.Downside is, you can only start the transformer without load.An interesting problem indeed.