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Transformer Input Voltage Reduction

Transformer Input Voltage Reduction

Transformer Input Voltage Reduction

(OP)
Hi folks,

Is the voltage input magnitude has an effect on the transformer inrush current?

If voltage is V(t) = Vm*sin(wt+theta)

at theta = 90 Deg, V(t) = Vm*sin(wt+90) = Vm*cos(wt)volts

assuming no residual flux on the core,

This yeilds to steady state flux which is

Flux (max) = Vm/(w*Np)

at Theta = 0 Deg, then V(t) = Vm*sin(wt+0)
                            = Vm*sin(wt)

Therefore maximum flux at 1/2 cycle is

Flux(max) = 2Vm/(w*Np)

If there is residual flux(Br)present it may go as high as

Flux (max) = 3Vm/(w*Np) depending on the amount of Br and its polarity at Theta=0 Deg.

My question, is reducing the input voltage going to significantly reduce the inrush current? Up to what percentage assuming negligible system impedance?

Putting a resistor in series sized to say develop an initial voltage drop of 50~60% is another way and will also increasing the inrush decay period (damping time), after which it is shorted out by a contactor. While closing in the contactor to shunt it out, a voltage spike may happen which in turn may saturate the transformer again. Is this correct?

If a transformer secondary windings are manufactured with taps, is it possible to provide on-load tap changing through contactors only assuming a dry type transformer with 208D-208/120Y of the 20kVA size.

RE: Transformer Input Voltage Reduction

Yes, voltage magnitude directly impacts the inrush magnitude.  Generator step-up transformers unit connected to the generator terminals exhibit no inrush when voltage is brought up with the generator.  Point-on-wave switching can also minimize inrush.  On the other hand, unless you are dealing with at least a few hundred MVA worth of transformer it probably isn't worth the effort to eliminate inrush.  The more load on the transformer the shorter the time the inrush rings.

RE: Transformer Input Voltage Reduction

(OP)
Actually, I was thinking of a way to softstart a transformer on a LV level using step resistance or any other form, it will work especially for a rareb application, but the cost of doing it and its counter productivity is something I have to bear in mind.

I have one engineer insisting of a 208-208V, Delta-delta isolation transformer, a special transformer that will have a special primary design to limit the inrush to within 5x only. On its secondary, another transformer will be connected who is having a high inrush. Since the isolation transformer is 1:1, once the downstream transformer is energized, its inrush will be seen as a "load inrush" and will be reflected at this isolation transformer's primary..so there is no mitigation happening. The isolation transformer will juts only be another impedance into the system. Is my concept correct?

RE: Transformer Input Voltage Reduction

Yeah, the 1:1 is just more impedance, more losses.  What is the problem with inrush that is worth spending anything to mitigate?  I've never run into any form of transformer inrush control outside the higher voltages of the bulk power system except for generator unit transformers.  I find it difficult to believe that you have an application so different that you need to worry about inrush.

RE: Transformer Input Voltage Reduction

(OP)
In reality, inrush current is a healthy current since this is required to energize the transformer core and it consists of parameters that are permament and almost uncontrollable.

Actually, I am doing a document for me to present to the stakeholders the kind of worth of trying to mitigate it and what are the counter productive effects and uncertainties of doing so. Its actually associated with the UPS problem I have posted earlier and the best way to solve the problem is to connect these input transformers (of the rectifiers) to normal AC sources and provide manual transfer switches for incoming supply diversity.  

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