Switching a transformer on and off with something to reduce the inrush current

[Ben-Louis van Vuuren]

Good day



I need some assistance.
I have a step up stepdown system to get power from my power source to my irrigation lands throughout my farm.
We only have solar power so fact being at night time the transformer’s no load current use about 10% of my batteries.
I want to switch the transformers off in the evenings and on in the mornings but the problem is the inrush current every time I switch on the transformers the inrush current trips the main breaker due to the high inrush.

I need something to start and stop the transformers without that inrush current.
My setup is as follows.
Transformer 1:

Step up
125kva

Primary 380v

Secondary 3000v



Transformer 2

Step down

100kva

Primary 3000v

Secondary 380v

Connected to Transformer’s 1 secondary



Transformer 3

Step down

50kva

Primary 3000v

Secondary 1000v

Also Connected to Transformer’s 1 secondary



Transformer 4

Step down

25kva

Primary 1000v

Secondary 380v

Connected to Transformer’s 3 secondary

Transformer 5

Step down

10kva

Primary 1000v

Secondary 380v

Also Connected to Transformer’s 3 secondary

I only want to switch on and of the main step up transformer.



Can you please assist me.

 

[crshears]

These fora are for professional engineers . . . then again, I'm not one and they tolerate me, so maybe you can get some help after all.

First question: Is this a new system or an existing one? In other words, did it used to work but now that you've changed something it doesn't anymore?

Second question: when you first energize the step-up transformer, are you at the same time also picking up load?

Observation: one step-up trafo at central location, step down trafos spread out around the lands, so you don't want to have to travel to each one every day to operate; makes sense.

Could you post a single line diagram of your system?

More information yields better answers.

 

[wcaseyharman]

In the utility world we use pre-insertion resistors. Basically a smaller breaker in series with a resistor that closes first to energize the windings. The resistor reduces the magnitude of the inrush current. The main breaker contacts close shortly after.

 

[LionelHutz]

We've sold soft-starters before to do this. A starter that can do an open loop voltage ramp works, one with a bypass contactor is best.

 

[waross]

A moulded case, thermal magnetic breaker of 225 Amps with adjustable magnetic trips, or fixed 10X magnetic trips.
This will give you 2250 Amps before the instantaneous element trips.
You may also add a thermal magnetic breaker on the secondary so that you may energize the transformer unloaded and then switch on the remote transformers.
Dual element or time delay fuses are also an option.

Reduced voltage and soft starts are most often used for large transformers and special instances.
125 KVA transformers are generally protected by thermal/magnetic breakers or time delay fuses.
It is only when the thermal magnetic or time delay fuses do not permit energization that reduced voltage schemes are implemented.
If you are already past this step, then resistors and a second breaker may be the most economical solution. (Manual operation.)

 

[Giulio Prette]

You can use
. Series Reactors
- Installing reactors in series with the transformer
- These limit the rate of current change
- Can be bypassed after startup using a bypass switch

 

[Ben-Louis van Vuuren]

These fora are for professional engineers . . . then again, I'm not one and they tolerate me, so maybe you can get some help after all.

First question: Is this a new system or an existing one? In other words, did it used to work but now that you've changed something it doesn't anymore?

Second question: when you first energize the step-up transformer, are you at the same time also picking up load?

Observation: one step-up trafo at central location, step down trafos spread out around the lands, so you don't want to have to travel to each one every day to operate; makes sense.

Could you post a single line diagram of your system?

More information yields better answers.

Its a old system and its working fine no problems I just need something to switch it on in the morning and of in the evening because the no load is using to much of my batteries at night time.

No I start the transformers and then after a few minutes I start the loads running from the transformers.

 

[crshears]

Hello Ben-Louis, and thank you for following up.

Have a look at what others here have posted; there should be something useful for you in there somewhere.

Please let us know what you try and how it works out.

 

[jghrist]

What controls the output voltage of the solar power? If you lower the voltage it will reduce the inrush.

 

[bacon4life]

Lowering the voltage from the inverter will also reduce losses in the transformers.

The statement about 10% current from the batteries seems surprising unless the battery capacity is quite small compared to the AC portion of the system. The current flowing to unloaded transformers will be mostly reactive, so the real power losses supplied by the batteries should be relatively small.

 

[Ben-Louis van Vuuren]

In the utility world we use pre-insertion resistors. Basically a smaller breaker in series with a resistor that closes first to energize the windings. The resistor reduces the magnitude of the inrush current. The main breaker contacts close shortly after.

What is the correct spesifications of the resistors that I can use? Then I can put them on a small 3phase contactor and after the transformer started I can use a larger contacter to bypass the resistors and connect both of the contactors on a wifi conroller so I can switch them witch my phone?

 

[wcaseyharman]

I personally have never sized them, and the one time we were looking at it the engineer was using a transient modeling software package to reduce resonance in a system.
However, my first inclination in your case would be to size the resistor to limit current to some around 5X the no load current of the transformer assuming the transformer looks like a short to ground at initial closure. That will bring the transformer to around 80% voltage at no load, well below saturation but high enough that when you close the main contacts it won’t be a huge jump. This is assuming you’re energizing these transformers without load.
I assume 1second should be plenty of time to wait before closing the main contacts.

 

[iop95]

Inverter need to be power-on over night?
In the morning any solar inverter start at some solar panels voltage and limit output current itself, no need for other parts.

 

[Ben-Louis van Vuuren]

I personally have never sized them, and the one time we were looking at it the engineer was using a transient modeling software package to reduce resonance in a system.
However, my first inclination in your case would be to size the resistor to limit current to some around 5X the no load current of the transformer assuming the transformer looks like a short to ground at initial closure. That will bring the transformer to around 80% voltage at no load, well below saturation but high enough that when you close the main contacts it won’t be a huge jump. This is assuming you’re energizing these transformers without load.
I assume 1second should be plenty of time to wait before closing the main contacts.

So why cant I then just use a VFD to energize the transformer and the switch it to bypass the VFD?

 

[wcaseyharman]

You might be able to do that, I am not a VFD expert and have never tried something like that myself. That would be prohibitively expensive in my world in the medium and high voltage systems I work on.
Others mentioned they’ve used a soft starter, so there’s some precedent for that sort of thing.