Inrush on Primary of Three-Winding Transformer When Other Two are In Use? 2

[HenryOhm]

I am working with an energy storage application. A three-winding transformer is in use. The utility feeds the primary winding medium voltage. One low-voltage secondary feeds an energy storage inverter with lithium-ion behind it. The third winding steps down the utility power and feeds a lower level of medium-voltage to the AC loads. When not on utility, the inverter and batteries step up and feed the AC loads through the two "secondary" windings. When the utility comes back on line, there will be a transition period as the inverter goes from discharge mode back to charge mode. This transition will lead to the AC loads transitioning from battery to utility power.

However, at the point that the inverter synchronizes with utility and the utility breaker closes back in, will the utility see little or no inrush on the primary (energized) winding? I want to say that there will be no inrush but I just can't get my head around this completely. Thanks.

 

[davidbeach]

A synchronous close into an energized transformer does not cause a magnetizing inrush.

 

[jrostron]

The inrush is generally caused by energizing the interconnection transformer to the grid. When this is energized, the inverters come on line rather seamlessly.

Yes, this inrush of the transformer can cause significant voltage dips. This is true for battery storage installations as well as solar farms. They all have a similar inrush problem as the connection transformer is common to both. Some StatComs also have this issue as well.

Estimating inrush and voltage sags or RVC can be very complex but there are some simple rules of thumb that will assist. One such technique is in the link below.

https://www.southernstatesllc.com/applications/connecting-solar-to-the-grid

All switching of transformers leaves residual magnetism or remnant flux. Switching to reenergize transformers at maximum flux exaggerates the inrush while switching at a reverse flux minimizes it. Determining the polarity of the flux is critical to determining when to close. One technique exists that eliminates the need to do this. It is the use of pre-insertion resistors. This technique requires no special sensors or controllers to implement.