Transformer Initial Inrush

How many transformers / inductive loads are down stream of the primary TX?

TonyS

There are two 2.5MVA Primary Transformers each stepped down to 660V connected to to Inverters connected to 2.4MVA DC Turtine Generators.

Nothing of the sort I've read or heard about and there's no logic supporting it. Check the accepted connection sequence of the inverters. Also possible is their internal over-voltage protection triggered at -random time- C/T connection, or not yet run-in bulk capacitors that caused the inrush overcurrent. Is the problem now reproducible?

Magnetic inrush will be the same in all cases.

Problem might be due to inadequate capacity elements like cables causing excessive voltage drop at inrush and the relay trips.

Since you have demonstrated that switching in stages overcomes the deficiency most easy solution would be to incorporate time delays so that in case of automatic restoration the time grading keeps the system stable.

Besides the protection relay time grading can also be accomplished by incorporating timers in the closing coil control circuits of the CB's.

Sorry AA, the maximum inrush current does vary according where in the sine wave the voltage is when it is applied. It is highest when it is being energized from the HV side at the instant the supply voltage passes through zero. Energizing a core-form transformer from the LV side may result in inrush currents approaching twice as high. The per unit inrush current to a shell-form transformer is approximately the same on the HV or LV side.

YES, . . you will get different inrush - with different transformers, especially when comparing new and efficient transformers with the older ones and less efficient ones. New transformers will have a high-grade, low-loss core that has a pronounced knee-point what promotes inrush. Also, new and highly efficient transformers have lower winding resistances - that further promote inrush, once the core is saturated. You will just have to re-set your relays, . . or to use better relays that recognise inrush . . . . Oleh

We're talking about the same transformer inrush current differences from when it's brand new and when been in service. Can you support any theory about it?

The initial inrush current varies according to when in the cycle it is energized.

More info is required to know what is causing the OC relay to operate, but my guess is that your windfarm consists of a number induction generators radially connected via buried three phase cables all leading to your transformer then out to the grid. If this is true then you need to account for the inrush current for the generators especially if the turbines are not operating near their synchronous speed when they are energized. I suspect this is the case because you have found that if you energize the turbines sequentially the relay doesn't operate. Sounds like a microprocessor programmed to sequentially apply the turbines might save you from running out there every time the wind starts blowing fast enough.

Thanks for your comments.

The wind turbine generators are DC and the Inverters do the Synch. Each turbine has its own transformer.

the turbines only turn if there is supply or else they mechanically lock out until supply returns.

Im thinking that the capasitor banks inside the inverters caused the inrush to be greater than normal because they had never been energised.

Also i think that the Networks protection was too tight.

Ill be returning in a few weeks to simulate a loss of grid supply to see if the system can restart on its own.

cheers