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Charging Power transformer through lower rated grid auto Transformer

Q: What will happen when we back-charge 220 kV / 20 kV, 500 MVA Generator transformer through 75 MVA, 220/132 kV Grid Auto Transformer. Whether the 75 MVA Grid Auto Transformer can successfully back-charge the 500 MVA Generator transformer? If not, what can be the optimum transformer size to back-charge the 500 MVA Generator transformer.

A: We have carried out analysis of charging 220 kv /110 kv/33 kv ICT from 110 kv side. As you are aware the application of results will differ based on the system parameters namely sc level, source imp, Transformer Rand L etc and most importantly on instant (v(t)) of switching. Our observations - when inrush current is maximum, the dip in bus voltage is less. But lasts for longer duration (and vice versa) To shield other consumers from experiencing voltage dip ,we isolated the bus section during charging. There was no abnormality observed during charging.

In your set up your if your 75 MVA transformer is rated for 132 kv/220 kv (?) there will be additional imp of 75 Mva transformer unlike in our case where 110 kv source is strong, with almost 35 kA sc level. Anyway, your system may experience a sharper dip. As long as auto transformer ratio is 2 charging from LV side should not be a problem.

Second alternative, (we have not tried so far) is use of controlled switching device to synchronize closing of Transformer /line /source breaker at voltage zero. Many EHV breaker suppliers claim it to be useful for capacitor /inductor switching.

To get rid of any doubt of insulation during first charging, you may also think of using a controlled 400 v source which should be capable of supplying reactive vars during magnetization of the system. (This one is tried alternative ,
We have gone from 440 v to 33 kv to 145 kv (including 2 X 90 mva Transformers and GIS bays) in one of our stations.

Case 1: Charging 500MVA transformer without considering inrush currents.

As secondary is open we just need to supply core losses for 500MVA transformer, which can be done easily by 75MVA. Core losses will be maximum of 50 MVA for assumed percentage impedance of 10% to 500MVA transformer.

Case 2: Inrush Currents:
Inrush is a phenomenon which occurs during starting. Core will be going into saturation. because of which shunt impedance will be zero. This is nothing but short circuit condition for 75 MVA transformer. Based on Percentage impedance short circuit currents can be calculated for 75MVA. In this way i should say above condition that is charging of 500MVA transformer with 75MVA is possible and without any worries about inrush. And there is no harm in operating transformer in this manner as Inrush won't last for long time. Shunt impedance of 500MVA transformer will be regained and once lag between flux and voltage is 90 degrees we will just have core losses.

As long as you can tolerate the impact of tripping the 75MVA transformer due to the large through currents then simply give it a go. If it trips you can apply temporary settings to the overcurrent relay and have another go.

But if this exercise is to be repeated each time the generator transformer is to be isolated, then you will need to confirm the first energization wasn't just a lucky point on wave that led you to success. You would need to energise at least 5 times to get some confidence in my opinion.

If the source impedance on the 132KV system is low - you had better warn the Grid-Co of the exercise well in advance so they can prepare for a longish (500msec+) voltage dip which may upset other customers/generators.

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