How to specify pw_mw for ext_grid

[angrysnail]

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Issue

I want to hold a specified value for ext_grid, by adjusting the pcs, pvs, even the load。 But the load, sgen, storage in pandapower， all need to specify a p_mw value, then after runpp(), the ext_grid p_mw value will be changed even I set ext_grid's controllable=False.
Any suggestion?

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