How to bid powerful NT hands.

North will pass.

South with 25HCP balanced will bid 2♣ 

North has a choice - relay with 2♦  and see what partner has got, but better might be to bid 2NT showing 8-10HCPs balanced..

If North does this South will jump to 6NT as 8 + 25 = 33HCPs.

If North bids 2♦ (because if balanced he wants the strong hand to declare) South will rebid 3NT to show 25-27 HCPs balanced and North can then raise to 6NT.

The complete range of balanced NT bids in Acol

12-14 1NT

15-16 1x -1y - 1NT

17-18 1x-1y-2NT

19 1x-1y-3NT

20-21 (22) 2NT

(22)-24 2♣ -2♦ -2NT

25-27 2♣ -2♦ -3NT - very rare statistically

28-30 2♣ -2♦ -4NT - even rarer

https://www.bridgebum.com/strong_2c.php is a good summary of options.