In the Georgia (US state, not country) there is one investor owned utility (Georgia Power) that is regulated like you mention by the state. They have to get rates and new plants and basically everything approved by the state Public Service Commission. We'll set aside how the PSC is basically a captured regulatory agency, but in theory they are highly regulated and don't just get to do what they want. GPC serves about little more than half the state's population, and about a quarter of the physical land mass

The rest of the state is served by either local cities with their own utilities or cooperatives. The cooperatives are not government agencies, they are privately owned non-profit corporations whose customers are the owners. These companies aren't regulated by the state PSC and have the ability to set their own rates unilaterally. The only regulation they have is that they report their rates to the PSC, but it's just a report, no approval

Technically they're almost all subject to regulation by the US Government as a condition of taking loans in the 30s/40s (and most of them have rolled that financing over and continue to use it to finance operations even today) but for the most part that regulation just says they have to at least charge enough to cover their interest, and can't sell their systems without the feds' approval

They could raise rates as much as they want, but the structure of a customer-owned company like that is the profits just get redistributed back to the customers, so there's no reason or incentive to do so

And, FWIW, that 23% number from the linked article is about right. GPC is usually 15-20% higher than most of the coops and city-owned systems in Georgia (there are some that are higher). And this is even with some of those coops and cities actually buying power wholesale from GPC