At one time, the five major groups and the power grid company were the same company, called the State Power Company. After the separation of electricity reform in 2002, the five major groups only operated power supply (power plants), and the power grid companies operated power grids. Then, in a sense, the power grid companies have become the parents of the five major groups. Most of the TV universities issued by the five major groups have to go online, that is, sell the electricity to the power grid company, and then the power grid company sells the electricity to users all over the country through its own lines.
This is from a commercial point of view, but from a technical point of view, power grid companies can restrict power plants. For example, the dispatching center can dispatch more or less power plants, and the dispatching can find any technical excuse.
However, this year, the national policy has been relaxed to support large users to purchase electricity directly. For example, steel mills can directly negotiate with the power plants of the five major groups, and electricity can be sent to factories through their own networks without the need for power grids. But this is just the tip of the iceberg.