A spinning turbine has mechanical energy. If an object's kinetic energy is added to the object's potential energy, the total is the object's mechanical energy.
An object's mechanical energy never changes, even though kinetic and potential energy are variable. This is because if kinetic energy increases, potential energy drops equivalently, and vice versa. To use the turbine example, if the turbine begins to spin faster, its kinetic energy increases while its potential energy decreases. If the turbine begins to spin more slowly, its potential energy increases while its kinetic energy decreases. Either way, the turbine's mechanical energy remains exactly the same because it is the sum of the two other varying energy types.