Your explanation about how conventional power stations make up for say a trip of one turbine is not entirel correct. No manual input is required to increase power from all the rest of the conventional stations  simply becaue all the turbine power, i.e. steam, is controlled by a speed sensitive governor on each turbine. Yes there is inertia, but like a car hitting a hill that's not enough to keep up the speed, you must open the throttle. Each turbine will automatically increase power to keep the turbine rotating at 3,000rpm . This speed is strictly controlled within fine limits. Voltage is also automatically controlled by voltage regulators.