5. Runaway

When the turbine-generator set trips, the turbine runs away. Runaway is a stable equilibrium point where the power delivered to the turbine shaft is equal to zero, and where the speed of rotation of the unit, for a Francis wheel, can reach 1.5 to 2 times the synchronous speed of rotation of the alternator. The runaway speed is a guarantee of the contract, as it determines the size of the alternator. Increasing the runaway speed increases the stress on the alternator rotor, which in turn increases the cost of the machine.

The theoretical stabilized runaway speed measured on the model depends on the valve opening. The no-load opening is the distributor opening corresponding to the runaway speed equal to the alternator synchronizing speed.

Figure