What Happens if the Prime Mover of Synchronous Generator Fails?

If we take a Thermal Power Plant, then steam Turbine serves the purpose of prime mover and coupled with Synchronous Generator. Now if the steam supply to the steam turbine is suddenly cut off while the Synchronous Generator is connected to Grid, what will happen?


The answer is simple. If all the Generator protections are working fine then the Generator will be isolated from the Grid by Reverse Power Protection Relay. But here we will assume that no protection is working and then in investigate the behavior of Synchronous Generator.

Here are the various possibilities for a synchronous generator connected to grid when Prime mover is suddenly decoupled. The behavior of Synchronous Generator depends on whether the field winding is excited or not.

Case1: Field Excitation is available

In case the field excitation of Generator is available when the steam supply to the turbine is cut off, the Synchronous Generator will draw power from the Grid and will behave as a Synchronous Motor rotating in the same direction.

Case2:Field Excitation is failed

In this case we will consider two types of Synchronous Generator- Salient Pole and Cylindrical Pole.

Cylindrical rotor machine: It will work as Induction Motor running in same direction with speed less than synchronous speed.

Salient pole machine: It will work as Reluctance Motor running in same direction at synchronous speed.

One more situation is possible when the Prime mover is coupled but the Excitation alone fails, What will happen then?

In this case, the Machine runs as an Induction Generator whose rotor will rotate at a speed higher than synchronous speed.

Based on construction Induction Motor and Synchronous Machine have many similarities. Both have stators wired to have rotating magnetic field. The main difference comes in rotor construction. Induction Motor requires closed path for induced rotor currents created due to flux cutting. This flux cutting arises as the machine is not rotating at synchronous speed, as Slip is not zero.

In Synchronous Machine  also closed paths are available. In salient pole machine the damper bars act like squirrel cage rotor as salient poles are laminated so core does not provide proper closed paths. In case of cylindrical rotor machine there is a solid steel core which provides a closed current path. So when machine loses synchronism these will provide paths for circulating currents and create magnetic field.

Now consider a Synchronous Generator connected to Grid. When the excitation fails the power delivering capacity of machine suddenly reduces. So the mechanical power input is greater than electrical power output. This imbalance will accelerate the rotor and force it to run above synchronous speed. The circulating currents and then magnetic field are created.  The machine will start to generate power by induction and hence will work as Induction Generator.

Why don’t we then operate Synchronous Generator as Induction Generator?

We cannot operate Synchronous Generator as an Induction Generator. This operation is disastrous as the circulating current will heat the core and may damage the insulations. There is also a considerable change in the reactive power flow. A Synchronous Generator which was generating lagging power is suddenly changed to a machine absorbing lagging power i.e. generating leading power. These sudden changes will certainly cause disturbances in the network.

It should be noted that failure in field does not mean rotor magnetic field is absent. The machine draws reactive energy from Grid to create magnetic field. This field is created by the circulating rotor currents.

In industries, these situations are prevented by the Generator Protection system which isolates the generator from the grid immediately after it senses the fault.

Thank you!

9 thoughts on “What Happens if the Prime Mover of Synchronous Generator Fails?”

  1. Dear admin! In case prime mover fails and excitation is present, due to same phase sequence direction of electromagnetic torque remains same which was opposite to direction of rotation for case of generator. But now, when machine behaves as motor, this electromagnetic torque opposes the direction of rotation and machines comes to halt and start rotating in opposition direction.

    Kindly let me know where am I wrong or if you are?

    • No, the machine will rotate in the same direction. Have you ever heard that reverse power protection operates after the power output of generator comes to zero? The phase relationship between the Ef and Vt will change and machine will continue to rotate in the same direction.

  2. Yes. I have idea about reverse power protection. It’s due to the fact that machine behaves as motor which was earlier behaving like generator and taking electrical input from grid rather than delivering to it.
    But the question remains whether motor rotates in same direction or opposite, just like case of plugging in DC motors. As per you direction remains same that means electromagnetic torque has to be reversed. So what exactly is causing this reversal of electromagnetic torque? How is that related to reverted phase relationship between induced emf and terminal voltage?

    • You draw phasor for motoring and generating mode.Things will be clear. If you still face doubt plrase let me know.

  3. Hello. What if the machine is being operated as a syn. motor and suddenly the 3 ph supply to the stator is cut off while still maintaining the field excitation? Would the machine behave as generator under action of stored inertia in the rotor and that of the load being driven?


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