Induction Generator connected to a standalone Synchronous generator

[b.shahvir]

Hi all,

This particular query is purely theoretical. Assume a standalone synchronous generator feeding power to a single induction machine (the only connected load) which is made to run as induction generator at super-synchronous speed. What will be the prevailing electrical conditions in such a case if the induction generator is assumed to feed active power back to the standalone Synchronous generator? will the synchronous generator run as a synchronous motor. How will reactive power exchange take place in such condition to provide magnetizing current to both machines? what will be the effect of such a set-up on;
1) Line terminal voltage
2) Magnetizing current & air-gap flux in both machines
3) Mechanical torque of both machines

Thanks & regards,
Shahvir

 

[Windadct]

Seems like some weird homework - both subsystems will behave depending on their control - i.e. you have 2 "generators" - if both are putting in energy ? Makes no sense to me - ether one has to motor, or in my mind without appropriate control - the system will oscillate.

 

[b.shahvir]

Seems like some weird homework - both subsystems will behave depending on their control - i.e. you have 2 "generators" - if both are putting in energy ? Makes no sense to me - ether one has to motor, or in my mind without appropriate control - the system will oscillate.

Weird as it may sound but I have already mentioned earlier that it is a purely theoretical assumption & no it's not a homework question. The problem is the 2nd generator is assumed to be a Induction generator & as such it cannot generate reactive power to supply magnetizing current to the synchronous machine which might attempt to act as a motor (if at all) or the whole system may react in a different manner. No doubt it is all hard to imagine so I put forth this query so that someone could throw light on a concept I might be missing out on.

 

[zoki85]

Haha. Sync machine can work in a generator regime or in a motor regime. Of course, the same holds for async machine. But, the important point are different ways of magnetic excitation. The excitation of sync machine is independent of network the machine is connected to while the excitation of async machine is dependent. No matter wether async works as motor (0<n<n_s) or as generator (n>n_s) it needs to be fed with reactive inductive power ( with magnetizing current) from network . In described case the network is already operating generator spinning at fixed speed n=n_s, and if you try to spin async machine at n>n_s, nothing will happen as regards reversing flow of the energy in the network.The async machine will continue to be perceived as passive inductive load from the standpoint of the sync generator.

 

[b.shahvir]

Haha. Sync machine can work in a generator regime or in a motor regime. Of course, the same holds for async machine. But, the important point are different ways of magnetic excitation. The excitation of sync machine is independent of network the machine is connected to while the excitation of async machine is dependent. No matter wether async works as motor (0<n<n_s) or as generator (n>n_s) it needs to be fed with reactive inductive power ( with magnetizing current) from network . In described case the network is already operating generator spinning at fixed speed n=n_s, and if you try to spin async machine at n>n_s, nothing will happen as regards reversing flow of the energy in the network.The async machine will continue to be perceived as passive inductive load from the standpoint of the sync generator.

Nice explanation but what will be the effect of spinning the async machine at n>ns on the foll;
1) Line terminal voltage
2) Magnetizing current & air-gap flux in both machines
3) Mechanical torque of both machines

 

[zoki85]

1) Very small steady state variance (small increase)
2) Magnetizing current same
3) Same steady state torque same for sync machine, and arbitrary (one you provide) for async machine

IOW, sync generator stays boss :)

 

[b.shahvir]

1) Very small steady state variance (small increase)
2) Magnetizing current same
3) Same steady state torque same for sync machine, and arbitrary (one you provide) for async machine

IOW, sync generator stays boss :)

But if line voltage increases, the air gap flux (which depends on voltage) & hence magnetizing current in async m/c stator will also increase. In my opinion if rotor of async m/c is spun above Ns, back emf E1 in async m/c stator winding will increase which will reduce the magnetizing current & hence air gap flux, terminal voltage would remain constant. This is because of the absence of balancing amp-turns current which could have flown in stator of async m/c in case a suitable active power consuming load were present. Is this assumption correct?

 

[zoki85]

If the machines are aproximately of same nominal power, line voltage change is negligible (less than 1%) and depends on async rotor construction and magnetic core loses in both cases.

 

[b.shahvir]

If the machines are aproximately of same nominal power, line voltage change is negligible (less than 1%) and depends on async rotor construction and magnetic core loses in both cases.

Ok, I have one more query. Suppose now a resistive load is connected to the system. The async m/c will now supply active power to the load (depending on rotor speed) by increasing the line terminal voltage such that it would partially relieve the sync m/c of supplying active power to the load. What I fail to understand is that with increase in line terminal voltage will the magnetizing current & hence air gap flux of async m/c increase in direct proportion or should it remain same as before?

 

[zoki85]

Suppose now a resistive load is connected to the system. The async m/c will now supply active power to the load (depending on rotor speed)

No, it will not

 

[b.shahvir]

No, it will not

But isn't this principle of regenerative braking in traction?

 

[zoki85]

But isn't this principle of regenerative braking in traction?

Yes,you're right. I thought you meant increasing torque, not load. Line voltage will depend how big is load with respect to the generator's reactances.

 

[b.shahvir]

In described case the network is already operating generator spinning at fixed speed n=n_s, and if you try to spin async machine at n>n_s, nothing will happen as regards reversing flow of the energy in the network.The async machine will continue to be perceived as passive inductive load from the standpoint of the sync generator.

Dear Zoki,
Can you explain the part as to how the async machine will continue to be perceived as passive inductive load from the standpoint of the sync generator? If the rotor of async generator spins above sync speed, the stator field flux will cut the 'leading' rotor conductors & induce an emf & current in the rotor bars. But in the absence of a load for eg., what will happen to the stator induced current? (due to transformer action). If the induced stator current is zero on no load, rotor current cannot flow although the rotor conductors are being cut by the stator magnetic field. But this is against the law of physics. I am confused, pls. explain