Rotor Angles in PSSE Dynamic Simulation 1

[electric87]

Hi Everyone, I have set up IEEE 39 bus system in PSSE. I am able to run the dynamics successfully. The only problem I got is the rotor angles which are increasing indefinitely when I apply a contingency. They increase upto some 100's of degrees.The speed deviations are also in the range of 1E-4. This is same with all the generators. Before applying the contingency, they are constant. Can Anyone tell me if this is an unstable behavior? Is there any better way to monitor the rotor angles? Every thing else seems to be good, the voltages and all the other values settle at constant values. Thank you

 

[electricpete]

I don't know that software, but fwiw if you have one bus with constant angle (swing bus) which ties through lines/transformers to generators whose angle is changing by 100's of degrees, that sounds unstable to me.

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(2B)+(2B)' ?

 

[electric87]

Thank you electricpete, Actually the angle of the swing bus is also changing along with the other buses.

 

[davidbeach]

Angles changing relative to what? If they all change together is there a problem?

 

[electricpete]

For the power flow algorithms that I am familiar with, you must pick a designated swing bus and establish constant phase for that bus (usually zero). It forms the phase reference for everything else. It seems to me that your software/program has not properly implemented the concept of a swing bus.

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(2B)+(2B)' ?

 

[electricpete]

Sorry, terminology "swing bus" applies to power flow / load flow, but maybe not to transient / stability calcs.

Nevertheless, there should be a bus somewhere in the calc that is identified as a reference.

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(2B)+(2B)' ?

 

[PHovnanian]

I'm not familiar with PSSE. But in general, there are two things that could be going on here. One is that you have actually modeled an unstable system and what you are seeing is (an approximation of) its real world behavior. Check for reasonable assumptions and values.

The other is that your model and/or initial conditions are failing to converge. Read the manual sections related to this topic and see if there are any specific requirements, restrictions or hints on ensuring that you've actually got a valid model.

 

[electric87]

I doubt the same, but the model converges well during steady state behavior. I will try to make sure that the model which I have built is valid.

@davidbeach, the manual says that the output which I am seeing are the relative angles. They remain at some constant values during the normal conditions, but when I apply some disturbance, they all change together. What I expect here is that they settle down at constant values determined by the new generator outputs, which is not happening.

 

[QBplanner]

Relative angle in PSS/E means two things one is relatve to whole system generators average weight angle. Another one is relative to one machine angle. If you chose the first one every generator in the IEEE 39-bus case will increase together, it means it is a synchronous with the rest of the system.
If it is the later, then the contingency you added will cause the testing generator OOS
.

 

[tlrols]

You probably need to look, as others have mentioned, at what you are using for an angle reference. Typically you can select the average of all machines or the system slack bus generator. Normally either is ok, unless your disturbance is close (electrically speaking) to the slack bus...if it is then all your angles may start to move.

You might want to post a plot here for some better feedback.. Basically you have a drifting angle plot described, which means you need to select a different reference, or not. The nature of the plot matters. Did you plot voltage? What does that look like? Are the plots of angle or voltage well damped? Is there "hash" on the plots? To be honest neither the absolute or relative angle value is particularly important...it is more the nature of what that angle is doing over time that matters.

 

[electric87]

The angle which I am using is the one relative to whole system generators average weight angle. I think I got the reason for this behavior of generator angle.

The voltages of all the generators are well damped and settle down at stable values. I found out from the tech support that the governor characteristics are responsible for continues change in angle. The angle would settle down if I used isochronous governor instead of one with droop characteristics which do not completely damp the speed deviations. I have attached a plot of rotor angles after a disturbance at 2 seconds.

 

[QBplanner]

they are ok