# So what physically causes my highly loaded system to blow up?

In my very hypothetical scenario, I'm simulating the IEEE 9 bus system, whose loads I am deliberately increasing linearly with time in order to make the system 'blow up'. Some unrealistic modifications I have done to this systems are:

• Disabled all protection mechanisms (didn't put any such mechanisms would be a more accurate statement)
• Generator maximum allowed powers and operation voltage ranges have been drastically increased (no chance of hitting the limits during the simulation).
• Used zero time lag governors which can sense any change in load powers and instantly increase the real power generation of the generators.
• Additional power lost in the transmission lines due to extra loading has also been accommodated for in these power increments.

Yet after all this, once my generator bus voltages dip below some critical values, say between 0.80pu to 0.75pu, the simulation eventually 'blows up', and the bus voltages start drastically oscillating.

I was hoping for a result in which the generator bus voltages dip until they reach 0pu (or close to zero). So physically what goes on which causes my system to 'blow up'?

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You are increasing the load until instability occurs. The transmission system becomes more and more stressed and at one point there is loss of synchronism between parts of the system. I.e., one or more generators goes out-of-step with the rest.

Plot the rotor angles of the machines in order to see which part of the system that is going out-of-step. When out-of-step occurs, there will be 0 voltage at points in the system, most likely inside transmission lines (or inside transformers).

Plot all bus voltages to see where the voltage becomes lowest during the out of step. There is a voltage dip each time there is an out-of-step going on in the system. Here you see repeated out-of-steps, causing the oscillating voltage.

You can repeat the simulation with dynamic options "Scan out of step conditions - Monitor and trip" enabled. The branches and transformers involving the out-of-step will then be tripped automatically and you can easily see where the out-of-step occurs. There will then be a system split, leading to two separate AC-systems.

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