Electromechanical oscillations
Electromechanical oscillations
(OP)
If one part of the network is weak, we can have electromechanical oscilaltion phenomena.
Is it really true that: one side of the circuitbreaker runs with 60 Hz and the other side "runs" at: (60 Hz - 20 mHz) + frequency oscillation with a period of 1 second of +/- 50 mHz?
Which means that the second side of the (open) circuit breaker shows a frequency modulated waveform (amplitude is constant) with carrier frequency of (60 mHz-20 mHz) and modulation period of one second.
I heard this from some colleagues... is that really true?
Is it really true that: one side of the circuitbreaker runs with 60 Hz and the other side "runs" at: (60 Hz - 20 mHz) + frequency oscillation with a period of 1 second of +/- 50 mHz?
Which means that the second side of the (open) circuit breaker shows a frequency modulated waveform (amplitude is constant) with carrier frequency of (60 mHz-20 mHz) and modulation period of one second.
I heard this from some colleagues... is that really true?






RE: Electromechanical oscillations
RE: Electromechanical oscillations
RE: Electromechanical oscillations
RE: Electromechanical oscillations
Your CB (and its line) is the only link between two areas of a big grid. Each area can be substituted by an equivalent generator. A big gen with a big inertia for the strong area and a little gen for the weak area, plus an impedance connecting them.
Suppose that something like a load is switched on. The absorbed power is greater then the produced one and the generators slow down until their frequency regulators operate to obtain again the equilibrium. So you have a transient during which the frequency goes down and up again. The electrical frequency is always the same in the system and it is governed by the mechanical frequency biggest generator. But the two generators are not synchronous during the transient because the smallest one oscillates more than the other.
When you open the CB, the two separated grids are governed by two indipendent generators so the frequency can be different.
RE: Electromechanical oscillations
that is! The CB (the line) is the only LINK between the 2 grids. If there is just another link, this cannot happen. Is that what you mean?
Let's suppose than that there is only my line as link. Is it really true that I can model the voltage waveforms like a frequency modulation... i.e. the voltage amplitude remains constant? Is there any theoretical background on that or it is just a simplification?
In a "power swing" , stable r not stable, there is also a modulation in the voltage amplitude..
Just wondering....
RE: Electromechanical oscillations
I'm just simulating an out of step condition and... Frequency is always the same in the whole system while voltage varies from point to point.
During a power swing you have an amplitude modulation of the voltage and not a frequency modulation.
Please refer to the impedance locus:
When your impedance point crosses the axis of your system, voltages and currents are exaclty the same as during a 3ph fault: low voltages (in phase opposition) and high currents.
When your point goes away from the axis, voltages increase and currents decrease.
So you have an "amplitude modulation" with constant frequency
RE: Electromechanical oscillations
I also believe that not only the frequency is modulated, but also the voltage amplitudes are modulated. So in my opinion, describing this phenomenon with a "pure frequency modulation" is wrong.
Is there any analitic (explicit) formula describing these oscillations?
RE: Electromechanical oscillations
The formula describing the phenomena are described in any book about the power system (electromechanical, transient, dynamic) stability.