Phase-shift, expressed in angle or time?

[powersys]

Hello,

I copied the following paragraph of text from:
https://www.allaboutcircuits.com/vol_2/chpt_10/7.html

For the more mathematically inclined, this principle may be expressed symbolically. Suppose that A represents one waveform and B another, both at the same frequency, but shifted 120o from each other in terms of phase. Let's call the 3rd harmonic of each waveform A' and B', respectively. The phase shift between A' and B' is not 120o (that is the phase shift between A and B), but 3 times that, because the A' and B' waveforms alternate three times as fast as A and B. The shift between waveforms is only accurately expressed in terms of phase angle when the same angular velocity is assumed. When relating waveforms of different frequency, the most accurate way to represent phase shift is in terms of time; and the time-shift between A' and B' is equivalent to 120o at a frequency three times lower, or 360o at the frequency of A' and B'.

Kindly advise the following:

[Q1] In practice, do you think the waveforms of a 3-phase system (any 3-phase system, not restricted to power generation) might have different frequency? Please provide a practical example.

[Q2] If the waveforms in a 3-phase system have slightly different frequency, do you think the phase shift between any two of them will be constant? If the phase shift is expressed in term of 'angle', will it be constant? If the phase shift is expressed in term of 'time', will it be constant?

Thank you very much

 

[karesz]

Hi,
In my opiniion; its absolutly yuninteresting that the expression is in angle or time!
Then both are in strong relation, only called others...
Signal periode is= 360 degre, these is= T i.e. in nsec`s.
A signal of 10 MHz has 360degre =as 100nsec, so is 120 degre = 33.33nsec. that is all.
K.

 

[FvM]

In my opinion it makes no sense to analyse a 3-phase system as having more than one (fundamental) frequency.

I don't understand, what the author wants to suggest with the alternative "angle or time". Clearly, "waveforms of different
frequency" are not part of the 3-phase system, although they can exist in a real application, e.g. PWM frequeny components
and also intermodulation products of mains and PWM frequency. Both are not harmonic components.

A practical problem can arise, if mains and "other" frequency components are close together and also interacting nonlinear,
respectivly causing intermodulation. But I guess, that's not the problem you're facing in your application.

 

[flatulent]

When a mains system is close to going unstable because of the load conditions harmonics appear in the current drawn. This is used to monitor the system and open circuit breakers before the generating station equipment is damaged.