frequency during power system disturbance
frequency during power system disturbance
(OP)
will we have the same frequency in all connected network during power system disturbances ?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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frequency during power system disturbance
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RE: frequency during power system disturbance
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: frequency during power system disturbance
One of the largest open pit mines in the world commenced operation on the island.
The utility engineers required the mine to install underfrequency meters to trip the main incoming 140KV breaker at the mine, in the event that the DC feed was lost and the mine load started pulling the frequency down on the hydro fed system.
As I said, this is a unique situation and Gunnars answer is correct in all normal situations and systems.
Respectfully
RE: frequency during power system disturbance
RE: frequency during power system disturbance
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: frequency during power system disturbance
Frequency is, by definition, "something over time". There is no "instant everywhere frequency".
Yes the frequency you measure everywhere on your network will be found to be the same over time.
The actual phase may be different, and in fact probably is different all over your network.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: frequency during power system disturbance
respectfully
RE: frequency during power system disturbance
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: frequency during power system disturbance
It requires an out-of-step (pole slipping) type of event or a severe power swing to cause this frequency difference. If the system hangs together, the frequency differences will be relatively minor and could be treated as a rapidly changing relative phase angle. During pole slipping there are two (or more) portions of the grip operating at different frequencies. This should last for a very short time as the protective system will segment the system to stop the pole slipping and once that happens there are two (or more) separate systems each with its own frequency.
RE: frequency during power system disturbance
I recall a discussion we had with one of the major power companies over here. There were two paper mills. Bot owned by same guy. He had two identical 36 MW direct (electrodes dipping into water) heated electrical steam vessels and he bought cheap electricity from this energy company. Cheap, because the energy company was allowed to cut his steam generators if frequency dropped below 49.85 Hz (50 Hz grid) and thus be able to manage difficult situations.
One of the vessels was stopped a lot more often than the other, which caused two paper machines to stop. When the owner asked power company why? They answered that "since the A mill was closer to the power station than the B mill was, it was only natural that the frequency dropped faster at the B mill"
I had been asked to participate at the meeting and I asked the power company guy to say again. He did. I asked if the meeting had understood what he said. It had. Then the paper mill owner sued the power company for contract breach.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: frequency during power system disturbance
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Sometimes I only open my mouth to swap feet...
RE: frequency during power system disturbance
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: frequency during power system disturbance
RE: frequency during power system disturbance
Regards
Marmite
RE: frequency during power system disturbance
Imagine two voltage waveforms at the same frequency and zero phase angle difference. Now shift one voltage waveform to 30 degrees lagging. Its next zero crossing will be 30/360 = 1/12 of a cycle later. If a voltage waveform instantaneously shifts from 0 to 30 degrees lag there has to be one half cycle where time between zero crossings is longer than normal. Frequency = 1/(time between zero crossing). Frequency is slower during that one cycle. Otherwise, how does the voltage go from 0 lag to 30 lag?
One-cycle measurements of frequency are not as good as averaging over several cycles. The change in frequency caused by a sudden phase shift would be barely discernable over three-four cycles.
For that one cycle, a slower frequency would be recorded.
RE: frequency during power system disturbance
But the question was about frequency. And I think that the OP shall tell us what he really meant by frequency.
I repeat what I said a few hours ago: "A seemingly simple question has evolved into a philosophical discussion relating to phase difference and whether a frequency should be considered measured over a complete period or defined as the derivative of instantaneous value."
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: frequency during power system disturbance
See http://en.wikipedia.org/wiki/Rack_railway
Now think of it in reverse. If the rack moves and the engines are still then that is how a power system works. The rack drives the gears at a constant speed (frequency). Generators push the rack, loads slow it. In practice consider the rack and gears to be made of rubber to allow for disturbances (faults, load changes), but the frequency is constant (speed of rack). Even though though one may be speeding up and the other slowing.
It depends on power flow, I live in UK. If there is a sudden power demand in England, frequency will drop, Scotland will restore balance by the fact that its frequency is high, resulting in power flow to England. System will be stable when power flow is zero, at a lower frequency. Difference in frequency is marginal, but that’s how it works.
Other use of analogy, to synchronise you have to march the speed of the gear and the rack and get the teeth in line.
RE: frequency during power system disturbance
Sorry. Had a big one to get over loss of Konrad, our cat.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: frequency during power system disturbance
I think we all agree that an action that initiates or terminates a phase shift will cause at least one-half cycle to be distorted. I think that it is spitting hairs to call this half cycle anomaly a different frequency.
Consider a power system with a branch circuit energizing a resistance heating load. Now start a motor with a bad power factor on the same circuit. We have a phase shift, right? Different frequency for a very short time? What about the voltage? It didn't shift, only the current shifted. But we have been hearing that a phase shift causes a different frequency.
Okay, now I have two different frequencies on the same circuit. The voltage frequency no longer equals the current frequency (for a short time).
I may change my mind at some point in the future, but for now, I am not quite ready to accept different voltage and current frequencies in the same circuit, rather than calling a phase shift a phase shift.
Let's call a phase shift a phase shift and accept that it causes a distortion in the wave form, but let's not split this poor hair any further.
Pole slipping is a little more interesting. I have never slipped a really big pole, but I imagine that a fourier analysis on the system would show both the system frequency and the frequency of the problem machine. I would expect the relative percentages of the two frequencies to vary at different locations throughout the system.
respectfully
RE: frequency during power system disturbance
RE: frequency during power system disturbance
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: frequency during power system disturbance
I am getting very tired of this.
If you are going to drag all sorts of frequency components into this discussion, then you have different frequencies all the time. I am, if you didn't get it, referring to harmonics.
You have to decide, Are you discussing frequency as measured with a frequency meter? Or are you discussing transients? Or frequency defined as d(alpha)/dt? Or are you trying to define some sort of Unidentified Frequency Object aka UFO?
We need to hear what the OP actually meant with his question. It may even be that he wasn't aware of the differences?
As to "local load shedding": There is no such thing. If grid frequency goes down, then all objects set to switch off at that frequency do switch off. No matter where in the grid they happen to be situated. There may exist load dependent extra conditions to help decide which object to shed. But then you have something else then frequency that decides.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: frequency during power system disturbance
RE: frequency during power system disturbance
Also, the harmonic frequencies are unchanging. While voltages or currents at harmonic frequencies might have changing magnitudes over the system, the harmonic frequencies themselves do not. The second harmonic on a 60 Hz fundamental system is 120 Hz where ever you are.
RE: frequency during power system disturbance
I know - never use irony without being very clear about it. But then, what use is it?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: frequency during power system disturbance
http://www.basler.com/downloads/loadshed.pdf
Regards
Marmite
RE: frequency during power system disturbance
Before we can get on the same page, we will have to agree on how to measure frequency. Do we measure peak to peak, full cycle zero crossings or half cycle zero crossings.
For example, we are familiar with the wave form of an asymetrical fault current with a DC offset. If we measure the peak to peak time of a complete cycle we will see our fundamental frequency. If we measure the zero crossing time of a complete cycle we will see a small drift in frequency as the phase shift introduced by the fault slowly decays. It can be seen that each succesive full cycle zero crossing is not at the same point on the wave form as the DC component decays.
Measuring the full cycle zero crossings will show a frequency different from the fundamental frequency but the frequency difference will follow the decay of the DC component. When the DC component has decayed to zero the zero crossing frequency difference will be zero and we will be back at fundamental frequency.
Determining the frequency by the half cycle zero crossings will give alternating high and low frequencies for every half cycle.
I still stick to my original thought that the frequency is basically the same everywhere on the system.
Different measurment techniques may show apparently different frequencies.
Even though a DC offset or a phase shift on a wave form may look like a frequency change to a relay using zero crossing techniques, the full cycle peak to peak may well still be the fundamental frequency.
I will accept the case of the "Hunting" generator as a differend frequency, because the speed of the generator is varying, but as the excursions are limited to about plus and minus 90 degrees, I am comtent to also call this a series of phase shifts.
Note that the Basler information is at some points discussing frequency drop when a section of the grid has become islanded on overloaded, and the frequency of the islanded portion is decaying, and at some points the information is discussing "apparent' frequency shifts as determined by a relay using zero crossing determination of frequency.
What's the point?
How do we measure frequency? Half cycle, full cycle, peak to peak, zero crossing, or average.
Can we accept the small timing variation when a phase shift is introduced or must we consider this a short duration frequency change.
When I want to pick up some more load on a paralleled generator, I open the governor. The shaft assumes a new relative position a few degrees ahead of its previous position. Technically, yes, to affect the relative position change, the frequency has to have increased a very small amount for a very small time.
I'd rather call a phase shift a phase shift, and not a very limited frequency increase for a very short time.
"I also tend to call a spade a spade, rather than speaking about the dog's hysterectomy."
This is an attempt at a little humor and I hope it is taken that way. I respect all the points of view that have been presented here. I agree with most of them.
Bottom line, how do we determine frequency.
Respectfully
RE: frequency during power system disturbance
periods occurring per unit time.
periodic function: A function that satisfies f (x) = f (x + nk) for
all x and for all integers n, k being a constant.
First of all you need a periodic waveform.