frequency during power system disturbance 1

[salem]

will we have the same frequency in all connected network during power system disturbances ?

 

[Skogsgurra]

Yes. There may be some phase differences, depending on how strong/weak the grid is and what currents are flowing. But frequency remains the same throughout the whole grid.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

I did see an exception once, but it is very rare. This was a large island with it's own network and distributed hydro generation and co-generation. It was also fed with a DC undersea cable.
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

 

[salem]

Let my explain more, what I mean is during transients in the system due to faults, swings, etc.. , do we the same frequency variations at all buses in the interconnected network that is if we have a frequency of a certain value at one point in the network then the same value is exactly measured at any other point in the network at the same time

 

[Skogsgurra]

Yes.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

There is some problem with the whole aspect of 'the' question.

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

 

[waross]

Yes.
respectfully

 

[Skogsgurra]

Yes.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

At the risk of contradicting the esteemed members above, it is possible to have different frequencies during certain system events. The difference in frequencies will be seen more as differences in phase angle, but the difference will be there.

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.

 

[Skogsgurra]

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. Since the latter method is both impractical and of little practical interest - I still say that frequency is the same throughout the system.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

I'm going to side with David on this one - under severe system disturbances the instantaneous frequency at differing locations on the grid can vary as power flows redistribute following a major event such as loss on a large interconnector or generating site. The instantaneous frequency at a node will rise as generators move in relative phase as the load angle changes with load. System-wide frequency will also move slightly as load (or infeed) increases or decreases. The events in Europe last year (or was it 2005?) almost certainly showed this immediately prior to the European grid separating.


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Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

It surely showed different frequencies after the separation. But before? Frequency? Measured? I do not believe it changed - not if we are talking frequency as we know it from frequency counters. No.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[davidbeach]

Frequency as measured by zero crossing or other whole cycle phenomena; probably not. Frequency as calculated from the instantaneous speed of the rotors of synchronous machines; absolutely. If the swing is stable it is probably sufficiently accurate to assume a constant frequency and varying phase angles, but once poles start to slip there are two, or more, frequencies on the system until the system becomes two or more independent systems.

 

[Marmite]

Food for thought: What is instantaneous frequency? The definition of 1Hz is the frequency of a periodic event having a duration of 1 second. Doesn't that imply a sampling interval of 1 second?! ie until you've measured a series of events for a second you don't know the true frequency. The load angles and the instantaneous speed of the individual rotors may oscillate above or below sychronous speed, but the system frequency is the same across the whole system.
Regards
Marmite

 

[rcw retired EE]

The average frequency over two or three cycles may be constant, or the same at all busses in the system. But frequency has to change for one or more cycles in order for a phase angle change to occur.

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.

 

[Skogsgurra]

Yes. I think that most of us understand that.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Hoxton]

This is getting very intellectual! How I explain generating power systems to mechanical engineers (lowest of low..) is think of a rack and pinion railway. For non rail enthusiasts, it is like a gear laid out flat, the engine goes along by driving a gear along the rack.

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.

 

[Skogsgurra]

Hear! Hear! Getting your teeth in line! That is what counts!

Sorry. Had a big one to get over loss of Konrad, our cat.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

I have been thionking today about phase shifts as they relate to frequency changes.
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

 

[apowerengr]

I agree - you can have short-term multiple frequencies during system disturbances. If not, how would underfrequency loadshedding be effective at a local level? Any underfrequency event would shed all protected load across the entire grid at the same time?