frequency tolerance

[danielb]

Good morning

I have a question to submit

I want know what is the tolerance range, of the frequency, at 60 Hz in the

power distribution and if there is a standard on this isuue

Kind regards and Thanks in advance

Daniel

 

[davidr43229]

Daniel,
This would largely depend on what state you are in, or Utility serving your area.. Texas says:
Voltage distortion of less than 2%, nor 3% of the fundamental frequency for any individual harmonic level.
Some Utility company do not publish their tolerences. I did a goggle search and found very little.
Just my $.02

 

[aja21]

I'm no expert by any means but in our hydro plants our boss never tolerates a full hertz on a digital meter. for whatever thats worth.

 

[jim dungar]

I know you can find this infomation in the IEEE Red book.

 

[don_resqcapt19]

aja21,

I'm no expert by any means but in our hydro plants our boss never tolerates a full hertz on a digital meter. for whatever thats worth.

Unless you are operating as a island you don't really have any control over this as there is nothing that your plant can to to change the frequency of the grid that you are connected to.
Don

 

[don_resqcapt19]

It is my understanding that the grid operators start getting very nervous with a frequency that is more than a few tenths away from 60 hertz.
Don

 

[winnie]

I did a bit of looking about. The search "north american grid frequency" pulls up a bunch of stuff.

http://www.ornl.gov/sci/btc/apps/Restructuring/ORNLTM200341.pdf was fascinating on a quick skim. To my neophyte eye, it looks like +-0.01Hz is the normal operating range, with +-0.02 Hz intentionally used to adjust long term averages.

Evidence of long term (over the course of a month) errors on the order 0.004Hz is enough to get some people worried about the grid.

-Jon

 

[don_resqcapt19]

You can download a real time grid monitor screen saver for the western interconnect here. It looks cool, but note that lately the server has been down a lot of the time.
Don

 

[dbuckley]

Have a look at IEEE Guide for Abnormal Frequency Protection for Power Generating Plants (PDF)

Heres a snippet:

The loss of a large unit at a time when the frequency is scheduled at 59.98 Hz can result in system frequency below 59.95 Hz. Frequency at this level is cause for concern, and, in some area control centers, triggers the declaration of a state of alert. The rate of occurrence of such an event on the Eastern Interconnection has been established to be once per month. In recent years, the frequency has descended below 59.90 Hz only once, during a time when there was a widespread period of very cold weather, resulting in fuel problems at many generating plants.

 

[coulter]

Heres a snippet

...In recent years, the frequency has descended below 59.90 Hz only once...

Really. It must be great to live in America. (giggle)

We have three 6500hp motors on VFDs. Generally run two at a time. Our agreement with the Utility is with three running, we trip the first at 59, the second at 58.7, the third at 58.5Hz.

In the last few months, I've seen 59 a few times, but nothing lower.

carl

 

[Physis 3]

I know very little about how frequency is either monitered or maintaned but my understanding, as clueless as it might be in this case, is that the utility in my area makes ajustments based on what synchronous clocks do. A synchronous motor's rpm is derived from the AC's frequency. I've been told that their frequency adjustments are made to compensate for time variances that will be somewhat obvious in this type of clock, over some time of course. Also, again, I'm not sure, but I think, there is no second to second, minute to minute or even hour to hour adjustment. Only adjustments based, essentially, on what a wall clock does. Who's gonna notice anyway? I'm not aware of any "guarranties". But I've never heard of anything even near a 1/2 cycle error.

I've also never heard of anything so sensitive to frequency that it would matter. Aside from things that would produce their own waveforms.

 

[dbuckley]

Your understanding is correct, the long term frequency is maintained outrageously close to 60Hz so clocks over the long term are right. They may be a few seconds out from time to time (I think they were something like 24 seconds adrift after the east coast blackout, at least for those places that stayed on), but long term, it's spot on. Most civilised countries have always done this, going back to the days of Mr Hammond (he of Hammond organ fame) who invented the synchronous electric motor clock prior to the great depression.

There was a thread recently about a microwave that (if my memory si working) wouldn't work as the frequency to 58.something from a genset. It just wouldn't play ball...

I'm in New Zealand, 50Hz-ville, and our generating utilities under fault conditions are required to stay online down to 48Hz, which is really really hard (and destructive) for coal fired plant. Normally, our clocks stay right too, but 'cos our grid is much smaller than even a ickle bit of the USA, frequency excursions can be somewhat larger.

 

[robbietan]

been in the generating plants all my (working) life. They have a trip setting of 59.7Hz and 60.3Hz. been fortunate enough to work with 60Hz generators.

in a grid, 59.7 Hz represents a major power supply trouble. Distributors often set 59.7 Hz for automatic load dropping

 

[don_resqcapt19]

Physis,

I know very little about how frequency is either monitered or maintaned but my understanding, as clueless as it might be in this case, is that the utility in my area makes ajustments based on what synchronous clocks do.

That is true, but it is not just the utility in your area...it is all of them in the same interconnect. There are only 3 interconnects in the US. East of the Rockies, west of the Rockies and Texas (living up to its other name...the lone star state). All of the utilities in an interconnect are physically connected to each other.
Don

 

[Physis 3]

Physis,

That is true, but it is not just the utility in your area...it is all of them in the same interconnect. There are only 3 interconnects in the US. East of the Rockies, west of the Rockies and Texas (living up to its other name...the lone star state). All of the utilities in an interconnect are physically connected to each other.
Don

That's something that's always interested me Don, not enough to actually try to find out I suppose, but what in the world is the method used to keep all these individual generators, or alternators, in synch?

I know that if you have a number of smaller AC making gadgets paralelled the frequency errors can cancel leaving an overall acurate sine wave, of course with a power loss.

But with the big stuff, how are these sourses kept synched?

By dbuckly:

going back to the days of Mr Hammond (he of Hammond organ fame) who invented the synchronous electric motor clock prior to the great depression.

I play guitar and am heavily interested in building music in general. This is very interesting to me that Hammond also invented the synchronous motor. I had no idea. I'm used to refering to these things as "Hammond church organs". Most people are familiar with earlier Carlos Santana, Abraxis and before, that's what they were using.

 

[SurfSide EC]

There was a thread recently about a microwave that (if my memory si working) wouldn't work as the frequency to 58.something from a genset. It just wouldn't play ball...

That was us, and after some tinkering we able to get things operational, though there was little to no change in the hz. Hopefully it was a one time deal with the micro not working because it was fully functional on the the return trip. We have had no other problems that I know of with the gensets ranging from 58-63 hz. (Sometimes we set the aircooled engines to a hz of 62.5 to allow for a load that will drop the engine speed and still maintain an opperable hz). The only other issue we have had with frquency tollerance is certain elevators do not like any sort of drop in volt/hz and we have to size genset accordingly.

 

[winnie]

Physis, not a complete answer, since that would involve system stability theory that is over my head, but something to keep in mind as part of the answer:

For most electrical rotating machines, the difference between a motor and a generator is how the machine is operated, _not_ the machine itself. An induction motor, connected to an 'overhauling load', meaning a mechanical load that tries to push the motor faster than its rated synchronous speed, will operate as a generator.

The large power plants generally use synchronous machines tied to their prime movers. In the steady state, a synchronous machine will always spin at a speed related to the supply frequency. The angle of the rotor relative to the rotating magnetic field determines the torque of the machine. Roughly: if the magnetic field is pulling the rotor around, then the machine operates as a motor, supplying mechanical power to its shaft (and any connected load), and if the rotor magnets are 'pulling' on the rotating magnetic field, then the machine absorbs mechanical power from its shaft (and any connected prime mover), producing electrical power.

So once the power plant is connected to the grid, the grid itself acts to synchronize the generators. Should a particular power plant change power output, then its generator would adjust rotor angle slightly to increase or reduce torque. So there would be a transient difference in generator speed versus grid frequency, but once the two come to equilibrium, they would both again be at the same speed.

When initially connecting power plants to the grid, there can be tremendous transient current flows as the magnetic forces synchronize the rotor to the grid. I don't really know the details of how operators deal with this.

I seem to recall a thread here or on ECN describing how plants get connected to the grid. Below is an interesting description of the grid:

http://www.electrical-contractor.ne...wflat/Forum/1/topic/003211/Number/0/site_id/1

-Jon

 

[dbuckley]

This is very interesting to me that Hammond also invented the synchronous motor. I had no idea. I'm used to refering to these things as "Hammond church organs". Most people are familiar with earlier Carlos Santana, Abraxis and before, that's what they were using.

Yep, to sell more of his synchronous motors he invented the Hammond organ, with the 'tonewheel' note generation system. The Hammond tonewheel organ is an instrument that is still highly revered and sought after, despite the fact they haven't been manufactured since 1974, one year after the inventor's death. (Though frankly, any of the modern Hammond type organs from Korg et al sound just as good as the real thing)

Of course, the fact these organs use synchronous motors as the frequency source makes for some interesting times. Those of us who sat under the stars in the gardens of Knebworth Castle in the UK in 1974 to watch Pink Floyd had an interesting night. The whole production ran off a number of diesel gensets, and back in 1974 hireable gensets had mechanical speed governors, which meant the frequency stability was not great. Bad enough that the organ dropped about a semitone when the band played. Which meant that the Farfisa organ was used for the whole night, rather than the Hammond, leading to a different sound for The Floyd...

This was also a problem for the then widely used StroboTuners, which had a disk driven by a synchronous motor. Fortunately, Korg rescued us from that mess too. I note that Roger Waters still uses real StroboTuners, I hope they are rigged to be independent of the mains frequency supply.

Am I glad than both portable power generation and music technology has improved in the intervening quarter century!!! A redundant pair of Cat gennys (usually called a twinpack) is an unbelievably rock solid source of power, they hold voltage and frequency at load steps of most of the rated output, fabulous tools.

 

[dbuckley]

When initially connecting power plants to the grid, there can be tremendous transient current flows as the magnetic forces synchronize the rotor to the grid. I don't really know the details of how operators deal with this.

The somewhat flippant and correct answer is very very carefully.

Before a generator is connected to the grid the generator is exactly synchronized to the grid. In the early days this was done with lamps, then with mechanical syncroscopes, and now with automatic synchronizers. But however you get the generator rotor into phase with the grid it's about to connect to, when the breakers are closed the "generator" will motor itself to the correct position, causing a mechanical shock. So the normal way to bring on a generator is to run the genny just a tadge faster than the grid, and at the co-incident point (lamps out or, syncrosope pointer on zero, whatever) the breakers are closed bringing the genny on-line.

With a small genset it's fairly easy, as a 2000A 480V ACB closes pretty quickly. But a 138KV (or similar) breaker is not an instant thing to close, and thus the close instruction has to be given in advance of expecting the close to take place, so the generator is approaching synchronization when the breaker is instructed. A lot of work was done to make breaker closing time consistent as if it was slow or fast the rotor would be in the wrong place and a correction would occur. It took lots of near misses before that was solved.

Old timers will tell you that the power plant shakes every time they bring a generator on line. Thees an old story (which I cant find a reference to) about a cogen plant at a factory (a Ford factory, if my memory serves) which was brought on-line out of sync to the grid, and the rotor went through the wall of the powerhouse, rolling off across fields for thousands of yards.

They don't call the grid "infinite" for nothing.

 

[ghostbuster]

We were performing some site tests on a CAT scan machine in a fairly large hospital.All of a sudden you could here the inverter frequency change.Monitors at the main board confirmed the frequency had dropped to 52 hz. for approx.3-5 min.I contacted the utility and they said this was a common thing(happens 2 to 3 times a day) and not to worry.

We were on large island ,population approx.500,000 people.All their power is derived from diesel.As step load is introduced, the diesels slow down each time ,according to the utility reps.

This must play havoc with some of the more high tech equipment!