# 60 Hertz Tolerance???

Discussion in 'Electronic Basics' started by Randy Gross, Nov 23, 2003.

1. ### Randy GrossGuest

Greetings,

This question concerns the accuracy of 60 cycles per second when it comes
to power use. Is this a constant or, is there a tolerance? What effect
would say 55cps or 65cps have on a circuit (analog)?

curious,
Randy Gross

2. ### Henry KolesnikGuest

As I understand it, it's quite accurate over the long term, electric clocks
keep very good tiem but it's not that good on the short terrm. If for some
reason the plant runs at 59 cps for say a second, they'll play catch up and
run at 61 cps for a second or may 60.5 for 2 seconds and so on. Perhaps an
electric utility power plant engineer will tell us. Unless the line
frequency is used for timing I doubt that the change you mention would have
any effect and I doube that you'd ever see that kind of change unless the
plant was having major problems.
hank wd5jfr

3. ### SteveGuest

I'm not a power plant engineer, but I have been an engineer on a ship with
power generators and highly accurate frequency controls.

I just happen to have a standard switch board freq. meter here in a
collection of old stuff. I looked at the range of the freq. meter and it is
from 55 to 65 hz. However the engineer controling the turbine and the
electrician on the switch board controls would never attempt to bring a
generator online until the frequency was stablized to withing a tenth of a
hz of 60 hz. It would also cause problems in paralelling two or more
generators if the frequency was off (out of syc).

Power plant operators on the grid can't tollerate anything out of syc. If a
generator slows down, it will trip off line ..

Then again, I have been on diesel generators out in the jungles of Cambodia
and all we had to aid in paralleling them was a light bulb between the two..
Light bulb is lit, your out of syc. Light bulb goes out, close the breaker.
If it's a bad light bulb, then the breaker will overload and trip out.

Sorry, I'm rambling..

Steve

4. ### Bob MastaGuest

The latest issue of The Industrial Physicist had a nice
article on the North American power grid, relating to
issues such as the recent blackout and deregulation.
It points out that frequency must be held fairly tight
because otherwise there will be enormous current
flows simply due to the mismatch. I passed the
article on to my brother-in-law, so I can't refer to
it right now, but as I recall they keep errors in the
0.01 Hz range if they can.

But as long as we are on the subject, let *me* ask
a question: The article says that the several separate
grids in North America are interconnected by *DC* lines.
This seems astounding to me ( but would at least allow
for frequency differences). I assume that means they
must have huge motor-generator sets or something
similar on either end. Does anyone know how this
works?

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

5. ### Henry KolesnikGuest

Interconnected AC grids are usually done DC to DC using rectifiers and
inverters.

6. ### Don LancasterGuest

I believe the short term accuracy can vary all over the lot, but that
they are very careful to make everything come out even every month.

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: fax 847-574-1462

Please visit my GURU's LAIR web site at http://www.tinaja.com

7. ### Daniel LangGuest

Do a Google search for Pacific Intertie

Also, take a look at:

http://tdworld.com/ar/power_new_thyristor_valve/

Daniel Lang

8. ### Don KellyGuest

-----------------
Frequency variations in themselves do not cause a stability problem if all
generators change frequency together. The problem occurs when differences in
frequency between machines occur and a sustained difference of any value
between machines will cause problems. Normally if a machine tends to
overspeed there will be torques produced to speed up other machines and slow
the fast unit. This sort of jockeying around goes on all the time but the
average is held to to 60Hz while the instantaneous frequency of the system
generally will be within 0.05 Hz. How close the frequency can be held
depends on the system, the number of units, and the control used. A single
generator will show far more variation than an large grid.
Other problems can occur with large frequency drifts. some of these may
include the loss of such things as critical feedwater pumps, resulting in
the shutdown of a generator, leading to further shutdowns, etc.
As for the DC transmission:
Believe it or not- that was done in the very first DC transmission system
(Theiry(?) in the 1920's) where a group of generators in series (i.e. a
cascade) were used to reach a decent transmission voltage and motors in
cascade driving other generators were used as loads. Insulation was a bit
messy.
Later developments included the use of mercury arc rectifiers as both
rectifiers and inverters and at present most systems use solid state
controlled rectifier/ inverters. The Source AC is stepped up to the desired
level, rectified and the resultant DC is inverted back toAC at the other
end. There are many systems in existence around the world. Most are intended
for long distance transfer of energy or for cases where long underwater
links are needed (30 miles of underwater cable is roughly equivalent to
300miles of overhead line at 60Hz). Some systems are "back to back"
providing an asysnchronous link. Some of the latter are at Eel River New
Brunswick (first commercial solid state converter) , between 50 and 60 Hz
Vancouver Island and long distance transmission exists in Manitoba, Quebec,
and in the Pacific region of the US among others.
Look up HVDC
one source is
http://www.siemens.com/page/1,3771,261226-1-12_2_255135-0,00.html
which gives a brief and limited explanation

9. ### Ralph & Diane BaroneGuest

Off the top of my head, I would say that plus or minus 0.1 Hz is about as
far as the frequency will normally wander. If your local power system is
at 55 or 65 Hz, then all hell has broken loose and your lights will
probably be going off real soon.

10. ### Watson A.Name - Watt Sun, Dark RemoverGuest

[snip]

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11. ### Don KellyGuest

-----------
From a stability analysis viewpoint you are correct. However, note that I
said "sustained" difference of any value is a problem (as I didn't want to
get into stability analysis per se). By the time there is such a frequency
difference between machines, it is too far too late to do anything but break
out the candles. Transient speed (and associated frequency) variations are
going on all the time- in analysis one simply looks at the speed differences
and these are best handled in terms of relative phase angles.
From a non-stability point, the system frequency will always be changing
due to load changes and the responses of the various prime movers. In
extreme situations the frequency change can be enough to cause problems as
indicated. In other cases changes in the power balance between sections of
the system can cause line tripping and islanding of the systems where the
islands may lose generation due to over or underspeed.
Thank you

12. ### Paul Hovnanian P.E.Guest

They usually resync the 60Hz 'clock' with a time standard every night
when there is some spare generating capacity available.

During the day, system operators can (usually) keep the system frequency
within +/- 1% on an instantaneous basis. Underfrequency is often used to
trip substation circuits off line for load shedding purposes. IIRC, some
rural circuits I used to work on were set to shed at 59.5 Hz. In urban
areas, this setting will be significantly lower, something like 58.5 or
59 Hz.

We were told in the power electronics class that I took some years back that
all grid interconnections between Texas and the rest of the nation were this
way: Something in the Texas state constitution forbid them from having AC
power lines crossing their state lines, but didn't mention anything about DC
lines and apparently it was easier to build a bunch of M-G sets (or -- more
recently -- their solid state equivalents) than change the state
constitution.

....or so I was told...

14. ### Randy GrossGuest

I guess I can take all this to mean: No Tolerance;-)

Randy Gross

<[email protected]>...
:
:
:
in
: message : > In article <[email protected]>,
: > mentioned...
: > >
: > >
: > >
: > > : > > > On Sun, 23 Nov 2003 14:04:59 +0000 (UTC), "Randy Gross"
: > > >
: > > -----------------
: > > Frequency variations in themselves do not cause a stability problem
if
: all
: > > generators change frequency together. The problem occurs when
: differences in
: > > frequency between machines occur and a sustained difference of any
: value
: > > between machines will cause problems. Normally if a machine tends to
: > > overspeed there will be torques produced to speed up other machines
and
: slow
: > > the fast unit. This sort of jockeying around goes on all the time but
: the
: > > average is held to to 60Hz while the instantaneous frequency of the
: system
: > > generally will be within 0.05 Hz. How close the frequency can be
held
: > > depends on the system, the number of units, and the control used. A
: single
: > > generator will show far more variation than an large grid.
: > > Other problems can occur with large frequency drifts. some of these
may
: > > include the loss of such things as critical feedwater pumps,
resulting
: in
: > > the shutdown of a generator, leading to further shutdowns, etc.
: >
: > One should not talk about frequency, but about relative phase angle,
: > leading or lagging, between generators.
: -----------
: From a stability analysis viewpoint you are correct. However, note that
I
: said "sustained" difference of any value is a problem (as I didn't want
to
: get into stability analysis per se). By the time there is such a
frequency
: difference between machines, it is too far too late to do anything but
break
: out the candles. Transient speed (and associated frequency) variations
are
: going on all the time- in analysis one simply looks at the speed
differences
: and these are best handled in terms of relative phase angles.
: From a non-stability point, the system frequency will always be changing
: due to load changes and the responses of the various prime movers. In
: extreme situations the frequency change can be enough to cause problems
as
: indicated. In other cases changes in the power balance between sections
of
: the system can cause line tripping and islanding of the systems where the
: islands may lose generation due to over or underspeed.
: Thank you
: --
: Don Kelly
:
: remove the urine to answer
:
:
:

15. ### Dan AkersGuest

Randy wrote;
"This question concerns the accuracy of 60 cycles per second when it
comes to power use. Is this a constant or, is there a tolerance? What
effect would say 55cps or 65cps have on a circuit (analog)?"
_____________________________________
Re;
On continental North America, the touted tolerance is +/- 0.2Hz. This
tolerance is more a credit to the large number of units operating in
parallel than to the accuracy of the individual generator governors.
There is no scheme to "make up lost cycles" to keep clocks accurate that
I've ever heard of. The maintenance of grid frequency has become much
more important than just the accuracy of synchro-motor clocks with the
advent of nuclear generating stations; namely due to maintaining the
required reactor coolant flow rate. A whole host of reactor protection
schemes (over power based on core delta temp. for example) are based on
the premise that the reactor coolant flow rate (proportional to grid
frequency) be within prescribed parameters. The typical low flow trip
setpoint is 91% design flow, which corresponds to about 54.6 Hz. The
typical low frequency relay trip setpoint is set above this to provide
"anticipatory" redundancy and reactor coolant pump motor protection;
typically in the neighborhood of 58-59 Hz.
It would be very unlikely that you would ever see a 5 Hz variation from
the normal 60 on a large scale, multi generator, grid. The grid would
be "dead" long before that, I suspect. The generating station trips, as
outlined above, come after numerous other substations would have already
"selectively" tripped off in an automatic attempt to reduce system load.
This usually starts at 59.5 Hz.
High frequency, is a very rare problem, since the problem implies that
load<supply. High frequency, from the generation stand point, is a
problem because of it's turbine generator over-speed implications. As
such, large turbo-generators typically auto trip on over speed at 1% or
so above rated synchronous speed; between 60.5 and 61Hz or so.
A 1 Hz variation in frequency on a large scale grid indicates a serious
a problem with the load-generation balance (usually only seen when
several generating units trip off in cascade fashion or sudden loss of a
very large load) because high demand is dealt with by allowing grid
voltage to droop rather than at the expense of frequency.

-Dan Akers

16. ### Don KellyGuest

-----------
As an aside, a typical generator governor will have a speed droop of , say,
3 to 7% from no load to full load. Without a droop, then power sharing
between machines is a problem. With multiple machines as you indicate, the
system speed changes are much smaller. I recall one utility (all hydro)which
required a special report rom operators if frequency drifted by more than
0.1Hz. In another, there was a central controller which compared the
electric clock to a standard mechanical clock which in turn was regularly
checked through the US time signal (WWV at the time). The reference clock
sent out pulses once a minute and generators would get nothing if the system
was within tolerance but otherwise would get hurry up or slow down signals.
These were biased for each machine (about 15-18 units in several
plants -many hydro) depending on load. This system worked well but after
interconnection with the Northwest Pool, it was replaced by standard load
frequency controllers as used in the associated system.

17. ### Steven SwiftGuest

DC-interties are based on stability and transient analysis of large
grids in the 1930s and 1940s. It all derives from control theory with
delays in the feedback loop.

It turns out that long AC lines, over a certain length, can not be made
stable.

My grandfather got his tenure by proving this (with a whole bunch of
grad students and huge number of analog computer runs). His work formed
a theoretical basis for the DC-intertie to California.

Steve

18. ### Steven SwiftGuest

Okay, I have been following this thread, so here's what I'll do.

Novatech's calibration system can continuously monitor frequency drift
(to about 1e-13) and since we are closed for a week at Christmas, I will
set the system to measure the 60Hz line. That will give a week's worth of
data (recorded every 10minutes). I'll post a graph.

Steve.