Power Grid Synchronization and Reactive Power
leko...@yahoo.com
Once a generator is connected/synchronized to a grid and everything is
running normally. Can there be a reverse reactive power flow from the
grid to the generator? What would happen in cases of reactive power
flow? How is this prevented/protected? Is this a part of
synchronization or does it come under transmission?
Thanks,
LS
phil-new...@ipal.net
This is something that reportedly happened (reverse reactive flow) during
the 2003 blackout in the midcentral to northeast US.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-200...@ipal.net |
|------------------------------------/-------------------------------------|
Salmon Egg
"phil-new...@ipal.net" <phil-new...@ipal.net> wrote:
> On 22 Mar 2007 07:23:04 -0700 leko...@yahoo.com wrote:
>
> | Once a generator is connected/synchronized to a grid and everything is
> | running normally. Can there be a reverse reactive power flow from the
> | grid to the generator? What would happen in cases of reactive power
> | flow? How is this prevented/protected? Is this a part of
> | synchronization or does it come under transmission?
>
> This is something that reportedly happened (reverse reactive flow) during
> the 2003 blackout in the midcentral to northeast US.
Without more explanation, these posts do not make sense to me.
Reactive current output from the generator can be either positive or
negative, depending upon the power factor of the load. Whichever it is, it
consists of power flow from the rotational energy of the shaft into
electrical output for about half a cycle and return of electrical power to
the shaft half a cycle later. If this exchange is large enough, ohmic
heating in various windings can become large enough to cause damage.
In any even, such reactive (non)power is normal, albeit costly, in power
systems. I can picture situations when such exchange can increase out of
control. But even real power can exceed safe operational limits.
Ext question?
Bill
-- Fermez le Bush--about two years to go.
Don Kelly
<leko...@yahoo.com> wrote in message
news:1174573384....@e65g2000hsc.googlegroups.com...
Yes there can be a reverse reactive flow. This is not a problem as long as
it is within the capability of the generator*. It won't affect the prime
mover. Reactive output of the generator is controlled through the excitation
system -trying to raise voltage will increase reactive generation (lagging
pf when seen as a generator) and lowering voltage will decrease reactive
flow or make it negative (leading pf).
Often an unloaded synchronous machine is run as a motor and its reactive
adjusted as desired to maintain a given voltage at its terminals or at
nearby loads.
*The capability or allowable operating range is different in the lagging
and leading regions as the limits are dictated by different factors -VA and
field current limits in the lagging region (+var output) and VA and a
"stability limit" in the leading region (low field current, -var output) and
is lower in the leading region.
--
Don Kelly dh...@shawcross.ca
remove the X to answer
phil-new...@ipal.net
| On 3/22/07 9:40 AM, in article etubh...@news1.newsguy.com,
| "phil-new...@ipal.net" <phil-new...@ipal.net> wrote:
|
|> On 22 Mar 2007 07:23:04 -0700 leko...@yahoo.com wrote:
|>
|> | Once a generator is connected/synchronized to a grid and everything is
|> | running normally. Can there be a reverse reactive power flow from the
|> | grid to the generator? What would happen in cases of reactive power
|> | flow? How is this prevented/protected? Is this a part of
|> | synchronization or does it come under transmission?
|>
|> This is something that reportedly happened (reverse reactive flow) during
|> the 2003 blackout in the midcentral to northeast US.
| Without more explanation, these posts do not make sense to me.
What I read was that the grid was actually looped in a circle involving
portions in Canada, and that real power was flowing along a different path
than reactive power, which resulted in some portions of the grid having
real power going in one direction and reactive power going the other way
over transmission line(s). How that could get established, I do not know.
I do not recall how long that situation existed, but what I do recall is
that the catastrophic failure took place within 2-3 minutes of this. What
I wonder is what caused this. Could it have been a change in frequency
that had happened just before? Could it have been some voltage or load
spikes seen earlier from such things as loss of a transmission line due to
a sag fault into a tree. I guess you'll have to google up all the reports
(I didn't keep the URLs) yourself and use your professional understanding
to make better heads and tails out of it than I possibly could (electricity
is mostly a hobby for me, not my profession).
| Reactive current output from the generator can be either positive or
| negative, depending upon the power factor of the load. Whichever it is, it
| consists of power flow from the rotational energy of the shaft into
| electrical output for about half a cycle and return of electrical power to
| the shaft half a cycle later. If this exchange is large enough, ohmic
| heating in various windings can become large enough to cause damage.
|
| In any even, such reactive (non)power is normal, albeit costly, in power
| systems. I can picture situations when such exchange can increase out of
| control. But even real power can exceed safe operational limits.
What about a situation where power demand is very high and more power is
being drawn from longer distances (say far into the midwest or central
parts of the US). How well does reactive load pass over that distance?
Could it be that closer generators had to take on more of the reactive
load relative to their power production?
AL BENSER
>Once the generator is connected to the grid, power (both active and
reactive) can flow in or out of that generator. You can not separate the
reactive power from the active!! both powers use real current amperes!!
The reactive power is always a component of the total power. So, yes
you can have reactive power flowing from the grid to the generator,
and that means that the current reverse direction and starts flowing
from the grid to the generator, converting it into a motor!!!
That would trigger all king of protective devices, etc . . . . and that has
nothing to do with the synchronization process when you connect the
generator to the grid. This can happen when the voltage of the generator
is too low vs. the grid.
As mentioned by other reply, the reactive power out of the generator
is controlled by its excitation, which vary the output current without
effecting the real mechanical power driving the generator.
Don Kelly
news:eu0ks...@news5.newsguy.com...
sure that, just as with the '65 outage, that IEEE (PAS) has something on
this. Reactive flow and power flow are not necessarily in the same
direction. For example a transmission line may have a real power flow in one
directions and at the same time, depending on terminal conditions:
a) + reactive into the line at each end
b) + reactive out of the line at each end
c) + reactive into the line at one end and + reactive out at the other end
This is determined mainly by voltages and line inductance and capacitance.
As far as drawing reactive from a distant source- can be done but there are
problems in terms of voltage regulation (also in some extreme cases ,
voltage collapse- not good.) and a rule of thumb would be "generate reactive
as close to the load as possible") Hence the use of capacitors or other
capacitive sources to counter inductive loads.
Closer sources will tend to take on more of the reactive load. The problem
is that, if some sources go too far lagging (+ reactive as a generator) and
exceed system reactive needs, then other sources may go leading and suck
vars. This will require a reduction of internal voltage generated (i.e
reduce field) and a reduction in the maximum power transfer- reducing
stability limits. Push it to far, and a bump happens and it is time to get
out the candles. It is best to try to keep all generators near the same
power factor or, at least, all of the same sign of reactive output.
--
Don Kelly dh...@shawcross.ca
remove the X to answer
----------------------------
Don Kelly
news:AYTMh.143$Du2.63@trnddc07...
>
> <leko...@yahoo.com> wrote in message
> news:1174573384....@e65g2000hsc.googlegroups.com...
>> Hi,
>>
>> Once a generator is connected/synchronized to a grid and everything is
>> running normally. Can there be a reverse reactive power flow from the
>> grid to the generator? What would happen in cases of reactive power
>> flow? How is this prevented/protected? Is this a part of
>> synchronization or does it come under transmission?
>>
>> Thanks,
>> LS
>
>
>>Once the generator is connected to the grid, power (both active and
> reactive) can flow in or out of that generator. You can not separate the
> reactive power from the active!! both powers use real current amperes!!
>
> The reactive power is always a component of the total power. So, yes
> you can have reactive power flowing from the grid to the generator,
> and that means that the current reverse direction and starts flowing
> from the grid to the generator, converting it into a motor!!!
Sorry, Al, I disagree strongly.
Your last paragraph below
"the reactive power out of the generator
> is controlled by its excitation, which vary the output current without
> effecting the real mechanical power driving the generator. "
is correct but doesn't jibe with your statement above.
"Reactive power" is a consequence of having inductive or capacitive loads.
The average power due to this is 0. The only effect on "real" power is the
effect on (I^2)*R losses which increase as one goes either lagging or
leading -. "reactive power" because its average is 0 will not affect the
prime mover except for the losses which will always be an additional load on
the prime mover and cannot make it motor. Only a reversal of the real power
will try to do this.
You do have real amperes and you do have real voltages. However, you also
have real phase differences.
--
Don Kelly dh...@shawcross.ca
remove the X to answer
----------------------------
>
Salmon Egg
<dh...@shaw.ca> wrote:
> Sorry, Al, I disagree strongly.
> Your last paragraph below
>
> "the reactive power out of the generator
REACTIVE POWER IS ZERO!!! What you call reactive power is an exchange of
power at a rate of 120Hz on a standard 60Hz. IT AVERAGES ZERO. The current
associated with this does heat conductors by R*I^2 losses. That loss,
however, IS REAL POWER.
leko...@yahoo.com
Thanks,
LS
Fred Lotte
> The reactive power is always a component of the total power. So, yes
> you can have reactive power flowing from the grid to the generator,
> and that means that the current reverse direction and starts flowing
> from the grid to the generator, converting it into a motor!!!
No,no,no!!!!!
It becomes a motor only if the total power summed over all phases is into the machine.
In any given phase, the instantaneous power will have a positive value sometimes during a cycle
and a negative value at other times depending on the phase relation of the voltage and current.
At 1 pf there will be no negative portion of the power waveform. At 0 pf the power waveform will
be centered on the 0 power axis and will be positive during 2 quarters of the cycle and equally
negative during the other 2 quarters of the cycle. (The product of the 2 sine waves is a double
frequency sine wave.)
If you calculate the instantaneous phase power as the product of the instantaneous voltage (L-N)
and the instantaneous (Line) current, then add the 3 values to get the total power, you will
find that for a *balanced* 3 phase system (voltage and current sinusoidal and equal from phase
to phase but displace by *exactly* 120 degrees from phase to phase) the total power will be
absolutely constant regardless of the power factor. Introduce *any* unbalance and the total
power will have a little ripple at 2x operating frequency. (And the shaft torque will have a
little ripple at 2x electrical frequency.)
If all phases are at 0 pf the total power is 0. The machine simple shuttles what may be large
amounts of instantaneous power from phase to phase. The turbine supplies the losses and
excitation. If it doesn't, then the pf won't be 0.
The machine does not motor until the total (sum of all phases) electrical power out of the
machine is negative at the same instant! It doesn't happen in a balanced system. To actually be
a motor the input electrical power supplies the losses (including excitation) and any mechanical
load.
--
Fred Lotte
flo...@nospam.stratos.net
Don Kelly
"Salmon Egg" <salm...@sbcglobal.net> wrote in message
news:C22A0AEA.6B43F%salm...@sbcglobal.net...
I know that, and I should have said "reactive volt amperes" but the term
"reactive power" is commonly used (real, reactive, and apparent power
terms and the "power" triangle were established before you and I were
around) and, as you indicate, are not strictly correct.
What I did say is (note the quotation marks):
"
"Reactive power" is a consequence of having inductive or capacitive loads.
The average power due to this is 0. The only effect on "real" power is the
effect on (I^2)*R losses which increase as one goes either lagging or
leading -. "reactive power" because its average is 0 will not affect the
prime mover except for the losses.....
"
This seems to be in line with what you said.
Salmon Egg
<dh...@shaw.ca> wrote:
> "Reactive power" is a consequence of having inductive or capacitive loads.
> The average power due to this is 0. The only effect on "real" power is the
> effect on (I^2)*R losses which increase as one goes either lagging or
> leading -. "reactive power" because its average is 0 will not affect the
> prime mover except for the losses.....
> "
>
> This seems to be in line with what you said.
I agree--we agree.
In a sense, consider power in a parallel resonant circuit. There is lots of
circulating power, but very little is being dissipated.
AL BENSER
imbalances nor
instantaneous conditions. I also know that, way before the grid could drive
a generator in
a power plant, multiple protective devices will be triggered to stop that
from taking place . . . .
"Fred Lotte" <flo...@nospam.stratos.net> wrote in message
news:flotte-DB44D7....@sn-ip.vsrv-sjc.supernews.net...
daestrom
>I do agree with you Fred!!! and the original question did not mention any
>imbalances nor
> instantaneous conditions. I also know that, way before the grid could
> drive a generator in
> a power plant, multiple protective devices will be triggered to stop that
> from taking place . . . .
>
Actually, my experience in power-plants disagrees, --slightly--. They *do*
motorize, *then* protective relaying trips them.
Most protective relaying schemes for generators include a reverse-power trip
(ANSI device code number 32-). These activate to trip the generator output
breakers when power flow reverses in the unit, after a short time delay.
A typical large steam turbine-generator unit (rated at say, 800 MW) will
reverse power trip when power flow reaches about -40 MW after about 15 to 30
seconds.
On many trip signals, the turbine-generator undergoes what's known as a
'sequential trip'. The turbine is tripped for some reason (low oil, thrust
wear, loss of vacumn, etc...) and the generator is not tripped directly from
the same signals. Instead, it is tripped later, ('sequentially') by the
generator's reverse power relaying as I explained above. This prevents the
residual steam in the separators / reheaters from overspeeding the unit.
Westinghouse came out with a notice about one series of reverse power relays
they have that warns if there is a very low power factor on the unit, it may
not operate at all. This could happen if there was a small lagging power
factor (say, about 0.9) when running at full load, then you lose the steam
supply. The MW will drop to zero and the unit will begin motorizing, but
the MVAR load may not drop enough to avoid the very low, non-tripping pf
condition.
In either case, operators are trained to look for it and prepare to trip the
unit manually.
Diesel-gen units also have a 32- device and although I can't speak from
direct experience, I expect gas turbines do as well (probably hydro as
well).
Motorizing a large generator isn't particularly risky for the generator if
it's already synchronized. It can take such operation indefinitely. A
couple of old steam units in a oil-burner near me have disconnected the
turbines from the generators and run the generators as 'synchronous
condensers' often. This is essentially 'motorizing' the generator 24/7.
The risk is to the 'prime mover' (turbine or engine). Steam turbines don't
mind it for a short time, but with little/no steam flow, the blading can
actually overheat (windage/friction). And if an 8 or 10 foot long blade
grows just a small amount percentage to heating, clearances can be reduced
and a rub occurs. And that's a bad thing.
Diesel-engines don't like being driven by the shaft, but I'm not exactly
sure what would fail first inside the engine.
daestrom
Don Kelly
"daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote in message
news:460aff40$0$17209$4c36...@roadrunner.com...
Ah, the problem is not the reversal of the energy flow as far as the
synchronous machine is concerned (or even an induction machine) but it does
exist in attempting to turn electricity back to coal or oil (wind and BS
are not problems -any government can do that)- which is something that
science or engineering has not been able to do.-but hey, physical
limitations shouldn't get in the way of "what if" concepts.
Shame on you for using common sense!
--
Don Kelly dh...@shawcross.ca
remove the X to answer
----------------------------
Bill Shymanski
"daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote in message
news:460aff40$0$17209$4c36...@roadrunner.com...
> Most protective relaying schemes for generators include a
reverse-power trip
> (ANSI device code number 32-). These activate to trip the generator
output
> breakers when power flow reverses in the unit, after a short time
delay.
>
> A typical large steam turbine-generator unit (rated at say, 800 MW)
will
> reverse power trip when power flow reaches about -40 MW after about 15
to 30
> seconds.
>
..
I'm not a protection designer, but for the mid-size hydro units that
I've seen protection schemes, none of them have reverse power. There's
even a stunt called "synchronous condensor" mode in which you close the
intake gates and blow the water out of the unit with compressed air, to
run it as a synchronous capacitor...since sometimes not all the water is
out, a little reverse power flows.
Bill
Fred Lotte
"daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:
> A typical large steam turbine-generator unit (rated at say, 800 MW) will
> reverse power trip when power flow reaches about -40 MW after about 15 to 30
> seconds.
Probably more like -4MW. I really don't think you can get to 10
MW let alone 40 unless you break vacuum on the turbine. (Maybe
not even then.)
40 MW is a lot of heat to put into the last few stages of a
turbine. Even taking out generator no load losses of say 8MW,
32MW is a lot. (I'm thinking of 2 pole machines here. Not really
calibrated to 4 pole.) (Are you thinking of cross compound 2
shaft nukees?)
> On many trip signals, the turbine-generator undergoes what's known as a
> 'sequential trip'.
Dead on truth. ANY MECHANICAL TRIP SHOULD BE SEQUENTIAL!!!!! We
used a logic string from valve limit switches to 'complete' the
trip sequence.
I never figured out what to do if a turbine valve sticks or, more
likely, one of those fussy limit switches fails to close.
I hate reverse power relays. Up until the 80's my company used
turbine exhaust hood temperature to trip when (if) motoring.
Sometime in the 80's the relay manufactures sold a bill-of-goods
to our planning and relay department and 32's started appearing.
After that, the operators were so afraid of setting off the 32
that they'd trip at up to 10% load when taking a machine off
line. Someday there will be a stuck valve and turbine parts will
joint the debris cloud in orbit.
> Motorizing a large generator isn't particularly risky for the generator if
> it's already synchronized. It can take such operation indefinitely. A
> couple of old steam units in a oil-burner near me have disconnected the
> turbines from the generators and run the generators as 'synchronous
> condensers' often. This is essentially 'motorizing' the generator 24/7.
Absolutely true. (I wrote the 'motoring' section for IEEE Std 95
in the '80s) A friend of mine on the same committee wrote the
following section on 'inadvertent energization' which is the most
serious form of 'misadventure' (his really great British word for
it) that can befall a large generator.
I've seen 2 rotors from generators that were inadvertently
energized (and pictures of a few others). One, a 72MVA 4 pole was
taken from 0 to about 300RPM in 'a few seconds'. It had shaft
mounted retaining rings. Any place that wedge had 'crawled' out
and touched the ring, it was burned by a heavy arc. Some wedges
were also burned. 3 weeks down time to chemically etch the burned
metal.
The second was a 800MVA 2 pole that went from 1200 to about 3400
when the generator breaker flashed over during shutdown. It was
motored for about 5 minutes while the plant got the dispatcher to
trip the far end of the line. The rotor body was discolored and
there were little stalactites of metal at the ends of the cross
pole slots. Some wedges were just starting to come out of the
slots. The breaker was open as far as the controls were concerned
and the 'fault' current wasn't that much different from normal
load current so all the normal protective schemes were blind to
the problem. Nobody wanted to crank open a 345KV switch under
load. I don't blame them. I don't know how long the outage was.
I did a couple studies to try to convert some old (1930's)
generators to synchronous condensers. The sticking points were:
1. How to start the thing (these were 62.5MVA 4 pole turbine
generators)
2. The thrust bearing is in the turbine.
3. How to do it all really cheap.
> The risk is to the 'prime mover' (turbine or engine). Steam turbines don't
> mind it for a short time, but with little/no steam flow, the blading can
> actually overheat (windage/friction).
If I had a dollar (as my dad used to say) for every time I said
those words, I could have retired a couple years earlier. The
risk to an engine is seriously higher because of the
reciprocating mass. Motoring does something bad to the crank or
connecting rods or wrist pins.
In article <caYOh.57021$mJ1....@newsfe22.lga>, "Bill Shymanski"
<wtsh...@mts.net> wrote:
> I'm not a protection designer, but for the mid-size hydro units that
> I've seen protection schemes, none of them have reverse power. There's
> even a stunt called "synchronous condensor" mode in which you close the
> intake gates and blow the water out of the unit with compressed air, to
> run it as a synchronous capacitor...since sometimes not all the water is
> out, a little reverse power flows.
A pumped storage plant that I helped place in service about 40
years ago actually has a scheme to depress the water out of the
turbine with compressed air (also mainly used during pump
starting) and run the rather large machines as synchronous
condensers. I don't think it's ever been used that way. You do
have to supply the machine's losses (F+W, excitation, core, I^2
*R) which may be 0.5% of rating. You also have to supply water to
the turbine seals.
I was at the plant once during a pump start when the dispatcher
asked us to hold off loading until the hour (about 15 minutes
away). I asked the operator to stop the sequence with the machine
synchronized to the system and the pump/turbine spinning in air
(about 10 MW load or so, nearly all generator losses). At 7
minutes til we started venting air. At about 3 minutes til we had
50MW load (pumped primed), 3 minutes to open the guard valve and
as the turbine wicket gates opened we went from 50 to 250MW in
about 5 seconds, right on the hour. When the plant was first
started, everybody was afraid that we'd boil the water with that
50MW waiting for the guard valve to open, but in 1000's of pump
starts it's never happened. I never figured out why.
--
Fred Lotte
flo...@nospam.stratos.net
Pablo
reverse power trip protection or thermal power protection on the machine
windings..reactive power power can result in heating which can cause
thermal limits to be esceed and thermal trip to occur...simple explanation,
little more difficult to engineer
daestrom
"Bill Shymanski" <wtsh...@mts.net> wrote in message
news:caYOh.57021$mJ1....@newsfe22.lga...
Good point. The idea is that it isn't the *generator* that cares, it's
whatever is driving it. Some hydro 'turbines' are even meant for this
(think 'pumped storage').
daestrom
daestrom
> In article <460aff40$0$17209$4c36...@roadrunner.com>,
> "daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:
>
>> A typical large steam turbine-generator unit (rated at say, 800 MW) will
>> reverse power trip when power flow reaches about -40 MW after about 15 to
>> 30
>> seconds.
>
> Probably more like -4MW. I really don't think you can get to 10
> MW let alone 40 unless you break vacuum on the turbine. (Maybe
> not even then.)
>
> 40 MW is a lot of heat to put into the last few stages of a
> turbine. Even taking out generator no load losses of say 8MW,
> 32MW is a lot. (I'm thinking of 2 pole machines here. Not really
> calibrated to 4 pole.) (Are you thinking of cross compound 2
> shaft nukees?)
>
Not quite, 4-pole, *single* shaft 'nukees'. One HP and three LP mounted all
in-line with the generator. Some of that 40MW (seen it on the oscillograph
traces) goes into the exciter I'm sure.
Regarding LP blades, you're right they don't 'like it'. GE warns about
running with less than 5% load for any length of time and they recommend not
breaking vacumn at all unless it's an emergency. Saw a nice video they
developed from one plant where they mounted the camera and strobe light in
the exhaust hood. After tripping, the last stages 'wave' back and forth
something awesome.
>> On many trip signals, the turbine-generator undergoes what's known as a
>> 'sequential trip'.
>
> Dead on truth. ANY MECHANICAL TRIP SHOULD BE SEQUENTIAL!!!!! We
> used a logic string from valve limit switches to 'complete' the
> trip sequence.
>
> I never figured out what to do if a turbine valve sticks or, more
> likely, one of those fussy limit switches fails to close.
>
In the units I've worked with, that is what the 'backup unit protection' is
for. Some times the 'anti-motoring' is a series of limit switches and the
backup is reverse power relay. Sometimes it's the other way around.
Usually 'backup' protection uses a different method of detecting the problem
from primary, whatever it is.
Also seen a series of valve limit switches in the close permissive. Trying
to protect from inadvertant energizing.
> I hate reverse power relays. Up until the 80's my company used
> turbine exhaust hood temperature to trip when (if) motoring.
> Sometime in the 80's the relay manufactures sold a bill-of-goods
> to our planning and relay department and 32's started appearing.
> After that, the operators were so afraid of setting off the 32
> that they'd trip at up to 10% load when taking a machine off
> line. Someday there will be a stuck valve and turbine parts will
> joint the debris cloud in orbit.
>
Guess it depends on how sensitive the 32 is set at. Can't imagine how long
you have to motor to get a high exhaust hood temp. On the 4-pole units I
work with, that would be a lot longer than the 32 relay.
Why operators afraid of setting off the 32? I've seen some plants that
basically depend on it when shutting down. Basically they say 'reduce the
steam flow and verify the turbine trips when MW goes below zero'. Some
bug-a-boo about letting the protective relays activate?
One of the things I've seen plants during outages is 'backfeed'. They open
the generator disconnects and bring line power backward into the step-up
transformer. Then with 24kV on the low side, they feed the 'hotel loads' of
the plant from the normal service transformer. I can't stress to them how
disasterous it would be if they missed that one step of 'open the generator
disconnects'. But your experience brings it into crystal-clear focus.
> I did a couple studies to try to convert some old (1930's)
> generators to synchronous condensers. The sticking points were:
> 1. How to start the thing (these were 62.5MVA 4 pole turbine
> generators)
IIRC, the steam unit near me installed an electric motor. They get them up
to about 3000 (2-pole unit) and then pull them into sync from there. Been
awhile though, maybe it was higher RPM.
<snip>
>
> I was at the plant once during a pump start when the dispatcher
> asked us to hold off loading until the hour (about 15 minutes
> away). I asked the operator to stop the sequence with the machine
> synchronized to the system and the pump/turbine spinning in air
> (about 10 MW load or so, nearly all generator losses). At 7
> minutes til we started venting air. At about 3 minutes til we had
> 50MW load (pumped primed), 3 minutes to open the guard valve and
> as the turbine wicket gates opened we went from 50 to 250MW in
> about 5 seconds, right on the hour. When the plant was first
> started, everybody was afraid that we'd boil the water with that
> 50MW waiting for the guard valve to open, but in 1000's of pump
> starts it's never happened. I never figured out why.
>
Well....
50MW for 3 minutes is about 15 teraJoules. If it *ALL* went into the water
(none into heating steel, or excitation or other losses), that would heat up
about 45 metric tonnes of water to boiling. But you mentioned about 10 MW
in generator losses, and certainly the steel runner and casing take a lot of
heat to warm up as well, and how much water is in one of those casings.
Anyway, seeing is believing (usually:-) and if it doesn't warm up that
much, it must be true. Mind you, I wouldn't want to test how long you could
go. From what I've seen of hydro-generators, the tolerances are tight on
the rotor of the generator (~ 1/8 inch or better). How tight is the fit on
the runner?
daestrom
Fred Lotte
"daestrom" <daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:
> 4-pole, *single* shaft 'nukees'. One HP and three LP mounted all
> in-line with the generator. Some of that 40MW (seen it on the oscillograph
> traces) goes into the exciter I'm sure.
I got an old tie clip that has that model on it. ;-)
At no load, I'd expect the excitation to be of the order of 2MW
not including F+W of the exciter.
The 2 pole machines that I have in mind (both about 800MVA &
600MW) had full load losses of about 2% or less. About half were
no load losses. With vacuum maintained the turbine losses were ?
but small. My original 4MW is probably about half of what it
should be but 40 still seems like a big number. If you saw it,
you saw it. I made it a practice when I was working to not doubt
what the operating types saw, only try to explain it. Sometime I
found that they were right and sometimes their observation or
instrument was off a little. But, we found out for each other's
mutual benefit and education.
> Can't imagine how long
> you have to motor to get a high exhaust hood temp. On the 4-pole units I
> work with, that would be a lot longer than the 32 relay.
Probably about the same. They were set to about 175dF. They could
also be a problem when going on line if the unit wasn't loaded
fast enough. I don't recall ever seeing one trip but I'd guess
anywhere from 5 to maybe 30 minutes depending on vacuum and any
number of other things that take place during turbine run up.
> Also seen a series of valve limit switches in the close permissive. Trying
> to protect from inadvertant energizing.
The best inadvertent energization protection is an open
disconnect switch and operators that check things out and think
before they close a breaker or switch. Every other scheme fails
eventually. In this day of remote indications and operation etc.
this is a really tough problem. A single failure can turn the
machine (including the turbine) into scrap.
> Why operators afraid of setting off the 32? I've seen some plants that
> basically depend on it when shutting down. Basically they say 'reduce the
> steam flow and verify the turbine trips when MW goes below zero'. Some
> bug-a-boo about letting the protective relays activate?
I think that's probably it. When the unit protection operates the
shutdown of everything is pretty much taken out of the operator's
control. They don't like that. I don't blame them. But, I wish
they'd calmly reduce the load to slightly motoring then trip the
breaker.
During a strike, when I really learned how to operate a power
plant, one of my fellow amateur operators in training would
actually check the rotor of the unit watthour meter to confirm
that he had 0 watts before he tripped a machine. I generally put
the watt meter pointer just below 0. These were some 62.5MW 4
pole machines from 1930 or so that were very forgiving trainers.
Protection was a differential relay and a smoke detector + an
operator with good peripheral vision to catch that sudden meter
twitch and a good ear for a disturbance in the hum. Those
operating in the turbine room basement or on the firing floor had
similar sensitivity for things that go bump in the pump or bang
in the boiler. ;-)
I also took a 250MW off line the same way.
> One of the things I've seen plants during outages is 'backfeed'. They open
> the generator disconnects and bring line power backward into the step-up
> transformer. Then with 24kV on the low side, they feed the 'hotel loads' of
> the plant from the normal service transformer. I can't stress to them how
> disasterous it would be if they missed that one step of 'open the generator
> disconnects'. But your experience brings it into crystal-clear focus.
I've done this many times when simulating operation to check the
unit protection. We didn't have generator disconnects so the
phase bus straps were removed for this. I always personally
inspected that the generator terminals were disconnected and
roped and flagged off limits before we did these tests.
> 50MW for 3 minutes is about 15 teraJoules. If it *ALL* went into the water
> (none into heating steel, or excitation or other losses), that would heat up
> about 45 metric tonnes of water to boiling. But you mentioned about 10 MW
> in generator losses, and certainly the steel runner and casing take a lot of
> heat to warm up as well, and how much water is in one of those casings.
>
> Anyway, seeing is believing (usually:-) and if it doesn't warm up that
> much, it must be true. Mind you, I wouldn't want to test how long you could
> go. From what I've seen of hydro-generators, the tolerances are tight on
> the rotor of the generator (~ 1/8 inch or better). How tight is the fit on
> the runner?
Well, according the the test engineer when we started the plant,
about 1% goes into noise. I can believe it. Also, there are some
8 inch recirculation lines that bypass the turbine and there is a
little leakage thru the wicket gates and the depression system
vents stay open at least til full prime (maybe longer, it's only
been about 40 years since I checked that system out). All I know
for sure is that it's never been a problem and the unit wattmeter
reads 50 MW almost exactly any of the few times I've watched a
pump start. The generator/motors are 220MVA/260KHP and regularly
pump at 230 to 250MW (which is at the 1.15 service factor limit).
The air gap is about an inch.
The turbine/pump (it's a single machine that looks a lot like a
pump) has an overall diameter of less than 15 ft. It's hard to
estimate because most of the time I've only seen it from the
draft tube which is about 6-8 ft in diameter (I'd estimate) or
stood on the head cover which is 218 inches +/-. Seal clearance
is probably of the order of 1/8 inch or less, probably less. The
seals are 'lubed' with really clean water from the high head
penstock. It's all encased in a huge monolith of concrete.
The original pump/turbine impellers were changed out about 15
years ago and I haven't attended a pump start with the new ones
so the current rule of thumb times and loading may be different.
The numbers were very repeatable with the old pumps.
--
Fred Lotte
flo...@nospam.stratos.net