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| Tim Perry 2007-10-18, 3:25 am |
|
"Paul Hovnanian P.E." <paul@hovnanian.com> wrote in message
news:471580EF.3DB280C8@hovnanian.com...
> Tim Perry wrote:
involved[color=darkred]
simply[color=darkred]
than[color=darkred]
resemblance to[color=darkred]
DC[color=darkred]
>
> This is a rectified load?
Yes, the usual six diode stacks on the secondary. 'normal' is 10 kV 5.2
amps.
>
> The harmonic distortion being measured is current or voltage?
>
The power co. was measuring both at the same time. (They dont usully test
customers generators but they were being nice to me and I suspect he was
intreagued by the problem.)
I think the waveform display was power but i'm not absolutely sure. It was
an average of a 15 min sample.
standard[color=darkred]
>
> The regulator is probably based on true RMS sensing.
>
> By 'standard' DVM, you mean a peak reading unit with a 1/SQRT(2) fudge
> factor?
Yes, both flukes. one was true RMS reading
an engineer writes:
"The transmitters do not present a linear load, This causes harmonics as
the transmitter wants more current when the Generator is ready to give it.
The biggest problem occurs on the odd number harmonics as the even number
harmonics will cancel each other. The triplet harmonics become additive,
therefore a neutral line that would normally not have any current draw will
suddenly have the current draw of these additive 3rd harmonics of all three
phases. This will cause the neutral lines to over heat and the generator to
over heat.
Possible solutions
1)Balance the load with more linear loads. The transmitter uses 51kW, what
is being done with the remaining 79kw capacity of the generator? Some
people put a purely resistive linear load on the generator to offset the
harmonics coming back into the generator. The down side of this approach is
that 51kW of resistance is going to add a lot of extra heat.
(in this case adding a resistive load showed no evidence of improving
operation)
2) Install Harmonic line filters on the output of the Genset to filter out
the harmonics. The manufacturers of the Harmonic filters should be able to
calculate what is needed to clean up the lines. The Genset manufacturer may
even be able to provide a recommendation."
It looks like option 2 is the way to proceed. It is a bitter pill as the
project is already over budget do to cost increases.
>
10%[color=darkred]
kW.[color=darkred]
>
> --
> Paul Hovnanian mailto:Paul@Hovnanian.com
> ------------------------------------------------------------------
> There are only 10 kinds of people in this world,
> those who understand binary and those who don't.
but do the 0011 kind know BCD?
| |
| operator jay 2007-10-18, 3:25 am |
|
"Tim Perry" <timperry@nospammeadelphia.net> wrote in message
news:4716d159$0$7477$4c368faf@roadrunner.com...
>
> "Paul Hovnanian P.E." <paul@hovnanian.com> wrote in message
> news:471580EF.3DB280C8@hovnanian.com...
[color=darkred]
> DC
>
> Yes, the usual six diode stacks on the secondary. 'normal' is 10 kV
> 5.2
> amps.
>
>
> an engineer writes:
>
> "The transmitters do not present a linear load, This causes
> harmonics as
> the transmitter wants more current when the Generator is ready to
> give it.
> The biggest problem occurs on the odd number harmonics as the even
> number
> harmonics will cancel each other. The triplet harmonics become
> additive,
> therefore a neutral line that would normally not have any current
> draw will
> suddenly have the current draw of these additive 3rd harmonics of
> all three
> phases. This will cause the neutral lines to over heat and the
> generator to
> over heat.
>
> Possible solutions
> 1)Balance the load with more linear loads. The transmitter uses
> 51kW, what
> is being done with the remaining 79kw capacity of the generator?
> Some
> people put a purely resistive linear load on the generator to offset
> the
> harmonics coming back into the generator. The down side of this
> approach is
> that 51kW of resistance is going to add a lot of extra heat.
>
> (in this case adding a resistive load showed no evidence of
> improving
> operation)
>
>
> 2) Install Harmonic line filters on the output of the Genset to
> filter out
> the harmonics. The manufacturers of the Harmonic filters should be
> able to
> calculate what is needed to clean up the lines. The Genset
> manufacturer may
> even be able to provide a recommendation."
>
>
> but do the 0011 kind know BCD?
A few comments:
Gensets can handle only a relatively small amount of capacitive
loading (leading pf), they go unstable. Rough rule of thumb,
capacitive kVAR equal to 10% of kVA rating is the limit, IIRC. Could
be more or less.
Gensets can handle only a moderate amount of harmonic current load.
The genset mfr can probably advise on capacitive current limitations
and - somewhat - on harmonic current limitations.
Maybe the impedance of the genset is an issue. Trying to draw
significant currents - at higher frequencies (5th, 7th, 11th, 13th) -
through the synchronous (inductive) reactance of the genset may cause
a fair voltage drop or flat topping of the voltage waveform and a
reduction of RMS voltage.
You say the power factor is 0.991, do we know the displacement power
factor, distortion power factor, and total power factor? Is the
displacement pf leading or lagging? Do we have an idea of the
harmonic spectra - mostly 5th, 7th, 11th, 13th? no dc? no even
harmonics? no triplens? Nicely balanced between phases? These
questions about harmonic spectra are just a bit of a fishing
expedition.
I have some reservations as to the benefits of adding more resistive
load. Mathematically this sounds nice because you are bringing the pf
closer to unity and reducing the % harmonic current. However you are
not reducing the actual number of amps of capacitive (or inductive as
the case may be) current nor the actual number of amps of harmonic
current and I suspect the actual number of amps of current is what is
most critical in each case. It is worth a shot though, a genset
company can bring around some resistive test loads easily enough, if
you have a splitter, breaker, or some place to connect them to while
the plant is still running.
If you put harmonic line filters on the output of the genset, be
careful. Simple filters are normally quite capacitive and can be bad
news for a genset as the genset does not like a capacitive load. Some
filters come on in stages, some are active harmonic cancellation, I
suspect these would be better suited to application with a genset.
The UPS guys learned this the hard way. Gensets had to be drastically
oversized to handle UPS because the gensets didn't like the harmonics
that UPS drew. So the UPS guys put filters on the front ends. That
cleared up the harmonics but now the gensets didn't like the
capacitive load that the UPS filter presented.
First thing I would try is to get a generator mfr / service guy to
come down and take a look for free, or for a bit of money if he's
bringing test banks too. Next thing would be to check out costs to
get filtering supplied and installed ($20-40k?). I would look for a
power quality type consultant if one could come down cheap enough that
it is worth the risk of getting non-conclusive results, maybe $5k.
Look for one with tons of experience with gensets, UPS, transformers,
harmonics, and power quality, and with a quality 3 phase power
monitoring / waveform capturing / transient capturing logger like a
Dranetz, BMI, RPM, or whatever.
Isn't 0011, 3? Then I guess the answer would be "no." 
j
| |
| Tim Perry 2007-10-18, 1:25 pm |
|
"operator jay" <none@none.none> wrote in message
news:L7CRi.2431$KX4.2315@newsfe14.lga...
>
> "Tim Perry" <timperry@nospammeadelphia.net> wrote in message
> news:4716d159$0$7477$4c368faf@roadrunner.com...
>
>
>
>
>
>
>
>
>
> A few comments:
>
> Gensets can handle only a relatively small amount of capacitive
> loading (leading pf), they go unstable. Rough rule of thumb,
> capacitive kVAR equal to 10% of kVA rating is the limit, IIRC. Could
> be more or less.
>
> Gensets can handle only a moderate amount of harmonic current load.
>
> The genset mfr can probably advise on capacitive current limitations
> and - somewhat - on harmonic current limitations.
>
> Maybe the impedance of the genset is an issue. Trying to draw
> significant currents - at higher frequencies (5th, 7th, 11th, 13th) -
> through the synchronous (inductive) reactance of the genset may cause
> a fair voltage drop or flat topping of the voltage waveform and a
> reduction of RMS voltage.
>
> You say the power factor is 0.991, do we know the displacement power
> factor, distortion power factor, and total power factor?
The buiilding measures .991 inductive. The major load is speced at .94.
additional loads are lighting, obstruction beacons (1,800 W, flashing), 2
cell companies with a/c cooling, building HVAC and 5,400 watts of antenna
deicers.
>Is the
> displacement pf leading or lagging? Do we have an idea of the
> harmonic spectra - mostly 5th, 7th, 11th, 13th? no dc?
5th an 7th, no even
> no even
> harmonics? no triplens? Nicely balanced between phases?
pretty well ballanced... the cell sites are single phase and were measuring
50 to 60 amps on a hot day with thier A/C running. The gen tests were on a
cool day.
>These
> questions about harmonic spectra are just a bit of a fishing
> expedition.
I realize i have incomplete data at this time. It's just a problem i am
facing that seemed to relate to the way the gizmo that made the W-H meter
slow down. Thats assuming the whole test jig wasnt rigged.
>
> I have some reservations as to the benefits of adding more resistive
> load. Mathematically this sounds nice because you are bringing the pf
> closer to unity and reducing the % harmonic current. However you are
> not reducing the actual number of amps of capacitive (or inductive as
> the case may be) current nor the actual number of amps of harmonic
> current and I suspect the actual number of amps of current is what is
> most critical in each case. It is worth a shot though, a genset
> company can bring around some resistive test loads easily enough, if
> you have a splitter, breaker, or some place to connect them to while
> the plant is still running.
No noticable effect, still had 5 to 10% reduction in power supply voltage
and the consiquent reduction in output power.
This location loses power several time a year. It also has had a couple of
mysterious 'brownouts" the power co. can't or won't explain. voltage on all
3 legs drops from a nominal 208 to 190. My equipment just shuts itself down
then. They told me their PF capacitor banks are non switching.
>
> If you put harmonic line filters on the output of the genset, be
> careful. Simple filters are normally quite capacitive and can be bad
> news for a genset as the genset does not like a capacitive load. Some
> filters come on in stages, some are active harmonic cancellation, I
> suspect these would be better suited to application with a genset.
> The UPS guys learned this the hard way. Gensets had to be drastically
> oversized to handle UPS because the gensets didn't like the harmonics
> that UPS drew. So the UPS guys put filters on the front ends. That
> cleared up the harmonics but now the gensets didn't like the
> capacitive load that the UPS filter presented.
>
> First thing I would try is to get a generator mfr / service guy to
> come down and take a look for free,
He was there. He pretty much demonstrated the that genset is performing to
spec.
> or for a bit of money if he's
> bringing test banks too. Next thing would be to check out costs to
> get filtering supplied and installed ($20-40k?).
gulp
'Corporate says: "we have these things running all over the country and
don't have this problem elsewhere"
> I would look for a
> power quality type consultant if one could come down cheap enough that
> it is worth the risk of getting non-conclusive results, maybe $5k.
> Look for one with tons of experience with gensets, UPS, transformers,
> harmonics, and power quality, and with a quality 3 phase power
> monitoring / waveform capturing / transient capturing logger like a
> Dranetz, BMI, RPM, or whatever.
>
> Isn't 0011, 3? Then I guess the answer would be "no."
well dosent that just half byte
>
>
> j
>
>
| |
| Paul Hovnanian P.E. 2007-10-18, 8:25 pm |
| Tim Perry wrote:
>
> "Paul Hovnanian P.E." <paul@hovnanian.com> wrote in message
> news:471580EF.3DB280C8@hovnanian.com...
> involved
> simply
> than
> resemblance to
> DC
>
> Yes, the usual six diode stacks on the secondary. 'normal' is 10 kV 5.2
> amps.
>
>
> The power co. was measuring both at the same time. (They dont usully test
> customers generators but they were being nice to me and I suspect he was
> intreagued by the problem.)
>
> I think the waveform display was power but i'm not absolutely sure. It was
> an average of a 15 min sample.
That doesn't make much sense. What you are interested in is an
instantaneous reading of both the harmonic currents and voltage
distortion. That will give you an idea of the source impedance at the
harmonic frequencies.
> standard
>
> Yes, both flukes. one was true RMS reading
>
> an engineer writes:
>
> "The transmitters do not present a linear load, This causes harmonics as
> the transmitter wants more current when the Generator is ready to give it.
> The biggest problem occurs on the odd number harmonics as the even number
> harmonics will cancel each other. The triplet harmonics become additive,
> therefore a neutral line that would normally not have any current draw will
> suddenly have the current draw of these additive 3rd harmonics of all three
> phases. This will cause the neutral lines to over heat and the generator to
> over heat.
The neutral current is only one problem caused by harmonic currents. The
above is correct, albeit somewhat poorly worded. But high harmonic
currents drawn on the phases can result in voltage distortion.
Particularly if the source has a higher impedance at harmonic
frequencies. This voltage distortion will be seen by every load on the
system and can cause strange load behavior.
> Possible solutions
> 1)Balance the load with more linear loads. The transmitter uses 51kW, what
> is being done with the remaining 79kw capacity of the generator? Some
> people put a purely resistive linear load on the generator to offset the
> harmonics coming back into the generator. The down side of this approach is
> that 51kW of resistance is going to add a lot of extra heat.
>
> (in this case adding a resistive load showed no evidence of improving
> operation)
>
> 2) Install Harmonic line filters on the output of the Genset to filter out
> the harmonics. The manufacturers of the Harmonic filters should be able to
> calculate what is needed to clean up the lines. The Genset manufacturer may
> even be able to provide a recommendation."
>
The filter vendors will probably have the best ideas about what sorts of
measurements need to be made to specify the correct solution.
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
f u cn rd ths u r usng unx
| |
| Tim Perry 2007-10-19, 3:25 am |
| > > > The harmonic distortion being measured is current or voltage?
test[color=darkred]
was[color=darkred]
>
> That doesn't make much sense. What you are interested in is an
> instantaneous reading of both the harmonic currents and voltage
> distortion. That will give you an idea of the source impedance at the
> harmonic frequencies.
At the time I didn't know what questions to ask. After talking to an
applications engineer today I believe it was 10% voltage distortion.
out[color=darkred]
able to[color=darkred]
may[color=darkred]
>
> The filter vendors will probably have the best ideas about what sorts of
> measurements need to be made to specify the correct solution.
>
> --
> Paul Hovnanian mailto:Paul@Hovnanian.com
> ------------------------------------------------------------------
> f u cn rd ths u r usng unx
The applications engineer of a filter vender spoke with me at length. I now
have an assignment to query the generator's aplication engineer about a
series inductor.
I appreciate the thoughtful comments. I was able to study some areas that
are new to me which allowed me to communicate better with the ap eng.
| |
| operator jay 2007-10-19, 3:25 am |
|
"Tim Perry" <timperry@nospammeadelphia.net> wrote in message
news:471784cc$0$25661$4c368faf@roadrunner.com...
>
> "operator jay" <none@none.none> wrote in message
> news:L7CRi.2431$KX4.2315@newsfe14.lga...
>
> The buiilding measures .991 inductive. The major load is speced at
> .94.
> additional loads are lighting, obstruction beacons (1,800 W,
> flashing), 2
> cell companies with a/c cooling, building HVAC and 5,400 watts of
> antenna
> deicers.
>
With a large 3 phase diode rectifier I could believe a dispalcement
power factor near unity if I had to. This rules out problems due to
leading pf. (Strangely, yesterday as I read '0.991' power factor, and
typed '0.991' power factor, I was thinking '0.911' power factor).
However there ought to be a whole bunch of harmonic current, and a
horrible distortion power factor. Normally that would be my first
suspect. I would expect that a genset supplying a bare (no filters or
harmonic mitigation) 3 phase rectifier might have to be rated between
1.4x and 4x the rectifier rating - these are derating numbers I had
heard from UPS mfrs, maybe 6 years ago, in regards to sizing gensets
that would feed UPS with their rectifying, high harmonic, front ends.
If you have a step up transformer in front of your rectifier, this
should act as a bit of harmonic filtering, like a series choke or
series inductor. Sometimes large rectifiers are fed via TWO
transformers, with some phase shifting (e.g. one Y-D transformer and
one D-D transformer) as this tidily causes a bunch of harmonic
cancellation. I think this would not be the case at your site as this
setup should remove the 5th and 7th harmonics.
If there are similar sites around the country with the same rectifier
and the same genset with the same controller, regulator, alternator,
and excitation, and no voltage problems, then I would have to suspect
a problem with THIS genset. If you know the current distortion you
might be able to ask the genset mfr if the genset ought to be able to
handle it. If the genset people say yes, the genset ought to be able
to handle it, then maybe there is a problem with the unit. If the
genset people say no, this genset can't handle those harmonics, then
maybe they can tell you what exactly the difference is between this
site and other sites where the gensets can handle it? (It's worth
knowing for future installations).
Another possibility is a resonance condition. If there is a
significant capacitor somewhere in the distribution (fed by the
generator) there is a chance that it makes a parallel resonance with
the genset inductive impedance at a critical frequency (e.g. somewhere
around 5th to 7th harmonic or even 11th to 13th). Voltage distortion
would be terrible (as it is seen to be). It is a slim chance and I
don't see anything that looks like a capacitive load in your
description of loads. I would continue to look into the genset first
before investigating this. If you are able to run the convertor on
genset power, with all the breakers to all other equipment open, and
the voltage problems persist, then resonance is very likely not the
problem.
>
> He was there. He pretty much demonstrated the that genset is
> performing to
> spec.
Well, is 10% voltage THD and 10% voltage drop 'to spec'? Or did he
show that the genset is operating 'to spec' under linear loads only?
These are rhetoric, and are the same issue as discusse above re:
should this genset be able to handle the current harmonics it is being
subjected to.
> 'Corporate says: "we have these things running all over the country
> and
> don't have this problem elsewhere"
Corporate has a point. I would discuss this with the genset people.
Does this genset have a lesser spec, or a lesser controller,
regulator, alternator, or excitation? Round up some detailed specs or
shop drawings for a couple of similar installations (similar loads,
similar load ratings, and similar genset) if you can.
> well dosent that just half byte
yes, that nibbles, to use the parlance
| |
| operator jay 2007-10-19, 3:25 am |
|
"Tim Perry" <timperry@nospammeadelphia.net> wrote in message
news:47182694$0$20650$4c368faf@roadrunner.com...
> test
> was
>
> At the time I didn't know what questions to ask. After talking to an
> applications engineer today I believe it was 10% voltage distortion.
>
I imagine that is correct, current distortion was probably way higher
assuming a full diode bridge rectifier with little or no filtering /
smoothing. Harmonic current could be equal to 40% or more of the
fundamental.
>
> out
> able to
> may
>
> The applications engineer of a filter vender spoke with me at
> length. I now
> have an assignment to query the generator's aplication engineer
> about a
> series inductor.
Is there a convertor transformer that steps voltage up from 208V to 7
or 8kV or so? If so the transformer ought to be very much like a
series inductor of a few percent - if the transformer is 5%IZ
impedance then it is somewhat like a series inductor of around 5%. A
little less inductive than a 5% inductor and a little more resistive,
but in the neighborhood.
>
> I appreciate the thoughtful comments. I was able to study some areas
> that
> are new to me which allowed me to communicate better with the ap
> eng.
>
>
>
If an active cancellation filter is deemed to be required, and is
horribly expensive, it may be worth asking the genset people if the
genset can be modified to handle the harmonic current. If it is a
matter of changing the controller it may be 'only' several thousand
dollars.
By the way, when current harmonic distortion is discussed in
'percentages' (e.g. %THD or "40% current distortion) it is a little
ambiguous as percent current distortion is sometimes measured as
current distortion = (combined total harmonic current of all harmonic
frequencies) / (RMS of total current)
and sometimes as
current distortion = (combined total harmonic current of all harmonic
frequencies) / (RMS of fundamental)
In the first case, a heavily distorted current can show up as, say,
80% harmonic distortion. In the second case the same current waveform
could be way over 100% harmonic distortion. For your waveform at full
load I would expect the discrepancy between the two measurements to be
much less than this and hopefully small enough to not be a concern.
If the utility technician can tell you the amps of harmonic current at
each frequency then that is completely unambiguous.
| |
| operator jay 2007-10-19, 3:25 am |
|
"operator jay" <none@none.none> wrote in message
news:%RXRi.1688$uk.765@newsfe21.lga...
>
> "Tim Perry" <timperry@nospammeadelphia.net> wrote in message
> news:47182694$0$20650$4c368faf@roadrunner.com...
>
> I imagine that is correct, current distortion was probably way
> higher assuming a full diode bridge rectifier with little or no
> filtering / smoothing. Harmonic current could be equal to 40% or
> more of the fundamental.
>
>
> Is there a convertor transformer that steps voltage up from 208V to
> 7 or 8kV or so? If so the transformer ought to be very much like a
> series inductor of a few percent - if the transformer is 5%IZ
> impedance then it is somewhat like a series inductor of around 5%.
> A little less inductive than a 5% inductor and a little more
> resistive, but in the neighborhood.
>
>
> If an active cancellation filter is deemed to be required, and is
> horribly expensive, it may be worth asking the genset people if the
> genset can be modified to handle the harmonic current. If it is a
> matter of changing the controller it may be 'only' several thousand
> dollars.
>
>
>
> By the way, when current harmonic distortion is discussed in
> 'percentages' (e.g. %THD or "40% current distortion) it is a little
> ambiguous as percent current distortion is sometimes measured as
>
> current distortion = (combined total harmonic current of all
> harmonic frequencies) / (RMS of total current)
>
> and sometimes as
>
> current distortion = (combined total harmonic current of all
> harmonic frequencies) / (RMS of fundamental)
>
> In the first case, a heavily distorted current can show up as, say,
> 80% harmonic distortion. In the second case the same current
> waveform could be way over 100% harmonic distortion. For your
> waveform at full load I would expect the discrepancy between the two
> measurements to be much less than this and hopefully small enough to
> not be a concern. If the utility technician can tell you the amps of
> harmonic current at each frequency then that is completely
> unambiguous.
>
Two more quick thoughts. It may be that adding a series inductor will
add to the problem. The inductive impedance of the genset (and step
up transformer (if there is one), and cabling) may be contributing to
the poor voltage (low RMS value and heavy distortion) at the rectifier
input. While more inductance may reduce harmonic current draw, I
think it could only do so at an increase of voltage problems. Since
it is the ac voltage peak that 'tops up' the dc capacitor in a
rectifier, the dc voltage would probably be very sensitive to the peak
value of the applied ac voltage. Your ac voltage is probably
'flat-topping', due to the impedances plus a possibly struggling
genset, and causing the low dc voltage. I would mention this to the
filter guy, he should be able to debunk this idea if it is bunk.
Also, you already have the genset people, filter people, and,
somewhat, the utility people involved. You may also want to call up
the vendor of the rectifier equipment. They may be able to say how
much harmonic current the thing is expected to draw, and what voltages
(RMS and distortion-wise) it may be able to tolerate. A four way
teleconference may help get things rolling.
j
| |
| Paul Hovnanian P.E. 2007-10-19, 8:25 pm |
| operator jay wrote:
>
[snip]
>
> Two more quick thoughts. It may be that adding a series inductor will
> add to the problem. The inductive impedance of the genset (and step
> up transformer (if there is one), and cabling) may be contributing to
> the poor voltage (low RMS value and heavy distortion) at the rectifier
> input. While more inductance may reduce harmonic current draw, I
> think it could only do so at an increase of voltage problems. Since
> it is the ac voltage peak that 'tops up' the dc capacitor in a
> rectifier, the dc voltage would probably be very sensitive to the peak
> value of the applied ac voltage. Your ac voltage is probably
> 'flat-topping', due to the impedances plus a possibly struggling
> genset, and causing the low dc voltage. I would mention this to the
> filter guy, he should be able to debunk this idea if it is bunk.
>
Exactly right. Most nonlinear loads of this type draw nearly constant
harmonic currents. If the voltage source is ideal (zero impedance at the
harmonic frequencies), the system voltage will remain a pure sine wave.
As the source impedance increases, harmonic currents will result in
harmonic voltage drops.
If a blocking filter is placed between the system and the one offending
load to block the currents, other loads may be protected, but the one
producing the currents will see highly distorted voltages. The best
filters offer a combination of a low impedance path for harmonics to
flow (such as a transformer with a tertiary delta winding) plus high
impedance to block the currents from being drawn from a common source
(the generator).
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Do not mold, findle or sputilate.
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