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Filtering harmonic distortion on a 2.8KV 50Hz 230v Petrol Generator to satisfy fussy UPS - Page 2

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Posted by Johnny B Good on April 29, 2009, 10:43 pm
 
2nd update on generator filter testing.



====snip of previous quotage====

 I've discovered some pertinent facts about standby generators and their
behaviour with reactive loading, notably capacitive (leading current)
loadings as per my interleaved comments below.

====more snippage====


 Much to my surprise, fitting the two 30uF capacitors to the genny's two
115v output windings made things much worse. The O/C voltage shot up to
280 odd on the 230v output and even reducing the (effective) 15uF
loading down to 4.7uF produced almost the same effect!


 This doesn't seem to be the issue, however, looking at the UPS cct
diagram suggests that its own input circuit is the real culprit since it
places some 10.4uF across the incoming mains supply, which, going by my
previous experimental results with a 4.7uF capacitor on the generator,
explains the cycling between battery and mains power behaviour of the
UPS.

 I've taken 5 of those 400VA transformers which I'd previously (and
erroneously) estimated to be about 1H inductance on the 240v primary
(but are in fact some 3.82H) and wired them across the feed to cancel
the UPS's capacitive loading. This did actually make some improvement in
that the UPS, when not powering a load, would switch back to the
generator feed and not keep cycling between generator and battery power.

 I think the real problem isn't so much the waveform as much as its
unstable level in the face of capacitive loads. Testing the UPS with its
normal loading still proved problematic, suggesting that more inductive
loading on the generator might resolve this issue.


 This might prove unnecessary, but it does have the merit that winding
an air cored choke for the required inductance (3.763mH) is a practical
exercise.

 The main lesson that has emerged from my experiments is that such cheap
gensets can behave very strangely with any sort of capacitive loading so
it's imperitive to avoid such loadings and, where this isn't possible,
compensate with an inductive load. In my case, placing some 955mH of
inductance across the genset's output doesn't seem to be quite
sufficient to cancel the UPS's own capacitive loading _and_ its attached
load.

 Although I seem to be alone in dealing with this issue, I'm pretty sure
there must be others who have also come face to face with this very same
problem and failed to get any meaningful insight. My test results just
might provide a practical workaround strategy to improve the voltage
stability of such emergency power sources when used to power UPS
protected loads (even when the UPSes can be programmed to be less
critical of the rather large voltage level variations that can result in
this circumstance).

 When I next get a chance to add another five transformers across the
genset power feed, I'll repeat the test and report back to this thread
in the next day or so.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


Posted by EXT on August 4, 2009, 12:23 am
 

As in everything, there are the cheap and crappy, and there are the
expensive with quality -- somewhere in the middle is the reasonable and
good-enough. It is hard to determine this in a generator - but - slip rings
is a giveaway for cheap with poor quality. Generator sets consist of an
engine and a generator head. You really need to buy a generator set that
combines a quality engine such as a Honda along with a good brushless
generator head, preferably a brand name. You cannot just look at one
component and ignore the other.


Posted by Johnny B Good on August 4, 2009, 2:54 am
 

====big snip====


 All that is true, but a slipring fed field design need not produce poor
results. The even cheaper designs based on an adapted asynchronous motor
which relies on the capacitive effect to excite the necessary field
currents and determine the output voltage for a given spin speed have
even worse regulation and frequency stability.

 I know about the slipring eliminating designs that allow an external
AVR to regulate a synchronously generated voltage to the same precision
as that of the 'cheap slipring' designs. This type is fine when you need
very long generator head lifetimes and greater reliability. However, for
a cheap commodity genset, the life of the slipring and brush gear will
far exceed the life of the prime mover.

 The charm of the slipring fed arrangement is that the AVR can be
designed to buck the capacitively induced excitation current (assuming I
haven't missed some vital fact that defeats this 'simple concept'),
whereas the brushless type would need the AVR circuitry to be
incorporated into the rotating field assembly itself.

 Although I have no circuit diagrams for this PowerCraft generator, I
suppose I could try a slightly more sophisticated version of my original
12v battery powered excitation test (the one where I was trying to
determine the cause of the 1500Hz ripple component that stands out so
markedly on no load).

 In this case I could make a simple Bridged output DC amplifier driven
from a potentiometer fed by a suitable bias voltage that would allow me
to feed the field with voltages over the range +12 through zero to -12v
whilst capacitively loading the generator output. This basic test should
reveal whether it _is_ possible to negate the effects of capacitive self
excitation by a modified AVR circuit.

 The trouble is, I have a horrible feeling that the problem I'm trying
to address is possibly not quite so simple as it seems. The one fact
that looms large in my mind being that, aside from residual magnetism
considerations, it doesn't matter which way the current flows in the
field winding, you'll get AC output regardless.

 The only control you then have being the magnitude and phase relative
to the poles on the rotor. I suspect an AVR module capable of driving
excitation current in either direction is only going to result in some
rather wild voltage amplitude excursions. I really could do with some
expert advice on this matter.

 The fact that the capacitive loading issue effect on power station
plant at Black Start capable stations which can arise with long length
unloaded transmission lines has been mentioned elsewhere in this
newsgroup rather suggests that any attempts by the AVR module to buck
the self excitation effect will be doomed to failure.

 I suspect that using an inductive load to swamp out random capacitive
loadings might be the only practical solution after all. Removing the
second UPS loading from the primary UPS's output circuit might be all
that is required to achieve sufficient succes with the inductive load
I've already assembled to make it worth the effort of getting hold of a
purpose made inductor to make the whole scheme work.

 Since I want to retire the old Upsonic anyway, I'll repeat my tests
next chance I get after doing so. I might have some encouraging news to
report in a few days time.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


Posted by Roger_Nickel on August 4, 2009, 10:43 pm
 On Tue, 04 Aug 2009 03:54:09 +0100, Johnny B Good wrote:


AVR output into the field is rectified AC?. If so then then there may be
a phasing problem . Your idea of DC exciting the field seems good.
Suggest that you do the correction in the feedback circuit rather than
hanging phase shifting components on the generator output. How much field
current do you need?. It only needs to be unipolar DC and the DC source
can be derived from the generator output. Automotive alternators get
their field current from the battery---so no issue with out of phase AC
components on the field current.

Posted by Johnny B Good on August 5, 2009, 12:57 am
 

 I rather suspect I may have answered the "vital fact that defeats this
'simple concept'" question above in the third and fourth paragraphs
below. :-(


 The reason I think this can work is that it seems the self excitation
problem only occurs with leading current and not lagging (inductive)
current loadings.


 Yes, undoubtedly, the current source used by the AVR module is a low
voltage (around 50 volts or so) rectified AC (this genset lacks electric
start, relying on recoil starting, hence no handy battery to eliminate
the reliance on residual magnetism), but this rectified ac source will
include a basic smoothing circuit (just a suitably sized electrolytic
cap across the rectifier output would be my guess), so the issue of the
phase of the ripple relative to the excitation current being fed to the
field won't arise. For the purposes of this exercise, the AVR might just
as well be energised from a battery.

 When I was testing to determine whether the 1500Hz noise on no-load was
the result of a modulation of excitation current from the AVR module, I
disconnected the AVR and attached a 12v dryfit battery directly to the
slipring contacts. Fortuitously, this was just the right amount of
excitation to produce 230v on no-load (which dropped to about 100v when
loaded with my 2.5KW electric kettle test load).

 Now, the important point to consider in this test is that I'd have
gotten the same result whichever way round I chose to connect the
battery. The subtle difference of phase relative to a fixed spot on the
generator shaft would have required the use of a strobe lamp to spot.

 As far as the load is concerned, the polarity of the excitation current
is totally immaterial, only its magnitude being the deciding factor in
just how strong the output voltage would be for any given resistive (or
inductive) loading.

 Unfortunately, capacitive loading above a trivial amount (a 4.7uF
capacitor across the 230v output was enough to raise the no-load voltage
up to some 270 odd volts which no amount of adjustment on the AVR's
trimpot would compensate). IOW, the AVR's job had been hijacked by a
capactive load that was half that which the UPS was switching in when
attempting to switch from battery power back to generator power.

 I know the UPS will continue to run in pass through mode when the
Upsonic UPS (and its load) are disconnected and I have those 8 400VA
230v transformer primaries (about 500mH's worth of inductance) wired
across the generator's output so my hope is that if I remove that second
UPS from the loading equation, I might discover that I've cracked the
problem.

 I'm planning on retiring that Upsonic unit anyway (it takes about 18
watts maintaining power when the battery packs are in good condition
which they no longer are so I suspect it might be taking a little bit
more than 18 watts by now).

 The last endurance test I ran on this unit a couple of days back lasted
just over 11 minutes on a 200 watt load which doesn't compare very well
to a test (with new batteries 11 years ago) which ran a 394 watt load
for over 33 minutes.

 Since I'd need to buy at least three 7AH 12v dryfit batteries to
refurbish just one of the two banks it can have installed and it has
started to mimic the SFX used in motion picture productions of the 40s
depicting scenes of Dr Frankenstein's Lab _and_ is a bit of overkill
anyway, I am going to decommision it in the next day or so.

 Once the Upsonic is out of the picture, I'll repeat my generator
testing yet again to see if _this_ time around the inductive loading
trick will actually work. If it does, you (collectively) will be the
first to know.

--
Regards, John.

 Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.


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