Forums

ADC/DAC for 50-400Hz 3-phase?

Started by Guy Macon May 9, 2006


I am doing some preliminary "back of the envelope" design work on 
a high-end split phase / three phase 50 Hz. to 400 Hz. AC power 
supply & analyzer.  Advice/comments/ridicule/brickbats welcome...

Generating the 2 or 3 phases is straightforward, but monitoring 
the current and voltage is a bit more tricky -- especially when
monitoring the far end of a transformer, line filter, or power
factor compensator.  I wish to determine the following values 
for a single-cycle non-repeating transient:

Phase A/B/C to N (Neutral) voltage
Phase A-B/B-C/C-A voltage
Phase A/B/C/N current
Phase A/B/C apparent power (VA)
Phase A/B/C RMS power
Phase A-B/B-C/C-A voltage phase shift
Phase A-B/B-C/C-A current phase shift
Phase A/B/C THD+N

The first thing that comes to mind is to use some high-end 24-bit, 
192 kHz audio ADCs, possibly interleaving if the sample rate needs 
to be higher.  I am leaning toward a sample rate that is divisible 
by two and three (for split-phase 180 degree and three phase 120 
degree measurements) instead of the traditional power of two sample 
rate.

Let's say that I wish to measure phase with 0.1 degree precision 
at 400 Hz.  That's a range of 3600 (0.0 degrees to 359.9 degrees).
So I need at least 3600 x 400 = 1.44 MSPS, right?

If I need that many samples per second using 192 kHz audio ADCs, 
I would have to interleave 8 of them per measurement channel, 
times 3 voltage measuring points and 4 current measuring points, 
or 56 of them total (and that's assuming that I can make the 
interleaving work right).  Suddenly the low-cost audio parts 
aren't looking so inviting...

Looking at faster parts, a quick Google search comes up with parts
such as the Analog Devices AD7621 (16-Bit, 3 MSPS) and AD7641 
(18-Bit, 2 MSPS) and the Linear Technology LTC2208 (16-bit, 130 MSPS).
Hmmmm... could a fast enough ADC allow me to measure multiple channels
with a single ADC?  Or do I really need simultaneous sampling?

Or could it be that I am on the wrong track and there is a better 
way than the brute-force lots-of-samples, lots-of-resolution lots-
of-processing-power solution that is the first thing to comes to mind?

-- 
Guy Macon
<http://www.guymacon.com/>

On Tue, 09 May 2006 07:29:25 +0000, Guy Macon
<http://www.guymacon.com/> wrote:


>Let's say that I wish to measure phase with 0.1 degree precision >at 400 Hz. That's a range of 3600 (0.0 degrees to 359.9 degrees). >So I need at least 3600 x 400 = 1.44 MSPS, right?
>If I need that many samples per second using 192 kHz audio ADCs, >I would have to interleave 8 of them per measurement channel, >times 3 voltage measuring points and 4 current measuring points, >or 56 of them total (and that's assuming that I can make the >interleaving work right). Suddenly the low-cost audio parts >aren't looking so inviting...
How are you actually going to measure the phase shift between phases or between voltage and current ? Trying to detect the time difference between the zero crossings would work in an ideal situation with perfectly clean sinusoids, however, in practice, the mains voltage is badly polluted by all kinds of noises. With even order harmonic distortion, the half waves are different, so it is difficult to even establish the zero-line. Trying to detect the top of the waveform would even be worse. So in practice, multiple mains cycles would be required to determine the phase difference to such accuracy. As long as the sampling frequency is not an _exact_ multiple of the mains frequency, a repetitive waveform will be accurately reconstructed anyway. Calculating the complex FFT for a sufficiently long sample should give the phase and magnitude for the fundamental and for each harmonics in the waveform. Thus, I do not think that you would need such huge sampling frequency for the phase measurement, but of course for transient recording the high sampling rate is desirable. There are a few problems with typical audio ADCs. The DC and low frequency characteristics seem to be poor, sometimes artificially limited at 3 Hz to avoid drift problems. This may be an issue if you want to measure any DC component on the mains (which will increase transformer noise and ultimately saturate the cores even with a few volts of imbalance). Even if some ADCs boost 192 kHz and 24 bits, the SNR quoted is at best around 120 dB and this is usually specified for the 20 kHz audio bandwidth only. The SNR for the full 90+ kHz bandwidth possible with 192 kHz sample rate can be much worse, corresponding to 18-20 bit ideal ADCs.
>Looking at faster parts, a quick Google search comes up with parts >such as the Analog Devices AD7621 (16-Bit, 3 MSPS) and AD7641 >(18-Bit, 2 MSPS) and the Linear Technology LTC2208 (16-bit, 130 MSPS). >Hmmmm... could a fast enough ADC allow me to measure multiple channels >with a single ADC? Or do I really need simultaneous sampling?
How did you intend the current measurement ? Have you looked at the current transformer phase and frequency response or are you planning to use shunting resistors in each phase and using separate floating power supplies for each ADC at mains phase potential and bring down the digital sample values in an optical fiber (and apparently feed the ADC with a common clock source using an other optical fiber) ? Paul
Guy Macon wrote:

> I am doing some preliminary "back of the envelope" design work on > a high-end split phase / three phase 50 Hz. to 400 Hz. AC power > supply & analyzer. Advice/comments/ridicule/brickbats welcome... >
<snip>
> > Let's say that I wish to measure phase with 0.1 degree precision > at 400 Hz. That's a range of 3600 (0.0 degrees to 359.9 degrees). > So I need at least 3600 x 400 = 1.44 MSPS, right?
Why do you need 0.1' / 24 bit ? Most standards specs I've seen quote harmonics only to a certain number, and if you are doing an analyser do you need to be 70x that ? Many Audio DACS have poor impulse response - a 20KHz response is gives out to the 50th harmonic of 400Hz. -jg
"Guy Macon" <http://www.guymacon.com/> wrote in message
news:NJednWDnvsBK2f3ZRVn_vQ@giganews.com...
> > > > I am doing some preliminary "back of the envelope" design work on > a high-end split phase / three phase 50 Hz. to 400 Hz. AC power > supply & analyzer. Advice/comments/ridicule/brickbats welcome... > > Generating the 2 or 3 phases is straightforward, but monitoring > the current and voltage is a bit more tricky -- especially when > monitoring the far end of a transformer, line filter, or power > factor compensator. I wish to determine the following values > for a single-cycle non-repeating transient: > > Phase A/B/C to N (Neutral) voltage > Phase A-B/B-C/C-A voltage > Phase A/B/C/N current > Phase A/B/C apparent power (VA) > Phase A/B/C RMS power > Phase A-B/B-C/C-A voltage phase shift > Phase A-B/B-C/C-A current phase shift > Phase A/B/C THD+N > > The first thing that comes to mind is to use some high-end 24-bit, > 192 kHz audio ADCs, possibly interleaving if the sample rate needs > to be higher. I am leaning toward a sample rate that is divisible > by two and three (for split-phase 180 degree and three phase 120 > degree measurements) instead of the traditional power of two sample > rate. > > Let's say that I wish to measure phase with 0.1 degree precision > at 400 Hz. That's a range of 3600 (0.0 degrees to 359.9 degrees). > So I need at least 3600 x 400 = 1.44 MSPS, right? > > If I need that many samples per second using 192 kHz audio ADCs, > I would have to interleave 8 of them per measurement channel, > times 3 voltage measuring points and 4 current measuring points, > or 56 of them total (and that's assuming that I can make the > interleaving work right). Suddenly the low-cost audio parts > aren't looking so inviting... > > Looking at faster parts, a quick Google search comes up with parts > such as the Analog Devices AD7621 (16-Bit, 3 MSPS) and AD7641 > (18-Bit, 2 MSPS) and the Linear Technology LTC2208 (16-bit, 130 MSPS). > Hmmmm... could a fast enough ADC allow me to measure multiple channels > with a single ADC? Or do I really need simultaneous sampling? > > Or could it be that I am on the wrong track and there is a better > way than the brute-force lots-of-samples, lots-of-resolution lots- > of-processing-power solution that is the first thing to comes to mind? > > -- > Guy Macon > <http://www.guymacon.com/> >
Hello Guy, You are indeed on the wrong track, To measure everything about a 400 Hz signal you need to sample at 800Hz and a bit - in real life you need more samples than this to deal with non ideal anti-alias filters etc. (and don't forget that uncertainty in the sample time (jitter) will affect accuracy just as much as amplitude errors). Most mains analysers allow you to measure up to 20th Harmonic - 1kHz for 50Hz mains, 8kHz for your 400Hz top limit. A 40kHz sample rate will do fine. I wouldn't use audio parts for this - they are a pain to multiplex. There are single chip parts with 4 and 8 way multiplexors that should work quite well. You get on much better measuring the phase if you use all the samples rather than just the few near the zero crossing - Google for Goertzel - there is a lot of 'noise' but you should find something. You don't need simultaneous sampling - just to know when your samples were sampled - you can calculate out any errors due to the exact sampling time. Michael Kellett www.mkesc.co.uk


Paul Keinanen wrote:
> >Guy Macon <http://www.guymacon.com/> wrote: > >>Let's say that I wish to measure phase with 0.1 degree precision >>at 400 Hz. That's a range of 3600 (0.0 degrees to 359.9 degrees). >>So I need at least 3600 x 400 = 1.44 MSPS, right? > >>If I need that many samples per second using 192 kHz audio ADCs, >>I would have to interleave 8 of them per measurement channel, >>times 3 voltage measuring points and 4 current measuring points, >>or 56 of them total (and that's assuming that I can make the >>interleaving work right). Suddenly the low-cost audio parts >>aren't looking so inviting... > >How are you actually going to measure the phase shift between phases >or between voltage and current ? > >Trying to detect the time difference between the zero crossings would >work in an ideal situation with perfectly clean sinusoids, however, in >practice, the mains voltage is badly polluted by all kinds of noises. >With even order harmonic distortion, the half waves are different, so >it is difficult to even establish the zero-line. > >Trying to detect the top of the waveform would even be worse.
What I usually do is to calculate a Fourier transform on the signal, discarded all harmonics leaving only the fundamental, calculate a reverse Fourier transform to reconstruct the signal, then measure the zero crossing.
>So in practice, multiple mains cycles would be required to determine >the phase difference to such accuracy. As long as the sampling >frequency is not an _exact_ multiple of the mains frequency, a >repetitive waveform will be accurately reconstructed anyway.
Alas, my customers really do want to measure transient waveforms. For example, when load X was suddenly connected to power, how many cycles did it take to recover?
>Calculating the complex FFT for a sufficiently long sample should give >the phase and magnitude for the fundamental and for each harmonics in >the waveform. > >Thus, I do not think that you would need such huge sampling frequency >for the phase measurement, but of course for transient recording the >high sampling rate is desirable. > >There are a few problems with typical audio ADCs. The DC and low >frequency characteristics seem to be poor, sometimes artificially >limited at 3 Hz to avoid drift problems. This may be an issue if you >want to measure any DC component on the mains (which will increase >transformer noise and ultimately saturate the cores even with a few >volts of imbalance).
Indeed it does, and one of the requirements of an AC power source is to shut down if there is more than a small amount of DC at the output. I am likely to design in a seperate circuit for that.
>Even if some ADCs boost 192 kHz and 24 bits, the SNR quoted is at best >around 120 dB and this is usually specified for the 20 kHz audio >bandwidth only. The SNR for the full 90+ kHz bandwidth possible with >192 kHz sample rate can be much worse, corresponding to 18-20 bit >ideal ADCs.
Audio bandwidth is fine; my signal is in the 50 to 400 Hz. range. 24 bits is massive overkill given the noise and distortion typical of real-world powerlines. I don't think I will end up using the cheap audio ADCs, but it won't be because of SNR or LF issues.
>How did you intend the current measurement ? Have you looked at the >current transformer phase and frequency response
Look here: http://www.pearsonelectronics.com/current-monitor-products/standard-current-monitor.htm


Jim Granville wrote:
> >Guy Macon wrote: > >> I am doing some preliminary "back of the envelope" design work on >> a high-end split phase / three phase 50 Hz. to 400 Hz. AC power >> supply & analyzer. Advice/comments/ridicule/brickbats welcome... >> ><snip> >> >> Let's say that I wish to measure phase with 0.1 degree precision >> at 400 Hz. That's a range of 3600 (0.0 degrees to 359.9 degrees). >> So I need at least 3600 x 400 = 1.44 MSPS, right? > >Why do you need 0.1' / 24 bit ? >Most standards specs I've seen quote harmonics only to a certain >number, and if you are doing an analyser do you need to be 70x that ? > >Many Audio DACS have poor impulse response - a 20KHz response is >gives out to the 50th harmonic of 400Hz.
24 bits is a huge overkill, it's just what the 192KHz low-cost audio ADCs put out. Measuring to a fraction of a degree is real; the users of these things tend to drive large inductive loads and to add capacitor banks to correct the power factor, and they do that by measuring the phase between voltage and current.


MK wrote:

>You are indeed on the wrong track,
<Big happy smile> Now *that's* the kind of advice that I was hoping for! Far better to find out now when I am doing preliminary design work...
>To measure everything about a 400 Hz signal you need to sample at 800Hz and >a bit - in real life you need more samples than this to deal with non ideal >anti-alias filters etc. (and don't forget that uncertainty in the sample >time (jitter) will affect accuracy just as much as amplitude errors). > >Most mains analysers allow you to measure up to 20th Harmonic - 1kHz for >50Hz mains, 8kHz for your 400Hz top limit. A 40kHz sample rate will do fine.
A California Instrument 9003iX does waveform analysis and waveform synthesis to the 51st harmonic and has a 16 Hz to 500 Hz Hz full power bandwidth. http://www.calinst.com/ixseries.html A Pacific Power Source 320AMX does waveform analysis and waveform synthesis to the 51st harmonic and has a 20-5000 Hz full power bandwidth. http://www.pacificpower.com/products/amx-series.aspx A Voltech PM300 measures up to 250kHz. A PM3000A goes to 1 MHz, and a PM6000 goes to 10MHz. http://www.voltech.com/products/pwr_anl/index.htm http://www.voltech.com/products/pwr_anl/pm3000/index.htm http://www.voltech.com/products/pwr_anl/PM6000/pm6frame.htm It seems that somebody out there seems to think that you have to go past a few KHz.
>You get on much better measuring the phase if you use all the samples rather >than just the few near the zero crossing - Google for Goertzel - there is a >lot of 'noise' but you should find something.
I was under the impression that the Goertzel Algorithm was a way to detect frequecies with less computation than a DFT or FFT. I usually do an FFT, throw out all the harmonics, reconsruct a fundamental-only version of the signal, and them meaure the zero crossing point.
>You don't need simultaneous sampling - just to know when your samples were >sampled - you can calculate out any errors due to the exact sampling time.
Hmmm. That makes sense, but I have never done it. If it turns out that I am wrong about bandwidth and can multiplex, I will revisit this.
>I wouldn't use audio parts for this - they are a pain to multiplex. >There are single chip parts with 4 and 8 way multiplexors that should >work quite well.
I doubt that I will be multiplexing. Too much bandwidth loss. If I end up using audio parts I may end up interleaving them!
Guy Macon wrote:
> Jim Granville wrote: > >>Guy Macon wrote: >> >> >>>I am doing some preliminary "back of the envelope" design work on >>>a high-end split phase / three phase 50 Hz. to 400 Hz. AC power >>>supply & analyzer. Advice/comments/ridicule/brickbats welcome... >>> >> >><snip> >> >>>Let's say that I wish to measure phase with 0.1 degree precision >>>at 400 Hz. That's a range of 3600 (0.0 degrees to 359.9 degrees). >>>So I need at least 3600 x 400 = 1.44 MSPS, right? >> >>Why do you need 0.1' / 24 bit ? >>Most standards specs I've seen quote harmonics only to a certain >>number, and if you are doing an analyser do you need to be 70x that ? >> >>Many Audio DACS have poor impulse response - a 20KHz response is >>gives out to the 50th harmonic of 400Hz. > > > 24 bits is a huge overkill, it's just what the 192KHz low-cost > audio ADCs put out. Measuring to a fraction of a degree is > real; the users of these things tend to drive large inductive > loads and to add capacitor banks to correct the power factor, > and they do that by measuring the phase between voltage and > current.
Could you not derive phase for power factor terms very accurately, by using a whole-cycles samples. You do not need _each_ sample to be 0,1', all you need is the phase of the 'best fit sine'. Quite a low number of samples will give you that : If you know the exact times of the samples. ~0,5us time delta is ~1 part in 5000, and 12 bit ADC is ~1 part in 4096 ad that's better than 0.1' That leaves you with transient capture, which sounds partly real, and partly 'bragging rights' stuff. For the numbers game, look at the TMS320F28x, they have 12bit ADCs with 12.5 Msps, and enough crunching you could do fancy compression on the transient info : Store the delta from a ideal sine - so you don't waste storage on clean waveforms.... Or ADIs newest ADuC7128 - that has 1 MSps ADCs, 12 bit, DACs and a DDS core as well. -jg
On Wed, 10 May 2006 09:09:08 +0000, Guy Macon
<"http://www.guymacon.com/"> wrote:

>A Voltech PM300 measures up to 250kHz. A PM3000A goes to 1 MHz, >and a PM6000 goes to 10MHz.
>It seems that somebody out there seems to think that you have >to go past a few KHz.
It depends what you want to measure, if you are interested in measuring lightning current waveforms, then at least 1 MHz sample rate should be used, since the current slew rate is several kA/us and the peak is reached in the order of 10-30 us. For transients in typical man made electric systems, the transient current is limited by the circuit inductances (including wire inductances) and hence the current slew rate. Voltage transients are limited by the stray capacitances. Thus you would have to analyse what kind of voltage and current transients are expected in your environment, before deciding the sample rate. Paul


I have been doing some more "back of the envelope" design work 
on a split phase / three phase 50 Hz to 400 Hz AC power source.
The comments some here have posted about ACD/DAC bandwidth have
been very helpful and are much appreciated.

I obtained a couple of different kinds of commercially available
AC Power sources and spent a couple of days running tests on 
them in order to see what I am competing against.  I found that
the real-world bandwidth limit isn't fixed at a particular 
roll-off but is instead slew-rate limited, and thus the small 
signal bandwidth -- and the number of useful harmonics that are 
used in the signal synthesis function -- is considerably better 
than the bandwidth at full voltage swing.

As a thought experiment, imagine a 400 Hz AC power source that 
has just enough output-DAC bandwidth to generate a 400 Hz sine
wave, bumping right up against the Nyquist limit.  This wouldn't 
be enough in the real world, where one very common set of 
waveforms that the users desire are sine waves with hard clipping 
at 1%, 5%, 10% THD etc.  Then again, some of them want square 
waves or triangle waves...  This requires more DAC bandwidth, but 
the question is, how much?  I am inclined to design in enough 
DAC bandwidth so that the slew-rate-limiting of the output stage 
dominates from 10% to 100% amplitude, and to give the waveform-
capture ADC a bandwidth of maybe twice that.  Comments?

As always, many of these decisions will change as the design 
progresses; I am just setting a starting point for the preliminary 
design work.

-- 
Guy Macon
<http://www.guymacon.com/>