[time-nuts] Simple AC mains zero-cross detector

Magnus Danielson magnus at rubidium.dyndns.org
Sat Dec 20 09:39:12 EST 2014

Yes, PMUs makes it a bit different. Here is a random paper someone wrote:

PMUs samples at some "high" frequency, mixes down the network frequency 
to base-band, filters away the mirror frequency before sub-sampling it 
into the configured sample-rate. The process is being controlled by the 
IEEE C37.118.1 standard, while the communication is described in IEEE 
C37.118.2 but an IEC 61850 extension provides for another way of 
conveying that data.

PMUs have proven themselves to outperform many of the normal frequency 
and ROCOF estimators, which became evident in the 2003 NE black-out 
scenario, as the SCADA data kept getting them on the wrong tracks, so 
after 8 months they just scrapped the frequency readings from the 
traditional equipment and just looked at the PMU data, they could sort 
the events out properly in time. Turns out that the details of how a 
particular vendor implements the frequency estimation and filtering can 
be devastating in getting comparable frequency measures, and thus 
loosing the observation precision needed to follow the aftermath properly.

The C37.118.1 has put a stringency on how filtering is to be done, as 
well as how frequency and ROCOF (Rate Of Change Of Frequency) is to be 
calculated. NIST has a small department focusing on the calibration of 
PMUs, and is working actively with the vendors to get them improved. 
Good folks and I have helped them a little with some ideas, amongst 
others to do through-zero calibrators.

For other events, Digital Fault Recorders (DFR) is being used. They are 
essentially memory oscilloscopes which have a more advanced trigger 
adapted to go off on all "interesting" events. Today DFRs is a legal 
requirement in some countries.

So, I do not completely agree that a through-zero measurement with a TIC 
has all the information, and for the information you do get, you would 
like to be as careful as the PMU folks about the group-delay of filters, 
time-compensation of processing and filters etc. to maintain good 
precision. There is reason to look at it and learn.


On 12/18/2014 11:59 PM, Mike Garvey wrote:
> There is an interesting article in the Nov/Dec issue of Inside GNSS
> describing the robust measurement of "...voltage and current phasors at
> widely dispersed locations in a power grid".  A Phase Measurement Unit
> measures and time stamps the voltage and current phasors "...thousands of
> times per second..." to an accuracy of <1 us using GPS.  The authors discuss
> several strategies for dealing with jamming and spoofing of the civil GPS
> signals.  It's a good read.
> See http://www.insidegnss.com/node/4281
> Mike
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Hal Murray
> Sent: Wednesday, December 17, 2014 3:28 AM
> To: Discussion of precise time and frequency measurement
> Cc: hmurray at megapathdsl.net
> Subject: Re: [time-nuts] Simple AC mains zero-cross detector
> csteinmetz at yandex.com said:
>> That one is not ideal for this task, because (i) its output pulse is
>> symmetrical about the mains zero cross, and (ii) the hysteresis zone
>> is not well characterized and will drift with temperature and input
>> voltage.  So, there is no edge that is well characterized in relation
>> to the AC mains zero cross.
> What are you going to do with data from the line accurate to 1 microsecond?
> --
> These are my opinions.  I hate spam.
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