Yes, PMUs makes it a bit different. Here is a random paper someone wrote:
https://rubidium.dyndns.org/~magnus/papers/KTH_paper1.pdf
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.
Cheers,
Magnus
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-boun...@febo.com] On Behalf Of Hal Murray
Sent: Wednesday, December 17, 2014 3:28 AM
To: Discussion of precise time and frequency measurement
Cc: hmur...@megapathdsl.net
Subject: Re: [time-nuts] Simple AC mains zero-cross detector
csteinm...@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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