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Environmental Monitoring (Environmental Monitoring)
Paoletti, Fiori - 15:49 Wednesday 04 September 2019 (46837) Print this report
50Hz mains sidebands: some facts

There are some (not all) 50Hz mains sidebands that couples with hrec; a good witness (also used for a 50Hz reduction through a feedforward) is the signal named ENV_CEB_ELECTRIC. This signal is the voltage drop between the electronic WI tower suspension crate and the tower's vacuum vessel (using a Stanford SR-560 10x differential amplifier).

These sidebands are usually generated by heavy loads (e.g. heaters) switched ON and OFF with a cycle of about 1s

Trying to reduce the sidebands in large families we can see that one is at +/- 1.25Hz (and its +/- 2.5Hz harmonic, period is 0.8S), the second is at +/- 1.7Hz (with its harmonics at +/- 3.4Hz and +/- 5.1Hz, period is 0.6S)

Using data of the last HVAC shutdown test (19 April 2019) we can see that none of the sidebands disappear in ENV_CEB_ELECTRIC; we can therefore exclude all HVAC systems (except the INJ that has been left on) as a possible source.

Another interesting hint is that this signal has no coherence at sidebands frequencies with the magnetometer located in the Mode Cleaner Building (ENV_MCB_MAG), this excludes a possible earth return current from the MCB HVAC machine through the MCB vacuum pipe towards the central building; instead there is coeherence with other sidebands, all very close to the 50Hz mains (e.g. +/- 0.53Hz), we should repeat the test made in this entry with a look at all visible sidebands.

 

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Paoletti - 14:01 Friday 06 September 2019 (46850) Print this report

Correlation shows that the 1.7Hz and 3.4Hz spaced sidebands seem originate from a load connected to the CEB_IPS_R mains line. A lower coherence is present in the other two CEB_IPS lines (S and T), while no coherence (or at least a very low one) is visible with all three CEB_UPS mains lines.

At the same time, during one ENEL IPS cut-off (e.g. 4 June 13:33:19 UTC + 43s) these sidebands do not disappear!

If we need to search for a ON-OFF controlled load with a period of 0.6s, this is not associated with the Virgo HVAC machines (exception: INJ HVAC - to be verifed) and could be connected to some UPS lines (the main one, or through some small ones local UPS)

Again: a look at data of the last HVAC shutdown test (19 April 2019) confirm that the MCB heater system generate many sidebands in the mains line, and the nearest of them to the 50Hz line (e.g. +/- 0.53Hz and +/-0.75Hz) are visible also in ENV_CEB_ELECTRIC signal.

Attached:

    coherence between ENV_CEB_ELECTRIC signal with other ENV signals
    coherence between Hrec_hoft_clean with other ENV signals
    spectrogram of the HVAC shut-down test focused to the MCB OFF time
    FFT of the ENV_CEB_ELECTRIC signal with (purple) and without (blue) IPS mains

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Paoletti, Fiori - 15:08 Wednesday 11 September 2019 (46894) Print this report
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Paoletti, Washimi, Sugimoto - 10:40 Thursday 03 October 2019 (47079) Print this report

One of the first task of ENV october commissioning shifts is to find the source of 50hz mains sidebands.

Yesterday, with the help of KAGRA colleagues and the use of their portable magnetic probe, we have sniffed all around the CEB near electrical panels (both UPS and IPS).

We found the highest sidebands were looking at the electrical panel in the TCS room, just on the left when entering in the room.

Labels on it shows that it drives both the TCS benches. Going near these lasts we indeed found the maximum of sidebands noise on one of the two cables entering each bench, and in detail 1.2Hz sidebands for the WEST bench and 1.7Hz for the NORTH bench.

 

As a further check we had a look to the data of the early morning (08:00 Local time) when there was again a change in the sidebands presence as seen in the past, and indeed we now found correlation with TCS benches environmental sensors.

TCS team is awared of this, they have planned a selective switch-off of all devices on the benches in order to find the culprit.

 

Side note: while sniffing in EE_room we found many 2.3Hz sidebands (with lot of harmonics) if the magnetic probe is over the Laser Amplifier.

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paoletti - 12:13 Friday 04 October 2019 (47094) Print this report

Please forget the last sentence: the 2.3Hz sidebands are a side effect of the reading system, they are not real.

Paoletti, Nardecchia - 11:57 Monday 07 October 2019 (47120) Print this report

Today we switched off the two CINOGY IR laser beam profilers (one for each TCS bench), because we suspected they were the 50Hz mains sidebands generators.

Indeed they are :-)

Here are the times:

  • 7:53 UTC WI
  • 7:57 UTC NI


Attached the FFTs before (Purple) and after (Blue) of the sensor ENV_CEB_ELECTRIC and of the magnetometer ENV_CEB_MAG_W.
The WI contributes to the 1.2 Hz sidebands, while the NI creates the 1.7 Hz sidebands with its harmonics and also a broadband noise from 40 to 60Hz (note that in this last spectrogram there is a magnetic spike every 5 minutes, we still have to find the origin ).


Let's see if this action will give some results in Hrec in terms of Mpcs (finger crossing)

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