HV Shielding


by stefanb1
Tags: power, sensors, shielding
stefanb1
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#1
Jan29-13, 09:12 AM
P: 6
Hi,

I am doing an university project that consists in building a robot that operates in a AC to DC convertor. The robot has to work under HV environment. I am looking at possibilities of shielding the electronics of this robot. Can you give me any suggestions or links to look at? Also, will ultrasonic sensors work in such an environment. I could not find any evidence that shows that this is not possible.

Looking forward to hearing from you!!

Many thanks!!!
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the_emi_guy
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#2
Jan30-13, 07:39 AM
P: 580
Can you be more specific? An AC to DC converter is a device, not an environment (at least in my mind).

How high is the voltage?

What are your concerns about the high voltage? Interference with your device? Safety?
Bobbywhy
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#3
Jan30-13, 07:08 PM
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stefanb1, Welcome to Physics Forums!

What does "the robot has to work under HV environment" mean? Also, please describe your plans for using ultrasonic sensors. To sense what, at what range? Active or passive? The more you share your design specifications, the better members can assist you.

Cheers,
Bobbywhy

stefanb1
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#4
Jan31-13, 04:39 AM
P: 6

HV Shielding


Hi,

thanks for the reply. the project is a proof of concept. the robot is suppose to be placed in an offshore HVDC convertor hall, roam around and take thermal images. the convertors use MMC technology based on IGBTs. there are 3 big convertors in the halls and the robots needs to roam around them. normally these 3 convertors are placed one near each other at the minimum phase to phase distance so basically the robot will need to be inside these corridors. that is why it needs shielding.

i am using a parallax ping ultrasonic sensor which uses pulses to gather information. by using pulses it minimizes the noise getting in the receiver transducer. to protect the sensor from EMI i am planning to use a screen like a grill that will let the ultrasonic waves in and out but not the electromagnetic interferences.

thanks
sophiecentaur
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#5
Jan31-13, 05:04 AM
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Quote Quote by stefanb1 View Post
Hi,

thanks for the reply. the project is a proof of concept. the robot is suppose to be placed in an offshore HVDC convertor hall, roam around and take thermal images. the convertors use MMC technology based on IGBTs. there are 3 big convertors in the halls and the robots needs to roam around them. normally these 3 convertors are placed one near each other at the minimum phase to phase distance so basically the robot will need to be inside these corridors. that is why it needs shielding.

i am using a parallax ping ultrasonic sensor which uses pulses to gather information. by using pulses it minimizes the noise getting in the receiver transducer. to protect the sensor from EMI i am planning to use a screen like a grill that will let the ultrasonic waves in and out but not the electromagnetic interferences.

thanks
What frequencies are likely to be present in the EMI you are dealing with?
stefanb1
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#6
Jan31-13, 05:20 AM
P: 6
the IGBTs switch around 2 kHz, but my sensor has a burst frequency of 40 kHz. I am worried about EMI reflecting ultrasonic? or am i wrong?
sophiecentaur
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#7
Jan31-13, 05:33 AM
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40kHZ is a high harmonic of 2kHz so I doubt there would be much EMI up there - except if there is significant arcing.
This sounds like an overalll expensive project so you should at least suggest a survey of the EM environment. This is 'real engineering' and the actual figures really count. Screening should be 'appropriate' and it would be a (engineeringly criminal ) waste to 'over-kill'. Have you researched data on EMI levels in similar plant? It would be worth brownie points in your final assessment, if you did (imo).
stefanb1
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#8
Jan31-13, 05:52 AM
P: 6
thanks for all the help!
Bobbywhy
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#9
Jan31-13, 06:11 AM
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A description of a HVDC convertor hall is here:
http://www.scribd.com/doc/14579976/h...station-layout

A 60 Hz converter will generate AC current harmonics corresponding to the 11th and 13th harmonics at 660 Hz and 780 Hz. Additionally the DC side would produce a 720 Hz side harmonic. The Electromagnetic Interference radiated from these currents will not interfere with the acoustic signals at 40 KHz. Why would they interfere: one is electromagnetic and the other is acoustic? You have not explained what the ultrasonic sensor will be used for.

Is your group the first to attempt using mobile robots to obtain thermal images? Since there are many HVDC systems already functioning around the world, have no others used robots for taking thermal images? How do all these others obtain thermal images?
stefanb1
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#10
Jan31-13, 10:10 AM
P: 6
Hi,

thanks again!! as i said the project is a proof of concept. these converters, especially the Siemens HVDC converter placed offshore is pretty amazing with all protection features against any problems such as partial discharge, loss of an IGBT module, etc. we planned this robot to measure temperature of the MMC modules using a thermal camera and then sending pictures wirelessly to a server that is also place offshore. this server then transmits via fibre optic to the onshore control centre.

the ultrasonic sensors will be used for navigation, distance measurement and obstacle avoidance purposes.
Bobbywhy
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#11
Jan31-13, 06:50 PM
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Thank you for a more detailed description of your proof of concept project.

The building surrounding a valve hall normally contains a steel structure designed to act as a Faraday cage to block the electromagnetic radiation which might cause radio interference outside the hall. However, the EM radiation inside the hall will be extremely strong.

I certainly agree that your robot will need to be shielded by its own Faraday cage to protect its electronic circuits. I agree that ultrasonic pulses will pass through a wire mesh screen unperturbed, while maintaining the integrity of the cage. Please see the below article for some predicted values of radiation in a valve hall, both power versus frequency and also field strengths. Before putting all trust in these simulated predictions, I would prefer to base design specification requirements on real in situ measurements of the EM radiation. Might it be simpler to install several fixed thermal cameras? How do other installations monitor the valve thermal profiles?

“EMC’09/Kyoto
Simulation of Radiated EMI in Valve Hall of
800-kV HVDC Converter Stations
V. CONCLUSIONS
(1) The radiated EMI calculation of converters based on the
method of moments (MOM) is proposed.
(2) Electric field patterns around converters are analysed.
(3) According to simulation results, the field strength
reaches 373V/m at 1m away from valve hall walls in a valve
hall, while this value can reach 231V/m in the control building.”

http://www.google.com/url?sa=t&rct=j...nxVD07imYX0Oxw

Cheers,
Bobbywhy
stefanb1
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#12
Feb4-13, 04:13 AM
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This is really helpful! Thanks a lot! There are other ways to monitor temperature there, but they are all static. We are trying to make a more dynamic and flexible one.
sophiecentaur
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#13
Feb4-13, 10:08 AM
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The em waves in such close proximity are not, strictly 'radiation'. Everything in the converter hall is very much near field and, as such, the impedance would be variable and far from that in free space. It would be necessary to look, not only at the E fields but also the H fields, which would be related by whatever the local impedance happened to be. You could imagine some pretty massive currents flowing on the surface of any screening box / cage and these could provide much more embarrassment at seams and joints than the effects of a 'mere' hundred volts or so from the few hundred volts /metre that have been measured.


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