IET
Decrease font size
Increase font size
Topic Title: 11kV isolation instrumentation transformer
Topic Summary: Manufacturer & What Spec
Created On: 10 May 2011 01:08 PM
Status: Read Only
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 10 May 2011 01:08 PM
User is offline View Users Profile Print this message



wrigpm

Posts: 8
Joined: 25 July 2008

We are designing a sensor to measure 11kV power lines as found in rural areas in fields. There is an electronics module (with 4 current transformers) that needs to be at the 'Hot' end near the lines and a processing module with a battery on the ground. The ground unit feds power to the top unit. We have been advised that in order to ensure safety we need a 2nd (the first being the current transformer) isolation break so intend to send the power to the top unit in AC (only 12V) and use a transformer to provide the isolation.

Can anyone suggest who might be able to make the transformer (coffee cup sized!) and to what spec it might need to meet. I have been told BSEN 60076 might apply but this is out of our field!

Any help appreciated.

Paul
 10 May 2011 03:01 PM
User is offline View Users Profile Print this message


Avatar for sfchew.
sfchew

Posts: 589
Joined: 10 December 2002

What is it that you are measuring? Voltage ? Current ? Power ? Why do you need to measure on the high voltage end?
What is the need of 4 CTs?

Regards
Chris Chew
 10 May 2011 03:04 PM
User is offline View Users Profile Print this message



wrigpm

Posts: 8
Joined: 25 July 2008

Chris

We are measuring current using Rigowski coils. We need to measure at the 11kV end 'because our customer wants to' - I can't really go any further than that for security reasons! 4 CT's as our customer wants to do 3 phases plus N/E.
 10 May 2011 05:44 PM
User is offline View Users Profile Print this message



simongallagher

Posts: 148
Joined: 28 July 2005

You will have to look very hard to find the N/E on 11 kV O/H!

Even if there was one, 3 CTs would be enough.

Where are you getting the 12v power from? Do you also have a VT up the pole?
 11 May 2011 04:12 AM
User is offline View Users Profile Print this message


Avatar for sfchew.
sfchew

Posts: 589
Joined: 10 December 2002

Chris We are measuring current using Rigowski coils. We need to measure at the 11kV end 'because our customer wants to' - I can't really go any further than that for security reasons! 4 CT's as our customer wants to do 3 phases plus N/E.


You are designing the sensor so the Rigowski coils are supplied by others. The key problem is to rigidly place the coils on the 11KV line conductors with sufficient clearance to avoid the high voltage from striking to the coils.

So there is no need to have a transformer for the sensor.

Regards
S F Chew
 11 May 2011 08:28 AM
User is offline View Users Profile Print this message



wrigpm

Posts: 8
Joined: 25 July 2008

Simon, I am fully aware of star/delta configurations! I did not say that this was ONLY going to be used on 11kV but it must be capable of going on 11kV. 3 CT's are not enough for our application - I cannot divulge what we are looking for but it is not what you are thinking!

The coils will be potted/moulded such that they are fully insulated to 11kV. However, we have been advised that this would not be sufficient as a safety case so another isolation barrier will be needed - hence the fiber and power feed transformer.

Chris, the 12V has to come from our power source on the ground as I mentioned above - we cannot have a VT on the line. A question re VT's - if the secondary winding were, say, 12V would it be considered completely safe - i.e. I could touch it?
 11 May 2011 04:42 PM
User is offline View Users Profile Print this message


Avatar for sfchew.
sfchew

Posts: 589
Joined: 10 December 2002

The picture is much clearer than the beginning.

12V is safe to touch.

Since the power is from the ground I assume you will need a transformer 230V/12V double winding with the relevant rating of current.

You should be able to source them easily.

Regards
Chris Chew
 11 May 2011 04:52 PM
User is offline View Users Profile Print this message



wrigpm

Posts: 8
Joined: 25 July 2008

Chris

The power is from a battery on the ground so we intend to go for a 12V/12V transformer. The problem is that we have been advised that we need a 2nd 'safety break' to ensure that a person on the ground is completely isolated from the 11kV. Apparently, this means an isolation between windings of about 75kV to account for lightning strikes on the lines!

Any advice on where to get one of those!?
 11 May 2011 09:34 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3377
Joined: 22 November 2007

Try Burrows Electrical in Chelmsford. They make all sorts of transformers. If they can not supply, I'm sure that they would know someone in the industry that could.

We've used them a few times for odd ball transformer problems such as stepping up and back down 230 to achieve longer cable runs to remote sites.


Stu
 13 May 2011 02:14 PM
User is offline View Users Profile Print this message


Avatar for JonathanHill.
JonathanHill

Posts: 225
Joined: 09 September 2002

Hi Paul,

This is a pretty specialised area and I'd commend a specialist instrument transformer manufacturer such as DK Moriarty or Transmag. I believe the relevant standard for this type of transformer is BS EN 60044 (60076 is for power transformers). These guys could also probably assist with an isolation transformer for your 12Vac supply to the Rogowski coil.

I'd have some concern on the ability of the Rogowski coil to drive its voltage output through a transformer, and more particularly what the impact would be on accuracy - may be worth asking the manufacturer.

I also have (greater) concerns on safety aspects. In my experience, I've never come across a ring-type CT on an HV system that didn't have an earth screen between the live conductor and the CT. This presumably will prevent capacitive coupling of dangerous voltages onto the CT secondary wiring. Also, as Chris Chew advises, there'll be a challenge in achieving a secure mounting for the CT.

Bearing in mind the safety issues, I'd strongly recommend that you get an HV design specialist onto your team.

-------------------------
Jonno
 14 May 2011 05:56 PM
User is offline View Users Profile Print this message



alancapon

Posts: 5700
Joined: 27 December 2005

Although not in common use at the lower HV voltages, I can't help thinking that you would be better off looking at an optical current sensor and not a Rogowski coil. That way, the connection between your electronics module and the line will be an optical fibre rather than cable.


Regards,

ALan.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.