03 AardEnergy – Current and Voltage Transformers

 

In this Blog

I have a Current Transformer and a Voltage Transformer to evaluate. In this blog there will be a description of the sensors and some analysis of the signals as well as some measurements to assess performance.

 

See previous Blogs…

http://www.element14.com/community/groups/arduino/blog/2015/11/23/1-aardenergy-kick-off--a-new-project

http://www.element14.com/community/groups/arduino/blog/2015/11/26/2-aardenergy-set-up-uno-and-due

Current Transformer

The current transformer (CT) was bought from Amazon; the SCT 013-030 is a 30A device that costs £8.75.

http://www.amazon.co.uk/gp/product/B005CTWE8A?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00

This part is made by YHDC and details can be found on their website:

http://www.yhdc.com/en/product/320/

The part supplied is fitted with an internal resistor that converts the output of the current transformer (CT) to a voltage with the scaling of ±30A input to ±1V output.

Check that the part supplied has the resistor before use. You can do this with by measuring the output resistance with a meter, it should read about 37R. It is clearly marked 30A/1V. You should note that a CT without a resistor will generate very high voltage – see the article linked below in the section on Transformers.

 

This CT has insulation rated for 1000VAC and we will only be using it clipped onto wire that is also insulated. It must be applied to a single live or neutral wire and our project will be limited to use with the meter connection on the supply to our house, like this:

Note that if you place the CT over an appliance cable that has live and neutral running through it, the magnetic fields cancel and the CT will read zero.

The electrical supply to my house is 15kW so in theory I could draw 65Arms through the meter and my 30A sensor would not be able to detect anything above 30A. Also we need to be mindful that the peak current of my sensor is 30A so the maximum rms current will be 21.2Arms. 65Arms requires 92A peak measurement. For my house this should be okay as we do not use electrical power for cooking or heating water. We have oil filled radiators but these are used on rare occasions. The peak consumption will come from the washing machine and the tumble drier. I am expecting my 30A sensor to be able to deal with these loads. However if you expect your loads to be higher you need to purchase a suitably rated sensor. For example if you want the full current capability of 65Arms the peak current will be 92A so you should buy a 100A sensor. There is a 100A/1V sensor in the YHDC product range and you should buy that.

It is a difficult choice to design a measurement system either for the maximum possible or the maximum you normally need. If I use a 100A sensor but normally I am measuring around 1Arms with occasional 20Arms maximum then I am using the sensor at the bottom of its range and suffer the inaccuracy that entails. However if my supply is normally around 1A but with 60Arms occasional consumption I have to use a 100A peak sensor and put up with inaccuracy at 1Arms.

Note that the jack plug is compatible with this cable mounted socket from Farnell 1280665

http://uk.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15001&langId=44&urlRequestType=Base&partNumber=1280665&storeId=10151

Voltage Transformer

The voltage transformer (VTx) was bought from Farnell 2112026:

http://uk.farnell.com/ideal-power/77db-06-09/adaptor-uk-9v-0-67a/dp/2112026?ost=2112026&selectedCategoryId=&categoryName=All+Categories&categoryNameResp=All+Categories

Follow the link to find the datasheet.

 

The transformer will operate at no load and the datasheet specifies the output as 11.6Vac ±3% with a nominal input of 240Vac. The output is specified as rms so the peak output will be nominally ±16.4V. However we should note that the electrical system in the UK has a nominal rating of 230Vac and a maximum of 10% which means 252Vac and ±358V peak. So the maximum output of the VTx will be ±17.2V

To measure the mains voltage, the VTx will be plugged into the ring main. To get the most accurate reading, choose a wall socket close to the meter to minimise voltage drops around the ring.

 

Note that the jack plug is compatible with this socket from Farnell 3648102

http://uk.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15001&langId=44&urlRequestType=Base&partNumber=3648102&storeId=10151

Some Test Results

The following tests we done under workshop conditions rather than home conditions. The CT and the VTx were compared with a precision current and voltage amplifier using a National Instruments NI USB 6008 DAQ logger with Signal Express. The load and the VTx were plugged into an extension cable and the CT was installed into the case that housed the instrumentation amplifiers.

Current Transformer Results

Direct measurement 7.83A

CT Measurement 257.29mV

Scaling 7.83/257.29 = 0.0304A/mV

Voltage Transformer Results

Direct measurement 236.38

Tx Measurement 11.346

Scaling 236.38/11.346 = 20.8V/V

Current Transformer Basics

This is a useful explanation about current transformers:

http://www.electronics-tutorials.ws/transformer/current-transformer.html

Next Steps

So far I have chosen sensors without much consideration of how the Arduino will handle the data. The plan is to use a meter chip with the Uno and to directly convert the sensor voltages (after suitable scaling) with the Due.  In the next blog I will evaluate a number of meter chips for use with the Uno…ADE7753, ADE7763, CS5463, MCP3905, MCP3909.