Hi Everyone,
I use a three-phase power quality meter to check for power problems in commercial facilities.

Many times to measure the current on larger switch boards is impossible with clamp on current transformers and is even sometimes impossible with the flex CTs, because of the many parallel conductors.

If I measure the three phases of one set of parallel conductors will I get true waveform and other quality related measurements?

Is this an acceptable practice?

This would make my life much easier.

Thanks,
Andy

 

Responding to Andy's Aug 6, 2:25pm query... additional info would help!

Are you questioning if the parameter (Amps, kW, kVAr, harmonic magnitudes) for the feeder is some multiple of the measurement taken for one set of parallel cables?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Hi Phil,

I realize the total amperage and wattage will be a multiple of the number of parallel circuits, but I am wondering if there is going to be anything that will affect the shape of the waveform, the % harmonics, or any other “quality” data by reading one set of parallel conductors versus reading all the phase conductors at once.

Thanks,
Andy

 

Replying to Andy's Tue, Aug 9, 3:12pm reply:

If the sending-end and receiving-end voltage triangles are similar and the load is balanced, meaning little neutral current, then, what you propose is acceptable.

Conversely, if there is considerable difference in the two triangles or there is a considerable neutral current, then, what you propose will introduce substantial error. Inaccuracy can be caused by type of cable, configuration, installation, mutual inductance affects, to name the major ones. However, there are solutions!

Can you read one set of parallel cable data at the sending end, and the other set at the receiving end?

BTW, what is the voltage and load? An arc furnace? Or other heavy current application?

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Hi Andy
There is simply no way to tell with an unspecified, real world load. For a common example, lots of places run mixed fluorescent, sodium and mercury lighting on individual phases with a mix these days of magnetic ballasts and electronic ballasts. I see no particular reason the trash on one phase would accurately represent the others. Unless the load is symmetrical and the source is too, it's really a crap shoot assuming one phase is the same as the others. And even when they are, transformer imperfections and parasitic variance cause imbalances. Actually the questions is, why would one phase represent the others.

And how does one assess them together? You can filter out the fundamental and sum the noise for quality and additive time coherent noise, and sample and do the math for the fundamental, but what does it mean? That depends on the load.

Regards
cww