Understanding Reactive Power Measurements with a Smart Meter

[Sebastian Neo]

Hi,

I was doing some test with the load bank today and I obtain a value which I don't quite understand.
I supply a 400V Line to Line, 50Hz to a smart meter and to the resistor load bank. The measured value by the smart meter was 500W and -40Var with a current of 0.71A. As the current increases, the real power increases and the reactive power decreases further.

I know that there is no such thing as ideal load but to have a negative reactive power does not make any sense.

Please kindly advice. Thank you

 

[anorlunda]

Negative VARs just means a capacitive load.

What doesn't make sense is so many VARs of either sign with just a smart meter and a resistor bank. There must be more to the story, something else in the setup that you didn't mention.

 

[Sebastian Neo]

the setup was quite simple. I am taking supply from the main power grid into my microgrid and to my smart meter which is connected to the resistor bank.
That's all for my setup

 

[jim hardy]

A couple of thoughts:

1. Can i assume single phase ?

500 watts and 40 vars is only 501.6 VA
what is the advertised accuracy of your smart meter ? Do your discrepancies fall outside its guaranteed performance?

2. With single phase 501.6VA at 400 volts is 1.254 amps not 0.71,
however
1.254 divided by √3 = 0.723, within 2% of what you report measuring.
So was my assumption of single phase perhaps wrong?

3. There's an ambiguity in your description.

As the current increases, the real power increases and the reactive power decreases further.

By 'decreases' do you mean becomes a larger negative number as in moving toward -50, -60, -70 vars?

4. If this is three phase recall that when PF = 1 the line current and line-line voltage are displaced by 30 degrees, which at first glance looks like a PF of about 0.87. That's why there's a two wattmeter method for measuring three phase power.
If you are trying to measure three phase power with a single phase "smart meter", it sounds to me like everything is behaving pretty much as it should.

5. So, what's your "smart meter" ? Got a link to its instruction manual ? And a sketch of your hookup?

 

[Sebastian Neo]

A couple of thoughts:

1. Can i assume single phase ?

500 watts and 40 vars is only 501.6 VA
what is the advertised accuracy of your smart meter ? Do your discrepancies fall outside its guaranteed performance?

2. With single phase 501.6VA at 400 volts is 1.254 amps not 0.71,
however
1.254 divided by √3 = 0.723, within 2% of what you report measuring.
So was my assumption of single phase perhaps wrong?

3. There's an ambiguity in your description.

By 'decreases' do you mean becomes a larger negative number as in moving toward -50, -60, -70 vars?

4. If this is three phase recall that when PF = 1 the line current and line-line voltage are displaced by 30 degrees, which at first glance looks like a PF of about 0.87. That's why there's a two wattmeter method for measuring three phase power.
If you are trying to measure three phase power with a single phase "smart meter", it sounds to me like everything is behaving pretty much as it should.

5. So, what's your "smart meter" ? Got a link to its instruction manual ? And a sketch of your hookup?

By 'decreases' do you mean becomes a larger negative number as in moving toward -50, -60, -70 vars?
Yes, that's what i meant.

I am using ABB A44 meter.

544W is total power for all 3 phase. single phase voltage is 230V. Current for Phase A is 0.71A. Power for 1 phase is 178W.

Total reactive power is -38.94. Phase A reactive power is -15.55 Var, Phase B is -4.92 Var and Phase C is -18.46 VAr