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Star point in inverter measurement

Edward Sciberras

Posted on 07.07.2016 14:10

Need to measure the output from an inverter using Sirius HS units. In the Power add-on manual, there is a note about wiring the setup, where it recommends against creating an artificial star point (pg 79) due to imbalances caused by "different impedances". Does this mean that the impedances between the channels do not match, and if so to what extent are they so different such that they give a noticeable result? Is this only valid for inverter outputs though? Why does this affect only the reactive power measurements then!

Bernhard Grasel
Product Manager Power & E-Mobility
Posted on 08.07.2016 14:00

Dear Edward!

For your application, measuring after an inverter, you have the following options:

1) Measuring in Delta Connection: You don't need a star-point adapter and can connect the voltage to the system as described in Power Manual. Please take care that the connection is exactly done according to Power Manual (p 72). Delta connection requires symmetrical load on all three phases and it's not possible to detect leakage currents.

2) Measuring in STAR Connection: If you want to measure in star connection the best way is to measure the phase voltages against the starpoint of the motor. In this configuration you can analyse additional to power also Symmetrical components, leakage currents, PWM modulation etc. If this is not possible a star point adapter is necessary to adjust the impedance (actually resistance) of the input amplifiers. If you don't use a starpoint adapter the voltage signal will float!! You will see this in Scope visualization. The voltage signal you see is wrong. It's not a stable PWM signal as it should be. Also results for reactive and apparent power can be wrong.

This is only valid for measurement after inverter. At grid measurement with 50Hz/60Hz the artificial starpoint can be done direct at low side of input amplifier (no need of a starpointadapter).

Best regards

Edward Sciberras

Posted on 11.10.2016 07:58

Hi Bernhard,

I'd like to follow this up with related question about the use of an artificial starpoint. In case of an unbalanced load at 50/60Hz, what are the repercussions of using an artificial star point? Is the voltage and apparent power then also incorrect as in the inverter measurement?

thanks,

ed

Bernhard Grasel
Product Manager Power & E-Mobility
Posted on 19.10.2016 12:51

Hi Ed,

At 50/60 Hz systems a starpointadapter is not required. You always will receive a stable signal, even at unbalanced loads, also doing the artificial starpoint via the low-side of voltage inputs. If you there is also a neutral line available it makes sense to connect this to the artificial starpoint.

greets,
Bernhard


Edward Sciberras

Posted on 20.10.2016 07:54

Thanks Bernhard!

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