how to find source impedance based on THDi and THDv

[LMAO]

Is there a way to know how much internal impedance I have based on Harmonic voltage and current? I can measure all harmonics (voltage and current) but do not have access to source data. It is generator.
Thanks,

 

[GoldDigger]

Is there a way to know how much internal impedance I have based on Harmonic voltage and current? I can measure all harmonics (voltage and current) but do not have access to source data. It is generator.
Thanks,

If you could be assured that the entire THDv was the result of non-linear load current and that the source impedance was purely resistive, or at least not strongly frequency dependent, then you would have some leverage on the calculation.

But, at a minimum, I would also want to know the THDv when the generator was subjected to a purely resistive load at maximum power (and also no load.)

 

[Jraef]

Is there a way to know how much internal impedance I have based on Harmonic voltage and current? I can measure all harmonics (voltage and current) but do not have access to source data. It is generator.
Thanks,

Trying to calculate source impedance backward only from a known THD value is, I suppose, theoretically possible, but I think would be like trying to push a rope up hill.

I would simply ask the generator mfr., they would know. If they no longer exist, you may need to find other similar generators that do exist and guesstimate it.

 

[Ingenieur]

how is the THD data presented
by fundamental freq and harmonics?
can you show us the data?
if the fund is >90% of the energy/power, I would say a good approximation of |Z| is possible but without time stamped data phase might not be possible

 

[rian0201]

i think you can calculate the source impedance but not by THD..


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[LMAO]

how is the THD data presented
by fundamental freq and harmonics?
can you show us the data?
if the fund is >90% of the energy/power, I would say a good approximation of |Z| is possible but without time stamped data phase might not be possible

it is a VFD running a 30HP motor at full load fed by a 440V 50Hz 60kVA gen. There is also a a passive harmonic filter upstream to the drive.

THDv=2.3%
THDi= 6.5%

 

[Ingenieur]

it is a VFD running a 30HP motor at full load fed by a 440V 50Hz 60kVA gen. There is also a a passive harmonic filter upstream to the drive.

THDv=2.3%
THDi= 6.5%

the VFD changes things
now it is non-linear
let me think about it

what are you trying to do?
model the system in a simulation program?

 

[LMAO]

the VFD changes things
now it is non-linear
let me think about it

what are you trying to do?
model the system in a simulation program?

It is long story. VFD is a prototype of what will be used in the field. IEE519 has different current thresholds for different input impedances (ISC/IL). I need to know what my generator input impedance is so I can add enough impedance to it to make 5% which is what customer has in field. Right now my system is not meeting IEEE because TDDi for 5% impedance (ISC/IL=20) is 5% but I believe my source impedance is too low resulting in high TDDi.

 

[Ingenieur]

a cheap and dirty approximation: Zs = V l-l / i fault
i fault may be hard to find in the gen data
but maximum starting KVA may give a good approximation

let's say starting kva = 60 kva x 10 = 600 kva or 790 A
so Zs = 440/790 ~ 0.55 Ohm (mostly inductive so 0.55j)