harmonics?

[pajce]

is there rule of thumb what causes harmonics in the system. Customer has single phase 240V motor pump and there is a cyclic sag in voltage and spike in current. THD is very hifg in range of 120% PREDOMINATE 3RD AND 5TH HARMONIC

 

[ozmosis]

No, there's no rule of thumb. It is basic physics.
A non-linear load will generate non-sinusoidal currents that will interact with your supply impedance to create voltage distortion.
Do you have a VFD running the motor pump? if yes, then this will be generating your harmonic distortion. If no VFD, then you need to look at other equipment on the distribution network you are measurieng.
Your % of distortion is only a problem when you understand the actual amps compared to the capacity of your supply. If 120%=2amps in a 200amp rated system, as an example, don't worry about it.
What is your question?

 

[pajce]

I don't have details about customer load yet. I understand there is motor that may have capacitor start. My question is for inrush of the motor is 2nd harmonic dominant as for the power transformers? Also what harmonics would come from motor load, say there is no VFD?

 

[Skogsgurra]

No point in measuring harmonics during start (inrush is always distorted). Are you making a problem out of a measurement you did? Or does the customer have a real problem?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[pajce]

OK. Customer hired Consultant who came back to Utility to complain about voltage dips. We set recorder on line for a week and recorded cyclic dips in voltage, one in particular went all the way to zero and yet current did not rise. I wonder if this was some kind of resonance, but I am not sure if resonance would occur only once and other times not. This is rural customer I assume grid is weak infeed. I have no more info at this time. But I was hoping to understand if this dips in voltage and spikes in current would be start of the motor and which harmonic is dominant for start of the motor Vs transformer?
What brings 3rd and 5th harmonic?

 

[Skogsgurra]

It is a rural grid? Then, there may very well be other loads where you have heavy motor starts outside of this customer's premises. So, the dips you see can be caused by a distant motor start or some other heavy momentary load.

It is, as I said, not meaningful to talk about harmonics during a transient. You get so differing results depending on sample frequency and sample length. But, in general, if you have a transformer inrush you usually see a DC component and also quite a lot of second harmonics. But you can also have none of it, depending on at what phase angle you switch the transformer.

Third and fifth are generally produced by non-linear loads - as are seventh, elevent, thirteenth etcetera. Third and other triplenes are usually not seen in a three-phase system unless there are loads connected phase-neutral.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[slavag]

Hi.
Small additional excplanation to harmonic sources:
Harmonic sources:
The most important harmonic sources are basically converters and power supplies
for numerous electrical equipment. This equipment is a source for harmonics, and at
the same time, its operation principles may be very sensitive to harmonics,
especially to voltage harmonics. Still, some devices can be designed to decrease
their characteristic harmonics.
Single-phase power supplies
A major harmonic concern in commercial buildings is that power supplies for
single-phase electronic equipment will produce too much distortion for the wiring.
Direct current power for modern electronic and microprocessor-based office
equipment is commonly derived from single-phase full-wave diode bridge rectifiers.
Modern technology for single-phase power supplies is based on switch-mode. A
distinctive characteristic of switch-mode power supplies is the very high thirdharmonic
content in the current. Other characteristic harmonics are the 5th and 7th
harmonics. Switch-mode power supplies are beginning to find applications in
fluorescent lighting systems.

Three-phase power converters:
Three-phase electronic power converters differ from single-phase converters mainly
because they do not generate the third harmonic or the third harmonic is quite small.
There are many designs and types of converters for AC or DC drives with different
power ratings. Harmonics may vary significantly between designs and operation
conditions. Still, some examples are given below.
Six-pulse and twelve-pulse converters
The most significant harmonics for six-pulse converters are the 5th, 7th, 11th and
13th. For twelve-pulse converters, the 11th, 13th, 23rd and 25th harmonics are the
most significant.
Regards.
Slava