Relationship between harmonics and sequence currents?

[norske]

Hello all,

Can some one clarify something for me. To my mind there must be a correlation between positive, negative and zero sequence currents with harmonic content. My understanding of +,-, and 0 seq currents is that their weighted contribution to a waveform can be used to produce any wave form shape.

Given that any complex wave form can be broken down to its fundemental and interger (harmonic) parts surely there must be a relationship between this these two methods of determing a complex waveshape make up.

Q1. Does any one know of a tabulated data aligning the harmoics with their respective seq currents?

Q2. Does any one know of any protection devices that use, say, a fast fourier transformation to determine harmonic components as the front end of determining fault type?

Lots of stuff here, thank you all in advance who have waded through my questions.

 

[SidiropoulosM]

The three symmetrical components of a phase current are, by definition, sinusoidal and have the same fundamental frequency as the phase current. You cannot combine them to produce "any waveform shape" as you suggest. You can only produce waveforms of one frequency, the fundamental.

The presence of harmonic content is caused by non-linearities in the system (magnetic saturation, rectifier operation etc) and is unrelated to unbalanced phase currents.

 

[Aakhrinaam]

Harmonics and sequence components have to be considered separately. Harmonic do actually exist where as sequence components provide a mathematical tool for analysing and calculating fault components.

For example , a sequence networks are driven by sources (positive seq. etc.) which are assumed to be producing components of fundamental frequency.
If you want to evaluate harmonics in the same way , you will have to create separate networks having "sequence" sources of that harmonic frequency.

No practical device (relay) would use FFT's to detect faults. FFT is too calculation intensive to be used for real time applications. A DFT working on the fundamental (sometimes a third harmonic too) are usually used. But mostly for HV/EHV protection , the operating time of the relay becomes critical and shorter filters are used.

There is some research going on in the field of wavelet's which is aimed at detection of high frequency components for fault detection but it will be some time before a relay based on that is available (reason for that is a lack of previous experience with high frequency components for fault detection).

 

[busbar]

Not sure this is exactly related, but it has been published that there is a harmonic relationship that can affect watthour-meter registration.
[tt]
1 2 3 4 5 6 7 8 9 10 11 12 13 order
+ - 0 + - 0 + - O + - 0 + component
[/tt]
Negative-sequence current will retard meter registration. Positive-sequence current will advance meter registration. Zero-sequence current will not directly affect meter registration.