Effect of Harmonics

Pl write point wise what exactly is your problem and what exactly you want.

Thank you for your interest

first of we want to perform capacitor stress analysis (in voltage and current)and i want to knowif that exceeding IEEE limits.( of 150 kvar capcitor).

regards

rahul

The manufacturer of the capacitors should be able to tell you what are acceptable limits for their product.

Please check this,

Effect of Harmonic

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First thought is....

1. Capacitor loss for effective series resistance r is given by [If² + Ih² ] x r. Where If = fundamental current ; Ih = total harmonic current : both r.m.s. values.
2. Loss at rated (60Hz?) current = Ir² x r.
3. Assuming r is same at fundamental as at harmonics, [If² + Ih² ] compared with Ir² should give an estimate of how close to rated capacity the bank is working.

As has been pointed out, the capacitor maker is the place to go for limits, corrections for working temperature etc.

67model

very much thanks

can i take assumptions that the rated kvar of capacitor is the product of capacitor voltage and capacitor current is we can take line voltage and current to capacitor (v&I METER SHOWN THERE) is same as capacitor voltage and current.then we have know capacitor stress and these analysis seems true.

2.to calculate Resonance frequency how can i calculate utility source impedance(transformer630kva11kv/433v) if capacitor reactance is known

please also tell me how cable and bus bar voltage drop relate to harmonics.

thanks and regards

rahul

Reference rahulgrove post #7,

1. Rated kVAr = rated voltage x rated current. Note these are rated values on capacitor rating plate, not actual working voltage etc. In this case, rated current = rated kVAr/rated voltage; NOT rated kVAr/line working voltage.
2. The transformer rating plate should have an impedance value on it in %. This is actually the primary voltage (in % rated primary voltage), at rated frequency,which gives rated secondary current with secondary short circuited. You have not told us your frequency - you talk about IEEE limits, which suggests 60 Hz, but 11 kV/433V suggests 50 Hz. Assuming 50 Hz, it is likely the impedance is 3.75%. Using that value one can estimate the source impedance seen at the 433V winding due to the transformer. The actual utility 11 kV source impedance would add to that value, it can be ignored as negligibly small for present purposes.
3. For transformer 50Hz, 4.75% Z, 630 kVA, 11000/433V ratio designed for rated load at 415V/240V secondary output voltage with 11000V primary, one can estimate rated current as 630/(0.415 x 1.732) = 876 amps with 433/√3 = 433/1.732 = 250 V open circuit. 100 % impedance would be 250/876 = 0.285 ohms. Hence 3.75% would be 0.285 x .0375 = 0.0107 ohms. This can be assumed to be the inductive reactance, since resistance is small in comparison.
4. If you calculate the inductance from that reactance, you can find a resonant frequency.
   

Thanks a lot. its really very best answer to my question ,sorry the transformer frequency is 50Hz.

keep in touch i need your help

thanks and regards

rahul

dear sir,

Now i am little bit doubt that about IEEE limits ie,

120% of peak voltage, including harmonics but excluding

transients

• 135% of nominal rms current based on rated kVAR and rated

voltage

but i want to calculate peak or rms voltage and rms current in which capacitor working then only i know stress please give me a clear solution for calculating capacitor working voltage and current

and i really sorry to distrurb you

regards

rahul

Ref Rahulgrove post #11,

A wide range of harmonic frequencies and amplitudes and phases is possible.

Take the simplest case, harmonics all (only) third harmonic.

Suppose the fundamental component is 100% rated. If the harmonic voltage is 20% of the fundamental rms, its peak voltage will add 20% to the fundamental peak voltage (assuming its phase is such that peaks coincide). This corresponds with the 120% peak limit you give. Transients due to switching would be excluded to protect against a presumption that the capacitor was designed for that rather than for a steady mix of low harmonics.

20% of fundamental voltage at 3 x fundamental would give 20% x 3 = 60% of the fundamental current. This would have a heating effect of 0.6 x 0.6 = 0.36 = 36% of the fundamental current in the capacitor series loss resistance (I²r), remembering that it is current² which determines the heating of the capacitors.

Not surprisingly, this 36% means fundamental loss + harmonic loss = 100 + 36 = 136% - which is very close to the 135% of nominal rms current limit you state.

It would appear the rating is based on a third harmonic current additional to expected fundamental current for the capacitance and fundamental voltage rating. I have a feeling (I have not checked!) that all 3rd harmonic gives the highest peak voltage compared to a mix of harmonics with the permitted 35% current excess over fundamental current.

Unfortunately, a peak voltage cannot be calculated without the phase as well as the amplitude of each harmonic. This would only be available if your harmonic ammeter gave the phases of each harmonic.

It would be much easier and more satisfactory to measure the actual peak voltage with an oscilloscope. With a 10:1 probe, most oscilloscopes will take 250V rms without clipping. If you do not have an oscilloscope, there are programs available for laptop/tabletop computer's audio inputs which can be very good if used with care - some have excellent analysis functions. At a pinch, one can record a .wav audio file and import to a spreadsheet program.

For the capacitors, more than voltage, the "killer" will be temperature rise giving excessive total temperature and possibly a peaky current due to harmonics above 3rd being exceptionally strong. The capacitor maker or bank maker will know the "hot spot" to measure and peak current restrictions.

thank you very much sir

keep in touch

Rahul