Nov25-10, 11:24 AM
I will just apologize to everyone in advance because of my English,, as well as for long post, I just don't know how shorten it ..
I'm expirencing some problems (that are more "theoretical" nature) with harmonic filters.. I can't seem to find how to properly calculate the values for inductance and capacitance of "notch" filter..
Actually, I have designed notch harmonic filters for 5th and 7th harmonic in EasyPower,, using thompson relation,, but my real problem is to determin reactive power request at the point of installation of filters..
Here You can see some "print screens" of my project (it's pretty simple one) that consists of 4 loads, 1 linear and 3 nonlinear.. transformer is able to atenuate "triplens" but, because at my loads 5th, 7th, 11th and 13th harmonics are dominant,, I still have significant current THDi at PCC point of sistem if filters are OFF..
impendance characteristics @ bus 3:
My problem here is, how to precisely calculate "capacitor bank" and how to determin "voltage" required by programme, please look at the filter data box (notch filter):
I've set aprox. voltage level of 430V (because main is 400V) just to perform calculation,, and it seemed to work, but I'm looking to find formula for it's precise calculation..
The only information provided by programmes "manual" and "help" is:
• Resistor: Resistance of resistor of the filter in ohms.
• Inductor: Real and imaginary impedances (ohmic resistance and inductive reactance) at system frequency in ohms.
• Capacitor Bank (1 and 2): The reactive power of capacitors for the system frequency in MVARs at the voltage level specified in kilovolts
… which doesn't helps me very much..
Other thing, about reactive power calculation,, my loads are set in kW and kVAr,, so if I just "add" those kVArs I'm not sure if I'm geting proper value for my capacitor bank.. Lets say, for example, if my linear load is set as: 100 kW and 20 kVAr, and other three, ASD 40 kW and 5 kVAr, Fluo. lighting 25 kW and 5 kVAr,, and PC center 30 kW and 5 kVAr,, simple problem is how should I properly add those to gain value for reactive power request by my loads.. I asume that linear load would have "ohmic" character, ASD drive "inductive",, and Fluo and PCs "capacitive" character,, so simple adding (20+5+5+5=35 kVAr) is definatelly questionable,, more proper way would be "phazor" addition.. but interesting fact is that if I set my filters capacitor bank to 0.035 MVAr it provides better THDi atteunation than it's if I set it to phazor value 20,616 kVAr,, I supose that programe is not able to differ those "load character" differences..
Once knowing this "voltage" and "capacitance" it's simple to calculate "inductance" using Thompsons relation..
Here are also the links for my calculation of those filters, as well as "reverse" calculation (that's the way I found to calculate my filters) of filters that are provided in example in the users guide of this book (I'm going to apologize again, now for two reasons, first is those materials are not in English, since I'm from adriatic region, but You can see/look only at calculations…. other reason for apologize is "messy" pdfs because my scanner just got broken, so I was using my camera)..
"primjer iz knjige" would be "example from the book" and "računanje filtera" would be my filter calculation..
example form the book (please notice that "2.filter" and "3.filter" are notch 5th and 7th filters respectively….. how come is Xc value different for both filters and it's close to 20 MVAr set by the load?……. there You can also see those voltages like 120 kV that are set "in decimal"):
calculation (please look on the page 3. for calculation of my filters,, and on page 2. for reverse calculation of filters from book example):
Every help is warmly wellcome :D :D ..
Kind Regards,, azzaman77..
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