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Lot Ndapona
08-29-2014 03:10 PM

Harmonics source detection method

What is the best method to use for Harmonics source detection in a power system? some body please help
08-29-2014 05:13 PM
Top #2
Jim Phipps, P.E.
08-29-2014 05:13 PM
A harmonic source (or group of sources) can be located by taking voltage and current distortion measurements at multiple points in a power system. As you move closer to the harmonic source while measuring high current distortion along the path where measurements are being taken, the voltage distortion will increase. As you move farther away from the harmonic source while measuring high current distortion along the path, the voltage distortion will decrease. If there are shunt capacitors that are in resonance, the voltage distortion may be very high at many points in the system which can complicate finding the harmonic sources; so it is important to identify the location of the capacitors and switch them off-line (if possible) when using this method. If the capacitors are in resonance and the voltage distortion is high, switching the capacitors off-line can make drastic improvements to the system voltage distortion level. Switching the capacitors off-line will also result in a lower system operating voltage which may require raising the system operating voltage using other methods (raising transformer taps or boosting the field excitation on synchronous machines).

You can also find harmonic source using other methods such as inspecting the types of loads that are connected to the power system. Any type of load that converts power with an switching power supply and rectifier, such as a variable speed motor drive, will be a harmonic source.
08-29-2014 07:57 PM
Top #3
Lot Ndapona
08-29-2014 07:57 PM
Jim P. Thank you for the response. Are there no formulas you can recommend?
08-29-2014 10:23 PM
Top #4
Jim Phipps, P.E.
08-29-2014 10:23 PM
Sorry, no generalized formulas. You have to be a skilled hunter and track down your prey.
08-30-2014 12:51 AM
Top #5
Spir Georges GHALI
08-30-2014 12:51 AM
Dear Mr. Ndapona ;

Refer to what said by Mr. Jim, I would add the following :

During the designing step of a Network, we can estimate the Harmonics that will be by using the following methods :
- We should know the specifications of all " Non-linear Loads " like : Variable Speed Drives, UPS, Electronic Ballasts, Electrical Chargers, Electronic Switching Devices, Computers, Faxes, Printers, ext.
- By these specifications, we can calculate the current's value of each Harmonic.
- Depending on the circulation of the Harmonics' Currents in any branch or at the output of the Source's side, we calculate the total current's value of each Harmonic, that means " Y2, Y3, Y4 ..... Yn ".
- After calculation the total Currents value " the " Fundamental Component " " Y1 ", we can calculate the percentage of " THD " ( Total Harmonic Distortion ) by using the following formula :

THD of Current " % " = { 100 x ( Y2^2 + Y3^2 + Y4^2 + .... + Yn^2 )^ 1/2 } / Y1

where the " THD-I " helps us to define correctly the : Cables Sections, Powers of the Sources, and do some modifications in the designing to minimize the influence of those Harmonics.
By the way, we can calculate the Distortion of the Voltage " THD-U " after knowing the Harmonics' Currents, but it's not easy, because we should calculate the " Harmonic Impedance " of each section and for each Frequency.

If the Network is in service, we can know all by using an Analyzer measurements Apparatus that helps us to know : the Percentage of each Harmonic " % or A ", the Total harmonic Distortion of Current - THD-I ", the Total harmonic Distortion of Voltage - THD-U ".
08-30-2014 03:22 AM
Top #6
Lot Ndapona
08-30-2014 03:22 AM
Thank you very much Spir Georges GHALI, I wait to hear from others if they do have extra to say or other alternative methods. but yours is very much appreciated
08-30-2014 06:18 AM
Top #7
08-30-2014 06:18 AM
Dear Mr.Lot Ndapona,
You have not clarified what is the kind of the "power system" concerned that has high Harmonics.
A - If the Power System is a Power Plant:
Highest level of harmonics are generated in a power plant if Generator Excitation is of Electronic Solid State Type and input power to the Excitation Panel is provided from Unit Auxiliary Transformer Secondary Side i.e., in parallel with Unit Auxiliary Loads.
If one the other hand, even though the Generator Excitation is of Electronic Solid State Type but input power to the Excitation Panel is provided from Generator Bus Duct, the effect of harmonics becomes so less as to go largely unnoticed. However, this requires arrangement for initial "flashing" of the Generator Field.
if you can throw some light on the type of Excitation System that the concerned Power Plant has, perhaps it may enable more useful comments to be offered by all.

B - If the "Power System" pertains to an industry, please describe what constitutes the System.
Are there high power Rectifiers?
If you kindly describe this aspect, perhaps it may aid brain storming to better put up suggestions to you.
08-30-2014 09:10 AM
Top #8
Ashish Srivastava
08-30-2014 09:10 AM
I think Spir have given valuable inputs. We are measuring back rms current ripple while testing our inverter to see the effect of converter harmonics on source side and current and voltage THD on 3 phase output side through analyzer...generally a product quality is satisfactory if we are measuring upto 20th harmonic.
Now if see it analytically a recursive discrete fourier transform(RDFT) is one of the first method used for frequency and Harmonics detection. A no of algorithms developed and published in IEEE etc based on kalman filter,PLL,Genetic algorithm,wavelet transform, PQ theory,neural network etc..
A DFT(discrete fourier transform) based approach for real time estimation of fundamental frequency,fundamental amplitude,harmonic amplitude and phases for a noisy signal even with time varying amplitude and frequency is available in some IEEE paper (4-5 years back) which is I think related to wild frequency generators/Alternators harmonics detection while fundamental frequency is also varying..
You may need to read yourself or meet some knowledgeable professor in this area
Best of luck...
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