Mains Harmonics(ZM Mhlongo)

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University of KwaZulu±Natal

School of Electrical, Electronic and Computer Engineering

Mains Harmonics

ZM Mhlongo

208528095



Abstract

This report investigates the mains distortion caused by non linear loads. A rectifier connected to a

resistive load is used as the non linear load. After observations are made with regards to performance

various harmonic curbing techniques are investigated. Inductive coupling and 5th

harmonic filter are

employed and the results are discussed in terms of the effect each has on the system. Simulink

SimPowerSystems library is used to simulate those circuits. The Analysis is done using the PowerGui FFT

analysis tool. A comparison is made with the schemes given in five circuits and simulations divided,

correspondingly to five parts.



Introduction

Electric utilities supply power at sinusoidally varying voltages, the frequency of these sinusoids is

ussually 50 or 60Hz. In South Africa the standard is 50Hz. There are basically two types of loads, linear

and non linear. The linear loads do not present a problem since the current drawn by them is sinusoidal

as well. The problem comes when the loads connected to the power supply are non linear, these devices

draw a non linear current and thus cause a distortion in the input current.

The resulting waveform can be approximated using sinusoids via the Fourier analysis. The resulting

sinusoids start at the power system frequency and appear as interger multiples of the fundamental

frequency. The effect of these devices is not very desirable and can cause a decay in porformance of thepower grid.

Theory

Definition

Harmonics are a mathematical way of describing distortion to a voltage or current waveform. Fourier

analysis allows for the use of sinusoids to represent any waveform by a sum of sinusoids. The term

harmonic refers to a component of a waveform that occurs at an integer multiple of the fundamental

frequency. Figure 1 illustrates how harmonics affect the fundamental current.

Figure 1: Effects of harmonics on the fundamental frequency sinusoid.





After a process of observing the load current and adjusting the resistive load, the current was set to 1A

with some ripple. This was achieved when the resistor was set at 300 ohms. The waveform of the

current through the load is shown in figure 3.

Figure 3 : Load Current Waveform

The supply voltage waveform is shown in figure 4. Some harmonic distortion is present, although not

alarmingly large. The rectifier shows minimal disturbance in the supply voltage.



Figure 4 : Supply Voltage Waveform

The supply current waveform differs much from a sinusoid, indicating large harmonic distortion present

in the current due to current blocking in the diodes in the rectifier. This is shown in figure 5.

Figure 5 : Supply Current Waveform



The load voltage is displayed in figure 6, with a ripple but centered to approximately 300V.

Figure 6 : Load Voltage Waveform

The ripple in the voltage is calculated in this way



Figure 7 : FFt Analysis Tool for Part 1

Using the Power GUI analysis tool the results obtained were analyzed to give the following

Fundamental value of input current(rms)

Total harmonic distortion

Total input current(rms) =

3. Harmonic Numbers Present

Current : 5, 7, 11, 13, 17, 19

Voltage : 2, 4



4. & 5.

Figure 8 : Diode Voltage and Current Waveform

For the diode current




6. Diode rating

A safety factor of 1.5 was used and the values obtained from the graph of the diodes current and

voltage waveforms, together with the FFT tool for the current.

The PIV was found to be approximately 325V







Figure 11 : Current Waveforms for circuit two.

The inrush of current during starting causes the capacitor to add a proportinal voltage this causes a

transient a during the circuits startup. The voltage observed during startup peaks at an excess of 600V.

After the current reaches a stedy varying waveform, the voltage stabilizes with some ripple.



Figure 12 : Voltage Waveform Across the Load

This ripple is larger by 22.2% when compared to that of part one.



Figure 13: FFT Analysis Results.





Current : From 2 to 19 are present

Voltage : None

Virtually no harmonics can be are present in the voltage waveform, the current shows more harmonics

and the effect of 2 to 19, as shown by the FFT tool, is evident.



Figure 14 : Diode Voltage & Current plot





6. Diode rating

PIV rating = 1.5(PIV )= 1.5*(346) = 519V

Irms rating = 1.5*I = 1.5*1.036 = 1.554A

Conclusion.

The addition of the capacitor reduces the harmonics in the supply voltage. This can be seen as a goodresult. However, the total harmonic distortion in the current waveform increased from 66.72% to

216.74%. This coupled with the fact that the current waveforms appear different from each other leads

to the conclusion that the capacitor is detrimental to the system.



Part 3

Inductors were added in the circuit of part two to form the third circuit. The effect of linking the rectifier

and supply with inductors is investigated next. The model used is shown in figure

Figure 15 : Model for Part 3



The voltage outpu from the supply is smoother, with less harmonics affecting it. As evident in figure 16.

Figure 16 : Supply Voltage Against time



The addition of the inductors positively influenced the durrent waveform, less harmonic distortion is

observed. Although the harmonics were reduced, the problem of different pahse current magnitudes is

not solved by this addition.

Figure 17 : Supply Current After the Addition of Inductors



Figure 18 : Load Voltage waveform

Besides the increase in the magnitude of the transient, a lower ripple is observed as compared to parts 1

and 2. The ripple voltage is 41% lower than that of part one.



Figure 19: FFT Results





Current : 5, 7, 11, 13

Voltage : None



Figure 20 : Diode Voltage and Current waveforms





6. Diode rating

PIV rating = 1.5(PIV )= 1.5*(329) = 493.5V

Irms rating = 1.5*I = 1.5*1.036 = 0.924A

Conclusion

The total harmonic distortion has decreased to 66.72%, from the 216.74% when there was no inductivelink. The diodes use will require a lower rating since the PIV is lower and the rms Input current as well.

The inductive link can be regarded as a good addition since it improved all aspects of the circuits.



Part 4

Part four investigates the effect of inductively linking the parallel RC load with the rectifier. The effects if

this is shown on the supply voltage and current as well as the load.

Figure 21 : Model Used to simulate Circuit Four

Figure 22 : Supply Voltage Plot Against Time



The supply voltage deteriorated with this new configuration. There is more distortion, although close

resemblance to a sinusoid is evident. The method used earlier, in circuit three is better.

Figure 23 : Supply Current Plot



Figure 24 : DC Output Voltage







Current : 3, 5, 7

Voltage : None







6. Diode rating

PIV rating = 1.5(PIV )= 1.5*(325) = 487.5V

Irms rating = 1.5*I = 1.5*0.578 = 0.867A

Conclusion

Although this scheme causes some distortion in the supply voltage waveform, the distortion is quite

small. The main benefit that differs from the previous scheme is that the current waveforms are the

same and the distortion is less (56.52% compared to 68.2%). The ripple voltage is the same

Part 5





The DC ripple voltage is varies and its maximum is :



While its minimum is





Current : 5

Voltage : 5







6. Diode rating

PIV rating = 1.5(PIV)= 1.5*(355) = 532.5V

Irms rating = 1.5*I = 1.5*1.206 = 1.8087A

Conclusion

The filter increases the distortion of the voltage and current. From the FFT analysis it can be seen that

the 5th

order harmonic which is supposed to be filtered, is the more prominent one compared to the

other harmonics. This might be a result of the limited elements available for its design. The input current

shows an increase also. The ratings of the diode have to be increased as well if the filter is in use

campared to the other schemes



Overall Conclusion

Harmonics are detrimental to the power system and the equipment adjacent to the device causing

them. Various schemes to minimize this were investigated, inductive coupling of the load, inductive

coupling of the source with the rectifier and the fifth order filter were simulated and analyzed in this

practical. Some results were rather difficult to explain. The differences between these schemes were

observed in the simulations. From the simulation, linking the rectifier and the supply with inductors

proved most effective albeit the different current waveforms for the different phases.

In an application where the current waveform similarity is crucial, the method of inductively linking the

load is prudent. The filter, although thought to be the best manner to handle harmonics, proved

ineffective and actually detrimental.



References

1.Ellis, R., 2001. Power System Harmonics. [Online].

http://samplecode.rockwellautomation.com/idc/groups/literature/documents/wp/mvb-wp011_-en-p.pdf [01 June

2011]

2. http://www.joliettech.com/abb_guide-to-harmonics-with-ac-drives.htm [01 June 2011]

3. Kreide, P., Power Quality [Online] http://www.docstoc.com/docs/21314824/REDUCTION-OF-LINE-

LOSSES-VOLTAGE-STABILIZATION-POWER-FACTOR [01 June 2011]

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Mains Harmonics(ZM Mhlongo)