**8.4 Measurements and results**

The measurement of the harmonics caused by PC's and any other nonlinear elements were conducted at each bus (points 1, 2, and 3) as indicated in the single line diagram.

At the first point shown in the single line diagram (Figure 4), three measurements have been conducted over three periods. The THD during the first period at point one was measured and the maximum total harmonic distortion (THD) was 10.6%. The THD during the second period at point one and the maximum total harmonic distortion (THD) was 10.7%. The THD during the third period at point one and the maximum total harmonic distortion (THD) is 10.8%

From the previous three readings it is clear that at normal load at point one the maximum total harmonic distortion of the three readings is 10.8%. Moreover, the harmonics effect appears in the current waveform as in Figure 5.

For the second point the THD during the first period indicated a maximum total harmonic distortion (THD) was 29%.

On the second measuring period the THD during the second period at point one and the maximum total harmonic distortion (THD) was 28.4%. The THD during the third time period at point two indicated maximum total harmonic distortion (THD) of 28.9%. From the previous three readings it is clear that at normal load at point two increases the harmonic flow, the maximum total harmonic distortion of the three readings is 28.9%. Moreover, the harmonics appears to distort the phase current waveform as in Figure 6.

Harmonics Effect in Industrial and University Environments 225

To avoid the injection of harmonics into the system, a harmonic filter must be designed to eliminate the highest harmonics order flowing and then installed at the near bus of such produced harmonic. It is noticed in table 6 that the highest THD measured was during the period where students were using laboratories of Building 14 at point 2 with the highest harmonic is of fifth order. While table 7 illustrates harmonics produced as to the total

Fig. 6. Current wave forms at point two

numbers of Pc's used.


Fig. 5. Current wave forms at point one

The sum of all these THD's = 83.8% is not equal the THD at point one which is 10.8%.The difference between these two values explains which is called harmonic cancellations. The phase angles of the magnetization current harmonics oppose the phase angles of the load current harmonics and such lead to harmonic cancellations. The maximum total harmonic distortion at point three is 20.6% which is less than that at point two (29%).The difference between the two values is caused also by harmonics cancellation. According to the instructions provided with the power quality analyzer manual and other standards which state that if the current THD is less than 20% the harmonic distortion is probably acceptable, the total harmonic distortion at point three (29%) was greater than 20%, so it is not acceptable and definitely will make adverse affect on the neutral line cable.

The sum of all these THD's = 83.8% is not equal the THD at point one which is 10.8%.The difference between these two values explains which is called harmonic cancellations. The phase angles of the magnetization current harmonics oppose the phase angles of the load current harmonics and such lead to harmonic cancellations. The maximum total harmonic distortion at point three is 20.6% which is less than that at point two (29%).The difference between the two values is caused also by harmonics cancellation. According to the instructions provided with the power quality analyzer manual and other standards which state that if the current THD is less than 20% the harmonic distortion is probably acceptable, the total harmonic distortion at point three (29%) was greater than 20%, so it is not

acceptable and definitely will make adverse affect on the neutral line cable.

Fig. 5. Current wave forms at point one

Fig. 6. Current wave forms at point two

To avoid the injection of harmonics into the system, a harmonic filter must be designed to eliminate the highest harmonics order flowing and then installed at the near bus of such produced harmonic. It is noticed in table 6 that the highest THD measured was during the period where students were using laboratories of Building 14 at point 2 with the highest harmonic is of fifth order. While table 7 illustrates harmonics produced as to the total numbers of Pc's used.

Harmonics Effect in Industrial and University Environments 227

Where, Irms, n is the amplitude of the harmonic component of order n (i.e., the nth harmonic). The numerator gives the RMS current due to all harmonics and I1 is the RMS value of fundamental component of current only. Given above is the mathematical form of the IEEE.

2

*Irms n*

,

(9.2)

(9.3)

(9.4)

= 9.36% (9.5)

2 1

*n* 1 *Irms* 

Where Irms, n is the amplitude of the harmonic component of order n (i.e., the nth harmonic) and Irms is the rms value of all the harmonics plus the fundamental component of the current. The later standard is referred in this study, because the apparatus used for analysis

The 3rd, 5th, 7th and 9th harmonics being the most significant, the definition of THD may be

The value of THD may be calculated for any number of computers using formula (9.3).

2 2 22 25,6 12,5 4,2 1,8 308,4

Figure 9 is showing the magnitude of individual harmonics, when 263 PCs in building 14

<sup>2222</sup> *Irms Irms Irms Irms* ,3 ,5 ,7 ,9 *Irms*

*I*

*n*

 

According to IEC standards, the mathematical form of THD is given below:

Irms = <sup>2</sup>

THD =

THD =

Fig. 7. Harmonic spectrums at point one when 263 PC's operating

RMS magnitude of 3rd Harmonic= 25.6 A RMS magnitude of 5th Harmonic= 12.5 A RMS magnitude of 7th Harmonic= 4.2 A RMS magnitude of 9th Harmonic= 1.8 A

were connected to the supply mains.

was based on IEC Standards.

modified and written as in 9.4

Irms = 308.4 A

Where

THD =


Table 6. Summary of the obtained results for the three points of Bldg. 14


Table 7. Harmonic magnitudes for different No. of PC's at point 1
