**10. Thd measurements discussion**

According to the previous measurements it has been observed that the total harmonic distortion at point two (29 %) is much greater than that at point one (10.8%). Since there is no load connected between these two points except the Uninterruptible power supply (UPS), it is considered that UPS is the main reason for this difference. The UPS can be considered to fit 'in-line' between the loads and the mains power supply. In addition to providing power protection to the loads, it should also protect the main power supply itself from getting any harmonics generated by the loads themselves. However, it is again not commonly known that UPS and their design being power electronics oriented, also generate harmonic pollution. For any UPS this is typically stated as Total Harmonic Distortion (THD). Care has to be taken when comparing different THD values as these can differ when contrasting the two different types of on-line UPS (transformer-based and transformer less) and also with regard to the percentage of load applied for each measurement.


Table 8. Magnitudes of harmonics for different numbers of PC's at point 3

Table 8 indicates the online value of THD is 19.7%. The difference of the calculated and experimental value of 0.37% as shown in table 9. This difference caused again by other odd harmonics being neglected, however, such low error proves the validity of measurement and it consequently plays a pivotal role for the accurate analysis of the odd harmonics.


Table 9. Comparison of calculated and experimental values when 263 PC'S were connected

Harmonics Effect in Industrial and University Environments 231

A series of tests personal computers in some buildings at King Fahd University of Petroleum and Minerals have been investigated in order to study the influence of these computers on the line current harmonics. The following conclusions can be drawn from the

The switch mode power supply (SMPS) used in personal computers draws a non linear current that is rush in harmonics currents. A high density of (SMPS) loads results in over loading of the neutral conductor and the overheating of the distribution transformers. The assessment of odd harmonics in current significant in magnitudes are represented by mathematical modeling a proved theoretically the decrease in THD in current at some points when increasing the number of PC's connected to these points. On the other hand, THD increased with increase the number of PC's on the other points of these buildings. According to this study the maximum THD found was 29% in the main student lab in building 14 and it was unstable and the minimum THD was found 1.1% in building 58. According to the instructions provided with the power quality analyzer Fluke 43 B manual which state that if the current THD is less than 20% the harmonic distortion is probably acceptable, the total harmonic distortion at point three of building 14 (29%) is greater than 20% is not acceptable and makes affect on the neutral line cable. To avoid the injection of

Due to the highly non sinusoidal nature of the input current waveform of personal computer, the high amplitude of harmonics currents are generated. These harmonics currents are of odd order because of half wave symmetry of the input current waveform. The magnitudes of the harmonics currents up to the seventh harmonics are

The phase angle of the harmonics currents of the input currents of different PC's vary to cause significant current cancelation. There are some cancelations in the higher order

The UPS (Uninterruptable power supply) in building 14 can be considered to fit 'in-line' between the loads and the mains power supply. In addition, to providing power protection to the loads, it should also protect the mains power supply itself from any harmonics generated by the loads themselves. However, it is again not commonly known that UPS themselves, by the way of their design, also generate harmonic pollution. For any UPS this is typically stated as Total Harmonic Distortion (THD). The care are has to be taken when comparing different THD values as these can differ when contrasting the two different types of on-line UPS (transformer-based and transformer less) and also with regard to the

Within a UPS it is the rectifier that connects to the mains power supply and converts the mains alternating current (ac) into the levels of direct current (dc) required to power the

For transformer-based UPS, rectifiers are typically six or twelve-pulse, dependent upon the thyristor number and configuration. A six-pulse rectifier at full load will typically generate a THD of around 29% and a 12-pulse around 8%. To reduce these values further a passive harmonic filter can be installed alongside the UPS. The obvious disadvantages of this approach being increased capital cost, wiring, installation, loss of efficiency and increased footprint. Harmonic filters can be added post-installation but further installation costs and

harmonics into the system, a harmonic filter must be installed.

percentage of load applied for each measurement.

inverter and charge the battery.

downtime need to be planned for.

results of this study.

significant.

harmonics.

Within a UPS it is rectifier that connects to the mains power supply and converts the mains alternating current (AC) into the levels of direct current (DC) required to power the inverter and charge the battery.

For transformer-based UPS, rectifiers are typically six or twelve-pulse, dependent upon the thyristor number and configuration. A six-pulse rectifier at full load will typically generate a THD of around 29% and a 12-pulse around 8%. To reduce these values further a passive harmonic filter can be installed alongside the UPS. The obvious disadvantages of this approach being increased capital cost, wiring, installation, loss of efficiency and increased footprint. Harmonic filters can be added post-installation but further installation costs and downtime need to be planned for.

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.
