**7. Power quality issues related to computers**

Although harmonics is the subject of several international standards [16], the difficulties of regulating harmonics emissions from small, individual devices employing switched mode technology, such as a personal computer, is compounded by the fact that the harmonic generation capabilities of these devices varies according to the number of parallel connected devices.

In the last decade, power quality has become a major issue for electric utilities and their customers, and indirectly to almost all manufacturers of equipment that depend on sinusoidal supply voltage waveforms. The proliferation of solid-state switching devices in both industry and home has increased the harmonic levels found on electricity supply systems. In the past, sources of harmonics were limited and in most cases tolerable. However today, virtually all high performance electronic equipment uses switched mode power converters, conducting current for part of a cycle through multiple paths in order to efficiently convert alternating current to direct current. In response to this increased demand some countries encouraged the development of more efficient ways of getting full benefits from electrical energy by various switching methods.

On the other hand a new technique of manufacturing circuits called integrated electronic circuits have been invented and led directly to increasingly complex systems such as in industry ,communications, domestic appliances … etc because its size, weight and cost. These circuits, although increased speed of operations and complexity of tasks, typically use a technique called switch mode power supplies [26- 29].

The majority of modern electronic units use switched mode power supplies (SMPS). The advantage–to the equipment manufacturer– is the size, cost and weight is significantly reduced and the power unit can be made in almost any required factor [28]. But unfortunately switch mode power supplies act as non–liner loads or they can be considered as a noise generators which get down the quality of the electricity supply.

Equipments with switch mode circuits are usually more sensitive to main variations and disturbances than liner loads [30]. For this reason power quality has become a major issue for electric utilities and their costumers and almost all manufactures of equipment that despond on sinusoidal supply voltage waveforms. The increasing of solid state switch devices such as Personal computers in both industry and homes will draw rather than continuous current from the supply; the power supply unit will draw pluses of current which contain large amount of harmonics [29, 32].

The switching mode power supplies used in personal computers are major sources of harmonic currents. An experiment [26, 28, and 32] has been done for different types of computers to measure the harmonic generated by each type and the result was scheduled as:

Harmonics Effect in Industrial and University Environments 221

on large power converters as sources of harmonics due to the high magnitude of the currents evolved. Nevertheless, harmonics generated by low power converters become significant when large numbers of converters are used simultaneously [14, 15]. Indubitably the personal computer is an ideal place for these converters. The power supplies employed in most personal computers are of the switching mode type. In switching mode power supply, the 60 Hz AC voltage is converted to into DC through a single phase rectifier and the output voltage of the rectifier is stepped down using AC to DC converter [13, 14]. The main advantage of the switching mode power supply over the traditional linear power supply is its high energy efficiency since the switching elements used (BJT's, MOSFETS) are

Personal computers impacts on power quality are noticeable due to the common place usage of switched mode power supplies (SMPS) for supplying processing electronics. Such power supplies are responsible for the generation of odd line current harmonics. However, in turn harmonically polluted line currents can distort supply voltages causing power quality problems for other consumers connected at a point of common coupling (PCC). Switched mode power supplies are by no means restricted to PCs and can be found in a variety of other widely used electrical equipment including low energy lighting, battery chargers, televisions and their peripherals. For power quality considerations, the PC is the most

On an individual basis, it represents one of the larger loads consuming typically 200-300

The last point is one of the major concerns of increasing PC usage and has resulted in power quality studies of the cumulative effects of many PCs connected to the same bus bar [27]. A recent study has shown that the line current harmonics from a single PC differed considerably to the harmonics generated collectively by several PCs of the same type. One widely held

Attenuation describes the reduction in harmonic magnitude, and change in phase angle, as load connected to a SMPS increases due to the change in spectrum of the line current pulse which widens to allow more power flow through the SMPS. Attenuation is also observed where several identical loads share the same impedance. Diversity describes a similar effect where a reduction or even cancellation of harmonics is possible due to loads of different levels, or connected through different impedances, presenting differing phase angles to the

The University consists consists of 60 buildings including students housing. Almost each student at the university has a laptop regardless of the number of PC's in each, laboratories, or rooms. The university in-house that there are around 6000 PCs distributed unequally . Most of the PS's in the university are of model HP, DELL, Compaq dc 7800P; they use Intel

theory regarding this effect introduces the concepts of attenuation and diversity [25].

supply. These findings have not been proved using large scale studies [25-28].

This investigation adapted the following steps which include: Obtaining all line diagrams of the area of investigation.

either completely off or completely on.

W per unit.

**8.1 Introduction** 

core 2 Dual processors.

**8.2 Measurement plan** 

significant item for a variety of reasons including.

The current drawn is affected by user behavior.

**8. Study proceedures and results** 

Large numbers of PCs can be connected to a single supply point.


Table 4. Magnitudes of harmonic currents of various PC's in mA [32]

The phase current is given by:

$$I\_P = \sqrt{I\_1^2} + I\_3^2 + I\_5^2 + I\_7^2 + I\_9^2 + I\_{11}^2 + I\_{13}^2 + I\_{15}^2 \tag{6.1}$$

The IBM XT™ has the lowest percent third harmonic current (74%). Since the computer is connected line-to-neutral in a 3-phase system, the neutral current is approximately equal to three times the vector sum of the third and ninth harmonic currents flowing in each phase.

$$I\_n = 3\sqrt{I\_3^2} + I\_9^2 = 367\,764\,mA\tag{6.2}$$

$$I\_P = 219\,\text{J}\,\text{226}\,mA \tag{6.3}$$

$$\frac{I\_n}{I\_P} = 1,677\tag{6.4}$$

The Mac PlusTM offers the worst case third harmonic current (87%). In a similar manner In & Ip are obtained.

$$I\_n = 223\,\text{J}\,\text{377}\tag{6.5}$$

$$I\_P = 130 \,\text{507} mA \tag{6.6}$$

$$\frac{I\_n}{I\_p} = 1,712\tag{6.7}$$

With a large number of personal computer loads, the neutral current is expected to be 1.7 times the phase current. It will certainly overload the neutral conductor that is designed to handle lower currents than the phase currents [14]. Non linear loads connected to a sinusoidal source inject harmonic currents into the source. Much attention has been focused

165.93 (100%)

122.45 (73.8%)

68.03 (41.0%)

26.71 (16.1%)

5.64 (3.4%)

3.98 (2 .4%)

8.46 (5 .1%)

8.29 (5. 0%)

Table 4. Magnitudes of harmonic currents of various PC's in mA [32]

I1

I3

I5

I7

I9

I11

I13

I15

Ip are obtained.

106.82 (100%)

89.31 (83.6%).

65.18 (61.0%)

40.51 (37 .9%)

21.25 (19.9%)

8.70 (8.1%)

2.95 (2 .8%)

3.46 (3.2%)

The phase current is given by:

**Mac IIsi IBM XT IBM PS/30 Mac Plus IBM PS/70** 

83.35 (100%)

72.30 (86.7%)

55.80 (66.9%)

36.53 (43.8%)

17.78 (21.3%)

3.31 (4.0%)

4.95 (5. 9%)

7.51 (9.0%)

541.88 (100%)

458.83 (84.7%)

333.54 (61.5%)

192.04 (35.4%)

71.18 (13.1%)

17.75 (3 .3%)

49.02 (9.0%)

48.01 (8.8%)

199.02 (100%)

152.63 (76.7%)

94.76 (47. 6%)

39.16 (19.7%)

10.39 (5.2%)

3.53 (1.8%)

1.49 (0. 7%)

2.30 (1.1%)

*<sup>P</sup>* 1 3 5 7 9 11 13 15 *I IIIIII I I* (6.1)

3 9 3 367,764 *nI II mA* (6.2)

219,226 *PI m A* (6.3)

*<sup>I</sup>* (6.4)

223,377 *nI* (6.5)

130,507 *PI m A* (6.6)

*<sup>I</sup>* (6.7)

222222 2 2

The IBM XT™ has the lowest percent third harmonic current (74%). Since the computer is connected line-to-neutral in a 3-phase system, the neutral current is approximately equal to three times the vector sum of the third and ninth harmonic currents flowing in each phase.

1,677 *<sup>n</sup>*

The Mac PlusTM offers the worst case third harmonic current (87%). In a similar manner In &

1,712 *<sup>n</sup>*

With a large number of personal computer loads, the neutral current is expected to be 1.7 times the phase current. It will certainly overload the neutral conductor that is designed to handle lower currents than the phase currents [14]. Non linear loads connected to a sinusoidal source inject harmonic currents into the source. Much attention has been focused

*p I*

2 2

*P I*

on large power converters as sources of harmonics due to the high magnitude of the currents evolved. Nevertheless, harmonics generated by low power converters become significant when large numbers of converters are used simultaneously [14, 15]. Indubitably the personal computer is an ideal place for these converters. The power supplies employed in most personal computers are of the switching mode type. In switching mode power supply, the 60 Hz AC voltage is converted to into DC through a single phase rectifier and the output voltage of the rectifier is stepped down using AC to DC converter [13, 14]. The main advantage of the switching mode power supply over the traditional linear power supply is its high energy efficiency since the switching elements used (BJT's, MOSFETS) are either completely off or completely on.

Personal computers impacts on power quality are noticeable due to the common place usage of switched mode power supplies (SMPS) for supplying processing electronics. Such power supplies are responsible for the generation of odd line current harmonics. However, in turn harmonically polluted line currents can distort supply voltages causing power quality problems for other consumers connected at a point of common coupling (PCC). Switched mode power supplies are by no means restricted to PCs and can be found in a variety of other widely used electrical equipment including low energy lighting, battery chargers, televisions and their peripherals. For power quality considerations, the PC is the most significant item for a variety of reasons including.


The last point is one of the major concerns of increasing PC usage and has resulted in power quality studies of the cumulative effects of many PCs connected to the same bus bar [27]. A recent study has shown that the line current harmonics from a single PC differed considerably to the harmonics generated collectively by several PCs of the same type. One widely held theory regarding this effect introduces the concepts of attenuation and diversity [25].

Attenuation describes the reduction in harmonic magnitude, and change in phase angle, as load connected to a SMPS increases due to the change in spectrum of the line current pulse which widens to allow more power flow through the SMPS. Attenuation is also observed where several identical loads share the same impedance. Diversity describes a similar effect where a reduction or even cancellation of harmonics is possible due to loads of different levels, or connected through different impedances, presenting differing phase angles to the supply. These findings have not been proved using large scale studies [25-28].
