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Transcript
INSTALLATIONS & CONTRACTING In view of the present Eskom power supply shortage and the change from filament lamps to compact fluorescent lamps (CFLs), it was decided to study the electric current behaviour of such lamps and other equipment used in the home. An electrically isolated digital oscilloscope was used for this purpose. Current wave shape of equipment in the home The resulting figures all use the same time scale of 2,5 ms per division, giving a total time span of 25 ms. This shows slightly more than one cycle of the 50 Hz mains supply, which has a duration 20 ms. For the 10 horizontal divisions, the change from the negative half to the positive half occurs at the end of the first division from the left, as shown in Fig. 1a. The positive maximum occurs at the end of division 3 with the change from positive to negative at the end of division 5 (the centre line). The negative maximum occurs at the end of division 7 with the change from negative to positive at the end of division 9 (similar to the end of division 1.) Lamps The larger trace in Fig. 1a is the mains voltage signal with a vertical by J J Kritzinger scale of 100 V per division. The maximum positive or negative value of 320 V corresponds to the normal mains supply of 226 V RMS. The smaller signal is proportional to the current drawn by a nominal 60 W incandescent filament lamp. It is measured as the voltage across a 0,5 Ω resistor in series with the neutral line. Thus the vertical scale of 100 mV per division represents a current scale of 200 mA per division. The positive or negative peak value of 400 mA gives an RMS value of 282 mA. When this is multiplied with the 226 V RMS the power is given as 64 W. The current signal for this Fig. 1a: Voltage: 100 V/div. with 60 W ref. current: 200 mA/div. 60 W filament lamp is used for amplitude and time reference of Figs. 1b to 1g and Figs. 2a to 2d. Figs. 1b, 1c and 1d show the current wave shape for 11 W, 14 W and 22 W nominal power CFLs respectively. The shape is similar in all three cases with the amplitude increasing with power rating. It starts with a fast rise to maximum approximately 2,5 ms prior to the maximum current point of the filament lamp. Within the following 2 ms it decreases to about quarter of the maximum as it passes the maximum filament current point, to return to zero 1,5 ms later. The initial peak for the 14 and 22 W CFLs is comparable in Fig. 1b: 11 W CFL with 60 W ref. current: 200 mA/div. March 2009 - Vector - Page 54 Fig. 1c: 14 W CFL with 60 W ref. current: 200 mA/div. Fig. 1d: 22 W CFL with 60 W ref. current: 200 mA/div. Fig. 1e: 22 W CFL switching noise. Note the different time scale for the top trace. Voltage: 2 V/div. Fig. 1f: 36 W 1,2 m fluorescent tube with modern electronic control and 60 W ref. current: 200 mA/div. Fig. 1g: 36 W 1,2 m fluorescent tube with old inductive ballast and 60 W ref. current: 200 mA/div. amplitude to the maximum of the 60 W filament lamp and obviously does not coincide with the supply voltage peak. It is at least three times larger than that for a 15 W filament lamp. Timing is similar for both positive and negative parts of a cycle and for all the tested CFLs from different manufacturers. The implication of this will be discussed in the section for combined equipment. Figs. 1b and 1c show the average value of 16 cycles but 1d is a single record to show the higher frequency noise current produced by the switching supply operation of the CFL. The result in Fig. 1e is obtained when the oscilloscope probe is placed next to the base of the CFL lamp. The top trace is at a much shorter time scale of 25 μs per division, to indicate the 35 kHz switching frequency. The bottom trace is at the standard 2,5 ms per division reference, showing that the switching operation is continuous but with higher intensity when drawing current from the mains. Fig. 1f shows the current of a1,2 m single 36 W fluorescent tube fitting with modern electronic control. The current wave March 2009 - Vector - Page 55 Fig. 2a: 1,4 GHz Celeron laptop with 60 W ref. current: 1A/div. Fig. 2b: Pentium 3 desktop PC, with and without CRT monitor with 60 W ref. current: 1 A/div. Fig. 2c: 51 cm CRT television with 60 W ref. current: 1 A/div. Fig. 2d: 93 cm LCD television, DSTV with 60 W ref. current: 1A/div. shape is fairly symmetrical with that of the filament lamp but of somewhat shorter duration and a faster rise at the start. The current peak is substantially less than that of the 60 W lamp while the light output is much more than from the filament lamp. In Fig. 1g the current of a 1,2 m single 36 W fluorescent tube fitting, with a very old inductive ballast, shows a very high peak current in comparison. The peak occurs almost at the time of minimum voltage, thus indicating a very poor power factor, but with no rapid current changes. Electronic equipment In this series of Figs, 2a to 2d, the current sensitivity is 1 A per vertical division and the 60 W filament lamp is used for comparison in each case. Fig. 2a shows the current wave shape of a Celeron 1,4 GHz laptop computer. The current is peaked with a width of approximately 2,5 ms but fairly symmetrical with respect to the supply voltage peak. Fig. 2b indicates conditions for a 533 MHz Pentium 3 desktop computer, with the 43 cm (17 inch) CRT monitor on and switched off. The current width and time of occurrence is similar to that of the laptop but with double the peak value. Fig. 2c, for an old 51 cm CRT television is similar when compared to the CRT monitor of the desktop computer. Fig. 2d is for a modern 93 cm LCD television. The shape is a reasonable approximation to a sine wave and occurs in phase with the supply voltage. The third trace, showing a small peak similar to Figs. 2a to 2c, is for a DSTV decoder. The peak value is similar to the peak value of the 60 W filament lamp. This is representative of other equipment such as a VCR, DVD or audio player. In the kitchen In this series, Figs. 3a to 3c, the current sensitivity is approximately 2,5 A per vertical division and a 700 W resistive load (a coffee machine) is used for comparison. These results were taken with a current transformer in the mains supply at the distribution board. Fig. 3a is for a refrigerator in comparison with the 700 W load. It does not have fast changes but is symmetrically later in time than the mains voltage peak by about 2 ms. This is owing to the inductance of the electric motor. March 2009 - Vector - Page 56 Fig. 3a: Refrigerator with 700 W ref. current: 2,5 A/div. Fig. 3b is for a microwave oven with rated output of 500 W. The graph is fairly smooth but with symmetry slightly earlier than the mains voltage peak. This is caused by the capacitor of the voltage doubling circuit, used to generate the 4 kV required by the magnetron. Combined equipment When different pieces of equipment are combined interesting results are obtained. As a starting point the current of two desktop computers is shown in Fig. 4a with the 700 W resistive load as comparison at current sensitivity of 2,5 A per division. Fig. 3b: Microwave oven with 700 W ref. current: 2,5 A/div. When 12 CFLs are switched on in addition, Fig. 4b is the result. This indicates that the two peaks are adjacent, with the overlap increasing the current of the two computers by only 10% - 15%. If the currents were coincident the peak value would be almost double. For Figs. 4c to 4e a current scale of approximately 5 A/div is used with a reference resistive load of 1800 W of an electric kettle. Fig. 4c shows the current for a 12 000 BTU air conditioner compared to that of the kettle. The slight variation from sinusoidal shape comes from the standby currents of March 2009 - Vector - Page 57 Fig. 4a: Two desktop PCs with 700 W ref. current: 2,5 A/div. Fig. 4b: Two desktop PCs and 12 CFLs with 700 W ref. current: 2,5 A/div. the electronic equipment. The current symmetry of the airconditioner lags that of the kettle by approximately 1 ms, caused by the motor inductance (similar to that for the refrigerator.) Fig. 4d shows the result when the conditions of 4b is added to 4c. It shows clearly that the current of the CFLs brings the resulting wave shape closer to that of the ideal, represented by that of the kettle. It thus performs a kind of time multiplexing and compensates somewhat for the current lag of the air conditioner. Fig. 4c: Air conditioner with 1800 W ref. current: 5 A/div. Fig. 4d: Air conditioner and two desktop PCs and then with 12 CFLs added, in comparison with the 1800 W ref. March 2009 - Vector - Page 58 Fig. 4e: Refrigerator, two desktop PCs and 12 CFLs and then with a 2500 W resistive load added, in comparison with the 1800 W ref. When the current of a large resistive load of 2500 W is added to the currents of a refrigerator, two laptop computers and 12 CFLs, Fig. 4e results. The addition of the currents can be seen, but the deviation from the ideal sine wave is not as significant as it is at lower resistive loads. Resistive loads of stoves, kettles, dishwashers and hot water cylinders are usually in the range of 2000 to 3000 W. In conclusion it appears that, until the peaked current of electronic equipment can be made more sinusoidal, the early peaked current of the CFLs perform a useful function to compensate for the lagging symmetry of electric motors, without significantly adding to the current peak. A penalty is the higher frequency harmonics introduced. If solar hot water systems become common, large resistive loads will diminish in residential areas so that the peaked current of electronic equipment may become significant at substations. It will be interesting to monitor the current wave shape at residential substations and to consider what influence this will have on fast acting current protection. Contact Johan Kritzinger, Tel 021 852-3592, [email protected] March 2009 - Vector - Page 59