Electrical Power Quality Experiments

Transcription

1 Session 3233 Electrical Power Quality Experiments Timothy L. Skvarenina Purdue University, West Lafayette, IN Abstract This paper describes several experiments that can be performed with a second generation power harmonics analyzer, such as the Fluke 43, or with an oscilloscope. Two of the experiments described herein involve transient phenomena, while the third demonstrates the capabilities of inexpensive UPS devices. Introduction Power quality problems may arise due to a variety of phenomena that occur on time scales ranging from less than a millisecond to steady-state. Each of these phenomenon cause different effects in the power system and require different solutions to avoid problems. A previous paper 1 demonstrated several power quality experiments that could be performed with a first generation power harmonics analyzer, such as the Fluke 41. Those experiments concentrated primarily on harmonics. This paper describes several additional experiments that can be performed with a second generation power harmonics analyzer, such as the Fluke 43, or with an oscilloscope. Two of the experiments described herein involve transient phenomena, while the third demonstrates the capabilities of inexpensive UPS devices. The emphasis on these experiments was to keep the cost relatively low and to provide the student with a good demonstration of the applicable phenomena. The paper will show typical lab setups, procedures, and results. Induction motor starting current and system voltage drop The first experiment demonstrates the starting current and system voltage drop of a three-phase induction motor. Figure 1 illustrates the set-up for this experiment. Two power sources are provided for the induction motor. The first is the three-phase supply from the bench (stiff source), while the second places a three-phase transformer between the bench supply and the motor (weak source). The stiff source is rated substantially higher than the motor, while the weak source is rated only slightly higher than the motor. By using the transformer, the effect of starting a large motor in a plant is obtained. The first part of this experiment is the observation of the starting current of the motor. It is well known that induction motors draw six to eight times rated current when starting 2. Furthermore, the duration of the starting current is affected both by the load on the motor and by the voltage that is applied to the motor. The load for the motor consisted of a DC generator that could be connected to the shaft of the motor as shown in Figure 1. A resistive load allows the generator load to be varied. The generator and resistive load were set to provide somewhat above rated motor load at normal operating speed. Of course the output of the generator is a function of Page

2 Figure 1: Induction motor experiment setup speed so the load was variable, increasing as the motor accelerated. Using a power quality meter, the student observes the starting current magnitude as a function of time, and by changing the load on the machine, the student observes how long the motor takes to start. Figures 2 through 5 illustrate how the starting current varies with load and source rating. Figure 2: Starting current for unloaded induction motor with stiff source Page

3 As shown in Figure 2, the motor starts very quickly when started with no load from a stiff voltage source. The results clearly show that the initial motor current is on the order of six times the steady-state current. When a load was added to the motor, as shown in Figure 3, two phenomena are observed. First, the motor takes longer to start and second, the steady-state current is somewhat higher than in Figure 2. Figure 3: Starting current for loaded induction motor with stiff source The data shown in these figures was obtained using a Fluke 43 meter, which has software to download the waveforms to a computer. The information can be displayed using the provided software and can be copied as a bitmap image into other programs or the data points can be copied to a spreadsheet for plotting and analysis. The curves shown herein were obtained by copying the data points to a spreadsheet as the bitmap images were not very readable. In addition, it was found that the downloaded waveform actually shows boundaries for the variables being displayed; i.e., it shows upper and lower limits. Thus, the curves in Figures 2 and 3 (and others that follow) consist of two series. This causes the beating effect displayed in the waveforms and also results in two lines (above and below the x-axis) prior to time zero in Figure 3. Figures 2 and 3 showed the motor starting current with a stiff source. This means the impedance of the source is low so the voltage doesn t drop significantly due to the starting current of the motor. To investigate the effects of a weak source, a 750 VA, three-phase transformer was placed between the source and the motor. Since the motor requires about 600 VA, this simulates starting a very large motor in a plant. The results with the weak source are shown in Figures 4 and 5. Page

4 Figure 4: Starting current for unloaded induction motor with weak source Figure 5: Starting current for loaded induction motor with weak source Page

5 Comparing Figure 4 to Figure 2 shows that the starting current lasts slightly longer when the motor is started with a weak source. Similarly, comparing Figure 5 to Figure 3 shows a significantly longer time for the starting current and a higher steady-state current. The ability of the Fluke 43 to display the starting current reinforces the student s theoretical knowledge from the text and helps the student to understand the effects of loads and source impedance on the starting characteristics of the motor. To further understand the reasons for the differences between Figures 2 and 4 and Figures 3 and 5, the supply voltage can be observed during operation of the motor. The Fluke F43 allows plotting of RMS voltages and currents for extended periods of time. For this study, the meter was started and then the motor was started. Figure 6 shows the results for an unloaded motor with a stiff source. When the motor is started there is an immediate drop from the normal 208 volts (line to line) down to 202 volts. Similarly the current at the bottom of the figure shows a transient. Once the motor starts, the source voltage is only slightly affected by the motor with a decrease well below 5%. The results of Figure 6 were copied to a spreadsheet and plotted, as shown in Figure 7. There it can be seen that the voltage supplied to the motor only drops about 1 volt and that, in fact, the random fluctuations in the source voltage are almost as large as the change due to the motor running. Starting the motor with a load yields similar results, although the decrease in voltage to the motor is slightly larger. For comparison, the motor was started and operated with the previously described load from the weak source. The results are shown in Figure 8. Figure 6: Fluke F43 display of RMS voltage and current during motor operation (no-load and stiff source) Page

6 Figure 7: Results of Figure 6 as plotted in a spreadsheet Figure 8: Fluke F43 display of RMS voltage and current during motor operation (full load and weak source) Page

7 Figure 8, like Figure 6, shows the meter display of the RMS voltage and current. In this case, the source impedance is high, due to the transformer. As a result, the voltage dips to 180 volts when the motor is started. Since the motor receives a much lower voltage, the starting current persists for a longer period of time, as was observed in Figure 5. Figure 9 shows the spreadsheet version of the results of Figure 8. In Figure 9, it can be observed that the steady-state motor voltage declines to about 203 volts while the motor is running. This is still within 5% of nominal, so the motor will run correctly once started. If this were a motor in a plant, problems such as voltage dimming might be observed during the starting of the motor. A soft-starter could be employed to eliminate such problems. Figure 9 Capture of switching transients Newer power quality meters offer the student the opportunity to observe transient phenomena in addition to harmonic analysis. Probably the most common power quality disturbance is the highfrequency ringing surge or transient that can occur with capacitor switching, lightning strikes, or transmission line switching. Such a transient can easily be observed in the laboratory and by repetitive operation, the student can observe how the transient varies with the time of the event on the voltage waveform. Figure 10 shows a photograph of an experimental setup to Page

8 Figure 10: Experiment to observe capacitor switching transients Figure 11: (a) circuit layout for capacitor switching and (b) equivalent circuit demonstrate capacitor switching transients, while Figure 11 shows a sketch of the setup and the equivalent circuit. As shown, a small transformer was used to provide series inductance and an ordinary household light switch was used to switch the capacitors into the circuit. As shown in the equivalent circuit of Figure 11, closing the switch places the capacitor in parallel with the source. Initially, the capacitor acts like a short circuit so the source voltage will incur a transient, unless the capacitor happens to be added when the source voltage waveform is passing through zero. In addition, the series inductance and capacitance will have a resonant frequency so the voltage will oscillate until the transient is damped out by the resistance in the system. Page

9 Figure 12: Switching transient in AC source voltage Figure 12 shows a spreadsheet plot of the data obtained from the power quality meter. As with the motor starting current, the meter downloads upper and lower limits for the instantaneous voltage. The transient, however, is quite clear at about 0.03 seconds. When the switch was closed, the instantaneous voltage immediately dropped to zero and then oscillated about the AC waveform, resulting in a negative peak voltage of about -250 volts. This very simple experiment provides the students with an example of a surge voltage. The transient can be captured on an oscilloscope if a power quality meter is not available; however, setting up the waveform capture is extremely easy on the power quality meter. Investigation of Uninterruptible Power Supply (UPS) operation Voltage transients, such as those shown above due to motor starting and capacitor switching, can play havoc with computers. One way to protect against voltage problems is the use of an Uninterruptible Power Supplies (UPS). UPS devices were once a major investment but they have become commonplace as their price has dropped to less than $50.00 to protect a single computer. However, there are a number of different type of UPS devices and the advertising of them is not always honest. In this experiment, the student investigates the capabilities of an inexpensive UPS device to gain a better understanding of exactly what protection is provided. Figure 13 shows three different modes of operation for UPS devices 2. In Figure 13 (a) a switch transfers the input voltage to the output until such time as the input varies beyond some preset limits. At that point the switch transfers to the inverter and the battery storage provides the output power through the inverter. In Figure 13 (b), there is a regulating transformer to provide a buck-boost capability which keeps the output voltage closer to its nominal value when the input varies. Finally, in Figure 13 (c), the inverter operates continuously providing the output voltage until such time as it is necessary to switch to line power due to a failure in the UPS. Page

10 Figure 14 shows the setup for this experiment. Here, the UPS is fed from a variable-voltage supply and provides voltage to an incandescent lamp. By varying the input voltage to the UPS, the student can determine how the UPS operates. Interestingly enough, although these are very inexpensive UPS devices, they were advertised as having buck-boost capability. As the students vary the line voltage, they quickly find out that there is no buck-boost because the output voltage is equal to the line voltage over a relatively wide range. In particular, the students were instructed to set the line voltage to 120 volts and then to slowly decrease it until the UPS began operating. Typically, they found the output voltage stayed equal to the input down to about 97 volts. They were then instructed to turn the voltage back up slowly. They found that the UPS did not restore line voltage until about 105 volts. Figure 13 Figure 14: Setup for UPS experiment Page

11 Similarly at the high end, the UPS did not start operating until about 132 volts and did not stop operating until the voltage dropped below about 127 volts. The students were asked to suggest a reason for the hysteresis in the operation of the UPS. The reason of course is to prevent repetitive operation of the transfer switch if the voltage dropped and then varied by a volt or two. Finally, the students were asked to observe the output waveform when the UPS was in the inverter mode. Figure 15 shows the voltage waveform to consist of a series of square pulses with zero spaces in between. With an incandescent lamp as the load, a definite flicker of the lamp intensity was observable when the UPS was in the inverter mode. In general, the students were quite surprised to find that the UPS device did not offer protection against voltage variations of up to and beyond 10% of the nominal 120 volts. Summary Figure 15: UPS output voltage when operating in inverter mode The availability of advanced power meters provides many new opportunities for students to observe phenomena that previously were very difficult to explore in the laboratory. This paper has described several small experiments that can be done with minimal investment in equipment if a power meter is available. Investigation of induction motor starting allows the student to observe the high starting current and to explore the effects of source impedance on the starting current and motor voltage. Transients due to capacitor switching can be readily demonstrated Page

12 with a fairly simple experimental setup. Finally, UPS device operation can be demonstrated and truth in advertising can be determined with the third experiment described in this paper. References 1. Skvarenina, T.L (1996). Development of a laboratory experiment to demonstrate power quality issues, 1996 Annual Conference Proceedings, American Society for Engineering Education, paper 2, session 2333, six pages (proceedings on CDROM) 2. Skvarenina, T.L. and DeWitt, W.E. (2001). Electrical Power and Controls. Prentice Hall, Biographical Information Tim Skvarenina was born in Chicago, Illinois on December 27, He received the BSEE and MSEE degrees from the Illinois Institute of Technology in 1969 and 1970 and the Ph.D. in electrical engineering from Purdue University in During his college career he worked four summers at U.S. Steel South Works as an assistant electrician, rewinding motors and installing electrical equipment. He then served 21 years in the U.S. Air Force, including six years designing, constructing, and inspecting electric power distribution projects for a variety of facilities. He spent five years teaching and researching pulsed power systems including railgun systems, high power switches, and magnetocumulative generator modeling. He also has four years experience in operations research, having conducted large scale systems analysis studies for the Strategic Defense Initiative. He has authored or coauthored over 25 papers in the areas of power systems, pulsed power systems, and engineering education. He is the primary author of one textbook and is the Editor-in-Chief of a Power Electronics Handbook.. In the fall of 1991, he joined the faculty of the School of Technology at Purdue University where he primarily teaches undergraduate courses in electrical machines and power systems. He is a senior member of the IEEE; a member of the American Society for Engineering Education (ASEE), Tau Beta Pi, and Eta Kappa Nu; and a registered professional engineer in the state of Colorado. He has served as Chair of the Central Indiana Chapter of the IEEE Power Engineering Society, Chair of the ASEE Energy Conversion & Conservation Division, Chair of the ASEE Professional Interest Council III, and as Vice President for Professional Interest Councils and member of the board of directors of ASEE. Page