EE 242 EXPERIMENT 5: COMPUTER SIMULATION OF THREE-PHASE CIRCUITS USING PSPICE SCHEMATICS 1

Transcription

1 EE 242 EXPERIMENT 5: COMPUTER SIMULATION OF THREE-PHASE CIRCUITS USING PSPICE SCHEMATICS 1 Objective: To build, simulate, and analyze three-phase circuits using OrCAD Capture Pspice Schematics under balanced and unbalanced conditions, and to understand the characteristic of 3-phase power transmission circuits. Prelab: Figure 1. Three-Phase Circuits with Line and Load Impedances 1. In Figure 1, let s assume that the three-phase circuits are balanced and each has a magnitude (peak value) of 170 V at 60Hz in the positive sequence with V a = 170 V 0 o. The line impedance is (1 + j10) Ω, and the load is (20 + j20). Find: a) the line currents (I a, I b, I c ) and the neutral current (I n ) in peak values b) the power loss in each line, including the neutral c) the power factor for each phase of the load. 2. Repeat problem 1, but let s now assume that the three-phase circuits are unbalanced and operating in the positive sequence with V a = 170 V 0 o. Use the same line impedance, but the load is now (20 + j20) Ω for phase A, (50 + j10) Ω for phase B, and (5 + j50) for phase c. 1 New experiment developed by Taufik, last revised 11/02/06 (John Saghri) 1

2 Background Electronic circuit design requires accurate methods for evaluating circuit performance. Because of the enormous complexity of modern integrated circuits, computer aided circuit analysis is essential and can provide information about circuit performance that is almost impossible to obtain with laboratory prototype measurements. Computer Simulations are used in industry to shorten the overall design process, since it is usually easier to study the influence of a parameter on the system behavior in simulation, as compared to accomplishing the same in the laboratory on a hardware breadboard. Cadence Pspice is software that allows you to perform circuit analysis by literally drawing the schematic of the circuit, and therefore can provide an intuitive insight of the circuit. Numerous circuit analysis tasks can be done using Cadence Pspice such as time domain analysis, dc sweep and ac sweep. In this experiment, we will learn how to build the schematic of three-phase circuits and then perform the time domain analysis on them using the Cadence Pspice. Getting Started With Capture Pspice Turn on the PC and under the Start menu in Windows, find Cadence SBP 15.7 program and select Design Entry CIS. Ignore the TLK0015 warning message that appears by clicking on ok. From the menu that appears next, choose Allegro PCB Design CIS L In the next window, select PCB Design Studio with Capture CIS and then hit OK. See Figure 3. Figure 3. Selection window at the start of the program You should now see Capture CIS main window. Go File on the menu and select New and then Project as shown in Figure 4. 2

3 Figure 4. Opening up a new project The New Project window should appear by now as shown in Figure 5. The default choice is Schematics. Please change this and select Analog or Mixed A/D instead. Then, assign a name to your new project, and specify where you want to save the associated files. Hit the Browse button if you want to store the files in another directory or path. A small window Create Pspice Project pops up. Select the Create a blank project option as shown in Figure 6. You should now see the main Schematic window on your screen as shown in Figure 7. You are now ready to draw the schematic of your circuit. Figure 5. New Project window 3

4 Figure 6. Create Pspice Project window Procedure: Figure 7. The main Schematic Window 1. Three-Phase Balanced Circuits a. Build the three-phase circuits of Figure 1 onto the Schematic window b. To get parts, click button on the right hand side menu. Alternatively, you could also get parts by going to the top menu, click on Place, and then select Part as shown in Figure 8. 4

5 Figure 8. Getting the parts for the Circuit Schematic c. If no library is shown on the Place Part window, then you will have to manually add the library by clicking the Add Library button (see Figure 9). Look for a library called Source and click on it (see Figure 10). The SOURCE library should now be listed on the Place Part window. Figure 9. The Place Part window 5

6 Figure 10. Obtaining a particular library d. The three-phase voltages are made up of three ac sinusoidal single-phase voltage sources as shown in Figure 11. Use Vsin under the SOURCE library to build the three-phase voltages. Once the Vsin part is on the schematic, double click on it to assign its parameter values: AC=0 DC=0 FREQ=60 PHASE=0 VAMPL=170 VOFF=0 Note that the other two Vsin voltages should have the same parameter values as above except their phases (for V2 and V3) should be -120 o and + 120, respectively (assuming the phase sequence is positive). Figure 11. Three-phase Voltage sources 6

7 e. Passive components such as Resistor, Inductor, or Capacitor can be found under the ANALOG library. For the given impedances in the Prelab part 1, determine the resistor and inductor values. These are the values that you will need to assign for the Resistors and Inductors on the schematic. Figure 12. Three-phase Voltage sources f. Connect a 0/source ground to the neutral points (node n and N in Figure 1). The ground can be obtained by clicking on the right side bar menu, and then select 0/SOURCE in the Place Ground window as shown in Figure 13 below. 7

8 Figure 13. Selecting the Ground g. Place one 0/SOURCES ground on the schematic as shown in Figure 14 Figure 14. Putting passive components h. Connect all the components with wire using the on the right bar menu, or alternatively the wire can also be obtained by going to Place on the top menu and then select Wire. Do not assume that Pspice will always automatically place a connecting wire between two components if you move them close to each other. It is advisable that you actually see the connecting wire between the components. 8

9 Figure 15. Wiring the components i. After the schematic is done, go to Pspice on the top menu, and select New Simulation Profile. A window appears asking you to name the simulation profile. Type in any name, but preferably something that relates to your schematic, such as three-phase. Then, hit OK and the following window appears. Figure 12. Simulation Setting window j. Enter the following values for the simulation settings and then hit OK: Run to time=1050ms, Start saving data after=1000ms, Maximum step size=0.1ms 9

10 Check the box for the Skip the initial transient bias point calculation (SKIPBP) k. Run Pspice Simulation by selecting Run under Pspice on the menu. Once the simulation is completed, a Probe window will appear as shown in Figure 13. However, if there is an error or more on your schematic then the simulation will stop. You should then go back to the schematic page and trouble-shoot the schematic. l. To show various waveforms (voltage, current, power) from the schematic, go back to the schematic window and then place the markers or probes to any place of your interest on the schematic. The probes are located just below the top menu and there are four probes available: voltage (V), voltage differential (V+V-), current (I) and power (W). Note that you should run your simulation again every time you add or remove probes. m. To observe the input voltage waveforms, place the Voltage markers on top of each Vsin symbol on your schematic. This will automatically generate the waveforms on the Probe window. Switch to the Probe window and you should see the waveforms of balanced three-phase voltages as shown in Figure 14. Copy and paste the plot into Word by going to Window and then Copy to Clipboard. To save ink on the printer select the following items in the Copy to Clipboard- Color Filter probe window as shown below. Load a new document in Microsoft Word and then paste the plot. 10

11 Figure 13. The probe window (note that your probe window will start from 1000 ms instead of 0 ns shown in this graph) Figure 14. Three-phase voltage waveforms 11

12 n. Remove voltage probes for Phase B and Phase C from the schematic, and add a current probe into Phase A. See Figure 15. o. Switch back to the Probe Window, you should now see the Phase A voltage and the line current A as shown in Figure 16. R1 L1 VOFF = 0 VAMPL = 170 FREQ = 60 V1 V I R1a L1a R2 L2 VOFF = 0 VAMPL = 170 FREQ = 60 V2 R2a L2a R3 L3 VOFF = 0 VAMPL = 170 FREQ = 60 V3 R3a L3a Rn Ln 0 Figure 15. Voltage and current probes or markers on Phase A Figure 16. Voltage and current waveforms on the Probe Window 12

13 p. Rescale the current waveform by a factor of 10 to see the current waveform more clearly. This is done by double clicking the name of the current waveform (I(R1) in Figure 16), a new window will appear. In the text box titled Trace Expression, you should see the name of the current waveform. Multiply the waveform by 10 by typing *10 following the name of the waveform. See Figure 17. Click on the OK button, and the current waveform should now be more visible as shown on Figure 18. Figure 17. Voltage and current waveforms on the Probe Window Figure 18. Voltage and current after rescaling the current waveform 13

14 q. Zoom in to the zero crossings (i.e., crossing of the waveform with the x-axis) of the voltage and current waveforms at a appropriate location similar to the area highlighted in the Figure 19 below. Use the Zoom Area function provided in the probe window. The Zoom Area is located on the upper left corner of the probe window. Click on and select an area around the adjacent zero crossing of both waveforms (both waveforms should be either rising or falling). Figure 20 shows the resulting plot after the area is zoomed. Figure 19. Voltage and current after rescaling the current waveform Figure 20. Voltage and current after rescaling the current waveform r. To determine the times when these zero crossings occur, you may use the cursors by clicking on the menu. There are two cursors which are movable by the use 14

15 of left click and right click of your mouse. At this point, the two cursors should be on one of the waveforms. To find out which waveform the cursors are currently on, look at the names of the waveforms (bottom left of the plot). If the legend of the waveform is surrounded by a square then the cursors will be assigned to the waveform. Also, if you look at the bottom right of the plot, you should also see a small window entitled Probe Cursor which shows the location of the cursors (x and y coordinates) on the plot. See Figure 21. Figure 21. Zooming in to the zero crossing points s. The Probe Cursor window consists of 3 rows and 2 columns. The first column shows the time in ms and the second column show the voltage in Volts and/or current in Amps. The third row shows the difference between the two x-points (row 3 column 1) and the two y points (row 3 column 2). See Figure 21 again. t. Use the right click of your mouse to move one cursor to find the zero crossing of the voltage as shown in Figure 21. Use the left hand click to measure the zero crossing of the current. Note that you won t be able to get exactly 0 for the y points, so do the best you can to get a number close to 0. Ask your instructor to verify your result and then print it out. u. From the zero crossing values that you just obtained, measure the power factor as seen by the source, i.e., power factor associated with the total impedance of he load and the line. Is it a lagging or leading power factor? v. Zoom to fit the plot by clicking on the upper right corner of the plot. Delete both waveforms from the plot, and plot the neutral current by placing the current probe on the neutral line on the schematic. Observe the value of the neutral current. 15

16 w. Delete the neutral current waveform from the plot, and, instead, add the load voltage from phase A to the plot. Switch to the Probe Window and you should see the load voltage waveform on the plot. 2. Three-Phase Unbalanced Circuits a. Build a three-phase unbalanced circuit using the same three-phase schematic of part 1. Change the load impedance to the values listed in Prelab part 2. b. Run the simulation and obtain the load voltage waveforms into a single plot. Copy and paste into Word. Note that because the circuit is unbalanced, the voltage at the load side of the neutral line is not the same as the voltage at its source side, i.e. at ground level. Hence, to obtain the load voltage waveform, you have to use the Differential Voltage probe or marker from the menu. With this probe, you will have to place two markers (since it will be measuring a differential voltage): V+ marker and V- marker. Place the V+ marker with the first click of your mouse to the top terminal of load resistor R1a and place the V- marker on the bottom terminal of load inductor L1a. c. Delete the load voltage waveforms and obtain the input voltage waveforms (i.e., the three phase voltages at the source side) into a single plot. Copy and paste into Word. d. Remove the input voltage waveforms and now plot the current waveforms (all line currents and neutral current) into a single plot. Rescale any current waveform if necessary to make all waveforms visible on the plot. Copy and paste into Word. e. Determine the power factor for each phase of the load by measuring the phase difference between the voltage across and the current through it. Note that the phase difference between the voltage across and the current through each of the three load phases should be equal (theoretically) to the angle of the corresponding load impedance. Discussion: 1. Besides the ones mentioned in the background section, list three other advantages of using Computer Simulation in circuit design process. 2. How do your simulation results compare with your calculated ones from the Prelab? 3. From section 1 part v, what is the value of the neutral current? Why? 4. What happened with the line current when the three-phase circuit is unbalanced? Why? 5. In this exercise, which three-phase circuits (balanced or unbalanced) require higher average power (Watts)? 16