Grid-Tied Home Energy Production Using a Solar or Wind Power Inverter without DC-to-DC Converter

Transcription

1 Exercise 3 Grid-Tied Home Energy Production Using a Solar or Wind Power Inverter without DC-to-DC Converter EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with grid-tied home energy production using a solar or wind power inverter that does not include a dc-to-dc converter. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Grid-tied home energy production using solar power or wind power Solar or wind power inverter implemented using the transformerless without dc-to-dc converter topology Solar or wind power inverter implemented using the LF transformer without dc-to-dc converter topology Operation of the MPP tracker in a solar power inverter implemented without a dc-to-dc converter DISCUSSION Grid-tied home energy production using solar power or wind power As mentioned in the introduction of this manual, grid-tied home energy production is the small-scale generation of electric power at a site that is wired to the local ac power network (i.e., the grid) from renewable resources such as wind and sunlight. Although some of the energy produced can be used locally, it is generally meant to be supplied to the grid. The typical installation for grid-tied home energy production minimally consists of a building (e.g., a home, a small commercial building, a farm building, etc.) that is wired to the local ac power network and equipped with a large solar panel installed on the rooftop or with a wind turbine mounted atop a vertical pole. Figure 26. Home that is wired to the local ac power network and equipped with large solar panels installed on the rooftop. Festo Didactic

2 Discussion Since solar panels and wind turbines produce dc power, a power converter is required in any installation for grid-tied home energy production to convert the dc power coming from the solar panel or the wind turbine into ac power that can be fed to the ac power network. The single-phase grid-tied inverter studied in the previous exercise of this manual is perfectly suited for this task. A maximum power point (MPP) tracker is also required in any installation for grid-tied home energy production to maximize the amount of energy produced by the solar panel or the wind turbine. The single-phase grid-tied inverter and the MPP tracker are often combined in a single device that is referred to as a solar power inverter (also referred to as a solar inverter or PV inverter) or a wind power inverter. Figure 27 shows the main elements of a basic installation for grid-tied home energy production using either solar power or wind power, and Figure 28 shows a picture of the wind power inverter in a typical installation for grid-tied home energy production. Grid Solar panel DC power Solar power inverter AC power Energy meter AC power Grid Wind turbine DC power Wind power inverter AC power Energy meter AC power Figure 27. Main elements of a basic installation for grid-tied home energy production using either solar power or wind power. 66 Festo Didactic

3 Discussion Figure 28. The solar power inverter (or the wind power inverter) is the keystone of any installation for grid-tied home energy production. Each of the two boxes in the installation shown above is a grid-tied inverter converting dc power from a wind turbine into ac power that is fed to the grid ( Solar or wind power inverter implemented using the transformerless without dc-to-dc converter topology The MPP tracker in the solar or wind power inverter shown in Figure 29 adjusts the active current command of the single-phase grid-tied inverter (i.e., the amount of active power fed to the ac power network) so that the solar panel or the wind turbine operates at the MPP (i.e., produces the maximal amount of power) no matter what the solar irradiance or wind speed is (the operation of the MPP tracker is covered later in this discussion). The reactive current command of the single-phase grid-tied inverter is 0 A to zero the exchange of reactive power between the inverter and the ac power network, thereby resulting in a power factor close to unity. Festo Didactic

4 Discussion Single-phase grid-tied inverter Current sensor I DC bus Current sensor I DC input E Voltage sensor Single-phase PWM inverter Filter E Voltage sensor Current control loop Local ac power network MPP tracker Active current command Reactive current command (0 A) Figure 29. Block diagram of a solar or wind power inverter using the transformerless without dc-to-dc converter topology. The solar or wind power inverter shown in Figure 29 uses the simplest possible topology because it only consists of the two devices that are strictly required to implement such a power inverter (i.e., a single-phase grid-tied inverter and an MPP tracker). Using the transformerless without dc-to-dc converter topology thus minimizes the weight, size, and cost of the solar or wind power inverter. It also provides the best power efficiency because the dc-to-ac power conversion is performed via a single device (the grid-tied inverter). Using the transformerless without dc-to-dc converter topology allows active power to flow in either direction. This not only enables dc power to be converted into ac power to feed the grid, but also enables ac power from the grid to be converted into dc power that can be stored into batteries. The energy stored in the batteries can be released later and converted back to ac power that is returned to the grid. In other words, this allows grid-to-battery and battery-to-grid energy exchanges. This feature, which might seem unimportant at first, could become an important step in the implementation of a smart grid as is discussed in the last exercise of this manual. On the other hand, the simplicity of the transformerless without dc-to-dc converter topology has some disadvantages. First, there is no galvanic isolation between the dc and ac sides of the solar or wind power inverter. This is acceptable when the local regulation does not require one of the dc power terminals of the solar panel or wind turbine to be grounded. However, when the local regulation does require grounding of one of the dc terminals of the solar panel or the wind turbine, galvanic isolation is required between the dc and ac sides of the solar or wind power inverter, thereby precluding the use of this solar or wind power inverter topology. When galvanic isolation is required 68 Festo Didactic

5 Discussion between the dc and ac sides of the inverter, a solar or wind power inverter implemented using the LF transformer without dc-to-dc converter topology can be used (this topology is described in the next section of this discussion). The transformerless without dc-to-dc converter topology requires the dc input voltage to be high enough to allow the single-phase grid-tied inverter to produce voltage at the local ac power network voltage value. This means a dc input voltage of at least 175 V when the local ac power network voltage is 120 V. This is even worse at local ac power network voltages of 220 V and 240 V, as the dc input voltage values required are at least 320 V and 350 V, respectively. The dc input voltage should not increase too much above the minimum values given above to avoid affecting the operation of the solar or wind power inverter. In other words, the dc input voltage range of operation is small. This is not problematic in solar power inverters because the dc voltage produced by a solar panel at various solar irradiance values is fairly constant when it operates at the MPP as shown in Figure W/m W/m 2 : MPP Current (A) 675 W/m W/m W/m 2 Voltage (V) Figure 30. E-I characteristic and MPP curve of a solar panel at various irradiance values. However, this is rather problematic in wind power inverters because the dc voltage produced by a wind turbine at various wind speeds values varies significantly when it operates at the MPP as shown in Figure 31. In other words, the transformerless without dc-to-dc converter topology is not well suited for operation with wind turbines because maximal power can be fed to the ac power network only over the very limited range of wind speeds at which the dc voltage produced by the wind turbine is close to the dc input voltage required (i.e., wind speeds between about 10 m/s and 11 m/s in Figure 31 when power is supplied to a 120 V ac power network). Furthermore, operation at wind speeds below a certain value (less than about 6 m/s in Figure 31 when power is supplied to a 120 V ac power network) is not possible because the dc voltage produced by the wind turbine is not sufficient to allow the grid-tied inverter to produce voltage at the local ac power network voltage value. Festo Didactic

6 Discussion Wind speed Generator current (A) : MPP 3 m/s 4 m/s 5 m/s 6 m/s 7 m/s 8 m/s 9 m/s 10 m/s 11 m/s 12 m/s Generator voltage (V) Figure 31. E-I characteristic and MPP curve of a wind turbine at various wind speeds. It is difficult to avoid injection of dc current in the local ac power network (this is commonly called dc injection) when the transformerless without dc-to-dc converter topology is used. This is because the ac side of the inverter is dc coupled to the local ac power network, and thus nothing prevents any dc voltage produced by the inverter (of course, such dc voltage is highly undesirable) from forcing dc current into the local ac power network. Leakage current can also flow at the dc power input of the solar or wind power inverter when the transformerless without dc-to-dc converter topology is used. This is due to some residual ac component in the dc input voltage (e.g., the solar panel voltage). With the transformerless without dc-to-dc converter topology, the MPP tracker operates by adjusting the active current command of the grid-tied inverter directly. This type of MPP tracker is usually more complex to implement. Table 7 summarizes the advantages and disadvantages related to the use of the transformerless without dc-to-dc converter topology in solar and wind power inverters. 70 Festo Didactic

7 Discussion Table 7. Advantages and disadvantages of a solar or wind power inverter implemented using the transformerless without dc-to-dc converter topology. Advantages Best efficiency Low weight Small size Low Cost Power flow is bidirectional (inverter-to-grid or grid-toinverter) Disadvantages No galvanic isolation between the dc and ac sides Requires high dc input voltage (i.e., dc input voltage must be high enough to allow the inverter to produce ac voltage at the local ac power network voltage value) Small dc input voltage range of operation (i.e., not well suited for operation with a wind turbine) Difficult to completely eliminate dc injection in local ac power network Leakage current to ground at dc input due to ac component in dc input voltage Complex MPP tracker (direct control of the active current command) Solar or wind power inverter implemented using the LF transformer without dc-to-dc converter topology As mentioned earlier in this discussion, when galvanic isolation is required between the dc and ac sides of the inverter, a solar or wind power inverter implemented using the LF transformer without dc-to-dc converter topology can be used. Figure 32 shows a block diagram of a solar or wind power inverter using the LF transformer without dc-to-dc converter topology (the same block diagram is valid for both the solar power inverter and the wind power inverter). Festo Didactic

8 Discussion Single-phase grid-tied inverter Current sensor I DC bus Current sensor I Voltage sensor DC input E Voltage sensor Singlephase PWM inverter Filter E Current control loop Local ac power network MPP tracker Active current command Reactive current command Figure 32. Block diagram of a solar or wind power inverter using the LF transformer without dc-to-dc converter topology. The LF transformer without dc-to-dc converter topology simply adds an LF power transformer to the transformerless without dc-to-dc converter topology shown in the previous section of this discussion. The current sensor of the current control loop is connected to the primary of the LF power transformer instead of the secondary. This arrangement prevents any non-linearity in the LF power transformer operation from affecting the performance of the current control loop. The reactive current command of the grid-tied inverter, instead of being set to 0 A as in the transformerless without dc-to-dc converter topology, is set in order to zero the exchange of reactive power between the inverter and the grid, therefore ensuring operation at unity power factor. The addition of the LF transformer not only provides galvanic isolation between the dc and ac sides of the solar or wind power inverter, but it also prevents dc injection in the local ac power network. Furthermore, the galvanic isolation allows one terminal of the solar panel or wind turbine to be grounded, thereby preventing any ac component in the dc input voltage. This results in no leakage current to ground at the dc input. By using a step-up (boost) LF transformer, the dc input voltage required to produce voltage at the ac power network voltage can be reduced to the desired value. On the other hand, the addition of the LF transformer significantly increases the weight, size, and cost of the solar or wind power inverter. Despite some additional power losses due to the addition of the LF transformer, high efficiency can generally be achieved in solar or wind power inverters implemented using the LF transformer without dc-to-dc converter topology. 72 Festo Didactic

9 Discussion Table 8 summarizes the advantages and disadvantages related to the use of the LF transformer without dc-to-dc converter topology in solar and wind power inverters. Table 8. Advantages and disadvantages of a solar or wind power inverter implemented using the LF transformer without dc-to-dc converter topology. Advantages Galvanic isolation between the dc and ac sides High efficiency No dc injection in local ac power network No leakage current to ground at dc input High dc input voltage no longer strictly required Disadvantages Small dc input voltage range of operation (i.e., not well suited for operation with a wind turbine) High weight Large size Cost of LF transformer Complex MPP tracker (direct control of the active current command) Power flow is bidirectional (inverter-to-grid or grid-to-inverter) Operation of the MPP tracker in a solar power inverter implemented without a dc-to-dc converter This section focuses only on the operation of the MPP tracker in solar power inverters implemented without dc-to-dc converter. This is due to the fact that both the transformerless without dc-to-dc converter topology and the LF transformer without dc-to-dc converter topology are rarely used to implement wind power inverters, as they are not well suited for this purpose. As mentioned earlier in this discussion, the MPP tracker in a solar power inverter without a dc-to-dc converter adjusts the active current command of the singlephase grid-tied inverter (i.e., the amount of active power fed to the ac power network) so that the solar panel operates at the MPP (i.e., produces the maximal amount of power) no matter the solar irradiance. To do so, the MPP tracker in a solar power inverter uses the perturb-andobserve (P&O) algorithm which slightly varies (perturb) the active current command of the grid-tied inverter and observes the effect on the power produced by the solar panel. The P&O algorithm repeats continually in order to find the value of the active current command which maximizes the power produced by the solar panel. Note that the P&O algorithm starts with a low value of active current command (i.e., a low dc current flowing through the solar panel) and increases the value of the active current command until the MPP is found, as shown in Figure 33. Festo Didactic

10 Discussion Current (A) E-I curve MPP The active current command of the grid-tied inverter starts at a low value and increases by steps until the MPP is found. Voltage (V) Power (W) Power curve MPP Voltage (V) Figure 33. E-I and power curves of a solar panel showing how the MPP tracker varies the active current command of the grid-tied inverter to make the dc input current vary and find the point where the power produced is maximum. Once the MPP tracker has found the MPP, the P&O algorithm still repeats continually to maintain the active current command of the grid-tied inverter at the value that maximizes the power which the solar panel produces. Consequently, the solar panel continues to operate at the MPP no matter what the solar irradiance is since the value of the active current command is readjusted automatically. Once the MPP tracker has found the MPP, the dc input current, voltage, and power (i.e., the solar panel current, voltage, and power) oscillate between two values which are located on both sides of the actual MPP, as shown in Figure 34. Current (A) Power (W) E-I curve Power curve The solar panel current, voltage, and power oscillate between two values when the P&O algorithm has found the MPP. Actual MPP Voltage (V) Figure 34. Once the MPP tracker has found the MPP, the dc input current, voltage, and power (i.e., the solar panel current, voltage, and power) oscillate between two values which are located on both sides of the actual MPP. 74 Festo Didactic

11 Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Grid-tied home energy production using a solar power inverter implemented with the LF transformer without dc-to-dc converter topology PROCEDURE High voltages are present in this laboratory exercise. Do not make or modify any banana jack connections with the power on unless otherwise specified. Set up and connections In this part of the exercise, you will set up and connect the equipment. 1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform the exercise. Install the equipment in the Workstation. Make sure that the main power switch on the Four-Quadrant Dynamometer/ Power Supply is set to the O (off) position, then connect its Power Input to an ac power outlet. 2. Connect the Power Input of the Data Acquisition and Control Interface to a 24 V ac power supply. Connect the Low Power Input of the Chopper/Inverter to the Power Input of the Data Acquisition and Control Interface. Turn the 24 V ac power supply on. 3. Connect the USB port of the Data Acquisition and Control Interface to a USB port of the host computer. Connect the USB port of the Four-Quadrant Dynamometer/Power Supply to a USB port of the host computer. 4. Set up the circuit shown in Figure 35. Use the Four-Quadrant Dynamometer/Power Supply to implement the solar panel emulator. Use the Filtering Inductors/Capacitors to implement the inverter output filter; the inductance and capacitance values to be used for,, and depend on your local ac power network (see table in the diagram). Use the 4700 µf capacitor of the Insulated DC-to-DC Converter to implement the second capacitor at the dc side of the inverter (set switch S1 on the circuit board of the Insulated DC-to-DC Converter to CAPacitor). Use the Single-Phase Transformer to implement the LF power transformer. Festo Didactic

12 Procedure Solar panel emulator DC side DC bus Single-phase PWM inverter 4700 µf To filter Filter 40 A LF power transformer AC side From the single phase PWM inverter Local ac power network * See note below N * The connections of the LF power transformer secondary shown in the diagram are for 120 V ac power network voltage. The LF power transformer secondary must be connected in series when the ac power network is either 220 V or 240 V. Local ac power network Voltage (V) Frequency (Hz) (mh) ( F) Figure 35. Solar power inverter implemented using the LF transformer topology. 76 Festo Didactic

13 Procedure 5. On the AC Power Network Interface, Model 8622, set the I/O switch of the AC Power Inlet to the O position, then connect the AC Power Inlet to an ac power outlet using the line cord supplied. The AC Power Inlet of the AC Power Network Interface is used to connect the the LF power transformer secondary to the local ac power network. 6. Connect the Digital Outputs of the Data Acquisition and Control Interface to the Switching Control Inputs of the Chopper/Inverter using a DB9 connector cable. On the Chopper/Inverter, set the Dumping switch to the O (off) position. Grid-tied home energy production using a solar power inverter implemented with the LF transformer without dc-to-dc converter topology In this part of the exercise, you will check the insulation provided by a solar power inverter implemented using the LF transformer topology. Then you will operate the solar power inverter at various solar irradiance values and observe how the voltages, currents, powers, dc-to-dc converter duty cycle, and efficiency vary. 7. Temporarily disconnect the solar panel inverter from the grid by disconnecting the leads at the AC Power Inlet on the AC Power Network Interface. Using an ohmmeter, measure the resistance between the dc side terminals of the solar power inverter and the ac side terminals. Do your measurements indicate that the dc side terminals of the solar power inverter are insulated from its ac side terminals? Yes No 8. Reconnect the leads at the AC Power Inlet on the AC Power Network Interface, then set the I/O switch of the AC Power Inlet to the I position. 9. On the Four-Quadrant Dynamometer/Power Supply, set the Operating Mode switch to Power Supply, then turn the module on. Festo Didactic

14 Procedure 10. Turn the host computer on, then start the LVDAC-EMS software. In the LVDAC-EMS Start-Up window, make sure that the Data Acquisition and Control Interface and the Four-Quadrant Dynamometer/Power Supply are detected. Make sure that the Computer-Based Instrumentation and Home Energy Production Control functions for the Data Acquisition and Control Interface are available. Also, make sure that the Standard Functions (C.B. control) and Solar Panel Emulator functions for the Four-Quadrant Dynamometer/Power Supply are available. Select the network voltage and frequency that correspond to the voltage and frequency of your local ac power network, then click the OK button to close the LVDAC-EMS Start- Up window. 11. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window, then make the following settings: Set the Function parameter to Solar Panel Emulator. This function makes the power supply in the Four-Quadrant Dynamometer/Power Supply operate like a solar panel. Set the Solar Irradiance parameter to 0 W/m 2. Set the Number of PV modules in series parameter to 7. This setting allows the solar panel emulator to generate an open-circuit voltage ( ) of 67.9 V (7 PV modules 9.7 V/PV module). Set the Number of PV modules in parallel parameter to 38. This setting allows the solar panel emulator to deliver a short-circuit current ( ) of 4 A (38 PV modules A/PV module). Start the solar panel emulator. 12. In LVDAC-EMS, open the Home Energy Production Control window, then make the following settings: Set the Function parameter to Solar Power Inverter (LF Transformer). Make sure that the MPP Tracker parameter is set to On. The Reactive Current Command, Controller Proportional Gain Kp and Controller Integral Gain Ki parameters should be set to 0.00, 6.0, and 250, respectively. a The Controller Proportional Gain Kp should be set to 4.0 when the ac power network voltage is 240 V. Start the solar power inverter. 78 Festo Didactic

15 Procedure 13. In LVDAC-EMS, open the Metering window. In the Options menu, select Acquisition Settings to open the corresponding dialog box. Set the Sampling Window to 8 cycles, then click OK to close the dialog box. Set meters to measure the voltage (E1), current (I1), active power (E1,I1), and reactive power (E1,I1) at the solar power inverter output. 14. Does the active power meter indicate that power is produced at this moment? Explain why. 15. In the Four-Quadrant Dynamometer/Power Supply window, set the solar irradiance to 700 W/m 2. In the Home Energy Production Control window, stop the solar power inverter. 16. In LVDAC-EMS, open the Oscilloscope window and make the settings required to observe the waveforms of the voltage at the dc input of the solar power inverter, voltage, and current. 17. Observe and describe the waveforms of the voltage at the dc input of the solar power inverter, voltage, and current displayed on the oscilloscope screen (before the solar power inverter is started). Festo Didactic

16 Procedure 18. Start the solar power inverter while observing the waveforms of the voltage at the dc input of the solar power inverter, voltage, and current displayed on the oscilloscope screen, as well as the values of the voltage, current, and power at the dc input of the solar power inverter displayed by the meters in the Home Energy Production Control window. If it is necessary to validate your observations, perform several start/stop cycles of the solar power inverter. From your observations, describe the operation of the solar power inverter. 19. In LVDAC-EMS, open the Phasor Analyzer window and make the settings required to observe the phasors of voltage and current. Use voltage as the reference phasor. 20. In the Four-Quadrant Dynamometer/Power Supply window, successively set the solar irradiance to 300, 400, 500, 600, 700, 800, 900, and 1000 W/m 2. For each solar irradiance, measure and record, using the Data Table, the voltage, current, and power at the dc input of the solar power inverter (i.e., the voltage, current, and power produced by the solar panel), as well as the voltage, current, active power, reactive power, and power factor at the solar power inverter output. Also record the solar irradiance. Finally, observe the phase shift between the voltage and the current using the Oscilloscope and the Phasor Analyzer. The values of voltage, current, and power are displayed by the meters in Home Energy Production Control window. a Note that when the solar irradiance level is low, the waveform of current is severely distorted. At low solar irradiance level, the waveform of the current at the output of the LF transformer is greatly affected by the magnetizing current (non-sinusoidal waveform) of the LF transformer. a If the MPP tracker takes too much time to determine the MPP (this may happen at low solar irradiance values), stop and restart the solar power inverter. 80 Festo Didactic

17 Procedure 21. Stop the solar power inverter, then the solar panel emulator. In the rest of this procedure, the solar panel emulator is also referred to as the solar panel. Using the data recorded, plot a curve of the power at the dc input of the solar power inverter as a function of the solar irradiance. On the same graph, plot the curves of the active power and current at the solar power inverter output as a function of the solar irradiance. 22. How does the power at the solar power inverter input (i.e., the power coming from the solar panel) vary with the solar irradiance? 23. How does the active power at the solar power inverter output vary with the solar irradiance? 24. How does the current at the solar power inverter output vary with the solar irradiance? 25. Is the observed variation of the power at the solar power inverter input (i.e., the power coming from the solar panel) as a function of the solar irradiance as expected? Explain why. Festo Didactic

18 Procedure 26. Start the solar power inverter, then the solar panel emulator. In the Four-Quadrant Dynamometer/Power Supply window, make sure that the Solar Irradiance parameter is set to 1000 W/m In the Metering window, observe that reactive power is not zero, therefore indicating that a significant amount of reactive power is exchanged between the inverter and the grid. What could be done to zero the reactive power at the solar power inverter output and restore unity power factor? 28. In the Home Energy Production Control window, adjust the reactive current command by 0.1 A steps until the reactive power is approximately zero. Record the value of the reactive current command. Reactive current command (required to zero ): A 29. Is the power factor close to unity? Yes No 30. Stop the solar power inverter, then the solar panel emulator. From the data measured in step 20, note that the voltage at the solar power inverter input is between about 45 V and 60 V. Also note that the PWM inverter operates at a dc bus voltage equal to voltage since there is no converter on the dc side of the inverter. Since these dc bus voltage values are not enough to allow the PWM inverter to directly produce sinewave voltage at the ac power network voltage value, what does this imply? 31. From the data recorded in step 20, calculate the efficiency of the solar power inverter for each solar irradiance level, then plot a curve of the inverter efficiency as a function of the active power at the inverter output. 82 Festo Didactic

19 Conclusion 32. How does the efficiency of the solar power inverter vary with the inverter output power? 33. Close LVDAC-EMS, then turn off all equipment. Remove all leads and cables. Set switch S1 on the circuit board of the Insulated DC-to-DC Converter to DC/DC. CONCLUSION In this exercise, you familiarized yourself with solar and wind power inverters implemented without a dc-to-dc converter at the dc input. You saw that a power converter is required in any installation for grid-tied home energy production to convert the dc power coming from the solar panel or the wind turbine into ac power. You also saw that a MPP tracker is required to maximize the amount of energy produced by the solar panel or the wind turbine. You were introduced to solar and wind power inverters implemented using the transformerless without dc-to-dc converter and LF transformer without dc-to-dc converter topologies. REVIEW QUESTIONS 1. Name the main elements of a basic installation for grid-tied home energy production? 2. Why is a power converter required in any installation for grid-tied home energy production? 3. What is the MPP tracker used for in any installation for grid-tied home energy production? Festo Didactic

20 Review Questions 4. Explain why the transformerless without dc-to-dc converter topology and the LF transformer without dc-to-dc converter topology are not well suited for operation with a wind turbine. 5. Which topology of solar or wind power inverters can be used when the local regulation requires one of the dc power terminals of the solar panel or wind turbine to be grounded? Why? 84 Festo Didactic