Introduction
Numerous electrical applications require an inverter, which converts Direct Current (DC) to Alternating Current (AC) power. Some of these applications require the stepping down of voltage levels when realizing this conversion, which signifies that the input DC power is greater than the AC power. Conversely, certain applications require the opposite: to step up voltage when performing the power conversion, which means that the input DC power is lower than the AC power. These requirements can be met through the use of Voltage Source Inverters and Current Source Inverters, respectively. However, some particular applications require to step down voltage for a period of time and to step up for another period through the course of the DC to AC conversion, and considering that VSI and CSI only operate for their specific application, they cannot be used to satisfy this requirement. A Z-Source converter is the solution required for this problem as it can convert DC to AC power with step up or step down voltages.
Purpose
A Z-Source inverter converts DC to three –phase AC power and has the capability to step down or step up voltages as required by the application. Moreover, it can operate over a wide range, which permits the process of the required application to be developed without stress.
Literature Review
A Z-Source inverter is a three-phase inverter that can be divided into two stages: a) DC-DC converter and b) three-phase inverter.
The DC-DC converter acts as a back-boost converter. Two identical inductors and capacitors, which are connected in X-shape, couple the inverter to the DC source. The input DC source connected to the Z-impedance can be either a voltage source or current source.
The three-phase inverter converts the DC resulting from the Z-impedance to a three-phase AC output. The inverter is composed of six switches that are responsible for the six vectors, called the active states or vectors. Furthermore, a short-circuit of the load terminals of the upper or lower device leads to two additional states called “zero states”.
Both active vectors and zero vectors result in the traditional voltage source inverter. However, in the Z-Source inverter, there is one additional state that occurs when a leg gets shorted (the upper and lower switches are on), as it produces an additional “zero state” called the “shoot-through zero state”. There are seven different ways trough which this state can be accomplished: shoot-through by one phase leg, combination of two legs, or all three legs combined. These states provide a unique feature that allows the Z-Source converter to act as a buck-bust converter.
The six switches for the three-phase inverter of the Z-Source inverter can be controlled using traditional Pulse Width Modulation (PWM), and this particular design utilizes Sinusoidal Pulse Width Modulation (SPWM). This technic is based on the comparison of a triangle carrier wave with three sine reference waves. The three sine waves need to have a 120° phase shift between any two phases in order to produce a symmetric three-phase AC signal. The SPWM technic provides sinusoidal waves AC output. Likewise, a sinusoidal output simplifies the process of filtering frequency components through a low pass filter.
Objectives
Design Approach
The design of a Z-source inverter can be divided into three main parts: the Z-impedance, the three-phase inverter, and the low pass filter. Additionally, a protection mechanism must be designed in order to cut off the power supply in case of the occurrence of a phase disconnection.
Three-phase Inverter
SPWM implementation. The three phase inverter switches in this design are controlled by SPWM signals. To implement this technic and achieve the best results for the SPWM signals, four different approaches were considered:
Use of the Arduino Due to generate the sample sine wave and triangle wave given that it has a Digital-Analog-Converter (DAC). The Matlab software was used to simulate the required waves and then transmit the data (sampled data) to the Arduino software. After the generation of the sine wave with a frequency 50Hz and the triangle wave with a frequency of 1050 Hz, a circuit using Op-amps to compare the triangular wave with sin wave was designed. Moreover, the circuit generated sister sine waves shifted 120° from each other. This approach was not a good method as it added unnecessary complications to the system, which contradicts the object of this project. Likewise, the use of the circuit may cause noise to affect the SPWM output signal.
Use of Matlab software to perform the comparison offline, then transmit the data to the Arduino software and use the PWM ports to generate the SPWM signals. Through this approach, six SPWM signals are generated to control the six switches. This approach created a problem with delay. Additionally, the data generated through Matlab is composed of ones and zeros, which represent when the signals should turn on and off. Hence, even if the delay problem could be fixed, the SPWM signals would not be able to control the switches as they should.
The third method involves using both Simulink and Matlab software. The triangle and sine waves would be generated and compared through Simulink, and the SPWM would also be generated. All SPWM signals data would be sent to the workspace in Matlab and the Arduino microcontroller would programed directly through Matlab. Using this approach the Arduino produced slightly better SPWM signals as the programing method changed. However, the result signals still could not control the switches as they should. The data that transferred from Simulink to the Matlab workspace became larger and this caused the microcontroller to slow down, considering there are six SPWM signals.
The forth and best approach consists of using Simulink to generate and compare the triangle wave with the sine waves to generate the SPWM signals. Simulink is used to directly program the Arduino. The advantages this approach include good SPWM signals generated from Arduino microcontroller. Furthermore, it simplifies the design as it is easier to set up and reprogram the microcontroller when there is a change in the simulation, given that the simulation and programing are realized through the same method. Moreover, for future development, the use of different frequencies for the triangle and sine waves or different SPWM can be done easily as this approach allows for the microcontroller to generate the same SPWM signals generated in the simulation. The disadvantage of this approach is that the waves used in the simulation need to be converted from continuous time domain to discrete time domain. This change causes the frequency to change, and in the discrete time domain the wave does not solely depend on the frequency and the phase, but also on the sampled period. Many experiments were performed using this approach to generate smoother triangle and sine waves with the same sampled period, as the sampled period has to be the same for all waves in order for them to be compared. The frequency modulation that has been used in this design is 21 Hz. Selecting sample periods that produce smoother waves for both triangle and sine is important in order to get the best SPWM signals that will open and close the switches at the right moment. There have been modifications in the frequency modulation as it needs some changes to produce similar SPWM signals from continuous time domain. The frequencies that have been used for the triangle and the sine waves are 1050 Hz and 5Hz, respectively. The sampled frequency for the waves is 48 KHz. Figure X shows the Simulink blocks for the generation and comparison of the triangle and sine waves. For all SPWM signals the triangle wave the phase shift is 0°. The phase of the sine waves phase differ for each leg switch. Switches 1 and 4 used a sine wave with a 0° phase shift. Switches 3 and 6 use a sine wave with a 120°phase shift. Switches 5 and 2 use a sine wave with a 240° phase shift. The comparison of these waves is used to produce SPWM signals. Each switch operates for a half cycle which allow the switches on the same leg to not at the same time.
Mosfet Driver. Since a Microcontroller has been used to produce the SPWM signals, a Mosfet driver is necessary to boost the current of the SPWM signals produced by the Arduino microcontroller. The Mosfet driver assures the switches will receive SPWM signals with enough current to operate continuously, which results in a smooth sinusoidal output. To energize the Mosfet driver, a BJT transistor is needed to connect to the power port of the Arduino Due microcontroller as it provides 3.3V, which is not enough voltage to energize the Mosfet driver since its minimum voltage is 4.5 V. The Arduino Due was chosen due to the first approach which requires DAC ports. However, the BJT transistor was removed from the design due to damage in one of the PWM Arduino Due ports. The Arduino Due was replaced by an Arduino Mega 2560, which has a good processor, low cost, and provides an option of 5V which is sufficient to power to the Mosfet driver. The Mosfet drivers used in this design are MIC4427 dual no inverting that provide 1.5A peak output current, and have operation voltage range of 4.5V – 18V.
Inverter Switches. The switches for the three-phase inverter are N-channel Mosfet switches. Mosfet switches are a good choice for this design since they can operate with high speed for the switch to open and close. Also, Mosfet switches operate at the voltages required for this design. However, for the prototype a low voltage has been chosen for testing. The Mosfet switches used in this design are PSMN022-30PL with the rated of 30 V and 22mΩ logic level Mosfet.
Z-impedance
The design of the Z-impedance can be completed in a couple of ways. The idea behind this approach is to find the value for the capacitor voltage; it uses the values of the three-phase inverter to find the needed values for the inductors and capacitors. The input DC voltage, output inverter AC voltage, output current, and switching frequency can be found from the simulation of the three-phase inverter. This approach was preferred over the other approaches as its implementation is considerably simpler.
The inductors and capacitors values, which were calculated through a long set of equations which can be found in the appendix, are 4.5mH and 0.3mF, respectively. These values are concordant to the 3-phase inverter values, and the inductors and capacitors can boost the DC voltage of the Z-Source inverter.
Output Filter
The filter for the Z-source inverter is a LC low pass filter. It is necessary to filter out the noise above the wanted frequency (f=12πLC). Filtering the output of the Z-source inverter, allows for the output frequency to be closer to the desired frequency, which is 60 Hz. Therefore, a low pass filter was chosen to remove harmonic components at higher frequencies. The design of the filter can be challenging as the lower the cutoff frequency, larger capacitance and inductances is needed to filter out the output of the Z-Source inverter. Consequently, the filter design becomes a tradeoff between the effectiveness of the filter and the cost and size of the components.
The values of the inductor and the capacitor for the output filter are 400μH and 100mF respectively.
Protection Mechanism
The protection mechanism is designed using PSIM software. Each phase is connected to a BJT transistor that acts as a switch. The BJT transistors control the relays, each of which is connected to a main relay that operates when any phase of the inverter is disconnected, effectively disconnecting the DC power supply from the circuit until all the three phases are in operation. The type of relay that was chosen for this design is normally open and normally closed. It is simple, low cost, and can operate at the required voltages. The advantage of the relay that it disconnects the power from the signal temporarily, until all phases are connected. The limitation of the current flowing to the BJT transistor is necessary given the BJT transistor can be damaged if it receives more current than its rated value. Measuring the RMS voltage of the inverter, getting the voltage value from the Relays data sheet, and the rated current of the energizing current is required to find the resistor value necessary limit the current level. Then, Ohm’s should be used: V=I*R.
In Figure 4, the sine source represents the output of each phase for the Z-source inverter. The BJT transistor selected is the MJE13007GOS-ND and a 54 kΩ resistor to limit the current flow to the BJT. Each transistor is connected from the source side to the G5LE-14 DC24 relay. Whenever any phase is disconnected, the relay switch changes from normally open to closed, which energizes the main switch to cut off the DC power from the circuit.
Results of Testing
SPWM Signals
Figure 5 it shows the simulation result for the comparison of the triangle wave with the sine waves. This is the result after modifying the frequencies and sampled frequency of the triangle and the sine waves. The frequency of the triangle is 1050 Hz with a sampled frequency of 48 kHz. The frequency of the sine wave is 5 Hz with a sampled frequency of 48 kHz. In the figure, all six signal are presented in order, from the signal of switch 1 gate to the signal switch 6 gate. Each SPWM signal is shifted 60 degrees from the previous SPWM signal, which allows for each SPWM signal to operate for only 180 degrees.
Figure 5. Simulation result of comparison of the triangle wave with the sine waves.
The SPWM signal generated through the microcontroller (actual result) is shown in the Figure 6. This figure shows the SPWM for switch 1.
Figure 6. SPWM generated for Switch 1.
Figure 7 is the same SPWM signal but zoomed in.
Figure 7. Zoom of SPWM generated for Switch 1
Inverter output
The output of the Z-Source inverter is shown in Figure 8. This is a simulation output for the Z-source inverter without the filter.
Figure 8. Z-Source Inverter output simulation without filter.
Figure 9 shows the current output of all three phases for the Z-Source of the inverter.
Figure 9. Three-phase current output of the Z-Source Inverter.
Figure 10 shows the output with a resistive load, without a filter. A low input voltage of approximately 7 V was used. The output of the inverter is approximately 7.12 Vpk-pk, which is about 5.034V RMS. Therefore, the efficiency of this design is approximately 71%.
Figure 10. Output with resistive load without filter
After connecting the filter, one of the Arduino due PWM ports was damaged due to reversed current flow to the microcontroller. Moreover, after replacing the Arduino, some components of the filter were damaged while testing.
Conclusions
The goal of this project was to design a Z-Source inverter capable of converting DC power to AC power using SPWM technic, and a protection mechanism was designed, which adds a unique feature to this project. The design process of the Z-source inverter was challenging considering the time limit set by this project, building and testing procedure, yet the project was successful. The Z-Source inverter has a unique feature as it can step up or step down when performing DC-AC conversion.
This project was divided into parts, in order to simplify the approach. The design phases were the DC-DC converter, the 3-phase inverter, the output filter and the protection mechanism. Converting DC to AC was accomplished through the use of the SPWM technic. There were some issues during testing of the prototype with output filter, specifically when measuring the harmonic components that required to be removed by the output filter. The protection mechanism helps the design to enhance system security.
The approach used in this design provides support for future improvements. For instance, in the SPWM technic, there are many methods used to generate SPWM signals. By performing the simulation and programing of the microcontroller using Simulink, the system improves. Moreover, for future developments, there is an interesting methodology that was considered in this design but was not implemented due to time limitations, which is the comparison of each sine wave with three triangle waves, as a method to improve the efficiency of the inverter. The three triangle waves have the same frequency and phase. Many other methodologies can be used and tested in design, which leads to numerous future improvement. It can be considered that this design approach overcame many obstacles that limited the development of the project through different methods.
