ENTRY 3: FYP 2 Reflection of progress 3

For this week, I am deciding to remove the bidirectional DC-DC  converter, because I can't get a better result. So, after that, I decided that I want to use the Buck converter for stepping down the DC bus voltage from the rectifier. The reason we need buck is to reduce the voltage in order to charge the battery for low power applications, which is 120Vdc. Next is we need to calculate the parameters for the buck converter. Based on my calculation, 

D=Vo/Vin=120/240=0.5

L=15.6mH and C=0.5μF

From that, we can get the result which is 120Vdc from the step down which is 240Vdc.

In this week 11, I already upgrade the result for the boost converter that I simulate for the last few weeks. From the separate design, we get the desired voltage that we want which is 440Vdc which will be fed to the inverter. the voltage is stepped up from the Grid and the battery which 120Vdc. The Grid is being stepped down first. We also try to put the controller for the boost converter by using the DC-DC PWM Generator to stabilized the output.

For without control, 

For with controller,

For this current week,  with the buck and the boost converter has been completed, therefore we will combine the circuit that has been designed before. The circuit is shown below,

The result for the rectifier,

Then for the buck converter result, 

Next result is the boost converter result for open loop,

Finally is the inverter before the filter,

After filter,

Voltage THD after the filter,

This simulation consists of 2 mode of operation which is the normal mode and the stored-energy mode operation. In this simulation, the circuit breaker is functioning as a timer which connected in the overall circuit to show the operation of the UPS when the main supply is in normal mode condition and when the main supply fails to operate the battery will supply the voltage which is the stored-energy mode operation. The circuit breaker is controlled using a timer. The timer has been set at 0.5 s. Before 0.5 s, the main supply keeps supplying power in normal operation, but once the timer is triggered, the supply automatically cuts off, and the Online UPS begins to operate.

In this simulation, the circuit breaker has been set to trigger at 0.5s. The voltage remains constant for 0.5s to prove that the main supply is functioning well. But, after 0.5s, the main supply has been cut-off or fail to operate, as shown below. From here, results on the normal mode can be seen in the On state while the UPS mode can be seen in the Off state condition.  The Figure 4.1 below is the waveform for the voltage source which is 240Vac RMS with the peak-peak voltage is 339V with the current waveform in Figure 2 when the timer become 0 the current will instantly become zero after 0.5s.

for current,

RECTIFIER

Based on the rectifier design before the uncontrolled rectifier, the input voltage, which is 240Vrms, will go through the rectifier until the timer reaches 0.5s. The purpose of this timer is to convert the operation from the normal mode into the stored-energy mode, where the supply will be cut-off, which will also affect the rectifier. The figure below is the output waveform for the rectifier before and after 0.5s.

Then, the Figure below shows the enlargement for the rectifier waveform. The value of the ripple can be reduced by using the suitable value for the capacitor and the resistor used. From Figure 4.2, the voltage ripple from the simulation is,

Ripple Voltage = 338.84 - 318.48 = 20.36V

The peak-to-peak ripple voltage from the theoretical calculation is 0.5V. Meanwhile, the value for the simulation design is 20.36V, which is different from the before combine the whole circuit. This possibly due to output from the battery and the impedance or load become higher after the rectifier combines with the other components.

BUCK CONVERTER

The peak-to-peak voltage 339Vdc from the rectifier design circuit will flow through the buck converter and stepped down to 120Vdc. It will be fed the boost converter and charging the battery until the timer reaches 0.5s. For the uncontrolled rectifier, the voltage output for this circuit is equivalent to the peak of the voltage source, which will become the input voltage for the buck converter. The Figure below is the output waveform for the buck converter before and after 0.5s.

Table 4.1    The simulation result of buck converter

Table 4.1 shows the result from the output waveform for the buck converter for both normal and stored-energy mode operation that is recorded from the simulation result. From the theoretical value, the buck converter supposedly to produce 120Vdc. However, the voltage produces for the normal mode, and stored-energy mode is 121.680Vdc and 119.776Vdc, which is near the theoretical.

BOOST CONVERTER

The process of boost operation from the boost converter is to step up the voltage. The 120Vdc that produces from the buck converter that receives the output from the rectifier that has been rectified and then, it also can be produced from the battery. Which the source from this both sources will be stepped up to 400Vdc. The Figure below is the output waveform for the boost converter before and after 0.5s.

Table 4.2    The simulation result of the boost converter

Table 4.2 shows the result from the output waveform for the boost converter for both normal and stored-energy mode operation that is recorded from the simulation result. From the theoretical value, the boost converter supposedly to produce 400Vdc. However, the voltage produces for the normal mode, and stored-energy mode is 416.011Vdc and 411.252Vdc, which is near the theoretical.

INVERTER WITH FILTER

The voltage output and current output of inverter with low pass filter are shown in Figure below after receiving the input voltage from the boost converter. The figure below shows the spectrum of the inverter with a THD of 1.43% with a maximum voltage 341.10V. Table 4.4 shows the parameter that is recorded from the simulation result for the normal and stored-energy mode operation.

For THD,

Table 4.4   The simulation result of inverter after filter

Then, from all the results above, the output voltage and current for the inverter with a filter for both normal and stored-energy mode operation are near the theoretical value which is 240Vac rms. LC filter is used to get a pure sine-wave waveform, which is the same as the input voltage.

In conclusion, the design that has been simulated, a full performance comparison between simulation for both difference mode, which is the normal mode and stored-energy mode results, has been discussed and analyzed. Based on these simulation results, the objective of building a backup system that consists of 5 main components, which is a rectifier, buck converter, PFC boost converter, and inverter, is achieved. This simulation of the open-loop UPS systems is successfully implemented with the two-mode of operation, which is the normal mode and the stored-energy mode operation. From this design, it meets the purpose of the Uninterruptible Power Supply (UPS), which is to take over as the main supply role to keep supplying power by cutting off the power supply by using a circuit breaker at the time of 0.5s. The UPS designed is to keep supplying power according to the load demand or provide electrical power when a normal utility system fails to operate. This system will continue supplying the load by using the battery whenever the power outage occurs. From the behaviour for both operations, it has a bit slight difference for the output voltage, which is the component in this design still needs improvement even though the output from both operations is near with the theoretical value. The battery output waveform from the battery also has a little bit ripple, which supposedly the battery output DC should not have ripple. This possibly due to the rectifier or the other components after combining the whole circuit. Therefore, both operations meet the theoretical, but further improvement might be needed to achieve a better result.

Based on the Online UPS that has been designed, there are still weaknesses and problems that need to be overcome. So, here are several recommendations that can be made in the UPS system for further improvement. Firstly, design the UPS system with a controller to improve the performance of the output waveform. By using the correct type of controller, the output voltage can be maintained even if the load is changed from full load to no load or vice versa. Furthermore, the focus for the improvement can be given to a DC-DC converter such boost converter. This recommendation is given since the output for the boost converter, which does not have a really good ripple. Other than that is to design a controller model for the rectifier to overcome voltage fluctuation in the system. By doing so, it can give the battery a longer life since the voltage used to charge the battery is always constant in any condition. Lastly, using less DC converter to use fewer power switches, which can cause losses and maybe it can affect the battery, which as shown from the waveform result for the battery, during discharging mode, the output DC voltage battery should not have ripple.