Test for the output of the universal bridge

Before developing the AC load part circuit, we began to design another circuit to test the universal bridge output which is connected to a 6 pulses PWM Generator. The designed circuit was shown below:

The line to line voltage and the line to line current in the three phase system is shown below:

Simulation model of the dc load part

After the dc load part was built successfully which is shown in the former blog, we begun to simulate the photovoltaic voltage output and the dc load output.

The curve of the current-voltage characteristics of PVA and the power-voltage characteristic of PVA is shown below:

Then the curves of the Time response of the PVA voltage, PAV current, PAV power and voltage at the dc load is also displayed below:

Test the model to supply for the DC load.

After the PVA model is built successfully, we can use it as a generator to supply power to a simple DC load. Today we were tried to design the dc load circuit and connect it to the PVA model. The designed circuit is shown below:

At the beginning, when we tested this circuit, the result is terrible and we can’t get stable output and the curves we obtained changed too much. They were far away from what we expected. Then we discussed this problem with our supervisor Dr Tang. After that, we found that the problem may caused by the wrong design in the filter so the next work we should do is to improve the design in filter.

In the above figure, it can be seen that in front of the filter, there is a diode.  It also has a little affect on the output voltage so we designed a circuit to test the output of this diode. The circuit is shown below:

When we simulated this design, the following figure can be obtained.

In the figure, it can be seen that at the beginning the voltage in diode increase fast and then it remains stable and to be 0.8 voltage. Thus at the dc load, the voltage should decrease fast at the beginning and then immediately the voltage reaches at a stable state. The initial value of the current is assigned to 0. In this case, the voltage has its maximum value. 

The development of the PVA functional model for the Simulink environment

Recently we are mainly contributed to design the PVA MODEL for GUI environment. It is the main part and the objective of this project is to develop the PVA functional model to simulate the photovoltaic solar cell output. This part uses the modeling stage 2 as a subsystem. The design circuit is shown below:

The Powergui block is used and necessary for any simulink model which contains SimPowerSystems blocks. It can store the equivalent simulink circuit. The other part in this model is mainly used to assign values to each variable in the photovoltaic cell voltage. In the above figure, it can be seen that Ns represents the number of the solar cells in series and is 8 in this model. Np represents the number of the solar cells in parallel and is 1. Tx and Sx are used to represent current value of temperature and solar irradiation and they are 15.9908 centigrade and 102.9493. The temperature is converted into Kelvin in equation of this model. The referent temperature Tc and irradiation Sc is given in the modeling stage 2 and the value is 20 centigrade and 100. According to the equation 1 which is:

It can be calculated that the maximum current is I_c=I_ph+I₀=5.1352 and the minimum current is: I_c=0. To make the voltage positive we use a Saturation component to limit the current in the range of 0 to 5.1. Then it can be obtained that the maximum voltage is: V_c=229.57 and the minimum voltage is: V_c=116.87.

The development of the modeling stage 2

Today, we were tried to design the modeling stage 2. The modeling stage 1 is a subsystem of the modeling stage 2 so this stage should contain the block of stage 1. This stage is used to multiply the cell voltage to form the full array voltage and then divide the array current by the number of the cell in parallel to obtain the cell current. And solar cells which we researched have 8 photovoltaic cells in series and only 1 line in parallel. The block circuit is shown below:

In this circuit, there are several new blocks used. This afternoon, we met our supervisor Dr Tang to discuss the problems we encountered when we assigned value to these blocks parameters. For the controlled voltage source is just used to change the input value into voltage value so the parameter in this block doesn’t need to change. For the Saturation part, only the current flow into this part, so the maximum and minimum value of the current should the calculated. According to the voltage equation, it can be found that the maximum value of current is 5.135 and the minimum value of the current is 0. Then for the transport delay part, Mr Tang suggested us to set the time delay to be 0.0001 so that enough test points can be got.

The development of the Modeling stage 1

Today, we used the Matlab-Simulink Gui Environment to build the first part of the model. The first modeling stage is used to simulate the output voltage and current of one single solar cell. This part is masked as one of subsystems of the last stage. The first stage was built successfully and the Simulink circuit is shown below.

For the Equation1 block and the Effect of Temperature & Solar Irradiation block, we also had two methods to build this part: One is to use Matlab Fun block to build this part and the other method is to use the math operation model in the Simulink to build the output voltage equation and the equation for the effect of the temperature and solar irradiation.

The code by using the first method is shown below:

The connection by using the second method is displayed:

Then this afternoon, we met with our supervisor Dr Tang to discuss about these two methods. We want to find which method is better. Mr. Tang advised us to use the Matlab function block to build this part because there is less component used in the method and this will occupy less CPU space and thus the processing speed can be improved. After the meeting, we tested these two circuits and it was observed that the first design circuit has high performance and faster processing speed. Thus we adopted the first method to develop this part.

Sustainable Development Aspects of the Photovoltaic Array Design

Recently, we were contributed to the Sustainable Development Aspects of the Photovoltaic Array Design. After reading materials, we found that there mainly two problems which restrict the application of the photovoltaic cell. One is the high price of solar cell and the other is the unsteady output voltage. Users are usually like to buy electricity from the utility grid because it is cheaper and steady. To decrease the solar cell price, one method is to use the thin-film cell technology and change the manufacturing process. A report states that the use of thin-film cells grows faster than the C-Si cell and has already comprised about 50% of U.S. production in 2006 compared the 6% in 2004. It is a good opportunity to use thin-film cell instead of C-Si cell. To obtain the steady output voltage, it was proposed that a switching system can be design to change the structural connection of the arrays and the cell array topology so that the required voltage can be established during the different periods of a day. If this design can be realized, the photovoltaic array will must have a more wide range of application.

Then we also learned the life cycle of the photovoltaic solar cell. The life-cycle of solar electric-energy generation mainly can be divided into 6 parts. They are material production; photovoltaic array production; balance of system production implemented by using inverter and transformers; system operation and maintenance; system decommissioning, and disposal. There is no doubt that the photovoltaic solar cells have immediate benefit for sustainability. The solar energy is sustainable, harmless, carbon free and environmental. It can be used as ideal energy source and will not cause environment pollution compared to the fossil energy. Compared to the wind and ocean thermal energy, the photovoltaic solar cells is more effective. Compared to the utility power, the photovoltaic solar cells can also be used in remote and isolated areas for water pumping and air conditioning. In these areas, it is difficult and expensive for the utility power to transport.

Using the Matlab M-file to simulate the photovoltaic array output

After learning the photovlaic array structure and analyzing the influence of the solar irradiation and ambient temperature, at first we used the Matlab M-file to simulate this part and test the output rather than use the Matlab-simulink GUI environment to build the circuit directly.

We edited the code which contains the information PVA equivalent circuit and the influence of the irradiation and the temperature. 

The following figures show the result of the above procedure. The first figure is the current-voltage characteristic of the PVA during the operation and the second figure is the PVA power-voltage characteristic:

The theoretical characteristic curve is shown below:

It can be seen that the simulation result matches theoretical value. Here the cure fitting factor A is assigned to be 110. The value A is used to change magnitude of output voltage to adjust the I-V characteristics. With the different test value of A, we can obtain different maximum powers. 

The influence of the solar radiation and operating temperature

After the analysis of the internal structure of the photo voltaic array, we began to study the external influence to the photo voltaic array. There are mainly two factors that restrict the operation of the photo voltaic: the solar irradiation level and the ambient temperature. These can be affected by sudden change in the weather and the seasons. In this project, the influence of these factors must be researched and designed in the circuit.

It was told that the solar cell operation temperature is changed as a function of solar irradiation level and the ambient temperature.The function is shown below:

The temperature coefficient C_TV is the temperature influence on the photovoltaic cell voltage and the temperature coefficient C_TI is the temperature influence on the photovoltaic cell current.

It should be noted that the constant T_a is used as the reference temperature and the constant S_c is the reference solar irradiation at the temperature T_a. Then the temperature change T_x is made. The value of β_T­ is 0.004 and the value γ­_T is 0.06. The reference temperature T_a is 20 centigrade and the irradiation at this temperature S_c is 100. To make the temperature unit same, the temperature in Celsius should be converted into Kelvin: T_aK=T_ac +273.

Then considering the change in solar irradiation, the solar irradiation change can cause the change of cell temperature and this change also should be considered. It can be found that due to the variation of the solar irradiation level, the photovoltaic voltage and current change factors is expressed by the following equations:

The constant α_s is the change in temperature due to the variation of the irradiation level. It is equal to 0.2 for the solar cells used. S_x is the change value of the irradiation level. The coefficient C_SV is the irradiation influence on the photovoltaic cell voltage and the coefficient C_SI is the irradiation influence on the photovoltaic cell current.

According to above analysis, the new photovoltaic voltage and the current can be obtained after the change in the solar irradiation level and the ambient temperature. The equations for the new value are shown below:

The value V_c and I_ph is used as the benchmark reference cell output voltage and the reference cell photocurrent. V_cx and I_phx is the new value at the new temperature Tx and the new irradiation level S_x.

Circuit design – equivalent circuit of the photovoltaic cell

Today, we were devoted to the theoretical analysis and the equivalent circuit design of the photovoltaic array.

A single solar cell can be represented by a simplified equivalent circuit model and the circuit is shown in the figure 1. The photovoltaic(PV) arrays are made up of many solar cells and they are combined in series or in parallel. In the figure, it should be noted that I­_ph­ is the photocurrent and it is a function of irradiation level and junction temperature. I_c is the cell output current and V­_c is the cell output voltage. The photovoltaic arrays are made up of many solar cells and they are combined in series or in parallel. Thus the array current is n times of cell current where n is the number of the cells connected in parallel and the full array voltage is the cell voltage multiplied by the number of the cells connected in series.

These are the theory of simulation model. Then, next work is to use the Matlab-Simulink GUI Environment to simulate the circuit and discuss the I-V and P-V characteristics. It can be found that the output voltage is affected by the load current and the solar irradiation level. 

These are the theory of simulation model. Then, next work is to use the Matlab-Simulink GUI Environment to simulate the circuit and discuss the I-V and P-V characteristics.