SOLAR EMULATOR AND SIMULATOR DESIGN FOR CUBESATS

63rd International Astronautical Congress, Naples, Italy. Copyright ©2012 by the International Astronautical Federation. All rights reserved.

* Undergraduate Student, Istanbul Technical University, Turkey, [email protected]

t Undergraduate Student, Istanbul Technical University, Turkey, [email protected]

:t Undergraduate Student, Istanbul Technical University, Turkey, [email protected]

§ Undergraduate Student, Istanbul Technical University, Turkey, [email protected]

** Professor, Istanbul Technical University, Turkey, [email protected]

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IAC-12-C3, 4, 9, x15321

SOLAR EMULATOR AND SIMULATOR DESIGN FOR CUBESATS

Mr. Mehmet Ertan Umit

Istanbul Technical University, Turkey, [email protected]

Mr. Mustafa Erdem Bas *, Mr. Isa Eray Akyol

t,

Mr. Mehmet Sevket Uludag

:t, Mr. Ahmet Berkant

Ecevit §, Dr. Alim Rustem Aslan

**

A highly efficient, redundant electrical power system is developed in Istanbul Technical University (ITU) Space

Systems Design and Testing Laboratory (SSDTL). A Hardware-in-the-loop (HIL) simulation platform is designed

for testing, calibration and qualification procedures of the power system. The system is based on a

MATLAB/Simulink model of space qualified solar cells. A DC/DC converter generates required power for any

given load depending on the solar cell model. An interface drives the converter by the information from the solar cell

model. The system is connected to developed power system, in order to measure its performance. This paper

explains the methods, trade-offs and the results of the project.

I. INTRODUCTION

TURKSAT 3USAT is one of the first

communications CubeSat with the main payload of

redundant transponders for voice communication.

Turkish satellite operator TURKSAT is the main

supporter and stakeholder of the project.

A redundant CubeSat was developed to increase

reliability. Master sub-systems, such as On-board

Computer, Modem and EPS are chosen from COTS

equipment with flight heritage. Secondary sub-systems

have been developed in ITU.

The EPS of the system uses sophisticated switching

mechanism to isolate the master EPS after a catastrophic

event. Three MPPT controllers have implemented for

maximum power tracking. 300F supercapacitors have

been used for energy storage. Sensors on the EPS

measure the current and voltage of different sources

such as solar panels in order to generate necessary

telemetry data.

II. DESIGN OF THE SIMULATOR

In order to test the EPS different simple methods

have implemented such as; analogue solar emulator,

LED based light source and outdoor tests on a cloudless

sunny day. Even though simple methods are more

robust and reliable, a more complex HIL was required

for a complete performance measurement of the power

system. To achieve that first a solar panel model is

generated using MATLAB/Simulink. The model

evaluates required voltage/current output of the panel

for any given load on different conditions. Then a

DC/DC converter system is developed to convert the

available electrical power into required Voltage/Current

condition. An interface and sensor system is developed

in order to drive the DC/DC converter depending on the

output of the model and send the load information to the

model.

Fig. 1: Work chart of the simulator system.

Different hardware and software components of the

system are presented on figure 1. Output of the

converter will be very close to real solar cell output on

orbit conditions.

III. DETERMINATION OF SOLAR CELLS’

PARAMETERS

In TURKSAT 3USAT project solar panels have

Spectrolab UJT solar cells. Spectrolab explained

modelling of the solar cells in detail1.

[1]


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[2]

Voc: Open Circuit Voltage

ISAT : Saturation Current

A: Cell Area

Eg: Band Gap Voltage

Fig.2: Equivalent circuit of a photovoltaic cell

2

Rs: 11 mohm (Series Resistance)

RSHB: 1.6x105 ohm (Shunt Resistance)

K1: 146.1 uA/uC

Eg: 1.2

Also the other parameters of PV cell model are

given in Application Note for Spectrolab GaAs solar

cell1 and datasheet

3.

IV. MATHEMATICAL MODEL OF SOLAR

CELLS

Solar cells can be explained and modelled as a non-

linear electrical power supply. The relationship between

voltage and current contains exponential terms and is

given at Equation 34.

[3]

I is cell current and V is cell voltage, IPH describes

“photocurrent”, IS is the saturation current of the cell

diodes, k is Boltzmann’s constant and q is the charge of

one electron. RSH means shunt resistance and RS means

series resistance. IPH changes with environmental factors

as shown in Equation 44.

I PH = [ISC + KI (TC −TRef )]λ [4]

At [4], ISC is the short circuit current of cell, which

determined before, KI is a constant, TRef means the base

temperature of the cell and λ is the solar insulation. Also

another variable, IS varies with reference temperature4.

When the solar cells combine to create a solar panel

Equation 3 express as Equation 5. Ns stands for cell

number in series connection, Np stands for number of

cells in parallel.

[5]

These equations are used to generate a solar cell

model on MATLAB. I-V graphics have generated and

compared with real data on Figure 4 and Figure 3. As

seen on Figure 3 model results correspond to real data

from manufacturer datasheet3.

Fig. 3: Output for mathematical model of solar cells.

Fig. 4: Typical characteristic from datasheet

3.

Additionally, PV solar model has been tested for

different environmental conditions such as irradiation

and temperature deviation. The dependency of solar cell

I-V characteristics to temperature and irradiation are

shown in Figure 5 and Figure 6.

Fig. 5: Changes on characteristic according to

varying temperature.


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Fig. 6: Changes on characteristic according to

varying irradiation.

Figure 5 shows that, solar model evaluates correct

output. For higher temperatures, ISC is higher and VOC is

lower. Figure 6 presents that, ISC has a bigger value for a

greater irradiation level.

V.DC-DC CONVERTER

DC - DC converters are being used in low power

electronic devices due to their efficiency and

simplicity.5

By using buck converter, voltage level can be

regulated to any desired lower voltage. One of the good

things about the buck converters is that the output is

controlled via PWM signal, which is very easy to

generate. PWM’s duty cycle is the ratio of conduction

time over period. Duty cycle is proportional with both

conduction time and output voltage. If duty cycle

increases, the output voltage will also increase and vice

versa. Buck converters schematic can be seen in Figure

7. While on conduction mode the buck converter

circuits inductance get started to store energy and when

the switch is off again, the load will use this stored

energy in the inductance. Also the capacitor at the

output side is for reduce the output voltage’s ripple.

Fig. 7: Buck converter circuit

6

Before manufacturing the simulator circuit, some

calculations had been done (Table 1) in order to

calculate required inductance and capacitance to obtain

required ripple values at every type of the PV cells.

Value of L and C are calculated using the given values

in Table 1 via equations below7.

[6]

[7]

[8]

[9]

[10]

Table 1: Inductance and capacitance values for different

PWM frequencies.

Regarding to Table 1, biggest inductance and

capacitance values are chosen to get the lowest voltage

and current ripple from the PV simulator.

VI.INTERFACE

The interface measures the load 100 times a second

using a current sense amplifier. The amplifier measures

the current and the voltage on a single transmission line.

With that information evaluating the load is very easy.

This load information is transferred into MATLAB

model. The model then evaluates the output depending

on the load and sends the output information to

interface. The interface modifies the PWM signal

depending on the output information and measures the

new output in a simple control loop.

PV Model at SIMULINK

In the PV Panel block, the data which are read and

sent by sensors used for determining the PV panel

output voltage and current at specific operation point

using the Equation 1, 2 and 3.


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Fig. 8: General SIMULINK model

The incoming data are parsed into useful

information. The system contains three different panel

and each panel sends voltage and current values which

make six sensor data. The data are read as 12 bytes at

every cycle via serial communication port and then

converted to the voltage and current values. The

transformation elaborated at Fig. 9.

After the reading of data, the load values are

determined by using with incoming current and voltage

values at PV Panel block. Also the desired voltage and

current values are calculated in this block via

mathematical PV model equations at specific panel

temperature, irradiation level and load condition.

After all these calculation, calculated voltage values

are compared with incoming load voltage. Then

command signal is generated according to difference

between voltage values. If incoming load voltage is

greater than calculated voltage value, a command send

to MBED via serial port which is “1” in character

format (49 in decimal format) for decreasing the output

voltage.

If incoming load voltage is smaller than calculated

voltage value, a command send to MBED via serial port

which is “2” in character format (50 in decimal format)

for increasing the output voltage. The command block

elaborated at Fig. 10.

Fig. 9: Parsing block in SIMULINK model

Fig. 10: The command generating block Generated commands are sent after determination via

serial port and the data reading procedure from

microcontroller, which reads the data from voltage and

current sensors starts again.

V. RESULTS

Simulator system has been tested with different

frequencies of PWM. It is clearly seen that (Figure 11-

16) at lower frequencies resolution gets better and

simulation result fits to the real PV cell I-V

characteristic.


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As shown in Figures 11 to 16, the results of open

circuit and short circuit operations are not as conditions'.

The solution of this problem is increasing the resolution

of measurements and having a more stable load.

Fig. 11: Voltage and Current measurements at 100kHz

PWM signal with 0.5% resolution (7 cell in series)












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Fig. 17: Work chart of electrical power system.

Fig. 18: Electrical power system(left) and simulator

hardware(right).

Fig. 19: TVAC test preparation.

Fig. 20: TVAC test operation.

VI. CONCLUSION

The aim of this project is designing a solar simulator

for testing the produced systems. Simulator consists of a

software part and a hardware part. In software part,

MATLAB-SIMULINK models are used, and the

hardware part contains a microprocessor, convertor and

a load. Work chart of electrical power system is given in

Figure 17. In Figure 18, full hardware of simulator and

EPS is shown. Also in Figure 19 and 20, scenes from

TVAC test can be seen.

For future works, simulator can be improved by

having a better load and increasing the resolution. The

way of increasing the resolution is using a faster

microcontroller which has a better clock speed.

VII. REFERENCES

[1] Spectrolab Solar Cells Application Note

http://www.spectrolab.com/appnotes/0902%20Analytica

l%20Model%20for%20C1MJ%20and%20C3MJ%20C

DO-100%20Cells%20and%20CCAs.pdf

[2] Symposium on , vol., no., pp.2392-2396, 4-7

June 2007 doi: 10.1109/ISIE.2007.4374981

URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&a

rnumber=4374981&isnumber=4374555

[3] Spectrolab Ultra Triple Junction Solar Cells’

Datasheet

http://www.spectrolab.com/DataSheets/cells/PV%20UT

J%20Cell%205-20-10.pdf

[4] Huan-Liang Tsai, Ci-Siang Tu, and Yi-Jie Su

(2008). Development of Generalized Photovoltaic

Model Using MATLAB/SIMULINK. In: Proceedings

of the World Congress on Engineering and Computer

Science, San Francisco, USA 22-24 October 2008, pp.1

[5] Sizikov, G.; Kolodny, A.; Fridman, E.G.;

Zelikson, M.; , "Efficiency optimization of integrated

DC-DC buck converters," Electronics, Circuits, and

Systems (ICECS), 2010 17th IEEE International

Conference on , vol., no., pp.1208-1211, 12-15 Dec.

2010 doi:10.1109/ICECS.2010.5724735

URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&a

rnumber=5724735&isnumber=5724436

[6] Texas Instruments Power Supply Topologies

Poster http://www.mti.tul.cz/files/vke/sluw001a.pdf

[7] MICROCHIP Buck Converter Design Example

http://simonthenerd.com/files/smps/SMPSBuckDesign_0

31809.pdf

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