Control Circuit For a Stand Alone Photovoltaic Led Road Lighting System Design And Implementation

Control Circuit for a Stand-alone Photovoltaic LED Road Lighting System: Design and Implementation

Hammad Abo-Zied Mohammed and Hamdy A. Ziedan

Electrical Engineering Department, Faculty of Engineering, Assiut University, Egypt.

Abstract

This paper presents an optimum design and implementation of the control circuits for a stand-alone photovoltaic (PV) LED road lighting system. Control circuit for this system adjusts Maximum Power Point Tracking (MPPT), which makes the road lighting system operated effectively and adjusts battery charging process. Also, control circuit function is considered to automatically switch ON lights at sunset. The simulation study is done by using the PSIM and Simulation of MATLAB software to achieve optimum design. Experimental work has been done in the Renewable Energy Laboratory at Assiut University, Egypt. Experimental results verify the feasibility of this design.

Keywords: Photovoltaic, Stand-alone, control circuit, LED lighting system, battery charging.

1. Introduction

Renewable energy is generated from natural resources; sun, wind, ocean and fuel cell. Initial cost of solar Energy is high; but advantages of low cost of running and maintenance are achieved. Solar energy is produced by sun and reached earth in the form of solar radiation. Amount of energy production depends on PV module efficiency and solar irradiation. Solar module current is proportional to solar irradiance [1 – 3].

PV lighting systems are usually stand-alone or "off-grid" systems as shown in Figure 1, [4, 5]. PV system design includes three aspects; (1) capacity design, (2) electrical circuit design and (3) control circuits design. Capacity design is to figure out the needed solar panel arrays, battery arrays and matching loads for year round effective operation of a stand-alone system. Electrical circuit design is to design a suitable electrical circuit to meet the requirement, [6, 7]. The first two aspects were explained in detail by the authors in reference [8].

This paper is aimed at designing and implementing of control circuit of stand-alone PV lighting system for highways and car parking lighting.

2. Design of control circuit

Control circuits consist of two main parts; First, control circuit of Maximum Power Point Tracking (MPPT). Second, control circuit of charging and discharging batteries as shown in Figure 2, [9, 10].

Figure 1. Stand-alone PV system components of Lighting System.

Figure 2. Block diagram of PV Lighting System.

2.1. Design of MPPT Control Circuit

MPPT control circuit is a fully electronic circuit including a microprocessor and DC/DC step down converter, for battery charging of the solar power system. Microprocessor tries to maximize power input from solar panel by controlling step down voltage ratio to keep solar panel operating at its maximum power point [13, 14]. The MPPT control circuit consists of the following main parts:

A. Current Sensor

Current sensor is a device that detects electrical current (AC or DC) and generates a signal proportional to it. Generated signals could be analog or digital. It can be utilized for control purpose as shown in Figure 3. In the present case, the generated signal from sensor is analog voltage. Current is measured by taking a signal from the circuit using a very small series resistance. To avoid power loss and compensate voltage drop in the circuit, a compensator with gain 10 is provided by a differential amplifier circuit. Resistance R1 of 0.05 Ω with rated power up to 6 W is used in Figure 3.

Figure 3. Current sensor circuit.

B. Voltage Sensor

Voltage can be sensed by voltage divider circuit, Figure 4, which is simple linear circuit with output voltage (Vout) expressed as a fraction of input voltage (Vin) as expressed by Eqn. 1.

(1)

(2)

where D is duty cycle.

C. Buck converter

Buck converter is step-down DC to DC converter based on current in inductor which is controlled by switches, usually MOSFET, as shown in Figure 5.

D. Microprocessor control

DC/DC converter is controlled by PIC16F877A microprocessor U1, Figure 6, [13, 14]. It is clocked at 20 MHz by the crystal XTAL1. PWM output of PIC on pin 16 is used to control duty cycle of the DC/DC converter which sets its voltage conversion ratio. Frequency of PWM is set to 100 KHz by PIC software. PWM duty cycle is controlled by PIC software to optimize power output from solar panels. MOSFET driver (2004A U2) is controlled by output pin 16 of PIC. PIC calculates solar power generated from solar panels through A/D converter on pins 2 and 3, Figure 6.

Circuit Description and operation

Buck converter (DC/DC converter) is made up of synchronous MOSFET switches Q1, energy storage devices inductor L1 and capacitors C1, C2, C10. Both high side and low side MOSFETs are IRFf250 N-fets. N-fets were chosen for their low Rdson to reduce resistance losses in DC/DC converter. Input of capacitors C1, C2 and output of capacitor C10 are low ESR capacitors to handle large current pulses from switching DC/DC converter. Inductor value is 33µH and it is sized to handle 11 A. D2 is fast recovery diode used to conduct the circulating current while low side MOSFET is turning on. C7 and R3 is a snubber network used to cut down on some of electrical noise generated by switching current in inductor. Diode D1 is added to allow system to be turned off when connected to battery. In PPT; Q1 is shut off at night when there is no enough solar power to charge battery. Microprocessor also should shut off when there is no longer any solar power to run it. D1 is installed, so it blocks reverse current flow. Synchronous MOSFET Q1 gate driver U2 is 2004A. It drives high and low side MOSFETs using PWM signal from microprocessor. Equivalent circuit of IC2004A is shown in Figure 7.

Figure 7. Equivalent circuit of IC2004A.

Figure 6. Simulation of MPPT control circuit.

ULN2004A are high voltage, high current darlington arrays, each containing seven open collector darlington pairs with common emitters. Each channel rated at 500 mA and can withstand peak currents of 600 mA. Suppression diodes are included for inductive load driving and inputs are pinned opposite.

R2 and R3 are voltage divider used to drop the input voltage into 5 V, which can be read by PIC. R5 is current sensor resistor. By using program MPLAB, communicate and interface code into PIC and run simple debugging commands can be done without removing processor. PIC has a build in serial port that used data from Peak Power Tracker (PPT). So, it can be logged externally.

If Maximum Power Point (MPP) of solar panel is fixed, it would be simple to design DC/DC converter with fixed conversion ratio. It converts MPP voltage down to battery voltage level. However, changing of MPP depends on amount of irradiation and temperature of solar panel. Microprocessor takes care of this by measuring input power from solar panel and changing the conversion ratio to keep solar panel at its MPP, Figure 6.

DC/DC converter is a buck converter; which takes a higher input voltage and converts it to a lower output voltage. Since this is switching converter topology. It doesn’t dissipate any power internally, except for some small resistive losses. That means, output power is equal to the input power, Pin = Pout. So, if power stays same and voltage drops then, output current must be greater than the input current. This is a synchronous switching converter, meaning it has low side and high side MOSFET switch. This is slightly more efficient than a converter with just a high side switch. N-fets are used for MOSFET switch because they have lower Rds on, which means less resistive losses in the switches. However, this causes problems with driving high side MOSFET. To fully turn on, an N-fet must have a gate to source voltage (Vgs) of roughly 8 V or greater. The high side switch in DC/DC converter has its source pin tied to input voltage so we have to generate a gate drive voltage of at least 8 V higher than the input voltage. This is taken care of by bootstrap capacitor connected to MOSFET driver 2004A. When low side switch is on, bootstrap capacitor is charged to the input voltage. Then, when high side switch is on, cap adds voltage to input voltage. So, double input voltage now drives high side switch. This works as long as the converter is switching but occasionally high side switch is turned on full time to directly connect solar panel to battery. Microprocessor, PIC16F877A, controls the conversion ratio of DC/DC converter. PIC generates 100 kHz, PWM signal with its internal PWM circuit. Duty cycle of PWM signal sets ratio of on time for high side MOSFET switch versus on time of low side MOSFET switch. On time ratio of switches sets the conversion ratio of input to output voltage of DC/DC converter.

PIC tries to set conversion ratio of DC/DC converter to allow solar panels to operate at their Maximum Power Point (MPP). Microprocessor does this using an iterative algorithm to maximize input power of solar panels. Input power from solar panel is calculated by measuring voltage and current with PIC’s A/D inputs and multiplying internally to get power. Solar panel voltage runs through a resistor divider network to get it down in 5 V range of PIC’s A/D converter. Solar panel current is measured with a current sense resistor and difference amplifier to condition signal before it is read by PIC’s A/D. Microprocessor transmits out serial port all values it has measured and calculated (volt, current and power) once a second to be logged by an external computer. Every 20 seconds PIC set conversion ratio of DC/DC converter to 100% or full on. This simulates direct connection between solar panels and battery. Solar panel input power are measured and compared with solar panel power at MPP to get the gain or boost of PPT.

2.2. Design of Control Circuit of Battery Charger

Function of solar charge controller is to regulate power flowing from a photovoltaic panel into a rechargeable battery. It features easy setup with one potentiometer for float voltage adjustment, an equalize function for periodic overcharging, and automatic temperature compensation for better battery charging over a wide range of temperatures. This circuit is able to handle reverse polarity connection of both battery and photovoltaic panel [6-11]. Simulation circuit for Battery Charger is shown in Figure 8, [11 – 14].

3. Experimental Technique

Experimental work has made in Renewable Energy Laboratory, Electrical Engineering Department, Assiut University, Egypt. Characteristics of PV Module are 83 W with a maximum open circuit voltage (Voc) of 19.7 V, maximum short circuit current (Isc) of 5.78 A, maximum power voltage (Vrate) of 15 V and maximum power current (Irate) of 5 A. These characteristics are obtained experimentally during the day time, Figure 9.

Conventional controller simply connects module and battery. Therefore, if module operates at 12 V, conventional controller artificially limits power production to 63 W. In this work, MPPT system operates module at 15V to extract full power at 83.

Figure 8. Simulation circuit for Battery Charger.

Figure 9. PV Module real characteristics as obtained experimentally (Assiut University).

4. Implementation Results and Discussion

4.1. Implementation of MPPT control circuit

Figure 10 shows the connection diagram of MPPT circuit which was implemented in the laboratory. Switching signal with frequency of 100 KHz was generated by PIC as shown in Figure 11.

Figure 10. Connection diagram of MPPT circuit (Assiut University).

Figure 11. Switching signal with 100 KHz frequency (Assiut University).

4.2. Implementation of control circuit of battery charger

Charging circuit is connected between PV module and battery. It consists of the following main parts as shown in Figure 12:

A. Voltage regulator

IC 78l05 was used to generate 5 V-DC. Output voltage signal of IC 78105 is shown in Figure 13. This voltage is used for all circuit parts which need DC voltage to operate such as comparator and flip-flop.

Figure 12. Connection diagram of battery charger (Assiut University).

B. Clock oscillation

Comparator IC1b was used to generate a square wave pulses. These pulses were used to trigger clock of flip-flop. Wave frequency was 37 Hz. Square wave pulses, clock oscillation, are shown in Figure 14.

Figure 13. Output voltage signal of IC 78L05 (Assiut University).

Figure 14. Square wave pulses, clock oscillation (Assiut University).

4.3. Charging and discharging states of battery

There are two different states for operation of battery charging circuit:

A. Charging state

Comparator IC1a compares battery voltage with a reference float voltage. When battery is empty (less than 12 V), non-inverting input (V+) will be higher than inverting input (V-), high gain of op-amp causes output to saturate at the highest positive voltage of (+5 V) as shown in Figure 15. Output of comparator will make flip-flop operate due to clock pulse as shown in Figure 16. Flip-flop output signal is shown in Figure 17. It will operate transistor Q1 which make IRF4905 operate and connect PV to battery to start charging as shown in Figure 18. When the battery exceeds 12 V. So, IC1a and flip-flop outputs drop to zero and battery charging comes to end.

B. Discharging state

In discharging state, battery voltage is higher than 12 V. Transistor Q1 will not operate and subsequently IRF4905 will not operate also and the battery continues discharging.

4.4. Voltage on charging state LED

In charging state, battery voltage is periodically, turning on and off as shown in Figure 19. Period of change depends on battery condition. When it is empty, turning off time will be very small, so change in red light can’t be noticed, but when its voltage is float voltage, turning off time increases to exceed the charging time value and LED light is turned to be green.

Figure 15. Output signal of comparator (Assiut University).

Figure 16. Comparing of comparator signal with the clock oscillation signal (Assiut University).

Figure 17. Output signal of flip-flop (Assiut University).

Figure 18. Battery voltage signal (Assiut University).

Figure 19. Voltage signal on charging state LED (Assiut University).

5. Conclusions

(1) A PV-based stand-alone system for highways and car parking lighting is designed and implemented in the laboratory.

(2) The control of the lighting system includes circuit for Maximum Power Point Tracking of solar irradiation and circuit for battery charging and discharging.

(3) The proposed lighting system is tested and worked satisfactorily.

6. Acknowledgment

The authors would like to thank Prof. Mazen Abdel-Salam of Assiut University, Egypt for his interest and help in this research work.

7. References

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Hammad Abo-Zied Mohammed received his B.Sc. and m.Sc. from Assiut university / Egypt in 1993 and 1998 respectively. He received the Ph.D. from Darmstadt university and Assiut university (Germany-Assiut) in 2004. His field in Electrical Engineering. From 1995 to 2000 he worked as a Teaching Assistant at Assiut university. He worked as a lecturer at the Department of Electrical Engineering, faculty of Engineering, Assiut University in 2004 and from 2010 to 2013. He worked as a lecturer at the Department of Electrical Engineering, faculty of Engineering, University of Omar El-Mouktar, Libya from 2005 to 2010. From 2013 till now, he work as Prof. assistance at the Department of Electrical Engineering, faculty of Engineering, Al jouf University, KSA. He taught numerous courses in utilization of energy, Electrical machines, electrical power plants, Electrical drive and power electronics. He has numerous publications in the fields of, power electronics, Electrical machines and renewable energy.

Hamdy A. ZIEDAN was born in New-Valley, Egypt, on 1981. He received the B.Sc. and M.Sc. degrees from Minia University, Egypt, in 2003 and in 2006 respectively. He was a Ph.D. student at [anonimizat] Laboratory, Faculty of Electrical Engineering, Czech Technical University in Prague, Czech Republic from 2009 to 2010. He received his Ph.D. degree in High Voltage Engineering from Faculty of Engineering, Assiut University, Egypt in 2011, according to a Governmental scholarship Program, which means a Scientific Co-operation between the Czech Technical University in Prague, Czech Republic and Assiut University, Egypt.

Since 2004, he has been with the Department of Electrical Engineering, Faculty of Engineering, Minia University and Assiut University as a Teaching Assistant, Lecturer Assistant, and since 2011, as an Assistant Professor. He was a Visiting Researcher at Czech Technical University in Prague, Czech Republic, from 2012 to 2013. He is an Assistant Professor at Electrical Engineering Department, Faculty of Engineering, AlJouf University, Saudi Arabia from 2014 until now. His research interests include High Voltage Engineering, Electric Power Systems and Renewable Energy.