optimized design of a photovoltaic cathodic protection - CiteSeerX

‫‪S. Kharzi, M. Haddadi, A. Malek, L. Barazane and M. M. Krishan‬‬

‫‪OPTIMIZED DESIGN OF A PHOTOVOLTAIC CATHODIC‬‬ ‫‪PROTECTION‬‬ ‫‪S. Kharzi, M. Haddadi and A. Malek‬‬ ‫‪Division Energie Solaire Photovoltaïque‬‬ ‫‪Centre de Développement des Energies Renouvelables,‬‬ ‫‪B.P. 62, Route de l’Observatoire, Bouzaréah, Alger, Algérie‬‬

‫‪and‬‬ ‫‪Laboratoire de Dispositifs de Communications et de Conversion Photovoltaïque‬‬ ‫‪Ecole Nationale Polytechnique,‬‬ ‫‪Rue Hassen Badi, El Harrach, Alger, Algérie‬‬

‫‪L. Barazane‬‬ ‫)‪University of Science and Technology Houari. Boumediene (USTHB‬‬ ‫‪Faculty of Electronic & Computing‬‬ ‫‪BP 32, El-Alia, Bab-Ezzouar 16111, Algiers, Algeria‬‬

‫*‪M. M. Krishan‬‬ ‫‪Department of Engineering of Information and Measurement Systems‬‬ ‫‪Al-Balq'a Applied University‬‬ ‫‪P.O.Box: 194 Ma'an 71111- Jordan‬‬

‫اﻟﺨﻼﺻـﺔ‪:‬‬ ‫ﻟﻘﺪ ﺗﺮآﺰ اهﺘﻤﺎﻣﻨﺎ ‪ -‬ﻓﻲ هﺬﻩ اﻟﻮرﻗﺔ ‪ -‬ﻋﻠﻰ اﻟﺤﻤﺎﻳﺔ اﻟﻤﻬﺒﻄﻴﺔ ﺑﺎﺳﺘﺨﺪام اﻟﻜﻬﺮوﺿﻮﺋﻴﺔ اﻟﻤﺴﺘﻌﻤﻠﺔ ﻓﻲ اﻟﻌﺪﻳﺪ ﻣﻦ اﻟﺘﻄﺒﻴﻘﺎت‪ .‬وﺗﻌﺘﺒﺮ اﻟﺤﻤﺎﻳﺔ اﻟﻤﻬﺒﻄﻴ ﺔ ﻣ ﻦ‬ ‫أآﺜﺮ اﻷﺳﺎﻟﻴﺐ اﻟﻤﺴﺘﺨﺪﻣﺔ ﻋﻠﻰ ﻧﻄﺎق واﺳﻊ ﻟﻤﻨﻊ اﻟﺘﺂآﻞ ﺑﺼﻮرة رﺋﻴﺴﻴﺔ ﻓﻲ ﺻﻨﺎﻋﺎت اﻟﻨﻔﻂ واﻟﻐﺎز‪ .‬وﺗﻘ ﺪم ه ﺬﻩ اﻟﻮرﻗ ﺔ ﺗﻘﺮﻳ ﺮًا ﺑ ﺸﺄن دراﺳ ﺔ ﺗ ﺼﻤﻴﻢ ﻣ ﻨﻈﻢ‬ ‫اﻟﺠﻬ ﺪ ﻣ ﻊ اﻟﻨ ﺎﺗﺞ اﻟﻤﺘﻐﻴ ﺮ ﺑﺎﺳ ﺘﺨﺪام ﺗﻌﻘ ﺐ ﻧﻘﻄ ﺔ اﻟﻄﺎﻗ ﺔ اﻟﻘ ﺼﻮى ﻣ ﻦ أﺟ ﻞ اﻟﺤﻤﺎﻳ ﺔ اﻟﻤﻬﺒﻄﻴ ﺔ اﻟﻜﻬﺮوﺿ ﻮﺋﻴﺔ و اﻟﺸﻤ ﺴﻴﺔ ‪ .‬وﻳﻘ ﻮم اﻟﻨﻈ ﺎم ﻋﻠ ﻰ اﺛﻨ ﻴﻦ ﻣ ﻦ‬ ‫اﻟﻤﺘﺤﻜﻤﺎت اﻟﺪﻗﻴﻘﺔ )‪ (MPPT‬ﺑﺎﺳﺘﺨﺪام اﻟﻤﺤﻮﻻت ‪ DC/DC‬اﻟﺨﺎﺿﻌﺔ ﻟﻠﺮﻗﺎﺑﺔ‪ .‬وﺗﺴﺘﺨﺪم أوﻟﻰ وﺣﺪات اﻟﻤﺘﺤﻜﻤﺎت اﻟﺪﻗﻴﻘﺔ ﻟﻤﻄﺎﺑﻘ ﺔ اﻟﺠﻬ ﺪ ﻋﻨ ﺪ ﻧﻘﻄ ﺔ اﻟﺤ ﺪ‬ ‫اﻷﻗﺼﻰ ﻣﻦ ﻗﺪرة اﻷﻟﻮاح اﻟﺸﻤﺴﻴﺔ ﻣﻊ ﺟﻬﺪ اﻟﺸﺤﻦ ﻟﻠﺒﻄﺎرﻳﺔ‪ .‬أﻣﺎ اﻟﻮﺣﺪة اﻟﺜﺎﻧﻴﺔ ﻓﺘﺴﺘﺨﺪم آﻮﺣﺪة دﻋﻢ ﺑﻴﻦ اﻟﺒﻄﺎرﻳﺎت واﻟﺤﻤ ﻞ ﻣ ﻦ أﺟ ﻞ ﺗ ﺄﻗﻠﻢ اﻟﻤ ﻨﻈﻢ ﻣ ﻊ‬ ‫ﺟﻤﻴ ﻊ ﺣ ﺎﻻت ﻧﻈ ﻢ اﻟﺤﻤﺎﻳ ﺔ اﻟﻤﻬﺒﻄﻴ ﺔ‪ .‬وﻗ ﺪ ﺟ ﺮت اﻟﻌ ﺎدة أن ﺗﻌﻤ ﻞ ﻧﻈ ﻢ اﻟﺤﻤﺎﻳ ﺔ اﻟﻤﻬﺒﻄﻴ ﺔ ﺑﺎﺳ ﺘﺨﺪام ﺗﻴ ﺎر اﻹﺧ ﺮاج اﻟﻤ ﺴﺘﻤﺮ اﻟ ﺬي ه ﻮ اﻟ ﺸﺮط اﻟ ﻀﺮوري‬ ‫ﻟﻠﺤﺼﻮل ﻋﻠﻰ ﺟﻬﺪ اﻟﻤﻨﺎﻋﺔ ﻟﻠﻬﻴﺎآﻞ وﺑﺎﻟﺘﺎﻟﻲ ﻣﻨﻊ ﺣﺪوث اﻟﺘﺂآﻞ‪ .‬وﻓﻲ ﺣﺎﻟﺘﻨﺎ هﺬﻩ ﻓﺈن اﻟﻌﻤﻞ ﻳﺘﻢ ﺑﺎﻟ ﻀﺒﻂ اﻟﺘﻠﻘ ﺎﺋﻲ ﻟﺠﻬ ﺪ اﻹﺧ ﺮاج ﻟﻠﻤﻬ ﺎﻳﺊ وه ﻮ ﻣﺤ ﻮل وﺣ ﺪة‬ ‫دﻋﻢ ‪ DC/DC‬ﻋﻦ ﻃﺮﻳﻖ اﻟﻤﺘﺤﻜﻢ اﻟﺪﻗﻴﻖ ﺑﺘﻮﻟﻴﺪ دورة اﻟﻌﻤﻞ اﻟﻤﻄﻠﻮﺑﺔ ﺑﺎﺳﺘﺨﺪام ﻣﻨﻔﺬ اﻹﺧﺮاج ﻟﺘﻀﻤﻴﻦ ﻋﺮض اﻟﻨﺒﺾ )‪ .(PWM‬وﻗ ﺪ ﻗ ﺪﻣﺖ ه ﺬﻩ اﻟﻮرﻗ ﺔ‬ ‫دراﺳﺔ اﻟﺘﺼﻤﻴﻢ اﻷﻣﺜﻞ ﻟﻬﺬا اﻟﻤﻨﻈﻢ ﻟﻠﺠﻬﺪ ذى اﻹﺧﺮاج اﻟﻤﺘﻐﻴﺮ‪ ،‬وذﻟﻚ ﻟﻠﺤﻤﺎﻳﺔ اﻟﻤﻬﺒﻄﻴﺔ ﺑﻮﺳﺎﻃﺔ اﻟﻜﻬﺮوﺿﻮﺋﻴﺔ واﻟﺸﻤﺴﻴﺔ‪.‬‬

‫‪* Corresponding authors:‬‬ ‫‪E-mails: [email protected], [email protected], [email protected], [email protected], [email protected]‬‬ ‫‪Paper Received September 30, 2009; Paper Revised March 21, 2009; Paper Accepted May 27, 2009‬‬

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‫‪The Arabian Journal for Science and Engineering, Volume 34, Number 2B‬‬

‫‪October 2009‬‬

S. Kharzi, M. Haddadi, A. Malek, L. Barazane and M. M. Krishan

ABSTRACT Among the large number of photovoltaic applications, we are interested in the cathodic protection; it’s one of the most widely used methods for corrosion prevention, mainly in the oil and gas industries. In this paper, we report on our study of the design of a voltage regulator with variable output and maximum power point tracker (MPPT) for solar photovoltaic cathodic protection. The system is based on two microcontroller controlled DC/DC converters. The first one is a buck used to match the maximum power point voltage of the solar panel to the battery charging voltage; and the second one is a buckboost used between the batteries and the load in order to adapt the regulator for all cases of the cathodic protection systems. Usually, cathodic protection systems are working with constant current output, which is the necessary condition to procure immunity voltage to the structures, thus preventing corrosion from occurring. In our case, the system is carried out by an automatic adjustment of the output voltage of the adapter, which is the buck-boost DC/DC converter, by means of the microcontroller by generating the required duty cycle by way of its pulse width modulation (PWM) output port. In this paper, a study of an optimum design of this voltage regulator with variable output for solar photovoltaic cathodic protection has been carried out. Key words: photovoltaic system, MPPT, cathodic protection, buck-boost, regulator, impressed current, impressed voltage

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OPTIMIZED DESIGN OF A PHOTOVOLTAIC CATHODIC PROTECTION 1. INTRODUCTION It is well known that all metallic structures buried or immersed, and even concreted, inevitably undergo the phenomenon of corrosion once plunged in an electrolyte. Cathodic protection (CP), after a good passive protection, is an efficient means of stopping the process of corrosion. It lowers the potential of the protected metallic structure to the value where the reaction of corrosion cannot take place; this potential is known as "threshold of immunity"[1,2]. In most cases, it requires energy from an electrical energy source to impress the current. This power might be obtained by utility grids if available, and a rectifier. The second alternative is to use a diesel generator and a rectifier, but this system is not maintenance free, whereas photovoltaic power supplies can operate maintenance free under all environmental conditions at any site [3]. Since power modules still have relatively low conversion efficiency, the overall system cost can be reduced using high efficiency power conditioners which, in addition, are designed to extract the maximum possible power from the PV module (maximum power point tracking) MPPT. For this purpose, a lot of work has been done [5–8]. In our case, a regulator circuit with MPPT for a photovoltaic solar energy systempowered cathodic protection system has been designed. The developed circuit allows the solar panel to operate at its maximum power point output (MPP). When the MPP changes, the microcontroller takes care of this and changes the conversion ratio (duty cycle) of the buck circuit to keep the solar panel at its MPP. The buck steps higher voltage panel down to the charging voltage of the battery. On the other hand, to stop corrosion entirely, one of the main conditions to be satisfied by an ICCP system is that the protected structure is to be fed by a constant current determined by the structure's metal, the area, and the surrounding medium. However, due to the environmental conditions, CP resistance changes significantly due to the anode-to-earth resistance being affected by the variations of the surrounding medium resistivity accordingly to extreme dry-to-wet conditions, which leads to the variation of the output current (impressed current). In recent years, there has been increasing interest in the development of efficient corrosion control to improve CP. Thus, several control circuits intended to regulate ICCP powered by solar photovoltaic energy have been designed and discussed [9–12]. In our work, to overcome this difficulty, an automatically regulated cathodic protection system with MPPT control is proposed. This system senses the variations of the surrounding medium resistivity by measuring the impressed voltage Ei of the pipe branch against a buried reference electrode Cu/CuSO4 (CSE) and adjusts automatically the (DC) output voltage of the adapter circuit (buck-boost) so that the current is kept nearly constant at the required level regardless of the soil resistivity variations. This system is developed around the microcontroller 16F877 which controls the output voltage of the buck-boost, the charging voltage of the battery and the maximum power point tracker of the photovoltaic generator [13–14]. The acquisition of the different parameters, as well as the measure of the impressed voltage against the buried reference electrode CSE, is allowed by the 16F877’s ADC ports. By using this suggested system, three important goals are achieved. The first one is the prevention of corrosion, because the metallic structure will always receive the exact current required for protection. The second goal is the reduction of maintenance costs and system costs by using high-efficiency power conditioners, which are the maximum power point tracking (MPPT) control. It is allowing us to maximize the output power of the PV array. Moreover, the proposed system will make use of a clean and renewable source of energy. In this study an optimized photovoltaic cathodic protection system has been developed for tuning output voltage of the interface adapter (buck-boost) to keep the current of protection (impressed current) nearly constant in order to reach the protection criterion. On the other hand, the cost effectiveness of our photovoltaic (PV) array powered system is improved by the efficient use of its generated electric power by way of the maximum power point tracker. In the remainder of this paper, Section 2 shows the system description, Section 3 presents the system for the MPPT, Section 4 introduces the regulated system in impressed current cathodic protection ICCP, where both the load regulator circuit and the proposed buck-boost command are described, Section 5 discusses the results obtained, and Section 6 presents conclusions. 2. SYSTEM DESCRIPTION The synoptic scheme of the ICCP system powered by PV solar energy is illustrated in Figure 1. Its main components are PV modules, which are an array of solar cell modules; a power conditioner, which allows the regulation and control of the batteries’ storage (BVR); the load voltage regulator (LVR), which consists of the cathodic protection system and the maximum power point tracking (MPPT); the load, and a group of storage batteries with sufficient capacity enough to supply the load (cathodic protection system) with the required energy. Auxiliary components include anodes and a copper-copper sulphate CSE reference electrode to indicate the state of corrosion of the pipeline.

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Figure 1. Synoptic diagram of the regulated cathodic protection system supplied by photovoltaic solar energy

3. THE MAXIMUM POWER POINT TRACKER SYSTEM A block diagram of the batteries voltage regulator circuit design with MPPT is shown in Figure 2. A buck type DC/DC converter is used to interface the PV output to the battery and to track the maximum power point of the PV modules.

Figure 2. Block diagram of the batteries voltage regulator circuit design with MPPT

The DC/DC converter is a buck converter which means it takes a higher input voltage and converts it to a lower output voltage. Since this is a switching converter topology, it doesn’t dissipate any power internally. That means the output power is equal to the input power. Therefore, if the watts stay the same and the voltage drops, then output current must be greater than the input current. The microcontroller PIC controls the conversion ratio of the DC/DC converter. It generates in our case a 50 kHz PWM signal with its internal PWM circuit. The ratio of the on time to the period of the commutation of the switch sets the conversion ratio of the input to the output voltage of the DC/DC converter (duty cycle). The PIC tries to set the conversion ratio of the DC/DC converter to allow the solar modules to operate at their Maximum Power Point (MPP). The input watts from the solar panel are calculated by measuring the voltage and current with the PIC’s A/D inputs and multiplying internally to get the watts. The solar panel voltage runs through a resistor divider network to get it down to the 5v range of the PIC’s A/D converter. The solar panel current is measured with a current sense resistor and difference amplifier to condition the signal before it is read by the PIC’s A/D. In each iteration, the DC/DC converter input voltage and current are measured and the input power is calculated. The input power is compared to its value calculated in the previous iteration and, according to the result, the duty cycle is changed. The MPP tracking process is shown in Figure 3. The starting points vary according to the atmospheric conditions, while the duty cycle is changed continuously, according to the above mentioned algorithm, resulting in the system steady-state operation around the maximum power point. The battery storage is monitored continuously and, when it reaches a predetermined level, the battery charging operation is stopped in order to prevent overcharging. Its voltage is read by the microcontroller PIC and with another voltage divider and the current flowing into the battery with another sense resistor and difference amplifier. The battery voltage is used by the PIC to tell when the battery is fully charged. If the battery is charged, the microcontroller rolls back the charging current to keep from overcharging the battery. The flowchart of the battery charge control program is shown in Figure 4. 4. THE REGULATED SYSTEM IN ICCP Concerning the specificity of this regulated system, the adjustment of the DC voltage is done continually and automatically. The basic circuit is a buck-boost converter. The driving signal of its main switch is obtained by means of the 16F877 microcontroller by way of its second PWM port. The PWM generate the required duty cycle in order to adjust the output voltage applied to the protected structure to reach the cathode potential (preventing the protection).

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Figure 5 shows the block diagram of the proposed optimized design of the regulated cathodic protection system supplied by photovoltaic solar energy. 4.1. Load Voltage Regulator Circuit This circuit supplies a controlled DC voltage to the load depending of the control signal voltage. The switching frequency was fixed at 50 kHz. One notes that the two DC/DC converter circuits (buck and buck-boost) must operate at the similar switching frequency because they use the same internal PWM circuit of the microcontroller. The duty cycle αbuck-boost of the chopper buck-boost is generated by the microcontroller’s PWM output signal according to the developed program so as to reach the criteria of the cathodic protection which is included between 1.5 V and 0.850 V in absolute value. This range represents the difference of the potential between the protected structure (the pipe in our case) and the reference electrode Cu/CuSO4 (CSE). Begin Set αbuck to 80%

Mesure de Vpv(k) et Ipv(k)

Ppv(k)= Vpv(k) .Ipv(k)

∆ Ppv=Ppv(k)-Ppv(k-1)

∆ Ppv>0 α buck (k − 1) > α buck ( k ) Yes

Yes

α buck ( k − 1) < α buck (k ) No No

α buck (k − 1) = αbuck (k) + ∆α buck

α buck (k − 1) = αbuck (k ) + ∆αbuck

α buck ( k − 1) = α buck ( k ) − ∆α buck

α buck ( k − 1) = α buck ( k ) − ∆α buck

Figure 3. The MPP tracking process Begin

Acquisition of Ibat,Vbat Vd=1.8V/elt., Vg=2.2V/elt.

No

Vbat=0

Yes

Vbat>Vg

No

Yes

Batteries disconnected from PV

Figure 4. The flowchart of the battery charge control program

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Figure 5. Block diagram of the regulated cathodic protection system

4.2. The Buck-Boost Command In our case, the buck-boost output voltage Vout must be able to insure the immunity voltage for the protected structure. The objective is the ability to continuously reach the conditions of protection in the minimum amount of time by using a simple control technique. It consists of the measuring of Ei, the impressed voltage (the cathode potential), against a buried reference electrode CSE. We act on the duty cycle to have the buck-boost adequate command. Consequently, the structure voltage (Ei) is kept at the effective protection value. Figure 6 shows the circuit model simulating the command.

Figure 6. Modelling and the simulation of the buck-boost command

The structure protection is assured if its voltage Ei against the reference electrode belongs in the range of V1 = 1.5V and V2 = 0.850V (in absolute value). The control is set according to [3]: If Ei − V1 〉 0

(1)

the output voltage should be equal to

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Vout (k) = Vout (k − 1) + (E i − V1 )

αbuck -boost =

(2)

Vout

(3)

(Vout + Vin )

According to Equation (3), the duty cycle becomes αbuck -boost =

Vout (k ) Vout (k ) + Vin

(4)

Therefore, the output voltage (the load voltage) increases if Es exceeds V1=1.5V. If E i − V2 〈 0

(5)

Vout (k ) = Vout (k − 1) + ( E i − V 2 )

(6)

then

As in the above, the duty cycle is calculated by Equation (2). The result is that the output voltage decreases if Ei drops below V2 = 0.850V. This kind of control is based on both the input voltage (battery voltage) and the buck-boost output voltage (the load voltage). This last one must be adjusted to reach the limits values imposed by the cathodic protection. Table 1. PV Module Specifications Under Standard Test Conditions (irradiation = 1 kW/m, T=25°C and A.M = 1.5) Type BPSX50, polycrystalline photovoltaic module

SX 50 BPSolar 50 W 21 V 16.8 V 3.23 A 2.97A 47 °C 0.0065 mA/°C -0.008 V/°C

Manufacturer Maximum power (Pmax) Open circuit voltage (Voc) Maximum power point voltage (Vmp) Short circuit current (Isc) Maximum power point current (Imp) NOCT Temperature coefficient of Isc Temperature coefficient of Voc Temperature coefficient of power Module efficiency under STC

-0.5 % /°C 12 %

5. RESULTS AND DISCUSSION 5.1. Simulation Result of the Maximum Power Point Tracker System The verification of the MPPT control algorithm result is done through simulation. The BP SOLAR BPSX50 PV module was chosen. It provides 50 watts of nominal maximum power. Its characteristics are given in Table 1. In our case, we have considered two modules in series. According to Figure 7 (c), the algorithm tends to adjust the new operating point of the photovoltaic PV module. The duty cycle brings back the system to the maximum power point. The steady state is thus reached. Another result which shows the effectiveness of the aptitude of the maximum power point tracker algorithm is illustrated by Figure 7 (c). The duty cycle αbuckvaries when the insolation varies. Both the instantaneous variation of the power of the PV module and the variation of the PV module voltage are also reported in both Figure 7 (b) and (d). The average values of the PV modules voltage are close to the maximum voltages values, which are close to 34V for 1000W/m2, 30V for 400W/m2, and 32 for 600W/m2. In the case of instantaneous variation of the power, the average values of the fluctuation of its form of wave are close to 100 W for 1000W/m2, 55 W for 600W/m2 , and 34 W for 400 W/m2. We notice that, according to the simulation result, the algorithm of MPPT is able to track easily the maximum power point.

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(a)

(b) T=25°C 1000W/m2

T=25°C 1000 W/m2

600 W/m2

400 W/m2

600 W/m2

400 W/m2

(c)

(d)

(e)

Figure 7. Simulation result of MPPT control (a) Insolation Variation (b) PV power variation (c) PV module characteristics and buck duty cycle variation (d) PV voltage variation and (e) PV current variation

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5.2. Simulation Result of the ICCP Control System In the following example, we consider the output voltage Vout = 24 V so αbuck-boost = 0.5. 1st case: Ei – V1 > 0 with V1 = 1.5V, Ei = 2.5 V if the measured impressed potential Ei is larger than 1.5 V. Here, we have been informed that the soil resistivity has changed without measuring it. Thus, the current has varied. To maintain it, we must adjust the buck-boost output voltage. Figure 8 shows that the buck-boost output voltage increases and becomes Vout = 25V and the duty cycle varies in the same way αbuck-boost = 0.5097.

Figure 8. Buck-boost output voltage and duty cycle in the 1st case

2nd case: Ei – V2 < 0 with V2 = 0.850V, Ei = 0,3V. Figure 9 shows that the buck-boost output voltage decreases and becomes Vout ≈ 23V and αbuck-boost also decreases (αbuck-boost = 0.4937) when Ei is smaller than 0.850V. 3rd case: Ei – V1 < 0 and Ei – V2 > 0 with V1 = 1.5V and V2 = 0.850V, Ei = 1V. In this case, Ei belongs to [0.850 V 1.5 V] (the allowed limits), and the buck-boost output voltage is kept unchanged. Vout = 24 V and α = 0.5 as shown by Figure 10.

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Figure 9. Generated output voltage and duty cycle in the 2nd case

Figure 10. Generated output voltage and duty cycle in the 3rd case

From there, one notices well that the simulated control circuit, according to the introduced cases, delivered the required buck-boost output voltage according to the generated duty cycle. It varies accordingly with the impressed voltage variation Ei. The duty cycle is stable. It increases in the 1st case, decreases in the 2nd case, and remains

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invariable in the last case. Proportionally, the buck-boost output voltage varies in the same way while tending to reach the protected structure immunity criteria. 6. APPLICATION OF A MICROCONTROLLER 16F877 TO OUR MODEL The microcontroller is used to develop our real time multitask application. We must assign priority levels to the tasks mentioned above. This must be achieved by our microcontroller. We also must synchronize between the tasks. We have to carry out an interrupt management program by including the specific concepts allowing communication between the various tasks.

Figure 11. Schematic design of the cathodic protection regulator circuit START

Set α (the duty cycle) for the specified output voltage:

α=

Vout (Vin + Vout )

α=0.5

Vout=24V

Acquisition: Ei : the pipeline measured voltage against Cu/CuSO4

Yes

Increase Vout , thus increase α : Vout(k)=Vout(k-1)+ (Ei – V1) and

α=

Vout ( k ) (Vin + Vout (k ) )

No

Ei – V1 > 0

No

keep α unchanged and Vout(k)=Vout(k-1)

Yes

Ei-V2 < 0

Decrease Vout and also α : Vout(k)= Vout(k-1) + (Ei – V2) and

α=

Vout (k )

(Vin + Vout (k ) )

Figure 12. Cathodic protection control and monitoring algorithm

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The classification of the tasks is as follows:

The PV module current and voltage acquisition. These read values are used to calculate the PV power module so that the microcontroller may accomplish the tracking of the maximum power point.

The batteries current and voltage acquisition. The voltage is used for the charge control of the batteries.

The control and the regulation of the cathodic protection which must operate permanently. This last one must have the highest order of priority (the 1st order of priority).

Figure 11 shows the electronic circuit of the regulator system. The cathodic protection control and monitoring algorithm flow chart is shown by Figure 12. 7. CONCLUSION An elaborate regulated photovoltaic cathodic protection system is suggested and discussed. Compared with a manually conventional adjusted system, this proposed regulator offers continuous and automatic adjustment of the applied voltage so that the buried pipelines receive the exact current required for protection against corrosion. On the one hand, the use of solar energy for this type of application brings a new security for the energy supply and an economic solution. As has been shown by this simulation, the PV module output delivered to a load (battery) can be maximized using MPPT control systems. It consists of a power conditioner to interface the PV output to the load (battery), and a control unit, which drives the power conditioner such that it extracts the maximum power from a PV module. It is a MPPT system for battery charging. On the other hand, the interface of adaptation between the battery and the cathodic protection system is a charge regulator which answers to the specificities of the ICCP. It is a buckboost type DC/DC converter. The driving signal of its main switch is obtained by means of the 16F877 microcontroller PWM output port. The PWM generate the required duty cycle in order to adjust the output voltage applied to the protected structure to reach the cathode potential (preventing the protection), and the variation of the duty cycle must be in accordance with the optimal values of the cathodic protection ranges from -0.850 to -1.5 V. Among the advantages of the novel system:

It saves energy because the voltage is automatically adjusted so that the output voltage never exceeds the required voltage and, thus, minimum dissipation is attained.

The proposed system will make use of a clean and renewable source of energy.

The use of the microcontroller of the microchip provides low-cost and low-power consumption.

NOMENCLATURE BVR

Battery voltage regulator

DC

Direct current

Ei

structure protected voltage measured against a reference electrode Cu/CuSO4 also called impressed voltage, V

f

Switching frequency, kHz

ICCP

Impressed current cathodic protection system

LVR

Load voltage regulator

MPP

Maximum power point of the photovoltaic module

MPPT

Maximum power point tracker

PV

Photovoltaic module (solar panel)

PWM

Pulse width modulation

Vin

Input voltage of the buck-boost circuit, V

Vout

Buck-boost output voltage, V

Vout(k)

Recently measured buck-boost output voltage value

Vout(k-1)

Previously measured buck-boost output voltage value

Cu/CuSO4, CSE

Reference electrode, Copper Sulphate Copper electrode

GREEK LETTERS

488

αbuck

Buck duty cycle

αbuck-boost

Buck-boost duty cycle


October 2009


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[16]

S. Kharzi, M. Haddadi, and A. Malek, “Conception d’un Dispositif de Protection Cathodique Alimenté par Energie Solaire Photovoltaïque”, Revue des Energies Renouvelables ICRESD_07 Tlemcen (2007).

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