POWER MANAGEMENT AND CONTROL FOR HYBRID PV/BATTERY DC MICROGRID

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1 International Journal of Electrical Engineering & Technology (IJEET) Volume 9, Issue 5, September-October 2018, pp , Article ID: IJEET_09_05_004 Available online at ISSN Print: and ISSN Online: Journal Impact Factor (2016): (Calculated by GISI) IAEME Publication POWER MANAGEMENT AND CONTROL FOR HYBRID PV/BATTERY DC MICROGRID R. D. Bhagiya Assistant Professor, Electrical Engineering Department, Government Engineering College, Bhuj, India Dr. R. M. Patel Professor, Electrical Engineering Department, MEFGI s FoPG, Rajkot, India ABSTRACT This study proposes an effective power management and control strategy for standalone PV/Battery hybrid dc microgrid. Maximum available power for a given instantaneous environmental conditions from a photovoltaic (PV) is extracted by maximum power point tracking (MPPT) algorithm. DC bus voltage is regulated by compensating the mismatch power between PV power generation and load demand by optimal charging and discharging of battery under stand alone mode condition. The double loop control is implemented for both PV boost converter and Battery buck/boost converter. The proposed work has been validated through MATLAB/Simulink results under different environmental and load conditions. Key words: DC microgrid, hybrid PV/Battery, mismatches power, reliability of power. Cite this Article: R.D. Bhagiya and Dr. R.M. Patel, Power Management and Control for Hybrid PV/Battery DC Microgrid. International Journal of Electrical Engineering & Technology, 9(5), 2018, pp INTRODUCTION The developments in power electronics and control technology, decreasing cost of photovoltaic (PV) panel, free cost of solar energy, and large storage capability of various storage devices have motivated the use of PV based energy power generation[1-2]. Rising price of fossil fuel and environmental concerns have made force to use renewable sources for power generation worldwide. A dc microgrid has many advantages because of many sources like PV is a dc type source and it can be easily interfaced and need less number of conversion stages as many domestic loads are of dc in nature. Availability of solar energy at almost every places; it has been became possible to provide electricity at remote places by standalone PV/Battery system. Dc bus voltage is not regulated; as load is not always matched with available PV power as PV power generation is changed with change in solar irradiation and 33 editor@iaeme.com

2 Power Management and Control for Hybrid PV/Battery DC Microgrid temperature. Power difference between available PV power generation and load demand should be stored or supplied as per requirement. Among all type of storage devices, battery is the best option because of its low cost, maintenance free, and modular in nature [3]. Since PV array and battery both are DC sources, this make easy interface with dc bus. For a stable, optimal, and reliable operation of a dc microgrid; PV array, battery, and load are controlled effectively. I-V and P-V characteristics of a solar cell are non-linear, which makes it difficult to determine the maximum power operating point on I-V characteristic. For maximum power extraction from the PV array, it must always be operated at or very close to the point on P-V curve where the product of the voltage and output current is the highest. This point is referred to as the maximum power point (MPP), and it is located around the knee of the P-V characteristic. Various types of MPPT algorithms are investigated and each has their own advantages and limitations [4]. Perturb and observe (P&O) and incremental conductance (INC) methods are generally used due to their ease in implementation and good performance [5]. DC-DC unidirectional converter is used to match the source impedance and load impedance for maximum PV power extraction. Boost converter topology is the best because of its low cost and high efficiency, but it has poor tracking capability under low irradiation conditions as its operating points become outside from its operating region for low values of irradiation [6-7].There are two ways to connect the battery with dc bus in PV/Battery hybrid standalone system. In first case, battery is directly connected to the dc bus and, hence the dc bus voltage is determined by the nominal battery voltage. This type of connection is generally used in PV based UPS systems. In this case, maximum power extraction from PV is not possible at all conditions. Also, large surge current may damage the battery during large load variations. The battery stack must be configured to obtain the required nominal dc bus voltage which results in a parameter matching problem and reduces the flexibility in matching the system components [8-9].Power balance is necessary among different sources, storages and loads in microgrid from stability and security point of view [10-11].More number of converters and complex control makes the system less stable and sluggish [12-13]. In this paper minimum possible numbers of converters are used. This paper proposes double loop control for PV unidirectional converter and battery bidirectional converter. 2. SYSTEM ARCHITACTURE A standalone microgrid consists of PV array, dc-dc unidirectional converter, battery, dc-dc bidirectional converter and loads is shown in figure 1. PV array converts solar energy into dc electrical energy. Maximum power available from PV array is extracted by unidirectional dcdc converter. The dc bus voltage depends on the load connected and available PV power at point of time. The battery maintains the dc bus voltage by charging/discharging mismatch power through dc-dc bidirectional converter. When available PV power is greater than load demand, the excess power is stored into the battery and deficit power is extracted from the battery. The size of the battery and PV array are selected as per the load curve of the microgrid in standalone mode editor@iaeme.com

3 R.D. Bhagiya and Dr. R.M. Patel DC-DC Boost Converter PV Array Vpv Ipv Battery MPPT Controller DC-DC Buck-boost Converter DC Load Ib Charging/ Discharging Controller DC bus Figure 1 Schematic diagram of the proposed microgrid 3. SYSTEM MODELING AND CONTROL STRATEGY 3.1. PV Array Modeling PV cell converts solar energy into electrical energy. PV module is made of series connected cells. Each PV module has specific parameters and performance like open circuit voltage, short circuit voltage, MPP operating point, diode ideality factor, values of series and shunt connected resistors and number of series connected cells. PV modules are connected in series and parallel as per the power requirement. Single diode model of PV cell is considered as shown in figure 2 [14]. RS ID Ish IPV Iph D Rsh RL VPV Figure 2 Single diode model of PV cell The current-voltage relationship of the PV module consists of N S series connected PV cell is represented as V + I R (q(v PV+IPVR s )/(NsAkTak) PV PV s I PV = Iph - I s (e - 1) - Rsh (1) Where, I ph is the current generated by incident light, I s is leakage current of diode, q is the charge of electron, R s & R sh are the series and shunt connected resistors respectively, Ns is the number of series connected PV cells, A is diode ideality factor, k is the Boltzmann s constant, Tak is the panel operating temperature and V PV and I PV are the PV module terminal voltage 35 editor@iaeme.com

4 Power Management and Control for Hybrid PV/Battery DC Microgrid and current respectively. Photocurrent I ph is dependent on solar irradiation and operating surface temperature, Trk of the module is expressed as G Iph=(Iscref + Ki(Tak - Trk)) Gref (2) Where, Iscref is the PV terminal current at reference irradiation, Gref = 1000W/m 2 and reference temperature of PV module, Trk=25 0 C and Ki is the temperature coefficient under short circuit condition. Leakage current is also dependent on operating temperature and material property of the PV cell is expressed as 1 1 qego - /Ak 3 Trk Tak Is=Irs(Tak/Trk) e (3) Where, Irs is the saturation diode current under reference irradiation and temperature and Ego is the band gap energy of the semiconductor material of PV cell. The P-V curves of a typical PV array at constant temperature of 25 0 C and different irradiation is shown in figure 3. Figure 3 P-V curve at constant temperature and different irradiation 3.2. Maximum Power Point Tracking of PV Array From the P-V curve, it is seen that maximum power is extracted when terminal voltage at PV array is set corresponding to maximum power value. Source impedance and load impedance are matched by dc-dc unidirectional converter by changing its duty cycle. Perturb and observe (P&O) algorithm is used to determine the reference value of PV array terminal voltage. When the operating voltage of the PV array is perturbed in a given direction and, if the power drawn from the PV array is increased, this indicates that the operating point needs to move toward the MPP and, hence, the operating voltage must be further perturbed in the same direction. Otherwise, if the power drawn from the PV array is decreased, the operating point needs to move away from the MPP and, hence, the direction of the operating voltage perturbation must be reversed. The Perturb and observe (P&O) algorithm is shown in figure editor@iaeme.com

5 R.D. Bhagiya and Dr. R.M. Patel START Read Vpv(k),Ipv(k) Calculate Ppv(k)=Vpv(k)*Ipv(k) N Ppv(k) Ppv(k-1) Y Y N Y N Vpv(k) Vpv(k-1) Vpv(k) Vpv(k-1) Vref = Vpv(k) - dv Vref = Vpv(k) + dv Vref = Vpv(k) + dv Vref = Vpv(k) - dv k = k + 1 Return to Start Figure 4 The Perturb and observe (P&O) algorithm for MPP Tracking 3.3. Maximum Power Point Tracking of PV Array Control of DC-DC Unidirectional Boost Converter The Perturb and observe (P&O) algorithm determines the reference value which is kept at the terminals of PV array for maximum power extraction. Proposed double loop control strategy determines the reference PV array current and controls the unidirectional dc-dc converter in such a way so that input voltage and current at input terminals of converter becomes equal to the MPP operating point of PV array for a given irradiation and temperature. Dc-dc PV converter and proposed control strategy are shown in figure 5. Ipv L D Vpv Cpv Q Cdc Vpvref Ipvref PI PI PWM Boost(Q) Vpv Ipv Figure 5 Double loop controls for PV converter Control of DC-DC Bidirectional Buck/Boost Converter Power is managed in proposed dc microgrid by absorbing the excess PV power by charging the battery and supplying the deficit power by discharging the battery. Maximum extraction of PV power is only possible when there is a storage device with dc- bus which has to be 37 editor@iaeme.com

6 Power Management and Control for Hybrid PV/Battery DC Microgrid regulated. Proposed double loop control strategy regulates the dc- bus voltage within permissible range by charging or discharging the battery using non-isolated dc-dc bidirectional converter. In this control strategy, dc-bus voltage is sensed and fed back to compare with the reference voltage; the voltage error is fed into voltage controller to generate current controller reference and creates control signal for the battery converter. Proposed control strategy with battery side converter is shown in figure 6. Q1 Ib Lb Cdc Vb Cb Q2 ref Ibref PI PI PWM Boost(Q2) Ib Figure 6 Double loop controls for battery converter Buck(Q1) 3.4. Design Parameters of the Microgrid System The proposed microgrid is made of PV array, dc-dc unidirectional converter, dc-dc bidirectional converter, battery and dc loads. The values of the design parameters of converters are shown in table 1. Table 1 Design parameters of the converters PV dc-dc unidirectional converter Battery dc-dc bidirectional converter Inductance, L 0.8 mh Inductance, Lb 2.75 mh Resistance of inductor, r L 0.05 Ω Resistance of inductor, r bl 0.01 Ω Capacitance, Cpv 500 µf Capacitance, Cb 500 µf Capacitance, Cdc 1000 µf Capacitance, Cdc 1000 µf Switching frequency, fsw 10 khz Switching frequency, fsw 10 khz The rating of the PV array consists of three parallel strings each made of seven series connected module, lithium ion battery and loads are shown in table 2. Table 2 Rating of the system components PV module Battery Load Maximum power, Pmax 220 W Nominal voltage, Vb 204 V Cells per module 60 Rated capacity 50 Ah Load 1 45Ω Voltage at maximum power Initial state of 29.2 V 80 % point, Vmp charge(soc) Current at maximum power 7.54 A point, Imp Internal resistance, r b Ω Load 2 25Ω Open circuit voltage, Voc 36.6 A Short circuit current, Isc 8.08 A 38 editor@iaeme.com

7 R.D. Bhagiya and Dr. R.M. Patel 4. RESULTS OF SIMULATION OF MICROGRID SYSTEM UNDER VARYING LOAD AND IRRADIATION Microgrid is simulated in MATLAB/Simulink software for two different load conditions and three solar irradiations at a same panel temperature of 25 0 C. The MPP tracking and effectiveness of battery charging/discharging algorithm is shown in figure 7. Load resistance, solar irradiation and temperature are initially kept at 45Ω, 600 W/m 2 and 25 0 respectively up to 1.5 second during simulation; the power demand is more than PV generation in this condition, so the mismatch power is drawn from the battery. Load and generation are equal for a solar irradiation of 800 W/m 2 ; hence there is no exchange of battery power with dc bus. Load is increased at 2.5 second with same irradiation of 800 W/m 2 ; battery is discharged to regulate dc bus. PV generation is increased at 3.5 second due to rise in irradiation from 800 W/m 2 to 1000 W/m 2 without change in load; but the demand is still excess than generation so battery is continued to supply deficit power. Load is decreased at 3.5 second without any change in level of solar irradiation; excess PV generation is stored into the battery. Figure 7 The MPP tracking and effectiveness of battery algorithm Performance analysis of the microgrid power management and control is shown in figure 8 for different load and PV power generation under MPP mode. Double loop controls of both PV and battery converters are performed effectively under stand alone mode. DC bus voltage is regulated at its desired level of 380 V within permissible variations. Transients in dc bus voltage are observed when there is a change in load; but not for the change in PV power generations editor@iaeme.com

8 Power Management and Control for Hybrid PV/Battery DC Microgrid Figure 8 Performance Analysis of Microgrid for different load and PV generation Scenarios 5. CONCLUSIONS A double loop control strategy is proposed for both PV and battery converters to regulate the dc bus voltage within limits under standalone mode. All possible realistic situations of irradiation and load were considered. Perturb and observe algorithm was used to determine the PV reference voltage for maximum power extraction from PV array. Outer loop of double loop control compared the actual PV voltage with calculated PV reference value and gave the PV reference value and inner loop compared this value with actual and determined the duty cycle for PV array dc-dc converter. Mismatched power between PV generation and demand was compensated by battery through double loop controlled dc-dc bidirectional converter. Battery had made standalone operation of PV array possible for varying irradiation and load. Results obtained indicate the required performance of proposed power management and control strategy under steady state and dynamic load and irradiation conditions. REFERENCES [1] Lasseter, Robert H. "Microgrids." Power Engineering Society Winter Meeting, IEEE. Vol. 1. IEEE, [2] Farid, K., Reza, I., Nikos, H., & Aris, D. (2008). Microgrid management control and operation aspect of microgrids. IEEE Power Energy Mag. [3] Xu, Lie, and Dong Chen. "Control and operation of a DC microgrid with variable generation and energy storage." IEEE Transactions on Power Delivery 26.4 (2011): [4] Femia, N., Petrone, G., Spagnuolo, G., & Vitelli, M. (2005). Optimization of perturb and observe maximum power point tracking method. IEEE transactions on power electronics, 20(4), editor@iaeme.com

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