R. Pon Vengatesh Assistant Professor Department of EEE
Mepco Schlenk Engineering College, Sivakasi, Tamilnadu, India [email protected]
Abstract— A green power revolution is essential for the country to achieve a sustainable energy base that will support the goals of economic development, energy security and environmental protection. Nowadays, Photo---Voltaic (PV) system plays a vital role in power generation systems. The paper deals with efficient simultaneous power management of solar renewable energy sources using Multiport DC-DC converter. The proposed converter reduces number of switches, controller circuit and each port can be conveniently linked to one controllable switch for proper operation of the system. Moreover, it has less components and simple in construction. All the ports are said to be connected in parallel and thus provides continuous supply of power to the load. The proposed converter is developed for simultaneous maximum power point tracking (MPPT) of PV modules at different operating conditions. A simple Perturbation and Observation (P&O) MPPT algorithm has been developed for continuous extraction of maximum power from PV panels. A single MPPT controller arrangement deals with the change in irradiation level of each port connection has been studied using Matlab-simulink environment. The specifications of the PV module have been obtained from the manufacturer datasheet (MS24250) for these analyses. Index Terms— Multiport DC-DC Converter, Maximum Power Point Tracking (MPPT), Perturbation and Observation, Solar energy.
I. INTRODUCTION ecently, conventional source of energy is depleting and rise in cost of energy, so distributed renewable energy sources are used in many applications. To achieve effective power management, multiport
connected DC-DC converter has been proposed here. This converter can be connected with different renewable sources like solar, wind, fuel cell for supplying power to load. In this work, the PV modules operating at different conditions which
can be connected to each port of the multiport converter. The Photo-voltaic technology uses photo-voltaic cells to convert sunlight directly into electricity with two stage configuration is used in [1], without polluting atmosphere and no emission of greenhouse gases. In earlier work, single PV system is connected to two or more switches with an isolated boost type converter in [2] which has some limitations like larger duty cycle ranges from 50% onwards. There are different types of isolated converter which uses a transformer with a separate winding for each source [3] and there are multiple ports connected to a single winding on one side of the transformer. An integrated power converter requires many power terminal feedback controllers [4] for their operation of the system. A several topologies were implemented with more number of switches that leads to high switching losses. The isolated half bridge converter is the mostly used topology which has (2n+2) switches [5] and (n+3) switches [6, 7] where n are the number of ports. But the number of switches can be reduced to 2n by using one source as a dc link or reducing switches on the secondary side of the transformer [8]. The combination of bridge rectifier along with boost converter is discussed in [9].The proposed parallel connected three ports converter system with PV sources requires less number of switches as compared to the existing topologies for continuous supply of power flow to enhance the reliable operation of the system. In this application, a higher level constant output voltage can be achieved through Matlab- simulation. The block diagram representation of the proposed system is shown in Fig.1. P1, P2, P3 are the sources taken from various ports and V1, V2, V3 are the PV voltage sources. A regulated boost converter circuit has been placed as the 2nd and 3rd port of the converter to improve the output power. Moreover all the input sources can deliver continuous power to the load. A filter circuit is too introduced for filtering the A.C components of the rectified output and allowing only the D.C components to the load. Since, the PV is non-linear in nature and to extract the maximum operating point, it is necessary to establish the MPPT controller in the proposed systems. The duty ratio (Kn) (for this study n=1, 2, 3) of converter varies with MPPT controller according to the source
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voltages. A Perturbation and Observation (P&O) MPPT algorithm [10] is incorporated for extracting possible maximum power. By sensing the voltages and currents from PV panels within the controller tend to adjust the duty cycle of the switches simultaneously .
Fig.1 Block diagram of the proposed system The paper has been organized as follows: The modes of operation of multiport converter and its topology are discussed in section II. In section III the design parameter of the multiport DC-DC converter and its simulation results have been discussed. The PV module characteristics under different operating conditions have been studied & plotted in section IV. Section V dealt with the integrated PV modules and converter circuit through MPPT controller. The Matlab simulation response under different operating condition of PV panels has been reported in section VI. Section VII concludes the finding of this research work.
II. PROPOSED MULTIPORT DC-DC CONVERTER In this proposed DC-DC converter, output voltage is
regulated by ON-time of switches, pulse width and switching frequencies (fs in Hz). Three ports are connected in parallel and each port is connected with controllable MOSFET switches (Sn), diodes (Dn) and an inductor (Ln) by bridge rectifier at the load terminal. Mutual inductance turns ratio is defined as N1:N2 where, N1 & N2 are the number of turns in coil 1 and coil 2. L1 ,L2 ,L3 are the inductors used as the storage element. Cm is the charging/discharging capacitor connection in series with mutual inductance. LC combination is required for filter purpose. C1 ,C2 ,C3 are the capacitors connected in parallel with the each and every ports in the input side of the multiport DC-DC converter and three source input currents are represented as i1,i2 i3. Multiport can also work as single input connected to the converter with all the negative terminals are said to be grounded to the common point. The Fig.2 shows the Matlab- simulink diagram of Multiport DC-DC converter circuit under different operating input voltage levels.
A. Different Modes of Operation: Mode I: When all the switches are ON (S1 S2 S3), the
source power flows within the converter, not to the load side. The capacitor connected in series with mutual inductance that is charged in the previous stage get discharges to the output side. Hence capacitor gets lower amount of energy during charging/discharging period. Mode II: Keep switch ON (S1) and at least any one of the remaining switch should be ON. Mode III: When all the switches are at OFF state then, there is a direct current flow from source to load side without involvements of any controllable switches and it requires bulk value of capacitor. Normally, this operation is not entertained for this system.
Under steady state operation conditions, the duty cycle of a converter can be calculated from the following equation:
⎥⎦
⎤⎢⎣
⎡−−=
i
nin V
VKK )1(1 ….. (1)
Where n=2, 3 and n≠1 and i=1,2,3,….n. Vn is the steady state voltage of the nth input port of the converter.
III. DESIGN SPECIFICATION OF THE CONVERTER In order to ensure the multiple sources, the necessary
conditions should be maintained. The switch Sn (n=2,3) should not be switch OFF before S1 is turned OFF. Otherwise L2, L3, … Ln will store energy continuously only through switch1. To meet this requirement, the following inequality should be satisfied. i) K1 ≤ Minimum of other two duty cycle of the switches (K2,K3 …Kn )
ii) V1 ≥ Maximum value of other port voltages (V2 , V3 ..Vn ) The inductance Lp can be calculated by,
)()(
ps
ppp If
KVL
Δ∗∗
= …. (2)
Where, p = 1, 2 …., m ∆Ip – desired current ripple of inductor Lp The output side inductor (L) is
L
s
I
TKVk
V
LΔ
⎟⎟⎠
⎞⎜⎜⎝
⎛ ∗⎟⎠⎞
⎜⎝⎛ −
=1
1ο
….. (3)
Ts – Switching Period
οVKVk )2( 11 ∗=
V0 is the output voltage (V) K1 is the duty cycle for S1 Capacitor can be calculated from,
)()(
ps
ppp Vf
KIC
Δ∗∗Δ
= …. (4)
∆Vp is the voltage ripple of ∆Ip
SOURCE m
SOURCE2
SOURCE1
. .
. .
Vn
MULTIPORT DC-DC
CONVERTER
V2
V1
Pn
P2
P1
. .
LOAD
MPPT TECHNIQUE
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Fig.2 Multiport DC-DC converter circuit under different operating input voltage level
TABLE I. DESIGN SPECIFICATION OF CONVERTER
COMPONENTS SPECIFICATION
Inductors(L1,L2,L3) 420µH,280µH,300µH
Capacitors(C1,C2,C3) 1000µF
Filter (L&C) 250mH,100µF
Turns ratio (N1 : N2) 1:4
Fig. 4(a) Pulse Generated from Switches (S1, S2, S3) & Output Voltage
Waveform
Fig 4(b) Output current waveform
Turns ratio for the mutual inductance and specification of
the converter components are represented in the above Table I. The simulated output responses of Multiport DC-DC converter circuit under different operating input voltage levels say V1=30V, V2=20V &V3 =10V are as shown in Fig.4 (a) and Fig.4(b). The output power generated is accounted as 171.2W.
IV. PV MODULE CHARACTERISTICS The multi-crystal PV module under various configurations has been simulated in MATLAB-Simulink environment [11]. The Power-Voltage (P-V) & Current-Voltage (I-V) characteristics curve under 1000W/m2, 800W/m2, 600W/m2 at constant temperature (T) 25°C is shown in Fig 5. Whenever there is a change in irradiation, power gets varied simultaneously. Once it reaches the maximum peak power, then the voltage starts decreases. In (I-V) characteristics curve there is a constant
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current rating for certain period until it reached its peak point. Later it start reduces.
Fig.5. Power-Voltage (P-V) & Current-Voltage (I-V) characteristics curve under 1000W/m2, 800W/m2, 600 W/m2 at constant temperature (T) 25°C
V. POWER GENERATED FROM SOLAR PANEL AND MULTIPORT DC-DC CONVERTER WITH MPPT CONTROLLER
If there is no power from source1, then condition (i) should be satisfied and S1 is tends to change the direction of the mutual inductance current (im). If the S1 is turned OFF, then im flows from the other sources of the port to the mutual inductance and charges the capacitor (Cm) in series with it.
When S1 is turned on, the capacitor gets discharges and hence there will be reverse flow of current im exists. The condition (ii) cannot be satisfied, when the solar power generated from source1 is less than Pmax1. At this stage, the duty cycle K1 will be increased to a higher value by the MPPT controller and the S1 function follows the first condition. Moreover, boost type switching regulators are connected with all ports in the multiport system except port1 for independent control of each port power through the MPPT (P&O algorithm) controller [12-15]. This can be achieved by a simple strategy, in which one of the duty cycle K1 can be updated at a time while the other duty cycles are fixed so that the source1 voltage and current can be controlled. This is further evaluated by keeping different updating frequencies for the duty cycles in different sources (d1<d2<d3).The updating frequency of switch1 is set to be higher range. MPPs for PV panel can be determined by frequently increasing the duty ratio from lower value to higher value.When the S1 is said to be ON, the capacitor gets charges and it discharges to the load during OFF condition. Hence there is a sinusoidal signal flows to the mutual inductance. Mutual Inductance will induce voltage in another side where the full bridge rectifier is placed (conversion stage) and then to the filter connected with a load. The capacitor (Cm) connected in series with mutual inductance can store the energy. Charging and discharging takes place by switching operation of multiport converter. The integrated PV modules and converter circuit through MPPT controller is shown in Fig.6.
Fig.6. An integrated PV modules and converter circuit through MPPT controller
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VI. SIMULATION RESULTS The Matlab simulation responses of input voltage and input current under different operating condition of PV panels at three ports has been studied and plotted in Fig. 7(a), (b), (c). The switching frequency for various switches should be updated. The updated frequencies are from 100 Hz to 2KHz.
(a) at Port1
(b) at Port2
(c) at Port3
Fig.7 Simulation response of Input Voltage & Input current at different ports
To vary the duty cycle of the switches, there must be a need for updating the frequency of the MOSFET switches and it should be a lower value for S1 rather than other two switches (S2 & S3). The Fig.8 shows the control pulse generated from MPPT controller for the proper operation of the system.
(a) at Switch (S1)
(b) at Switch (S2)
(c) at Switch (S3)
Fig.8 Control pulse generated from MPPT controller In this converter design, a capacitor in parallel at the input port side is used for storing more energy. Moreover, the duty cycle ranges gets varied by changing the frequencies. Oscillations will be smaller level that are caused by MPPT algorithm in which the duty cycle ratio changes slightly around optimal duty ratio.Such Oscillations are lower when compared to the average power value which is acceptable.
Fig.9 Simulation response of input current of the mutual inductance (im) & output voltage & current waveform.
The input current (im) of mutual inductance is considered to be in sinusoidal in nature and the output (Vo& Io) waveform is shown in the above Fig.9
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Fig.10 Efficiency(%) vs Power(W)
TABLE II. SIMULATED OUTPUT FOR THE PROPOSED CONVERTER
Port Input Vpanel(V) Ipanel(A) Vout(V) Iout(A) η
Port 1 at 800W/m2 30.05 6.569
289.1
1.401
89.88% Port 2 at 700W/m2 26.7 6.01
Port 3 at 500W/m2 26.2 3.54
For the different power rating, the calculated efficiency values are shown in the Fig.10. Input ports irradiation and its panel voltage, current values are mentioned in the above Table II. The obtained efficiency is 89.81%
VII. CONCLUSION This paper investigates the performance evaluation of Multiport DC-DC converter for simultaneous power management of multiple PV modules applications. Moreover, the Photo-Voltaic module has been modeled and simulated in MATLAB-Simulink environment and the performance characteristics (V-I & P-V) of PV module under different operating conditions were also studied. The simulation results show that an increase in irradiance causes an increase in the module’s output current. The proposed multiport DC-DC converter uses minimum number of switches which reduces the switching losses and also less complexity than other full bridge isolated converter.The proposed system is employed with simple topology having single MPPT controller than two or more controller schemes. Here, the conventional P&O was implemented to track the maximum output power under different operating conditions of PV panels. The proposed converter deals with a single controller which can automatically adjust the duty cycle values for various switching condition by updating frequency function. The simulated results illustrated that the MPPT controller gives the better results.
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