Cascaded Converter based Hybrid Power Supply for PV Assisted Light Electric Vehicle

Abstract

The lighter power electronic interface for PV, battery and super capacitor-based power supply to PV assisted light EV greatly enhance efficiency of drive system. This paper presents a novel cascaded converter-based PV- Battery-Super capacitor power supply interface with reduced battery and supercapacitor power converters which is controlled through model-referred duty estimation-based PI control for maximum power absorption from PV source, accurate current reference generation for battery and super capacitor, faster current control, and accurate DC bus regulation. The simulation of proposed converter interface and control were carried out in MATLAB / SIMULINK environment. Simulation results proved the robustness of control with 2.7 percent regulation in DC bus voltage, 0.05 sec and 5 sec transient settlement times for supercapacitor and battery bus voltages.

Description

Field and background of the inventionEfficiency and size of power train is the key factor for productiveness of light motor solar electric vehicle. With the inclusion of photovoltaic (PV) source and battery power supply, it is the power electronic converter size that det.ermines the size of power train. In this regard the present study focuses on two aspects of power electronic interface for hybrid PV, battery and super capacitor power supply to motor drive of electric vehicle (EV). First, the study of topologies towards lighter interface and second, efficient control of the multiple source system. Sources such as Wind power, PV power, power from fuel cell are reviewed for application to light solar EV. Multi-input non-isolated converters are proposed for DC bus integration in which fuzzy logic control determines the coordinated power share. Battery storage along with super capacitor assistance combined control for average current control and ripple current control is developed. The features of bi-directional power flow are provided to the interfacing converters for to facilitate for regeneration during braking operation. Interfacing DC bus with three level converters is developed for with reduced component size. Usage of same interface for charging of battery is developed which also facilitate for charging operation with high efficiency. Another vertical in topologies is identified with the power from multiple sources is integrated in single stage conversion. This method has an additional advantage that it does not need maximum power point tracking (MPPT) dc-dc converter. Further, there are processes which aimed at optimal size of power sources, power conversion in both the directions, and inclusion of multiple functions for one converter, analyzing thermal withstand capabilities and inclusion of other functions for the main power converter. All these topologies have merits of multiple source interfaces but at the cost of complex interface.Brief description of the systemThe primary objective is to develop a novel topology with reduced size of power switches and filter components for two stage conversion of Battery power for interfacing PV and Battery sources to electric vehicle power train. The conventional integration includes injection of power from PV source, battery and super capacitor in to DC bus of the drive as parallel components as shown in Fig. 1. A boost converter with maximum power point tracking control is employed to deliver PV power in to DC bus. Power switches Spy, Dpv along with filter elements Lpv and Cpv facilitates this operation. Bi-directional power control converters are employed for integrating battery source power and supercapacitor in to DC bus. Series and parallel switches in these converters facilitate for respective buck or boost operation. The modified topology which integrates super capacitor in one stage and battery power in two stages in to DC bus is shown in Fig. 2. With converter for PV integration remaining same as conventional process, the battery power integration is modified as a two-stage process. In the first stage, battery supplies power to super capacitor bus. In the second stage, super capacitor bus supply battery power along with power from super capacitor in to DC bus. In this process, the sizing of the converters in each of the cascaded stages ,is reduced significantly for. similar performance as that of conventional integration. This reduction in size is owing from the change in Bus voltage levels between which the converters are placed compared to conventional integration.The equivalent voltage across the inductor condition of PV converter when the switch Spv if ON is given as Vpv. During this condition, The dynamics of PV converter are depicted in equations in (1)Equation (1)The output current of the PV array as function of terminal voltage is given asEquation (2)Where, A- material constant- of P V panel The condition for extracting maximum power from P-V and I-V curves is obtained as:Equation(3)Considering Equation (4)Therefore, at maximum power condition,Equation(5)In the real time sampling, equation (1) and equation (5), were considered asEquation(6)Equation(7)Thus, the duty for next sampling instant is obtained from equations (6) and (7)The dynamics of Battery converter are depicted in equations in (8) and (9) as follows:Equation(8)In real time sampling, these are considered as,Equation(9)Given the reference value of current iBat* the duty ratio to be needed for next sample is given as Equation(10)The dynamics of super capacitor converter are depicted in equations in (11) and (12) as follows:Equation (11)Discretizing the differential equation,Equation(12)Given the reference value of current isc*, duty ratio to be needed for next sample is obtained as: Equation (13)The dynamics of stage 1 conversion are depicted in equations in (14) and (15) as follows:Equation (14)In real time sampling, it is considered as,Equation(15)The duty ratio needed for next sampling instant is obtained asEquation (16)The model derived duty (MDD) control is depicted in Fig. 3. The PV interface converter is independently controlled with respect to battery and super capacitor converters. This converter always feed the DC bus with maximum available power at any given instant of time. The other two sources viz. battery and super capacitor should share the rest of power demand as per load at any given instant of time. The proposed current regulation scheme is explained as follows:The control algorithm has two stages viz. generating reference currents for battery and supercapacitor, and obtaining respective gating control signals. The DC bus regulation error given to PI controller provides reference current to be generated. Average value of reference current is given as reference to battery converter inductor. The ripple extracted from reference current is given as reference to super capacitor converter.The cascaded converter with MDD control scheme along with application of control scheme to conventional topology were simulated in MATLAB / SIMULINK. A resistive load controlled by external signal source was considered to provide time varying load similar, to electric vehicle drive cycles which include acceleration, constant velocity and deceleration of vehicle. The system simulation parameters are provided in Table-I which include parameters of sources and filter elements. The non-idealities such as series parasitic resistance for filter components is considered to be 0.5 percent of its nominal value. Power switches are considered with snubber and parasitic ON state resistance. A random signal-controlled resistance created a load current demand as shown in Fig. 5. The current supplied by PV source is according to MPP algorithm at varying irradiance as seen from Fig. 5. The rest of current demand by the load is supplied by combination of battery and super capacitor as shown in Fig. 5. Owing to the slower response of battery current because of its chemical properties it can be seen from Fig. 5 that at instants where there is sudden increase in load current such as at time interval near to 50 sec and sudden decrease in load current demand such as at time interval near to 230 sec, respective c-rate of the battery limits current of the battery. At such instants, the reaction of super capacitor could be absorbed in "terms'"of sudden supply of current or sudden absorption of current to meet the load demand. During constant load current which is met by battery and PV current, super capacitor only floats without any power supply or absorption which can be observed from fig. 5. Also, for gradual increase, or decrease of load current, battery current is adjusted quickly to meet the load demand as seen from Fig. 5 at intervals 120 - 125 sec and 180 - 190 sec respectively. A similar response is observed with conventional topology as well. Voltages at DC bus, super capacitor bus and battery bank are shown in Fig. 6 which are regulated to designed magnitude with accurate tracking which is observed from Fig. 6. The regulation and voltage stress are discussed in detail in next subsection. A highly accurate regulation is observed for various load values which accounts for magnitude close to five percent. With voltages regulated very accurate to nominal values it is evident from Fig. 7 that the power share among the sources follows the pattern as similar to current share. It is also evident from Fig. 7 that, the irradiance dependent PV power is supplied to DC bus. The rest of the power demand is shared between battery and super capacitor. Similar to the response of current the sudden variations in load power are instantaneously'tackled by 'super capacitor while battery adjusts to meet the corresponding load power demand.From the variations of load power demand, a worst-case sudden decrease in load is considered at 150 sec. owing to this sudden change DC bus voltage pertaining to conventional converter topology is shown in Fig. 8. The DC bus voltage is raised to 49.9 V and is regulated to nominal 48 V at 155 sec. Therefore, the worst-case voltage regulation in this case is obtained as 3.125 percent and transient time for settling back is 5 sec. The similar load change for cascaded converter topology resulted in DC bus voltage change as shown in fig. 9. The DC bus voltage is raised to 48.9 V and is regulated to nominal 48 V at 155 sec. Therefore, the worst-case voltage regulation in this case is obtained as 2.7 percent and transient time for settling back is 5 sec. Thus, the cascaded converter topology resulted in better voltage regulation for worst case variation in nominal range load. Similarly, the voltage "stress on the''super capacitor'is also studied. In this case, the worst case demand is observed at 40 sec where the load is suddenly increased. An instantaneous drop in voltage to 17.69 V voltage followed by raise to 18.32 V is observed for super capacitor voltage with conventional converter topology as shown in Fig. 10. The percentage voltage stress forthis worst case variation is obtained to be 2.2 percent. For similar situation, with cascased converter topology, An instantaneous drop in voltage to 17.95 V voltage followed is observed as shown in Fig. 11. In this case the percentage voltage stress is only 1.6 percent. Thus, along with better DC bus regulation, voltage stress performance of super capacitor is also found improved with cascaded converter topology by applying proposed control.Therefore, the reduction in filter inductor size is determined in terms of percentage change in which is obtained as 30 percent.A comparison is made to evaluate sizing of battery interface converter. The inductor at battery interface as calculated from (19) and (20) and verified from Fig. 12 and Fig. 13 a great reduction in inductor size is obtained with cascaded converter. Also, the series switch S2 is evaluated for voltage stress at OFF condition. Considering the voltage stress with conventional topology as 1 per unit, the voltage stress with cascaded converter topology is obtained to be only 0.16 per unit.1 Therefore, reduction in sizing of battery interface components with equivalent performance is achieved with the cascaded converter with MDD control.Comparative conclusions with respect to existing topologies are provided in Table I and Table II. Reduction in filter sizing is compared in Table III.Table I. Comparison of DC Bus regulationTable II. Comparison of Super capacitor voltage stressTable III. Battery interface converter parameters comparisonSummary of the inventionThe lighter power electronic interface for PV, battery and super capacitor-based power supply to PV assisted light EV greatly enhance efficiency of drive system. This paper presents a novel cascaded converter-based PV- Battery-Super capacitor power supply interface with reduced battery and supercapacitor power converters which is controlled through model referred duty estimation-based PI control for maximum power absorption from PV source, accurate current reference generation for battery and super capacitor, faster current control, and accurate DC bus regulation. The present work proposes cascaded converter topology and a model referred duty estimation based-PI control for efficient and cost-effective solution for PV- battery-super capacitor interface for light solar EV. The novel contributions of the work include:- Development of two stage cascaded power conversion topology for integrating battery and supercapacitor in to power supply- This topology reduced the size of inductor for integrating battery in to supply system- This topology also reduced stress on the power switches of battery interface compared to conventional interface- Development of novel control algorithm which is model based PI control for current regulation from battery and super capacitor.- Better regulation of DC bus and super capacitor bus with proposed controlThe topology and mathematical model of proposed two stages cascaded converter-based power supply, control structure for the proposed converter, simulation results, and performance validation are presented which proved that the proposed cascaded converter topology for hybrid power supply along with proposed model based PI regulator resulted in improved performance of the light electric vehicle power supply system with improved DC bus regulation by 5 percent, improved transient behavior with settling times of 0.05 sec with reduced interface size by 30 percent.

Claims

1. Development of two stage cascaded power conversion topology for integrating battery and supercapacitor in to power supply.

2. Reduced size of inductor by 30 percent for integrating battery in to supply system.

3. Reduced stress on the power switches of battery interface by 84 percent compared tb conventional interface.

4. Model based PI control for current regulation from battery and super capacitor.

5. Improved regulation of DC bus and super capacitor bus by 5 percent with proposed control.