PV based Bidirectional Converter for Traction drive application
S. Sreenu[1], Dr. N Bhoopal[2], Artham Murali[3]
[1]Assistant Professor, Dept of Electrical Engineering, BVRIT, Hyderabad, Telangana, INDIA
[2]Professor, Dept of Electrical Engineering, BVRIT, Hyderabad, Telangana, INDIA
[3]Assistant Professor, Dept of Electrical Engineering, BVRIT, Hyderabad, Telangana, INDIA Abstract: In this paper integration of traction machine with
converter topology to have bidirectional power flow between electric vehicle and grid with an additional component PV connected in parallel to battery of the existing system is presented. The inductances of the traction motor winding are used for bidirectional converter operation to transfer power by eliminating the need of external inductors of charging and vehicle to grid converter operations. The proposed converter system can also be used for transferring power between a single-phase ac grid and the vehicle in either direction without any extra component.
By this the size and weight of the electrical power train system is reduced. To reduce the switching stresses we are implementing interleaving technique. This concept we are implementing on MATLAB/Simulink.
I. INTRODUCTION
In a wide sense, all the vitality used to fuel the today autos has originated from the sun, having been aggregated along a great many years as concoction vitality in fossil fills. Be that as it may, obviously, the developing enthusiasm toward sunlight based vitality is identified with its element as a renewable source [1].
Likewise in this sense, sun oriented vitality could be utilized for auto drive as a part of various routes: i) in aberrant path, to create bio-fills (for customary vehicles) or hydrogen (for power device vehicles, or vehicles with ICE fuelled with hydrogen or a blend of methane and hydrogen); ii) in roundabout route, as a renewable source to deliver power, to energize Electrical Vehicles (EV) or Hybrid Electric Vehicle (HEV) module; iii) in direct route, by method for photovoltaic boards mounted on the auto (the utilization of warm boards has not been considered yet for car applications: keeping in mind the end goal to accomplish adequate change proficiency, concentrating advances ought to be embraced, that appear to be fairly unfeasible in an auto). The multipurpose utilization of the force electronic converter in the drive train of an electric vehicle has turned into an intriguing subject for minimizing the framework size, weight, and cost [2-4].
The weight and size of the converter are testing issues on account of on-board chargers which generally gives the adaptability of charging the vehicle anyplace. The vehicle is not driven amid the time of charging, and henceforth, the footing engine and inverter of the power train can be utilized as a basic part of the converter.
The windings of the footing engine can serve as the inductors of the force converter alongside force gadgets of the footing inverter to exchange power [5-6]. The force converter of the electric vehicle can draw power from the lattice when it requires, furthermore can convey energy to the network in the pinnacle
time when the framework needs control. Amid a critical part of the day, most vehicles stay inert in the parking area when the coordinated force converter can utilize the footing engine and its drive to exchange energy to the framework.
II. INTEGRATED MOTOR DRIVES AND BATTERY CHARGERS IN VEHICLE APPLICATIONS Battery Chargers in Vehicle Applications There are diverse orders for battery chargers in vehicle applications. They are partitioned in two principle classifications of on-board and off-board sorts.
The onboard sort gives the opportunity to the client to charge wherever that an attachment is accessible and the second one has favorable position of high-power charging that is quick charging.
The subject of this proposal is ready battery chargers, so the off- board sorts are not tended to here [7]. Galvanic disconnection from the utility network separates the charger sorts to disengaged or non-segregated sorts. At the main section, a short clarification is given to give more knowledge with respect to this point.
Another generally utilized grouping is as indicated by charger power level [8]. Three force levels are utilized as a part of this characterization in which a harsh estimation of the charging time can be considered.
Case of Integrated Motor Drives and Battery Chargers Fig. 1 demonstrates a schematic graph of a PHEV with parallel arrangement (both the inward ignition motor and the electric engine can drive the vehicle at the same time) for instance of a vehicle with a lattice associated battery charger. The electrical part incorporates the lattice associated battery charger, battery, inverter, motor and control system. It is here assumed that during charging time the vehicle is not driven and during driving time it is not possible to charge the battery pack except for regeneration at braking. In a classical electrical device arrangement in the vehicle, there are separate inverter and charger circuits for traction and charging from an external source.
Figure 1: A simple diagram of the traction system of a parallel plug-in hybrid electric vehicle
A traction system based on an AC motor and a three-phase inverter is shown in Fig. 2. In several schemes a DC/DC converter is used in the system also. The battery power will be transferred to the motor through the inverter. Bi-directional operation of the inverter allows energy restoration to the battery during braking. Different types of integrated chargers are reported by academia or industry. Some of them that are more relevant to the current project are reviewed and compared in this chapter.
Figure 2: Electrical traction in a vehicle based on an AC motor and a three-phase inverter
Fig. 3 demonstrates the useful schematic graph of this non- detached incorporated charger framework. By the method for modest transfers the machine windings are reconfigured to be inductors in the charging mode.
Figure 3: Non-isolated single-phase integrated charger based on an induction motor drive system
The integrated charger is a complicated solution in both hardware and control implementation. Moreover, the traction system is not fully utilized in the charger circuit. The DC/DC boost converter and some parts of the SRM and its driver are not used in the charger circuit.
Figure 4: Single-phase integrated charger based on a SRM drive system
III. SYSTEM ANALYSIS
The converter configuration shown in Fig. 5can is operated as a simple boost converter if the frequency and duty cycle of the bottom three IGBTs and the top three IGBTs are the same [9].
The system matrix of the converter is
𝐴 =
−𝑟𝐿
𝐿
−𝐷′ 𝐿
−𝐷′ 𝐶
−1 𝑅𝐿𝐶
… … … … 1
Where 𝑟𝐿 is the internal resistance of the inductor, D is duty cycle,𝐷′ is (1 − D), L is the combination of the motor windings, and 𝑅𝐿 is the combined resistance on the grid side.
The system stability can be analyzed using the system matrix. For the input voltage range of 100–250 V with the desired output voltage of 650 V, the real part of the system poles are observed to be negative. The system is stable within these input–output voltage ranges.
Fig 5: Circuit with Switch 2 and Switch5 in State 2 for V2G boost or G2V buck operation with vehicle side inductors
interleaved
The transfer functions of duty cycle to output voltage and of duty cycle to battery current can be derived as follows from the small signal analysis of the converter with a PI controller
𝑣𝑜
𝑑 =
−𝐼𝑏𝑎𝑡𝑡
𝐶 𝑠 − 𝐷′𝑉𝑜
𝐼𝑏𝑎𝑡𝑡𝐿 𝑆2+ 1
𝑅𝐿𝐶 𝑆 +𝐷′2 𝐿𝐶
… … … 2
𝐼𝑏𝑎𝑡𝑡
𝑑 = 𝑉𝑜
𝐿 𝑆 + 1
𝑅𝐿𝐶+𝐼𝑏𝑎𝑡𝑡𝐷′ 𝑉𝑜𝐶 𝑆2+ 1
𝑅𝐿𝐶 𝑆 +𝐷′2 𝐿𝐶
… … … … . .3
The converter characteristics in terms of parameter sensitivity and bandwidth analysis can be analyzed with the closed-loop transfer functions. The corner frequency of the closed-loop system is
𝑓 = 𝐷′
𝐿𝐶… … … 4
The frequency response of the system for 60% duty cycle with the bandwidth of 6 kHz is shown in Fig. 9. The response would change with change in duty cycle which suggests the parameter dependency of the system transfer function. The converter system with feedback control as shown in Fig. 8 depends on the duty cycle nonlinearly, which makes it challenging to design this converter within the stability and bandwidth limits. The bode diagram is from the analytical model, which has been used to select the converter parameters.
Fig. 8: System block diagram with closed-loop feedback control
IV. SIMULATION RESULTS
In automotive applications, different kinds and ratings of electric machines are used. The applicability of the concept on different electric machines used in automotive applications is tested with simulation models that would provide the most realistic predictions.
Simulation with an Induction Machine
An induction machine is also a common type of electric machine type used in traction applications. A 10 HP, three-phase induction machine where the neutral point is available has been chosen as a traction machine for experimental verification. Dynamic simulation using MATLAB/Simulink has been done with the inductances of the machine windings. The power transfer characteristics and the interleaving technique for distributing the input currents into the three-phase windings can be analyzed with this simulation.
1) Mode 1 and Mode 2 Simulation
Mode 1 is for power flow from the battery to the dc grid and Mode 2 is for power flow from the dc grid to battery. In the simulation, three inductors with the same values of the winding inductance of the induction machine have been used to build the converter as the topology of Fig. 5 for V2G boost mode of operation. The simulation block diagram is shown in Fig. 9(a).
The simulation for V2G buck mode of operation using the configuration of Fig. 4 has been also done; the simulation block diagram for this mode is shown in Fig. 9(b).
Fig 9(a): V2G boost mode simulation of converter with MATLAB/Simulink
Fig 9(b): Integrated converter and induction machine operation with dc grid with V2G buck mode of operation
Fig 10: Control network for bidirectional converter The simulation parameters for boost operation are: input voltage is 200 V, output reference voltage is 260 V, maximum input current limit is 30 A, induction machine phase inductance is 5 mH, output capacitor is 3300 μF, load resistance is 20 Ω, and PWM switching frequency is 20 kHz. From the simulation result shown in Fig. 11(a), it is observed that the output voltage is following the reference voltage of 260 V in the boost mode of Fig. 3. The simulation parameters for buck operation are: input voltage is 400 V, output reference voltage is 200 V, maximum input current limit is 30 A, induction machine phase inductance is 5 mH, load resistance is 10 Ω, and PWM switching frequency is 20 kHz. From the simulation result shown in Fig. 11(b).
Fig. 11(a): Output voltage in Mode 1 of the integrated motor/converter: for boost operation
Fig 11(b): Output voltage in Mode 2 of the integrated motor/converter for buck operation
In the case of boost operation, the output power level is 4 kW.
The interleaving technique has been applied in the battery side of the system as shown in Fig. 3 and Fig. 9(a). The result in Fig. 12 shows that the input current can be equally shared through the windings of the three-phase induction machine.
Fig. 12: Input current in boost mode of operation and shared input currents in the three-phase windings on the machine In the case of buck operation, the interleaving technique has been applied in load side of the system. The results in Fig. 13 show that the output current can be equally shared through the windings of the three phase induction machine.
Fig 13: Output current in buck mode of operation and shared output currents in the three-phase windings on the machine 2) Mode 4 and Mode 5 Simulation
Mode 4 and Mode 5 allow power flow from the battery to a single phase ac grid and from a single phase ac grid to battery, respectively. In the simulation, three inductors of the same values as that of the induction machine have been used in the topology.
The simulation models for Mode 4 and Mode 5 are given in Fig.
14(a) and (b).
Fig 14(a): Power flow between the battery and an AC grid:
from battery to ac grid (Mode 4) (b) from ac grid to battery (Mode 5)
Fig 14(b): Power flow between the battery and an AC grid:
from battery to ac grid: from ac grid to battery (Mode 5) For power transfer between the battery and an AC grid, the converter is configured in two stages with a dc/dc converter using one phase leg followed by an H-bridge inverter that interfaces with the ac grid. A single phase PLL algorithm has been developed to synchronize the inverter with the single-phase grid.
In the PLL algorithm, the grid voltage is first shifted by 90o and then dq-transformation on the grid and the shifted voltages gives the d-axis and q-axis voltages. To lock the phase, the q-axis voltage has been kept at zero by using a loop filter. The converter is operated in the current controlled mode when interfaced with the grid. Current regulation in the dq domain has been used in the grid connected mode using grid current feedback converted to dq current. The amount of power transferred from grid to vehicle and V2G depend on the id and iq current commands. The current regulator design is based on the following dynamic equations:
𝑣𝑑 𝑡 = 𝑅𝑖𝑑 𝑡 + 𝐿𝑑𝑖𝑑
𝑑𝑡 − 𝑤𝐿𝑖𝑞 𝑡 − 𝑒𝑑 𝑡 … … … … . .5 𝑣𝑞 𝑡 = 𝑅𝑖𝑞 𝑡 + 𝐿𝑑𝑖𝑞
𝑑𝑡 − 𝑤𝐿𝑖𝑑 𝑡 − 𝑒𝑞 𝑡 … … … … . .6
Fig. 15: Voltages and currents in Mode 4: grid voltage and grid current
Fig. 15 shows the grid voltage and grid current in Mode 4 with a current command id of 30 A and iq of zero. In this case, the power is being transferred from the vehicle to the single-phase ac grid.
Fig 16: Voltages and currents in Mode 5: grid voltage and grid current
Fig. 16(a) and (b) shows the grid voltage and grid current in Mode 5; in this case, the current command id is –30 A and iq is zero. Power is being transferred from the single-phase ac grid to the vehicle.
V. Bidirectional Converter for Photovoltaic Systems with Energy Storage
Topology of the Proposed Converter
The circuit diagram of the proposed converter is shown in Fig. 17, which consists of a low-voltage side (LVS) circuit and a high- voltage-side (HVS) circuit connected by a high-frequency transformer. The LVS consists of two ports, an energy storage capacitor Cs, the primary winding of the transformer, and an LCL-resonant circuit consisting of two inductors Lr and Lp and a capacitor Cr, where Lp includes the added inductance Lp1 and the leakage inductance of the transformer Lp. The HVS consists of the secondary winding of the transformer and a full bridge rectifier implemented with the diodes Ds1∼Ds4. The transformer’s turn ratio is defined as: n = Np/Ns, where Np and Ns represent the numbers of turns of the primary and secondary windings, respectively. Among the switches, S1 is called the main switch because it not only controls the power generated by the source connected to Port 1 (P1) but also changes the direction of the current flowing through the transformer.
Fig 17: Proposed isolated three-port bidirectional dc–dc converter for a PV and battery system
In this project, the two ports on the LVS are connected to a PV panel and a battery. To simplify the analysis, the proposed converter is analyzed by two separate converters: one is a single- switch LCL-resonant converter, and the other is the battery- related buck and boost converter consisting of L2, S2, and S3.
Buck and Boost Converter for Battery
The buck and boost converter consists of the inductor L2, switches S2 and S3, and capacitor Cs. When the generated solar power is larger than the power required by the load, S3 is inactive and S2 is switched on to form the buck converter. Then, the surplus energy generated from the PV panel is stored in the battery. In contrast, when the generated solar power is less than the power required by the load, S2 is switched off and S3 is switched on to form the boost converter. The battery is discharged to Cs to provide the deficient energy required by the load.
Power Management of the Proposed Controller
Two controllers are needed to manage the power in the LVS.
Their objectives are to regulate the output dc-link voltage to a constant value and manage the power for the two sources, respectively [12-15]. According to the availability of the solar power, there are three working scenarios of the converter, as illustrated in Fig. 18.
Three Working Scenarios
Scenario 1 (p1 ≥ pout): the available solar power is more than the load demand. The PV converter works in the MPPT mode; the battery is charged so that the dc-link voltage is controlled at a constant value.
Scenario 2 (0 < p1 < pout): there is solar radiation, but the solar power is not sufficient to supply the load. The PV panel is controlled in the MPPT mode by the MPPT algorithm described later. On the other hand, the deficient power is supplied by the battery, which is discharged by the boost converter, so that the dc- link voltage can be maintained at a constant value.
Scenario 3 (p1 = 0): there is no solar power available and, thus, the battery is discharged to supply the load. The active switches are S1 and S3. Proper controllers are designed to manage the power of the system in different scenarios.
VI. SIMULATION RESULTS
Simulations are carried in MATLAB/Simulink to validate the proposed converter and the controllers. The parameters of the converter are as follows: transformer turn ratio n = 5:14, Lr = 3.3 μH, Lp = 3.5 μH, and Cr = 0.22 μF. A PV panel is used, whose open-circuit voltage Voc and short-circuit current Isc are 22 V and 3.15 A, respectively. The nominal voltage and internal resistance rb of the battery are 7.5 V and 0.16 Ω, respectively.
The on-time of the switch S1, i.e., ton, is 3 μs, and the switching frequency varies in a range of 100–170 kHz. The resistive load RL = 100 Ω. The desired dc-link voltage V*dc and the nominal power of the load are 50 V and 25 W,
respectively.
Fig 18: simulation design of proposed three-port Dc/Dc converter
Fig 19: Overall block diagram of the controllers To test the dynamic characteristic of the controllers, the solar radiation is step changed to examine the responses of the dc-link voltage and output power of the PV panel, as shown in Fig. 20.
Fig. 20(a) shows that the initial solar radiation is zero and there is no power generated by the PV panel, as shown in Fig. 20(c). This indicates that the converter works in Scenario 1 and all of the power is supplied by discharging the battery. Fig. 2(b) shows that the dc-link voltage quickly reaches its reference value of 50 V.
Fig 20: Step responses: (a) profile of solar radiation
Fig 20(b): Step responses: dc-link voltage response
Fig 20(c): Step responses: PV power response
Scenario 1 does not terminate until the solar radiation changes from zero to 400 W/m2 at the first second. After that, the maximum power generated by the PV panel is 20 W, which is less than the load demand of 25 W. Thus, the battery still works in the discharge mode to provide the deficient power required by the load, and the variation of the dc-link voltage is negligible during the transition. From 1 to 1.5 s, the converter works in Scenario 2. The PV panel generates the maximum power, as indicated in Fig. 20(c). At 1.5 s, the solar radiation is changed from 400 to 600 W/m2, which corresponds to 32-W maximum power. Then, the battery stops discharging and starts to absorb the surplus power generated by the PV panel. It takes some time to change the direction of the battery current, which not only results in an approximately 2- V overshoot in the dc-link voltage but also leads to the PV power generated less than the ideal maximum power during the transient period, as shown in Fig. 10(c). After 0.3 s, both the dc-link voltage and PV power reach the desired value and the ideal maximum power point (MPP), respectively.
VII. CONCLUSION
The converter reconfiguration concept is useful in minimizing the size and parts in the power train of an electric vehicle. The machine-converter coupled simulation results showed that the integrated converter can be used for the power transfer with versatility without significantly extra power elements. The proposed converter has been used for simultaneous power management of multiple energy sources, i.e., a PV panel and a battery, in this Project. Simulation results have shown that the converter is not only capable of MPPT for the PV panel when there is solar radiation but also can control the charge/discharge of the battery to maintain the dc-link voltage at a constant value.
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Author’s Profile:
S.SREENU was born in Nagarjuna Sager, India, in 1989.
He received the B.Tech and M.Tech. degrees from JNTUH University, India, in 2010 and 2014 respectively, and currently working as an Assistant Professor at “B.V.Raju Institute of Technology”, Narsapur,Medak, India.
Dr. N.Bhoopal was born in Medak, India, in 1971. He received Doctorate from JNTUH in 2012, M.Tech. from Osmania University in 2004, B.tech from JNTUA 1997, and currently working as an Professor at “B.V.Raju Institute of Technology”, Narsapur,Medak, India.
ARTHAM MURALI was born in Patancheru, India, in 1990. He received the B.Tech and M.Tech. degrees from JNTUH University, India, in 2011 and 2014 respectively, and currently working as an Assistant Professor at
“B.V.Raju Institute of Technology”, Narsapur,Medak, India.
