Design of a Bidirectional Buck-Boost DC/DC Converter for a Series Hybrid Electric Vehicle Using PSCAD/EMTDC D. R. Northcott
S. Filizadeh
A. R. Chevrefils
Westward Industries Ltd. St. Francois Xavier, Canada
[email protected]
Dept. of Electrical and Computer Eng. University of Manitoba Winnipeg, Canada
[email protected]
Manitoba HVDC Research Center Winnipeg, Canada
[email protected]
Abstract — A process for the design and development of a bidirectional buck-boost dc/dc converter for use as a generator controller in a series hybrid electric vehicle is presented. The converter allows a single permanent magnet dc (PMDC) electric machine to be used for both engine starting and generating modes. The power electronics and the control system methodology are studied and refined using the PSCAD/EMTDC transient simulator as a design tool. Several operation scenarios are studied using parametric studies. A control system is developed for which the parameters are selected and optimized using nonlinear simplex simulation based optimization. The converter and optimized control system are tested under a simulated scenario to verify acceptable functionality and performance.
advantage of design tools such as simulation based optimization is to be presented.
Battery +
PMDC Machine
Engine
IB
DC/DC Converter
In this hybrid architecture the dc/dc converter plays an important role in regulating the power flow in the system. The battery bank voltage will vary with the operating conditions of the vehicle. Since the battery is directly connected to the main electrical node in the system it will make up the difference of current coming from the dc/dc converter and going into the motor drive as follows:
I B = I MD − I DCDC
PMSM Drive Motor
Fig. 1 - Architecture of the series hybrid vehicle under study
II.
DESIGN SPECIFICATIONS
The dc/dc converter must be designed to function within the system detailed in Fig. 1. The required specifications are given in Table I. TABLE I. Parameter
(1)
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DC/DC CONVERTER SPECIFICATIONS Voltage specification
Current specification
Engine starting mode
Motor-generator output DC bus input
Although this arrangement saves the cost of a converter at the battery terminals, the analysis of the dc/dc converter operation becomes more complicated and the performance of its control system becomes more important. A design process that relies on accurate simulations of the system and takes
978-1-4244-2601-0/09/$25.00 ©2009 IEEE
IMD
Motor Drive
INTRODUCTION
A hybrid gas-electric vehicle uses an energy storage device in concert with an internal combustion engine to provide propulsion to the vehicle; thereby offering performance and operational benefits not possible using only a single source of energy [1], [2], [3]. In this paper a bidirectional dc/dc converter will be developed to control power flow between the engine and the dc bus of a series hybrid electric vehicle whose architecture is shown in Fig. 1.
Electrical Node
IDCDC
Keywords - Electromagnetic transient simulation, hybrid electric vehicle, modeling, optimization, road-vehicle electric propulsion.
I.
-
0 to 40V 60 to 80V
0 to 50A 0 to 50A
Engine generating mode
Motor-generator input DC bus output
60 to 72V 60 to 80V
0 to 150A 0 to 100A
III.
THE BIDIRECTIONAL BUCK-BOOST DC/DC CONVERTER
105
100 Calculated Efficiency (%)
The bidirectional buck-boost dc/dc converter circuit to be used is shown in Fig. 2. The power electronic switch S1 is used for boost converting while the switch S2 is used for the buck conversion mode, where the two modes control power flowing in opposite directions. In this design, the buck converter mode is used for engine starting while the boost converter mode is used to control the electric current from the generator into the batteries and the motor drive.
smallest capacitor of 360µF is used for this analysis, while the generator is run at 80% of rated speed to produce the worst case input voltage of 60V. The switching frequency is varied from 4000Hz to 14000Hz and the duty cycle is adjusted from 0 to 0.5 so that the converter capability can be studied. The dc/dc converter efficiency plots are shown in Fig. 3.
95
90
Increased Switching Frequency 85
80
75 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
Boost Duty Cycle 4kHz
Fig. 2 - The bidirectional buck-boost converter
IV.
Parameter Inductor ‘L’ Capacitor ‘C’
V.
8kHz
10kHz
12kHz
14kHz
Fig. 3 - Efficiency versus duty cycle for multiple switching frequencies
DESIGN CONSIDERATIONS
There are a number of practical considerations that have been made in order to facilitate the selection of components for the design. The inductor and capacitor should be selected from commercially available products. Two decisions will be made from practical considerations at the outset, which are summarized in Table II. To save on component count and weight, the armature inductance of the permanent magnet dc (PMDC) machine will be used as the only inductance, and the capacitor will be selected from a range of values in [5]. TABLE II.
6kHz
INDUCTOR AND CAPACITOR VALUES Voltage Specification PMDC armature inductance = 30µH Select from {360µF, 490µF, 600µF, 730µF}
CONVERTER DESIGN AND SIMULATION
A simulation case is developed using the PSCAD/EMTDC transient simulator, which is capable of capturing transient effects of the converter switching as well as macroscopic effects from the vehicle drivetrain. Parametric studies will be undertaken to select the design parameters. Then a closed-loop controller will be implemented to control the boost operation, the parameters of which will be tuned using optimization methods. A. Selection of the converter switching frequency The analysis in this section refers to a worst case set of conditions where the generator voltage is low (i.e. low engine rpm) and the batteries are also at their lowest state of charge. The battery bank is modeled using a 60V voltage source with a 0.2Ω internal resistance in order to achieve the specified maximum converter output condition of 80V, 100A. The
The measure of efficiency discussed here is based upon the simulation program library insulated gate bipolar junction transistor (IGBT) switch model and parameters, which has not been further adjusted to model any specific IGBT. Therefore these numbers are not of significance on their own, but are useful for relative comparisons between different switching frequencies and duty cycles. By analyzing the efficiency plots in Fig. 3 it is noted that the efficiency declines both as switching frequency is decreased and as duty cycle is increased. It is also determined that a higher switching frequency can produce slightly more output current at the same duty cycle but will create more dynamic losses, and so there exists a trade-off. From this analysis it is discovered that with a 10 kHz switching frequency the duty cycle can be constrained from 0 to 0.4, and the specified output current of 100A can be achieved with the efficiency always greater than 95%. Since unnecessarily high switching frequencies will cause greater switching losses and result in more waste heat generated, 10 kHz is selected. B.
Selection of the dc link capacitor The simulation case was run using all the different values of dc link capacitance given in the specifications. The unfiltered output voltage was measured and analyzed using a duty cycle of 0.4, a switching frequency of 10 kHz, and the minimum generator voltage for each possible capacitance value. Relative cost, weight, and volume were calculated using the capacitor specification sheet [5] and are presented in Table III. Ripple voltage may not be the most important variable for this decision, but weight and volume reduction are always important in the alternative vehicle sector, and so the 490µF capacitor has been selected here.
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TABLE III. Capacitance (µF) 360 490 600 730
devices on the dc bus, and converter over-current, which could cause the converter to damage itself. For these reasons, a three variable control scheme is devised to limit or control the output voltage, output current, and output power as shown in Fig. 5.
DC LINK CAPACITOR COMPARISON
Ripple (Vpk-pk) 11.09 8.41 6.97 5.78
Relative cost 1.00 1.19 1.39 1.59
Relative weight 1.00 1.11 1.33 1.44
Relative volume 1.00 1.24 1.49 1.75
C. Buck converter operation Since the boost converter mode is significantly more complicated in operation and design than the buck mode, the design choices made until now have been toward achieving boost mode specifications. Now it is important to evaluate the suitability of these choices for engine starting operation using the buck converter mode of operation. The simulation model is now modified slightly in order to test against the remaining specifications in Table I. A voltage source of 60V is applied to the dc link and a resistor of 0.8Ω is used on the generator side to achieve the converter engine starting condition of 40V, 50A. The current on the generator side of the converter is plotted for a buck duty cycle of 0.68 in Fig. 4. Note that the inductive nature of a dc motor armature will serve to filter this current, whereas this test simply uses an ideal 0.8Ω resistor. Nevertheless these simulation results verify the ability of the design to achieve the specified 50A starting current.
For this scheme, the voltage controller will be set to the safe maximum battery voltage, the current controller will be set to the safe maximum converter current, and the power controller will be varied as required by the vehicle control scheme. For all three of these control modes, an increase in duty cycle will cause an increase to the control variables. Because of this fact, the minimum block will automatically activate the controller with the lowest output, thereby respecting the upper limits of voltage, current, and the present power set point. Under this scheme, the power control mode will be active under normal conditions, but when the vehicle is under hard acceleration or regenerative braking, the current controller or the voltage controller may take over momentarily to prevent an undesired over-current or over-voltage condition. The control system will now be implemented in simulation and connected to the boost converter in PSCAD/EMTDC. Initially, the controllers will be connected and optimized individually before all three are connected via the minimum block and tested together as a complete system.
80 70 Generator Armature Current (A)
Fig. 5 – Boost converter control system block diagram
60 50 40 30 20 10 0 0
20
40
60
80
100
120
140
Time (uS) Igen (Filtered)
Igen
Fig. 4 - Generator starting current
VI.
BOOST MODE CONTROL SYSTEM DEVELOPMENT
Now that the converter has been designed to achieve a set of maximum operational requirements, the next step is to design a control system for the boost converter. It will be important to accurately and responsively control the power flowing from the engine because of the reasons discussed in Section I. A control system will be developed for the boost converter mode of operation which will be tuned and optimized using the simulation case.
B. PMDC Generator Model For the control system development, a PMDC machine will be modeled by using the standard library dc machine model from PSCAD/EMTDC. This machine will represent the generator used to drive the input voltage for the boost converter. Since the standard dc motor model has a field winding, and a PMDC machine does not, the field circuit will be fixed in such a way to achieve the open circuit voltage vs. rotational speed characteristic of the PMDC motor to be used [7]. The PSCAD circuit diagram is shown in Fig. 6.
A. High Level Design The boost converter shall control the power flow from the engine to the main dc bus, and hence the batteries by varying its duty cycle. While maintaining the desired power flow is important, there are a few additional problems to avoid during operation, such as battery over-voltage, which may damage
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This can be linearized by first passing the PI output through the following characteristic before sending it on to the gate driver:
DL =
DNL DNL + 1
(3)
Where DNL is the non-linearized duty cycle from the PI controller and DL is the duty cycle which will linearize the response of the converter for the PI controller’s efforts. This will make tuning the PI controller easier and should allow improved controller performance.
Fig. 6 – Modeling a PMDC generator using the standard PSCAD dc machine model
The rotor speed is fixed at 80% rotational speed (corresponding to 2880 rpm). Under this arrangement the simulation model will account for the transients in the electrical circuit, but neglect the dynamics in the mechanical system. It is assumed that the constant power control scheme and the fast response of the converter will effectively isolate the engine from any transients that are slow enough to affect the high inertia mechanical system for the studies being undertaken here. Therefore a mechanical model will not be implemented. The armature resistance and inductance for the model are adjusted to match values provided by the motor manufacturer. C. Boost Converter Linearization As mentioned earlier, the selected boost converter topology exhibits a nonlinear voltage response. The formula relating the voltage boost factor to the duty cycle [6] is given as:
Vout 1 = Vin 1 − D
D. Voltage Controller Implementation and Tuning Voltage control is implemented in PSCAD using the standard PI control block. A simple voltage source and resistor model is used to represent the battery bank. Several PI parameters are chosen intuitively, using a trial and error process, until the system begins to converge after a set point bump test. This will set up the initial conditions to be improved upon during the optimization process. The PSCAD circuit diagram for the system under study is shown in Fig. 7. The variables to be controlled will be the output voltage, known as Vout, and the load current, which can represent the variable power consumed by the drive motor to accelerate the vehicle. Now it is necessary to develop an optimization strategy by which the performance of the control system can be analyzed. The system will undergo a short test cycle whereby the controller set point is adjusted suddenly, and then the load current is adjusted a short time later. The controller’s ability to react to these two stresses will be used by the optimization routine to choose better PI control tuning parameters. The test schedule is summarized in Table IV.
(2)
Fig. 7 - Buck-boost dc/dc converter optimization circuit
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120
0.4 0.35
100
VOLTAGE CONTROLLER OPTIMIZATION TEST CYCLE Voltage set point (V) 75 65 65
Load current (A) 0 0 65
80
0.25 0.2
60 0.15 40
0.1 0.05
100
20 0
90 0
80
-0.05 0
10
20
30
40
50
60
Trial #
70 Voltage (V)
P, I Values
Time (ms) 0 25 35
0.3 Objective Values
TABLE IV.
Objective
60
P
I
Fig. 10 - Optimization progress for voltage controller
50 40 30
100
20
90
10
80
0 0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
70
0.05
VoutSetpoint
Voltage (V)
Time (s) Vout
Fig. 8 - Voltage control results with initial parameters
60 50 40 30
The performance of the controller using initial parameters is shown in Fig. 8. As can be seen, the controller has some oscillatory performance from the set point change, and is also affected by the load change at 35ms. The initial overshoot will be ignored as it is mainly a consequence of the initialization of the simulation. Any normal system startup would gently ramp the set point from the existing dc bus conditions. The objective function for the optimization will scale and accumulate control loop error from 10ms to 50ms. Through multiple trials the optimization will successively and automatically (i.e. without designer intervention) improve the performance by adjusting the P and I variables until a chosen tolerance has been reached [8]. The Simplex Optimum Run control block is configured as shown in Fig. 9.
Fig. 9 - Simplex optimum run configuration
The objective function for this optimization is simply the square of the controller error. Squaring the error is done to make all errors positive for the accumulator and can also be shown to speed the convergence of the optimization.
20 10 0 0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
Time (s) VoutSetpoint
Vout
Fig. 11 - Optimized voltage controller performance
As seen in Fig. 10, the optimization eventually converges toward a better set of parameters for the system. Fig. 11 shows the new tuning parameters in action. It is up to the designer to decide if this result is acceptable, or if additional optimizations should be run after changing the objective function, modifying the control scheme, or using different initial parameters. The same optimization procedure is run for the current and power controllers. Each time, the optimization routine produces a better set of parameters by using the initial guess that is determined interactively. E. Full Control System Evaluation A test strategy is developed to vary the output power set point and the load current in such a way to test all three control modes when they are used in a combined control system. The voltage and current set points are fixed to the maximum desired voltage and current, while the power set point and the load current undergo quickly ramped changes during the test. This test schedule is summarized in Table V.
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TABLE V. Load Current 40 A 40 A -100 A
Time 0 ms 200 ms 350 ms
information. The acceptable operation of the system was verified in buck mode in order to allow for starting the engine by using the generator as a motor. Finally, a closed loop control system was formulated and the parameters were optimized to improve some system performance metrics. The converter and control system were simulated using a simple battery model and current source to test the functionality of the developed control scheme.
CONTROL SYSTEM TEST SCHEDULE Pout set point 5 kW 8 kW 8 kW
Iout set point 100 A 100 A 100 A
Vout set point 90 V 90 V 90 V
120
10
REFERENCES
9 8 7
80
6 60
5 4
40
[2] [3]
3
[4]
2
20
Power Mode 0 0.00
[1]
Power (kW)
Voltage (V), Current (A)
100
Current Mode
Voltage Mode
1
[5]
0 0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
Time (s) Iout
Vout
[6]
Pout
Fig. 12 - Results of the test simulation
[7]
As detailed in Fig. 12, the system initially operates in power control mode, then enters current control mode when the power set point is increased toward 8 kW. This is because under this condition, 8 kW is not achievable without violating the current limit of 100A. Then at approximately 0.35s the system enters voltage control mode when a large amount of current is injected into the battery from the external source, which simulates a heavy regenerative braking condition. The duty cycle must be cut back dramatically in order to keep the voltage at the 90V maximum.
[8]
As can be seen from the data, controller overshoot on the order of 10% for a short time can be expected from this control scheme. This is because the PI controllers do not have integrator ramp-up limiting, and as a result there exists an adjustment period between mode transitions. This can either be made acceptable by ensuring a safety margin in the set point, or through some additional modification and tuning of the control scheme. A possible solution could involve back-calculating and setting the integrator of the PI controller upon the transition between control modes. VII. CONCLUSIONS A process for the design and development of a bidirectional dc/dc converter as a module for a series hybrid vehicle was presented. An electromagnetic transient simulation model was developed using PSCAD/EMTDC to study several transient and steady-state operation characteristics during the design process. In boost mode, the possible effects of switching frequency and duty cycle on the maximum current output and converter efficiency were studied. The effects of different dc link capacitor values on the output ripple voltage were also evaluated and several design decisions were made based on this
M. Ehsani, Y. Gao, S. E. Gay, and A. Emadi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles, Fundamentals, Theory, and Design. New York: CRC Press, 2005. J. M. Miller, Propulsion Systems for Hybrid Vehicles, UK: IEE, 2004. Emadi, K. Rajashekara, S. S. Williamson, and S. M. Lukic, “Topological overview of hybrid electric and fuel cell vehicular power system architectures and configurations,” IEEE Transactions on Vehicular Technology, vol. 54, no. 3, pp. 763-770, May 2005. N. Mohan, T. M. Undealand, W. P. Robbins, Power electronics: converters, applications and control, 3rd Ed., Wiley, 2003. Cornell-Dubilier, Type 947C High Capacitance, High Current, DC Link Capacitance, Metallized Polypropylene Dielectric, [Online] Available: http://www.cde.com/catalogs/947C.pdf [Accessed: Feb. 27, 2009]. M. H. Rashid, Power Electronics: Circuits, devices, and applications, 3rd ed. Upper Saddle River, NJ: Pearson Education, 2003. Northcott, D.R., Filizadeh, S., Electromagnetic Transient Simulation of Hybrid Electric Vehicles”, IEEE International Symposium on Industrial Electronics, pp. 833-838, June 2007. M. Gole, S. Filizadeh, P. L. Wilson, R. W. Menzies, "OptimizationEnabled Electromagnetic Transient Simulation," IEEE Tran. Power Delivery, vol. 20, no. 1, pp. 512-518, January 2005.
D. R. Northcott (M’06) received his B.Sc. and M.Sc. degrees in electrical engineering from the University of Manitoba in 2005 and 2007 respectively. He is currently working for Westward Industries as an Electrical Design Engineer developing a series hybrid GO-4 parking patrol vehicle. His areas of interest include power electronics, digital control systems, and hybrid electric vehicles. S. Filizadeh (M'05) received his B.Sc. and M.Sc. degrees in electrical engineering from Sharif University of Technology in 1996 and 1998, respectively. In 2004, he obtained his Ph.D. degree in electrical engineering from the University of Manitoba, where he currently is an assistant professor. His areas of interest include transient simulation of power systems, power electronics, hybrid and electric drives and nonlinear optimization. Dr. Filizadeh is a registered professional engineer (P. Eng.) in the province of Manitoba. A. R. Chevrefils (M’05) received his B.Sc. and M.Sc. from the University of Manitoba in 2005 and 2008 respectively. He is currently working for the Manitoba HVDC Research Center. His areas of interest are power systems, power electronics, simulation, and control.
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