An Increased Efficiency Series Hybrid Electric Bus Using Decoupled DC-Link Voltages

Craig B. Rogers, Fang Z. Peng Michigan State University

Department of Electrical and Computer Engineering 2120 Engineering Building East Lansing, MI 48824

Email: rogerscr@msu.edu

Abstract: This paper proposes a new main circuit topology, the back-to-back current fed quasi-Z-source architecture, for the series hybrid electric bus (SHEB) using decoupled dc-link voltages, which will greatly improve system efficiency. This work is based on urban drive cycle operating conditions, the optimal operating region of the internal combustion engine, and the varying voltage requirement of traction motors. The paper presents how the efficiency of the SHEB can be significantly enhanced by decoupling the dc-link voltage at the generator end of the topology from that of the traction inverter end. The analyzed results verify the proposed circuit for SHEB applications.

Keywords - Hybrid Electric, Hybrid Electric Vehicle System, Quasi-Z-Source Inverter

I. INTRODUCTION: Hybrid vehicles have made a sizable contribution to the improvement in fuel economy of vehicles and one such application of this has been the urban commuter bus. By the very nature of the repeated acceleration and deceleration of the urban drive cycle, this type of bus, is an excellent choice for the application of hybrid technology. The traditional hybrid bus has a dc-link shown in figure 1, where the generator inverter and the traction motor inverter share the common DC-Link [5] - [7]. Therefore, the DC-Link voltage present at the traction motor inverter is at all times equal to the DC-Link voltage present at the generator inverter; even though, the desired voltage of these two separate inverters is almost always different from each other. This paper presents a new topology that decouples the DC-Link voltage at one end of the topology from that at the other end and achieves reduced power losses as a result. Before proceeding, however, there are several constraints which need to be discussed which are relevant to this work. Constraint 1: The first such constrain relates to the internal combustion engine (ICE) of the bus. As seen in figure 2, the most efficient torque speed operational region of a typical diesel engine is between 1212 to 1575 revolutions per minute and 900 to 1650 newton meters of torque. For a diesel engine, this region may achieve efficiency as high as

Figure 1. Traditional back-to-back voltage source inverter to voltage source inverter (VSI-VSI) series hybrid electric configuration.

55% but the efficiency decreases rather rapidly for operation points outside of this region. Since the efficiency of the ICE is sensitive to its torque – speed operation point, a significant amount of savings can be achieved by ensuring that the ICE operating point does not deviate from this region. Unfortunately, maintaining the engine speed within this narrow region likewise holds the generator voltage

Figure 2. Typical torque vs. speed curves of a diesel ICE. Highest efficiency operation is shown in the center most enclosed line spanning the region from 1212 to 1575 revolutions per minute and 900 to 1650 newton

meters of torque.

978-1-4673-1408-4/12/$31.00 ©2012 IEEE

within a narrow region. Adding to this, it is usually desirable to generate at a higher voltage and this combination results in a DC-Link that is held at a high voltage. Constraint 2: It is desirable to use a battery pack that is significantly below the DC-Link Voltage since adding more battery cells in series decreases the reliability of the battery stack and increases its cost. To accommodate the current charging requirements of a commuter bus, several battery stacks must be placed in parallel with each other. As a result of this, increasing the voltage of the battery pack requires adding more cells in series with each of the several parallel stacks. One can see how increasing the voltage of the battery pack can quickly increase the number of cells required. Constraint 3: The voltage required by the traction motors varies with the speed of the motors (low voltage required for low speed) and the normal operating speeds of the commuter bus are significantly below full speed for the vast majority of time as shown in the Environment Protection Agency’s Urban Dynamometer Drive Schedule (EPA-UDDS) in figure 3. Constraint 4: Equations (1) and (2) show the turn on and turn off losses of an IGBT [7]. (1)

(2)

where,

trN = rated rise time; Vcc = DC-Link voltage; IcM = maximum collector current; IcN = rated collector current; Fs = switching frequency;

tfN = rated fall time. As seen in these equations, switching power loss is directly proportional to Vcc, the DC-Link voltage. These constraints lead to the DC-Link being held at a high voltage and therefore unnecessarily high switching losses in the traction inverters. By decoupling the DC-Link voltage at the generator inverter from that at the traction inverters, the efficiency of many of the required operating

Figure 3. EPA Urban Dynamometer Driving Schedule (UDDS)

points can be significantly improved. It is desirable at this point to compare the losses of the proposed topology with that of the traditional series hybrid bus topology.

II. BASE LINE

The following conditions were used in the calculation of power losses of the traditional topology.

Conditions: Generator line-to-line voltage is 429 Vrms; rectified generator voltage is 700 Vdc; traction motor (TM) line-to-line voltage sweeps from 29 Vrms to 429 Vrms; switching frequency is 10 kHz; power factor is 90%; battery pack voltage is 350 Vdc; switching, conduction and reverse recovery losses of the inverters and converter are based upon typical 1200-V/ 450-A IGBTs with freewheeling diodes. The traction inverter (TI) DC-Link voltage is at all times held equal to the generator inverter (GI) DC-Link voltage of 700 Vdc. One generator with its GI and two traction motors each with its own TI are assumed.

Turn-on loss, Pon, turn-off loss, Poff, IGBT conduction loss, Pi, Diode conduction loss, Pd, and diode reverse recovery loss, Prr, of the inverters and converter were calculated in a manner similar to that in [7] and the inductor loss was approximated by (3) which has provided a reasonable loss approximation for inductors tested in the lab.

(3)

where,

Pind_dc/dc is the power loss in the DC/DC converter inductor;

scN

cMrNccon F

IItVP

2

81 ⋅=

⎟⎟⎠

⎞⎜⎜⎝

⎛⋅

⋅⋅⋅=cN

sfNcMccoff IFtIVP

812

( )( )0057.010043.0 //_ +−≈ dcdcdcdcdcind BPP

Pdc is the DC power being absorbed or provided by the battery pack;

Bdc/dc is the boost factor of the DC/DC converter.

The total range of operating points for the hybrid vehicle is very wide. The bus speed can range from 0 to 80 mph and the traction motor torque can range from a large negative value to a large positive value for all bus speeds. Also, the choice can be made to draw power from the ICE and generator, from the battery pack, or some combination of the two. The inputs to the operating point decision model are gas or brake pedal position, bus speed, and battery state of charge (SOC). For any given bus driver pedal position input, which corresponds to a requested positive or negative traction motor (TM) torque, there is an infinite combination of generator and or battery power levels from which to choose from. For this analysis, a reasonable range of such operating points was sought. To illustrate a representative set of operating points, the TM power was swept from 16.7 kW to 250 kW while holding the TMs at full current. Conducting the analysis at full current is representative of full acceleration which is normal for these heavy vehicles. The sweep of traction motor power from 16.7 kW to 250 kW is representative of 0 to 80 mph which includes the entire range of speed for the bus. For this sweep of TM operating points, three different scenarios will be shown:

Scenario 1 holds the generator power steady at 50 kW throughout the TM power sweep while the battery pack provides or absorbs the difference between the generator power and TM power;

Scenario 2 holds the generator power steady at 100 kW while, again, the battery makes up the difference, and

Scenario 3 holds the generator power steady at 150 kW and the battery pack makes up the difference.

Figure 4 shows this power sweep for the traditional hybrid topology.

III. TOPOLOGY ANALYSIS Several topologies were analyzed with the desire to find an improved topology that can increase operating efficiencies by decoupling the DC-Link voltage at one end of the topology from that at the other end. The most effective topology devised, as shown in figure 5, will be referred to as the back to back current fed quasi “Z” source inverter topology (CF-qZSI). Current Fed Quasi “Z” Source Inverter (CF-qZSI): Like the other “Z” source type inverters, the inverter switching devices include an additional zero switching state that is not allowed with the traditional inverters. This additional switching state is frequently referred to as the

Figure 4. Power loss curves for the traditional back-to-back VSI-VSI series hybrid bus.

Figure 5. Back-to-back current fed qZSI topology using reverse blocking

IGBTs (RB-IGBT).

shoot-through state or short circuit zero state when both the top and bottom switches of a phase leg are turned on at the same time. This switching state would destroy the switching devices of the traditional inverter but the switching currents are safely limited in the impedance network of the “Z” source inverters. [2], [3], [6]. The CF-qZSI can be distinguished from the voltage fed qZSI (VF-qZSI) in that the CF-qZSI has a small capacitor bank in parallel with the generator or TM and uses reverse blocking IGBTs (RB-IGBT) while the VF-qZSI uses the traditional IGBT with a freewheeling diode. Both the voltage fed and current fed topologies are shown in [4]. The operation and switching states used to control the circuit configuration at the traction motor end of figure 5 (from the battery to the traction motors) is different than that at the generator end (from the battery to the generator). Therefore, each will be discussed separately below:

(a) State I– Active State: DA

(b) State II– Short Zero State: Dsh

(c) State III– Open Zero State: DOP

Figure 6. Operational states of the CF-qZSI [1].

A. Traction Motor End: The operation states of the traction motor end of the back-to-back topology are shown in Figure 6 (a), (b), and (c) where 6 (a) shows the CF-qZSI active state, figure 6 (b) shows the shoot-through state (short circuit zero state), and figure 6 (c) shows the open (open circuit zero) state. Figure 7 shows the voltage boost or buck results as a function of these states. When the inverter uses only active states and not zero states, the DC output voltage is equal to the DC input voltage. If boosting this voltage is desired, the active duty cycle will be reduced and short circuit zero states will be added. If bucking is desired, the active duty cycle will be reduced and open circuit zero states will be added. In the graph of Figure 7, the red line (known as Mode 1 or upper boundary) uses purely active states and short circuit zero states while the blue line (known as Mode 2 or lower boundary) is purely active states and open circuit zero states. Between these two boundary lines, a combination of all three states can be used.

Figure 7. Vout/Vin vs. active duty cycle. Mode 1 uses active states and short circuit zero states but not open circuit zero states, Mode 2 uses active

states and open circuit zero states but not short circuit zero states.[1]

At the traction inverter end of the topology, it is this buck and boost capability that allows the energy saving operation of maintaining the DC-Link voltage across the traction inverters at the minimum level required for the given traction motor speed.

B. Generator End: The operation of the qZSI configuration at the generator end of the topology is a bit different. The qZSI at this end is used to buck the generator voltage down to the level of the battery by using the active and open circuit zero states.

AGIDC

batt DV

V=

− (4)

where, DA is the active duty cycle of the generator inverter switches, Vbatt is the battery voltage, and VDC-GI is the DC-Link voltage across the generator inverter. In this case, as the active duty cycle is reduced from 100%, it is the open circuit zero state that is increased and the short circuit zero state is not used while pushing power from the generator to the battery and traction inverter. Figure 8 shows the generator end of the topology. This end of the topology is not a complete mirror image of that at the traction motor end. The reverse blocking IGBTs (RB-IGBT) are flipped over allowing the top DC-Link rail to be polarized with a positive voltage from the generator and an additional IGBT with a freewheeling diode has been added in the middle of the “Z” source impedance network. This switch “S1” remains open during normal operation and is only closed when power is to be pushed from the battery to the generator to start the diesel engine. Figures 9 (a) and (b) show the states that are used to start the engine. Since the generator inverter uses RB-IGBTs, current in this inverter

Figure 8. Generator end of the back-to-back topology.

(a) Open circuit zero state with S1 closed.

(b) Active state with S1 open.

Figure 9. The switching states used to start the diesel ICE.

cannot change directions as it can with the traditional voltage source inverter (VSI). Therefore, to reverse power flow to use the generator as a starter motor, it is the voltage polarity present at the generator inverter that must be inverted.

The switching states of figure 9 are used to achieve this. During the Open circuit zero state of figure 9 (a), the inverter’s switches are opened and the switch S1 closes. This puts the inductors LG1 and LG2 in parallel with the voltage of capacitors CG1 and CG2 which charges the inductors. During the state of figure 9 (b), the inverter switches re-enter the active states and the IGBT S1 opens again. This pushes the continuous current of the inductors LG1 and LG2 to the generator which now functions as a starter motor in this mode.

IV. CALCULATION OF LOSSES FOR THE PROPOSED BACK-TO-BACK CF-QZSI TOPOLOGY

The power losses for the back-to-back CF-qZSI topology were calculated under the same conditions as those of the traditional hybrid bus of figure 1, except that the DC-Link voltage present at the TI varies from 47 Vdc to 700 Vdc as the bus speed varies from 0 mph to 80 mph. The traditional hybrid bus holds this voltage high throughout the entire speed range. Also, there are several other differences that must be taken into account. The CF-qZSI uses reverse blocking IGBTs (RB-IGBT) instead of the common IGBT module with the anti-parallel freewheeling diode. The RB-IGBT has slightly higher forward conduction losses, but does not have the losses from an anti-parallel freewheeling diode. The qZSI impedance network on the generator inverter side includes an IGBT module (IGBT and its anti-parallel diode) which does not exist in the traditional hybrid topology and the switching, conduction, and reverse recovery losses of this device have to be considered for the modes under which they turn on, off, or conduct accordingly. You will also notice the two inductors of this impedance network and an inductor connected to the battery node. The losses of these inductors have also been estimated in the same manner as that of (3) based upon the power flow through them and the core losses (hysteresis and eddy) associated with the ΔB which results from boost or buck functions. Also, the capacitors of the qZSI impedance network are equivalent to those of the traditional topology and the comparative losses were found to be inconsequential and therefore, the capacitor losses of both the traditional and proposed topologies are ignored in this study. Likewise, losses are calculated for the three inductors on the TM side of the battery node as well as the conduction and reverse recovery losses of the diode in that impedance network. Also, the TI has an additional switching state that the traditional VSI does not have [2] [3]. This shoot-through state, in which both the top and bottom switching devices of a phase leg are gated on at the same time, provides an important function for this unique type of inverter. This state allows the TI to boost the voltage seen at the TI input, and the losses of this additional switching state also must be included in the analysis. The following summarizes the loss effects of the traditional hybrid vs. those of the proposed back to back CF-qZSI:

The proposed topology has additional losses that result from additional components and switching states:

• “Z” source impedance network inductors and inductors connected to the battery node;

• Reverse blocking feature of the inverter switching devices;

• Additional IGBT module included in the generator end “Z” source network;

G

D1

S1

• Additional diode included in the traction motor end “Z” source impedance network, and

• Additional switching states of the “Z” source type inverters.

The proposed topology has lower losses that result from:

• Not having a separate DC/DC converter connected to the battery, and

• Ability to decouple the DC-Link voltage present at opposite ends of the topology.

It is then necessary to aggregate all of the positive and negative loss effects of both the traditional hybrid topology and the proposed topology for comparison. Below in figure 10, are the losses of the new proposed topology under the same conditions as shown in figure 4 for the traditional hybrid topology.

Figure 10. Power loss curves for the proposed back-to-back CF-qZSI series hybrid bus.

For convenience, a set of comparative graphs are presented in figure 11 (a), (b), and (c).

(a) Power loss comparison, traction motor power sweep, generator power held at 50kW

(b) Power loss comparison, traction motor power sweep, generator power held at 100kW

(c) Power loss comparison, traction motor power sweep, generator power held at 150kW

Figure 11. Power loss comparisons between the traditional topology and the proposed topology under three different scenarios (a), (b), and (c).

V. CONCLUSION The proposed topology (back-to-back CF-qZSI series hybrid bus) offers significant opportunities for optimization and reduction in power losses for a bus operating within the EPA’s UDDS drive cycles. These reductions in power losses of the power electronics switching devices and inductors result from decoupling the DC-Link voltage present at one end of the topology from that at the other end which is not possible with the traditional series hybrid topology.

[1] Qin Lei; Shuitao Yang; Fang Zheng Peng; Inoshita, R.; , "Application of current-fed quasi-Z-Source Inverter for traction drive of hybrid electric vehicles," Vehicle Power and Propulsion Conference, 2009. VPPC '09. IEEE , vol., no., pp.754-760, 7-10 Sept. 2009

[2] F. Z. Peng, “Z-source inverter,” IEEE Transactions on Industry Applications, vol. 39, No. 2, pp. 504-510, March/April 2003.

[3] F. Z. Peng, X. Yuan, X. Fang, Z. Qian, “Z-source inverter for adjustable speed drives,” in IEEE Power electronics Letters, Vol. 1, No. 2, June 2003

[4] Anderson, J.; Peng, F.Z.; , "Four quasi-Z-Source inverters," Power Electronics Specialists Conference, 2008. PESC 2008. IEEE , vol., no., pp.2743-2749, 15-19 June 2008

[5] C. C. Chan, “The state of the art of electric, hybrid, and fuel cell vehicles,” Proceedings of the IEEE, Vol. 95, Issue 4, 2007, pp. 704-718

[6] K. Holland, F.Z. Peng, “Control strategy for fuel cell vehicle traction drive systems using the Z-source inverter,” Vehicle Power and Propulsion, IEEE Conference, 2005

[7] T.M. Jahns, V. Blasko, “Recent advances in power electronics technology for industrial and traction machine drives,” Proceedings of the IEEE, Vol. 89, Issue 6, pp. 963-975, 2001

[8] Casanellas, F.; , "Losses in PWM inverters using IGBTs," Electric Power Applications, IEE Proceedings, vol.141, no.5, pp.235-239, Sep 1994

3.05.07.09.0

11.013.015.0

Pow

er L

oss

(kW

)

Traction Motor Power (kW)

Traditional topology vs. Proposed topology:

Inverter, Converter & Inductor Power Loss

Traditional; Generator Pwr=150kW

CFqZSI; Generator Pwr=150kW