dc voltage control of inverter interfaced dual active
TRANSCRIPT
DC Voltage Control of Inverter Interfaced Dual
Active Bridge Converter for V2L applications
Yunting Liu
CURENT, Department of EECS
The University of Tennessee
Knoxville, TN, USA
Xiaorui Wang
Department of ECE
Michigan State University
East Lansing, MI, USA
Wei Qian
Department of ECE
Michigan State University
East Lansing, MI, USA
Ameer Janabi
Department of ECE
Michigan State University
East Lansing, MI, USA
Bingsen Wang
Department of ECE
Michigan State University
East Lansing, MI, USA
Xi Lu
Research and Innovation Center
Ford Motor Company
Dearborn, MI, USA
Ke Zou
Research and Innovation Center
Ford Motor Company
Dearborn, MI, USA
Chingchi Chen
Research and Innovation Center
Ford Motor Company
Dearborn, MI, USA
Abstract—This paper proposed a dc voltage control for
inverter interfaced dual active bridge converter. This converter is
used in SiC based on-board charger for electric vehicle. The
inverter interfaced dual active bridge converter has a dc link as a
middle stage of AC/DC conversion. This dc-link voltage is unstable
without control. Especially in applications like vehicle-to-load
(V2L) applications, the converter is required to operate under
both 120-volt ac load and 480-volt ac load so the vehicle can export
power to external loads. The large variation of dc link voltage
requires a robust general dc voltage control to guarantee a safe
operation of load change. This paper analytically derives the
small-signal model of the inverter interfaced dual active bridge
converter at wide range of operating point. Based on the small-
signal model, a robust dc voltage control is proposed to adapt to
the wide range of dc link voltage. The proposed dc voltage control
is verified by a 25-kW SiC based inverter interfaced dual active
bridge converter prototype.
Keywords—inverter interfaced Dual Active Bridge (DAB), dc
voltage control, small-signal model, vehicle to load (V2L)
I. INTRODUCTION
The inverter interfaced dual active bridge (DAB) converter has been widely used in electric vehicle on-board charger applications[1]. The inverter interfaced DAB converter consists of a three-phase front-end and two H-bridges as shown in Fig.1. The front-end three-phase inverter interacts with ac grid. The feature of this topology is that it has a wide range of operation and has a chance to adapt to multi-functions. This inverter interfaced DAB converter has been proposed with multi-functions in [2]. The converter discussed in [2] performed various functionalities such as battery charging (G2V), vehicle to grid (V2G) regenerative capability, and vehicle-to-load (V2L) feature. With the V2L function, the converter can output different types of voltage, such as 120-volt single-phase and 480-volt three-phase.
However, the multi-function concept increases the complexity of DAB control. In order to enhance the DAB with multi-function, we need to consider control algorithms for many operating points. To switch control algorithms from different operating points may consume valuable computation time of DSP. This becomes severe for wide-bandgap (WBG) based converters since normally the control algorithms are required to complete in one switching cycle. The switching frequency of WBG based converters normally falls in the range of 100 kHz to 500 kHz, which is 10 times faster than Si based converters. The fast switching requires the general control for multiple operating points, and even multiple functions. A faster and simpler controller is needed for WBG based inverter interfaced DAB system so that the converter can benefit most from the fast switching of WBG devices.
In scenarios like V2L applications, the converter is required to operate under both 120-volt ac load and 480-volt ac load. The wide range of dc-link voltage (VC1) challenges the control system to adapt to wide range of operating points. However, few literatures discussed the dc voltage control especially in V2L applications. Many existing literatures discussed the small-signal model of DAB converters[3]–[5]. A few other papers discussed the small-signal model of inverter-interfaced DAB converters[6], [7]. The focus of existing literatures is to develop a general model of DAB to understand the dynamics of DAB. Some control algorithms are established on the small-signal model for DAB[8]–[11]. The control objectives are normally transformer current or output voltage. The dc bus voltage control
Fang Z. Peng
Center for Advanced Power Systems
Florida State University
Tallahassee, FL, USA
HV
Batt
P1
P2
P3
P4
Q1
Q2
Q3
Q4
S1
S2
S3
S4
C1 C2
Ls
Lleak
1:1
Lbat
Vbat
Q3
Q4
Fig. 1. The inverter interfaced DAB converter.
978-1-7281-3761-2/19/$31.00 ©2019 IEEE 319
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for inverter interfaced DAB has been discussed in [12], [13]. The voltage control in these papers are too complicated to adapt to multi-function converters.
This paper proposed a general dc voltage control for inverter interfaced DAB converter that is suitable for a wide range of operating points. This paper is organized as follows. The small-signal model of inverter interfaced DAB converter that considers the primary side bus voltage as an independent state variable is derived in Section II. Based on the small-signal model of the converter, the wide range operating point for multi-functions are assessed in Section III. A robust dc voltage control is proposed in Section IV to cover all the operating points for the converter. Some experiment results to verify the effectiveness of the proposed method are presented in Section V.
II. SMALL-SIGNAL MODEL OF INVERTER INTERFACED DAB DC
VOLTAGE DYNAMICS
To simplify the small-signal modeling of the converter, the topology is reduced to single phase as shown in Fig.2. To further simplify the analysis, we assume that Lbat is small and the C2 voltage is the same as the battery voltage. Therefore, VC2 is no long a state variable. The high-frequency transformer model is replaced by the power transfer function,
𝑃 =𝑛𝑉C1𝑉C2
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘𝐷(1 − 𝐷) ()
Where 𝑛 = 1 is the turns ratio of the transformer for this paper. 𝑓𝑠 is the switching frequency. 𝑉C1 and 𝑉C2 are the capacitor voltages. 𝐷 is the phase shift between primary voltage and secondary side voltage. The single-phase shift control is adopted in this paper. the DAB is modeled with the power transfer function in this paper because the high frequency transformer is externally presenting a power transfer function in lower frequency domain. If considering the internal dynamic of the high frequency transformer, the conventional averaging model is not suitable since the averaging window (one switching cycle) will eliminate all the high frequency components. For example, the leakage inductance current becomes zero after averaging. This makes the averaging model unsuitable for internal dynamics of the DAB. C1 voltage 𝑣𝐶1 and Ls current 𝑖𝑠 are selected as state variables in this paper. The state equations are,
{
𝑝 =
𝑣𝐶1𝑣𝐶2
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘𝐷(1 − 𝐷)
𝑝 = 𝑝0 + 𝑝𝐶1
𝑝𝐶1 = 𝐶1𝑣𝐶1𝑑𝑣𝐶1
𝑑𝑡
𝑝0 = 𝑖𝑠𝑣𝑠
𝐿𝑠𝑑𝑖𝑠
𝑑𝑡= 𝑞𝑣𝐶1 − 𝑣𝑠
(2)
Where 𝑞 is the equivalent duty cycle of H-bridge Q1-Q4. Eq.(2) can be simplified as
{𝐶1𝑣𝐶1
𝑑𝑣𝐶1
𝑑𝑡=
𝑣𝐶1𝑣𝐶2
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘𝐷(1 − 𝐷) − 𝑖𝑠𝑣𝑠
𝐿𝑠𝑑𝑖𝑠
𝑑𝑡= 𝑞𝑣𝐶1 − 𝑣𝑠
(3)
Replace 𝑥 = �̃� + 𝑥𝑒 where 𝑥 = 𝐷, 𝑞 , 𝑣𝐶1 or 𝑖𝑠 . 𝑥𝑒 is the state variable values at steady state.
{
𝐶1(�̃�𝐶1 + 𝑉𝐶1)
𝑑(�̃�𝐶1+𝑉𝐶1)
𝑑𝑡
=(�̃�𝐶1+𝑉𝐶1)𝑣𝐶2
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘(�̃� + 𝐷) (1 − (�̃� + 𝐷)) − (𝑖�̃� + 𝐼𝑠)𝑣𝑠
𝐿𝑠𝑑(�̃�𝑠+𝐼𝑠)
𝑑𝑡= 𝑞(�̃�𝐶1 + 𝑉𝐶1) − 𝑣𝑠
(4)
Eq. (4) can be simplified as
{
𝐶1𝑉𝐶1
𝑑(�̃�𝐶1)
𝑑𝑡
=(�̃�𝐶1+𝑉𝐶1)𝑣𝐶2
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘(�̃� + 𝐷 − 𝐷2 − 2𝐷�̃�) − (𝑖�̃� + 𝐼𝑠)𝑣𝑠
𝐿𝑠𝑑(�̃�𝑠)
𝑑𝑡= 𝑞�̃�𝐶1 + �̃�𝑉𝐶1 + 𝑞𝑉𝐶1 − 𝑣𝑠
(5)
Since the steady state has
{0 =
𝑣𝐶1𝑣𝐶2
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘𝐷(1 − 𝐷) − 𝑖𝑠𝑣𝑠
0 = 𝑣𝑠 − 𝑞𝑣𝐶1 (6)
Therefore,
{𝐶1𝑉𝐶1
𝑑�̃�𝐶1
𝑑𝑡=
𝑉𝐶1𝑣𝐶2(1−2𝐷)
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘�̃� +
𝑣𝐶2𝐷(1−𝐷)
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘�̃�𝐶1 − 𝑖̃𝑠𝑣𝑠
𝐿𝑠𝑑�̃�𝑠
𝑑𝑡= 𝑞�̃�𝐶1 + �̃�𝑉𝐶1
(7)
Take Laplace transform to (7),
{
𝑠𝐶1𝑉𝐶1�̃�𝐶1(𝑠) =𝑉𝐶1𝑉𝐶2(1−2𝐷)
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘�̃�(𝑠) +
𝑉𝐶2𝐷(1−𝐷)
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘�̃�𝐶1(𝑠) − 𝐼𝑠(𝑠)𝑣𝑠
𝑠𝐿𝑠𝐼𝑠(𝑠) = 𝑞�̃�𝐶1(𝑠) + �̃�(𝑠)𝑉𝐶1
(8)
Therefore,
(𝑠𝐶1𝑉𝐶1 −𝑉𝐶2𝐷(1−𝐷)
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘+
𝑞𝑣𝑠
𝑠𝐿𝑠)𝑉𝐶1(𝑠)
�̃�(𝑠)=
𝑉𝐶1𝑉𝐶2(1−2𝐷)
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘 (9)
(𝑠𝐶1𝑉𝐶1 −𝑉𝐶2𝐷(1−𝐷)
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘+
𝑞𝑣𝑠
𝑠𝐿𝑠)𝑉𝐶1(𝑠)
�̃�(𝑠)=
𝑉𝐶1𝑣𝑠
𝑠𝐿𝑠 (10)
The transfer functions are
𝐻1(𝑠) =𝑉𝐶1(𝑠)
�̃�(𝑠)=
𝑠𝐿𝑠𝑉𝐶1𝑉𝐶2(1−2𝐷)
2𝑠2𝐿𝑠𝐿𝑙𝑒𝑎𝑘𝐶1𝑉𝐶1𝑓𝑠−𝑠𝐿𝑠𝑉𝐶2𝐷(1−𝐷)+2𝑣𝑠2
𝑉𝐶1𝑓𝑠𝐿𝑙𝑒𝑎𝑘
(11)
𝐻2(𝑠) =𝑉𝐶1(𝑠)
�̃�(𝑠)=
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘𝑉𝐶1𝑣𝑠
2𝑠2𝐿𝑠𝐿𝑙𝑒𝑎𝑘𝐶1𝑉𝐶1𝑓𝑠−𝑠𝐿𝑠𝑉𝐶2𝐷(1−𝐷)+2𝑣𝑠2
𝑉𝐶1𝑓𝑠𝐿𝑙𝑒𝑎𝑘
()
HV
Batt
P1
P2
P3
P4
Q1
Q2
Q3
Q4
S1
S2
S3
S4
C1 C2
Vs Ls Lleak
1:1
Lbat
Vbat
is
Fig. 2. The single-phase inverter interfaced DAB converter.
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III. OPERATING POINT ASSESSMENT
We can see from (11) that the transfer function 𝐻1(𝑠) is a function of VC1 and D. D may vary from 0 to 0.5. In V2L applications, VC1 may vary from 170 V to 700 V to adapt to 120-V load and 480-V load. The key parameters of the converter that under analysis in this paper is summarized in Table I.
The bode plot of 120-V load transfer function 𝐻1(𝑠) and 480-V load transfer function 𝐻1(𝑠) are shown in Fig.3. The bode plot of 120-V load transfer function 𝐻2(𝑠) and 480-V load transfer function 𝐻2(𝑠) are shown in Fig.4. The phase shift D is discretized in every 15 degree. We can see from the figures that the transfer function does not deviate from each other too much at two operating points. Therefore, we could rely on a controller with identical parameters to regulate the state variables.
IV. ROBUST DC VOLTAGE CONTROL FOR WIDE RANGE
OPERATION
As seen in (8), the state variable �̃�𝐶1(𝑠) is coupled with
𝐼𝑠(𝑠). In order to decompose the state variables, the controller is
designed to assign a fast loop to control 𝐼𝑠(𝑠) and a slow loop to
control �̃�𝐶1(𝑠) . The control objective of the fast loop is ac voltage is real system since the output of converter should be controlled as a voltage source. The control objective remains to
𝐼𝑠(𝑠) in this paper since the test is conducted under resistive load. The output ac voltage is proportional to output current in
this specific scenario. 𝐼𝑠(𝑠)𝑣𝑠 term in (8) could be deemed as zero since Is has reached to steady state in the fast control loop.
Also, 𝑞�̃�𝐶1(𝑠) term in (8) could be deemed as zero since VC1
changes very slow in the fast loop. Therefore, �̃�𝐶1(𝑠) can be
controlled by �̃�(𝑠) independently and 𝐼𝑠(𝑠) can be controlled
by �̃�(𝑠) independently. Eq. (8) can be simplified as
{𝑠𝐶1𝑉𝐶1�̃�𝐶1(𝑠) =
𝑉𝐶1𝑉𝐶2(1−2𝐷)
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘�̃�(𝑠) +
𝑉𝐶2𝐷(1−𝐷)
2𝑓𝑠𝐿𝑙𝑒𝑎𝑘�̃�𝐶1(𝑠)
𝑠𝐿𝑠𝐼𝑠(𝑠) = �̃�(𝑠)𝑉𝐶1
()
The transfer functions are
𝐻(𝑠) =𝑉𝐶1(𝑠)
�̃�(𝑠)=
𝑉𝐶1𝑉𝐶2(1−2𝐷)
2𝑠𝑓𝑠𝐿𝑙𝑒𝑎𝑘𝐶1𝑉𝐶1−𝑉𝐶2𝐷(1−2𝐷) ()
𝐺(𝑠) =𝐼𝑠(𝑠)
�̃�(𝑠)=
𝑉𝐶1
𝑠𝐿𝑠 ()
The H-bridge Q1-Q4 are used to control Is. The dual active bridge is used to control VC1. The bode plot of 120-V load transfer function 𝐻(𝑠) and 480-V load transfer function 𝐻(𝑠) are shown in Fig.5. The bode plot of 120-V load transfer function 𝐺(𝑠) and 480-V load transfer function 𝐺(𝑠) are shown in Fig.6.
A PI controller is designed for the VC1 regulation and another PI controller is designed for Is regulation. The control strategy is shown in Fig.7. The parameters of PI controller are summarized in Table II. The open-loop bode plots after implementing the PI controls are shown in Fig. 8-9. We can see from the bode plot that the phase of zero cross point is less than 180 degree for all operating points. This guarantees that the general PI controller is suitable for both 120-V load and 480-V load.
V. EXPERIMENT
To verify the effectiveness of the proposed dc voltage control, the scale-down experiments are conducted. The experiment circuit is similar to Fig.1. The ac terminals are
TABLE I. KEY PARAMETERS
Filtering
inductor,
Ls
10
μH
DC
Capacitor,
C1
3300
μF
Battery
voltage,
VC2
350
V
Leakage
inductance,
Lleak
22
μH
Switching
frequency,
fs
100
kHz
Load
voltage, Vs
120,
480
V
Capacitor
voltage,
VC1
170,
700
V
Phase
shift, D
0 ~
0.5
-100
0
100
200
1 10 102 103 104 105
-270
-180
-90
0
90
Bode Diagram
Frequency (rad/s)
Pha
se (
deg)
Mag
nitu
de (
dB)
H1 for 120-V load
H1 for 480-V load
Fig. 3. The transfer function H1 bode plot for 120-V load and 480-V load.
-100
0
100
200
300
-360
-270
-180
-90
0
Bode Diagram
Frequency (rad/s)
Phase
(deg)
Mag
nit
ude (
dB
)
1 10 102 103 104 105
H2 for 480-V loadH2 for 120-V load
Fig. 4. The transfer function H2 bode plot for 120-V load and 480-V load.
TABLE II. PARAMETERS FOR PI CONTROLLERS
HPI = KP + KI/s
GPI = KP + KI/s
KP KI KP KI
0.01 1 0.001 1
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connected to resistive loads in wye-connection with resistant of
12 Ω each phase. The battery voltage is 130 V. The load voltage is set to 80 and 140 V. The PI controller parameters for these two operating points are the same. In order to demonstrate that the PI controller parameters are the same, the experiment sets a hot dc link voltage change from 80 V to 140 V and backward. The key parameters of the experiment setup are summarized in Table III. The control is implemented in TI-DSP TMS320F28379D. The CPU clock is 200 MHz. The sampling frequency is 100 kHz. The equivalent PI controller parameters in continuous time domain is defined by,
𝐾𝑃𝐷𝑆𝑃 = 𝐾𝑃
𝐶𝑜𝑛𝑡. ()
𝐾𝐼𝐷𝑆𝑃 = 𝐾𝐼
𝐶𝑜𝑛𝑡. 𝑓𝑠𝑎𝑚⁄ ()
The PI controller equivalent parameters are the same as Table II.
-50
0
50
100
150
-180
-135
-90
Bode Diagram
Frequency (rad/s)
Pha
se (
deg)
Mag
nitu
de (
dB)
1 10 102 103 1040.1
H for 120-V load
H for 480-V load
Fig. 5. The transfer function H(s) bode plot for 120-V load and 480-V load.
-100
-50
0
50
100
150
-180
-135
-90
Bode Diagram
Frequency (rad/s)
G×GPI for 120-V load
G×GPI for 480-V load
10 102 103 104 105 106
Pha
se (
deg)
Mag
nit
ude (
dB)
Fig. 9. The open-loop bode plots after implementing the PI control for Is
under 120-V load and 480-V load.
PI+
–
VC1*
VC1 Vdc1
ia
ib
ic
+
+
+
ia*
ib*
ic*
PI
–
–
–
ia,b,c
Slow loop
Fast loop
( )H s
Q
D
( )G s
Fig. 7. Complete control strategy.
-100
-50
0
50
100
150
-270
-180
-90
Bode Diagram
Frequency (rad/s)
Phase
(deg)
Mag
nit
ude (
dB)
1 10 102 103 1040.1
H×HPI for 120-V load
H×HPI for 480-V load
Fig. 8. The open-loop bode plots after implementing the PI control for VC1
under 120-V load and 480-V load.
-50
0
50
100
150
-91
-90.5
-90
-89.5
-89
Bode Diagram
Frequency (rad/s)
G for 120-V load
G for 480-V load
102 103 104 105 106 107
Phase
(deg)
Mag
nit
ude (
dB
)
Fig. 6. The transfer function G(s) bode plot for 120-V load and 480-V load.
TABLE III. KEY PARAMETERS OF EXPERIMENT SETUP
Filtering inductor, Ls 10 μH
DC Capacitor, C1 3300 μF
Battery voltage, VC2 130 V
Leakage inductance, Lleak 22 μH
Switching frequency, fs 100 kHz
Load voltage, Vs 50, 90 V
Capacitor voltage, VC1 80, 140 V
Load resistance, Rabc 12 Ω
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The corresponding experiment results are shown in Fig. 10-13. VPrim is the primary side voltage of transformer which is close to C1 bus. VSec is the secondary side voltage of transformer which is close to C2 bus. Ileak is the transformer current which is measured on primary side. Fig.10 shows the dc voltage jumps from 140-volt to 80-volt under the proposed controller. Fig.11 is the zoom-in details of the jump-down dynamics with the leakage inductance current waveform. Fig.12 shows the dc voltage jumps from 80-volt to 140-volt. Fig.13 is the zoom-in details of the jump-up dynamics with the leakage inductance current waveform. The experiments indicate that the controller parameter is validated to both operating points of the dc-link voltage.
VI. CONCLUSION
This paper proposed a robust dc voltage control for inverter interleaved DAB. The small-signal model of the inverter interfaced DAB converter is derived at a wide range of operating point. Based on the small-signal model, a robust dc voltage control is proposed to adapt to the wide range of dc link voltage. The proposed dc voltage control is verified by a SiC based inverter interfaced dual active bridge converter prototype. The experiment results showed that the proposed control could cover a wide range of dc link voltage.
ACKNOWLEDGMENT
The authors would like to thank Ford Motor Company for the technical and fellowship support.
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VC1 Load change
Vab,bc
Ia
Fig. 10. DC voltage jumps from 140-volt to 80-volt under the proposed
controller.
Load changeVC1
VSec
VPrim
Ileak
Fig. 11. The zoom-in details of the jump-down dynamics with the leakage
inductance current waveform.
Load changeVC1
Vab,bc
Ia
Fig. 12. DC voltage jumps from 80-volt to 140-volt under the proposed
controller.
Load changeVC1
VPrim
VSec
Ileak
Fig. 13. The zoom-in details of the jump-up dynamics with the leakage
inductance current waveform.
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