This paper proposes a new gridtied power converter for battery energy storage, which is composed of a 2stage DCDC converter and a PWM inverter. The 2stage DCDC converter is composed of an LLC resonant converter connected in cascade with a 2quadrant hybridswitching chopper. The LLC resonant converter operates in constant duty ratio, while the 2quadrant hybridswitching chopper operates in variable duty ratio for voltage regulation. The operation of proposed system was verified through computer simulations. Based on computer simulations, a hardware prototype was built and tested to confirm the technical feasibility of proposed system. The proposed system could have relatively higher efficiency and smaller size than the existing system.
1. Introduction
The output power of renewable energy source, such as a wind power and Photovoltaic power, varies intermittently depending on weather conditions. Also timing discrepancy occurs between the power generation and the power demand. In order to eliminate this weak point, a battery energy storage, which is composed of power converter and battery, is provided rapidly
[1

4]
.
The power converter for battery energy storage regulates the charging and discharging current, and controls the active and reactive current flowing into or from the grid. The power converter for battery energy storage requires high performance, high efficiency, and small size. It also requires electrical isolation between the AC side and battery side for safety purpose
[5

9]
.
The power converter for battery energy storage can be designed with various topologies according to the power rating, performance, efficiency and safety. The simplest topology can be designed with a PWM converter and 3phase transformer, in which the modulation index of PWM converter is controlled according to the voltage variation of battery.
The insertion of DCDC converter between the PWM converter and the battery can offer wider operation range and more flexible control
[10

13]
. However, the system efficiency is relatively lower due to the rise of switching loss. In order to reduce the switching loss, many researchers already proposed replacing the hardswitching DCDC converter with a softswitching one
[13

16]
. However, careful attention should be paid to the resonant problem and harmonics generation due to adding the softswitching circuit.
This paper proposes a new gridtied power converter for the battery energy storage, which is composed of a 2stage DCDC converter and a PWM inverter. The 2stage DCDC converter consists of an LLC HalfBridge resonant converter and a 2quadrant hybridswitching chopper. Simulation model was developed to analyze the operation of proposed converter. Based on simulation results, a 10kW hardware prototype was built and tested to confirm the hardware implementation.
2. Proposed Power Converter
 2.1. System configuration
The power converter for battery energy storage can be simply designed with a PWM converter with 3phase transformer. This converter has simple structure and high efficiency, but the harmonic level of output current increases when the modulation index of PWM converter is low. So, the operation range of battery voltage is narrow, and the flexibility of current control is small.
In order to solve these weak points, a DCDC converter was inserted between the 3phase PWM converter and the battery as shown in
Fig. 1(a)
. The PWM converter maintains the DC link voltage constant, while the DCDC converter controls the charging and discharging current of battery.
This converter has wide range of current control which offers independent control in the AC current and the charging and discharging current. Also, harmonic reduction is possible. However, the system efficiency is low and the system size is bulky due to inserting one more stage.
In order to eliminate these weak points, many researchers proposed replacing the nonisolated hardswitching converter with an isolated softswitching converter, and replacing the line frequency transformer with the coupling reactors as shown in
Fig. 1(b)
. A typical softswitching converter is an active clamp converter, which has rather higher switching loss
[17]
.
Power Converter for Battery Charger
Fig. 2
shows whole structure of the proposed power converter including the control system for each stage. The PWM inverter controls the active power and the DC link voltage. The 2quadrant hybridswitching DCDC converter operates with variable duty ratio to control the charging and discharging current, while the LLC resonant converter operates with a fixed duty ratio. The proposed DCDC converter offers higher efficiency with soft switching in LLC resonant converter and hybrid switching in 2quadrant chopper
[18]
.
Configuration of Proposed Power Converter
 2.2. LLC Resonant converter
The DCDC converter for charging and discharging the battery requires stable power control, highly efficient power conversion, and reliable power transfer regardless the voltage variation in battery. Also reduction of physical size is a key issue, which can be implemented by the isolated highfrequency transformer. In this paper LLC resonant converter was selected among all the softswitching resonant converters.
The LLC resonant converter transfers power with relatively high efficiency through resonant phenomenon. The LLC resonant converter can operate in ZVS mode. It does not require connecting an additional inductance for resonance. And it has narrow variation in operation frequency regardless the load variation.
Fig. 3
shows the operation mode of LLC resonant converter. The converter operation is divided into four modes according to the time interval.
In mode 1, IGBT switch S
_{aH}
turns on and IGBT switch S
_{aL}
turns off as shown in
Fig. 3(a)
. The primary current flows through the transistor in S
_{aH}
, the magnetizing inductance L
_{p}
, and the DC capacitor C
_{dc}
. And the secondary current flows through the resonant capacitance C
_{r}
, the filter capacitor, the diode in S
_{bL}
, the leakage inductance L
_{r}
and the magnetizing inductance L
_{p}
. The source power in primary side is transferred to the secondary side, and the resonant capacitor C
_{r}
is charged.
In mode 2, both IGBT switches S
_{aH}
and S
_{aL}
turn off as shown in
Fig. 3(b)
. The primary current flows through the DC capacitor C
_{dc}
, the diode in S
_{aL}
, the magnetizing inductance L
_{p}
. The secondary current flows through the resonant capacitor Cr, the diode in S
_{bL}
, the leakage inductance L
_{r}
, and the magnetizing inductance L
_{s}
. No power transfer occurs during this mode. When S
_{aH}
turns off and the secondary current starts to flow through the diode in S
_{aL}
, ZVS occurs in S
_{aL}
. The energy in resonant capacitor Cr is discharged.
Charging Operation in LLC Resonant Converter
In mode 3, IGBT switch S
_{aH}
turns off and IGBT switch S
_{aL}
turns on as shown in
Fig. 3(c)
. The primary current flows through the DC capacitor C
_{dc}
, the magnetizing inductance L
_{p}
, and the transistor in S
_{aL}
. The secondary current flows through the leakage inductance L
_{r}
, the diode in S
_{bH}
, the resonant capacitance C
_{r}
, and the magnetizing inductance L
_{s}
. The source power in primary side is transferred to the secondary side, and the resonant capacitor C
_{r}
is charged.
In mode 4, both IGBT switches S
_{aH}
and S
_{aL}
turn off as shown in
Fig. 3(d)
. The primary current flows through the diode in S
_{aH}
, the DC capacitor C
_{dc}
, and the magnetizing inductance L
_{p}
. The secondary current flows through the leakage inductance L
_{r}
, the diode in switch S
_{bH}
, the resonant capacitance C
_{r}
, and the magnetizing inductance L
_{s}
. No power transfer occurs during this mode. As the current through S
_{aL}
turns off and the secondary current starts to flow through diode in S
_{bH}
, ZVS occurs in S
_{bH}
. The energy in resonant capacitor C
_{r}
is discharged.
 2.3. Hybrid switching chopper
Considering high efficiency and simple control, the LLC resonant converter operates with constant duty ratio and the 2quadrant hybridswitching chopper operates with variable duty ratio. Generally, IGBT has good performance in turnon and conduction period, while FET has good performance in turnoff period. The IGBT tail current during the turnoff period can be removed by hybrid switching, which offers reduction of the switching loss.
Fig. 4
shows the gating pulses for hybrid switching in the 2quadrant chopper. The shuntconnected FET removes the IGBT tail current during turnoff period. Since the FET is only involved in turnoff period, the duration of gating pulse is about 2μs and the instant current rating is about 3 or 4 times normal rating. Also, the switching loss can be reduced about 2%.
Gate pulse for Hybridswitching
In case of charging, the IGBT switch for charging turns on while the FET for charging remains offstate in
Fig. 4(a)
. The IGBT switch for charging turns off 2μs ahead the original turnoff instance. The FET turns on right before the turnoff instance of IGBT in
Fig. 4(b)
. In case of discharging, the IGBT switch for discharging turns on, while the FET for discharging remains offstate in
Fig. 4(c)
. The IGBT for discharging turns off 2μs ahead the original turnoff instance. The FET turns on right before the turnoff instance of IGBT in
Fig. 4(d)
.
Hybridswitching in 2Quadrant Chopper
 2.4. Highfrequency transformer design
The design of highfrequency transformer is very important to reduce the switching loss and to implement the softswitching. A sectiontype highfrequency transformer was chosen to utilize the leakage and magnetizing inductance. So, additional inductor connected in the secondary side can be removed, which is very critical to determine the size of DCDC converter.
In the LLC resonant converter, ZVS operation is closely related to the collectoremitter capacitance
C_{ce}
of primaryside IGBT switch, the diode junction capacitance
C_{j}
of secondaryside IGBT switch, and magnetizing inductance
L_{m}
. Therefore, it is critical for improving the performance and efficiency of LLC resonant converter to determine the magnetizing inductance properly. Since the IGBT collectoremitter capacitance
C_{ce}
becomes larger in the low linevoltage, the magnetizing inductance
L_{m}
should have small value for ZVS operation in whole range of input voltage. However, too small value of
L_{m}
brings about large conduction loss. So, in order to minimize the total loss, the value of
L_{m}
has to be determined by trading off the switching loss with the conduction loss.
The IGBT collectoremitter capacitance
C_{ce}
varies according to the magnitude of collectoremitter blocking voltage
V_{ce}
. The relationship can be expressed by the following equation.
Where,
C_{o}
is the value of
C_{ce}
at the experimental voltage
V_{o}
.
Also, the diode junction capacitance
C_{j}
has identical characteristic. According to Eq. (1), the collectoremitter capacitance
C_{ce}
increases sharply as the drainsource voltage
V_{ce}
, that is line voltage decreases.
Fig. 6
shows an equivalent circuit to determine the value of magnetizing inductance
L_{m}
for minimizing the loss. Where, the
C_{r}
and
C_{dc}
are much larger than the value of
C_{ce}
and
C_{j}
.
Equivalent Circuit for Resonant Converter
The switching loss of LLC resonant converter at
n
th switching period can be obtained by the following equation.
Where,
k_{r}
is the total number of switching during a halfperiod of line voltage.
If the starting voltage of ZVS is same as the input voltage
V_{in}
and its phase angle is
θ
, the number of switching from zero to
θ
is expressed by the following equation.
So, the switching loss during a half period of line voltage can be represented by the following equation.
If the switch turns off at the maximum value of primaryside magnetizing inductance and the switching period is defined by
t_{fr}
, the turnoff loss can be defined by the following equation.
The conduction loss at the
nth
switching period can be represented by the following equation.
Where,
R_{dsr}
is the conduction resistance of FET switch.
The average conduction loss during a halfperiod of line voltage can be represented by the following equation.
During the deadtime
T_{dead}
, the magnetizing current is used for the displacement current in resonant capacitor and the ZVS current of primaryside switch. Therefore, the magnetizing inductance
L_{m}
for ZVS operation under the given input voltage
V_{in}
can be expressed by the following equation.
Where, the resonant frequency
f_{r}
is determined same as the switching frequency
f_{s}
.
3. Simulation Analysis
The performance of proposed power converter was verified by simulations with PSCAD/EMTDC. In order to analyze the battery operation during charging and discharging, a nonlinear model for was derived based on the charging and discharging characteristic curves. The system parameters of proposed power converter are described in
Table 1
and the circuit parameters for DCDC converter are described in
Table 2
.
System Parameters of Proposed Charger
System Parameters of Proposed Charger
Circuit Parameters of Proposed Charger
Circuit Parameters of Proposed Charger
Fig. 7
shows the simulation results to verify the operation of proposed converter.
Fig. 7(a)
shows the phaseA voltage and current, and the DC link voltage during charging mode. The input voltage is in phase with the input current, which means that the power factor correction is properly operated. And the DC link voltage is maintained as a constant value of 700V.
Fig. 7(b)
shows the phaseA voltage and current, and the DC link voltage during discharging mode. The input voltage is 180 degree out phase with the input current, which means that the polarity of input current is negative during discharging. And the DC link voltage is maintained as a constant value of 700V. So, it is clear that the proposed converter can operates properly under accurate charging and discharging control.
Simulation Waveform of Gridtied Inverter
Fig. 8
shows the operation waveforms of LLC resonant converter. It is known that IGBT switch properly operates with a switching frequency of 50kHz. The primary and secondary resonant currents have sinusoidal waveforms without severe noise and ringing due to the highfrequency switching.
Simulation Waveform of LLC Resonant Converter
Fig. 9
shows the operation waveforms of 2quadrant chopper. The DC link voltage shows transient phenomena at the instance of power flow reversal.
Operation Analysis of 2Quadrant Chopper
4. Hardware Experiment
A prototype of 10kW power converter for battery energy storage was built with commercially available components as shown in
Fig. 10
. The prototype was connected to the 3phase 380V line voltage and coupled with the leadacid battery for charging and discharging experiments.
Hardware Prototype with 10kW Rating
Fig. 11
shows the experimental results to verify the operation of proposed converter.
Fig. 11(a)
shows the phaseA voltage and current, the phase angle reference signal, and the DC link voltage in charging mode. The input voltage is in phase with the input current, which means that the power factor correction is properly operated. And the DC link voltage is maintained as a constant value of 700V. It is clear that the PLL (phaselocked loop) operates accurately for converter control.
Fig. 11(b)
shows the phaseA voltage and current, and the DC link voltage in discharging mode. The input voltage is 180 degree out phase with the input current, which means that the polarity of input current is negative during discharging. And the DC link voltage is maintained as a constant value of 700V. So, it is clear that the proposed converter can operates properly under accurate charging and discharging control.
Experimental Waveform of Gridtied Inverter
Fig. 12
shows the operation waveforms of the LLC resonant converter, in which the voltages across the resonant capacitor
C_{r}
in the primary and secondary side are shown, and the resonant current in the primary and secondary side are also shown. It is known that IGBT switch properly operates with a switching frequency of 50 kHz. The resonant currents have sinusoidal waveforms without severe distortion and ringing due to the highfrequency switching.
Experimental Waveform of LLC Resonant
Fig. 13
shows the operation waveforms of hybridswitching in 2quadrant chopper.
Fig. 13(a)
shows the voltage and current waveforms of IGBT and FET, which confirms the hybridswitching behavior. The 2quadrant chopper can operates with the switching frequency of 25 kHz without any problem. Using the hybrid switching scheme the switching loss can be reduced by about 2%.
Experimental Waveform of Hybridswitching
Fig. 14
shows the measured efficiency of proposed power converter with respect to the battery output power in the charging and discharging operation. Generally the efficiency of power converter rises as the output power goes up. In the rated power of 10kW, the efficiency is about 92.5% in discharging and 92% in discharging.
Efficiency Analysis of Proposed DCDC Power Converter
5. Discussion
The ACside harmonics of the proposed power converter is almost same as that of the existing power converter, but the batteryside ripple current is much lower because the DCDC converter operates with high frequency switching.
The size of 3phase 60Hz 10kVA transformer is about 38cm x 17cm x 36cm that is equal to 22356cm
^{3}
, whereas the size of 50kHz 5kVA highfrequency transformer is about 10cm x 10cm x 12cm that is equal to 1200cm
^{3}
. Since two units are connected in shunt to handle 10kVA rating, total size is 2400 cm
^{3}
. The size of each coupling reactor is about 12cm x 12cm x 16cm that is equal to 2304cm
^{3}
. Total size for three units is 6912 cm
^{3}
. Therefore, the net size could be reduced down to 40%.
The efficiency of nonisolated hardswitching DCDC converter is about 90%. Considering the efficiency of linefrequency transformer is about 97%, the overall efficiency of the existing hardswitching power converter is about 87%. This value is about 5% lower than that of the proposed DCDC power converter. Also, the efficiency of proposed softswitching DCDC converter is about 1.5% higher than that of other softswitching converters, because the first stage operates in fixed duty and the second stage operates in hybridswitching scheme.
6. Conclusion
In this paper a new gridtied power converter for battery energy storage was proposed, which consists of a PWM inverter and a 2stage DCDC converter. The 2stage converter is composed of an LLC resonant converter connected in cascade with a 2quadrant hybridswitching chopper. The LLC resonant converter operates in constant duty ratio, while the 2quadrant hybridswitching chopper operates in variable duty ratio for voltage regulation.
The operation of proposed system was verified through computer simulations. Based on computer simulations, a hardware prototype was built and tested to confirm the technical feasibility of proposed system. The proposed system could have relatively higher efficiency and smaller size than the existing system.
Acknowledgements
This work (Grant No. 00045468) was supported by Business for Cooperative R&D between Industry, Academy, and Research Institute funded Korea Small and Medium Business Administration in 2012 and KEPCO(Korea Electric Power Corporation).
BIO
DoHyun Kim He received his B.S. degree in Electrical Engineering from Myongji University. He is currently pursuing his Ph.D. degree at Myongji University. His current research interests include Bidirectional Intelligent Semiconductor Transformer for Smart Grid.
YoonSeok Lee He received his B.S. degree in Electrical Engineering from Myongji University. He is currently pursuing his M.S. degree at Myongji University. His research interests include power electronics applications for distributed generation and microgrids.
ByungMoon Han He received his B.S. degree in Electrical Engineering from Seoul National University, Seoul, Korea, in 1976, and his M.S. and Ph.D. degrees from Arizona State University, USA, in 1988 and 1992, respectively. He was with the Westinghouse Electric Corporation as a Senior Research Engineer in the Science and Technology Center, Pittsburg, PA, USA. He is currently a Professor in the Department of Electrical Engineering, Myongji University, Seoul, Korea. His current research interests include power electronics applications for FACTS, custom power, distributed generation, and microgrid.
JuYong Kim He received his B.S, M.S and Ph.D. degrees in Electrical Engineering from Kyungbook University. He is currently Principal Researcher in KEPCO.
WooGyu Chae He received his B.S. degrees in Electrical Engineering from Sungkyunkwan University and M.S. degrees in Electrical Engineering from Chungbook University. He is currently Senior Researcher in KEPCO.
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