design and hardware implementation of a cost-effective battery energy storage system

www.as‐se.org/ijpres International Journal of Power and Renewable Energy Systems Volume 1, 2014

24

Design and Hardware Implementation of a Cost‐Effective Battery Energy Storage System Chao‐Tsung Ma*1, Chin‐Lung Hsieh2 1Applied Power Electronics Systems Research Group, DEE, CEECS, National United University, #1 Lien‐Da, Kung‐Ching Li, Miao‐Li City, 36003, Taiwan 2Institute of Nuclear Energy Research, Atomic Energy Council, Taiwan *Corresponding author: Chao‐Tsung Ma, Tel: 886‐37‐381369; Fax: 886‐37‐327887

E‐mail: [email protected]

Received 9 April 2014; Revised 24 May 2014; Accepted 27 May 2014; Published 11 June 2014 © 2014 American Society of Science and Engineering Abstract

This paper describes a design concept concerning the smart utilization of the power conversion system (PCS) in a battery energy storage system (BESS) widely used in advanced micro‐grid (MG) operations. In the proposed control scheme, besides the charging and discharging control functions, the BESS can be operated as an active power filter (APF) for harmonic‐current compensation or as a static synchronous compensator (STATCOM) for voltage regulation in MG systems. Some grid‐scale BESS designed with unity power factor method are mainly for balancing the dynamic real power flow in critical MG operations via charging and discharging their battery banks. Under this application scenario, the average utilization rate of the BESS entire asset is normally very low. To eliminate this shortcoming, the proposed control scheme aiming at optimally using the BESS hardware system for multiple control functions is termed flexible battery energy storage system (FBESS). The FBESS attempts to utilize the available inverter capacity for accomplishing the additional APF and STATCOM functionalities via digital control techniques. In this paper, the mathematical model of FBESS and its related power flow controllers are firstly developed and a set of simulation studies on a simple MG network are then carried out in Matlab/Simulink software environment. Typical measured results on a dSPACE1104 based hardware system are presented with brief discussions to demonstrate the feasibility of the proposed control scheme.

Keywords

Flexible Battery Energy Storage System; Energy Conversion System; Renewable Energy; Micro‐Grid

Introduction

Renewable energy based distributed generations (REBDG) have been recognized to play an important role for the achievement of some energy policy goals, such as reduction in high‐polluting power generations and global greenhouse gas emissions and improved diversity and security of energy supply. In recent years, the penetration level of REBDG has been rapidly increased [1]. Of the known REBDG systems, photovoltaic (PV) devices, wind turbine generators (WTG) and fuel cells (FC) systems are the most popular ones [2]‐[4]. Based on the EPRI reports concerning research projects on the distributed generations (DG) and micro‐grids (MG), the development of high‐efficiency, cost‐effective devices and the design of intelligent control algorithms for advanced operations of MG systems are receiving a lot of attention. A basic MG system normally comprised of several REBDG units [5]‐[6] working with a wide variety of renewable energy sources, local loads, energy storage systems and various advanced controllers to be operated in grid‐connected or islanding operating modes. It has been well accepted that DG units and MG systems equipped with fast‐response compensators are important requirements to maintain the balance between the area load and generated power, and to guarantee the stability and quality of electrical power supply at an acceptable level. It is important to note that using DG and the concept of MG has a number of attractive advantages in many aspects; however, the real power output from most of micro sources are changing from time to time and thus cannot be precisely scheduled. In other words, from a system point of view the MG with renewable energy resources is not a completely controllable power source. To ensure a secure system operation and a high‐profile power quality, suitable energy storage systems (ESS) with some advanced system control schemes and better coordination and communication systems will be required [7]‐[8]. Conventionally, to minimize the adverse effects of the MG on the distribution system (DS) when there is a severe fault initiated on the MG side or a voltage dip/swell event a static switch (SS) is commonly used to disconnect the MG from the DS. This follows that some immediate control actions must be initiated and completed as desired to guarantee a successful seamless transferring process and an acceptable stability level of the system. To make the most benefits from applying the above mentioned concepts of using ESS and REBDG in MG, it is important and a vital need to develop feasible control schemes for better operating the MG

International Journal of Power and Renewable Energy Systems Volume 1, 2014 www.as‐se.org/ijpres

25

both in grid‐connected and islanding operating modes.

Recently, the fast development in energy technologies, power electronics and digital control technologies is providing better flexibility of using the above mentioned ESS in the ever‐changing power systems to not only increase electric energy production but also help to enhance the power system stability via better system control capability. In the open literature, power quality control issues of the DG unit in MG operations have been addressed by a number of researches. In the aspect of power flow control and voltage regulation, some researches on line interactive inverters have been reported [9]‐[11]. Issues regarding the application of high‐efficiency, grid‐level battery energy storage systems (BESS) in MG are investigated in [12]. Of the reported control methods for BESS inverter systems, advanced charging and discharging controllers with high power factor have been investigated in [13]. This paper puts forward a design concept concerning the optimal utilization of the BESS system working in some advanced micro‐grid operations. In the proposed control scheme, besides the regulation of charging and discharging power the BESS inverter can be simultaneously operated as an APF for current harmonics compensation of nonlinear loads and as a STATCOM for reactive power compensation within its rated capacity. Due to the fact that the control functions of APF and STATCOM may be required 24 hours in a day the proposed design has the potential to provide additional financial benefits to the MG owners by allowing full utilization of the expensive asset of the BESS inverter systems. It is important to note that the proposed multifunctional BESS inverter scheme has been practically constructed and tested in the laboratory of applied power electronics research group (APERG), National United University, Taiwan. The following sections of the paper are organized as follows: following Section 1, the Section 2 briefly describes the concept of micro‐grid systems and its operating modes. Section 3 and 4 describe the controller design issues for the proposed FBESS system to be operated as a multifunctional power interface. Section 5 addresses system parameters and signals for controllers, while Section 6 presents the simulation and experimental test cases and typical results of various control functions based on a set of given system operating conditions. Finally, a brief conclusion is given in the last Section.

PV

Wind

With 20 x 12V Battery Bank

BidirectionalInverter

#1

Local ESS

AC-DCConverter

Load #1

~

AC Grid

Load #2

NonlinearLoad

BidirectionalInverter

#2

Local ESS

DC-DCConverter

BidirectionalVSC

InverterFBESS

P1 , Q 1

P2 , Q2

Pn , Qn

P Q

P-QDistribution

P-QPower Decision

Power ManagementController

Operating Condition

SS

Micro-Grid

MGFeeder

HarmonicCurrents

P&Q

FIG. 1. SIMPLE SYSTEM DIAGRAM OF A MICRO‐GRID WITH VARIOUS DGS AND THE PROPOSED FBESS.

Micro-Grid Systems

As electric distribution technology moves into the new era the requirements of energy delivery and management will be very different. These differences are being driven from both the demand side where higher energy efficiency, reliability and availability are desired, and from the utility side where the integration of distributed generation and certain advanced energy management technologies must be accommodated. Distribution systems possessing distributed generations, energy storage systems and controllable loads with the ability to operate in both grid‐connected and standalone modes are an important class of the so‐called MG systems [14]. A conceptual MG system configuration with the proposed FBESS installed is shown in Fig. 1. It is well known that MG strives for optimized operation of the aggregated distribution systems by coordinating the DGs and load resources not only when


26

connected to the main grid but also in a stand‐alone mode. In either modes of operation, advanced local controls, energy management and protection technologies are required for robustness and reliability. While the power management criteria and control objective functions are developed based on the needs of each application, in general the overall objective is to optimize operating performance and cost in the grid‐connected mode, while ensuring the given performance requirements in the stand‐alone mode.

FBESS Operating Principles And Control Methods

The operating principles and control concepts of the proposed FBESS are actually derived from that of shunt‐type flexible ac transmission system (FACTS) devices. In this paper, a basic FBESS configuration which consists of a 3‐phase converter using 6 IGBT devices is chosen to introduce the proposed control scheme and the FBESS operating principles. The 3‐phase voltage source converter (VSC) in the FBESS (Fig. 1) is designed to be operated from a DC link voltage provided by a battery bank. In normal operations, the active power can be controlled in either direction between the AC terminals of the converter and the grid to regulate the DC charging and discharging power of the battery bank. It is clear that with this hardware topology, the converter can also generate or absorb reactive power independently at its AC output terminals to affect system voltages or simply act as an APF. In this study, three control functions are designed for the FBESS, i.e. charging and discharging control, reactive power regulation for the MG and harmonic currents compensation for the local nonlinear load. The three control functions can be activated simultaneously or individually. Because the proposed control requirements (real/reactive power or harmonic currents) are directly related to the control of currents shunt connecting type is a reasonable choice.

Design Issues of FBESS Controllers

Current controlled voltage source converter (VSC) is normally used for interfacing the shunt‐type BESS and distribution networks. In this study, the command signals for the FBESS, which are current signals in nature, will include the information of charging and discharging power and reactive power demanded by the MG or the voltage fluctuation at load‐side and also harmonic currents if required. To allow for performing multifunctional control tasks, current controlled VSC is selected in this study for its fast dynamic response, accurate performance and ease of implementation. The inner‐loop current control techniques of VSC used in this paper is based on analysis of voltage and current vector components in a special d‐q reference frame. To decompose voltage and current components in a rotating reference frame, calculation of instantaneous angle of voltage or current is needed. To obtain this angle, phase‐locked loop (PLL) is commonly used in VSC's control loop [15]. However, employing PLL has some disadvantages such as problems due to synchronization of FBESS with the grid and elimination of a wide range of frequencies which is not favourable in FBESS's applications as an APF. In addition, the PLL is very sensitive to noises and disturbances. To solve this problem, in this paper, the synchronization algorithm uses the instantaneous angle of load voltage calculated directly by decomposing voltage vector components in a stationary reference frame. Removing PLL from control circuit of current controlled inverter also presents a new control method in this application case. In this control strategy, synchronization problems will be resolved and dynamic response of FBESS can be improved.

System Modelling and FBESS Control Systems

System Modelling

To achieve a multifunctional control scheme, mathematical models concerning the voltage and current components of the FBESS inverter system are firstly derived in stationary reference frame. Based on the detailed FBESS inverter circuit shown in Fig. 2, the relationships among voltage and current parameters can be expressed as follows:

dC

N

dcV

A

B

C

1S

2S

3S

4S

5S

6S

L oaI

n

a

b

c

R

ocI

obI anV

cnV

bnV

FBESS

dC

N

dcV

A

B

C

1S

2S

3S

4S

5S

6S

L oaI

n

a

b

c

R

ocI

obI anV

cnV

bnV

dC

N

dcV

A

B

C

1S

2S

3S

4S

5S

6S

L oaI

n

a

b

c

R

ocI

obI anV

cnV

bnV

FBESS

FIG. 2. FBESS INVERTER HARDWARE SYSTEM AND PARAMETERS.


27

1 11

2 22 1 1

13 2 2

1 11

2 2

( )

Oa

AN an

ObBN bn R

CN cnOc

dIL

dt V VdI

L V V Vdt

V VdI

Ldt

(1)

where,

oa

R ob

oc

RI

V RI

RI

(2)

Using the defined switching functions for the three‐phase PWM inverter shown in (3), the complete inverter model can be reached after some mathematical manipulations.

1

12

1

cona

tmAN

dc conbBN

tmCN

conc

tm

V

VV

V VV

VV

V

V

(3)

For the convenience of taking derivative operations on the above mathematical models, (1) can be rewritten as (4) using (2) and (3).

0 , , , ,pwm con ABC o abc o abc abc n

dK AV L I RI Av

dt (4)

The A in the above equation is the constant matrix in (1) and the small signal model of the FBESS system (4) can be simply expressed as follows.

0 , , , ,pwm con ABC o abc o abc abc n

dK AV L i Ri Av

dt (5)

As addressed previously, the design of FBESS power flow controllers are based on the equivalent quantities in d‐q axis. Thus the (5) can be expressed in the following d‐q frame (6) on Park’s transformation theory.

0

,

,

con d od oqpwm

con q oq od

od d

oq q

V i idK L L

V i idt

i vR

i v

(6)

In a synchronous rotating reference frame, when the d‐q output voltages of the FBESS inverter are in synchronous

with the grid voltages, the q‐axis voltage component becomes zero. This leads to the following results: dv v ,

0qv and (6) becomes the form of (7).

00

,

,

con d od odoqpwm

con q oq oqod

V i ii vdK L L R

V i iidt (7)

Finally, one can obtain the system model as shown in (8) and (9).

1 , [( ) ]odcon d pwm od oq

diV K Ri L Li v

dt (8)

1 , [( ) ]oq

con q pwm oq od

diV K Ri L Li

dt (9)


28

Based on the model derived in (8) and (9), the current commands for the real and reactive power flow control can be directly obtained as follows.

0 ,pwm con d od oq odK V sLI LI RI V (10)

0 ,pwm con q oq od oqK V sLI LI RI (11)

Choosing the odI and oqI as the output variables, (10) and (11) can be expressed as follows.

1 ,( )od pwm con d oqI K V V LI

sL R (12)

1 ,( )oq pwm con q odI K V LI

sL R (13)

To perform the function of load harmonic currents compensation, a simple low‐pass filter can be used to separate the fundamental and harmonic components in load currents and obtain the d‐q axis harmonic components to be

compensated. After all the current components are obtained in d‐q frame, *odI and *

oqI , the current commands (PWM

signals) in three‐phase frame can be achieved by using the inverse Park’s and Clarke’s transformations as follows.

1

01

1 2 3 2

1 2 3

2

32

*

*

*

*

*

/ /

/ /

a

b

c

oq

od

i

i

i

cos sin i

sin cos i

(14)

Based on the above derivation, the overall FBESS control system block diagram with system parameters can be constructed as shown in Fig. 3.

conavconbv concv

conqv condvOqi

OdicbOai ,,

cbLai ,,

Lqi

LdiLqhi

Ldhi

*Oqi

0qi

0di

cbOai ,,

fL fC

cbLai ,,

, ,ab bc cav cbSai ,,

AC Micro Grid

*Odi

*Q

Q

*(charging/discharging)P

(charging/discharging)P

FIG. 3. THE OVERALL FBESS CONTROL SYSTEM BLOCK DIAGRAM.


29

Case Studies and Results

Simulation and Experimental Tests

To simulate a realistic operation scenario in MG network, both of the linear and nonlinear loads may be connected or disconnected to the MG distribution network and the harmonic distortion of currents is varying with various load conditions. In this study, the load harmonics are set to be totally compensated by the FBESS when the APF function is activated. Since the principle of the proposed control technique used in FBESS is based on separating active and reactive current components in rotating synchronous reference frame known as the d‐q components, in all test conditions only phase‐a parameters (voltage and current) of the AC grid are shown. To demonstrate the performance of the proposed FBESS in regulating the reactive power and real power (charging/discharging currents), the related parameters of the power grid (MG) are shown.

To further verify the performance of the proposed control scheme, the FBESS system is experimentally tested as configured in Fig.4. In the hardware setup, an industrial level digital controller (dSPACE1104) based 1kW three‐phase grid‐connected FBESS is constructed. Test conditions and parameters are the same as that used in simulation cases. All the required controllers proposed in this paper are implemented with dSPACE1104. The sensed currents and voltages acquired to the dSPACE1104 and the control signals output to the driving circuit are using home‐made signal acquisition circuits. Both of the sampling frequency and the switching frequency are set at 24 kHz. The following subsections present the results of simulation and experimental studies on the FBESS.

The related system data concerning the FBESS and the arrangement of local load are given below.

System capacity of the FBESS: 1 kVA

Grid voltage: 110 V/60 Hz

Inverter parameters: switching frequency: 24 kHz, output filter: 6 mH/ 4.7uF, dcV =240 V, C= 1000 uF.

P‐Q controllers: PI controllers, Kp: 0.005, Ki: 79.14 designed with dSPACE1104 control desk functions.

Load: A set of nonlinear load, implemented by a three‐phase AC/DC rectifier with an output power of 500 W.

Control functions: In this paper, constant charging/discharging control, reactive power tracking and APF functions are discussed.

PC &dSPACE 1104

FBESS3L-6S

INVERTER

BatteryBank

Scope

FIG.4. THE EXPERIMENTAL SETUP OF DSPACE CONTROLLED FBESS.

Results

In simulation cases, the FBESS link is connected to the power network at t=0.0 sec. At this moment a full‐wave AC/DC converter with an output of 500 W is added to PCC and it follows that the charging (500 W) and discharging (‐500 W) commands are initiated at the simulation times of t=0.35 sec. and t=0.55 sec. respectively. To test the feasibility of the FBESS in performing multiple control functions, a positive reactive power of 500 VAr and ‐500 VAr commands are initiated at the simulation times of t=0.4 sec. and t=0.6 sec. respectively. Fig.5 shows the tracking results of both the charging/discharging and the reactive power. As can be seen in Fig. 5, there are nine


30

time‐intervals in the complete simulation cycle. The corresponding parameters of the battery bank are shown in Fig. 6. Fig. 7 shows a set of detailed voltage and current waveforms of the test system including the results when the APF function is ON and OFF. Fig. 8 shows the measured results with the same system and control arrangement as that presented in Fig. 5, while Fig. 9 and Fig. 10 respectively show the measured results with the same operating conditions presented in the simulation cases and the results of Fig. 6 and Fig. 7. It can be seen from the results presented in this paper that after the connection of FBESS link and with the multiple control function being activated, besides the charging/discharging of the battery bank both the reactive power tracking and APF functions can be carried out simultaneously if desired. The source current becomes sinusoidal after the harmonic currents are fully compensated by the FBESS link, as shown in Fig.9 and Fig. 10.

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8

-60

-40

-20

0

20

40

60

t(s):0.05S/div

* &Q Q

* &P P

P,Q

: 200 / : 200 /P W div Q V A R div

(1) (2) (3) (4) (5)

(6) (7) (8) (9)

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8

-60

-40

-20

0

20

40

60

t(s):0.05S/div

* &Q Q

* &P P

P,Q

: 200 / : 200 /P W div Q V A R div

(1) (2) (3) (4) (5)

(6) (7) (8) (9)

FIG. 5. THE TRACKING RESULTS OF THE CHARGING/DISCHARGING AND THE REACTIVE POWER.

dcI

dcV

V,I

0 2dc dcV : 6 V / div I : A / div

t(s):0.05S/div

(1) (2) (3) (4) (5)

(6) (7) (8) (9)

FIG. 6. THE CORRESPONDING DC VOLTAGE AND CURRENT OF THE BATTERY.

0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4

-10

-5

0

5

10 sisv

:100 / :5 /s sv V div i A div

(1) (2)

t(s):10ms/div

v,i

0.4 0.41 0.42 0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.5

-10

-5

0

5

10 sisv

:100 / :5 /s sv V div i A div

(3) (4)

v,i

t(s):10ms/div (a) At the transition from (1) to (2) with APF:OFF. (b) At the transition from (3) to (4) with APF:OFF.


31

0.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58 0.59 0.6

-10

-5

0

5

10 sisv

:100 / : 5 /s sv V div i A div

(5) (6)v,

i

t(s):10ms/div0.55 0.56 0.57 0.58 0.59 0.6 0.61 0.62 0.63 0.64 0.65

-10

-5

0

5

10 sisv


(7) (8)

v,i

t(s):10ms/div (c) At the transition from (5) to (6) with APF:OFF. (d) At the transition from (7) to (8) with APF:OFF.

0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4

-10

-5

0

5

10 sisv


(1) (2)

v,i

t(s):10ms/div0.4 0.41 0.42 0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.5

-10

-5

0

5

10 sisv


(3) (4)

v,i

t(s):10ms/div (e) At the transition from (1) to (2).with APF:ON. (f) At the transition from (3) to (4).with APF:ON.

0.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58 0.59 0.6

-10

-5

0

5

10 sisv


(5) (6)

v,i

t(s):10ms/div0.6 0.61 0.62 0.63 0.64 0.65 0.66 0.67 0.68 0.69 0.7

-10

-5

0

5

10 sisv


(7) (8)

v,i

t(s):10ms/div (g) At the transition from (5) to (6).with APF:ON. (h) At the transition from (7) to (8).with APF:ON.

0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4

-10

-5

0

5

10 loadisv

:100 / : 5 /s loadv V div i A div

v,i

t(s):10ms/div (i) The voltage and current waveforms of the nonlinear load.

FIG. 7. A SET OF DETAILED VOLTAGE AND CURRENT WAVEFORMS (PHASE‐A) OF THE POWER GRID INCLUDING THE RESULTS WHEN THE APF FUNCTION IS ON AND OFF.


32

t : 1s / div P : 250W / div Q : 250VAR / div

* &Q Q

* &P P

(1) (2) (3) (4) (5)

(6) (7) (8) (9)

t : 1s / div P : 250W / div Q : 250VAR / div

* &Q Q

* &P P

(1) (2) (3) (4) (5)

(6) (7) (8) (9)

FIG. 8. THE MEASURED TRACKING RESULTS OF BOTH THE CHARGING/DISCHARGING AND THE REACTIVE POWER.

dcV

dcI

dc dct : 1s / div V : 90V / div I : 2A / div

(1) (2) (3) (4) (5)

(6) (7) (8) (9)

FIG. 9. THE MEASURED DC VOLTAGE AND CURRENT OF THE BATTERY.

(1) (2) sisv

s st : 10ms / div v : 90V / div i : 2.4A / div

(3) (4) sisv


(a) At the transition from (1) to (2) with APF:OFF. (b) At the transition from (3) to (4) with APF:OFF.

(5) (6) sisv


(7) (8) sisv


(c) At the transition from (5) to (6) with APF:OFF. (d) At the transition from (7) to (8) with APF:OFF.


33

(1) (2) sisv


(3) (4) sisv


(e) At the transition from (1) to (2) with APF:ON. (f) At the transition from (3) to (4) with APF:ON.

(5) (6) sisv


(7) (8) sisv


(g) At the transition from (5) to (6) with APF:ON. (h) At the transition from (7) to (8) with APF:ON.

loadisv

s loadt : 10ms / div v : 90V / div i : 2.4A / div

(i) The measured voltage and current waveforms of the nonlinear load.

FIG. 10. A SET OF MEASURED VOLTAGE AND CURRENT WAVEFORMS (PHASE‐A) OF THE POWER GRID INCLUDING THE RESULTS WHEN THE APF FUNCTION IS ON AND OFF.

Conclusions

This paper has demonstrated a cost‐effective BESS in which multiple control functions are integrated into the proposed FBESS via digital control techniques to increase the average system utilization rate without any extra hardware units. Unlike the conventional BESS designed with unity power factor for charging and discharging its battery bank the proposed FBESS designed with P‐Q decoupled control scheme can be utilized as an APF for harmonic currents compensation and a STATCOM for power factor correction of the local load or voltage regulation at the PCC. It is important to note that as there are no filters used in the d‐q control loops of the designed FBESS link, satisfactory dynamic response can be achieved in performing various control functions of FBESS on conventional PI controllers. Based on the simulation studies with a simple MG network carried out in Matlab/Simulink software environment and experimental tests on dSPACE1104 based 1kVA hardware systems, the


34

proposed design concept concerning the feasibility and merits of optimal utilization of the three‐phase inverter interfaced FBESS has been verified.

ACKNOWLEDGMENT

The authors would like to acknowledge the financial support of the Institute of Nuclear Energy Research (INER), Atomic Energy Council, Taiwan R.O.C. for this study through: NL1021020.

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[13] N. Eghtedarpour, and E. Farjah, “Distributed charge/discharge control of energy storages in a renewable‐energy‐based DC

micro‐grid,” IET Renewable Power Generation, Vol. 8, No. 1, pp. 45‐57, 2014.

[14] H. Nikkhajoei, and R.H. Lasseter, “Distributed Generation Interface to the CERTS Microgrid,” IEEE Transactions on Power

Delivery, Vol. 24, no. 3, pp 1598 ‐1608, 2009.

[15] S. K. Chung, “A phase tracking system for three phase utility interface inverters,” IEEE Transactions on Power Electronics,

vol.15, no.3, pp.431‐438, May, 2000.

Chao‐Tsung Ma received his B.S. degree in Electrical Engineering from the National Taiwan University of Science and Technology, Taipei, Taiwan R.O.C. in 1989 and his M.S. and Ph.D. degrees from the department of ECE, the University of Missouri, Columbia, USA in 1992 and the department of EEE, the University of Strathclyde, Glasgow, UK in 2000 respectively. He has become a member of IEEE since 2003. His employment experiences include the R&D engineer in GeoTech Company, Taiwan and many years in teaching technical courses in the fields of power systems and applications of power electronics. He is currently a member in the Editorial Board of 8 International Journals, i.e., IJPSO, IJEEE, IJEE, IJARER, AJEEE, IJTEPE, AIE and Frontiers in


35

Energy Research. He is also the head of Applied Power Electronics Systems Research Group (APESRG) in the department of Electrical Engineering, CEECS, National United University, Miao‐Li, Taiwan. His special research fields of interest include applied power electronics, custom power, distributed generations, micro‐grids, power quality, power system control and flexible ac transmission systems (FACTS).

Chin‐Lung Hsieh is currently with the Institute of Nuclear Energy Research, Atomic Energy Council, Taiwan R.O.C. His special research fields of interest include the design of grid‐scale battery energy storage systems (BESS), vanadium redox flow battery (VRFB), application of hybrid ESS in distributed generations and micro‐grids.

design and hardware implementation of a cost-effective battery energy storage system

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