on a dc microgrid with supercapacitor-battery energy...

中華民國第三十一屆電力工程研討會台灣台南 2010 年 12 月 3-4 日

具超電容/蓄電池儲能系統之直流微電網

On a DC Microgrid with Supercapacitor/Battery Energy Storage System 陳建豪張淵智呂臨佑廖聰明

C. H. Chen Y. C. Chang L. Y. Lu C. M. Liaw 清華大學電機工程系

Department of Electrical Engineering National Tsing Hua University, Hsinchu, Taiwan, ROC

Tel: +886-3-5162188; Fax: +886-3-5715971 E-mail:[email protected]

摘要

本文旨在建構一含超電容/鋰離子蓄電池儲能系統之直流微電網。首先，主電源經單象限直流/直流升壓型轉換器建立400V 之共同直流匯流排。超電容組與電池組分別經由雙向直流/直流轉換器介接至共同直流匯流排以從事充放電操作。所有直流/直流轉換器皆採行簡易強健控制以得到良好調節及強健之直流輸出電壓。最後，建構後級單相三線式負載變頻器，由 400V直流匯流排電壓轉換控制輸出 220V/110V之單相交流電壓，於系統參數變動及未知非線性負載下具有良好之220V/110V 輸出弦波電壓波形。所建微電網系統之操作控制將適當安排，並以一些實測結果驗證其整體系統操控性能。

關鍵詞：微電網、介面轉換器、儲能系統、蓄電池、超級電容、變頻器、單相三線、強健控制、數位控制、開關式磁阻發電機。

Abstract This paper presents the establishment of a DC micro-grid with

energy storage system consisting of super-capacitor and Li-ion battery banks. First, the 400V common DC grid is established by a DC/DC boost converter from the main source. Then a super-capacitor bank and a battery bank are respectively interfaced to the common DC grid via its bi-directional DC/DC converter for making discharging and charging works. The simple robust control is applied to yield well-regulated and robust DC output voltages of all DC-DC converters. Finally, a followed 1P3W inverter is developed to yield 220V/110V AC outputs from the 400V DC grid. The simple robust control approach is proposed to yield good inverter sinusoidal 220V/110V output voltage waveforms under varied system parameters and unknown nonlinear loads. The system operation control of the developed micro-grid system is properly arranged. And the whole system operating performance is assessed experimentally. Keywords: Micro-grid, interface converter, energy storage system, battery, super-capacitor, inverter, single-phase three-wire, robust control, digital control, switched-reluctance generator.

I. Introduction Owing to having many merits in energy utilization and

reduction of emissions, micro-grid [1-4] has been gradually received interests in academic researches and practical applications. Compared to AC micro-grid, DC micro-grid can allow longer grid length, and the constituted generation and energy storage devices can easily be connected to the common DC grid via AC/DC or DC/DC converters [4]. The commonly available sources [5,6] for micro-grid may include wind generator, micro-turbine driven generator, photovoltaic cell and fuel-cell, etc.

Till now, there were some bi-directional DC-DC converters [7-11] being applied in motor drives, electrical vehicles (EVs), renewable power system, etc. In [7], a bi-directional DC-DC converter is used to perform the voltage control as well as the DC-link regenerative braking operation for EVs. In [8], two bi-directional buck-boost

converter topologies are studied and compared. Each of them allows stepping up or down the battery voltage level according to motor drive operation modes. In the developed DC micro-grid, the half-bridge bi-directional DC-DC converter is adopted to form the interface converters for battery and super-capacitor storage devices.

Among the commonly used energy storage devices, battery and supercapacitor are the most popularly employed ones for medium and small scale systems [12-16]. The super-capacitor belongs to power type storage source with short duration and fast dynamic response, whereas the battery is long time storage with sluggish response. Recently, many works [14-16] have been done to develop battery-supercapacitor hybrid source for vehicles, PV systems, emergency applications, power distribution systems and uninterruptible power supply.

Inverter [17-26] is an essential equipment to provide variable-voltage and/or variable-frequency AC sources for many power electronic plants. In recent years, inverters are extensively developed and applied as interface converters for distributed power system or micro-grid power system. Many types of PWM inverters are specifically designed to convert photovoltaic cell, solid oxide fuel cell or battery DC source into AC voltage [17-20]. For domestic appliances, the single-phase three-wire (1P3W) inverter is desirable to generate the commonly used 220V/110V AC sources. In the established micro-grid, a 1P3W inverter [20] is established and used as a test load of the micro-grid.

The output characteristics of an inverter are affected by the adopted switching methods. For the PWM switching schemes, the carrier-based PWM is the simplest one [17-19,21,23]. To meet some specific applications, many modified SPWM schemes are developed [17-19]. Basically, modified SPWM methods can yield the specially specified harmonic spectrum, and also the slightly increased DC-link voltage utilization. To meet more strict performance requirements for some plants, the current-controlled PWM (CCPWM) scheme must be adopted. In [23] and [24], a robust ramp comparison and a robust hysteresis CCPWM schemes have been presented. In addition to the PWM switching scheme, the dynamic control is also important to yield good inverter output dynamic response [17-26].

The inverter output is provided by DC-link voltage via proper PWM switching method, hence its output waveform is highly affected by the DC-link voltage characteristics. In [27], suitable harmonics are injected into the PWM switching scheme to compensate the inverter output low frequency harmonics. To yield better voltage waveform for powering the load, it is generally required to add a suitably designed output filter [28-30] in the inverter output. In [28], the nonlinear load current characteristics are considered in the design of output filters. As to [29], the output filter state feedback control is made to achieve the inverter output voltage instantaneous value control, and thus to reduce the inverter equivalent output impedance. For avoiding leg

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short circuit, the proper dead-time should be inserted in the switching signals for the upper and lower switches in a leg. This dead-time will lead to the voltage waveform distortion at current zero crossing instants [17]. Some existing dead-time compensation methods can be found in [31].

In this paper, a DC micro-grid with super-capacitor/ battery energy storage devices is established. Its 400V common DC grid is established by a DC/DC boost converter from the main source, the DC power supply as a substitute or an available SRG [32].The proposed energy storage system a super-capacitor bank and a battery bank. Each of these storage devices is interfaced to the common DC-grid via a bi-directional half-bridge DC-DC converter. In addition, a 1P3W load inverter is developed for generating 220V/110V AC sources from the common DC

grid to the loads. The control scheme for each constituted power stage is properly designed [33] to have satisfactory static and dynamic responses.

II. System Configuration Figs. 1(a) and 1(b) shows the developed experimental

micro-grid system and all its control schemes. Its 400V common DC grid voltage is established by a front-end DC-DC boost converter from the possible DC sources In this study, a commercial power supply with 48V output is first used as the main source for the micro-grid. Then an available practical SRG [32] is used to further evaluate the operation characteristics of the established micro-grid. As to the proposed energy storage system, it consists of a 48V

AC sources:1P/3W 220V/110V

SRGconverter

+

-oC )( oV

Battery bankwithwith

DC/DCDC/DCconverter)converter)

DC/DCconverter

+

-dcV

Boost(Buck/boost)

DC/ACconverters

IM

riT ω,)400( V

)48( V

Load

~~Utility

grid

Bilateralinverter

Power flowSW

Grid-connected system

Super capacitor

withwithDC/DCDC/DC

converterconverter

EnergyEnergystoragestoragesystemsystem

SRGSRG

SCBatterybank

)48( V )48( V

dV

1'

2

1

3

42'

3'

4'

1'

2

1

3

42'

3'

4'

Othersources

Other renewable,distributed sources

and interface converters

dC

7Q 9Q

8Q

11Q

10Q 12Q

7D 9D 11D

dcv

V400

dcC

dcP

oL

oC

oC

ABR

Bv

oL

AR

BR

LC

A

B

N

Av

ABv

A

B

oL

8D 10D 12D

1Q

2Q

3Q

4Q

5Q

6Q

1D 3D 5D

2D 4D 6D

L

dCdv

dPLi

scv

LiL

bv

LLi

ABP

AP

BP

oP

(a)

Li

contv

)/( 64 QQ

iε*Lri

)(sGci

ik

sawv

vε*drv

)(sGcv*dcv

*dcv 1

ABvAv

)/( 53 QQ

iε

*Lri

)(sGci*Li

sawv

vε*brv

)(sGcv*bv

scb 'v/'v

*bv 1

bsc v/v

cIY

N

iε

*Lri

)(sGci*Li 1

sawv

)(sWv

vε

*dcrv

)(sGcv*dcv

dcv

*dcv 1)Q( 2

L'i

L'i

L'i

Li

*Li

*Li

*Li

contv

ABv′

vdε

*ABdv

*ABv 1*

ABv )(sGcv ( )127 Q,Q( )118 Q,Q

)(sGcvcontv

( )9Qvcε

*Adv

*Bv 1

ABv′

AB v*v ′=

vk

ik

vk vk

( )10QB'v

vk

dc'v

dc'v

)(sWv )(sWi

charging) floatingfor 48 52( Vv,Vv scb ==

V52≤

)(sWi)(sWv

C

*Li 1

bv V48≤scv,)(sWi

)(sWvd

)(sWvc

contv

contv

(b)

Fig. 1. The developed experimental micro-grid system: (a) system configuration and schematic; (b) control schemes.

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super-capacitor module and a 48V Li-ion battery module. The bi-directional half-bridge DC-DC converters with buck/boost functions are used as their interface converters to the 400V DC-bus.

A followed 1P3W load inverter [33] is constructed to supply 220V/110V AC sources for domestic appliances. Good voltage waveforms are obtained by applying the proposed simple robust control method.

For compactness, all power stages are constructed using two three-phase IGBT power modules CM100RL-12NF manufactured by Mitsubishi Company. And digital controls of all the constituted power stages in the developed micro-grid system are realized in a common DSP.

In system operation, as the main source failure occurs ( V375≤dcv is detected), the super-capacitor bank is arranged to discharge first until V40≤scv is detected. Then the battery bank discharges for long duration until the condition of V.42≤bv Both the super-capacitor bank and the battery bank can be charged from the DC grid. The discharging and charging discrimination control scheme is shown in Fig. 2. A common digital control environment by DSP TI TMS320F28335 is used to realize all digital control algorithms of this system.

Vvdc 375≤

Y

N

Y

Ndcv′)( 2Q

)( 4Q

Vvdc 400=Y

Y

Vvsc 40≤N

N

Vvb 42≤

N

Y

)( 6Q

)( 3Q

)( 5Q

dcv′

dcv

scv+ −

bv+ −

dv+

−

Vvsc 40≤

1S

2S

Fig. 2. Discharging and charging discrimination control scheme.

III. Constituted System Components

3.1 Main Source Boost DC-DC Interface Converter A. Power Circuit

As shown in Fig. 1(a), the non-isolated conventional DC-DC boost converter is adopted for the main source interface converter. One transistor and one diode in a leg of three phase IGBT module CM100RL-12NF (Mitsubishi Company, 600V, DC),( A100=CI A(pulsed)200=CMI ) are used for implementing this converter. The other two legs of this module are used to construct the interface converters for the battery bank and the super-capacitor in the proposed energy storage system. The DC-DC boost converter is operated under CCM, and its power circuit components are designed step by step as follows: (a) Rated output: dP = η/dcP = 1kW/0.9 = 1.11kW, where

9.0=η is defined as the conversion efficiency from input dP to output dcP .

(b) Duty ratio 88.0=D . (c) Average rated converter output current /1000=dcI

A,78.2)4009.0( =× average inductor current =LaI A,15.23)1(/ =−DIdc and average switch current A37.20== LaSa DII .

(d) Letting the inductor current ripple A86.36/ =≤Δ LaL Ii under the switching frequency of ,Hz30kfs = the energy storage inductor L can be found to be

H97.303 μ≥L . The inductor is wound using AWG#10 with 43 turns on a toroidal core T400-26D (Micrometals). The measured inductances of the wound inductor using HIOKI 3532-50 LCR meter are

Hz,60/H)874.24( μ .Hz30/H)03.467( kμ (e) Selecting the output filtering capacitor to be =dcC

VDC,450/F2200μ the output voltage ripple is:

V033.01030102200160

40088.036 =××××

×==Δ

−sdcdedc

dc fCRDVv

where Ω=== 1601000/400/ 22 dcdcde PVR denotes the equivalent DC-link load resistance. B. Control Schemes

The system control block of the boost DC-DC converter is shown in Fig. 1(b). It consists of outer voltage loop and inner current loop. The current loop is basically a ramp-comparison CCPWM (RC-CCPWM) scheme. The inductor current Li can closely track its command *Li generated from voltage loop.

The designed controllers are listed below [33]: (a) Current controller:

ssKKsG IiPici

0.2333.0)( +=+= , ss

WsWi

ii 3100318.01

5.01

)(−×+

=+

=τ

(b) Voltage controller

s..

sKKsG Ivpvcv 92870)( +=+=

,s.

.s

WsWv

vv 310079501

6501

)(−×+

=+

=τ

The voltage PI feedback controller is quantitatively designed at the given nominal point ( V400=dcV ,

Ω= 200dcR ) to obtain the following voltage regulation control requirements: maximum dip dmVΔ = 12V and ret = 800ms (where dmredc Vtv Δ=Δ 1.0)( ) due to a step load power change of W200=Δ dcP .

3.2 Bi-directional Half-bridge DC-DC Converter The employed 48V BMOD0083-P048 supercapacitor

bank is manufactured by Maxwell Company, America. And the 48V/15Ah lithium-ion battery bank manufactured by LIFEBATT Company is used as longer duration energy storage device. Both of these two devices are connected to the common DC grid of the developed micro-grid via the similar bi-directional half-bridge DC-DC converter as shown in Fig. 3(a). The equivalent circuits in discharging and charging modes of the bi-directional half-bridge DC-DC converter are respectively depicted in Fig. 3(b) and Fig. 3(c), and its operation modes are listed as below: (i) Discharging mode (from storage device to common

DC grid): converter. DC-DCboost construst : , 12 DQ off.y permanentl :1Q

(ii) Charging mode (from common DC grid to storage device):

converter. DC-DCbuck construst : , 21 DQ off.y permanentl :2Q

The common DC grid voltage and battery voltage in Fig. 3 respectively are V400=dcV and V48=bV . The boost converter has been designed previously. It can be used in discharging mode of bi-directional half-bridge DC-DC converter as shown in Fig. 3(b). Hence, only the buck converter during charging mode needs to be designed for the bi-directional half-bridge converter, and it is designed as follows:

1327


bv dC

dcvL dcC2Q

1Q

1D

2D

bv dC

dcvL

LidcC

2Q

1Q

1D

2D

dci

bv dC

dcvL

LidcC

2Q

1Q

1D

2D

dci

bP

dcP dcP

bP

P

N

B U V W

1Q

D

S

3Q 5Q

2Q 4Q 6Q

1D 3D 5D

2D 4D 6D

dcR

bR

Fig. 3. Bi-directional half-bridge DC-DC interface converter for supercapacitor and battery banks: (a) power circuit; (b) equivalent circuit in discharging mode; (c) equivalent circuit in charging mode. A. Charging Mode:

In Fig. 3(c), the devices 1Q , 2D and the inductor L constitute a buck converter to act as charger. It is desired to step down the DC-link voltage V400=dcV to V.48=bV The buck converter is designed to operate under CCM with the following assumptions: (a) Constant current A8=bI is designed to charge the

battery or supercapacitor. (b) Duty ratio ..D 120= (c) Switching frequency Hz.30kfs = ,the average inductor

current =LaI ,8A=bI and the average switch current A96.0== bSa DII . By letting the inductor current ripple A,6.15/ ==Δ LaL Ii the peak inductor current is

=Δ+= LLaLm iIi 5.0 =+ 6.18 A.6.9 Inductor L can be found as: H67.366 μ≥L . The inductor designed in Sec. 3.1 is also used here.

(d) By /F2200μ=dC VDC450 , the output voltage ripple can be found as: V1071.5 3−×=Δ bv .

B. Power Semiconductor Devices The interface converters of the SC and battery banks in

the proposed energy storage system are constructed using the remaining two legs of the same IGBT module CM100RL-12NF, which has been used to construct the boost DC-DC interface converter of the common DC grid. C. Control Schemes and Parameters

The control blocks of the bi-directional half-bridge DC-DC converter in discharging and charging modes for both SC and battery banks are also shown in Fig. 1(b), it consists of outer voltage loop and inner current loop, and the latter is basically a ramp-comparison CCPWM (RC-CCPWM) scheme. For the multi-loop configured control system shown in Fig. 1(b), its inner current loop possesses higher bandwidth than those of outer loop. Thus, the sampling rates should be properly set in performing the digital realization of controllers. In each control loop, the basic PI feedback control is augmented by a robust error cancellation control to enhance the voltage and current command tracking performance.

The discharging and charging operations are arranged as follows:

Discharging: The discharging operations from storage devices to common DC grid are made until the allowed minimum voltages 40=scv V for SC bank and

42=bv V for battery bank reach. Charging: Initially, the constant current charging is

made according the set current. As the condition ( =scv V48 and Vvb 52= ) reaches, the charging is changed to constant voltage floating mode.

The set parameters of all control schemes are:

(i) Boost converter during discharging mode Voltage loop (sampling rate=3kHz), V/V0064580.Kv =

s..sGcv92870)( += ,

s.

.s

WsW

v

vv 3

10079501

6501

)(−

×+=

+=

τ

Current loop (sampling rate = 30kHz), V/A060.Ki =

,023330)(s..sGci +=

s..

sW

sWi

ii 3

1003180150

1)( −

×+=

+=

τ

(ii) Buck converter during charging mode: Voltage loop (sampling rate = 3kHz), V/V050.Kv =

s.sGcv515)( += ,

s..

sWsW

v

vv 31015901

801

)( −×+=

+=

τ

Current loop (sampling rate = 30kHz), V/A060.Ki =

,103)(s

sGci += s.

.s

WsWi

ii 3

10031801

6501

)( −×+

=+

=τ

3.3 Single-phase Three-wire Load Inverter A. System Configuration

The proposed 1P3W inverter is shown in Fig. 1(a), its major system variables and components are listed below: (a) System voltages: DC-link voltage V,400=dcV AC outputs

== BA VV Hz,60/V110 .Hz60/V220=ABV (b) Inverter load: the arranged total test loads shown in Fig. 1(a)

rated to be about 1000W. Linear load: resistor; nonlinear load: rectified and capacitive filtered resistor with VDC400/F4700μ=LC .

(c) Power circuit: it is constructed using the three inverter legs of an IGBT module CM100RL-12NF (Mitsubishi) ( 600V, 200A(peak)100A(DC), , T = 25°C).

(d) Output filter: (i) inductors: they are wound on the toroidal cores T250-26 (Micrometals) using the wire #AWG 12. The measured inductances using HIOKI 3532-50 LCR meter are Hz,60/H86.483 μ=oL Hz;30/H28.478 kμ (ii) capacitor:

VAC.400/F10μ The low-pass cutoff frequency can be found as: Hz301.2)2(/1 kCLf ooc == π .

B. Control Schemes As generally recognized, to yield high performance of an inverter-fed plant, the cascade control structure with inner current loop and outer voltage loop is usually adopted. For simplification, a robust voltage waveform control scheme without current loop as shown in Fig. 1(b) is proposed for the developed inverter system. Its major features are introduced below: (i) Master DM control scheme

The waveform control for ABv generated from the outer two IGBT module legs is handled by the master differential mode (DM) control scheme. (ii) Slave CM control scheme

The balance between the two voltages Av and Bv is regulated via the PWM switching of the center-leg IGBTs by the proposed slave common mode (CM) control scheme indicated in Fig. 1(b). For saving the sensors, the voltage

Bv′ is synthesized from the sensed ABv′ and Av′ ( AABB vvv ′−′=′ ). Then AB vv ′=

* is used as a reference, and it is closely followed by the voltage Bv′ using the developed simple robust control approach.

IV. Experimental Evaluation of Whole Micro-grid Having properly arranged the system operation control

schemes of the developed micro-grid system as shown in Fig. 1 and Fig. 2, the whole system operating performance is assessed experimentally. The test loads of the load 1P3W inverter are arranged in Fig. 1, wherein the nonlinear load is represented by a rectified and capacitive filtered resistor ( AL RC // ). And V48=dV and V400=dcV are set.

Figs. 4(a) and 4(b) show the measured 220V output

1328


with its command ( ,ABv *ABv ), two 110V outputs Av and Bv under the nonlinear )0( ≠LC and unbalanced load case of Ω,57( =AR F,4700μ=LC W),36.416=AP Ω,200( =BR W),53.63=BP Ω,90( =ABR W)61.538=ABP by PI and robust controls ( ,65.0=vdW 7.0=vcW ). The measured steady-state

characteristics are summarized in Table 1. Good quality in the output voltage waveforms (220V and 110V) is possessed by the developed load 1P3W inverter system.

*ABv

ABv

ms5

V100

ms5

V100

V0620%5156V5220 .V.THD.V dc,ABABvrms,AB −===

V5230%7039V05108 .V.THD.V dc,AAvrms,A −===

V6300%71211V12113 .V.THD.V dc,BBvrms,B −===

(a)

(b)

Av

Bv

Fig. 4. Measured inverter output under nonlinear and unbalanced loads by PI and robust controls ( ,65.0=vdW 7.0 =vcW ): (a)

),( *ABAB vv under ( ,90Ω=ABR W);61.538=ABP (b) two 110V outputs Av and Bv under ,57( Ω=AR F,4700μ=LC W)36.416=AP and ( ,200Ω=BR W53.63=BP ).

Table1: Measured steady-state characteristics ),,,( ηodcd PPP of the

established 1P3W inverter powered by front-end DC-DC converter under different load conditions

)(ΩAR

)(ΩBR

)(ΩABR

F)(μLC

(V)dcV

(W)dP

(W)dcP

(W)oP (%)

η

57 100 200 0 401.5 737.52 640.91 583.26 79.08

57 200 90 0 401.1 1017.61 886.52 806.73 79.28

57 100 200 4700 401.2 1002.03 869.76 784.52 78.29

57 200 90 4700 401.0 1296.38 1129.16 1018.5 78.56

※ do P/PΔη , power,input DC=dP .PPPP ABBAo ++== poweroutput total In dynamic behavior, Figs. 5(a) and 5(b) depict the

measured voltage and current of 220V output ( ,ABv ABi ) and DC-link voltage dcv under linear and unbalanced loads of Ω,57( =AR Ω,200=BR Ω150=ABR ) due to a step load resistance change of )Ω90150( →Ω=ABR by PI and robust controls ( ,65.0=vdW 7.0=vcW ). Good regulations in the DC-link voltage and the inverter output voltage are observed from the results.

The micro-grid is stably operated at steady-state with the inverter loads of ( Ω= 57AR , F;0μ=LC ;200Ω=BR

Ω= 90ABR ). When the main DC source failure is detected ( =1S “H”), the SC bank is controlled to support power ( 4Q is operated), the measured results are shown in Figs. 6(a) and 6(b). When the SC bank stops discharging as its voltage is smaller than V,40 the triggering signal ( =2S “H”) is set to let the battery bank be started its discharging to continuously support the load power ( 6Q is operated). The results are shown in Figs. 7(a) and 7(b) at loads of ( Ω= 57AR , F;0μ=LC ;200Ω=BR Ω= 90ABR ). From the experimental results one can be aware that the developed micro-grid can continuously provide the load power of 1P3W inverter as the main DC source failure occurs, and two energy storage devices support the power immediately.

V. SRG-Fed DC Grid and 1P3W Load Inverter For observing the operating features of the established

micro-grid system more practically, an available switched reluctance generator (SRG) [32] is employed as a main source. The complete configuration of the SRG powered micro-grid system is shown in Fig. 8. A SRG with higher rated speed can be directly driven in micro-turbine, ISG or flywheel system. For low speed prime mover such as wind turbine, a speeding up gear box is normally equipped. In the studied experimental system, an inverter-fed induction motor drive is employed as an alternative of prime mover. The well-designed SRG converts the mechanical energy to yield a robust 48V DC source. Then a common 400V DC-grid is formed using a boost DC-DC converter. Similarly the 1P3W inverter developed in Sec. 3.3 is also employed here as a test load.

5AV,100

0ms2

00ms5

V15

dcv

(a)

(b)

00V4

ABi

ABv

Fig. 5. Measured results of the 1P3W load inverter under linear and unbalanced loads of Ω,200Ω57( == BA R,R )150Ω=ABR due to a step load resistance change of Ω90150 →Ω=ABR by PI and robust controls ( 65.0=vdW , 7.0=vcW ): (a) ABAB iv , ; (b) dcv .

(a)

1S

00ms5

V50

V2

dcv

V400

ABv

V150

ms20

V21S

BA vv ,

(b) Fig. 6. Measured results as the main DC source failure occurs and SC bank supports power: (a) common DC bus voltage dcv , triggered signal 1S ; (b) ,ABv ABi under loads of ( Ω= 57AR ,

F;0μ=LC Ω= 200BR ; Ω= 90ABR ) and triggered signal 1S .

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(a)

2S

00ms5

V50

V2

dcv

V400

ABv

V150

ms20

V2

BA vv ,

(b)

2S

Fig. 7. Measured results as the SC bank stops discharging and the battery supports power: (a) common DC bus voltage dcv , triggered signal 1S ; (b) ,ABv ABi under loads of ( Ω= 57AR ,

F;0μ=LC Ω= 200BR ; Ω= 90ABR ) and triggered signal 1S .

Inv IM60Hz 220V

AC1φmP

1Q

2Q

1D

2D

3Q

4Q4D

3D 5Q

6Q6D

5D 7Q

8Q8D

7D

1W 2W 3W 4W

2i 3i 4i1i

di

eV dvdC

SRG and conveter

rωiT

Prime mover

Excitationsource

dP

CommonDC grid

400V

SRG

)( θΔosNAH

Isolation circuits

Positionsensingscheme

Commutationtuning scheme

Phase current

commandgenerator

Current controllers

Voltagecontrolscheme

∑*dvcIvε

dv′2q 4q 6q 8q

8q6q4q2q 1q 3q 5q 7q

BHrθ

AH BH EC

DSP

EC

41 ~ ii

*2i*3i*4i

*1i *2i *3i *4i

Encoderinterface

D/A

A/Ddv

convertersload

Other

Gear box

Energystoragesystems

7Q 9Q

8Q

11Q

10Q 12Q

7D 9D 11D

dcvdcC

dcP Output filter LoadoL

oC

oC

ABR

Bv

oL

AR

BR

LC

A

B

N

Av

ABv

A

B

oL

8D 10D 12D

IGBT Module 2

1Q

2Q

3Q

4Q

5Q

6Q

1D 3D 5D

2D 4D 6D

L

Li

IGBT Module 1

SC bank Battery bank

scv+-

Li

-

L

bv+

LLi

Boost+BESS+ 1P3W inverter

Boost+BESS+ 1P3W inverterSRG

ABP

AP

oP

BP

Fig. 8. System configuration of SRG fed micro-grid system.

By letting the SRG be driven stably at 6000rpm, Figs.

9(a) and 9(b) depict the measured DC-link voltage dv and the phase-1 winding current 1i of the SRG, the 1P3W inverter outputs ( ABAB vv ,* ) and ( Av , Bv ) at the loads of ( ;133Ω=ABR ,200Ω=AR F;0μ=LC Ω= 400BR ). The measured key steady-state characteristics are: (i) boost converter: V,15.48=dV W,86.602=dP V,18.400=dcV W,524=dcP

== ddc PP /η ;%93.86 (ii) 1P3W load inverter: total output power W,86.452=oP %42.86/ == dco PPη . Fig. 10 depicts the DC-link voltage ,dv common DC grid voltage dcv and

measured voltage and current of 220V output ( ,vAB ABi ) under linear and unbalanced loads of ( ;200Ω=ABR

,200Ω=AR F;0μ=LC Ω= 400BR ), due to a step load resistance change of Ω133200 →Ω=ABR .

VI. Conclusions This paper has presented the development of an

experimental DC micro-gird consisting of a common DC grid established by main source interfaced by boost converter, an energy storage system and a single-phase three-wire load inverter. Good whole system performance has been verified experimentally.

All power circuits in the developed micro-grid are constructed in two three-phase IGBT modules. One module is used to construct the main source interface DC-DC boost converter and two bi-directional DC-DC converters for interfacing two storage devices. The other IGBT module is employed to form a 1P3W load inverter. Moreover, the digital control algorithms of all power stages are realized in a common DSP. Hence, the established micro-grid system is very compact.

The 400V common DC grid voltage is established by the standard boost DC-DC converter from main DC source. And the micro-grid storage buffer is supported by an energy storage system containing a super-capacitor bank and a battery bank, which respectively provides the short-period fast and long-duration sluggish discharging powers. As to the 1P3W load inverter, the outer two legs of IGBT module are PWM switched to yield the 220V AC output, and the center leg is PWM controlled to maintain the balance between two 110V AC outputs. Thanks to the proper control design, good and robust control characteristics for all constituted power stages are achieved.

dv

1i

ms5.0

ABv

*ABv

Av

Bv

ms5 Fig. 9. Measured results at the loads of ( ;133Ω=ABR ,200Ω=AR

F;0μ=LC Ω= 400BR ), W,86.602=dP W,524=dcP ;%93.86/ == ddc PPη W,86.452=oP %42.86/ == dco PPη : (a) the DC-link voltage dv and the phase-1 winding current 1i of the SRG; (b) the inverter

outputs ( ,*ABv ABv ) and ( Av , Bv ).

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dv

dcv

ABv

ABi

0ms2 Fig. 10. Measured results under the loads of ( Ω= 200ABR ;

Ω= 200AR , 0=LC ; Ω= 400BR ), due to a step load resistance change of Ω133200 →Ω=ABR ( W86.452W15.333 →=oP ) : (a) DC-link voltage dv ; (b) common DC grid voltage dcv ; (c)

,ABv ABi .

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