06063844

8132019 06063844

httpslidepdfcomreaderfull06063844 17

A Grid Synchronization Method for Droop

Controlled Distributed Energy Resources ConvertersChia-Tse Lee Rui-Pei Jiang Po-Tai Cheng

CENTER FOR A DVANCED P OWER T ECHNOLOGIES DEPARTMENT OF E LECTRICAL E NGINEERING

NATIONAL T SING H UA U NIVERSITY HSINCHU TAIWAN

AbstractmdashWith the high penetration of distributed generationsystem many control methods have been widely discussed formanaging the power flows between these distributed energyresources converters in islanded or grid-connected operationmodes The grid synchronization method has been also elabo-rately discussed for single grid-connected converter However itis not often explored for the multi-converter oriented systemIn this paper a grid synchronization method for the multi-converter oriented distributed generation system is proposedThe proposed grid synchronization method can cooperate with

minus 1038389 1103925 minus 983769907317 droop controls and all the distributed energy

sources converters regulate their own phase angles and voltagemagnitude at the same speed Thus the original power flowdetermined by these droop controllers can be maintained duringthe operation of grid synchronization Its operation principle isexplained and experimental test results are presented to validatethe effectiveness of the proposed grid synchronization method

Index TermsmdashDistributed generation systems droop controlgrid synchronization Microgrid

I INTRODUCTION

With the awareness and need of low carbon emissions

renewable resources have become a significant research topic

recently Considering the generation scale and characteris-

tics of these renewable resources the concept of distributed

generation for these renewable resources have been proposedand discussed rather than conventional centralized generation

Therefore distributed generation systems (DGSs) such as

microgrids smartgrids have been developed to transform this

abstract concept into a practical application [1] [2] [3]

The control frameworks of distributed energy resources

converters (DERCs) in DGS have been explored over the past

years and the frequently discussed frameworks are master-

slave and droop controls [4] [5] [6] [7] [8] [9] The

master-slave controlled DGS must assign a converter to be the

master converter and control it as a voltage source converter

The rest of the converters in this system are controlled as

current source converters Because this master converter acts

as a virtual inertia [2] it will pick up most dynamic power

flows in DGS Therefore the power capacity of this masterconverter should be physically large to ride-through all the

transients and dynamics in this system On the other hand

the droop controlled DGS allows multiple voltage source

converters operating in DGS at the same time The transient

and dynamic power flows can be shared with these droop

controlled converters

Traditionally the real power-frequency droop ( minus1038389 droop)

control and the reactive power-voltage droop (1103925 minus 907317 droop)

are generally adopted in the droop controlled DGS [5] [6]

[7] The minus 1038389 droop control can achieve accurate real power

sharing results However the 1103925 minus 907317 droop control is highly

dependent on the line impedances seen from the converters

Therefore the 1103925minus 983769907317 droop control method has been proposed

to introduce one more dynamic relationship between the

converterrsquos reactive power and voltage magnitude [10] This

improved reactive power sharing control can be insensitive to

the unequal line impedances and improve the reactive power

sharing Furthermore it can be easily applied to converterswith different power capacities which is suitable for the rdquoplug-

and-playrdquo operation

Furthermore one of the most significant issues for DGS is

the control methods in different operation modes Many papers

have been presented for the controls of islanded mode and

grid-connected mode Another significant issue is grid syn-

chronization The grid synchronization method has been elab-

orately discussed for single grid-connected converters [11]

However it is not often explored for multi-converter oriented

systems or droop controlled DGS Reference [12] proposes

the grid synchronization control method for the conventional

minus 1038389 1103925 minus 907317 droop controlled microgrid

This paper continues from previous work exploring the grid

synchronization process for the multi-converter oriented and minus 1038389 1103925 minus

983769907317 droop controlled DGS The DERCs in DGS

achieve grid synchronization in an autonomous operation

During the transition of grid synchronization the power flows

can be controlled the same as those originally determined by

the autonomous droop controllers in islanded mode with negli-

gible transients The proposed control methods are explained

and the simulation and experimental test results are presented

to verify the effectiveness of the proposed control methods

I I DISTRIBUTED G ENERATION S YSTEM S TRUCTURE

Fig 1 shows the structure of the DGS The DGS consists of

several distributed energy resources converters (DERCs) line

impedances and load Because of the distributed location of DERCs these minus line impedances are to emulate the power

lines with different distances This DGS is connected to the

utility grid through a bypass switch As this bypass switch is

opened the distributed generation system operates in islanded

mode On the other hand it operates at grid-connected mode

as this bypass switch is closed

The overall control structure of the DGS is also shown in

Fig 1 Every DERC in DGS is controlled by its autonomous

978-1-4577-0541-011$2600 copy2011 IEEE 743

8132019 06063844


Utility Grid

PWM

Modulatoramp Gate driver

C o m m u n i c a t i o n

U n i t s

To autonomous

controller x+1

Bypass Switch

V G G R1+jX 1

Load

R x +jX x

Distributed Generation System

V 1 1

C f

L f

V x x

C f

L f

V PCC PCC

Autonomous controller x

Voltage

amp

Current

Controller

Droop-based

Power Sharing

Controller

Grid

Synchronization

Controller

Main controller

Detection amp

Calculation of

V PCC PCC amp

V G G

Central Console

Fig 1 Distributed generation system structure

controller including three main parts voltage and current

controller droop-based power sharing controller and grid syn-

chronization controller These autonomous controllers cooper-ate with the signals from main controller which are calculated

from the terminal voltages of the bypass switch 907317 10383891038389 ang 10383891038389

and 907317 1103925ang1103925 and are transmitted by the communication units

Also the operation modes of these autonomous controllers

are controlled by the grid synchronization sequence signals

which are commanded through the central console of DGS

The detailed control block diagrams and operations of grid

synchronization method are shown as follows

III GRI D S YNCHRONIZATION M ETHOD

The main focus of this paper is to present an autonomous

grid synchronization method based on the existing minus1038389 droop

and 1103925 minus

983769907317 droop controls for the multi-converter orientedDGS The grid synchronization is achieved by changing all

DERCsrsquo operation frequencies phase angles and voltage

magnitudes at the same speed in an autonomous manner such

that the relative phase angle differences and voltage magnitude

differences between all the DERCs are maintained during the

grid synchronization process The real power and the reactive

power flows originally determined by the droop controls can

be therefore maintained at the same values with negligible

transients The control block diagrams and the operations

of proposed grid synchronization method are explained as

follows

A Main controller

Fig 2 shows the control block diagram of the main con-troller The main controller senses the feedbacked signals

907317 1103925983084907317 and 907317 10383891038389983084907317 to calculate the information required

by all the DERCrsquos autonomous controllers This required

information includes the frequency of utility grid 1103925 phase

angle difference and voltage magnitude difference 907317

between the utility grid and the point of common coupling

(PCC)

The phase-locked loops use PI controllers to control the

d-axis voltages of utility grid and PCC at 983088 [13] and the

0 25 5 75 10372

375

378

381

0 25 5 75 10

0

0 25 5 75 10100

5

10

15

103838911039251103925 [radsec]

907317 [rad]

983088983086983093

minus983088983086983093

minus

907317 [V]

[sec]

Frequency restoration engages

Fig 3 The information derived by the main controller

frequencies 1103925 and 10383891038389 are derived and used to transform

the feedbacked phase voltages into qd-axis voltages under thesynchronous reference frame The voltage magnitudes of the

utility grid and the PCC (907317 1103925 and 907317 10383891038389 ) are calculated by

these qd-axis voltages under their individual synchronous ref-

erence frames and the required voltage magnitude difference

907317 is thus derived

The phase angle difference can be derived by equa-

tion (1) which is based on the sum and difference formulas

of a trigonometric function to eliminate the sine function of

9830801103925 983083 10383891038389 983081times9830839830801103925 983083 10383891038389 983081 and to obtain the sine function

of 9830801103925 minus 10383891038389 983081times 983083 9830801103925 minus 10383891038389 983081 As a result equals

the phase angle difference 1103925 minus 10383891038389 as 10383891038389 is regulated

to the same value as 1103925

983131907317

1103925

907317 1103925

983133 983101

983131 907317 1103925 9831399831519831559830801103925 times 983083 1103925983081minus907317 1103925 9831559831459831509830801103925 times 983083 1103925983081

983133983131907317

10383891038389

907317 10383891038389

983133 983101

983131 907317 10383891038389 983139983151983155983080 10383891038389 times 983083 10383891038389 983081minus907317 10383891038389 983155983145983150983080 10383891038389 times 983083 10383891038389 983081

983133

983101 983155983145983150minus983089983131 983089

907317 1103925907317 10383891038389

983080907317 1103925 times 907317

10383891038389 minus 907317 1103925 times 907317

10383891038389 983081983133

983101 9830801103925 minus 10383891038389 983081 times 983083 9830801103925 minus 10383891038389 983081(1)

Fig 3 shows the information derived during the main con-

trollerrsquos operation The operation frequencies 1103925 and 10383891038389

is detected by the phase-locked loops and the phase angle

difference and the voltage magnitude difference 907317

can then be calculated as shown in Fig 3 Before 983101 983093 983155983141983139 the

frequency of the PCC ( 10383891038389 ) deviates from the frequency of the utility frequency (1103925) and the phase angle difference

continuously varies between minus and After the frequency

restoration is engaged and 10383891038389 is restored the same value

as 1103925 stops varying and equals to the angle difference

1103925 minus 10383891038389 as shown in equation (1) Therefore the main

controller of DGS detects these required information and then

transmits them to all the autonomous controller through the

communication units to accomplish the grid synchronization

as shown in Fig 2

744

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V diff

diff

G

Main controller

V PCCabc

V Gabc V G qe

V G qe

X2

V G de

V G de

X05

V G V G

V PCC qe

V PCC qe

V PCC de

V PCC de

V PCC V PCC

V diff V diff

X2

X2

X

2

X05

Voltage magnitude

difference calculation

V G qs

V G ds

V PCC qs

V PCC ds

Sin-1 diff

Phase angle


V G V PCC

1

abcto

qde

V PCCa

V PCCb

V PCCc

V PCC

qe

V PCC de

LPF

0 PI

2 PLL

PCC

abc

to

qde

V Ga

V Gb

V Gc

V G qe

V G de

LPF

0 PI

2 PLL

G

Fig 2 Control block diagram of main controller

B Autonomous controller

The detailed control block diagram of DERCrsquos autonomous

controller is given in Fig 4 The minus1038389 droop control and 1103925minus983769907317

droop control are implemented by equation (2) where and

are the droop coefficients 1038389 983088 983769907317 983088 and 907317 983088 are nominal

frequency nominal 983769907317 and nominal voltage magnitude respec-tively and 983088 and 1103925983088 are the real power and reactive power

set-points which are related to the power capacity of DERC

In equation (2) 1038389 and 983769907317 are added for grid synchronization

which will be explained next The frequency and 983769907317 restoration

controllers are based on equation (3) to regulate 983088 and 1103925983088

in equation (2) where and are the restoration

speed related gains and and 1103925 are the DERCrsquos power

capacity related scaling gains With the aforementioned con-

trollers in equation (2) and equation (3) the minus1038389 and 1103925minus 983769907317

droop controls can accomplish real and reactive power sharing

and the operation frequency 1038389 and 983769907317 can be regulated back

to 1038389 983088 and 983769907317 983088 respectively in an autonomous manner

1038389 lowast 983101 1038389 983088 minus sdot 983080 983088 minus 983081 983083 1038389

983769907317 lowast 983101 983769907317 983088 minus sdot 9830801103925983088 minus1103925983081 983083 983769907317

907317 lowast 983101 907317 983088 983083

int 983769907317 lowast

(2)

983088 983101 9830801038389 983088 minus 1038389 983081

1103925983088 983101 1103925983080 983769907317 983088 minus

983769907317 983081

(3)


grid synchronization method based on the existing minus 1038389

droop and 1103925 minus 983769907317 droop controls To accomplish the grid

synchronization the voltage magnitude equalization and phase

synchronization controllers are proposed as equation (4) and

equation (5)

983769907317 983101 sdot 907317 983083

int 907317 (4)

1038389 983101 sdot 983083

int

983101

983163 if 983101 983089983084

983088 if 983101 983088983086

(5)

Islanded mode

Phase

synchronization

Grid-connected mode

Grid Synchronization Method

Bypass switch closesGS

Power management control

Frequency restoration

Voltage magnitude

equalization

V restoration

time

Fig 5 Grid synchronization sequence

The voltage magnitude equalization is designed to main-

tain 907317 to 983088 907317 transmitted from the main controller

represents the voltage magnitude difference between 907317 1103925 and

907317 10383891038389 and it can be regulated by lifting up or pulling down

all the DERCsrsquo operation voltage magnitudes at the same

speed Therefore 983769907317 in equation (4) representing the voltage

magnitude change with respect to the time variation is derived

from the PI regulation of 907317 and injected to the 1103925minus

983769907317

droop control in equation (2) to achieve the voltage magnitude

equalization In the same manner the phase synchronization is

implemented by injecting 1038389 the the PI regulation of to

the minus1038389 droop control in equation (2) Because of the voltage

magnitude equalization and phase synchronization change all

DERCsrsquo voltage magnitudes and phase angles at the same

pace the relative voltage magnitude differences and phase

angle differences between all the DERCs are maintained such

that the real power and reactive power sharing results are not

affected during these operations

To complete the grid synchronization and minimize the

transient power flows at the instant of grid-connection the

operation frequency phase angle and voltage magnitude of

907317 10383891038389 should be regulated the same value as those of 907317 1103925before the bypass switch is closed Therefore the grid syn-

chronization sequence shown as Fig 5 is commanded from

the central console to control the operation modes of the grid

synchronization process

Fig 6 and Fig 7 show the aforementioned grid synchroniza-

tion process with the computer simulation Before 983101 983092983088 983155983141983139

the bypass switch is opened the DGS is operating in islanded

mode All the DERCs are controlled by minus 1038389 1103925 minus 983769907317 droop

controls to achieve power sharing As the DERCs are operated

745

8132019 06063844



Voltage PI Controller

amp

Predictive Current

Controller

V diff

diff

G

GS V ref

MUX 1

0

diff

0 PI

GS

f S

Phase

synchronization

PS

Voltage magnitudeequalization

V diff PI V S

P-f droop P x

m x P 0x

f 0x

f x

f S f x

s

1

Q x

n x Q0x

V 0x V x

V 0x V x

V x V x

V S Q-V droop

0 V 0x K Qresx Q Rx

s

1 Q0x V x

V restoration


f 0x

f x

K Presx P Rx s

1 P 0x G

2

From Central

Console

From Main

Controller

Fig 4 Control block diagram of autonomous controller

0 10 20 30 40 50595

5975

60

6025

605

0 10 20 30 40 50minus02

minus01

0

01

02

03

0 10 20 30 40 50minus5

0

5

10

[ H z ]

[ r a d ]

[ V ]

Step load changes 1103925 983101 983089 Bypass switch closes

103838911039251103925

907317

907317

Time[sec]

Fig 6 The responses of main controller during grid synchronization process

in the 1103925 minus 983769907317 droop control where 983088 is assigned to 983769907317 983088 thereactive power sharing can be improved compared with the

reactive power sharing of 1103925 minus 907317 droop control [10] Owing

to that the operation of 1103925 minus 983769907317 droop control can result in

the output voltage variations of the DGS the engagement of

voltage magnitude equalization can regulate all the DERCsrsquo

voltage magnitudes at the same speed and thus maintain

the voltage magnitude of DGS without disturbing the power

sharing results Also the frequency restoration is activated

by assigning 1103925

983090 to 1038389 983088 to maintain the operation frequency

of PCC ( 10383891038389 983101 9830901038389 10383891038389 ) the same as that of utility

grid (1103925 983101 9830901038389 1103925) Note that a step load change from

983090983089983088983088983127 983083 983089983088983088983088 983126983105983122 to 983091983089983088983088983127 983083 983089983088983088983088 983126983105983122 occurs at

983101 983089983093 983155983141983139

To satisfy the necessity for grid-connection the phase angleof PCC ( 10383891038389 ) needs to be synchronized to the phase angle

of utility grid (1103925) At 983101 983091983088983155983141983139 DGS starts to synchronize

10383891038389 to 1103925 by transmitting the enabling signal to every

autonomous controller of DERC from the central console All

the DERCs in DGS then start to adjust their phase angles at

the same speed by 1038389 983089 and 1038389 983090 which is generated by the PI

regulation of the input variable as shown in Fig 7(a)

In the end ℎ10383891038389 can be regulated to 983088 which means

10383891038389 is aligned to 1103925 as 1038389 983089 and 1038389 983090 is backed and kept at

0 10 20 30 40 500

1000

2000

3000

0 10 20 30 40 505999

59995

60

60005

0 10 20 30 40 50minus002

0

002

004

006

[ W ]

[ H z ]

[ H z ]


1

2

1

2

1 2

Time[sec]

(a)

0 10 20 30 40 500

300

600

900

1200

0 10 20 30 40 50minus2

minus1

0

1

0 10 20 30 40 50minus5

0

5

10

0 10 20 30 40 50179

181

183

185

187

[ V A R ]

[ V s e c ]

[ V s e c ]

[

V ]


1

2

983769 1

983769 2

983769 1 983769 2

lowast

1 lowast2

Time[sec]

(b)

Fig 7 The responses of DERCs during grid synchronization process

746

8132019 06063844


983088 The power sharing results are also not affected during the

operation of phase angle synchronization

After the frequency voltage magnitude and phase angle

of 907317 10383891038389 and 907317 1103925 are synchronized as shown in Fig 6 the

bypass switch is closed at 983101 983092983088983155983141983139 by transmitting an

enable signal from central console and the DGS goes into

the grid-connected mode with tolerable transients power flowsThe grid synchronization process shown in Fig 7 verify that

the operation of minus 1038389 1103925 minus 983769907317 droop controlled DGS can

be transferred from islanded mode to grid-connected mode

without affecting the original power sharing results by the

proposed grid synchronization method

IV EXPERIMENTAL T EST R ESULTS

The DGS test benches are constructed to validate the effec-

tiveness of the proposed grid synchronization control method

The system configuration is the same as shown in Fig 1 and

the detailed descriptions of this DGS are stated as follows

∙ The system voltage is 907317 minus 983101 983090983090983088 983126983154983149983155 and the

frequency is 983094983088983112983162 Two DERCs are constructed in thisDGS and their power line impedances are set as 983089 983083 983089 983101 983088983086983090 983083 983088983086983091983095983095Ω and 983090 983083 983090 983101 983088983086983090 983083 983088983086983091983095983095Ω

The total of load of 983096983088983088 983127 is applied

∙ The DERCs are three-phase hard-switched PWM con-

verters whose switching frequency 1038389 ℎ 983101 983089983088983147983112983162

output filter inductor 983101 983090983149983112 output filter capacitor

983101 983089983088 983110 The DC bus voltage of DERC is supported

by DC power supply 62024P-600-8

∙ The main controller and the autonomous controllers

are implemented with the digital signal processor

TMS320F28335 and the sampling frequency is pro-

grammed at 1038389 907317 983101 983090983088983147983112983162 The coefficients of

main controller and autonomous controllers are given in

TABLE I∙ The bypass switch in Fig 1 can be implemented with

different topologies [14] [15] and the circuit breaker-

based (CB-based) switch is adopted in this experimental

test benches

∙ The communication interfaces are implemented with RS-

232 to transmit and receive data among the central con-

sole main controller and autonomous controllers The

bandwidth of these communication units are set at about

983096983088983112983162

Fig 8 shows the experimental test results of the proposed

grid synchronization method The detected information (1103925

10383891038389 and 907317 ) in the main controller shown in

Fig 8(a) are to investigate the operations and responses of

proposed grid synchronization method As these two DERCsin DGS are connected and operated in the islanded mode the

minus 1038389 droop control in every autonomous controller works

individually and then the operation frequency of the PCC

( 10383891038389 ) is internally decided and deviated from the utility

frequency 1103925 This operation frequency difference between

10383891038389 and 1103925 results in the variation of before the

frequency restoration is activated This variation is stopped

and controlled as long as the frequency restoration is activated

and the 10383891038389 is regulated the same as 1103925 as shown in

377

020sec

Voltage magnitude equalization

Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

377

0

G

PCC

diff

10V

2 DERCs

are connected

V diff

4rad

4radsec

4radsec

(a) Detected information in the main controller ( 1038389 11039251103925 X-axis 298308810383899830871103925907317Y-axis 4110392598308710383899830871103925907317 907317 X-axis 298308810383899830871103925907317 Y-axis 411039259830871103925907317 907317 X-axis 298308810383899830871103925907317 Y-axis 19830889830871103925907317)

0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 1

Q1

1

1000VAR

4radsec

4VsecV 1

2 DERCs

are connected

(b) Responses in the DERC1 ( 1 X-axis 2983088

10383899830871103925907317 Y-axis 1983088983088983088

98308711039259073171 X-axis 298308810383899830871103925907317 Y-axis 1983088983088983088 9830871103925907317 lowast

1 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast1

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)

0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 2

Q2

2

1000VAR

4radsec

4VsecV 2

2 DERCs

are connected

(c) Responses in the DERC2 ( 2 X-axis 298308810383899830871103925907317 Y-axis 198308898308898308898308711039259073172 X-axis 298308810383899830871103925907317 Y-axis 1983088983088983088 9830871103925907317 lowast

2 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast2

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)

Fig 8 Experimental test results of grid synchronization process

747

8132019 06063844


TABLE IRELATED PARAMETERS OF MAIN CONTROLLER AND AUTONOMOUS

CONTROLLERS

Main controller

PLL 983101 9830889830869830882983093 983101 1

Autonomous controllers

minus droop control 1 983101 2 983101 minus983091 times 1983088minus6 983154983137983140983087983114

minus 983769 droop control 1 983101 2 983101 minus983093 times 1983088minus4 1983087983105983155983141983139

Frequency restoration 10383891 1 983101 10383892 2 983101 66666

983769 restoration 11 983101 22 983101 4983088983088

Phase angle synchronization 983101 983088983086983088983096 983101 98308898308616

Voltage magnitude equalization 983101 9830889830864 983101 983088983086983091983093

Synchronous voltage PI controller 983101 983088983086983088983091 983101 4

Predictive current controller 1103925 983101 983091983088

Fig 8(a) 907317 and can then be regulated to 983088 by the

operation of voltage magnitude equalization and phase angle

synchronization respectively Note that 10383891038389 is increased to

be higher than 1103925 by the PI-based phase angle synchronization

to reduce the phase angle difference and then 10383891038389 and1103925 are the same again after the phase angle difference

is decreased and controlled at 983088

Fig 8(b) and Fig 8(c) show the DERC1 and DERC2rsquos

output power flow ( 1103925) and their operation frequency and983769907317 commands ( 983769907317 ) individually lowast is the combination

of minus 1038389 droop controlrsquos output 9830901038389 and the phase angle

synchronizationrsquos output 9830901038389 as shown in Fig 4 and 983769907317 lowastis the combination of 1103925 minus

983769907317 droop controlrsquos output 983769907317 and

the voltage magnitude equalizationrsquos output 983769907317 As shown in

Fig 8(b) and Fig 8(c) the DERCsrsquo lowast and 983769907317 lowast can be affected

by the operation of voltage magnitude equalization and phase

angle synchronization individually and their phase angle and

voltage magnitude of output voltages are accordingly affected

However these DERCs operate these grid synchronizationcontrols at the same instant and change the phase angle and

voltage magnitude of different DERCs at the same speed

Therefore the original power sharing results of islanded

operation is not affected during these grid synchronization

operations as shown in Fig 8(b) and Fig 8(c)

After the frequencies voltage magnitudes and phase angles

of the utility grid and the PCC are regulated and synchronized

the same the central console send out the enabling signal and

the bypass switch is then closed As shown in Fig 8(b) and

Fig 8(c) the power flow of these DERCs are still maintained

after DGS is operated in the grid-connected mode and their

transient power flows are mitigated by the proposed grid

synchronization method

Fig 9 compares the line-to-line voltage of the utility gridPCC DERC1 and DERC2 at different instance Note that

only a-to-b voltages are shown in Fig 9 Before the volt-

age magnitude equalization is activated the voltage magni-

tude of utility grid and PCC are 907317 1103925983084907317 983101 983090983089983090983086983094 983126983154983149983155 and

907317 10383891038389983084907317 983101 983090983088983094983086983095 983126983154983149983155 This voltage magnitude difference

can be pull back by the voltage magnitude equalization and

these voltage magnitudes become 907317 1103925983084907317 983101 983090983089983092983086983088 983126983154983149983155 and

907317 10383891038389983084907317 983101 983090983089983093983086983088 983126983154983149983155 However the phase angle of 907317 1103925983084907317

leads that of 907317 10383891038389983084907317 at 983096983093983086983096∘ By using the phase angle

synchronization this phase angle difference is regulated and

decreased as shown in Fig 9 and then the bypass switch

is closed as the voltage magnitude and the phase angles are

synchronized

V CONCLUSION

The minus 1038389 1103925 minus 983769907317 droop controls have been proposed

and discussed for their insensitivity to the unequal line

impedances and improved power sharing capability This paper

presents the grid synchronization for minus 1038389 1103925 minus 983769907317 droop

controlled DGS The proposed grid synchronization method

allows all the minus 1038389 1103925 minus 983769907317 droop controlled DERCs to

adjust their frequencies voltage magnitudes and phase an-

gles synchronously The relative differences between DERCsrsquo

voltage magnitudes and phase angles are not affected and the

original power sharing results under islanded operation mode

can be maintained during grid synchronization process Thus

the proposed method allows the multi-converter oriented DGS

to be changed from islanded mode to grid-connected mode

with negligible transient power flows and without affecting

the original droop controlled power sharing results to achieve

a smooth mode transfer Simulation and laboratory test results

are also presented to show effectiveness of this work

ACKNOWLEDGMENT

This research is funded by the National Science Council of

Taiwan under grant NSC-98-3114-E-007-004

REFERENCES

[1] R Lasseter ldquoMicrogridsrdquo in Proc IEEE Power Engineering SocietyWinter Meeting 2002 pp 305ndash308

[2] M Barnes J Kondoh H Asano J Oyarzabal G VentakaramananR Lasseter N Hatziargyriou and T Green ldquoReal-world microgrids-

an overviewrdquo in IEEE International Conference on System of Systems Engineering 2007 pp 1ndash8[3] F Katiraei R Iravani N Hatziargyriou and A Dimeas ldquoMicrogrids

managementrdquo IEEE Power and Energy Magazine vol 6 no 3 pp54ndash65 MayJun 2008

[4] C L Chen Y Wang J S Lai Y S Lee and D Martin ldquoDesignof parallel inverters for smooth mode transfer microgrid applicationsrdquo

IEEE Transactions on Power Electronics vol 25 no 1 pp 6ndash15 Jan2010

[5] M C Chandrokar D M Divan and R Adapa ldquoControl of parallelconnected inverters in standalone ac supply systemsrdquo IEEE Transactionson Industry Applications vol 29 no 1 pp 136ndash143 JanFeb 1993

[6] M C Chandrokar D M Divan and B Banerjee ldquoControl of distributedups systemsrdquo in Proc IEEE Power Electronics Specialists Conference1994 pp 197ndash204

[7] P Piagi and R Lasseter ldquoAutonomous control of microgridsrdquo in Proc IEEE Power Engineering Society General Meeting 2006 p 8pp

[8] J M Guerrero L G de Vicuna J Matas M Castilla and J MiretldquoOutput impedance design of parallel-connected ups inverters with wire-

less load-sharing controlrdquo IEEE Transactions on Industrial Electronicsvol 52 no 4 pp 1126ndash1135 Aug 2005

[9] C K Sao and P W Lehn ldquoAutonomous load sharing of voltage sourceconvertersrdquo IEEE Transactions on Power Delivery vol 20 no 2 pp1009ndash1016 Apr 2005

[10] C T Lee C C Chu and P T Cheng ldquoA new droop control methodfor the autonomous operation of distributed energy resource interfaceconvertersrdquo in Proc IEEE Energy Conversion Congress and Exposition(ECCE) 2010 pp 702ndash709

[11] F Blaabjerg R Teodorescu M Liserre and A V Timbus ldquoOverviewof control and grid synchronization for distributed power generationsystemsrdquo IEEE Transactions on Industrial Electronics vol 53 no 5pp 1398ndash1409 Oct 2006

748

8132019 06063844


0

0

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

Before voltage magnitude equalization After voltage magnitude equalization After phase angle synchronization After grid connection

Islanded mode

Phase

synchronization

Grid-connected mode

Bypass switch closes

Frequency

restoration

Voltage magnitude

equalizationtime

P-f Q-V

droop control

10msec200V

10msec200V

10msec200V

10msec200V

0

Fig 9 The variations of line-to-line voltage of the utility grid PCC DERC1 and DERC2 during the grid synchronization process ( 983084 103838911039251103925983084 1103925 1983084 1103925 2983084 X-axis 198308810383899830871103925907317 Y-axis 29830889830889830871103925907317)

[12] J M Guerrero J C Vasquez J Matas L G de Vicuna and M CastillaldquoHierarchical control of droop-controlled ac and dc microgrids-a generalapproach toward standardizationrdquo IEEE Transactions on Industrial

Electronics vol 58 no 1 pp 158ndash172 Jan 2011[13] L N Arruda S M Silva and B J C Filho ldquoPll structures for utility

connected systemsrdquo in Proc IEEE Industry Applications ConferenceThirty-Sixth IAS Annual Meeting 2001 pp 2655ndash2660

[14] B Kroposki C Pink J Lynch V John S M Dandiel E Benedict andI Vihinen ldquoDevelopement of a high-speed static switch for distributedenergy and microgrid applicationsrdquo in Power Conversion Conference -

Nagoya 2007 PCC rsquo07 Apr 2007 pp 1418ndash1423[15] Z Yang H Liao C Wu and H Xu ldquoAnalysis and selection of switch

for double modes inverter in micro-grid systemrdquo in Electrical Machinesand Systems 2008 ICEMS 2008 International Conference on Oct2008 pp 1778ndash1781

749

8132019 06063844


Utility Grid

PWM

Modulatoramp Gate driver

C o m m u n i c a t i o n

U n i t s

To autonomous

controller x+1

Bypass Switch

V G G R1+jX 1

Load

R x +jX x

Distributed Generation System

V 1 1

C f

L f

V x x

C f

L f

V PCC PCC


Voltage

amp

Current

Controller

Droop-based

Power Sharing

Controller

Grid

Synchronization

Controller

Main controller

Detection amp

Calculation of

V PCC PCC amp

V G G

Central Console

Fig 1 Distributed generation system structure

controller including three main parts voltage and current

controller droop-based power sharing controller and grid syn-

chronization controller These autonomous controllers cooper-ate with the signals from main controller which are calculated

from the terminal voltages of the bypass switch 907317 10383891038389 ang 10383891038389

and 907317 1103925ang1103925 and are transmitted by the communication units

Also the operation modes of these autonomous controllers

are controlled by the grid synchronization sequence signals

which are commanded through the central console of DGS

The detailed control block diagrams and operations of grid

synchronization method are shown as follows

III GRI D S YNCHRONIZATION M ETHOD


grid synchronization method based on the existing minus1038389 droop

and 1103925 minus

983769907317 droop controls for the multi-converter orientedDGS The grid synchronization is achieved by changing all

DERCsrsquo operation frequencies phase angles and voltage

magnitudes at the same speed in an autonomous manner such

that the relative phase angle differences and voltage magnitude

differences between all the DERCs are maintained during the

grid synchronization process The real power and the reactive

power flows originally determined by the droop controls can

be therefore maintained at the same values with negligible

transients The control block diagrams and the operations

of proposed grid synchronization method are explained as

follows

A Main controller

Fig 2 shows the control block diagram of the main con-troller The main controller senses the feedbacked signals

907317 1103925983084907317 and 907317 10383891038389983084907317 to calculate the information required

by all the DERCrsquos autonomous controllers This required

information includes the frequency of utility grid 1103925 phase

angle difference and voltage magnitude difference 907317

between the utility grid and the point of common coupling

(PCC)

The phase-locked loops use PI controllers to control the

d-axis voltages of utility grid and PCC at 983088 [13] and the

0 25 5 75 10372

375

378

381

0 25 5 75 10

0

0 25 5 75 10100

5

10

15

103838911039251103925 [radsec]

907317 [rad]

983088983086983093

minus983088983086983093

minus

907317 [V]

[sec]

Frequency restoration engages

Fig 3 The information derived by the main controller

frequencies 1103925 and 10383891038389 are derived and used to transform

the feedbacked phase voltages into qd-axis voltages under thesynchronous reference frame The voltage magnitudes of the

utility grid and the PCC (907317 1103925 and 907317 10383891038389 ) are calculated by

these qd-axis voltages under their individual synchronous ref-

erence frames and the required voltage magnitude difference

907317 is thus derived

The phase angle difference can be derived by equa-

tion (1) which is based on the sum and difference formulas

of a trigonometric function to eliminate the sine function of

9830801103925 983083 10383891038389 983081times9830839830801103925 983083 10383891038389 983081 and to obtain the sine function

of 9830801103925 minus 10383891038389 983081times 983083 9830801103925 minus 10383891038389 983081 As a result equals

the phase angle difference 1103925 minus 10383891038389 as 10383891038389 is regulated

to the same value as 1103925

983131907317

1103925

907317 1103925

983133 983101

983131 907317 1103925 9831399831519831559830801103925 times 983083 1103925983081minus907317 1103925 9831559831459831509830801103925 times 983083 1103925983081

983133983131907317

10383891038389

907317 10383891038389

983133 983101

983131 907317 10383891038389 983139983151983155983080 10383891038389 times 983083 10383891038389 983081minus907317 10383891038389 983155983145983150983080 10383891038389 times 983083 10383891038389 983081

983133

983101 983155983145983150minus983089983131 983089

907317 1103925907317 10383891038389

983080907317 1103925 times 907317

10383891038389 minus 907317 1103925 times 907317

10383891038389 983081983133

983101 9830801103925 minus 10383891038389 983081 times 983083 9830801103925 minus 10383891038389 983081(1)

Fig 3 shows the information derived during the main con-

trollerrsquos operation The operation frequencies 1103925 and 10383891038389

is detected by the phase-locked loops and the phase angle

difference and the voltage magnitude difference 907317

can then be calculated as shown in Fig 3 Before 983101 983093 983155983141983139 the

frequency of the PCC ( 10383891038389 ) deviates from the frequency of the utility frequency (1103925) and the phase angle difference

continuously varies between minus and After the frequency

restoration is engaged and 10383891038389 is restored the same value

as 1103925 stops varying and equals to the angle difference

1103925 minus 10383891038389 as shown in equation (1) Therefore the main

controller of DGS detects these required information and then

transmits them to all the autonomous controller through the

communication units to accomplish the grid synchronization

as shown in Fig 2

744

8132019 06063844


V diff

diff

G

Main controller

V PCCabc

V Gabc V G qe

V G qe

X2

V G de

V G de

X05

V G V G

V PCC qe

V PCC qe

V PCC de

V PCC de

V PCC V PCC

V diff V diff

X2

X2

X

2

X05

Voltage magnitude


V G qs

V G ds

V PCC qs

V PCC ds

Sin-1 diff

Phase angle


V G V PCC

1

abcto

qde

V PCCa

V PCCb

V PCCc

V PCC

qe

V PCC de

LPF

0 PI

2 PLL

PCC

abc

to

qde

V Ga

V Gb

V Gc

V G qe

V G de

LPF

0 PI

2 PLL

G





















907317 lowast 983101 907317 983088 983083

int 983769907317 lowast

(2)

983088 983101 9830801038389 983088 minus 1038389 983081

1103925983088 983101 1103925983080 983769907317 983088 minus

983769907317 983081

(3)






equation (5)

983769907317 983101 sdot 907317 983083

int 907317 (4)

1038389 983101 sdot 983083

int

983101

983163 if 983101 983089983084

983088 if 983101 983088983086

(5)

Islanded mode

Phase

synchronization

Grid-connected mode





Voltage magnitude

equalization

V restoration

time










983769907317























745

8132019 06063844




amp

Predictive Current

Controller

V diff

diff

G

GS V ref

MUX 1

0

diff

0 PI

GS

f S

Phase

synchronization

PS


V diff PI V S

P-f droop P x

m x P 0x

f 0x

f x

f S f x

s

1

Q x

n x Q0x

V 0x V x

V 0x V x

V x V x

V S Q-V droop

0 V 0x K Qresx Q Rx

s

1 Q0x V x

V restoration


f 0x

f x

K Presx P Rx s

1 P 0x G

2

From Central

Console

From Main

Controller


0 10 20 30 40 50595

5975

60

6025

605

0 10 20 30 40 50minus02

minus01

0

01

02

03

0 10 20 30 40 50minus5

0

5

10

[ H z ]

[ r a d ]

[ V ]


103838911039251103925

907317

907317

Time[sec]














983090983089983088983088983127 983083 983089983088983088983088 983126983105983122 to 983091983089983088983088983127 983083 983089983088983088983088 983126983105983122 occurs at

983101 983089983093 983155983141983139










0 10 20 30 40 500

1000

2000

3000

0 10 20 30 40 505999

59995

60

60005

0 10 20 30 40 50minus002

0

002

004

006

[ W ]

[ H z ]

[ H z ]


1

2

1

2

1 2

Time[sec]

(a)

0 10 20 30 40 500

300

600

900

1200

0 10 20 30 40 50minus2

minus1

0

1

0 10 20 30 40 50minus5

0

5

10

0 10 20 30 40 50179

181

183

185

187

[ V A R ]

[ V s e c ]

[ V s e c ]

[

V ]


1

2

983769 1

983769 2

983769 1 983769 2

lowast

1 lowast2

Time[sec]

(b)


746

8132019 06063844


































test benches





983096983088983112983162














377

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

377

0

G

PCC

diff

10V

2 DERCs

are connected

V diff

4rad

4radsec

4radsec


0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 1

Q1

1

1000VAR

4radsec

4VsecV 1

2 DERCs

are connected


10383899830871103925907317 Y-axis 1983088983088983088


1 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast1

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)

0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 2

Q2

2

1000VAR

4radsec

4VsecV 2

2 DERCs

are connected


2 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast2

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)


747

8132019 06063844



CONTROLLERS

Main controller

PLL 983101 9830889830869830882983093 983101 1





983769 restoration 11 983101 22 983101 4983088983088














































synchronized

V CONCLUSION


















ACKNOWLEDGMENT



REFERENCES















748

8132019 06063844


0

0

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab


Islanded mode

Phase

synchronization

Grid-connected mode


Frequency

restoration

Voltage magnitude

equalizationtime

P-f Q-V

droop control

10msec200V

10msec200V

10msec200V

10msec200V

0








749

8132019 06063844


V diff

diff

G

Main controller

V PCCabc

V Gabc V G qe

V G qe

X2

V G de

V G de

X05

V G V G

V PCC qe

V PCC qe

V PCC de

V PCC de

V PCC V PCC

V diff V diff

X2

X2

X

2

X05

Voltage magnitude


V G qs

V G ds

V PCC qs

V PCC ds

Sin-1 diff

Phase angle


V G V PCC

1

abcto

qde

V PCCa

V PCCb

V PCCc

V PCC

qe

V PCC de

LPF

0 PI

2 PLL

PCC

abc

to

qde

V Ga

V Gb

V Gc

V G qe

V G de

LPF

0 PI

2 PLL

G





















907317 lowast 983101 907317 983088 983083

int 983769907317 lowast

(2)

983088 983101 9830801038389 983088 minus 1038389 983081

1103925983088 983101 1103925983080 983769907317 983088 minus

983769907317 983081

(3)






equation (5)

983769907317 983101 sdot 907317 983083

int 907317 (4)

1038389 983101 sdot 983083

int

983101

983163 if 983101 983089983084

983088 if 983101 983088983086

(5)

Islanded mode

Phase

synchronization

Grid-connected mode





Voltage magnitude

equalization

V restoration

time










983769907317























745

8132019 06063844




amp

Predictive Current

Controller

V diff

diff

G

GS V ref

MUX 1

0

diff

0 PI

GS

f S

Phase

synchronization

PS


V diff PI V S

P-f droop P x

m x P 0x

f 0x

f x

f S f x

s

1

Q x

n x Q0x

V 0x V x

V 0x V x

V x V x

V S Q-V droop

0 V 0x K Qresx Q Rx

s

1 Q0x V x

V restoration


f 0x

f x

K Presx P Rx s

1 P 0x G

2

From Central

Console

From Main

Controller


0 10 20 30 40 50595

5975

60

6025

605

0 10 20 30 40 50minus02

minus01

0

01

02

03

0 10 20 30 40 50minus5

0

5

10

[ H z ]

[ r a d ]

[ V ]


103838911039251103925

907317

907317

Time[sec]














983090983089983088983088983127 983083 983089983088983088983088 983126983105983122 to 983091983089983088983088983127 983083 983089983088983088983088 983126983105983122 occurs at

983101 983089983093 983155983141983139










0 10 20 30 40 500

1000

2000

3000

0 10 20 30 40 505999

59995

60

60005

0 10 20 30 40 50minus002

0

002

004

006

[ W ]

[ H z ]

[ H z ]


1

2

1

2

1 2

Time[sec]

(a)

0 10 20 30 40 500

300

600

900

1200

0 10 20 30 40 50minus2

minus1

0

1

0 10 20 30 40 50minus5

0

5

10

0 10 20 30 40 50179

181

183

185

187

[ V A R ]

[ V s e c ]

[ V s e c ]

[

V ]


1

2

983769 1

983769 2

983769 1 983769 2

lowast

1 lowast2

Time[sec]

(b)


746

8132019 06063844


































test benches





983096983088983112983162














377

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

377

0

G

PCC

diff

10V

2 DERCs

are connected

V diff

4rad

4radsec

4radsec


0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 1

Q1

1

1000VAR

4radsec

4VsecV 1

2 DERCs

are connected


10383899830871103925907317 Y-axis 1983088983088983088


1 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast1

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)

0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 2

Q2

2

1000VAR

4radsec

4VsecV 2

2 DERCs

are connected


2 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast2

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)


747

8132019 06063844



CONTROLLERS

Main controller

PLL 983101 9830889830869830882983093 983101 1





983769 restoration 11 983101 22 983101 4983088983088














































synchronized

V CONCLUSION


















ACKNOWLEDGMENT



REFERENCES















748

8132019 06063844


0

0

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab


Islanded mode

Phase

synchronization

Grid-connected mode


Frequency

restoration

Voltage magnitude

equalizationtime

P-f Q-V

droop control

10msec200V

10msec200V

10msec200V

10msec200V

0








749

8132019 06063844




amp

Predictive Current

Controller

V diff

diff

G

GS V ref

MUX 1

0

diff

0 PI

GS

f S

Phase

synchronization

PS


V diff PI V S

P-f droop P x

m x P 0x

f 0x

f x

f S f x

s

1

Q x

n x Q0x

V 0x V x

V 0x V x

V x V x

V S Q-V droop

0 V 0x K Qresx Q Rx

s

1 Q0x V x

V restoration


f 0x

f x

K Presx P Rx s

1 P 0x G

2

From Central

Console

From Main

Controller


0 10 20 30 40 50595

5975

60

6025

605

0 10 20 30 40 50minus02

minus01

0

01

02

03

0 10 20 30 40 50minus5

0

5

10

[ H z ]

[ r a d ]

[ V ]


103838911039251103925

907317

907317

Time[sec]














983090983089983088983088983127 983083 983089983088983088983088 983126983105983122 to 983091983089983088983088983127 983083 983089983088983088983088 983126983105983122 occurs at

983101 983089983093 983155983141983139










0 10 20 30 40 500

1000

2000

3000

0 10 20 30 40 505999

59995

60

60005

0 10 20 30 40 50minus002

0

002

004

006

[ W ]

[ H z ]

[ H z ]


1

2

1

2

1 2

Time[sec]

(a)

0 10 20 30 40 500

300

600

900

1200

0 10 20 30 40 50minus2

minus1

0

1

0 10 20 30 40 50minus5

0

5

10

0 10 20 30 40 50179

181

183

185

187

[ V A R ]

[ V s e c ]

[ V s e c ]

[

V ]


1

2

983769 1

983769 2

983769 1 983769 2

lowast

1 lowast2

Time[sec]

(b)


746

8132019 06063844


































test benches





983096983088983112983162














377

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

377

0

G

PCC

diff

10V

2 DERCs

are connected

V diff

4rad

4radsec

4radsec


0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 1

Q1

1

1000VAR

4radsec

4VsecV 1

2 DERCs

are connected


10383899830871103925907317 Y-axis 1983088983088983088


1 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast1

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)

0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 2

Q2

2

1000VAR

4radsec

4VsecV 2

2 DERCs

are connected


2 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast2

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)


747

8132019 06063844



CONTROLLERS

Main controller

PLL 983101 9830889830869830882983093 983101 1





983769 restoration 11 983101 22 983101 4983088983088














































synchronized

V CONCLUSION


















ACKNOWLEDGMENT



REFERENCES















748

8132019 06063844


0

0

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab


Islanded mode

Phase

synchronization

Grid-connected mode


Frequency

restoration

Voltage magnitude

equalizationtime

P-f Q-V

droop control

10msec200V

10msec200V

10msec200V

10msec200V

0








749

8132019 06063844


































test benches





983096983088983112983162














377

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

377

0

G

PCC

diff

10V

2 DERCs

are connected

V diff

4rad

4radsec

4radsec


0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 1

Q1

1

1000VAR

4radsec

4VsecV 1

2 DERCs

are connected


10383899830871103925907317 Y-axis 1983088983088983088


1 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast1

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)

0

020sec


Frequency

restoration

Phase angle

synchronization

Bypass switch

is closed

0

377

1000W P 2

Q2

2

1000VAR

4radsec

4VsecV 2

2 DERCs

are connected


2 X-axis 298308810383899830871103925907317 Y-axis

4110392598308710383899830871103925907317 983769 lowast2

X-axis 298308810383899830871103925907317 Y-axis 498308710383899830871103925907317)


747

8132019 06063844



CONTROLLERS

Main controller

PLL 983101 9830889830869830882983093 983101 1





983769 restoration 11 983101 22 983101 4983088983088














































synchronized

V CONCLUSION


















ACKNOWLEDGMENT



REFERENCES















748

8132019 06063844


0

0

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab


Islanded mode

Phase

synchronization

Grid-connected mode


Frequency

restoration

Voltage magnitude

equalizationtime

P-f Q-V

droop control

10msec200V

10msec200V

10msec200V

10msec200V

0








749

8132019 06063844



CONTROLLERS

Main controller

PLL 983101 9830889830869830882983093 983101 1





983769 restoration 11 983101 22 983101 4983088983088














































synchronized

V CONCLUSION


















ACKNOWLEDGMENT



REFERENCES















748

8132019 06063844


0

0

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab


Islanded mode

Phase

synchronization

Grid-connected mode


Frequency

restoration

Voltage magnitude

equalizationtime

P-f Q-V

droop control

10msec200V

10msec200V

10msec200V

10msec200V

0








749

8132019 06063844


0

0

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab

V Gab V PCCab

V DERC1ab V DERC2ab


Islanded mode

Phase

synchronization

Grid-connected mode


Frequency

restoration

Voltage magnitude

equalizationtime

P-f Q-V

droop control

10msec200V

10msec200V

10msec200V

10msec200V

0








749

06063844

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