Uninterrupted Switching based on VSG Control
between Grid-connected and Stand-alone
Operation of Single-Phase Grid-Tied Inverter
Kodai Nishikawa1, Keisuke Kusaka2 and Jun-ichi Itoh1
1 Department of Science of Technology Innovation, Nagaoka University of Technology, Nagaoka, Japan
2 Department of Electrical, Electronics and Information Engineering, Nagaoka University of Technology, Nagaoka, Japan*E-mail: knishikawa@nagaokaut.ac.jp, kusaka@vos.nagaokaut.ac.jp, itoh@vos.nagaokaut.ac.jp
Abstract— This paper proposes an uninterrupted
switching method between a grid-connected operation and astand-alone operation by the output-frequency command
switching of a single-phase grid-tied inverter. The proposed
switching is characterized by a virtual synchronous
generator (VSG) control that switches from a gridconnected operation to a stand-alone operation. The VSG
control achieves the uninterrupted switching. However, theVSG control makes a frequency fluctuation during thestand-alone operation. In addition, it is impossible to lockthe voltage phase. This is the cause of inrush current at theswitching to the grid-connected operation. The proposed
switching method uses multiple control systems dependingon the operations. A constant voltage constant frequency
control is applied to suppress the frequency fluctuation
during the stand-alone operation. Moreover, a phase-locked
loop is applied to suppress the inrush current at theswitching to the grid-connected operation. The proposed
switching method is demonstrated by the experiments witha 5-kVA prototype. The experimental result shows thatuninterrupted switching is achieved by the proposed
switching method.
Keywords— Single-phase inverter, Grid-tied inverter,
Stand-alone operation, VSG control

I.

INTRODUCTION

In recent years, the introduction of renewable energies
such as wind and solar power generation has been
expanding. Then, the use of grid-tied inverters has been
increasing for a grid-connection of the power generationequipment to the power grid. Grid-tied inverters
generally control the output current with a gridconnection [1–3]. On the other hand, the inverters control
the output voltage by a constant voltage constantfrequency (CVCF) control in a stand-alone operation [4–5]. Thus, it is necessary to switch the control between thecurrent control and the voltage control system in order toswitch between grid-connected and stand-alone
operations. The control switching causes distortions andinterruptions of the output voltage.
A virtual synchronous generator (VSG) control is oneof the control methods to achieve uninterrupted switching
between the grid-connected and stand-alone operations[6–7]. The VSG control not only improves grid stabilities
by the simulation of inertia of the synchronous generator[8–12] but also connects to the power grid using thevoltage control system. Thus, the control systemswitching is not required for the operation switching.
However, the frequency fluctuation occurs in the output
frequency. In addition, the voltage surge occurs in theload voltage. The effect of the frequency fluctuation is aninrush current between the grid-tied inverter and thepower grid. The inrush current may occur with switching
to the grid-connected operation because the output phase
is not locked to the grid phase by a phase-locked loop
(PLL). This cause is the impossibility of arbitrary
frequency control because the output frequency iscontrolled according to the power deviation between the
power command and the output power. The voltage surge
may occur with switching to the stand-alone operation
because the voltage of the grid-tied inductance fluctuates
due to the sudden change of the inverter output current by
the disconnection from the power grid. This cause is theimpossibility of the power grid current control because
there is the power deviation between the load power andthe output power. These power deviations occur because
the load power is unknown.
In this paper, the uninterrupted switching method is
proposed using the frequency command switching. The
proposed method uses multiple voltage control systems
corresponding to the operation modes, such as the gridconnected or the stand-alone operation. Thus, arbitrary
frequency control is possible. The VSG control is used on
the grid-connected operation and the switching to standalone operation. The power command of VSG isdetermined by the supply power from the power grid tothe load. Thus, the load voltage surge is decreased at thedisconnection from the power grid. The CVCF control isapplied to the stand-alone operation. Furthermore, the
PLL is applied for the switching from the stand-alone
operation to the grid-connected operation. The output
frequency is determined within any allowable frequency
fluctuation. The uninterrupted switching operation isdemonstrated by the proposed method with a 5-kVA
prototype.
II.

UNINTERRUPTED SWITCHING CONTROL

Figure 1 shows the circuit configuration of a singlephase grid-tied inverter to which the proposed switching
method is applied. This circuit has a DC power supply on
the input side and supplies power to the load on theoutput side. In addition, a magnetic contactor (MC) is
connected between the load and the power grid on the
output side for disconnecting and synchronizing to thepower grid.
A. VSG control
Figure 2 shows the synchronous generator model usedby the VSG control. The VSG output power Pout is
controlled to the output power command Pcommand by thevoltage controller.
Figure 3 shows the voltage phasor diagram of themodel shown in． Fig. 2. Equation 1 presents the output
current phasor Iout of the synchronous generator.
VSG sin q − j ( VSG cosq − VGrid )(1),
I out =
w LSG
where VSG is an absolute value of the synchronous
generator voltage phasor, VGrid is an absolute value of thepower grid voltage phasor, q is a phase difference
between VSG and VGrid, w is an angular frequency of thepower grid, LSG is the synchronous inductance of thesynchronous generator. Thus, the output active power P
and reactive power Q by the synchronous generator isexpressed as
V V sin q(2),
P = SG Grid
2w LSGQ=

VSG VGrid cosq − VGrid

Lout
VDC

Cfil

Fig. 1. Circuit configuration of single-phase grid-tied
inverter with MC for the operation switching between
grid-connected and stand-alone operation.
Synchronous
generator

PcommanddwR Pcommand − Pout − D (wR − wGrid )
=
dt
J

Pout

where wR is an angular frequency of output voltage, wGrid
is an angular frequency of power grid, Pcommand is a powercommand value, Pout is output power, J is the virtualinertia, and D is a damping coefficient.
The phase difference q is changed by increasing or
decreasing the output angular frequency wR. The output
power is controlled by changing the output angularfrequency.
B. Conventional Switching Method
Figure 5 shows the uninterrupted switching method
between the grid-connected and stand-alone operation
using the VSG control proposed in [7]. The speed
regulation factor and the time constant of the governor
low-pass filter are expressed  and TG, respectively. In
the control method of [7], the power command value isswitched following the switching between grid-connected
and stand-alone operations. In the grid-connected
operation and the uninterrupted switching to the standalone operation (Spower = 1), the inverter outputs thepower according to the power command value. In thestand-alone operation (Spower = 3), the output frequency

GridFig. 2. Synchronous generator model that is connected
to a power grid via a grid-tied inductor.
・

VSG
・

VLSG
・

VGrid

q

Fig. 3. Voltage phasor diagram of the synchronous
generator model.

D
Pcommand

+

-

-

(4),

LSG

SG

(3).

Figure 4 shows the VSG control block diagram for theswing equation of the synchronous generator. The output
frequency command is determined from the deviation
between the power command value and the output power.
Equation 4 presents the swing equation of thesimulated synchronous generator.

VGrid
R

2

2w LSG

MC

Lfil

Pout

1
Js

Dw + wR
+

1/s

qR

wGrid

Fig. 4. VSG control block diagram.

changes by the speed regulation rate of the governor
according to the difference between the power command
value and the output power. Then, a load frequency
control is applied to fix the difference from the power
grid frequency. In the uninterrupted switching operationfrom stand-alone operation to grid-connected operation(Spower = 2), the output frequency is synchronized to thepower grid frequency. At the same time, the powercommand value is added to DwR for making the momentof phase synchronization for switching to the gridconnected operation.
The output power during stand-alone operation isdetermined by the required power of the load in the VSG
control. The output frequency fluctuates because there isa power difference between the output power commandand the output power. Thus, an additional control system
is required to suppress fluctuations, such as the loadfrequency control used by the conventional method. In
addition, the output frequency is not directly controlled
by the conventional control method. It is impossible tofix the output voltage phase with the power grid when

Gridconnected
operation

Pout*

DwR
Matching the gridfrequency control

Load Frequency
Control

Governor

wR
wO

wGrid

wR
+

wO

Pout

-

+

PI

wR

wR - wO
wO

2
3

Spower+PI-

Power command
switching in [7]

1

+

+

VSG control+

0.5Pmax

100

D- + Pcommand

1
1+TGs



+ -

-

Dw + wR

1
Js

wGrid

Pout

wO : Rated frequency

+

1/s

qR
PLL

vGrid

Fig. 5. Conventional uninterrupted switching method between the grid-connected operation and the stand-alone operationin [7] using the output power command switching.
Q-V droop control+

Qcommand: 0

KQ

Qout

VGrid ++

1, 3

V: 200 Vrms

Proposed control block

Sfreqency

vCONV = Vout sinqR

VSG control
Power command
generator

IGrid+Pcommand + +
-

Pout

VGrid

Dw +

1
Js

wR
+w

Grid

1

Sfreqency

1p.u.(50 Hz) 2(wPLL)

q
VGrid

1

Qout

÷

PWM
Generetor

Inverter

VDC

qR

Lout

Sin
table

Cfil

q
+ - Grid

Lfil

PLL(Fig. 7)

Square root
table

+

Load

100 Hz

50 Hz

BPF

BPF

iGrid
MC

2w(Lout+Lfil)
Vout

Qout calculation(Eq. 3)

1/s

wPLL 3Sin
table

Pout calculation(Eq. 2)vCONV

Frequency command
switchingD

KPcommand

VDC

Vout

2

1

2w(Lout+Lfil)

+

-

IGrid

Cos
table

Square root
table

+

100 Hz

50 Hz

BPF

BPF

vGrid

Amplitude acquisition

Fig. 6. Experimental control block diagram with proposed uninterrupted switching method using the frequency command
switching and the power command generator.
switching from the stand-alone operation to the gridconnected operation. Thus, it is necessary to switch when
the output voltage phase is synchronized with the power
grid. An inrush current is caused by the voltage
difference between the output voltage and the power gridvoltage at the moment of synchronization. The voltage
difference occurs because there is a delay in thesynchronization with the power grid due to the use of MC.
C. Proposed switching method
Figure 6 shows the control system of the single-phasegrid-tied inverter with the proposed uninterrupted
switching method using the output frequency command
switching. In the conventional method, the output powercommand is switched at the time of the operation
switching. Hence, there are the problems such as theoutput frequency fluctuation during the stand-alone
operation and that phase synchronization is not possible
during synchronizing. The output frequency command isdirectly switched according to the operation in the
proposed method. Thus, the suppression of the output
frequency fluctuation and the phase synchronization areachieved.
The explanation of each operation is described below.
(i) Output power control during grid-connected operationand switching to stand-alone operation (Sfrequency = 1)
The output power is controlled by VSG control. In thiscase, the power command value Pcommand is expressed asfollows from the grid current amplitude IGrid and thephase difference between the grid voltage and the output
voltage.
Pcommand = K Pcommand I Grid VGrid +Vout VGrid

2 f Grid ( Lout + L fil )sin q (5),where KPcommand is the gain that makes the product of
current and voltage to power, VGrid is the grid voltage
amplitude, fGrid is the power grid frequency, Lout is a grid-

tied inductance, Lfil is a filter inductance, and q is thephase difference between the power grid and the output
voltage.
The output current of the inverter cancels the gridcurrent iGrid because Pcommand is kept constant and thephase difference q is increased. This Pcommand makes it
possible to suppress the voltage surge that occurs when
the load is disconnected from the power grid.
(ii) Output voltage control during stand-alone operation(Sfrequency = 2)
The output voltage command of the inverter isdetermined by a constant output frequency and a constantvoltage amplitude during stand-alone operation. The
voltage is output without frequency fluctuation due to theeffect of VSG control.
(iii) Switching from stand-alone operation to gridconnected operation (Sfrequency = 3)
There is a phase difference between the inverter output
voltage and the grid voltage since the inverter is notsynchronized with the power grid during stand-alone
operation. Hence, it is necessary to synchronize the phaseof the inverter output voltage during switching fromstand-alone operation to grid-connected operation. In the
proposed method, phase synchronization is achieved byusing a zero-crossing detection-based PLL.
Figure 7 shows the block diagram of the frequency
command generation using the zero-crossing detection
PLL. The zero-crossing detection-based PLL shown in
Figure 7 (a) detects the zero-crossing of the grid voltage
and obtains the phase difference fR from the output
voltage phase during stand-alone operation. The phase is
synchronized by generating the output frequency so thatthe phase difference becomes zero using the PI controller
shown in Figure 7 (b). Thus, it is not necessary to turn on
the MC in a moment when the phases of the output
voltage and the grid voltage overlap as in theconventional method. The MC operation delay is not a
problem for the uninterrupted switching.
Assuming that the power grid frequency is constant,
the time required for phase synchronization is mainly
determined by the proportional gain and the allowablefrequency fluctuation. The allowable frequency
fluctuation Dwmax Dwmin is determined by the assumed
load condition. The operating time in the region where
the proportional controller output is saturated Dtdiff_psat isexpressed as
Dtdiff_psat =1 f0 − fborderf Grid Df psat

(6),

where f0 is the initial phase difference between the output
voltage and the power grid voltage at the moment of thesynchronization start, fborder is the boundary phasedifference of whether the proportional controller output issaturated and Dfpsat is the amount of change in the phasedifference per cycle of the power grid frequency. The
boundary phase difference fborder and the amount ofchange in the phase difference per cycle Dfpsat are
expressed such as

Output voltage

Grid voltage

VmaxsinfR
0V

Zero-cross detectionfR(a) Conceptual diagram

Controller

fR_command
0 rad

+
-

Kp_PLL1s

0++

wRated

1
s

qGrid
wPLL

Dwmax (50+ Hz) w- Grid Circuit
+
Dw +
1 fR

Dwmin

s

Ki_PLL==(b) Control block
Fig. 7. Frequency command generation method by thezero-cross detection-based PLL.
fborder =Dwmax
K p _ PLL

(7),

Df psat =Dwmax + wRated − wGrid
fGrid

(8),

where Kp_PLL is the proportional gain of the PLL and
wRated is the rated angular frequency of the power grid.
The operating time in the region where the integral
controller is working Dtdiff_pi is estimated by the
proportional gain. In this case, the amount of change inthe phase difference per cycle Dfpi is expressed as
Df pi =

K p _ PLLfR + w Rated − wGrid
f Grid(9).

The operation of (9) is repeated N times until the phasedifference q reaches the phase synchronization threshold
qThereshold. The operating time Dtdiff_small is expressed as
Dtdiff_pi =Nf Grid(10).

The estimated operation time is the sum of Dtdiff_psat andDtdiff_pi.
The amplitude values of the power grid voltage andcurrent are acquired only for the 50 Hz component using
a bandpass filter (BPF). The output voltage command isdetermined by the amplitude and phase. The output
voltage is controlled by an open-loop voltage controller.
The procedure for switching from the start of the gridconnected operation to the stand-alone operation is asshown below.
(1) The power grid frequency and phase is obtained by
PLL.
(2) The operation of the grid-tied inverter is started with
the Sfrequency being one at the moment when the power gridphase becomes zero rad.

(3) Sfrequency is switched from one to two at the moment
when the current amplitude of the power grid IGrid
becomes equal to or less than the switching thresholdvalue IGrid_Threshold. The operation is switched to the standalone operation.
(4) The MC is turned off.
In this procedure, the reason why the operation starts atzero rad is to prevent the inrush current caused by thevoltage difference from the power grid voltage.
The procedure for switching from the stand-alone
operation to the grid-connected operation is shown below.
(1) Sfrequency is switched from two to three. The output
voltage phase and the output frequency are synchronizedwith the power grid by PLL.
(2) The MC is turned on at the moment when the phasedifference with the grid voltage q becomes equal to orless than the threshold value qThreshold.
III.

Table 1. Experimental condition.
Rated PowerP rated
5 kVA
350 V

Grid voltage

V Grid200 VRMS

Utility frequency

f Grid50 Hz

Switching frequency

f SW

10 kHzGrid-tied inductor

L out

2 mH

Filter inductor

L fil

340 H

Filter capacitor

C fil

6 F

Percent impedance

%Z

7.9%

Percent admitance

%Y

1.5%

Damping coefficientD

0.048 J/rad

Vertual Inertia

J

0.055 kgm

Q -V droop gain

KQ

0.1 V/var

K Pcommand

0.5p.u.

I Grid_Threshold

5A

q Threshold

0.015 radPLL
PI gain

K p_PLL

62.8 s

K i_PLL

987 s-1

Rated grid frequency

w Rated rad/s

Allowable frequency
fluctuation

Dw max2 rad/s

Dw min-2 rad/s

Power command
generator gain
Threshold ofgrid current
Threshold ofphase different

Table 1 shows the experimental conditions, and Table
2 shows the control parameters. The switching operations
from grid-connected to stand-alone operation and fromstand-alone to grid-connected operation are confirmedwith the 5-kVA prototype with the proposed switching
method.

B. Switching from stand-alone operation to gridconnected operation
Figure 10 shows the waveforms when the operation isswitched to a grid-connected operation. The output
voltage phase is not synchronized with the phase of thegrid voltage during stand-alone operation. Then, a
synchronization operation is performed by PLL. After thephase synchronization, the MC is turned on to switch
grid-tied operation and supply power to the load by thepower grid. In Figure 10 (a), the output current is reduced
to almost zero amperes by the switching operation. Thisis because the phase difference between the grid voltage

V DC

Table 2. Controller parameter.

EXPERIMENTAL RESULT

A. Switching from grid-connected operation to standalone operation
Figure 9 shows the waveforms when the operation isswitched to a stand-alone operation. Before the inverter
operation starts, the power is supplied to the load fromthe power grid. The grid current iGrid corresponds to theload.
The inverter is connected to the grid and starts
supplying power to the load. The waveform shows thatthe inverter output current starts to increase at the zerocrossing of the grid voltage. Thus, the inverter starts
operation at the grid phase of zero rad. Also, the gridcurrent amplitude is reduced as the inverter output
current amplitude increases.
Next, the MC turns off and switches to stand-alone
operation when the grid current iGrid is reduced to thethreshold IGrid_Threshold. However, the amplitude of the gridcurrent is less than 5 A, which is the threshold of the gridcurrent amplitude about 40 ms before the turn-off signal
is output. This is due to an error in the current amplitudedetection.

DC-link voltage

Inverter: ON
0V

0V

2

-1

MC: OFF
20 ms
MC signal [-]Grid voltage [300 V/div]
Grid current [20 A/div]

0A
0A

IGrid_Threshold
Output current [20 A/div]
20 ms/div

Sfrequency = 1 MC Signal: OFF Sfrequency = 2

Fig. 9. Switching operation waveform from the grid-connected operation to the stand-alone operation.
and the output voltage is around zero rad. It takes 25 msfrom the time the turn-on signal is output to the MC tothe time it turns on. However, the phase synchronization
by PLL makes it possible to switch to the grid-connection
without inrush current due to phase difference. In Figure
10 (b), the synchronization is completed in about 580
millisecond after the start of synchronization under thecondition that there is a phase difference of about  rad.
when the PLL synchronization is started.
C. Uninterrupted switching
Figure 11 shows the waveforms during operation
switching with the load voltage vLoad. The power is
supplied to the load continuously after the MC is turned

MC:turn-on
25 ms

MC signal [-]0V

Phase difference:about  radStart
synchronization
MC:turn-on

580 msPhase difference, q [ rad/div]

Grid voltage, vGrid [300 V/div]

0V

0V

Grid voltage, vGrid [300 V/div]

0V

Grid current, iGrid [20 A/div]
0A

0A

0A

0A

Output current, iLout [20 A/div]
20 ms/div

Grid current, iGrid [20 A/div]

Output current, iLout [20 A/div]

100 ms/div

Sfrequency = 3

MC signal:onSfrequency = 2

Sfrequency = 3(a) Switching to the grid-connected operation(b) Phase synchronization control by the PLL
Fig. 10. Switching operation waveform from the stand-alone operation to the grid-connected operation.
MC: OFF

MC: OFF
Grid voltage, vGrid [300 V/div]

0V

Grid voltage, vGrid [300 V/div]
0V

Grid current, iGrid [20 A/div]

0A

Load voltage, vLoad [300 V/div]

0V

Grid current, iGrid [20 A/div]

0A
0V

Load current, iLoad [20 A/div]

0A

Load voltage, vLoad [300 V/div]

0A

Load current, iLoad [20 A/div]

5 ms/div

Sfrequency = 1

5 ms/div

Sfrequency = 2

Sfrequency = 3(a) Switching to the stand-alone operation(b) Switching to the grid-connected operation
Fig. 11. Uninterrupted switching operation waveform.
MC: OFF

MC: OFF

Grid voltage, vGrid [300 V/div]

Grid voltage, vGrid [300 V/div]

0V

0V
0A

Grid current, iGrid [20 A/div]
Load voltage, vLoad [300 V/div]

Grid current, iGrid [20 A/div]

0A

Load voltage, vLoad [300 V/div]

0V

0V

0A

0A

Load current, iLout [20 A/div]

Load current, iLout [20 A/div]

5 ms/div

5 ms/div(a) Grid current value at MC turn-off is 35 A(b) Grid current value at MC turn-off is 20 A
Fig. 12. Comparison of the load voltage surge by the difference current at MC turn-off.

on or off. The load voltage waveform at the moment ofswitching is used to confirm uninterrupted switching.
Figure 11 (a) shows that there is no surge or interruption
in the load voltage at the moment of switching from gridconnected to stand-alone operation. Similarly, Figure 11(b) shows that there is no surge or interruption in the load
voltage at the moment of switching from grid-connected
to stand-alone operation.
Figure 12 shows the load voltage surge without surge
reduction power command at the moment of switchingfrom grid-connected to stand-alone operation. In Figure
12 (a), the voltage surge value is about 90 V. In Figure 12(b), the voltage surge value is about 60 V. It is checkedthat a large load surge voltage is generated when the gridcurrent is not reduced.
These results show that the uninterruptible switching
between grid-connected and stand-alone operations isachieved by the proposed method by switching the output
frequency command.
IV.

CONCLUSION

In this paper, the uninterrupted switching method is
proposed using the frequency command switching for the
purpose of supplying power to the load at the operation
swathing without interruption. The proposed method uses

multiple voltage control systems such as the VSG control
and the CVCF control. It solves the problems of theconventional VSG control, such as the frequency
fluctuation during the stand-alone operation and phasesynchronization at switching to the grid-connected
operation. In addition, the inrush current due to a gridconnected delay of MC is prevented. The uninterrupted
switching of the proposed method is demonstrated by 5kVA prototype verification.
In the future, we will study the application of thismethod to nonlinear loads.
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