Engineering, 2013, 5, 44-49
doi:10.4236/eng.2013.51b008 Published Online January 2013 (http://www.SciRP.org/journal/eng)
An Integrated Control Strategy Adopt ing Droop Control
with Virtual Inductance in Microgrid
Jianjun Su1, Jieyun Zhe ng 2, Demin Cui1, Xiaobo Li1, Zhijian Hu2, Chengxue Zhang2
1Dezhou Power Supply Company, Shandong Electric Power Group Co., Dezhou, China
2Scho ol of Electri cal Engineering, Wuhan University, Wuhan, China
Received 2013
ABSTRACT
As there exists sorts of distributed generators in microgrid, an integrated control strategy containing different control
methods against corresponding generators should be applied. The strategy in this paper involves PQ control and droop
control methods. The former aims at letting ge nerato rs like P V output maxi mu m power. T he latter stems from inverter
parallel technique and app lies to controlling generators which can keep the network voltage steady to make the parallel
system rea ch t he mini mu m ci r c ula tio n p o int. Due to the unworthiness of droop control applied in low-voltage microgrid
of which the impedance ratio is rather high, the paper adopts the d ro op contr ol i ntro duci ng vi rtual ge ner ator and vir tual
impedance. Based on theore tical anal ysis, simulatio n in Matlab is also implemented to verify the feasibilit y of the strat-
egy.
Keywords: Microgrid; Integrated Control; PQ Control; Droop Control; Virtual Impedance
1. Introduction
With the demand for energy expanding and the concer n
about global warming growing deep, more and more dis-
tributed generators including fuel cell, wind power and
photovo ltaic ce ll at al. get wid ely used [1] . Thus we come
to a definition of microgrid, namely, a network group
consisting of DG units which are on the distribution
network side and provide energy for local areas, energy
storage devices, and loads [2,3]. Microgrid mainly oper-
ates in two modes: grid mode and island mode.
The control mode of microgrid is classified into two:
principle-subordinate control and peer-to-peer control.
According to this thought, droop control which is based
on the droop characteristic of traditional grid is mainly
used [4,5].In the practical application of microgrid, nev-
ertheless, there is a diversity of DG units ranging from
PV to DG unit s. Due to this, distinct co ntrol methods are
designed for each kind of DG units, that’s integrated
control strate gy [6].
Reference [5] introduces virtual impedance, however it
doesn’t give a detailed description of the realization of
this improve ment.
This paper not only introduces virtual impedance into
the integrated control of microgrid, but also specifies on
its detailed realization. Depart from analyzing its feasi-
bility, the paper verifies the strategy b y simulation.
2. Structure of Microgrid Controllers
As can be seen from Figure 1, the controller of DG is
composed of the controller which compounds grid vol-
tage reference and grid current reference and UI tracking
controller. The pape r will specify on the former.
3. Integrated Control Strategy of Microgrid
When microgrid is connected into power distribution
network, every DG unit gets controlled with PQ control
method as the voltage and the frequency of the system
have been adjusted by infinite power grid and it’s the
most important for DG units to keep power balance
among each other. When microgrid is disconnected from
power distribution network, there’s a need to maintain
the a mpli tud e and freq uenc y o f t he vol tage o f po wer gr id
Figure 1. Control diagram of droop control with virtual
inductance in Microgrid system.

J. J. SU ET AL.
45
As a result, droop control is adopted to provide reference
for grid volta ge a nd its frequenc y.
3.1. PQ Control Method
PQ control method is based on the feed-forward de-
coupling of dq transformation and realizes maximum
power output of DG by adjusting active current and reac-
tive current to track reference current [7].
The equation of reference current is:
ref
dref
d
ref
qref
d
P
iU
Q
iU
=



= −


(1)
Then by feed-forward decoupling of quadrature direct
axis current, we get reference for outer loop voltage.
That’s to make the inverter reach the reference output
power using the classical voltage and current dual-loop
control.
3.2. Droop Control Method
As DG units are connected to PCC with isolating trans-
formers, DC components of their injection currents are
no more than 5 percent of rated output currents [8]. The
sketch diagram of power transmission of a DG is shown
as Figure 2.
Where, P oint A is the output point of a DG unit whic h
is consisted of a DG and its filtering system. Point B is
the input point of power distribution network. The im-
pedance of the intermediate transmission line is
L
ZR jX= +. Assume that the injected power of point A
is
SP jQ= +
, the output power of the DG can be ex-
pressed as fo l l ows.
1 122
22
[ (cos)sin]E REEEX
PRX
δδ −+
=+
(2)
1 122
22
[(cos )sin]E XEEER
QRX
δδ −−
=+
(3)
The feasibility of droop control is based on the line
being inductive. The precondition is often satisfied by
setting the parameters of the dual-loop c ontroller without
virtual impedance [9]. The equivalent output impedance
of the inverter is
AB
S=P+jQ
Z L =R+jX
E
1 ∠δ E
2 ∠0
Figure 2 . Power trans mission di agram.
2
32
(1 )
()
pwmpwmppwm i
Ls
LCskkCskk kskk k
Zs
+ +++
=
(4)
where, k is the scale parameter of current control in dual-
loop control,
p
k
and
i
k
are the scale parameter and
integral parameter of voltage control respectively.
pwm
k
is the magnification coefficient of the inverter and we
can take that
2
dc
pwm
V
k=. We can choose parameters
which make Z(s) inductive in the frequency range of
50Hz and the i nductive ra nge is not that wide .
3.3. Droop Control Method with Virtual
Impedance
As microgrid is mostly low voltage network, the imped-
ance ratio of its lines is rather large. From (3), we know
that with large impedance ratio, the value of
1 12
(cos )EX EE δ −
is relatively small, while the influ-
ence of
12 sinEER
δ
which is also affected by active
power increases.
In the parallel s ystem o f DG units, whe n the o utput ac-
tive power differs from each other, it may occur that
some DG units absorb reactive power while others re-
lease [10]. To solve this problem, we need to decouple P
and Q. One solution is to make use of coordinate trans-
formation [11], but it involves impedance ratio which is
difficult to acquire sometimes. One solution is to design
parameters of the controller to turn output impedance
inductive which nevertheless is at the mercy of the para-
meter design of dual-loop control. Besides those, the
solution of introducing virtual impedance should be the
best.
The advanced droop control embodies the thought of
equivalent control. According to it, a DG in microgrid is
equivalent to a virtual generator with virtual impedance,
which is sho wn in Figure 3.
Reference [5] has specified on the principle and the
feasibility of the advanced method. This paper will tell
its detailed r e a liz a tion.
Figure 3. Equivalent Microgrid system with virtual induc-
tanc e .

J. J. SU ET AL.
46
4. Realization of Advanced Droop Control
4.1. Structure of Droop Control with Virtual
Impedance
The control model of droop control is made of three parts:
dq transformation and reference power compound, ref-
erence voltage and frequency compound, voltage and
curr ent dua l-loop co ntro l. According to equivalent thought,
we have
2
Q QIX
ξξ
= −
(5)
The instant reference voltage of original DG is
*/ C
DG DG
C
Ve Ldidt
s
ξξ
ω
ω
= −+ (6)
where,
C
C
s
ω
ω +
is the low-pass filter for re straining high-
freq uenc y noise in the virt ual line .
4.2. Simulation of Droop Control with Virtual
Impedance
In the power compound module, introduce the output
current of DG units. Compound output reactive power
Q
ξ
of the virtual generator with the dq components of
the output voltage of DG units. Meanwhile, cross mul-
tiply the dq components of output current to get
2
I
and
multiply by X
ξ
. On the basis of the logical relation in
(6), we get reference reactive power for DG units.
The active power compound module needn’t modifi-
cation as
PP
ξ
=
. According to (6), on the basis of orig-
inal dq components of reference voltage, minus respec-
tively the voltage drop of output currents down the vir-
tual impedance and get new reference output voltage.
Where, the drop is achieved by dq transforming the out-
put current, going through the differential part and a
transfer function of a filter with a filtering capacitor, fi-
nally going t hrough a p roportional el ement of
L
ξ
.
According to equi valent princi ple, we have
22
()( )
DG DG
DG DGDG
QX PXQX
EE E
EE E
ξξ ξ
ξ
= ++≈+
(7)
Dep ending on the t hought o f averaging, thus we have
max min
max minmax min
1[() ()]
2
DG DGDG DG
Q Q
EE EX
EE
ξξ
=++ +
(8)
According to parameters in simulation, assume
100 L mH
ξ
=
,
567 EV
ξ
=
.
5. Simulation Analysis
Establish a model of a system having 4 paralleled DG
units shown in Figure 4.
Figure 4 . S tructure of simulati on model.
PV resources are both controlled with PQ control me-
thod. Energy storage device (ESD) is both controlled
with droop control method and advanced droop control
method. The advanced method sets the line between DG
and PCC inductive by designing the parameters of the
controller, while the original realizes power decouple by
introduc ing virtual impedance.
The voltage of two ESD is 800 V. Their rated power is
1 kW. Reference output voltage is 380 V. Filtering in-
ductance is 50 mH and capacity is 20 μF. With regard to
line impedance,
0.641 /
L
R km= Ω
, 0.101 /
L
X km=Ω
and the length is 50 m. The PWM carrier frequency is
6000 Hz.
The action time of switches is: the illumination inten-
sity of PV decreases at 0.167 s, microgrid becomes island
mode at 0.3 s, Load 4 is applied at 0.5s and cut at 0.8 s,
microgrid is again con nected with distributio n network at
1s. Simulation step:
5
5 10 s
−
×
. Simulation algorithm:
ode23. Simulation time: 2 s.
5.1. Simulation Analysis of Integrated Control
According to PQ control principle, for droop control,
1
p
K=
,
0.5
i
K=
, as to current control,
5K=
. For P Q
control,
155
p
K=
and
1
i
K=
.
Simulation results are as follows. Figure 5(a) and
Figure 5(b) are the output voltage of ESD 1 and PV 1
respectively.
As can be seen, in grid mode, due to the effect of the
voltage of distribution network, the waveforms are steady
and fluctuate little when illumination changes. At 0.3 s
microgrid get s into island mode, the voltage deceases but
becomes stable immediately. The amplitude of voltage
changes but keeps sine curve at 0.5 s and 0.8 s when
Load 4 is applied and cut. Th is s ugge sts t hat o utp ut vo ltage
of DG can be controlled instantaneously to newly be-
come steady with integrated control strategy. The wave-
forms of microgrid getting into island mode are shown in
Figure 6, taking ESD 1 and PV 1 for example.

J. J. SU ET AL.
47
The output active power and reactive power of each
DG unit is shown in Figure 7. As it can be seen, the
power curves of two energy storage device are almost the
same and immediately reach a steady value whenever the
switches act, which will provide reference voltage for
controlling two PV units. As PV2 is close to Load 4, its
waveform is a little different from the one of PV1. But
they all become steady swiftly, which suggests that PQ
control is fit for PV.
5.2. Simulation Analysis of Integrated Control
Adopting Advanced Droop Control
Modify the simulation mod el acco rd ing to 3.2 . T his time,
0.1
p
K=
,
0.008
i
K=
, as to current control,
5K=
. As
to PQ control, for PV1,
160
p
K=
and
1
i
K=
. For PV2,
173
p
K=
and 1
i
K=.
Figure 8(a) and Figure 8(b) are the filtered output
voltage of ESD 1 and PV 1 respectively.
(a) (b)
Figure 5 . O utput v o ltage o f each di stributed generator.
(a) (b)
Figure 6 . Waveform of output vol tage.
(a) (b)
Figure 7 . Output power of each DG.

J. J. SU ET AL.
48
(a) (b)
Figure 8 . O utput v o ltage o f each di strib uted generator.
(a) (b)
Figure 9 . Waveform of output vol tage.
(a) (b)
Figure 1 0. Output power of each DG.
The adjusting is swift, their waveforms of voltage
when switches act are shown in Figure 9. The output
active power and reactive power of each DG unit is
sho wn in Figure 10.
With virtual impedance, the power of PV units fluc-
tuates little. Besides, in comparison with the reactive
power of original droop control, the fluctuation range of
the energy storage device greatly reduces, which indi-
cates that output power is better controlled by integrated
control strategy with advanced droop control.
6. Conclusion
Upon parallel system of microgrid with energy storage
device and PV, this paper makes a study of integrated
control strategy depending on characteristics of diverse
DG units. What’s more, virtual impedance is introduced
into droop control to get rid of the restrain of original
control on the line impedance. Besides that, specific rea-
lization is given in the paper. The results of simulation
demonstrate s that, the new int egrated contro l strateg y can
keep output voltage of DG units steady in grid mode and

J. J. SU ET AL.
49
island mode, and realize swift transition between the two
modes. Furthermore, with virtual impedance, the strategy
can better guarantee the stability of output reactive power
of each DG uni ts to realize the better decoupling control
of active power and reactive power.
7. Acknowledgements
This work was financially supported by the Ph.D. Pro-
grams Foundation of Ministry of Education of China
(20110141110032).
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