Aalborg Universitet
Low-Voltage Ride-Through Operation of Power Converters in Grid-Interactive
Microgrids by Using Negative-Sequence Droop Control
Zhao, Xin; Guerrero, Josep M.; Savaghebi, Mehdi; Quintero, Juan Carlos Vasquez; Wu,
Xiaohua; Sun, Kai
Published in:
I E E E Transactions on Power Electronics
DOI (link to publication from Publisher):
10.1109/TPEL.2016.2570204
Publication date:
2017
Document Version
Early version, also known as pre-print
Link to publication from Aalborg University
Citation for published version (APA):
Zhao, X., Guerrero, J. M., Savaghebi, M., Quintero, J. C. V., Wu, X., & Sun, K. (2017). Low-Voltage Ride-
Through Operation of Power Converters in Grid-Interactive Microgrids by Using Negative-Sequence Droop
Control. I E E E Transactions on Power Electronics, 32(4), 3128 - 3142 .
https://doi.org/10.1109/TPEL.2016.2570204
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IEEE Transactions on Power Electronics
www.microgrids.et.aau.dk
Abstract—Due to the increasing penetration level of microgrids
(MGs), it becomes a critical issue for MGs to help sustaining
power system stability. Therefore, ancillary services, such as the
low-voltage ride-through (LVRT) capability should be
incorporated in MGs in order to guarantee stable operation of the
utility grid during grid faults. In this paper, a LVRT control
strategy based on positive/negative sequence droop control is
proposed for grid-interactive MGs to ride-through voltage sags
with not only inductive/resistive, but also complex line impedance.
By using the proposed control strategy, MGs can support the grid
voltage, make profits, and also ride-through the voltage dip
during the whole fault period. A two layer hierarchical control
strategy is proposed in this paper. The primary controller consists
of voltage and current inner loops, a conventional droop control
and a virtual impedance loop, while the secondary controller is
based on a positive/negative sequence droop scheme which is able
to coordinate the power injection during voltage sags.
Experimental results are obtained to verify the effectiveness of the
proposed control strategy.
Index Terms—Grid-interactive microgrid, hierarchical control,
low-voltage ride-through, negative sequence droop control.
I. INTRODUCTION
RIVEN by the economic, political, and environmental
issues, renewable energy sources (RESs), such as wind
turbines (WT) and photovoltaic (PV) arrays, combined with
energy storage systems (ESSs), such as batteries,
super-capacitors and flywheels, are integrated into the future
distribution networks such as microgrids (MGs) [1], as shown
in Fig. 1.
Manuscript received October 15, 2015; revised December 22, 2015;
accepted May 11, 2016. This work was supported by the Danish Energy
Technology Development and Demonstration Program (EUDP) through the
Sino-Danish Project “Microgrid Technology Research and Demonstration”
(meter.et.aau.dk), and by the International Science & Technology Cooperation
Program of China, project Number: 2014DFG62610.
X. Zhao, J. M. Guerrero, M. Savaghebi, and J. C. Vasquez are with the
Department of Energy Technology, Aalborg University, Aalborg 9220,
Denmark (e-mail: xzh@et.aau.dk; joz@et.aau.dk; mes@et.aau.dk;
juq@et.aau.dk).
X. Wu is with the Key Laboratory of Power Electronics for Energy
Conservation and Motor Drive, Department of Electrical Engineering,
Northwestern Polytechnical University, Xi’an 710072, China (e-mail:
wxh@nwpu.edu.cn).
K. Sun is with the State Key Laboratory of Power Systems, Tsinghua
University, Beijing 100084, China (e-mail: sun-kai@mail.tsinghua.edu.cn).
Fig. 1. General structure of a grid-interactive microgrid.
Thanks to advanced power electronic systems, MGs can not
only energize the local loads, but also deliver electricity with
high reliability and quality to the grid. However, due to the
continuously increasing capacity of grid-interactive MGs, these
small scale power systems now play a more crucial role than
ever before concerning low-voltage ride-through (LVRT)
capability. Unfortunately, most of the existing LVRT control
strategies [2]−[6] mainly focused on wind farms or large PV
plants, meanwhile the current LVRT practice of MGs is simply
disconnect them from the grid once faults are detected [7], [8].
This practice suffers from several drawbacks. The first is that
this passive strategy will not be an economical option, since the
MGs operate in islanded mode during the faults. As a
consequence, power generated in the MGs may be wasted, if,
for instance, ESSs are fully charged. The second is that the
abnormal voltage caused by disconnecting MG with the grid [9]
may lead to potential damage to the electrical equipment which
is not desired in both safety and power quality point of view.
The third drawback of isolating the MGs is that a reconnection
process, which may lead to excessive inrush current [10], is
required after the fault is cleared. Thus, to help MGs smoothly
Utility Grid
Photovaltic
WindTurbine
Battery
Flywheel
Microgrid
Renewable
Energy System
Energy Storage
System
+
-
+
-
V
o
V
i
+
-
+
-
V
o
V
i
Low-Voltage Ride-Through Operation of Power
Converters in Grid-Interactive Microgrids by
Using Negative-Sequence Droop Control
Xin Zhao, Student Member, IEEE, Josep M. Guerrero, Fellow, IEEE, Mehdi Savaghebi, Senior
Member, IEEE, Juan C. Vasquez, Senior Member, IEEE, Xiaohua Wu, Kai Sun, Senior Member, IEEE
D
IEEE Transactions on Power Electronics
2
over the faults without losing profits and to enhance power
system reliability, a LVRT control strategy is proposed in this
paper to aid the MGs not only ride-through the grid faults, but
also support the grid voltage, generate profits, and eliminate the
voltage abnormality during the whole fault period.
Conventionally, LVRT in PV plants is achieved by means of
controlling output current of the grid-interactive converter
which phase is synchronized with the grid by using
phase-locked-loop (PLL), meanwhile a current loop ensures
power injection accuracy and current quality issues [11], [12].
In [11], a reactive current injection (RCI) strategy is proposed
for single-phase grid-connected PV inverter. In this case, the
reactive current reference is set according to the grid code while
the active current is limited by considering the converter
capacity. LVRT of a three-phase PV converter has been
discussed in [13], due to the presence of asymmetrical grid
faults, negative sequence current is also injected to suppress the
negative sequence voltage. Besides, LVRT in wind farms
usually has the objective of injecting balanced three-phase
currents or nullifying the 100Hz power oscillations by
controlling the grid side converter (GSC) [13]−[15]. In [14], a
LVRT control strategy, which is the synthesis of
demagnetization and virtual resistance control, is proposed to
eliminate the disturbance of stator flux and limit the stator side
current. A flexible voltage support strategy is proposed in [6],
which can equip the converters with the capability of positive
sequence voltage recovery and negative sequence voltage
suppression. Later, a LVRT strategy, which has a similar
objective as that in [6], is illustrated in [16] by taking the
network impedance impact into account. However, all the
aforementioned control strategies are proposed for converters
operating in current-controlled mode, while the LVRT
capability for the widely used droop-controlled converters [11],
[17]−[18] in MGs is barely studied.
Moreover, the aim of this paper is to provide a LVRT
strategy for MGs which has different objectives with the LVRT
method utilized in the conventional PV/WT power plant. In
MGs, high power quality is usually required at the AC bus to
ensure the normal operation of the local electrical loads. This
means that it is critical to maintain both voltage and frequency
at the AC bus constant. However, in most of the publications
[2], [11]−[15], the line impedance, which has a great influence
on the amplitude and phase angle of the compensation voltage,
is simply deemed as pure inductive (medium/high-voltage grid)
or pure resistive (low-voltage grid). Therefore, the AC bus
voltage cannot be compensated accurately, and the AC bus
power quality cannot be maintained at a satisfied level during
voltage sags. In order to address this problem, complex line
impedance is considered to control the amplitude and phase
angle of the current injected by the converters.
Thus, to embed the voltage-controlled converters with
LVRT capability, negative sequence droop control is proposed
in this paper to make the MGs not only maintain connected with
the utility grid under voltage sags, but also support the grid
voltage by injecting positive and negative sequence power with
a satisfied power sharing accuracy among the distributed
converters.
Fig. 2. German grid code requirements. (a) LVRT capability and (b) reactive
power support capability.
The rest of the paper is organized as follows. In Section II, a
brief introduction to the existing grid code is presented to assist
designing of the proposed controller. Section III analyses the
voltage and current phasors by using symmetrical component
theory. Section IV shows the overall hierarchical control
scheme while the proposed LVRT controller is illustrated in
Section V. Section VI studies the small signal stability of the
proposed negative sequence control system. Section VII
provides the simulation and experimental results of a lab-scale
MG that consists of two parallel converters connected to the
grid. Finally, the conclusions are presented in Section VIII.
II. GRID CODE REQUIREMENTS
Conventionally, the distributed generators (DGs) are
required to disconnect from the grid when voltage sags occur
and to reconnect to the grid when faults are cleared [7], [8],
[19]. However, with the increasing penetration level of the
grid-interactive MGs, it is preferred that MG could also
maintain active power delivery and provide reactive power
support during the period of voltage sag, since it may alleviate
the potential instability problems. Thus, many countries have
revised their grid codes to accommodate with the increasing
capacity of RES. Spain, German and Denmark have already
published the LVRT and reactive current injection
requirements for grid-connected RESs in 2005, 2007 and 2010,
respectively [20]−[22]. As an example, the German E.ON NetZ
code is shown in Fig. 2 [21]. Although this requirement is
designed for high voltage grid, it is applicable to low-voltage
grid since they have similar concepts.
It can be seen in Fig. 2(a) that only when the grid voltage
falls below the red curve, DGs are allowed to disconnect with
the grid. Otherwise, DGs should inject a certain amount of
reactive power which is defined in Fig. 2(b). As shown in Fig.
2(b), when the grid voltage is lower than 0.9V
N
, 1% drop of the
Disconnect
upon Agreement
Stay
Connected
Time/s
Fault occurs
10%20% 40%-20%-40%
-100%
Deadband
Voltage Sag/Swell
(a)
(b)
k=(ΔI
B
/I
N
)/(ΔV/V
N
) ≥ 2
ΔV/V
N
ΔI
B
/I
N
Voltage (%)
80%
60%
40%
20%
0%
100%
-1 0 1 2 3
IEEE Transactions on Power Electronics
3
grid voltage requires at least k% increase of the injected current.
If needed, it should be capable of supplying 1 p.u. of reactive
current. The corresponding equations are given as follows.
0, 0.9
, 0.9 0.5 , 2
, 0.5
gN
g
ref N N g N
N
N g N
VV
V
I k k I V V V k
V
I V V
(1)
where V
N
is the nominal grid voltage, I
N
is the converter rated
current, ΔV is the depth of voltage sag, and ΔI
B
is the increment
of reactive current after fault occurs. Note that the constant k
should be no less than 2 p.u. according to the German grid code
[21].
III. VOLTAGE AND CURRENT PHASOR ANALYSIS BASED ON
SYMMETRICAL COMPONENTS DECOMPOSITION
According to the symmetrical sequence theory [23], the
instantaneous voltage/current can be represented by positive
sequence, negative sequence and zero sequence components.
0
0
0
aa
aa
b b b b
c c c c
xx
xx
x x x x
x x x x
(2)
where x
a
, x
b
, and x
c
represent the abc component of voltage or
current phasors, and superscript “+”, “−” and “0” denote the
positive, negative and zero sequence, respectively. Zero
sequence components are neglected, since the MG is
considered three-phase three-wire in this paper.
Based on (2), the instantaneous active and reactive power
can be expressed as
2
()p P P p v i v i v i v i
(3)
2
()q Q Q q v i v i v i v i
(4)
/ / /
v v v
(5)
/ / /
v v v
(6)
/ / /
T
i i i
(7)
where P
and Q
are positive and negative sequence
active and reactive power, p
2ω
and q
2ω
are oscillation terms of
active and reactive power, v
and i
represent for the
positive and negative sequence components of voltage and
current, respectively, and denotes the corresponding
orthogonal vector.
From (3) to (7), it can be seen that the active and reactive
power in positive and negative sequence are independent from
each other. Consequently, different control strategies can be
designed to separately control the positive and negative
sequence power regarding different applications, e.g.,
maximum positive sequence voltage restoration, maximum
voltage unbalance mitigation, and so forth. Note that the
presence of oscillating power (p
2ω
and q
2ω
) is due to the
interaction between voltage and current in different sequences.
The power flow between the converter and the grid is shown
in Fig. 3. In the figure, v
g
and v
c
are the grid voltage and
Fig. 3. Power flow diagram between the converter and the grid.
Fig. 4. Steady-state phasor diagram with only reactive current injection under
complex line impedance condition.
Fig. 5. Steady-state phasor diagram with both active and reactive current
injection under complex line impedance condition. (a) positive sequence
diagram and (b) negative sequence diagram.
converter voltage, respectively. Z
g
and Z
o
are the grid
impedance and the converter closed-loop output impedance,
respectively.
Generally, Z
g
and Z
o
are considered mainly inductive due to
the large output inductor [24]. However, this is not always true,
since Z
o
also depends on the adopted control strategy [25],
while Z
g
can be mainly resistive in low-voltage grids [18]. In
fact, MGs may locate far from the grid, where non-negligible
inductive and resistive line impedance may also present [26].
Due to the aforementioned facts, line impedance is considered
complex in this paper. In this case, if only reactive current is
injected during the voltage sag, the compensated voltage will
not be in phase with v
g
, as shown in Fig. 4. In this figure, v
sag
is
the depth of the voltage sag, v
Lg
is the voltage drop on grid
impedance, I
injected
and v
com
are the compensation current and
Z
g
I
g
LOAD
Z
o
I
L
I
o
v
c
δ
v
g
0
I
g
I
L
v
Lg
I
injected
v
sag
v
c
δ
I
o2
v
com
v
g
0
δ
I
o1
I
g
I
L
θ
sag
v
injected o
II
θ
(a)
(b)
I
injected
v
sag
v
Lg
v
c
δI
o2
v
com
v
g
0
δ
I
o1
com
v
IEEE Transactions on Power Electronics
4
Fig. 6. Hierarchical control scheme.
voltage, respectively, while I
o1
and I
o2
are the converter output
current before and after the sag, respectively.
Positive and negative sequence phasor diagrams with both
active and reactive current injection are illustrated in Fig. 5.
Note that θ is the angle of line impedance. In Fig. 5(a), phase
angle of the injected current is θ rather than 90°, i.e. the
converter should inject not only reactive power, but also active
power to support the positive sequence voltage. Meanwhile,
Fig. 5(b) shows that the angle between negative sequence
voltage and injected current is 180°−θ, i.e., the converter should
inject negative sequence reactive power and absorb negative
sequence active power simultaneously.
Thus, not only positive sequence active/reactive power, but
also negative sequence active/reactive power is needed to
recover and balance the load side voltage under asymmetrical
voltage sags. Therefore, by using the negative/positive
sequence droop control, which is proposed in Section V, the
converter voltage reference can be modified to control the
power injected by the converter.
Based on the abovementioned analysis, the injected current
during the voltage sag can be written as
2o L g
I I I
(8)
sag sag
g g g
gg
vv
I I I
ZZ
(9)
where I
g
+
and I
g
are the injected positive and negative sequence
current, respectively, v
sag
+
and v
sag
are the positive and negative
sequence sag voltage, respectively.
From (8) and (9), it can be seen that during the voltage sag,
the amount of positive and negative sequence current needed to
restore the positive sequence voltage or eliminate the negative
sequence voltage is inversely proportional to the grid
impedance Z
g
. Thus, an extra inductor is implemented between
the MG and the grid to limit the grid current and also to emulate
the leakage inductor of the isolation transformer. However, this
inductor will also induce extra voltage drop across it which is
not preferred in normal conditions. Consequently, its value
should be chosen carefully based on voltage and current ratings
of the system.
IV. HIERARCHICAL CONTROL SCHEME
In this paper, the grid-interactive MG includes several
voltage-controlled converters connected to the grid through LC
filters, as shown in Fig. 6. A hierarchical control algorithm
[27], [28] which consists of primary and secondary control is
proposed in this paper.
A. Primary control loop
As shown in Fig. 6, the primary controller includes
voltage/current inner loop, droop control loop and virtual
impedance loop. These three control loops are implemented in
two-phase stationary reference frame (αβ) to reduce
computational burden.
Both voltage and current controllers are based on
proportional+resonant (PR) controllers to provide satisfactory
tracking performance for positive and negative sequence
sinusoidal signals [29], [30].
PI based droop control [30], [31] is implemented during
normal operation mode, since the whole system operates in
grid-interactive mode. Note that the active and reactive power
is calculated in primary loop and is followed by low pass filters
with 5Hz cut-off frequency which can filter out the power
ripples.
A virtual impedance loop [28] is implemented to enhance the
power sharing accuracy among the distributed converters, and
also to make the system more damped without sacrificing
system efficiency.
B. Secondary control loop
Secondary control mainly contributes to the positive and
negative sequence power injection under voltage sags in terms
Secondary Control level
Current loop
PR Controller
PWM
Droop Control
with Power
Decoupling
Virtual
Impedance
_
i
L
v
C
i
o
Power
Calculation
V
g
V*
Three-phase
converter
L
con
Z
g
Isolation
Transformer
Grid
Local Load
More Converters
C
To Secondary
Controller
Primary Control level
Power Stage
Voltage loop
PR Controller
DC
link
+
_
_
+
+
Line impedance
I
g
V
g
I
g
Communication Link
Primary Controller Primary Controller
V
Ref+
V
Ref−
V
Ref+
V
Ref−
