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Received August 7, 2021, accepted August 26, 2021, date of publication August 30, 2021, date of current version September 9, 2021.
Digital Object Identifier 10.1109/ACCESS.2021.3109050
A Comprehensive Review of Control Strategies to
Overcome Challenges During LVRT in PV Systems
JYOTI JOSHI 1, ANURAG KUMAR SWAMI1, VIBHU JATELY 2, (Member, IEEE),
AND BRIAN AZZOPARDI 2, (Senior Member, IEEE)
1Department of Electrical Engineering, College of Technology, G. B. Pant University of Agriculture and Technology, Pantnagar 263145, India
2MCAST Energy Research Group, Institute of Engineering and Transport, Malta College of Arts, Science and Technology, PLA 9032 Paola, Malta
Corresponding author: Jyoti Joshi (jjyotij25@gmail.com)
This work was supported in part by the European Commission H2020 TWINNING Joint Universal activities for Mediterranean PV
integration Excellence (JUMP2Excel) Project under Grant 810809.
ABSTRACT Due to the high penetration of grid-connected photovoltaic (GCPV) systems, the network
operators are regularly updating the grid codes to ensure that the operation of GCPV systems will assist in
maintaining grid stability. Among these, low-voltage-ride-through (LVRT) is an essential attribute of PV
inverters that allows them to remain connected with the grid during short-term disturbances in the grid
voltage. Hence, PV inverters are equipped with control strategies that secure their smooth operation through
this ride-through period as per the specified grid code. However, during the injection of reactive power
under LVRT condition, various challenges have been observed, such as inverter overcurrent, unbalance
phase voltages at the point of common coupling (PCC), overvoltage in healthy phases, oscillations in
active, reactive power and dc-link voltage, distortion in injected currents and poor dynamic response of
the system. Several strategies are found in the literature to overcome these challenges associated with LVRT.
This paper critically reviews the recent challenges and the associated strategies under LVRT conditions in
GCPV inverters. The drawbacks associated with the conventional current control strategies are investigated
in MATLAB/Simulink environment. The advanced LVRT control strategies are categorized and analyzed
under different types of grid faults. The work categorizes the state-of-the-art LVRT techniques on the basis
of the synchronization methods, current injection techniques and dc-link voltage control strategies. It is found
that state-of-the-art control strategies like OVSS/OCCIDGS provides improved voltage support and current
limitation, which results in smooth LVRT operation by injecting currents of enhanced power quality.
INDEX TERMS Current reference generation, dc-link voltage control, grid-connected PV, low-voltage-ride-
through, current limitation, voltage unbalance.
I. INTRODUCTION
During recent years, the penetration of distributed generation
(DG) based grid-connected photovoltaic (GCPV) systems
have exponentially increased [1]. This is due to its vari-
ous advantages such as low generation cost, zero carbon
emissions, enhancing the grid reliability and alleviating the
network capacity. On the other hand, the sporadic power
generation of the DG PV system can jeopardize its normal
operation leading to voltage variations, increased energy and
reactive power losses. Moreover, these PV systems are oper-
ated within a specified voltage range, which helps maintain
grid stability [2]. Hence, the network operators continuously
The associate editor coordinating the review of this manuscript and
approving it for publication was B. Chitti Babu .
develop and update the grid codes to minimize the adverse
effects of distributed generating resources, like PV, wind,
etc., on the power system [3], [4]. Among these grid codes,
LVRT is an essential requirement among grid-connected PV
inverters. Fundamentally, LVRT is a control action in GCPV
inverters that allows them to stay connected with the utility
during a short-term sag in the grid voltage [5]–[8]. Under
normal operating conditions, the PV system is operated at
maximum power point and injects active power into the
grid [9]–[12]. However, during LVRT, the large GCPV sys-
tems connected at higher voltages inject reactive power to
maintain grid stability [13], [14]. Moreover, small capacity
GCPV system is generally connected to a low-voltage net-
work and their inverter control action is designed in such a
way to give preference to the injection of active power under
121804 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ VOLUME 9, 2021
J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
FIGURE 1. LVRT Grid codes in different countries [34].
LVRT due to the small X/R ratio of the low-voltage network.
To limit the scope of this paper, the authors have reviewed
the control strategies that give preference to the injection of
reactive power under LVRT.
LVRT requirement is essentially a voltage versus time
characteristic, which shows the minimum period required to
withstand a voltage drop level. The LVRT of certain grid
codes requires an immediate revamping of active and reactive
power to the pre-fault values after the voltage has recovered
to its nominal value. Other LVRT grid codes require an
increased reactive power injection by PVs to provide voltage
support to the grid. The operators demand this grid support
due to the increasing PV penetration level in the transmis-
sion network. Many countries like Germany, China, UK,
Italy, Denmark, etc., are continuously updating their LVRT
grid codes based on their grid infrastructure to cope with
the rapidly expanding use of renewable energy resources,
as shown in Figure 1 [15]. According to the German code,
the PV inverter should ride through the fault for a maximum
of 0.15s under severe faults, i.e., when the grid voltage has
dropped to zero. This code allows the PV units to remain
connected without any nuisance tripping if the voltage at the
point of common coupling (PCC) has been able to recover
to 90% of its rated value within 1.5s after a fault. On the
other hand, China allows an additional time of 0.475s when
the PCC voltage reaches 20% of its rated value. Therefore,
the PV units should remain connected for China if the PCC
voltage reaches 90% within 2s of its collapse.
Moreover, in German code, if the grid voltage is between
90% – 50%, the DG unit should inject reactive current as a
function of voltage sag. If the voltage sag is more than 50%,
the DG unit should inject 100% of its reactive current [16].
Chinese grid codes are less stringent as compared to German.
The former allows a commensurate reactive power injection
when the grid voltage is between 90% – 20%. If the grid
voltage falls below 20%, the PV inverters should inject 100%
of their reactive power, as shown in Figure 2. This distinction
between the German and Chinese grid codes is apparently due
to the difference in penetration levels of PV units within these
two countries. The Chinese codes may also need revision as
the level of distributed generation is on a constant rise in
China. The grid codes for various countries under high PV
penetration are reviewed in [17], [18].
FIGURE 2. Grid codes for reactive power injection [16].
Several methods are present to enhance the fault ride-
through (FRT) capability of PV systems by using additional
components like energy storage systems (battery energy stor-
age systems, capacitor energy storage systems), fault current
limiters and static synchronous compensator (STATCOM)
[19]–[21]. However, the energy storage systems do not con-
sider the injection of reactive current and FACTS devices
like STATCOM only inject reactive power to support the
grid during fault [22], [23]. Moreover, the overall cost and
complexity of the system increase because of the addition of
these hardware components. Recently, the researchers have
also used computational methods like fuzzy logic control
(FLC) and optimization techniques, which help in adjusting
the inverter’s power references and improve the performance
of the inverter controller [24]–[27].
Though these computational methods are efficient and
help address the FRT problems, they enhance the system’s
complexity. However, in light of the issues mentioned above,
the modified inverter control techniques are gaining more
attention to meet the grid code requirements at a lower cost
and better accuracy [28], [29]. Further, the use of these mod-
ified inverter control techniques also aids in improving the
system speed and its dynamic response [30].
During recent years, several review articles have shone a
light on the LVRT capability of GCPV systems [16], [18],
[21], [31]–[49]. However, none of the articles have pro-
vided a detailed classification and critically reviewed the
recently developed modified inverter control techniques for
the LVRT capability of PV systems. This paper high-
lights the differences among the recently published review
articles in Table 1 to show the existing research gap
clearly.
The proposed work will provide the readers with an
exhaustive review of the various control strategies proposed
to date that overcome challenges present during LVRT and
provide avenues for future work. The key novelty features of
the manuscript are:
Certain key objectives are identified that are required dur-
ing the LVRT condition. Finally, recently developed modified
control techniques are classified based on these objectives.
The proposed work has provided a critical review of the
various inverter control strategies and their advantages and
potential shortcomings.
VOLUME 9, 2021 121805
J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
TABLE 1. Review articles on Low-voltage-ride-through for PV systems.
Since the outer loop dc-link voltage control plays a
vital role during LVRT condition, an exhaustive comparison
between the recently developed dc-link voltage control strate-
gies along with their potential demerits is presented.
The rest of the paper is organized as: Section II discusses
the challenges associated with LVRT. Section III critically
reviews the recently developed current control techniques.
Section IV classifies and compares various dc-link volt-
age control strategies along with their merits and demerits.
Section V provides a discussion on the future aspects of
the control strategies during the LVRT condition. Finally,
in Section VI, the conclusion of the work is encapsulated.
II. CHALLENGES UNDER LVRT
As previously discussed, appropriate reactive power is
injected into the grid based on the specified grid code to
ensure grid stability. The LVRT control action is initiated
when the grid voltage drops below its rated value [50]–[54].
121806 VOLUME 9, 2021
J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
Hence, a fast and reliable dip detection method is essential
under LVRT condition. This dip detection is usually accom-
plished by a phase-locked loop (PLL).
Synchronous reference frame-based PLL (SRF-PLL) is
commonly used for measuring the RMS values of the grid
voltage during normal operating and under balanced fault
conditions. A major drawback within the SRF-PLL is its
inability to accurately detect the grid voltage dip under unbal-
anced grid faults.
This inability stems from the presence of negative
sequence components, which are rich in higher-order har-
monics under unbalanced sag conditions. Several researchers
have suggested improvements in conventional SRF-PLL by
mainly focusing on increasing the noise elimination capa-
bility in the conventional SRF PLL, thereby enhancing their
filtering capability [55]–[59].
In [55], a double decoupled synchronous reference
frame (DDSRF) based PLL is proposed to detect funda-
mental frequency positive sequence (FFPS) component of
grid voltage under polluted grid conditions. The technique
employs a double synchronous reference frame (DSRF) with
a decoupling cell which enables the decoupling of positive
and negative sequence components. In [56], an improved
phase-locked loop (EPLL) is proposed, with enhanced fre-
quency flexibility. The EPLL exhibits superior performance
even under frequency divergence of the grid voltage from its
theoretical value. This EPLL has a high tolerance to noise
and harmonics as compared to the conventional PLL. In [57],
a moving average filter (MAF) is used to eliminate the rip-
ples caused by negative sequence components for extracting
the fundamental frequency positive sequence (FFPS) compo-
nent in the synchronous domain. Another attractive approach
for synchronization, namely, multiple complex coefficient
phase-locked loop (MCCF-PLL), which uses complex coef-
ficient filters (CCFs), is presented in [58]. The CCFs have
an inherent property of sequence separation, and therefore,
these do not require a sequence separation method or decou-
pling cell. In [59], a dual second-order generalized integrator
(DSOGI) based synchronization technique is presented that
evaluates the positive sequence component of grid voltage
and eradicates the harmonics during polluted conditions.
Although, PLLs with enhanced filtering capability pos-
sess various advantages in accurately detecting the sag in
grid voltage, but at a cost of increasing the overall com-
plexity of the system [68]. To overcome this, researchers
have formulated control strategies that eliminate the use of
PLL [69]. In [70], a control strategy is proposed to overcome
the problems related to power quality. As the control tech-
nique does not use a phase-locked loop, the system complex-
ity is significantly reduced thereby improving the dynamic
response of the system. Another control strategy is suggested
for GCPV inverters without using PLL showing satisfactory
performance under symmetrical and asymmetrical voltage
sag conditions [71]. The proposed control scheme is relatively
simpler and free from jitter. A LVRT technique that uses
an arbitrary angle instead of a PLL is proposed in [72].
The positive sequence of this angle is obtained by integrating
the angular frequency of the grid.
Once a sag in the RMS value of the grid voltage is detected,
efficient current reference generation strategies are formu-
lated based on the grid codes [73]. The use of current control
strategy helps in limiting the magnitude of the injected cur-
rents, mitigating the double grid frequency oscillations within
the injected power, providing voltage support at the PCC and
ensuring that the injected currents are of low total harmonic
distortion (THD) [74]–[76].
Another important task during LVRT under unbalanced
fault is to design an efficient dc-link voltage control strat-
egy to prevent inverter shutdown due to overcurrent and
to ensure reliable operation of the inverter. This control
strategy also prevents overvoltage in the dc-link capacitor
during power imbalance occurring under unbalanced fault
conditions [77], [78].
To summarize, under LVRT it is essential to quickly detect
the voltage dip, initiate appropriate control action to limit
the inverter current amplitude as well as determine precise
active/reactive power references to provide voltage support
at PCC and to ensure power balance. This entails a carefully
designed dc-link voltage controller to avoid overvoltage in
the dc-link capacitor. Due to the importance of LVRT in
PV inverters which contributes toward grid stability, a broad
categorization of LVRT techniques is done, based on the
following key objectives:
a. Quick dip detection (PLL): Advanced PLLs, notch filters
or repetitive controllers are generally used to quickly deter-
mine the sag in grid voltage. Several other advanced PLLs
have been proposed and reviewed in [60]–[67]. Hence, in this
paper, the importance and key attributes of various PLLs
which are widely used under LVRT condition, are briefly
described in section II.
b. Current control strategy: Formulation of a current con-
trol strategy is vital: to limit the amplitude of the injected
currents, to provide voltage support and to mitigate double
grid-frequency oscillations in injected powers under balanced
and unbalanced fault conditions. In this paper, the current
control strategies are further classified based on specific
objectives that are essential under LVRT.
c. DC-link voltage control: The dc-link voltage control
helps in reducing the oscillations in the dc-link capacitor
which is detrimental to capacitor life [79]. Moreover, this
outer loop control also helps in maintaining the power balance
between the dc and ac side. A detailed classification and
discussion on the recently developed dc-link voltage control
strategies are also carried out ahead.
III. CURRENT REFERENCE GENERATION (CRG)
According to the grid code, a well-designed current ref-
erence generation (CRG) must be formulated to deliver
the required power components (active and reactive) to the
grid [80]. Under normal grid conditions, the objective of
the current reference generation strategy is to improve the
quality of the power components being injected into the grid
VOLUME 9, 2021 121807
J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
FIGURE 3. Behavior of conventional current reference generation
strategies in pu under unbalanced fault for a 2kW GCPV system: (a) Grid
voltage, injected currents in (b) IARC, (c) AARC, (d) PNSC and (e) BPSC.
that can be easily delivered by conventional CRG strategies.
However, the conventional CRG strategies such as instanta-
neous active-reactive control (IARC), average active-reactive
control (AARC), positive-negative sequence control (PNSC)
and balanced positive sequence control (BPSC) require mod-
ifications to ensure continuous operation under unbalanced
grid faults [81]. This is because these conventional CRG
strategies do not provide additional support such as cur-
rent limitation, voltage support, which are necessary during
LVRT operation [82]. It can be observed from Figure 3, that
all conventional CRG strategies result in high peak current
amplitude under unbalanced grid fault as no provision is made
for limiting the peak amplitude of the inverter currents. This
can trigger the overcurrent protection devices of the inverter
and can result in the disconnection of the PV system. Hence,
under unbalanced grid voltage conditions, the major task
of the CRG technique during LVRT: is to provide voltage
support at the point-of-common coupling (PCC) and to limit
the amplitude of the injected currents to ensure continuous
safe operation of the PV inverter [83], [84].
The importance of the voltage support, current limitation
and dc-link voltage control strategies is explained under
two types of faults: unbalanced and balanced grid voltage
conditions. In the first case, the strategies are tested under
an unbalanced grid voltage condition by reducing the grid
voltage of phase A to 0.5pu at t =0.35s. In the second case,
a balanced phase drop in the grid voltage is considered by
reducing the phase voltages to 0.5pu at t =0.35pu.
Since this paper focuses on comparing the recently devel-
oped CRG strategies under LVRT conditions, therefore, this
article majorly classifies these techniques into two categories
based on the objectives stated above in Section II. The cur-
rent reference generation strategies that are discussed in the
following sub-sections can be implemented in stationary,
synchronously rotating or natural reference frame as shown
in Figure 4.
FIGURE 4. Generic circuit diagram of a three-phase two-stage GCPV
System.
A. VOLTAGE SUPPORT STRATEGIES (VSS)
According to LVRT grid codes, maximum and minimum
voltage limits at the PCC must be specified to ensure the
stable operation of GCPV systems under fault conditions.
By injecting reactive power, the CRG strategies provide
voltage support and help PV systems stay connected to the
grid. Under balanced grid voltage sag, the voltage support
strategies should be designed to equally raise the voltages
in all phases. This is achieved by increasing the positive
sequence voltage amplitude at the inverter side. Additionally,
the phase voltage equalization is another important objec-
tive under unbalanced sag conditions. This is so because
an equal rise in the phase voltages can trigger overvoltage
121808 VOLUME 9, 2021
J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
FIGURE 5. Response of voltage support strategy in pu at the PCC under
(a) unbalanced and (b) balanced grid faults.
protection, as the healthy phase voltage can easily surpass
the maximum permissible voltage limit. By increasing the
amplitude of negative sequence voltage, phase equalization is
achieved.
The efficient control of the ratio of positive and negative
sequence components in the reference currents helps in pro-
viding voltage support at the PCC under both unbalanced
and balanced types of faults, as shown in Figure 5(a)–(b),
respectively.
Hence, a current reference generation strategy that pro-
vides voltage support at the PCC should be carefully
formulated.
The following sub-sections discuss the recently developed
voltage support control strategies during LVRT under bal-
anced and unbalanced grid faults.
1) FLEXIBLE VOLTAGE SUPPORT CONTROL (FVSC) [85]
In [85], [86], a flexible voltage support current reference
generation control strategy is proposed. The voltage support
is provided by increasing the positive sequence voltage and
minimizing the negative sequence of grid voltage, simultane-
ously, to reduce the unbalance factor (n) in (1).
n=V−
V+(1)
where, V+=qv+2
α+v+2
β,is the positive sequence and
V−=qv−2
α+v−2
βis the negative sequence voltage at PCC
evaluated under stationary reference frame.
For flexible voltage support, the proposed strategy injects
both positive and negative sequence voltage into the grid
by adaptively varying their magnitude, under unbalanced
grid conditions. The injected reactive current references are
formulated as in (2) and (3).
i∗
αq=2
3Qref
k+v+
β+k−v−
β
k+v+
α2+v+
β2+k−v−
α2+v−
β2(2)
i∗
βq=2
3Qref −k+v+
β−k−v−
β
k+v+
α2+v+
β2+k−v−
α2+v−
β2(3)
where, k+and k−are the control parameters to balance the
positive and negative sequence voltage components, respec-
tively, and k−=1−k+.
Taking the value of k+close to 1 will increase the injection
of the positive sequence component and result in a constant
injection of the negative sequence component. This aids in
raising the voltage profile in each phase, under balanced
voltage sag. On the other hand, under severe voltage sags,
the value of k+is chosen close to zero to achieve injection
of a constant positive sequence and to decrease the magni-
tude of the negative sequence component resulting in voltage
equalization at the PCC. The positive and negative sequence
voltage amplitudes at PCC, are dependent on the voltage drop
due to grid side inductance as in (4) and (5), respectively.
V+=V+
g+2
3Qref
ωLgV+k+
k+(V+)2+k−(V−)2(4)
V−=V−
g−2
3Qref
ωLgV−k−
k+(V+)2+k−(V−)2(5)
where, ωis the grid angular frequency and Qref is the reactive
power reference. Lgis the grid side inductance, whereas, V+
g
and V−
gare the positive and negative sequence component of
the grid voltage, respectively.
Although the proposed strategy provides enhanced volt-
age support, evidently it demands the calculation of grid
impedance. Moreover, within this strategy, the maximum
allowable inverter current that can be injected into the grid
has not been considered.
2) VOLTAGE SUPPORT CAPABILITY IN DISTRIBUTED
GENERATED INVERTERS (VSCDGI) [87]
A strategy is proposed in [85] to equally raise the phase
voltage without designing a voltage control loop. This is a
major drawback as the reference reactive power Qref and the
control parameter k+are calculated without the knowledge
of PCC voltage. As previously discussed, for stable operation
the maximum and minimum values of phase voltages should
be within the limits as per the specified grid codes. To this
effect, a method is proposed in [87] which employs a voltage
control loop to determine the values of Qref and k+in (6) and
(7), respectively.
Qref =3
2
V∗
p(V∗
p−Vgp)−V∗
n(V∗
n−Vgn)
ωLg
(6)
k+=V∗
n(V∗
p−Vgp)
V∗
pVgn −V∗
nVgp
(7)
VOLUME 9, 2021 121809
J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
where, V∗
pand V∗
nare the references for positive and nega-
tive sequence voltages at PCC, respectively. V∗
pand V∗
nare
determined from the type of sag characteristic based on the
lower (V∗
L) and upper (V∗
H) boundary values, where, V∗
L=
min (Va,Vb,Vc)and V∗
H=max (Va,Vb,Vc)=(V∗
L+1V).
where, 1V=max (Va,Vb,Vc)−min (Va,Vb,Vc).Vgp and
Vgn is the positive and negative sequence component of the
grid voltage, respectively.
To provide better voltage support, the reactive reference
currents are formulated, in αβ reference frame as in (8)-(9).
i∗
αq=2
3
k+v+
β+(1 −k+)v−
β
k+V+2+(1 −k+)V−2Qref (8)
i∗
βq= −2
3
kqv+
α+(1 −kq)v−
α
k+V+2+(1 −k+)V−2Qref (9)
where, k+is the balancing factor which can take any value
between 0 and 1.
3) REACTIVE POWER CONTROL OF DISTRIBUTED
GENERATION INVERTERS (RPCDGI) [88]
In [87], the strategy was primarily focused on providing
voltage support under symmetrical voltage sags. In [88], this
limitation was overcome by proposing a control strategy that
works well under unbalanced grid voltage conditions too. The
technique increases the positive sequence voltage component
by injecting the positive sequence reactive power through the
inductor which in turn increases the PCC voltage by a voltage
variation of ωLgI+. On the contrary, to reduce the nega-
tive sequence component of the PCC voltage, the negative
sequence reactive power is injected which reduces the PCC
voltage by ωLgI−. By simultaneously, raising and reduc-
ing the positive and negative sequence voltage, respectively,
the voltage unbalance is minimized. The CRG equations to
flexibly regulate the positive and negative reactive power are
given in (10) and (11).
i∗
α=2
3[v+
β
v+
α2+v+
β2Q++v−
β
v−
α2+v−
β2Q−] (10)
i∗
β= −2
3[v+
α
v+
α2+v+
β2Q++v−
α
v−
α2+v−
β2Q−] (11)
Further to ensure the PCC voltages stay within the limit,
the positive and negative sequence reactive power references
are formulated in (12) and (13), respectively.
Q+=3
2V+∗hV+∗−V+
gi
ωLg
(12)
Q−=3
2V−∗hV−∗−V−
gi
ωLg
(13)
where, V+∗and V−∗are the desired positive and nega-
tive sequence voltages, respectively and are further evaluated
by carefully determining the maximum and minimum value
between phase voltages.
4) FLEXIBLE VOLTAGE SUPPORT WITH IMBALANCE
MITIGATION IN DISTRIBUTED GENERATION
INVERTERS (FVSDGI) [89]
In [89], the injection of reactive current by using a three-
level T-type inverter for medium switching frequency and
low-voltage applications, has been considered. The proposed
strategy employed DDSRF-PLL to extract the positive and
negative sequence current components at PCC. The refer-
ence currents are generated by combining both positive and
negative sequence components of PCC currents and are in
accordance with the maximum and minimum voltage limits
at PCC. Under balanced voltage sags, the PCC voltages are
equally raised with the help of a positive sequence regulator.
On the other hand, under unbalanced voltage conditions, two
PCC voltage setpoints are determined and flexible control of
both positive and negative sequence current regulator helps in
achieving voltage equalization. Under deep voltage sag con-
ditions, a current saturation strategy is activated which helps
in only injecting the positive sequence current thereby avoid-
ing overcurrent. The active and reactive current references
in d-q reference frame are formulated as in (14) and (15),
respectively.
i+
d∗=0 (14)
i+
q∗= −(I+)∗(15)
As shown in (14)-(15), under severe grid fault conditions
the injection of active current is taken as zero, hence only
reactive current injection is considered.
i−
d∗=I−∗ ¯v−
q
V−!(16)
i−
q∗=I−∗ −¯v−
d
V−!(17)
In (16)-(17),¯v−
dand ¯v−
qare the filtered negative sequence
voltages in d-q reference frame. I+∗and (I−)∗are refer-
ence of positive and negative current amplitude, respectively,
obtained from voltage control loop, whereas V−is the phasor
sum of negative sequence filtered voltages in d-q reference
frame.
5) INDIVIDUAL PHASE CURRENT CONTROL TO AVOID
OVERVOLTAGE (IPCC) [90]
It is evident that random injection of positive and nega-
tive sequence components without monitoring the voltage
drop in each phase can result in overvoltage in healthy
phases. In [90], a scheme is proposed to avoid overvoltage
in healthy phases by independently controlling the current
in each phase. Evidently, the injection of balanced reactive
currents under unbalanced voltage sags results in overvoltage
in healthy phases, hence, the strategy is based on updated
European grid code which requires the injection of unbal-
anced reactive currents to assist towards grid stability [91].
The injection of reactive current is based on the amount of
voltage drop in the faulty phase to ensure that the healthy
121810 VOLUME 9, 2021
J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
FIGURE 6. Block diagram for active and reactive current references.
phases remain unaffected. The reactive current is obtained as
the output of the droop controller and is given as in (18).
ˆ
iR−x=droop |dex|ˆ
In,where x ∈(a,b,c) (18)
where, droop coefficient is a constant and is evaluated as per
the grid codes, dexis the amount of deviation in the phase
voltage from the nominal value and ˆ
Inis the nominal current
of the inverter as shown in Figure 6.
6) ADVANCE VOLTAGE SUPPORT CONTROL (AVSC) [92]
In [92], a strategy is proposed which is suitable for both
inductive and resistive grids and hence injects both active and
reactive power into the grid during fault conditions.
Under unbalanced grid voltage conditions, the VSS limits
the phase voltages at PCC by setting the maximum and
minimum voltage limits according to the grid codes. The
positive and negative sequence reference currents for active
and reactive power for any X/R ratio are given in (19)–(22),
respectively.
I+
p=Rg
X2
g+R2
g×1V+
ref (19)
I−
p=Rg
X2
g+R2
g×1V−
ref (20)
I+
q=Xg
X2
g+R2
g×1V+
ref (21)
I+
q=−Xg
X2
g+R2
g×1V−
ref (22)
where, 1V+
ref and 1V−
ref are the positive and negative
sequence voltage drop, respectively, due to the grid-side
inductance and resistance. Xgand Rgare the inductance and
resistance of grid, respectively. It is evident from (19)-(22),
for the inductive grid, there is no contribution from the active
current component. For such an instance, this strategy aims
to inject maximum active power and regulates the phase
voltages, simultaneously. However, the injected active power
would suffer from oscillations under severe unbalanced grid
conditions.
7) POSITIVE AND NEGATIVE SEQUENCE VOLTAGE
SUPPORT STRATEGY (PNSVSS) [93]
In [93], a strategy is designed for both inductive and resistive
grids which helps in raising the positive sequence voltage,
reducing the negative sequence voltages, and maximizing
the difference between these two sequences. The increase in
the positive sequence component helps in raising the voltage
magnitude and reducing the negative sequence component
aids towards phase equalization. The additional objective
of maximizing the difference between these two sequences
ensures full utilization of inverter capacity as it injects the
rated current as well as provides voltage support.
The active and reactive reference currents are formulated
as in (23)-(26).
i∗
α(p)=2
3
v+
α
v+
α2+v+
β2P++v−
α
v−
α2+v−
β2P−
(23)
i∗
β(p)=2
3
v+
β
v+
α2+v+
β2P++v−
β
v−
α2+v−
β2P−
(24)
i∗
α(q)=2
3
v+
β
v+
α2+v+
β2Q++v−
β
v−
α2+v−
β2Q−
(25)
i∗
β(q)=2
3
−v+
α
v+
α2+v+
β2Q++−v−
α
v−
α2+v−
β2Q−
(26)
The amplitude of positive and negative sequence voltage at
the PCC is obtained as in (27) and (28), respectively.
V+=RgI+
p+ωLI+
q+r(V+
g)2−(ωLI+
p−RgI+
q)2(27)
V−=RgI−
p−ωLI−
q+r(V−
g)2−(ωLI−
p−RgI−
q)2(28)
where, I+
p,I+
q,I−
pand I−
qare the positive and nega-
tive sequence components of active and reactive currents,
respectively.
8) MAXIMIZING VOLTAGE SUPPORT IN LOWEST
PHASE (MVSLP) [94]
In [94], [95], a voltage support strategy is proposed by max-
imizing the RMS value of the most sagged phase voltage
and reducing the risk of an under-voltage disconnection, dur-
ing unbalanced grid sag conditions. The scheme works well
regardless of the grid impedance and maximizes the inverter’s
capability by injecting the rated current. The reference cur-
rents are formulated as in (29)-(30).
i∗
α=I+
p
V+v+
α+I+
q
V+v+
β(29)
i∗
β=I+
p
V+v+
β−I+
q
V+v+
α(30)
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It can be observed from (29)-(30), that balanced currents
are injected into the grid, as only the positive sequence
component is being considered. Hence, voltage imbalance
remains the major drawback of this method.
9) MAXIMIZE REACTIVE CURRENT INJECTION TO AVOID
OVER VOLTAGE (MRCAO) [97]
In [96], the strategy ensures simultaneous injection of
the maximum value of the positive sequence component of
the reactive current to achieve maximum voltage rise in the
faulted phase and the injection of negative sequence compo-
nent of the reactive current to ensure phase equalization. The
major demerit of this strategy is that it requires a reliable eval-
uation of the grid impedance and the controller operates in
the open-loop. To overcome this drawback, a voltage control
loop is incorporated in [97] to avoid overvoltage in healthy
phases. The strategy uses two PI controllers, one to inject the
maximum rated current in the disturbed phase, and the other
to avoid overvoltage in healthy phases. The current reference
generation equations are formulated by using the normalized
values of the positive and negative sequence voltages as
in (31) and (32).
i∗
α=Ip
V+v+
α+I+
q
V+v+
β+I−
q
V−v−
β(31)
i∗
β=Ip
V+v+
β−I+
q
V+v+
α−I−
q
V−v−
α(32)
Here Ip,helps in injecting the active power, I+
qis used to
balance the phase currents and I−
qprevents overvoltage in the
healthy phase.
10) MULTIPLE OBJECTIVE VOLTAGE SUPPORT
STRATEGY (MOVSS) [98]
In [98], a similar VSS is proposed for inductive and resistive
grids that minimize the imbalance in voltage by reducing and
increasing the negative and positive sequence component of
reactive power, respectively as in (33) and (34).
1V+=V+
g−V+
PCC =RgI+
d+ωLgI+
q(33)
1V−=V−
g+V+
PCC =RgI−
d−ωLgI−
q(34)
1V+and 1V−determine the voltage support from the
utility to the point of common coupling. The positive and
negative sequence of active and reactive reference currents
are formulated in SRF as in (35)-(38).
i+
d=2
3
P+
V+
PCC
(35)
i−
d=2
3
P−
V−
PCC
(36)
i+
q=2
3
Q+
V+
PCC
(37)
i−
d=2
3
Q−
V−
PCC
(38)
The increment in the positive sequence and decrement in
the negative sequence component of PCC voltage is achieved
by carefully determining the reactive power references as
in (39) and (40), respectively.
Q+=3
2
Rg
X2
g+R2
g×V+
PCC 1V+(39)
Q−= −3
2
Rg
X2
g+R2
g×V−
PCC 1V−(40)
11) OPTIMAL VOLTAGE SUPPORT STRATEGY (OVSS) [99]
Similar to strategies proposed in [93]–[98], the control strat-
egy in [99] is also based on the minimization of voltage unbal-
ance factor (n). However, the optimal solution is obtained
based on the knowledge of the impedance angle of the
injected current as in (41).
θinj =θg=tan−1ωLg
Rg
(41)
The optimal positive sequence active and reactive current
references are given by (42) and (43), respectively.
i+
p=i+
p(opt)=Icos θinj (42)
i+
q=i+
q(opt)=Icos θinj (43)
where, Iis a predetermined current value that will limit the
amplitude of the inverter current. Based on (42) and (43), the
positive and negative sequence components of PCC voltage
are determined as in (44) and (45), respectively.
V+=V+
g+IqR2
g+ωLg2(44)
V−=V−
g−nIqR2
g+ωLg2(45)
Apart from the above-mentioned VSS, several other
improvements have been proposed to provide enhanced volt-
age support under unbalanced faults [100]–[105]. In [100],
the injection of both active and reactive current is based
on the severity of voltage sags so that the inverter rating
is not exceeded. In [101], the voltage unbalance factor is
minimized by employing droop control. The scheme injects
the positive and negative sequence components of active
and reactive powers to ensure that the PCC voltage remains
within specified limits. A symmetric component decoupled
control strategy (SCDCS) for a three-phase four-wire sys-
tem is proposed in [102]. The strategy injects the active
power by utilizing the positive sequence component of the
inverter current. Moreover, the negative and zero sequence
components are utilized to provide local voltage support and
unbalance correction. It can be concluded that the knowledge
of grid impedance is imperative in deciding the proper VSS
i.e., for an inductive grid, the injection of reactive power is
preferred which helps in raising the phase voltages as opposed
to the preference given to the injection of active power for
a resistive grid [103]. In [104], a model predictive current
controller (MPCC) is proposed to enhance the VSS under
different grid faults. In this controller, the voltage limit targets
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TABLE 2. Comparison between voltage support strategies under low-voltage-ride-through condition.
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are achieved by including the zero-sequence compo-
nent of voltage in the current references. An improved
communication-less control strategy for voltage unbalance
mitigation is proposed in [105]. In this scheme, the grid
impedance estimation is not required and the LV network
is imitated by choosing the line impedances to ensure that
the X/R ratio is selected close to one. The above-mentioned
voltage support strategies are compiled based on certain key
performance parameters in Table 2.
B. CURRENT LIMITATION STRATEGIES (CLS)
Another challenge that exists under low-voltage-ride-through
condition is to ensure that the peak amplitude of the inverter
currents does not exceed beyond the inverter rated capacity.
To elaborate on this concept, consider if there is a short-
term voltage sag in one of the grid phases. To ensure power
balance between dc and ac network the faulty phase inverter
current increases and keeps injecting the same power coming
from the dc side. If the amplitude of the faulty phase current
exceeds beyond the rating of the inverter, protection devices
within the inverter will switch off the inverter for its safety.
This interruption in the operation of the inverter will prevent
the ride-through operation of the PV inverter. Hence, the cur-
rent limitation is an important objective under LVRT that
limits the amplitude of the injected currents to the rated value,
to avoid the operation of overcurrent protection devices. The
response of current limitation strategies under unbalanced
and balanced grid faults are shown in Figure 7 (a) and (b),
respectively. In the following sub-sections various control
strategies are discussed that provide over current limitation
under balanced and unbalanced grid faults.
1) TWO DISCRETE PARAMETER CONTROL (TDPC) [106]
In [106], a current control strategy is proposed which formu-
lates a generalized current reference expression by combining
various conventional CRG techniques with the help of two
discrete control parameters (α, β). The optimum power qual-
ity characteristics can be obtained by carefully choosing the
values of αand βin the range of (-1, 1). To obtain the opti-
mum power quality characteristics for a specific condition,
values of αand βcan be used for the chosen CRG strategy
as given in Table 3.
In [99], reference equations are formulated as in (46)-(48).
iref =i+
ref +i−
ref (46)
i+
ref =Pv+
v+
2+(1+α)βv+v−+αv−
2(47)
i−
ref =αPv−
v+
2+(1+α)βv+v−+αv−
2(48)
where, i+
ref and i−
ref are the positive and negative sequence ref-
erence current vectors, respectively and P indicates the active
power reference. By replacing the different values of αand β
in (46)-(48), conventional current schemes can be obtained.
This type of flexible control strategy can be most promising
FIGURE 7. Response of current limitation strategy in pu at the PCC under
(a) unbalanced and (b) balanced grid faults.
TABLE 3. Values of α, β for conventional CRG strategies.
to meet future LVRT requirements. Although efficient, this
strategy does not provide any regulation on the minimum set
point in the reduction of the inverter overcurrent.
2) MINIMUM PEAK GRID INJECTION CURRENT
CONTROL (MPGICC) [107]
In [107], a strategy is proposed to minimize the power quality
problems and help in determining the minimum peak currents
during polluted grid conditions. The instantaneous phase cur-
rents are obtained as in (49)-(51).
ia=2
3.P∗
V+.(1+αncosωt)
1+β(1+α)ncos (2ωt)+αn2(49)
ib=2
3.P∗
V+.
cosωt−2π
3+αncos ωt+2π
3
1+β(1+α)ncos (2ωt)+αn2(50)
ic=2
3.P∗
V+.
cosωt+2π
3+αncos ωt−2π
3
1+β(1+α)ncos (2ωt)+αn2(51)
where, P∗, is the reference power signal and nis the voltage
unbalance factor (VUF) of (1), which can take any value
between 0-1. It is evident from (49)-(51) that the peak values
of the currents are dependent on the values of αand β.
By precisely choosing the values of these two control param-
eters, the peak currents are minimized. The proposed scheme
is extremely useful in balanced conditions, however, during
longer periods of voltage sags of more than one second,
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J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
the injected currents are distorted due to the presence of
negative sequence component.
3) REDUCE RISK OF OVERCURRENT PROTECTION
(RROCP) [108]
Based on the conventional positive-negative sequence con-
trol (PNSC) method, the strategy in [108] injects negative
sequence inductive currents to effectively control the peaks in
the current waveforms. The peak currents of the three phases
are formulated as in (52)–(54).
IaPeak =rI2
p+I2
n+2IpIncos ϕ(52)
IbPeak =sI2
p+I2
n+2IpIncos (ϕ+4π
3)(53)
IbPeak =sI2
p+I2
n+2IpIncos (ϕ−4π
3)(54)
where,
ϕ= ∅2− ∅n− ∅1−∅p
∅1= −tan−1v+
d
v+
q
,∅2= −tan−1v−
d
v−
q
and
∅n= −π
2,∅p= −tan−1i+
d
i+
q
(55)
where, v+
d,v+
q,v−
dand v−
qare the positive and negative
sequence component of grid voltages, respectively, in the
synchronously rotating reference frame. Similarly, i+
d,i+
q,i−
d
and i−
qare the positive and negative sequence component of
grid currents, respectively. ∅1and ∅2are the phase angles of
the positive and negative sequence voltages with respect to the
reference axis. ∅pand ∅nare the phase angles of the positive
and the negative sequence currents, respectively, whereas Ip
and Inare obtained using (56) and (57), respectively.
Ip=i+
d2+i+
q2(56)
In=i−
d(57)
It can be observed from (52)-(54), that the peak values of
the currents are dependent on ϕ. To limit the peak amplitude,
the phase currents should not exceed the maximum value of
current Imax from (58) and hence Imax is set below the thresh-
old value to avoid the operation of overcurrent protection.
Imax =max (IaPeak,IbPeak ,IcPeak) (58)
4) ZERO SEQUENCE CURRENT CONTROL (ZSCC) [109]
In [109], a control strategy is proposed by considering
zero-sequence component to ameliorate the power quality
issues in a grid-connected distributed generation system.
Normally, the conventional current control schemes have
four control variables (i+
d,i+
q,i−
dand i−
q) in a three-wire sys-
tem. The control strategy of [109] has six control variables
(i+
d,i+
q,i−
d,i−
q,i0
Re and i0
Im) for a four or six-wire converter
system to achieve better performance under unbalanced
grid conditions. With the injection of the zero-sequence
current component, two additional controls of freedom are
obtained to improve the power quality characteristics.
The scheme is essentially divided into two objectives:
objective 1, in which the oscillations in active and reactive
power are removed and objective 2, where the oscillations in
active power and negative sequence current are eliminated at
the same time. The current references for objective 1 are given
as in (59)-(62).
i+
d=2
3.P∗
v+
d−v−
d.(1 −v−
d/v+
d);i−
d=v−
d
v+
d
.i+
d(59)
i+
q=2
3.Q∗
−v+
d+v−
d2/v+
d;i−
q= −v−
d
v+
d
.i+
q(60)
i0
Re =2
3.P∗−¯
P
v0
Re
(61)
i0
Im =v+
d.i−
q−v−
d.i+
q
v0
Re
(62)
Using (59)-(62), the oscillations in active and reactive
power can be eliminated. On the other hand, the reference
currents for objective 2 are given as in (63)-(66).
i+
d=2
3.P∗
v+
d−v−
d;i−
d=0 (63)
i+
q=2
3.Q∗
−v+
d;i−
q=0 (64)
i0
Re =−v−
d.i+
d
v0
Re
(65)
i0
Im =0 (66)
It can be seen from (63)-(66) that, reference currents
contain only positive and zero sequence components under
unbalanced grid conditions. The proposed control strategy
helps in removing the oscillations in active and reactive power
for a three-phase four-wire system. Furthermore, it also helps
in reducing the current amplitude in the faulty phase.
The proposed strategy is advantageous in terms of power
controllability, at the cost of increased computational burden
due to two extra control objectives.
5) FLEXIBLE PEAK CURRENT LIMITING CONTROL
(FPCLC) [110]
In [110], [111], a fully flexible current controller is proposed
that limits the peak currents to improve the ride through
services by injecting positive and negative components of the
active and reactive powers, P+,P−,Q+and Q−,respectively.
The control scheme ensures that the injected currents do
not surpass the inverter rated current and avoid overcurrent
tripping of the PV inverter to guarantee its safe and reliable
operation. The positive and negative sequence currents are
derived in SRF as in (67)-(70).
I+
p=2
3
P+
V+=2
3
kPP
V+(67)
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I−
p=2
3
P−
V−=2
3
(1 −kP)P
V−(68)
I+
q=2
3
Q+
V+=2
3
kqQ
V+(69)
I−
q=2
3
Q−
V−=2
3
(1 −kq)Q
V−(70)
From (67)-(70), there are four parameters P+,P−,Q+,Q−,
and hence several combinations are possible to limit the
peak currents. In [110], the relation among these variables is
established and the control gains are defined as in (71)-(73).
kP=P+
Pand kQ=Q+
Q(71)
P+=kPP,P−=(1 −kP)P(72)
Q+=kQQ,Q−=1−kQQ(73)
where, P=P++P−and Q=Q++Q−and kPand kqare
the active and reactive control gain, respectively.
It is observed that the phase currents Ia,Iband Iccorre-
spond to a unique solution of Qa,Qband Qc, respectively. The
maximum value among the phase currents is then determined
to ensure safe operation of the inverter as in (74).
Qmin =min {Qa,Qb,Qc}⇒max {Ia,Ib,Ic}=I(max)(74)
A generalized expression is derived, to evaluate the reactive
powers for each phase to limit the peak current as in (75)-(78).
Q=−2xP +qy3I(max)nV +2−(2zP)2
2y(75)
x=kP+kq−2kPkqnsin ˆϕ(76)
y=k2
qh1+2ncos ˆϕ+n2i−2kq1+ncos ˆϕ (77)
z=kP1−ncos ˆϕ+kq1+ncos ˆϕ
+kPkqhn2−1i−1 (78)
And the different values for Qa,Qband QCare obtained
from the three distinct values of ˆϕas in (79).
ˆϕ=ϕ, ϕ +2
3π, ϕ −2
3π(79)
The reactive power reference will be the minimum value
among Qa,Qband QCand once this reference is determined,
the positive and negative sequence of active and reactive
powers, i.e., P+,P−,Q+and Q−, respectively can be known.
This strategy is advantageous in terms of its flexibility
and capability to balance positive and negative components
of the active and reactive power at the same time while
restricting the currents to a safe value. It is applicable to all
sizes of power converters having different ratings. But the
major drawback of this strategy is its increased complexity as
compared with other control schemes as it highly depends on
the VUF and the phase angle between sequences which may
have limited practical applications. Moreover, the proposed
strategy does not provide zero active power oscillations.
6) PEAK CURRENT LIMIT CONTROL (PCLC) [112]
In [112], a strategy is proposed to avoid overcurrent pro-
tection by providing peak current limitation (PCL) of neg-
ative sequence current. To guarantee that the highest current
does not exceed the pre-defined value (Imax), the maximum
amplitude of the negative sequence current injection (I−
PCL ) is
calculated as in (80).
I−
PCL = −I+cos ϕ+k4π
3+I+2[cos2(ϕ+k4π
3)−1]
+I2
max
k=
0,−π
3≥ϕ < π
3
1,π
3≥ϕ < π
−1, π ≥ϕ < 5π
3
and
ϕ= ∅n+∅p+ ∅1− ∅2(80)
Symbols have their usual meanings as in [108]. The injec-
tion of active and reactive current is flexible; hence the strat-
egy is useful in satisfying the requirements of commonly
available grid codes. Moreover, by injecting a specific combi-
nation of active and reactive currents, this method eliminates
the ripples in active power.
7) LIMIT-THE-CURRENT CONTROL STRATEGY (LCCS) [113]
To overcome the drawback mentioned in [110], a control
strategy is proposed in [113], which is independent of VUF
and the phase angle. This strategy provides flexible control
to ensure proper regulation in the injection of the power com-
ponents and limits the current to avoid nuisance tripping of
the inverters. The peak values of currents during normal and
abnormal grid conditions are determined from (81) and (82),
respectively.
I∗
balanced =2P∗
3V+(81)
I∗
unbalanced =2P∗
3V+−V−(82)
It can be seen from (81)-(82) that the presence of the nega-
tive sequence component under unbalance voltage condition
results in higher peaks in current. Hence, minimization of
these peaks in current by formulating the current references
in the stationary reference frame is obtained as in (83)-(86).
i∗
α(p)=2
3
I∗
Prv+
α2+v+
β2
v+
α2+v+
β2+kpv−
α2+v−
β2
×v+
α+kpv−
α (83)
i∗
β(p)=2
3
I∗
Prv+
α2+v+
β2
v+
α2+v+
β2+kpv−
α2+v−
β2
×hv+
β+kpv−
βi (84)
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i∗
α(q)=2
3
I∗
qru+
α2+u+
β2
v+
α2+v+
β2+kpv−
α2+v−
β2
×hv+
β+kqv−
βi (85)
i∗
β(q)=2
3
I∗
qru+
α2+u+
β2
v+
α2+v+
β2+kpv−
α2+v−
β2
×−v+
α−kqv−
α (86)
where, I∗
Pand I∗
qdenotes the active and reactive current
references, respectively. The reference of the maximum cur-
rent (I∗
max ), in (83)-(86) is obtained from (87).
I∗
max
=2
3v
u
u
u
t"I∗
pV+2
V+2+kpV−2#2
+"I∗
qV+2
V+2+kqV−2#2
+2
3v
u
u
t"kpI∗
pV+V−
V+2+kpV−2#2
+"kqI∗
qV+V−
V+2+kqV−2#2
(87)
For different values of kpand kq, peak values of cur-
rents are obtained, and the proposed scheme reduces the
current peaks under polluted grid conditions. The maximum
value of current in (87) is determined from the active and
reactive current references and the positive and negative
sequence components of voltage at PCC. To ensure that the
current stays within the permissible limit the current refer-
ences are formulated in (88), where Irated and Imax represents
the rated current value of the inverter and the maximum
among the three-phase currents, i.e., Imax =max {Ia,Ib,Ic},
respectively.
ˆ
i∗
a
ˆ
i∗
b
ˆ
i∗
c
=Irated
Imax
ˆ
i∗
α(p)+ˆ
i∗
α(q)
−ˆ
i∗
α(p)+ˆ
i∗
α(q).2+√3ˆ
i∗
β(p)+ˆ
i∗
β(q).2
−ˆ
i∗
α(p)+ˆ
i∗
α(q).2−√3ˆ
i∗
β(p)+ˆ
i∗
β(q).2
(88)
Here, ˆ
i∗
a,ˆ
i∗
b,ˆ
i∗
care the current references in the natural
reference frame. The maximum value of the current reference
in (88) is Irated under severe grid fault. As compared to the
control strategy in [110], this scheme is simpler as it is inde-
pendent of the voltage unbalance factor and angle between
component sequences which can provide flexible regulation
in injected powers and limitation in current amplitudes to
avoid overcurrent protection.
8) POSITIVE AND NEGATIVE SEQUENCE G AND B
CONTROL (PNGBC) [114]
In [114], positive and negative sequence conductance (G) and
susceptance (B) based control method is proposed to achieve
multiple objectives like current limitation, minimization of
oscillation in active and reactive powers as in (89)-(90).
g−=kGg+(89)
b−=kBb+(90)
where, kGand kBare the proportional ratio between positive
sequence and negative sequence of G and B, respectively and
g+,g−,b+and b−are the positive and negative sequence
components of susceptance and conductance, which are
obtained using (91) and (92), respectively.
g+=2
3
P
v+
2−kGv−
2(91)
b+=2
3
Q
v+
2−kBv−
2(92)
where, kG,kBcan take any values between -1 to 1. Once g+
and b+are calculated (g+
cal ,b+
cal ), the current amplitude of
each phase can be easily determined. Further, the maximum
phase current (Imax ) is calculated as in (93).
Imax =max Iamp,Ibmp ,Icmp(93)
where, Iamp,Ibmp and Icmp are the current amplitude in phase
a, b and c, respectively. Then the appropriate value of current
is selected based on the converter capacity as Ilim. To avoid
the operation of overcurrent protection devices the values of
g+and b+are determined from (94)-(95), respectively.
g+=
g+
cal ,Imax ≤Ilim
Ilim
Imax
g+
cal ,Imax >Ilim
(94)
b+=
b+
cal ,Imax ≤Ilim
Ilim
Imax
g+
cal ,Imax >Ilim
(95)
If the maximum current Imax is less than Ilim, the over-
current control is avoided. On the other hand, when Imax is
greater than Ilim, the current is proportionally decreased based
on the ratio of (Ilim/Imax ) and thus prevents overcurrent with
the maximum phase current being limited to Ilim.
9) SINUSOIDAL CURRENT INJECTION STRATEGY
(SCIS) [115]
A control strategy that eliminates the double grid fre-
quency oscillation in active power and dc-link voltage with
the capability of injecting sinusoidal current is proposed
in [115]–[117]. The strategy formulates flexible active and
reactive current references, based on PNSC strategy, under
unbalanced fault. It also limits the injected current to the
rated value during faults. Moreover, this scheme involves
a non MPPT operating mode under severe faults when the
maximum power from the PV array results in overcurrent in
the inverter.
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The reference currents are formulated in the stationary
reference frame by taking four key parameters kαp,kβp,kαq
and kβq(96)-(99).
iαP=v+
α−v−
α
v+2
α+v+2
β+kαP(90 v−2
α+v−2
βP∗(96)
iβP=v+
β−v−
β
v+2
α+v+2
β+kβPv−2
α+v−2
βP∗(97)
iαQ= − v+
α⊥+v−
α⊥
v+2
α⊥+v+2
β⊥+kαQv−2
α⊥+v−2
β⊥Q∗(98)
iβQ= − v+
β⊥+v−
β⊥
v+2
α⊥+v+2
β⊥+kβQv−2
α⊥+v−2
β⊥Q∗(99)
where, P∗is obtained from the dc-link voltage control loop
and Q∗is the required reactive power during fault condition.
The values of these parameters in (96)-(99) are chosen either
+1 or -1 to modify the active and reactive current references
according to grid specifications as in Table 4. As evident
from (96)-(99), the use of mode 2 is suggested, to utilize the
inverter’s rated capacity.
TABLE 4. Different modes for utilizing inverter’s rated capacity.
Once the voltage sag occurs, the controller determines
the inverter pseudo power, namely, the new nominal
power (NNP) of the inverter which is determined by the
voltage sag depth. The NNP is evaluated as in (100).
NNP =√Vp−√Vn
Vbase
S(100)
where, the nominal power is denoted by S, Vbase is the base
voltage and it is equal to the RMS value of line-line grid
voltage, Vp=v+2
α+v+2
βand Vn=v−2
α+v−2
β. Based on the
per-unit depth in voltage sag, the reactive power is calculated
as per the Chinese grid code as in (101).
Q=0,if Vpu >0.9
Q=S×1.5×0.9−Vpu,if 0.2<Vpu <0.9
Q=1.05 ×S,if Vpu <0.2
(101)
where, Vpu =qv2
α+v2
β
Vb. To avoid overcurrent, the new ref-
erence power (Pmax) to be injected into the grid is Pmax =
pNNP2−Q2. Under severe faults, if (Q>NNP),Qis
selected as NNP, and the reference power Pmax is taken
as 0, which means only reactive power is injected. This is
because of the low nominal power of the inverter and is
not capable of delivering active power to the grid to avoid
overcurrent. However, the control strategy allows double
grid frequency oscillations within the reactive power. More-
over, the smooth transition from MPPT to de-rated MPPT is
not achieved [118]. To remove these oscillations in reactive
power, under normal and abnormal grid conditions for a
low voltage distribution grid, a robust Kalman filter (RKF)
is employed in [119]. A smooth transition from MPPT to
de-rated MPPT is achieved with the help of this strategy.
The function of RKF is to calculate the magnitude of the
fundamental load component (FLC) from the load current,
which enhances the system dynamics under load perturba-
tion. The KF is the mathematical approach, which works
through a prediction and correction module.
10) MULTI-OBJECTIVE CONTROL STRATEGY (MOCS) [120]
In [120], the control algorithm simultaneously mitigates the
challenges associated with power quality and provides over-
current limitation. To achieve the control objectives, current
references are formulated in the stationary reference frame as
given in (102)-(103) [96].
I∗
α=2
3
k+
pv+
α+k−
pv−
αP∗
k+
pV+2+k−
pV−2
+
k+
qv+
β+k−
qv−
βQ∗
k+
qV+2+k−
qV−2
(102)
I∗
β=2
3
k+
pv+
β+k−
pv−
βP∗
k+
pV+2+k−
pV−2
−
k+
qv+
α+k−
qv−
αQ∗
k+
qV+2+k−
qV−2
(103)
where, k+
p,k−
p,k+
qand k−
qare the four variable parameters.
P∗and Q∗are the active power and reactive power references,
respectively. By using (102) and (103), the injected reference
current can be determined from the positive and negative-
sequence components of the active and reactive currents
(I+
p,I−
p,I+
qand I−
q), respectively. The current amplitude in
each phase is determined as in (104)-(106), as shown at the
bottom of the next page.
The maximum values of the phase current (Imax ) is evalu-
ated using (107).
Imax =v
u
u
tV+2−2V+V−x+V−2
V+2I+
p2+I+
q2
(107)
where, x=min ncos (θ),cos θ−2π
3,cos θ+2π
3o.
It can be observed from (107), that the minimum value of
xresults in the maximum value of phase currents. To protect
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the inverter against overcurrent,
Imax ≤Irated (108)
By using (107) and (108) current limitation is guaranteed.
By substituting the value of I+
p=I+
p max and Imax =Irated
in (107), the maximum active current (I+
p max ) is obtained
as in (109).
I+
p max =v
u
u
tV+2(Irated )2
V+2−2V+V−x+V−2−I+
qGC 2(109)
where, I+
qGC represents the positive sequence reactive current,
which is defined by the grid code during voltage sag. Under
LVRT condition, to prioritize the injection of reactive power
the value of I+
pis always less than I+
pmax .
However, in the case of low-power production, if I+
pis
less than the I+
p max ,the rated current capacity of the inverter
is not fully utilized. Therefore, the amplitude of the refer-
ence reactive current is increased to fully utilize the current
capacity of the inverter to provide maximum voltage support.
By substituting Imax =Irated in (107), I+
qis determined
as in (110).
I+
q=v
u
u
tV+2(Irated )2
V+2−2V+V−x+V−2−I+
p2(110)
11) PEAK CURRENT CONTROL WITH RESCALING FACTOR
(PCCRF) [121]
In [121], zero oscillations in active power are achieved at
the expense of higher peak currents in one or two phases.
Hence, to limit these currents, a rescaling factor (krs) is used
to formulate the current references as in (111).
krs =
Irms
I∗
rms−max
,if I ∗
rms−max >1
1,if I ∗
rms−max ≤1
(111)
where, Irms is the rms value of the nominal current of the
inverter and I∗
rms−max is the maximum rms value of the
three-phase current references.
The current references are determined as in (112).
¯
i∗
a
¯
i∗
b
¯
i∗
c
=krs
i∗
a
i∗
b
i∗
c
(112)
where, ¯
i∗
a,¯
i∗
band ¯
i∗
care the current references in natural ref-
erence frame, after rescaling. The error and the instantaneous
phase current are then tracked using the proportional (PR)
controller and the voltage references are generated in station-
ary (αβ) reference frame.
12) OVER CURRENT CONTROL IN DISTRIBUTED
GENERATION SYSTEMS (OCCIDGS) [99]
A strategy to limit the maximum inverter current to avoid
overcurrent protection is proposed in [99]. The strategy deter-
mines the maximum safe current of the inverter based on
the minimum value of the angles among the three phases as
in (113).
Imax =p1−2nx +n2qI+
p2+I+
q2(113)
where, n is the voltage unbalance factor. Also x=
min ncos (∅),cos ∅ − 2
3π,cos ∅ + 2
3πo
The proposed strategy also provides maximum voltage
support by ensuring that the current injection is based on the
chosen injection angle θinj, for which the amplitudes of the
positive-sequence currents i+
pand i+
qis defined as in (114)
and (115), respectively.
i+
p=i+
p(opt)=Icos θinj (114)
i+
q=i+
q(opt)=Icos θinj (115)
where,
I=Irated
√1−2nx +n2(116)
Apart from the above-mentioned strategies, several other
improvements have been proposed to provide current limi-
tation under unbalanced grid voltage conditions. A current
reference generation strategy is proposed in compliance with
the recently developed grid codes (CRGGC) in which the pos-
itive and negative sequence reactive currents are injected in
proportion with the change in positive and negative sequence
Ia=v
u
u
tV+2−2V+V−cos (θ)+V−2
V+2I+
p2+I+
q2(104)
Ib=v
u
u
u
tV+2−2V+V−cos θ−2π
3+V−2
V+2I+
p2+I+
q2(105)
Ic=v
u
u
u
tV+2−2V+V−cos θ+2π
3+V−2
V+2I+
p2+I+
q2(106)
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voltage [122]. The distribution factors used in the strategy
for active power reference and reactive power reference are
designed explicitly in accordance with the modern grid codes.
The proposed strategy utilizes the converter’s full capacity,
avoids overvoltage at the PCC, and reduces the unbalance
factor. In [123], another strategy is proposed that maxi-
mizes the power delivery and provides current limitation.
The strategy employs a DDSRF to extract the positive and
negative sequence of voltages and currents. The proposed
DDSRF based PNS extractor exhibits faster response and
lower total harmonic distortion (THD) compared to other
techniques. In [124], a CRG scheme is proposed which min-
imizes the oscillations in active and reactive powers. A FOPI
(Fractional-order PI) controller instead of the conventional
PI, PR controllers, is employed to obtain the zero steady-state
error in the stationary reference frame which improves the
response time. In [125], [126], a control strategy is proposed
that helps in maximizing the power capability of PV inverter.
The flexible current injection strategy is developed by ensur-
ing a proper balance between positive and negative sequence
components. The strategy limits the current to its rated value
and avoids the oscillations in active power. Table 5 presents
a comparison of recently developed current limitation strate-
gies, based on their distinct characteristics.
IV. DC-LINK VOLTAGE CONTROL
The design of an efficient dc-link voltage control loop is
essential during LVRT operation. Under normal operating
conditions, the power extracted from the PV array is delivered
to the grid through a dc-link capacitor to ensure that the
power balance is achieved. It is well-known, that the reactive
power reference during LVRT under faulty grid conditions is
determined from the grid codes, whereas the active power ref-
erence is dependent on the inverter power rating. The injected
active power to the grid (Pinj), should follow the reference
power (P∗) when there is no sag present, i.e., when P∗≥Pinj.
The power imbalance occurs in the system when there is
inequality between the reference power (P∗) and the injected
active power. This usually occurs under unbalanced voltage
sag conditions, as the inverter capacity is mostly utilized to
inject reactive power, and the MPP power from the PV array
cannot be fed to the grid. To overcome this, the MPPT is ter-
minated as the active power injection capability of the inverter
is now reduced. If the MPPT is still operating, the power
imbalance may give rise to overvoltage across the dc-link
capacitor that may result in the deterioration of the capacitor
and thus reduce its life. To safeguard the dc-link capacitor
from overvoltage, a constant dc-link voltage is achieved, and
power balance is ensured by active power curtailment. This is
done by reducing the power extracted from the PV array by
shifting the point of operation away from the MPP on the P-V
curve to a new reduced reference power operating point [127].
Single-stage GCPV systems are self-protected as the oper-
ating point shifts to a new point in the I-V curve to curtail
down the active power under voltage sag conditions [128].
Nevertheless, in two-stage systems, the MPPT operation is
FIGURE 8. Response of dc-link voltage control strategy in pu across
dc-link capacitor under (a) unbalanced and (b) balanced grid faults.
performed by dc-dc converter [129], hence, the system is not
self-protected. A separate control loop is required to protect
the over voltages in the dc-link capacitor. The response of
constant dc-link voltage control strategy under unbalanced
and balanced grid faults are shown in Figure 8 (a) and (b),
respectively.
On the other hand, there are several challenges associated
with providing a constant dc-link voltage under unbalanced
sag conditions [130]–[132]. Under deep grid voltage sag
conditions, a constant dc-link voltage results in the injection
of non-sinusoidal unbalanced currents which is due to a low
modulation index [133].
It is well-known that if a fixed reference value of the
dc-link voltage is chosen for the worst condition, it results
in high switching and inductor losses in two-stage PV sys-
tems [134]. In [135], an adaptive dc-link voltage technique
is suggested that shows that the PV system may have an
increase in the lifetime of 75.76% as compared to the fixed
dc-link control strategy. Hence, there exists a trade-off when
operating the dc-link capacitor at a fixed or variable voltage.
Therefore, the dc-link voltage control strategies are classified
into two sub-sections, namely constant and adaptive dc-link
voltage control. This paper focuses on discussing the recently
developed dc-link voltage control strategies for two-stage PV
systems to limit the scope of the proposed study.
A. CONSTANT DC-LINK VOLTAGE CONTROL
This section discusses the recently developed control strate-
gies, to maintain a constant dc-link voltage. These methods
mitigate the double grid frequency oscillations within the dc-
link voltage with improved dynamic response during fault
conditions.
1) INJECTION OF LESS POWER DURING SAG (ILPDS) [127]
In [127], three solutions are suggested to limit the dc-link
overvoltage by reducing the active power from:
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TABLE 5. Comparison between current limitation strategies under low-voltage-ride-through condition.
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FIGURE 9. Approximation of new operating point.
Short-circuiting the PV (P =0), Open-circuiting the PV
(P =0) and extracting non-MPP power from the PV array
(P 6= 0). In the first two methods, no power is extracted from
the PV array, hence, only reactive power is injected into the
grid. However, in the third method, less power, as compared
to the pre-fault MPP power, is injected into the grid by
controlling the dc-dc converter. The controlling of the dc-
dc converter is done in such a way that the power generated
by the PV array matches the injected power to the grid. The
operating point moves to a new point to obtain power balance.
To ensure that the point of operation moves to the right-side of
MPP on the P-V curve, a positive voltage step 1vpv is added
to vmpp as in (117).
vnew =vmpp +1vpv (117)
Faster dynamics are obtained by regulating the energy
stored in the dc-link capacitor ( 1
2CV2
dc). In Figure 9,
pnew_est ,vnew_est are the estimated power and voltage in the
triangle, respectively. From Figure 9, vnew_est can be evalu-
ated as in (118).
vnew_est =pnew_est
Pmpp vmpp −voc+voc (118)
where, pmpp and vmpp are the power and voltage at MPP,
respectively before the fault. The new estimated power,
pnew_est is evaluated from the active current reference
as in (119).
pnew_est ∼pout =edidref (119)
Simplifying (115) and (117), the new operating point and
the voltage difference between the MPP and the new operat-
ing can be estimated using (120) and (121), respectively.
vnew_est =edidref
Pmpp vmpp −voc+voc (120)
and
1vpv_est =vnew_est −vmpp (121)
The 1vpv_est in (120) is added to the feedforward controller
before the limiter as in Figure 10. The limiter gives the
positive values for 1vpv to obtain the vnew on the right-side of
FIGURE 10. Controller to obtain non-MPP operating point.
the PV curve. Moreover, the estimation of duty cycle (dest ) is
determined as in (122).
dest =1−vnewest
v∗
dc
(122)
This scheme helps in injecting reduced power to the dc-link
capacitor by moving the point of operation away from the
MPP of the PV curve and has the advantage of injecting
balanced currents even under faulty grid conditions.
2) FEEDBACK LINEARIZING CONTROL WITH SLIDING
MODE COMPENSATION (FLCSMC) [136]
Several strategies have been proposed that use feedback
linearizing control (FLC) in GCPV systems. However,
the performance of FLC has not been investigated during
the non-MPP mode of operation during grid faults. In [136],
a robust FLC strategy is used, which employs sliding mode
control to deal with the uncertainties during low-voltage-ride-
through in GCPV systems. The proposed strategy controls
the active and reactive power under LVRT and maintains a
constant dc-link voltage.
In the case of asymmetrical grid conditions, FLC controls
the active and reactive power to fulfill all the LVRT require-
ments and ensures constant dc-link voltage. The active and
reactive power references are given as in (123).
(P∗=|s|q1−I∗2
r
Q∗=|s|I∗
r
(123)
where, S is rated apparent power of the grid. To provide
voltage support to the grid I∗
ris the injected reactive current
as per the grid code.
In this mode, the power regulation is done to track ref-
erence trajectories given in (123). The proposed feedback
sliding control is given as in (124).
i∗
◦=C(−kvev+ ˙v∗
dc −αv
Csgn (sv))) (124)
where, kvis the positive control gain, evdenotes the tracking
error, αvis the sliding gain and svrepresents the sliding
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surface for dc-link voltage control as in (125).
sv=ev(t)+kvZt
0
ev(τ)dτ) (125)
The proposed controller results in a constant dc-link volt-
age when subjected to external disturbances like irradiance.
This is because of the compensation provided by the sliding
control within the feedback system. Hence, the proposed
controller is superior to a conventional PI controller, which
requires its control gains to be adjusted for all the uncertain-
ties to achieve proper tuning.
3) NON-MPPT ALGORITHM WITH MCPC CONTROL
(NMMCPCC) [137]
In [137], the hybrid control strategy is a combination of model
current predictive control (MCPC) algorithm along with a
non-MPPT algorithm. The MCPC algorithm minimizes the
overcurrent in GCPV inverter and injects symmetrical cur-
rents even under faults. To eliminate the dc-link overvoltage
problem, the non-MPPT algorithm evaluates the adjusted
power for the PV array and a new duty cycle is acquired. The
revised duty cycle is then used by the converter controller for
proper tuning the output of PV array.
To alleviate the double grid frequency oscillations in
dc-link voltage, a feedforward compensation is incorporated.
The control diagram for non-MPPT mode is in Figure 11.
FIGURE 11. Control diagram for non-MPPT mode.
In non-MPPT mode, the duty ratio under the fault condition
(U∗
g<UN), is obtained as in (126).
D∗
ref =D+hUPV −U∗
ref ikp+ki
s+UDC −UDC_ref
×kp_dc
ki_dc
s(126)
where, UDC denotes the dc-link voltage and U∗
ref is the ref-
erence voltage of non-MPPT mode. U∗
ref is obtained by the
following set of equations given in (127).
Here, the fault voltages in the d-q frame of reference are
represented by U∗
gd ,U∗
gq.
P∗
PV =AU∗
ref ISC "1−C1 e
U∗
ref
(MC2UOC )−1!#
P∗
PV =1.5U∗
gqI2
N−I2
qset
U∗
g=rU∗
gd 2+U∗
gq2
A=NS
Sref 1+αt−tref
(127)
In comparison to the conventional dual-loop control (outer
voltage control loop and inner current control loop), this
scheme eliminates the use of inner loop PI controller, PWM
module and sequence separation techniques which result in
balanced injected currents even under unbalanced fault con-
ditions. The dc-link voltage is maintained at a constant level
and double harmonics components are removed by using
feedforward compensation.
Several other controllers have been proposed that help in
maintaining a constant dc-link voltage [138]–[140]. By using
the power references in (123) a constant dc-link voltage strat-
egy is proposed in [141]. The strategy for PV inverter is devel-
oped based on a robust model predictive control. To achieve
robustness, a disturbance compensator is employed in the sys-
tem, which alleviates the tracking errors in the steady-state.
In [142], an improved dynamic voltage regulation (IDVR)
method is proposed to regulate the dc-link voltage with the
help of a sliding mode controller along with a disturbance
observer (SMC +DOB) in dc microgrids. The SMC ensures
that the dc-link voltage is kept constant even in the presence
of uncertainties and disturbances. To remove the chattering
problem due to SMC, a saturation function is employed in
place of the signum function. The use of an observer for the
dc-link current helped in reducing the cost by removing the
dc current sensor which helped in improving the reliability of
the controller. In [143], a particle swarm optimization (PSO)
based dc-link voltage control of a two-stage PV is proposed.
A PI controller is employed to maintain the constant dc-link
voltage and the parameters of this PI controller are obtained
with the help of the optimization technique which helps
in improving the dynamic response of the dc-link voltage.
Another metaheuristic approach, namely the whale optimiza-
tion technique (WOADCVC) is proposed in [144] for opti-
mum tuning of the dc-link PI controller. It was reported that
among other meta-heuristic approaches whale optimization
algorithm (WOA) is best for tuning the PI controller.
B. ADAPTIVE DC-LINK VOLTAGE CONTROL
Although a constant dc-link voltage helps in enhancing
the life of the dc-link capacitor, a variable dc-link voltage
controller can assist in maintaining the modulation index
within a certain range. By efficiently controlling the mod-
ulation index, high-quality current can be injected into the
grid. An adaptive dc-link voltage control can also help in
injecting more power as compared to a constant dc-link
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voltage controller. This section discusses the recently devel-
oped control methods that adaptively vary the dc-link voltage.
1) VOLTAGE DROP RATIO BASED CONTROL (VDRBC) [145]
In [145], an adaptive dc-link voltage control method is for-
mulated by ensuring that the inverter operates at a high mod-
ulation index in the linear region. The use of high modulation
index helps in the injection of sinusoidal balanced currents
into the grid which improves the output power quality. Under
normal operating conditions, using the conventional control
strategy, the dc-link voltage is fixed at a constant value.
However, even under balanced voltage sag conditions,
the proposed strategy follows the variable dc-link voltage
reference (V0
dc), unlike the conventional strategy in which
the dc-link voltage reference is fixed at a constant value.
The variation in the update dc-link voltage reference V0
dc is
dependent on the voltage dip as expressed in (128).
Vpv ≤V0
dc =λ∗V∗
dc (128)
where, V∗
dc is the reference value for the dc-link voltage con-
trol and λis the voltage drop ratio tracked by the PLL. Further,
the dc-link capacitor voltage is controlled by regulating the
input and output current of the capacitor as in (129).
U=R(ii−io)dt
C(129)
where, iiis the input current to the capacitor and iois the
output current. It is worth noting that the output current
in (129) is fixed to avoid the nuisance tripping of the inverter.
Hence, the current regulation is achieved by the input current.
As mentioned earlier, this strategy controls the dc-link
voltage which ensures a high modulation index. However,
under asymmetrical voltage sag conditions, the modula-
tion index can be in the over modulation region, especially
when the dc-link voltage reduces below a certain value.
Hence, the operation in the over modulation is avoided by
checking the maximum voltage difference between any two
phases. Unlike the symmetrical voltage drop, double grid
frequency oscillations occur in the case of unsymmetrical
voltage drop. Therefore, under two-phase voltage drop con-
ditions, the maximum and minimum values of the dc-link
voltage are calculated as in (130).
va=V1sin ωtand vb=V2sin (ωt+2π/3)(130)
where, V1and V2in (128) denote the peak values of output
voltages.
The maximum phase difference between phase A and
phase B is given by (131), and the minimum value of the
dc-link voltage to avoid over modulation can be determined
using (132). The circuit diagram of the adaptive dc-link
voltage controller is shown in Figure 12. The strategy is
applicable for both, balanced and unbalanced grid voltage
conditions and a well-designed PIR controller is used for the
dc-link voltage control loop.
max (va−vb)=qV2
1+V2
2+V1V2(131)
Vdcmin =0.866qV2
1+V2
2+V1V2(132)
FIGURE 12. Adaptive dc-link voltage controller.
2) INTERWEAVED DFSOGI CONTROL (IDFSOGI) [146]
In [146], the dc-link voltage is adjusted with respect to the
variations in PCC voltage. This adjustable dc-link voltage
controller: minimizes the switching losses in the power con-
verter devices, helps in reducing high frequency I2R losses in
the inductor and results in the reduction of ripple current. The
reference duty ratio of the converter is evaluated as in (133).
Dref (k)=1−VPVref (k)
VDC (k)(133)
The reference dc-link voltage is determined using (134).
VDCref =µ√3VZ(134)
where VZ=q2(v2
sa+v2
sa+v2
sa)
3, is the phase voltage amplitude.
For an appropriate control action, the dc-link voltage must
be about 10% greater than the voltage at the PCC. Hence,
in (134) the value of µis considered as 1.1. Switching losses
in the inverter and the boost converter are dependent on
the dc-link voltage, hence by keeping the dc-link voltage
variable, these losses can be minimized.
The total energy loss (E) is obtained as in (135). Here,
Pswitch on,Pswitch off are the instantaneous power loss, when
switch is on and off, respectively and ton,toff the total on-time
and off-time, respectively.
E=Zton
0
Pswitch ondt +Pswitch off dt =1
6VDC IVSC ton +toff
(135)
The advantage of variable dc-link voltage is the minimiza-
tion of high frequency ripple current in the inductor. The
ripple current is expressed as in (136).
1I∝(Vs−VDC )(136)
It can be seen in (136), that the ripple current is dependent
on the difference of instantaneous PCC line voltage (Vs) and
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J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
dc-link voltage (VDC ). The fixed dc-link voltage produces
higher ripples in inductor current. As a result, the grid current
is also influenced by these ripple currents.
By keeping the dc-link voltage close to the grid line volt-
age, these ripple currents can be reduced. With the help of the
proposed strategy, more power is fed to the grid as compared
to the injection of less power using the conventional control
strategy with fixed dc-link voltage. The controller also results
in a low THD of less than 5% in the presence of nonlinear
load.
3) CPI BASED DC-LINK VOLTAGE CONTROL
(CPIDCVC) [147]
It is clear now that the switching losses are dependent on the
value of the dc-link voltage. In the case of fixed dc-link volt-
age, the switching losses are higher under both, normal and
unbalanced grid conditions. Hence, another adaptive dc-link
voltage control strategy is proposed in [147]. This strategy
reduces the switching losses by adaptively changing the ref-
erence dc-link voltage with respect to the PCC voltage. The
reference value of the dc-link voltage is obtained as in (137).
VDCref =τVpcc,where τ > 1 (137)
To ensure that dc-link voltage remains higher as compared
to the PCC voltage, the value of τis taken as 1.1 as in [146].
Another strategy is proposed in [135] which reduces the
dc-link voltage to its minimum possible value to inject more
power into the grid. To avoid the operation of the inverter in
over modulation region, a linearization strategy is employed
which helps in improving the transient and dynamic perfor-
mance of the system. In [148], another attractive approach is
presented, in which an adaptive PI controller is used to obtain
different control targets like stability, dynamic response,
disturbance rejection and low overshoot. In this scheme,
the control gains of the PI controller are adjusted adaptively
by employing an anti-wind-up scheme, which effectively
reduces the transients in the dc-link voltage. A comparative
table on the above-mentioned dc-link voltage control strate-
gies is prepared, based on their distinct characteristics as
in Table 6.
V. OTHER MISCELLANEOUS CONTROL STRATEGIES
Apart from the above-discussed control strategies, few addi-
tional challenges exist that are addressed by the following
control strategies.
A. VOLTAGE COMPENSATION CALCULATION CONTROL
STRATEGY (VCCCS)
In [149], a multi-objective strategy implemented in the d-q
reference frame is formulated. The strategy performs well
under symmetrical and asymmetrical grid voltage conditions.
It helps in generating sinusoidal voltage and currents and
alleviating the need for a switch for a transition from MPPT
to non-MPPT mode. Inverter currents are limited by adjusting
the reference dc-link voltage (V∗
dc), thereby utilizing the pos-
itive sequence of d component. The q-component is utilized
to supply the reactive power.
FIGURE 13. Control structure of the voltage compensation method.
FIGURE 14. Voltage compensation calculation unit.
A voltage compensation calculation (VCC) unit is devel-
oped to curtail down the active power during voltage sag.
A new dc-link reference (V∗
d) is obtained by adding a com-
pensating value (Vcom) to the optimum value (Vopt ).
By taking the tolerance of 10%, the compensating voltage
for the positive sequence is obtained as in (138)
Vcom−p= −1Vdp(Vdp −0.9) (138)
Similarly, the compensating voltage for negative sequence
is obtained as in (139)
Vcom−n= −1Vdn(−Vdn −0.1) (139)
Here, Vdp and Vdn are the positive and negative sequence
voltage of d component after fault, respectively. By utilizing
(138) and (139) it is ensured that, the V∗
dc is always less than
Voc. where, Voc is the open-circuit voltage of the PV array.
The control and calculation unit of the voltage compensation
method is shown in Figure 13 and 14, respectively.
B. KRUSH-KUHN-TUCKER BASED CONTROL (KKTBC)
Another optimization strategy in the d-q frame of reference
to generate current references by employing Karush-Kuhn-
Tucker (KKT) is proposed by [150]. This strategy is designed
by considering the X/R ratio of the system which helps
in differentiating between the weak and stiff grid. It also
provides voltage support by enhancing the positive sequence
component and minimizes the negative sequence component.
To prevent the activation of overcurrent protection in the
inverter, the necessary condition is given in (140).
Imax =pI+2+I−2=max(Ia(peak),Ib(peak ),Ic(peak )(140)
Although the condition in (140) is necessary, it does not
guarantee the prevention of overcurrent protection. Hence, an
inequality constraint of (141) is also considered as opposed
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TABLE 6. Comparison between DC-link voltage control strategies under low-voltage-ride-through condition.
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J. Joshi et al.: Comprehensive Review of Control Strategies to Overcome Challenges During LVRT in PV Systems
to the strategy proposed in [54].
Imax ≤Ioc (141)
where, Ioc is the overcurrent protection threshold that the
inverter switches can sustain.
The optimal solutions by employing KKT are obtained as
in (142)–(145).
i+
d∗=Imax
R
pR2+(ωL)2(142)
i+
q∗=Imax
ωL
pR2+(ωL)2(143)
i−
d∗= −Imax
R
pR2+(ωL)2(144)
i−
q∗= −Imax
ωL
pR2+(ωL)2(145)
C. ACTIVE AND REACTIVE CURRENT INJECTION BASED
CONTROL (AARCIBC)
The disadvantages of the traditional LVRT control scheme
are:
1) It is less effective for low voltage distribution net-
works (LVDN) as the resistive component is prominent
in this type of network.
2) The existing resources are not fully utilized. The dc-link
capacitor can be utilized to absorb or release a certain
amount of energy in transient voltage event, which has
not been pondered in previous works.
Considering the drawbacks of conventional strategies,
an improved LVRT strategy for LVDN is proposed in [151].
In this, a mathematical analysis is carried out to prove that
the active current injection (ACI) in LVDN with a high R/X
ratio, is as effective as reactive current injection (RCI) in high
X/R ratio networks to provide voltage support. Under normal
operating conditions, RCI is employed to support the voltage
at PCC. However, under severe grid fault, the ACI supports
the PCC voltage in LVDN.
The optimization problem is formulated as in (146) – (150).
v=hIref
pv Iref
dIref
qUref
dc iT(146)
Obj :arg max Id+Epv (147)
Subject to Pref
pv ≤Pmpp
pv (148)
Iref
q=min 2.Ug−Urated
Urated
.Irated ,Irated !
(149)
Iref
d≤q1.12.I2
rated −Iref 2
q(150)
Umin
dc ≤Uref
dc ≤Umax
dc (151)
where, vis the vector of decision variables, which includes
PV output current reference, current references of d and q
component and reference dc-link voltage.
The main aim is to maximize the ACI during faults and
PV energy harvesting. The environmental constraint in (145)
ensures that PV reference power should not exceed the PV
power at MPP under fault conditions. Using (149), Iref
qis
determined under LVRT condition, where Ugis the RMS
phase voltage at PCC. To maximize the ACI, the maximum
allowable output current of GCPV inverter is set to be 1.1 pu
during unbalanced grid conditions. The maximum injected
active power of the GCPV inverter is obtained as in (152).
Pmax
g=s1.12−2−2Ug
Urated 2
.Ug
Urated
.Po(152)
where, Poand Urated are the rated output power and RMS
phase voltage of GCPV inverter, respectively. Based on the
different PCC voltage and environmental conditions, three
modes of operations are proposed. In mode 1, when Ug≥0.9
pu, the PV generator works under normal operating condi-
tions with MPPT execution.
In mode 2, when Ug≤0.9 pu, and Pmax
g≤Pmpp
pv , the PV
inverter is operating under LVRT. It fulfills RCI requirements
as per grid code and the remaining power capacity of the
GCPV inverter is utilized through ACI. During this mode,
the dc-link capacitor also stores some extra PV energy.
In mode 3, when Ug≤0.9 pu, and Pmax
g≥Pmpp
pv , LVRT
control is activated and MPPT is maintained. Moreover,
the dc-link voltage is released to fulfill the ACI requirements.
In this mode to avoid over-modulation, in a three-phase sys-
tem, the dc-link voltage is maintained as in (153).
Uref
dc ≥Umin
dc =2√2Ug(153)
D. REACTIVE POWER SUPPORT WITH APC (RPSWAPC)
A strategy for a LV network with low X/R ratio is presented
in [152]. The scheme presents novel reactive power sup-
port that works well for under and over voltage conditions
by considering the grid impedance. The controller shifts to
APC mode if the reactive power support is not sufficient to
ensure the PCC voltage does not go beyond the over voltage
limits. Injection of active power is given more priority than
reactive power for better voltage support. To provide better
voltage support, active power is reduced during over voltage
conditions. Hence, the scheme also works well under high-
voltage-ride-through (HVRT). Unlike conventional peak cur-
rent limiter, this scheme directly calculates the peak values in
injected current and minimizes the active and reactive power
references. While minimizing the powers, reactive power is
given more priority, however, under severe voltage sag, both
power references are minimized.
Furthermore, under unbalanced grid conditions, it employs
both sequence components for better voltage support. The
strategy provides a smooth ride-through operation even for
sudden grid faults, without any current overshoots.
The PCC voltage amplitude is given in (154) by assuming
a small power angle.
VPCC ∼
=Vg+(PPV −PL)Rg
VPCC +(QPV −QL)Xg
VPCC
(154)
To remove the active oscillations and reducing the ripples
of dc-link voltage, the reference currents are formulated in
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the d-q reference frame as in (155)–(158).
iref +