Low-Voltage Ride-Through of Single-Phase Transformerless Photovoltaic Inverters

TL;DR: In this article, the LVRT capability of three mainstream single-phase transformerless PV inverters under grid faults is explored in order to map future challenges, and control strategies with reactive power injection are also discussed.

Abstract: Transformerless photovoltaic (PV) inverters are going to be more widely adopted in order to achieve high efficiency, as the penetration level of PV systems is continuously booming. However, problems may arise in highly PV-integrated distribution systems. For example, a sudden stoppage of all PV systems due to anti-islanding protection may contribute to grid disturbances. Thus, standards featuring with ancillary services for the next-generation PV systems are under a revision in some countries. The future PV systems have to provide a full range of services as what the conventional power plants do, e.g., low-voltage ride-through (LVRT) under grid faults and grid support service. In order to map future challenges, the LVRT capability of three mainstream single-phase transformerless PV inverters under grid faults is explored in this paper. Control strategies with reactive power injection are also discussed. The selected inverters are the full-bridge (FB) inverter with bipolar modulation, the FB inverter with dc bypass, and the Highly Efficient and Reliable Inverter Concept (HERIC). A 1-kW single-phase grid-connected PV system is analyzed to verify the discussions. The tests confirmed that, although the HERIC inverter is the best candidate in terms of efficiency, it is not very particularly feasible in case of a voltage sag. The other two topologies are capable of providing reactive current during LVRT. A benchmarking of those inverters is also provided in this paper, which offers the possibility to select appropriate devices and to further optimize the transformerless system.

Summary

A. Current Control Loop

• For the current control loop, as detailed in Fig. 4, the existing control methods, such as Proportional Resonant (PR), Resonant Control (RSC), Repetitive Controller (RC), and Deadbeat Controller (DB) can be adopted directly, since they are capable to track sinusoidal signals without steady-state errors [14], [17], [36]-[39].
• As it is shown in Fig. 4, the implementation of a PI-based current control loop in the synchronous rotating reference frame requires a signal generation system, which can produce a quadrature component corresponding to the input, and thus the complexity increases [37].
• By introducing Harmonic Compensators (HC) for the controller [13], [14] and adding passive damping for the filter, an enhancement of the current controller tracking performance can be achieved.
• In fact, the PV systems have been dominated by residential applications with low rated power and low voltage grid.

C. Reactive Power Injection Strategies

• The “Power Profiles” unit in Fig. 3 is used to generate the average active power and reactive power references for the power controllers, and subsequently, the references are controlled to produce the grid current reference as discussed previously.
• Through reactive power injection during LVRT, the grid voltage can be stabilized and also an avoidance of PV power generation can be achieved [2], [9], [21].
• Thus, the following presents the reactive injection strategies for single-phase systems, starting with an overview of possible reactive power injection strategies for three-phase applications.
• Since there is an interaction between voltage sequences and current sequences under grid faults, either the controlled active power or the controlled reactive power will present oscillations [56].
• Thus, in [56], the zero-sequence control path has been introduced to further increase the control freedoms and to eliminate the oscillations in the controlled power.

1) Constant Peak Current Strategy

• There is no risk of inverter shutdown due to overcurrent protection, since the peak of the injected grid current is kept constant during LVRT.
• The injected reactive current level (Iq) is calculated according to Fig. According to Fig. 6, the PV inverter should generate full reactive power (Iq=IN) when vg < 0.5 p.u..
• The phasor diagram for this control strategy is shown in Fig. 7(b), from which it can be observed that the output active power decreases (Id<IN and Vg<Vgn) during LVRT.

2) Constant Active Current Strategy

• Another control possibility under LVRT operation is to keep the active current constant.
• With this reactive power injection strategy, the amplitude of the injected current may exceed the inverter limitation (Imax).
• In such a case, the PV system should also de-rate the active power output in order to generate enough reactive power.
• Otherwise, over-rated operations may introduce failures to the whole system and shorten the inverter serving time, and thus the maintenance cost increases.

3) Constant Average Active Power Strategy

• Similar to the constant active current control strategy, a more intuitive way to maximize output energy (i.e., to deliver maximum active power) is to keep the average active power constant during LVRT.
• Thus, the following constraint should be satisfied to avoid inverter shutdown due to overcurrent protection.
• It is observed in Fig. 8 that an effective power calculation method in terms of fast dynamic response and accurate computation, together with an advanced synchronization unit, can contribute to the LVRT performance of the entire system.
• The average power calculations are based on the Discrete Fourier Transformation (DFT).

A. Simulation Results

• Simulations are firstly tested in MATLAB using PLECS blockset for the modelling.
• As it is shown in Fig. 10 and Fig. 11, in a wide range of grid voltage level, the FB-Bipolar inverter can provide required reactive power during LVRT operation.
• Since the HERIC inverter is disconnected from the grid when the transformerless inverter is also short-circuited in order to avoid leakage currents, the inverter can only operate at unity power factor (i.e. no reactive power injection capability) [13], [28].

B. Experimental Tests (FB System)

• Fig. 12 and Fig. 13 show the experimental results for a singlephase FB system.
• Since the constant peak current control strategy is used in the tests, the amplitude of the grid current is kept constant during LVRT (Fig. 13(c)), which validates its effectiveness.
• Nevertheless, those tests demonstrate the effectiveness of the power control method and the reactive power injection strategy used in this paper in terms of fast response and feasible compliance to the upcoming grid requirements.
• A benchmarking of those inverters has also been presented in terms of efficiency, LVRT capability with reactive power injection, current stresses and leakage current rejection.
• Moreover, due to the high switching frequency for the extra devices of the FB-DCBP, high current stresses might appear and further introduce failures to the whole system.

Figures

TABLE I SIMULATION AND EXPERIMENTAL PARAMETERS.

Fig. 5. Low voltage ride-through requirements defined in different countries covering a wide range of applications [9]-[12], [17]-[19].

Fig. 6. Reactive current injection requirements for medium- and/or highvoltage wind turbine power systems defined in E.ON grid code [11], [12].

Fig. 7. Representations of the grid current and the grid voltage of a single-phase PV system with different reactive power injection strategies (vg ≥0.5 p.u.).

Fig. 12. Common mode voltage of a 1 kW full-bridge inverter (DC voltage: 400 V) with different modulation strategies: vCMV [500 V/div].

Fig. 13. Low voltage ride through operation of a 1 kW single-phase full-bridge system with bipolar modulation and constant peak current control strategy (0.43 p.u. voltage sag): (a) grid voltage vg [100 V/div] and grid current ig [5 A/div], (b) average active power P [500 W/div] and average reactive power Q [500 Var/div], and (c) transient behavior of grid voltage vg [100 V/div] and grid current ig [5 A/div].

Fig. 8. Closed loop control system of a single-phase transformerless system with low voltage ride through capability based on the single-phase PQ theory and PR+HC current controller.

Fig. 10. Average current stresses of IGBT devices in the three transformerless PV inverters with different voltage levels: I- normal operation (0.9 p.u.≤vg < 1.1 p.u.), II-LVRT with constant peak current control (0.5 p.u.≤vg < 0.9 p.u.), and III-full reactive power injection (vg < 0.5 p.u.).

Fig. 9. Experimental setup of a single-phase full-bridge system.

Fig. 1. A single-phase Full-Bridge (FB) grid-connected PV system with an LCL-filter.

Fig. 2. Two main grid-connected transformerless PV systems with LCL-filter (SD-IGBT module, S-IGBT, D-Diode).

TABLE II BENCHMARKING OF THE THREE TRANSFORMERLESS INVERTERS.

Fig. 11. Performance of the three grid-connected transformerless PV systems in low voltage ride through operation (0.43 p.u. voltage sag): grid voltage vg [V], grid current ig [30×A], active power P [W], reactive power Q [Var], and common mode voltage vCMV [V].

Fig. 4. Implementation of current control loop for single-phase single-stage systems in different reference frames.

Fig. 3. Hardware schematic and control diagram of single-phase transformerless grid-connected PV systems with low voltage ride-through capability.

Full text

Aalborg Universitet
Low Voltage Ride-Through of Single-Phase Transformerless Photovoltaic Inverters
Yang, Yongheng; Blaabjerg, Frede; Wang, Huai
Published in:
IEEE Transactions on Industry Applications
DOI (link to publication from Publisher):
10.1109/TIA.2013.2282966
Publication date:
2014
Document Version
Early version, also known as pre-print
Link to publication from Aalborg University
Citation for published version (APA):
Yang, Y., Blaabjerg, F., & Wang, H. (2014). Low Voltage Ride-Through of Single-Phase Transformerless
Photovoltaic Inverters. IEEE Transactions on Industry Applications, 50(3), 1942-1952.
https://doi.org/10.1109/TIA.2013.2282966
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Digital Object Identifier (DOI): 10.1109/TIA.2013.2282966
IEEE Transactions on Industry Applications, in press, online available on 20 September 2013
Low Voltage Ride-Through of Single-Phase Transformerless Photovoltaic Inverters
Yongheng Yang
Frede Blaabjerg
Huai Wang
Suggested Citation
Y. Yang, F. Blaabjerg, and H. Wang "Low voltage ride-through of single-phase transformerless
photovoltaic inverters,” IEEE Trans. Industry Applications, in press.

Abstract – Tra
nsformerless photovoltaic (PV) inverters are
going to be more widely adopted in order to achieve high
efficiency, as the penetration level of PV systems is continuously
booming. However, problems may arise in highly PV-integrated
distribution systems. For example, a sudden stoppage of all PV
systems due to anti-islanding protection may contribute to grid
disturbances. Thus, standards featuring with ancillary services for
the next generation PV systems are under a revision in some
countries. The future PV systems have to provide a full range of
services as what the conventional power plants do, e.g. Low
Voltage Ride-Through (LVRT) under grid faults and grid support
service. In order to map future challenges, the LVRT capability of
three mainstream single-phase transformerless PV inverters
under grid faults are explored in this paper. Control strategies
with reactive power injection are also discussed. The selected
inverters are the full-bridge inverter with bipolar modulation, the
full-bridge inverter with DC bypass and the Highly Efficient and
Reliable Inverter Concept (HERIC). A 1 kW single-phase grid-
connected PV system is analyzed to verify the discussions. The
tests confirmed that, although the HERIC inverter is the best
candidate in terms of efficiency, it is not very special feasible in
case of a voltage sag. The other two topologies are capable of
providing reactive current during LVRT. A benchmarking of
those inverters is also provided in this paper, which offers the
possibility to select appropriate devices and to further optimize the
transformerless system.
Index Terms – Low voltage ride-through, grid support, single-
phase systems, photovoltaic (PV), transformerless inverters,
reactive power injection, efficiency, leakage current elimination.
I. INTRODUCTION
HE YEAR of 2012 has been another year for an extraord-
inary growth of photovoltaic (PV) systems with total
global operating capacity reaching the 100 GW milestone [1].
However, this high penetration level of PV systems may also
introduce negative impacts on the grid. Concerns like power
quality issues, the efficiency and the emerging reliability are
becoming of high interest and intense importance [2]-[9]. Thus,
many grid codes have been released to regulate PV systems
integration with the distributed grid [10]-[20]. Since PV
systems are typically connected to low-voltage and/or medium-
voltage distributed networks, the grid standards are mainly
focused on power quality issues, frequency stability and voltage
stability [13]. It is required that PV systems should cease to
energizing local loads in presence of a grid fault, e.g. a voltage
sag and a frequency disturbance [13], [17], which is known as
an anti-islanding protection.
Due to the still declined PV cell price and the advanced
power electronics technology, the penetration degree is going
to be much higher. In view of this, the impact of highly
penetrated PV systems, even serving low-voltage networks, on
the grid cannot be neglected anymore. A sudden stoppage of all
grid-connected PV systems in an unintentional islanding
operation mode could trigger much more severe grid problems
than the initial event, e.g. power outages and voltage flickers
[2], [10], [21]. In order to solve the potential issues, several
European countries have updated the grid codes for low- or
medium-voltage systems. The next generation PV systems have
to provide a full range of services as what the conventional
power plants do. For instance, the German grid code requires
that the generation systems connected to the medium- or high-
voltage networks should have LVRT capability under grid
faults [12], [17]. In the new Italian grid code, the generation
units connected to low-voltage grid with the nominal power
exceeding 6 kW have to ride through grid voltage faults [18].
Other countries like Japan [19]-[22] are undertaking a revision
of their current active grid standards in order to accept more PV
energy in the line. However, some standard committees, e.g.
IEEE Standard Committee, still have some catching up to do
[23].
Besides the ancillary services, achieving high efficiency and
high
reliability are always required in PV systems in order to
reduce energy losses and extend service time [3], [7], [8], [24].
Compared to conventional PV systems, transformerless
systems are increasing in popularity, especially in European
markets, because of the high efficiency [13], [25]-[35]. Many
transformerless topologies are derived by adding extra power
devices into the Full-Bridge (FB) inverter. For example, the FB
inverter with DC bypass (FB-DCBP) adds two power devices
at the DC-side [26], [27]; while the HERIC topology provides
an AC bypass leg [29]. Considering the fast growth of grid-
connected PV systems, it is better for the next generation
Low Voltage Ride-Through of Single-Phase
Transformerless Photovoltaic Inverters
Yongheng Yang, Student Member, IEEE, Frede Blaabjerg, Fellow, IEEE, and Huai Wang, Member, IEEE
T
Manuscript received
July 3, 2013; revised September 2, 2013; accepted
September 6, 2013. Paper 2013
-IPCC-572.R1, presented a
t the 2013 IEEE
Energy Conversion and Exposition, Denver, CO, USA, September 15
-
19, and
approved for publication in the IEEE
TRANSACTIONS ON I
NDUSTRY
APPLICATIONS
by the Industrial Power Converter Committee of the IEEE
Industry Applications Society.
Y. Ya
ng, F. Blaabjerg, and H. Wang are with the Department of Energy
Technology, Aalborg University, DK
-9220 Aalborg East, Denmark. (e
-mail:
yoy@et.aau.dk
; fbl@et.aau.dk; hwa@et.aau.dk).
Color versions of one or more of the figures in this paper are available online
at
http://ieeexplore.ieee.org.
Digital Object Identifier

transformerless PV inverters to equip with LVRT capability in
order to fulfill the upcoming requirements efficiently and
reliably.
Current stresses, power losses on the switching devices and
dynamic responses of transformerless inverters are dependent
on the topology configuration in both normal operation and
LVRT operation mode. Thus, it is necessary to explore the
performance of these PV systems under different conditions. In
this paper, three transformerless PV inverters – FB inverter with
bipolar modulation (FB-Bipolar), FB-DCBP inverter and the
HERIC inverter are studied in terms of current stresses,
efficiency, and LVRT capability with reactive power injection.
Firstly, a brief introduction of the selected inverters is given.
Then, the focus is shifted to the control of transformerless PV
systems under grid faults. Control strategies and reactive power
injection possibilities for single-phase PV systems are
discussed in § III. Simulation results of LVRT operation
examples are demonstrated in § IV, as well as experimental
tests of a FB inverter system. A benchmarking of the selected
inverters mainly in terms of leakage current elimination, LVRT
capability and efficiency is presented before the conclusions.
II. S
INGLE-PHASE TRANSFORMERLESS PV INVERTERS
Underpin
ned by the advanced and dedicated control
methods, the PV inverters are responsible for converting DC
source generated from PV panels to AC source efficiently and
reliably. A widely adopted single-phase PV inverter is the FB
topology as shown in Fig. 1, where it is connected to the grid
through an LCL-filter in order to ensure the injected current
quality. There are two main modulation strategies available for
this inverter: a) unipolar modulation scheme and b) bipolar
modulation scheme.
When the transformer is removed from a grid-connected PV
system, safety concerns (e.g. leakage current) will arise since
the lack of galvanic isolations. Thus, transformerless inverters
should eliminate or at least reduce the leakage current, e.g. by
including passive damping components and/or by modifying
the modulations [26]. In the light of this, the FB-Bipolar is more
feasible in single-phase transformerless PV applications.
However, in every switching period, there are reactive power
exchange between the LCL-filter and the capacitor C
PV
and also
core losses in the output LCL-filter, leading to a low efficiency
of up to 96.5% [13].
In order to further improve the efficiency and reduce the
leakage current, a tremendous number of transformerless
topologies have been developed [13], [25]-[35], most of which
are based on the FB inverter as it is shown in Fig. 1. As afore-
mentioned, the first priority of a transformerless inverter is to
avoid the generation of a varying instantaneous Common-Mode
Voltage (CMV, v
CMV
), since the CMV will induce a common-
mode current (leakage current). The relationships can simply be
described as,
2

AO BO
CMV
vv
v
, (1)
CMV
CMV P
dv
iC
dt
, (2)
where v
AO
and v
BO
are the voltages of the two midpoints of a FB
inverter shown in Fig. 1, i
CMV
is the common-mode current, and
C
P
is the stray capacitor between PV panels and the ground.
Besides those solutions to limit the leakage current by adding
passive damping components and by modifying the modulation
techniques, the elimination can also be achieved either by
disconnecting the PV panels from the inverter or by providing
a bypass leg at the AC side. For instance, the FB-DCBP inverter
patented by Ingeteam [26], [27] shown in Fig. 2(a) disconnects
the PV panels from the inverter using four extra devices (two
switching devices SD
5
, SD
6
and two diodes D
7
, D
8
); while the
HERIC inverter (Fig. 2(b)) by Sunways [29] provides an AC
bypass using two extra switching devices (SD
5
, SD
6
). There
have been other transformerless topologies reported in the
literature. Some are based on the multi-level topologies [31]-
[33], and some are derived by optimizing traditional transform-
erless inverters [34], [35].
In respect to the modulation of a transformerless inverter, it
should not generate a varying CMV. With a dedicated
modulation scheme for those inverters, there is no reactive
power exchange between the LCL-filter and the capacitor C
PV
at zero-voltage states, and thus higher efficiency is achieved.
However, extra power losses, including switching losses and
conduction losses, will appear on the required additional
Full-bridge
C
PV
O
v
g
LCL- Filter
D
1
D
2
D
3
D
4
A
B
PV Panels
i
PV
C
P
S
1
S
2
S
3
S
4
i
CMV
Ground
v
CMV
Fig.
1. A single-phase Full-Bridge (FB) grid-connected PV system
with an
LCL
-filter.
DC Bypass
C
PV1
v
g
C
PV2
SD
5
SD
6
D
7
D
8
Full-Bridge
i
PV
LCL- Filter
SD
1
SD
2
SD
3
SD
4
O
A
B
PV Panels
C
P
(a) Full-bridge with DC bypass topology [26], [27]
AC Bypass
SD
5
SD
6
Full-Bridge
i
PV
C
PV
v
g
SD
1
SD
2
SD
3
SD
4
LCL- Filter
O
A
B
PV Panels
C
P
(b) Highly efficient and reliable inverter concept, HERIC [29]
Fig.
2. Two main grid-connected transformerless PV systems with LCL
-filter
(SD
-IGBT module, S-IGBT, D-Diode).

switching devices in these inverters as shown in Fig. 2.
Moreover, the power losses of an individual switching device
are dependent on its commutation frequency, which differs with
inverter topologies, and its electrical stress. For example, the
extra devices, S
5
and S
6
in the FB-DCBP inverter are
commutated at a high switching frequency (e.g., 10 kHz); while
those in the HERIC inverter commutate at the line fundamental
frequency (e.g., 50 Hz). Since the total power losses will further
introduce redistributions of both current and thermal stresses on
the devices among these inverters, the efficiency and the
lifetime will be affected [3], [7].
Concerning LVRT operation, the control systems and the
dynamic response of the above inverters possibly differ with the
configurations and the modulation schemes. They may have a
significant impact on the capability of reactive power injection
to support the grid voltage recovery under grid faults.
Moreover, the overstresses on the switching devices may also
cause failures during LVRT and thus increase the maintenance
cost. Those aspects should be taken into consideration for the
design and operation of transformerless PV systems. Thus,
essentially, this paper explores the performance of the
mainstream transformerless inverters with the consideration of
such operation conditions.
III. C
ONTROL OF TRANSFORMERLESS PV INVERTERS
UNDER
GRID FAULTS
According to the grid requirements, the design of next
generation transformerless PV systems should take into account
not only the shape of grid current (power quality issues), but also
the behavior of reactive power injection under grid faults. Fig. 3
shows the hardware schematic and overall control structure of a
single-phase single-stage transformerless PV system with
LVRT capability.
Typically, the control strategy applied to a single-phase grid-
connected system includes two cascaded loops [13], [14]:
a) An inner current control loop, which has th
e
responsibilities of power quality issues and current
protection of the inverter and,
b) An outer voltage control (or power control) loop, in
which the grid voltage is controlled to generate desired
current references for the inner control loop
.
A. C
urrent Control Loop
For the current control loop, as detailed in Fig. 4, the existing
control methods, such as Proportional Resonant (PR), Resonant
Control (RSC), Repetitive Controller (RC), and Deadbeat
Controller (DB) can be adopted directly, since they are capable
to track sinusoidal signals without steady-state errors [14], [17],
[36]-[39]. Further, applying the Park transformation (αβÆdq)
leads to the possibility of Proportional Integral (PI) controllers
to regulate the injected current, and afterwards, the modulation
reference v
*
inv
can be obtained by means of the inverse Park
transformation (dqÆαβ) [37], [40]. However, as it is shown in
Fig. 4, the implementation of a PI-based current control loop in
the synchronous rotating reference frame requires a signal
generation system, which can produce a quadrature component
corresponding to the input, and thus the complexity increases
[37]. Since the current control loop is responsible for the power
quality, this responsibility should also be effective and valid in
the design of current controllers and also the LCL-filter. By
introducing Harmonic Compensators (HC) for the controller
[13], [14] and adding passive damping for the filter, an
enhancement of the current controller tracking performance can
be achieved.
(a) power calculation and signal generation
P Controller
Q
*
Q Controller
Power
Calculation
Orthogonal
Signal
Generator
Orthogonal
Signal
Generator
i
g
v
g
i
q
P
Q
MPPT
i
d
P
Q
v
gαβ
i
αβ
* v
gd
v
gq
i
d
*
i
q
PI
PI
dqÆαβ
*
i
q
*
i
d
i
g
(b) inner current control loop
(c) outer control loop for current reference generation
stationary reference frame synchronous rotating reference frame
i
αβ
αβÆdq
i
g
*
PR, RSC,
RC, or DB
v
inv
*
v
inv
*
-ωL
ωL
HC
Fig.
4. Implementation of current control loop for single-phase single-
stage
systems in different reference frames.
C
f
i
pv
S
1
L
if
Grid
L
g
Full-Bridge Inverter
S
2
S
3
S
4
L
gf
LCL-Filter
Acquisition & Calculation
Current Controller Power Controllers
Sag Detection
Power Profiles
i
g
v
g
i
g
P
Q
P
*
Q
*
i
pv
v
pv
Sag Signal
v
pv
v
gα
S
1
S
2
R
s
R
L
Fault Generator
v
inv
*
i
g
*
v
gβ
DC Bypass Switches
AC Bypass Switches
C
pv
PWM
v
inv
i
g
Acquisition
i
pv
v
pv
i
pv
PV Panels
D
1
D
2
D
3
D
4
v
PWM
*
R
g
v
g
A
B
O
Fig.
3. Hardware schematic and control diagram of single-phase transformerless grid-connected PV systems with low voltage ride-through capability.

Frequently asked questions

Q1. What have the authors contributed in "Low voltage ride-through of single-phase transformerless photovoltaic inverters" ?

The future PV systems have to provide a full range of services as what the conventional power plants do, e. g. Low Voltage Ride-Through ( LVRT ) under grid faults and grid support service. In order to map future challenges, the LVRT capability of three mainstream single-phase transformerless PV inverters under grid faults are explored in this paper. A benchmarking of those inverters is also provided in this paper, which offers the possibility to select appropriate devices and to further optimize the transformerless system. 

Q2. Why is the PR+HC controller selected in this paper?

Since the PR+HC controller presents a good performance in terms of accurate tracking (harmonic rejection) [13], [14], [38], [39], this controller is selected in this paper as the inner current controller. 

Q3. Why is the SOGI-PLL selected as the synchronization unit?

In this paper, the SOGI based Phase Locked Loop (SOGI-PLL) has been selected as the synchronization unit because of its robustness [9], [13], [14], [45]. 

Q4. How can a synchronous controller be improved?

By introducing Harmonic Compensators (HC) for the controller [13], [14] and adding passive damping for the filter, an enhancement of the current controller tracking performance can be achieved. 

Q5. What is the way to ride-through voltage fault?

For this inverter, a possible way to ride-through voltage fault is to modify the modulation scheme during LVRT but at the cost of reducing efficiency. 

Q6. What is the transfer function of the current controller?

4. The transfer function of this current controller can be given as,22 2 23,5,70 0 rh i p r h k ssG s k k s s h , (3)in which kp is the proportional gain, kr is the fundamental resonant control gain, krh is the control gain for h-order resonant controller (h = 3, 5, 7) and ω0 is the grid fundamental frequency. 

Q7. What is the amplitude of the grid current during LVRT?

Since the constant peak current control strategy is used in the tests, the amplitude of the grid current is kept constant during LVRT (Fig. 13(c)), which validates its effectiveness. 

Q8. What is the role of the current control loop?

Since the current control loop is responsible for the power quality, this responsibility should also be effective and valid in the design of current controllers and also the LCL-filter. 

Q9. What is the voltage fault in a FB system?

A voltage fault (0.43 p.u.) is generated by switching S1 and S2 of the sag generator shown in Fig. 3 and the experimental setup of a FB system shown in Fig. 

Q10. What are the possible reactive power injection strategies for three-phase systems?

For three-phase applications, the reactive power injection strategies can be summarized as: 1) unity power factor control strategy, 2) positive and negative sequence control strategy, 3) constant active power control strategy and 4) constant reactive power control strategy [13], [14], [21], [50]-[55]. 

Q11. Why is the FB-DCBP inverter so expensive?

due to the high switchingfrequency for the extra devices of the FB-DCBP, high current stresses might appear and further introduce failures to the whole system. 

Q12. What is the LVRT demand for wind turbine power systems?

Although the LVRT demand ( by German grid code) shown in Fig. 5 and Fig. 6 is initially set for medium and/or high voltage applications – wind turbine power systems, it is worth mentioning that low voltage PV systems are already on an upward track to dominate in the electricity generation [1], [2]. 

Q13. What is the phasor diagram for a PV inverter?

In order to avoid inverter shutdown due to over-current protection, the following condition should be fulfilled during the design and the operation of a PV inverter,22 max1 1 g N Ik v The author, (6)where vg is the grid voltage and k ≥ 2 p.u.. 

Q14. What is the LVRT capability of the FB-DCBP inverter?

A benchmarking of those inverters has also been presented in terms of efficiency, LVRT capability with reactive power injection, current stresses and leakage current rejection. 

Q15. How much voltage sag is the average reactive power Q*?

In the cases, the voltage sag is 0.43 p.u., and thus according to Fig. 6 and Fig. 7, the average reactive power Q* should be 490.2 

Q16. What are the main control methods for the current control loop?

For the current control loop, as detailed in Fig. 4, the existing control methods, such as Proportional Resonant (PR), Resonant Control (RSC), Repetitive Controller (RC), and Deadbeat Controller (DB) can be adopted directly, since they are capable to track sinusoidal signals without steady-state errors [14], [17], [36]-[39]. 

Q17. What is the average power reference in a normal operation mode?

In the normal operation mode, the average active power reference P* is the output of a Maximum Power Point Tracking (MPPT) system, as shown in Fig. 4 and the system is required to operate at unity power factor (i.e. Q* = 0 Var). 

Q18. What is the averaged reactive power reference Q*?

According to the requirements defined in Fig. 6, the averaged reactive power reference Q* is a function of the grid voltage level in LVRT operation mode. 

Q19. What are the advantages of the droop-based control method?

in regard to the above control methods, e.g. the PQ control strategies, a fast voltage sag detection and an accurate synchronization system will strongly contribute to the dynamic performance and the stability margin of the whole control systems. 

Q20. What is the purpose of the current control loop?

it offers the possibilities to add control methods into this loop to shape the grid current in LVRT operation mode with the purpose of reactive power injection. 

Q21. What is the voltage sag of the FB-Bipolar inverter?

As it is shown in Fig. 10 and Fig. 11, in a wide range of grid voltage level, the FB-Bipolar inverter can provide required reactive power during LVRT operation.