Aalborg Universitet
Asymmetrical Reactive Power Capability of Modular Multilevel Cascade Converter
Based STATCOMs for Offshore Wind Farm
Tanaka, Takaaki; Ma, Ke; Wang, Huai; Blaabjerg, Frede
Published in:
IEEE Transactions on Power Electronics
DOI (link to publication from Publisher):
10.1109/TPEL.2018.2866398
Publication date:
2019
Document Version
Accepted author manuscript, peer reviewed version
Link to publication from Aalborg University
Citation for published version (APA):
Tanaka, T., Ma, K., Wang, H., & Blaabjerg, F. (2019). Asymmetrical Reactive Power Capability of Modular
Multilevel Cascade Converter Based STATCOMs for Offshore Wind Farm. IEEE Transactions on Power
Electronics, 34(6), 5147-5164. [8443113]. https://doi.org/10.1109/TPEL.2018.2866398
General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners
and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
- Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
- You may not further distribute the material or use it for any profit-making activity or commercial gain
- You may freely distribute the URL identifying the publication in the public portal -
Take down policy
If you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access to
the work immediately and investigate your claim.
IEEE TRANSACTION ON POWER ELECTRONICS
ο
Abstractβ Modular Multilevel Cascade Converters (MMCC)
are becoming attractive solutions as high voltage Static
Synchronous Compensators (STATCOMs) for power plants in
renewable energy generation, in order to satisfy the strict grid
codes under both normal and grid-fault conditions. This paper
investigates the performances of potentially used four
configurations of the MMCC family for the STATCOM in
large-scale offshore wind power plants, with special focus on
asymmetrical Low Voltage Ride Through (LVRT) capability
under grid faults. Specifications and the component sizing of each
type of practical 80 MVar / 33 kV scaled MMCC-STATCOM are
carefully designed and compared. The total cost and volume are
compared based on total power semiconductor chip area and total
energy stored of the passive components. Asymmetrical reactive
power delivering operation of the MMCC family considering the
dc-link capacitor voltage balancing method is solved
mathematically in order to quantitatively understand the
performance limitations and behaviors. The electro-thermal
stress of the power modules used in each type of the MMCC for a
practical 80 MVar /33 kV STATCOM is analyzed. The
asymmetrical reactive power capability of the MMCC solutions is
compared under different scenarios of grid faults, with the
considerations of the device temperature limits and also voltage
saturation. It is found that the MMCC configuration with Double
Star Bridge Cells becomes the most attractive circuit
configuration for the STATCOM application based on the
obtained results.
Index Termsβ Static VAR compensators, Reactive power,
Wind power generation, MMCC, STATCOM, Asymmetrical grid
faults
I. INTRODUCTION
HE capacity of renewable energy generation has continued
to grow in the last decade, and it will become 2.5 TW in
2020 [1]. In accordance with constructions of large-scale
renewable energy generation systems such as solar PV and
Manuscript received Apr. 11, 2018; revised Jun. 23, 2018; accepted Aug. 6,
2018.
Takaaki Tanaka is with Corporate R&D Headquarters, Fuji Electric Co., Ltd,
1910064 Tokyo, Japan (e-mail: tanaka-takaaki@fujielectric.com).
Ke Ma is with the Department of Electrical Engineering, Shanghai Jiao
Tong University, 200240 Shanghai, China (e-mail: kema@sjtu.edu.cn).
Huai Wang, and Frede Blaabjerg are with the Department of Energy
Technology, Aalborg University, 9220 Aalborg, Denmark (e-mail:
hwa@et.aau.dk, fbl@et.aau.dk).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org..
wind power plants, stricter grid codes under both normal
operation and grid fault conditions are demanded by
Transmission System Operators (TSO) in most countries [2].
Offshore wind power plants have become one of the major
renewable energy sources in Europe with strong wind
conditions because of the advantages such as constant and high
wind velocity as well as extensive offshore area. However, the
generated electrical power has to be transmitted to the Point of
Common Cupping (PCC) onshore by long-distance submarine
cables, which arise a large amount of reactive power if Medium
to High Voltage Alternative Current (MVAC-HVAC)
transmission system is selected. In order to compensate enough
reactive power and satisfy the grid codes, SVC or STATCOMs
have to be installed on the onshore side of the wind power
plant.
The Modular Multilevel Cascade Converters (MMCC)
family could be suitable solutions in the case of the
high-voltage and high-power STATCOM application. They
have significant advantages, compared to the conventional
two-level or three-level voltage source converters which have
series-connected bipolar-power semiconductor devices, such as
lower harmonic distortions, transformer-less configuration at
medium voltage level, and modular/ redundancy design.
Nevertheless, voltage-balancing for a large number of DC-link
capacitors in converter cells are still challenging to be achieved
for MMCCs, especially under asymmetrical grid faults [4-7].
MMCC solutions with Single Star Bridge Cells (SSBC) and
Single Delta Bridge Cells (SDBC) have been reported to be
used for STATCOM and battery energy management system
application [8], [9]. They can keep operating under an
asymmetrical grid fault by activating the voltage-balancing
control methods for converter cells such as zero-sequence AC
voltage injection method for SSBC [10], [11] and
zero-sequence AC current injection method for SDBC [12].
However, in order to avoid the voltage saturation and over
junction temperature, these voltage-balancing methods could
result in increased voltage and current stress of the converters
and thereby they may compromise the reactive power
delivering capability of the MMCCs when they are practically
designed for wind farms.
MMCC solutions with Double Star Chopper Cells (DSCC)
and Double Star Bridge cells (DSBC) have been reported to be
used for Back-to-Back converters such as Medium Voltage
motor drive and High Voltage Direct Current (HVDC)
transmission applications typically, but they can also be used
for a STATCOM application. They can keep operating under
Asymmetrical Reactive Power Capability of
Modular Multilevel Cascade Converter (MMCC)
based STATCOMs for Offshore Wind Farm
Takaaki Tanaka, Member, IEEE, Ke Ma, Senior Member, IEEE,
Huai Wang, Senior Member, IEEE, Frede Blaabjerg, Fellow, IEEE
T
IEEE TRANSACTION ON POWER ELECTRONICS
Fig. 1. A typical offshore wind power plant with an MMCC-STATCOM.
(a) LVRT requirement by different countries.
(b) Additional reactive current requirement during LVRT
Fig. 2. Reactive power requirements of large-scale generating plants under
grid fault
Fault
Z
F
Z
S
Shoreline
Bus A (400 kV)
MMCC
STATCOM
Submarine cable
Offshore
wind farm
External gridHVAC transmission system
Bus D (33 kV)
Bus B(220 kV)
Bus C
(33 kV)
Shunt
reactor
TR1
TR2
150 500 1000 1500 2000
20
90
100
Time (ms)
Voltage (%)
Germany
(Regulation: Type 2)
Denmark
UK
0.5 0.9 1.0
100%
Voltage drop:
Ξ
V/V
g
(p.u.)
Required additional reactive
current:ΞI
Q
/I
R
(%)
Dead band
0
asymmetrical grid faults by activating the voltage-balancing
control using circulating dc current having two degrees of
freedom [13-15]. This voltage-balancing method also results in
increased current stress of the converters. However, this
method may have higher reactive power delivering capability
compared with an SSBC and SDBC because of more flexibility
in the circulating dc current.
A lot of authors have proposed many useful control schemes
and design methods for each type of MMCC solution until now.
However, the optimum MMCC solution for the STATCOM
application is still an open question because comprehensively
comparison between four types of the MMCC solutions has not
been done [16, 17, 28, 29]. In addition to total cost and volume
of the MMCC solutions, the asymmetrical reactive power
delivering capability under grid faults becomes more and more
important for the STATCOM application.
This paper clarifies the performances of potentially used four
configurations of the MMCC family with SSBC, SDBC, DSCC,
and DSBC for the STATCOM in large-scale offshore wind
power plants, with special focus on asymmetrical Low Voltage
Ride Through (LVRT) capability under grid faults. In section II,
the system configuration of typical offshore wind power plant
and system grid fault scenarios are summarized. In section III,
specifications and the component sizing of each type of
practical 80 MVar / 33 kV scaled MMCC-STATCOM are
carefully designed and compared. The total cost and volume are
compared based on total power semiconductor chip area and
total energy stored of the passive components. In section IV,
the mathematical formulation for the STATCOM based on the
MMCC solutions under asymmetrical compensation operation
is developed which contributes to quantitative understanding of
the performance limitations and circuit behaviors under the
asymmetrical compensation. In section V, the electro-thermal
stresses of actual power modules used in each type of the
MMCC with practical controls are analyzed in detail. The
asymmetrical reactive power capacity focusing on the MMCC
solutions is compared under different scenarios of grid faults,
with the consideration of device temperature limits and voltage
saturations. Finally, in section VI, most attractive MMCC
solution for the STATCOM application is suggested based on
obtained results.
II. TYPICAL OFFSHORE WIND PLANT AND SYSTEM FAULT
SCENARIOS
A. The system configuration for analysis
Fig. 1 shows the system configuration of a typical offshore
wind power plant and an MMCC-based STATCOM. The
generated active power from the offshore wind farm needs to be
provided to the Point of Common Coupling (PCC) as Bus A
(400 kV in this case) by an HVAC transmission system (220
kV in this case) with long distance submarine cables. Reactive
power induced by the submarine cable is compensated by the
full-scale converters of wind turbines, the shunt reactor, and the
STATCOM connected to Bus B via a delta-star transformer.
Other power generators and loads beside the wind power plant
may also be connected to Bus A.
B. Reactive power requirement under grid fault
Besides the normal operation, Transmission System
Operators (TSOs) in different countries have issued strict grid
supporting requirement for the growing large-scale renewable
power plant such as the offshore wind power plant under grid
fault which is specified in Fig. 2 [18], [19]. According to the
grid codes, the offshore wind power plant has to keep the
operation regarding the voltage sag under grid fault as shown
Fig. 2 (a), and in the case of German and Danish codes should
be able to inject additional reactive current to support the
recovery of grid voltage sag which is also in Fig. 2 (b). The
reactive current reference is only defined as positive-sequence
component because recent grid codes do not require
negative-sequence current to compensate the asymmetrical grid
fault voltage recovering. However, this feature may become a
future requirement [20]. In this paper, the positive sequence
reactive current injection capabilities of each type of MMCC
solution under grid faults are analyzed.
IEEE TRANSACTION ON POWER ELECTRONICS
TABLE I
PHASOR DIAGRAM AND VECTOR DEFINITIONS OF DIFFERENT FAULT
SCENARIOS ON PCC (BUS A)
Fault types
Phasor diagram
diffinitions
Vecter deffinitions
(a)
Three-phase-
to-ground fault
ξ΄Έ
οο¨ξ€΄ο£ο¨
ο
ξ΄Έ
οο©ξ€΄ο£ο¨
οο
ξ₯³
ξ₯΄
 ο ξ΅
ξ¦Ύ
ξ₯΅
ξ₯΄

ξ΄Έ
οοͺο£ο¨
οο
ξ₯³
ξ₯΄
 ο
ξ΅
ξ¦Ύ
ξ₯΅
ξ₯΄

(b)
Single-phase-
to-ground fault
ξ΄Έ
οο¨ξ€΄ο£ο¨
ο
ξ΄Έ
οο©ξ€΄ο£ο¨
οο
ξ₯³
ξ₯΄
ο ξ΅
ξ¦Ύ
ξ₯΅
ξ₯΄
ξ΄Έ
οοͺο£ο¨
οο
ξ₯³
ξ₯΄
ο
ξ΅
ξ¦Ύ
ξ₯΅
ξ₯΄
(c)
Phase-to-phase
short-circuit fault
ξ΄Έ
οο¨ξ€΄ο£ο¨
οξ₯³
ξ΄Έ
οο©ξ€΄ο£ο¨
οο
ξ₯³
ξ₯΄
ο ξ΅
ξ¦Ύ
ξ₯΅
ξ₯΄

ξ΄Έ
οοͺο£ο¨
οο
ξ₯³
ξ₯΄
ο
ξ΅
ξ¦Ύ
ξ₯΅
ξ₯΄

(d)
Two-phase-
to-ground fault
ξ΄Έ
οο¨ξ€΄ο£ο¨
οξ₯³
ξ΄Έ
οο©ξ€΄ο£ο¨
οο
ξ₯³
ξ₯΄
 ο ξ΅
ξ¦Ύ
ξ₯΅
ξ₯΄

ξ΄Έ
οοͺο£ο¨
οο
ξ₯³
ξ₯΄
 ο
ξ΅
ξ¦Ύ
ξ₯΅
ξ₯΄

TABLE II
SEQUENCE VOLTAGE AMPLITUDE DEFINITION OF DIFFERENT GRID FAULTS
SCENARIOS ON BUS C (THE V
S
IS THE RATED VOLTAGE ON BUS C)
Fault types
Each sequence voltage vector
(b)
Single-phase-
to-ground fault
σ°―
ξ΄Έ
οο€
οΎ
ξ΄Έ
οο€
οΏ
ξ΄Έ
ο΄
σ°°οξ΄Έ
ο
ξ³
ξ³
ξ³
ξ³
ξ³

ξ₯΅
ο
ξ₯΄
ξ₯΅

ξ₯Έ
ο
ξ₯³
ξ₯Έ
ο
ξ΅ο¬

ξ₯΄
ξ¦Ύ
ξ₯΅
ο
ξ₯³
ξ₯΄
ξ¦Ύ
ξ₯΅
ο°
ξ₯²
ξ³
ξ³
ξ³
ξ³
ξ³
(c)
Phase-to-phase
short-circuit fault
σ°―
ξ΄Έ
οο€
οΎ
ξ΄Έ
οο€
οΏ
ξ΄Έ
ο΄
σ°°οξ΄Έ
ο
ξ³
ξ³
ξ³
ξ³
ξ³
ξ³

ξ₯΄
ο
ξ₯³
ξ₯΄
ο

ξ₯Ά
ο
ξ₯³
ξ₯Ά
ο
ξ΅
σ°§
ο
ξ¦Ύ
ξ₯΅ξ΄¦
ξ₯Ά
ο
ξ¦Ύ
ξ₯΅
ξ₯Ά
σ°¨
ξ₯²
ξ³
ξ³
ξ³
ξ³
ξ³
ξ³
(d)
Two-phase-
to-ground fault
σ°―
ξ΄Έ
οο€
οΎ
ξ΄Έ
οο€
οΏ
ξ΄Έ
ο΄
σ°°οξ΄Έ
ο
ξ³
ξ³
ξ³
ξ³
ξ³
ξ₯΄
ξ₯΅
 ο
ξ₯³
ξ₯΅
ο

ξ₯Έ
ο
ξ₯³
ξ₯Έ
ο
ξ΅ο¬ο

ξ₯΄
ξ¦Ύ
ξ₯΅
ο
ξ₯³
ξ₯΄
ξ¦Ύ
ξ₯΅
ο°
ξ₯²
ξ³
ξ³
ξ³
ξ³
ξ³
V
Su_pu
V
Sv_pu
V
Sw_pu
V
Sv_pu
V
Sw_pu
V
Su_pu
V
Sv_pu
V
Sw_pu
V
Su_pu
V
Sv_pu
V
Sw_pu
V
Su_pu
(a) SSBC
(b) SDBC
(c) DSCC (d) DSBC
Fig. 3. Circuit configurations of the MMCC family for a STATCOM
application
L
ac
Cell u1
Cell u2
Cell ux
Cell v1
Cell v2
Cell vx
Cell w1
Cell w2
Cell wx
Bus C (33kV)
u phase
cluster
i
S,u
v phase
cluster
w phase
cluster
v
un
v
vn
v
wn
i
S,v
i
S,w
v
S,u,
v
S,v,
v
S,w
v
S,un
v
S,vn
v
S,wn
Virtual neutral point
at Bus C (grid side)
n: Neutral point in converter
M:
C
wx
v
wx
S
ax
S
b_wx
S
cx
S
d_wx
D
a_wx
D
c_wx
D
b_wx
D
d_wx
v
Cwx
β»x is total cell counts in
a cluster
Cell u1
Cell u2
Cell ux
Cell v1
Cell v2
Cell vx
Cell w1
Cell w2
Cell wx
u phase
cluster
v phase
cluster
w phase
cluster
Bus C (33kV)
i
vw
i
wu
i
uv
i
S,u
i
S,v
i
S,w
v
uv
v
vw
v
wu
L
ac
v
S,uv,
v
S,vw,
v
S,wu
C
wx
v
wx
S
a_wx
S
b_wx
S
c_wx
S
d_wx
D
a_wx
D
c_wx
D
b_wx
D
d_wx
v
Cwx
β»x is total cell counts in
a cluster
Cell u
u1
Cell u
ux
u phase upper arm
Cell u
l1
Cell u
lx
u phase lower arm
Cell v
u1
Cell v
ux
v phase upper arm
Cell v
l1
Cell v
lx
v phase lower arm
Cell w
u1
Cell w
ux
w phase upper arm
Cell w
l1
Cell w
lx
w phase lower arm
Bus C (33kV)
i
uu
i
ul
i
vu
i
vl
i
wu
i
wl
i
S,u
P
N
ul
i
S,v
i
S,w
wl
uu
vu
wu
L
ac
vl
v
UN
v
PU
v
VN
v
PV
v
WN
v
PW
v
S,u,
v
S,v,
v
S,w
M: Virtual neutral point at Bus C (grid side)
C
wlx
v
wlx
S
a_wlx
S
b_wlx
D
a_wlx
D
b_wlx
v
Cwlx
β»x is total cell counts in a arm
C
wlx
v
wlx
S
a_wlx
S
b_wlx
S
c_wlx
S
d_wlx
D
a_wlx
D
c_wlx
D
b_wlx
D
d_wlx
v
Cwlx
or
C. Grid fault scenarios
Table I shows the representative grid fault voltage phasors
and vectors corresponded to three-phase-to-ground fault,
single-phase-to-ground fault, phase-to-phase short-circuit fault
and two-phase-to-ground fault [21], [22]. It is assumed that the
short-circuit faults happen somewhere on a feeder with the line
impedance Z
F
to Bus A (PCC) in Fig. 1. The line impedance
from the PCC to the grid with a higher voltage level is Z
s
. A
voltage dip severity D is determined by the ratio of the Z
s
and Z
F
with positive-, negative- and zero-sequence impedance. In
order to simplify the grid fault scenarios, the D is considered as
a real part only and more details are explained and classified in
[23], [24].
In this paper, three asymmetrical grid-fault scenarios are
chosen as shown in Table I (b), (c) and (d). Where the
asymmetrical grid fault voltage on Bus A propagated to Bus B,
the Bus A and Bus B voltages do not appear significantly
different due to the used neutral point grounded wye-wye-delta
transformer TR1. However, the voltage on Bus B shows
different characteristics, when it is propagated to Bus C, which
IEEE TRANSACTION ON POWER ELECTRONICS
TABLE III
THE MMCC SPECIFICATIONS FOR THE CASE STUDY.
Circuit type of MMCC
SSBC
SDBC
DSCC
DSBC
Rated power Q
r
Β±80 MVA
Rated line-to-line voltage V
s
33 kVrms (Source angular frequency
ο·
s
: 2ο°Γ50 rad/s)
Rated DC-link voltage each cell V
C,dc
2600 Vdc
Nominal output voltage each cell
AC 1450 Vrms
DC 0 Vdc
AC 725 Vrms
DC 1300 Vdc
AC 1450 Vrms
DC 0 Vdc
Equivalent switching frequency f
eq_sw
10 kHz (with Phase Shift PWM)
Number of total cells N
cell
39
( 13 cells/cluster )
69
( 23 cells/cluster )
156
(26 series/arm)
78
(13 series/arm)
Number of total switching devices N
sw
156
276
312
312
Rated output current of each cell I
r
1400 Arms
808 Arms
700 Arms
700 Arms
Carrier frequency f
c
380 Hz
215 Hz
190 Hz
190 Hz
Total energy stored in
interconnection inductor E
L
15 kJ
( L
ac
= 2.6 mH )
15 kJ
( L
ac
= 7.8 mH )
15 kJ
( L
ac
= 5.2 mH )
15 kJ
( L
ac
= 5.2 mH )
Total energy stored in
dc-link capacitor E
C
1.6 MJ
( C
x
= 12 mF )
1.6 MJ
( C
x
= 7.0 mF )
6.3 MJ
( C
x
= 12 mF )
1.6 MJ
( C
x
= 6.0 mF )
IGBT module
MBN1500FH45F
( 4500V/1500A )
MBN900D45A
( 4500V/900A )
MBN800H45E2
( 4500V/800A )
MBN800H45E2
( 4500V/800A )
is seen by the STATCOM due to the used delta-wye
transformer TR2. Table II shows the asymmetrical grid fault
scenarios on Bus C corresponding with each grid fault. The
ξ΄Έ
σ°
οο€
οΎ
ξ‘ξ΄Έ
σ°
οο€
οΏ
ξξξξξξ΄Έ
σ°
ο΄
are positive-, negative and zero-sequence
component of the voltage, respectively, which are defined as
scenarios used in this paper.
III. SPECIFICATIONS OF THE MMCC-STATCOMS FOR STUDY
An 80 MVar / 33 kV case study for a practical STATCOM
application is chosen in this paper. Fig. 3 shows circuit
configurations of the STATCOM based on MMCC with SSBC,
SDBC, DSCC, and DSBC, respectively. Table III shows the
specifications, the cell numbers, and key components. The
design procedure is given below.
A. Basic structure and power semiconductor device
The rated DC-link voltage V
C,dc
of each converter cell in the
four types of the MMCC solutions are designed to be the same
at 2600 Vdc where widely used 4.5 kV IGBT modules are
selected for each converter cell in this case study. The nominal
output AC voltage each converter cell in the SSBC, SDBC and
DSBC is designed at 1450 Vrms with the nominal modulation
factor
ο‘
n
= 0.8. The margin of the modulation factor (0.2) is
determined by the voltage drop of the output impedance,
current control dynamics, pulse width limitation due to dead
time, and modular redundancy. However, the circuit
configuration of the converter cell for the DSCC is chopper
converter, which cannot output minus voltage. Because +/-
output voltage is also required for the DSCC based STATCOM,
the output voltage in each chopper converter cell is
superimposed with the half value of the rated DC-voltage ( i.e.
1300 Vdc ). When the above design guideline is followed, the
nominal output AC voltage each converter cell in the DSCC
becomes AC 725 Vrms with the nominal modulation factor for
AC component
ο‘
n
= 0.8.
The cell converter counts N
cell
of the MMCC solutions with
SSBC, SDBC, DSCC, and DSBC are expressed by the
equations in Table IV, respectively. Total number of switching
devices N
sw
for each MMCC topology is derived by each cell
circuit type and N
cell
.
Rated output currents I
r
in each cell among the MMCC
solutions are also expressed by the equations in Table IV,
respectively. The current ratings of the IGBT modules in each
of the cell converter among the MMCC solutions are selected
depending on the I
r
. It is worth to note that the N
sw
and N
cell
among the MMCC solutions are different. However, the
equivalent total power semiconductor chip area calculated by
the total IGBT module counts, the current capacity of each
IGBT module and the rated voltage of each IGBT module,
which strongly influence the total cost of the STATCOM,
become same values approximately.
B. Modulation type and frequencies for PWM
Phase-shift PWM modulation is chosen because of the
advantage that the electro-thermal stresses of the IGBT
modules and capacitors are equally distributed among the cells
in the same cluster (or arm). The equivalent switching
frequency of the MMCC f
eq_sw
is designed to be the same at 10
kHz. In this result, the carrier frequency f
c
of the MMCC
solutions with SSBC and SDBC are 380 Hz and 215 Hz,
respectively. The carrier frequency f
c
of the MMCC solutions
with DSCC and DSBC are the same 190 Hz. It is noted that f
c
of
each MMCC solution should not be an integer multiple of the
fundamental frequency in order to avoid diverging the
