第 17 卷
第2 期
电 机 与 控 制 学 报
ELECTRI C MACHINES AND CONTROL
2013 年 2 月
Vol. 17 No. 2
Feb． 2013
用于离岸风场高压直流输电的并网型多电平拓扑研究
费拉斯， 许烈， 李永东
（ 清华大学 电机工程与应用电子技术系，北京 100084）
摘 要: 提出一种新型的风力发电变换器拓扑结构 。可用于离岸型海上风电场，并可采用高压直流
输电的方式接入电网。其中，离岸部分包括 5 个单元结构，每一个单元结构均由一个永磁同步电机
( permanent magnet synchronous generators，PMSG) 和一个三电平中点钳位 ( 3 － level neutral point
clamped，3L － NPC) 变换器组成，通过最大功率点跟踪 ( maximum power point tracking，MPPT) 算法
控制 3L － NPC 变换器，进而控制 PMSG 运行速度。 离岸平台由半桥结构串联组成，用于平衡各离
岸单元电压并控制总体直流侧电压为给定值。 超高压直流电缆将被用于离岸侧和岸侧的功率传
输。在岸侧，采用一种模块化多电平变换器( modular multilevel converter，MMC) 实现直流到交流的
逆变，并将系统接入电网。
关键词: 永磁同步电机; 三电平中点钳位变换器; 半桥结构; 模块化多电平变换器; 系列直流风电场
中图分类号： TM 46
文献标志码： A
文章编号： 1007－ 449X（ 2013） 02－ 0007－ 07
Grid-connected multilevel topology for HVDC offshore wind farm
FIRAS Obeidat， XU Lie， LI Yong-dong
（ Electrical Engineering Department，Tsinghua University，Beijing 100084，China）
Abstract： This paper presents a topology to connect HVDC offshore wind farm into the grid； the offshore
part consists of five units，and each one contains permanent magnet synchronous generator （ PMSG） connected to a three-level neutral point clamped converter （ 3L-NPC） which works as a rectifier to control the
generator speed by using a maximum power point tracking （ MPPT） control． The offshore platform consists of a half bridge connected in series to balance the voltage between units and to collect the voltage in
order to get the desired DC voltage． The HVDC cable will be used to transfer power from the offshore side
to the onshore side． A modular multilevel converter （ MMC） is used in the onshore side to convert the
voltage from DC to AC voltage and to connect the offshore wind farm into the Grid．
Key words： permanent magnet synchronous generators； three-level neutral point clamped converter； half
bridge； modular multilevel converter； series DC wind farm
0
Introduction
Currently there is a rapid increase on the demand
of energy in most of countries especially in those countries which have growing economies，and because of
environmental problems which are produced from coal
and oil； countries are now looking for renewable energy
sources to face this demand and to decrease the greenhouse effect and reduce CO2 emissions．
The research focus of renewable energy is on wind
收稿日期： 2012 － 10 － 15
作者简介: 费拉斯( 1979—) ，男，博士研究生，研究方向为电力电子技术、新能源发电;
许 烈( 1980—) ，男，博士，讲师，研究方向为电力电子技术、矩阵变换器、新能源发电;
李永东( 1962—) ，男，博士，教授，研究方向为电力电子技术、电机控制，新能源发电等。
通讯作者: Firas Obeidat( 费拉斯)
电
8
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第 17 卷
and solar energy and many plants have been erected to
through one leg MMC and each submodule is 3L －
harness these two kinds especially in wind energy． The
NPC，other papers used DC / DC converters connected
first wind power plants have been mainly erected on-
in series to get HVDC in the offshore side like in ［16］
shore，but now many are being erected offshore． And
which used full bridge isolated boost （ FBIB） converter
the number of offshore wind farms is increasing for
and ［17 － 18］ which used quasi － Z － source DC / DC
many reasons such as： even with large investment on
converter． The generator can be a multi phase PMSM
wind energy，onshore wind plants can’t grow due to a
and each three phase connected to an AC / DC convert-
lack of space to erect new farms； onshore farms are an
er and then all of the converters are connected in series
eyesore and effect the landscape； more energy cab be
to get the desired HVDC as used in ［19］．
extracted from offshore farms because offshore wind are
In this paper，a series DC offshore wind farm lay-
more constant and effective； and there are no noise
out is presented． The offshore side consists of a number
complaints as offshore wind farms tend to be located far
of units，each unit consist of a generator connected to
from populated areas．
an AC / DC converter． And all of the units are connect-
The erected offshore wind farms use high voltage
ed in series through a DC offshore platform． HVDC ca-
AC （ HVAC） technology to transfer energy from off-
ble is used to transfer the power from the offshore to the
shore to onshore，but research tends to use high voltage
onshore side and an DC / AC converter is used to con-
DC （ HVDC） technology to transfer energy from off-
nect the offshore wind farm with the grid．
shore to onshore for many transmission and power gencan be avoided； the cross section of the cable in the
Series DC Offshore W ind Farm
Configuration
eration reasons such as： by using HVDC reactive power
1
case of HVDC is less than in the case of HVAC for the
The configuration of the series DC offshore wind
same power； there is no resonance between the capaci-
farm which proposed in this paper is illustrated in
tance of the cable and the inductive reactance of the
Fig． 1． This configuration can be divided into a wind
grid； the frequencies in both end are independent； and
generator level，an offshore platform （ collection lev-
a short circuit can’t be transferred by using HVDC［1 － 2］．
el） ，a transmission cable，and an onshore converter．
Offshore wind farms can be classified into two
Offshore%Side
types according to their connection，series and parallel． Each of these connections has advantages and dis-
Onshore%Side
DC%Cable
PMSG
SM
AC/DC
uap
WT
advantages． For example，a series connection can re-
PMSG
AC/DC
PMSG
AC/DC
E
2
SM
SM
SM
WT
lation index to reduce the losses of the converters． In a
steps to get the desired voltage which reflects on the
［3］
cost and the weight of the offshore platform
．
SM
SM
SM
SM
ib
PMSG
AC/DC
E
2
WT
ic
PMSG
AC/DC
WT
SM
SM
SM
SM
SM
uan
ubn
SM
SM
SM
SM
ucn
SM
SM
SM
DC%Cable
Phase%a
parallel connection the transmission losses are high due
to the high current． Thus this connection needs more
SM
SM
ucp
WT
type of connection the losses for converters are high so
the designer must be careful when choosing the modu-
SM
ia
duce the transmission losses and there is no need for
transformers to get large power density，but for this
SM
ubp
Fig． 1
Phase%b
Phasec
submodule
SM
S1
VC
S2
General configuration for the proposed layout
The wind turbine connected to a direct driven permanent magnet synchronous generator （ DD － PMSG）
Many papers explained general HVDC configura-
and the stator of the generator connected to a 3L － NPC
tions for offshore wind farms like in ［4 － 8 ］； others
converter which is used to convert the voltage from AC
used parallel connection in offshore side to get HVDC
to DC as shown in Fig． 2．
like in ［9 － 12］． And others used series connection in
For this layout the wind farm consists of five u-
offshore side to get HVDC； ［13］ used a medium fre-
nits． Each unit is connected from the DC side to a half
quency transformer （ MFT ） and bridge rectifier，in
bridge and the half bridges are connected in series to
［14 － 15］ the wind turbines are connected in series
collect 40 kV in the offshore platform．
费拉斯等： 用于离岸风场高压直流输电的并网型多电平拓扑研究
第2 期
9
wind speeds． So the variable speed generator is more
suitable than a fixed speed generator in the wind generation system．
PMSG
Wind%Turbine%Output%Power
WT
Fig． 2
C
pm3
Block diagram for wind generator
connected to 3L-NPC
The DC transmission cable can be a single core
v3>v2>v3
B
pm2
A
pm1
v3
v2
Optimal%Linc
v1
（ single core） or a double core （ bipolar ） ． A single
棕m1 棕m2 棕m3
core system uses electrodes for sea or ground return，
Wind%Turbine%Shaft%Speed
but reducing the cable and laying cost． This kind of
Fig． 3
cable system has some disadvantages such as： creation
wind turbine characteristics
of an electromagnetic field and gas emission from the e-
When the wind speed is v1 ，the maximum output
lectrodes． A double core system is commonly used in
mechanical power from the wind turbine will be p m1 and
the design，but also includes electrodes if one of the
the shaft speed will be w m1 ，but when the wind speed
cables is damaged ［20］．
changes to v2 ，the mechanical power will not be the
Five levels modular multilevel converter （ MMC）
maximum for the shaft speed w m1 ，so the speed will in-
is used to integrate the offshore wind farm into the grid
crease to reach w m2 in order to get the maximum output
and convert the HVDC to HVAC．
mechanical power for the wind turbine which is p m2 ．
2
In order to keep the wind generation system oper-
Wind Generator Model
2. 1
ating around the points A，B，and C （ which are the
points when the optimal line crosses the p m － w m
Wind Turbine Model
The mechanical power captured by the wind tur-
（ MPPT ） must be applied． So for this purpose the
bine from the wind can be described as ［21］：
P m = 0. 5 ρAv3 C p （ λ ，β）
curves） ； a maximum power point tracking control
（ 1）
MPPT method in［22］will be applied in this paper．
2. 2
Where
3
ρ is the air density （ kg / m ） ； A is the swept area
（ m ） ； C P is the power coefficient of the turbine，which
2
PMSM Model
The mathematical model is similar to that of the
wound rotor synchronous motor． Since there is no ex-
is a function of tip speed ratio （ λ ） and pitch angle
（ β） ； v is the wind / water current speed （ m / s） ．
ternal source connected to the rotor side and variation
The power coefficient and the tip speed ratio can
there is no need to include the rotor voltage equations．
in the rotor flux with respect to time is negligible，so
A rotor reference frame is used to derive the model of
be given as
(
)
－ 12. 5
116
－ 0. 4 β － 5 e λ i ， （ 2）
C p （ λ ，β） = 0. 22
λi
ωm R
．
（ 3）
λ=
v
Where ω m is the wind turbine speed （ rad / sec） ，
R is the radius of the turbine blades （ m） ，and
1
1
0. 035
=
－
．
λ i λ + 0. 008 β 1 + β3
（ 4）
From equation （ 1） ，it can be seen that the output
the PMSM［23］．
The electrical dynamic equation in terms of phase
variables can be written as
v a = R a i a + pλ a ，
v b = R b i b + pλ b ，
（ 5）
v c = R c i c + pλ c ．
While the flux linkage equations are
λ a = L aa i a + L ab i b + L ac i c + λ ma ，
mechanical power for the wind turbine is a function of
λ b = L ab i a + L bb i b + L bc i c + λ mb ，
rotor speed． Fig． 3 shows this relation for different
λ c = L ac i a + L bc i b + L cc i c + λ mc ．
（ 6）
电
10
机
与
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第 17 卷
Considering symmetry of mutual inductances such
regulated by PI controller，the output of speed regula-
as L ab = L ba ，self inductances L aa = L bb = L cc and flux
tor is used as reference for i q ． i q and i d are also regula-
linkage λ ma = λ mb = λ mc = λ m ．
For this model，input power p i can be represented
ted by PI controller and the output of them is used as
as
ting the coupling components．
reference for the SVPWM of the converter after subtrac-
（ 7）
p i = va ia + vb ib + vc ic ．
ted as
v d = （ R s + L d p） i d － w r L q i q ．
-
+
PI
+
PI
Nref -
-
（ 8）
姿m
× +
+
×
( 32 ) ( P2 ) （ λ i
m q
N
Fig． 5
+ （ Ld － Lq ） id iq ） ．
（ 10）
The equation for the machine dynamics is
P
（ T － T L ） = （ Jpω r + Bω r ） ．
2
3. 2
（ 11）
link / Matlab．
× Lq
+
+
Theta
1/s
id
× Ld-Lq
Rs
棕e
×
Ld
+
-
Offshore Platform Control
The offshore platform is half bridges connected in
series，this platform has two functions，the first is to
collect the voltage to the desired HVDC，and the secunits for the DC sides of the generator converter DC
sides．
The controller includes measuring the DC voltages
voltage． For each sampling time，four of the units must
1/Ld
flux
id
The control model for PMSM side converter
for the lowest DC voltage with respect to reference DC
Va，b，c
abc-dq
PMSM
and then sorting them． After that，get the percentage
Rs
iq
兹g
PMSM
Measurment
ond to balance the DC voltages for all of the generation
Fig． 4 shows the modeling of PMSG using Simu-
棕e
ia，b，c
id
iq
The electrical torque T e ，can be represented as
Te =
Lq
dq-abc
cal speed． And the instantaneous power can be derived
（ 9）
PMSM
Converter
+
chronous inductances，respectively． w r is motor electri-
3
p i = （ vq iq + vd id ） ．
2
SVPWM
+
PI
Ld
Where L d and L q are called d － and q － axis syn-
as
Va，b，c
dq-abc
+
v q = （ R s + L q p） i q + w r L d i d + w r λ m ，
+
id
The voltage equations in d － q axis can be presen-
id
1/Lq
+
3p/2
+
TL
1/s
+
- 1/J 1/s
-
be connected to the system and the fifth one which is
id
Te
id
p
F
dq-abc
Ia，b，c
棕e
the lowest one is bypassed depending on its percentage
with respect to the reference DC voltage．
1/s
Theta
3. 3
Grid Side Control
Fig． 6 shows the control model for the converter
flux
connected to the grid based on FOC． In this model i q
Fig． 4
3
Modeling of PMSM
Offshore Wind Farm Control
equal to zero to inject zero reactive power to the grid
and the DC voltage is the outer loop and regulated by
PI controller and the output of DC voltage regulator is
The control of the proposed offshore wind farm
used as reference for i d ，i q and i d are also regulated by
control can be divided into three parts： the machine
PI controller and the output of them is used as refer-
and its converter control，an offshore platform control，
ence for the phase Disposition sinusoidal PWM （ PD －
and a grid side converter control．
SPWM） of the converter after subtracting the coupling
3. 1
component．
Generator Side Converter Control．
Fig． 5 shows the control model for the PMSM con-
In order to get five levels waveform in MMC ac
nected to a space vector pulse width modulation （ SVP-
voltage side PD － SPWM method is applied． Four tri-
WM） converter， based on a field oriented control
angle carrier waveforms displace symmetrically with re-
（ FOC） ． In this model i d is set equal to zero in order to
spect to the zero axis． Then compare the reference volt-
get maximum torque，the speed is the outer loop and
age with the carrier waveforms the voltage level is de-
费拉斯等： 用于离岸风场高压直流输电的并网型多电平拓扑研究
第2 期
11
termined in the ac voltage side． The direction of the
also includes PD － PWM method and voltage balancing
current and the capacitor voltages for each SM must be
for the capacitors of each sub module． The parameters
measured and the level number determines how many
for the MMC are： submodule capacitor = 2. 7 mF，in-
SMs from the upper leg and how many SMs from the
ductance for each arm = 3 mH and resistance for each
［24］
lower leg that must be charged or bypassed
Vdc* +
+
PI
iq
PI
+
PI
-
．
+ +
棕Lsiq
- +
+
棕Lsid
-
Va，b，c
dq-琢茁
琢茁-abc
Gna
Converter
SVPWM
+
dq-琢茁
vq
id
iq
Va，b，c
control the DC voltage to 40 kV． Figure 8 shows the
phase voltage for the AC side of the MMC which clear
to line voltage for the same side． Fig． 10 shows DC
琢茁-abc
ia，b，c
dq-琢茁
25 kV and the frequency = 50 Hz． the MMC is used to
that it is five levels，and Figure 9 show nine levels line
PLL
vd
arm = 1 Ω．
For this simulation the line grid voltage is V rms =
voltage for the MMC from the cable side and it is clear
琢茁-abc
Grid
that it take some time before reaching to the desired
voltage．
Fig． 6
The control model for the rectifier
3
connected to the grid
×104
All the procedure of PD － PWM method and balancing the capacitors voltages is shown in Fig． 7．
Vphase%/%V
2
1
0
-1
-2
Modulating%Reference%Generator
-3
1
1 1.02 1.04 1.06 1.08 1.1 1.12 1.14 1.16 1.18 1.2
0.5
t/s
0
Fig． 8
Phase voltage for the ac side of the MMC
-0.5
-1
1
Level4
0
Level3
-0.5
Level2
-1
Vabc%/%V
Level5
0.5
Level1
nnp
t/s
Detemine%the%number%of%required%SMs%in%the
upper%arm（nnp） and%lower%arm（nlow）
nnp+nlow=4
Fig． 9
inp
If%（inp>0），select%nnp%SMs%with%the%lowest%values%of%vCup
If%（inp<0），select%nnp%SMs%with%the%highest%values%of%vCup
Sort
If%（ilow>0），select%nlow%SMs%with%the%lowest%values%of%vClow
If%（ilow<0），select%nlow%SMs%with%the%highest%values%of%vClow
Sort
ilow
Switching%Functions
Fig． 7
4
Vcup1
Vcup2
Vcup3
Vcup4
Vclow1
Vclow2
Vclow3
Vclow4
Gating%Signal To%IG%BTs%Gates
Generator
VDC%/%V
nlow
×104
4
3
2
1
0
-1
-2
-3
-4
1 1.02 1.04 1.06 1.08 1.1 1.12 1.14 1.16 1.18 1.2
voltage for the ac side of the MMC
×104
5
4.5
4
3.5
3
2.5
2
0
1
0.5
t/s
PD-PWM method for MMC
Fig． 10
Simulation Results
DC voltage for MMC side
For the offshore side each 2MW wind generator
The whole simulation structure is built － up in
worked for different wind speeds，the left column from
simulink / Matlab including grid block，R s = 0. 03 Ω
and L s = 3 mH block which represent the equivalent in-
figure 11 shows the wind profile for each wind generator at 0. 7 sec the wind changed to another value，the
ductance and resistance of the AC reactors between
right column from the same figure represents the corre-
power source and rectifier， respectively．
Rectifier
sponding wind generator speed． It can be seen that the
block includes five levels MMC used IGBT inside，and
speed for each generator varies according to wind
与
控
制
学
speed； this is because a MPPT control is applied． Ethe 3L － NPC converter side，which is proof that the
shown in figure 11． And the difference between them is
0
1
0.5
1
0.5
1.5
t/s
1
0.5
1.5
1.5
30
20
10
0
0
t/s
Speed4
12
11
10
9
0
1
0.5
t/s
12
10
8
0
1
0.5
1.5
Vab2
Vab3
0.5
1
1.5
1
1.5
t/s
0.5
1.5
0
0.5
1
1.5
t/s
30
20
10
0
0
0
1
0.5
1.5
1%000
500
0
-500
-1%000
0
1
0.5
1.5
0.5
1
1.5
t/s
1%000
500
0
-500
-1%000
0
1
t/s
1.5
t/s
1
0.5
1.5
Fig． 14
Phase currents for PMSGs
t/s
5
Left column wind profile and right
Conclusions
The simulation results are performed for a series
15%000
DC offshore wind farm layout，in this work a three
10%000
10%000
phase 3L － NPC converter is used in the offshore side
VDC4
VDC1
1
0.5
15%000
5%000
0
0
1
0.5
5%000
0
0
1.5
to convert the voltage from AC to DC for each unit． A
1
0.5
t/s
1.5
t/s
half bridge connected in series to represent the offshore
15%000
15%000
platform． A five level MMC is used to integrate the off-
10%000
10%000
shore wind farm with the grid． From the obtained re-
5%000
sults it can be seen that the control systems for the 3L
VDC5
VDC2
0
1%000
500
0
-500
-1%000
t/s
t/s
0.5
1%000
500
0
-500
-1%000
column wind generator speed
5%000
0
0
1
0.5
1.5
0
0
1.5
8%320
8%310
8%300
8%290
8%280
8%720
0
VDC（1，2，3，4，5）
5%000
1
0.5
t/s
Fig． 12
1.5
－ NPC，offshore platform，and MMC worked correctly．
t/s
10%000
0
0
1
0.5
t/s
15%000
VDC3
Vabc4
1%000
500
0
-500
-1%000
line voltage the 3L-NPC converter sides
t/s
t/s
Fig． 11
Vab1
t/s
30
20
10
0
0
30
20
10
0
0
Speed3
11
10
9
8
0
1.5
1.5
Vabc5
1
0.5
1
0.5
Vabc2
Speed2
10
8
0
0
Fig． 13
Vab3
Speed1
1.5
t/s
Speed5
wind%profile1
wind%profile2
wind%profile3
wind%profile4
wind%profile5
1
0.5
1.5
t/s
from one value to another due to the sudden change．
30
20
10
0
1
t/s
1
0.5
and there is oscillation when the wind speed change
12
10
8
0
0.5
0
-1
0
crease or decrease depends on the wind speed change
1.5
0
-1
0
1.5
1
t/s
t/s
1
worked correctly． Figure 14 shows the phase currents
0.5
1
0
-1
0
sides which proves that the control for this converter
0
-1
0
1.5
t/s
1
shows the line voltages for the 3L － NPC converter
for the PMSGs． It can be seen that the currents in-
1
-1
0
control of the offshore platform worked correctly as
very small as shown also in the same figure． Figure 13
第 17 卷
1
Vab1
qual DC voltage for all of the five units are obtained for
报
Vab4
机
Vab5
电
12
References：
［1］ N． M． KIRBY，LIE XU，MARTIN LUCKETT，WERNER SIEP-
1
0.5
t/s
DC voltage the 3L-NPC converter sides
1.5
MANN． HVDC transmission for large offshore windfarms［J］．
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