Slides - Power Systems Engineering Research Center

HVDC Transmission Systems
Based on Modular Multilevel
Converters
Maryam Saeedifard
Georgia Institute of Technology
([email protected])
PSERC Webinar
February 3, 2015
Presentation Outline
• Introduction to HVDC Transmission Systems
• Converter Requirements for HVDC Transmission
Systems
• The Modular Multilevel Converter (MMC)
- Features
- Operational Challenges
- Solutions
• Future Work
2
Introduction: AC Corridor’s
Power Flow Control
Boost or control ac voltage
(V)
Ref: ABB
Reduce line reactance
(X)
Regulate phase angle
(δ)
3
Introduction: DC Corridor’s
Power Flow Control
PP==
VDC. IDC
HVDC: High Voltage Direct Current Transmission
4
Introduction: AC vs DC Transmission
AC Transmission
×
Loading a function of Z
×
Charging current a function of
voltage level and cable
capacitance
×
Distance limitation
×
3 cables
DC Transmission
 Power flow controlled
 No charging current effect or
need for shunt compensation
 No distance limitation
 2 cables
5
Introduction: AC vs DC Transmission
• Due to reactive power charging, AC transfer capacity is dramatically
reduced with distance
• DC transfer capacity is almost independent of distance
Ref: ABB
6
Introduction: Overhead Line Transmission
Investment vs Cost
Ref: ABB
7
Introduction: Types of HVDC Systems
• Point-to-Point Systems
- Overhead lines
- Subsea or underground cables
HVDC Converter Station
HVDC Converter Station
• Back-to-Back Systems
- Interconnection of asynchronous AC grids
8
Introduction: Basics of HVDC Systems
Ref: ABB
9
HVDC Technology: Converter Requirements
Shortcomings:
×
×
Ref: ABB
Harmonic distortion
Switching frequency and power
losses
10
HVDC Technology: Converter Requirements
+

2
AC grid
_
+

2
_
 Staircase voltage waveform ==> Reduced harmonic distortion and
filtering size
 Low switching frequency ==> High efficiency
11
The MMC
Small Number of Sub-Modules
SubModule (SM)
Large Number of Sub-Modules
Features:
 Modular and scalable design
 Smooth and sinusoidal waveform
 Increased reliability and redundancy
×
Challenges:
SM capacitor voltage balancing
× Circulating currents
12
Equivalent Circuit of an MMC
i dc
+
+
SM1
SM2
v upa
SM1
SM1
SM2
SM2
SM
S1
SMn
SMn
SMn
-
Lo
iupa
VSM
Lo
Lo
Lo
Lo
vdc
Lo
SM1
ilowa
+
SM2
SM1
SM1
SM2
SM2
v lowa
−
SMn
SMn
+
−
ia
ib
ic
D1
C
S2
D2
+
vC
−
va
vb
vc
SMn
-
13
SM Capacitor Voltage Balancing
i dc
+
+
SM1
SM1
SM1
SM2
SM2
SM2
v upa
SM
S1
SMn
SMn
SMn
-
Lo
iupa
VSM
Lo
Lo
Lo
Lo
vdc
Lo
SM1
ilowa
+
SM2
SM1
SM1
SM2
SM2
v lowa
−
SMn
SMn
+
−
ia
ib
ic
D1
C
S2
D2
+
vC
−
va
vb
vc
SMn
-
14
SM Capacitor Voltage Balancing
Example: Five-Level MMC
15
Circulating Current Control
High circulating current:



Rating value/size of components
SM capacitor voltage ripple
Power losses
16
Circulating Current Control
• Circulating current – contains 2nd harmonic
predominantly
• Controllers to eliminate circulating current:
• Proportional Resonant (PR) Controller
• Predictive Circulating Current Controller
17
Circulating Current Control: PR Controller
• Circulating current dynamics:
diz ,abc
Lo
+ Roiz ,abc = vz ,abc ≈ mz ,abcVdc
dt
• PR Controller:
K i1s
Ki 2 s
K p1 + 2
+ 2
2
s + ωn1 s + ωn22
• ωn1 and ωn2 are tuned to 2nd and 4th harmonic.
18
Circulating Current Control: PR Controller
vc, p ,1,abc
vc, p , 2, abc
Software
SM Capacitor Voltage
Balancing
vc, p ,n ,abc
Measured
upper-arm
SM Capacitor
Voltages
Hardware
θe
iqe
iabc
i p ,abc
Ac-side current controller
e
θe
ωe Leqiˆd
iqe,ref
K
Kp + i
s
e
d
i
Kp +
Ki
s
ide ,ref
i p ,abc
in,a b c
iz ,abc ,ref
1
2
Vdc
2
iz ,abc
u
e
d
v
2
Vdc
K i1
K i2
+
s + ω n21 s 2 + ω n22
2
1
SM2
SMn
2
i p ,abc
Measured arm
in ,abc
currents
m0e = 0
v z ,abc
1
Vdc
vc, n ,1, abc
vc, n, 2, abc
vc,n,n ,abc
Measured
lower-arm
SM Capacitor
Voltages
mz ,abc
Circulating current controller
in ,abc
PWM
Generator-1
Upper-arm
switching
signals
mde
ωe Leq iˆqe
K p1 +
2
mabc
qd 0 → abc
e
d
SM1
mqe
eq
abc → qd 0
in,a b c
vqe
uqe
Phase-b
Phase-a
i p ,abc
1
Phase-c
PWM
Generator-2
1
2
SM1
SM2
Lower-arm
switching
signals
SMn
SM Capacitor Voltage
Balancing
Si1
SMi
~Si1
C
vci VS
i-th Sub-module
19
Circulating Current Control: PR Controller
20
Circulating Current Control:
Predictive Current Controller
From KVL:
diupa
Vdc
di
− vupa= l
+ Ria + L a + vsa ,
2
dt
dt
Vdc
di
di
− vlowa = l lowa − Ria − L a − vsa
2
dt
dt
Discrete model of the ac-side phase current:
Discrete model of the ac-side phase current:
ia (k + 1) =

1  vlowa (k + 1) − vupa (k + 1)
L'

− vsa (k + 1) + ia (k ) 
K' 
2
Ts

l
L' = + L
2
K'=
L'
+R
Ts
Discrete model for circulating current and SM capacitor voltages:
Ts
iDiscrete
k
+
=
− vlowa (k + 1)
− vupacapacitor
(
1)
(k + 1) ) + iz (k )
for
(Vdcmodel
z
2l
Vcij (k + 1) = Vcij (k ) +
voltages:
il (k )
Ts
C
21
Predictive Circulating Current Control of MMC
Prediction based on cost function minimization:
i dc
+
+
SM1
SM2
v upa
SM1
SM1
SM2
SM2
SM
S1
SMn
SMn
SMn
-
Lo
iupa
VSM
Lo
Lo
Lo
Lo
vdc
Lo
SM1
ilowa
+
SM2
SM1
SM1
SM2
SM2
v lowa
−
SMn
SMn
+
−
ia
ib
ic

V 
=
J λ  ∑ Vcij − dc  + λz izj
n 
 i
D1
C
S2
D2
+
vC
−
va
vb
vc
SMn
-
22
Closed-Loop Control of MMC-HVDC
Transmission Line
MMC-2
MMC-1
23
Predictive Control of MMC-HVDC
24
DC-Side Fault in MMC-HVDC Systems
25
SM Technologies: Normal Operation
Full-Bridge SM
Clamp-Double SM
26
SM Technologies: DC-side Short-Circuit
Fault Operation
Full-Bridge SM
Clamp-Double SM
27
DC-side Short-Circuit Fault Operation
of Full-Bridge MMC
28
DC-Side Fault in MMC-HVDC Systems:
Full-Bridge MMC Case
29
Power Losses for Various SM Circuits
Power Losses
of Single SMtype MMCs
Normalized
with Respect
to Half-Bridge
MMC
30
Hybrid Design of MMC-HVDC Systems
31
DC-Side Fault in MMC-HVDC Systems:
Hybrid MMC Case
32
Power Losses for Various Hybrid MMCs
Power Losses
of Hybrid
MMCs
Normalized
with Respect
to Half-Bridge
MMC
33
Future Work
• Control and protection of multi-terminal HVDC systems based on
the MMC
• Accurate and efficient modeling and simulation tools for MMCHVDC systems
• Operation of the MMC-HVDC systems under fault conditions
34
Acknowledgement
• This presentation contains data and graphs from ABB
publications/presentations available on the public domain including:
• Mats Larsson, Corporate Research, ABB Switzerland Ltd, “HVDC
and HVDC Light: An alternative power transmission
system”,Symposium on Control & Modeling of Alternative Energy
Systems, April 2, 2009.
• Gunnar Persson, Senior Project Manager,Power Systems - HVDC,
ABB AB Sweden, “HVDC Converter Operations and Performance,
Classic and VSC”, Dhaka, September 18, 2011.
35