HVDC Converter Transformer Saturation in
Hybrid AC/DC Transmission Caused by
Coupled Transmission Lines
Shuoting Zhang, Yalong Li, Bo Liu, Xiaojie Shi, Fred Wang
The University of Tennessee, Knoxville
MOTIVATION Increase the power transfer capability of long AC transmission lines
Investigate converter transformer saturation in hybrid AC/DC:
Coupled transmission lines induce zero sequence current
Zero sequence flow into DC side as a fundamental frequency current (i60)
LCC converts i60 to be DC current components on the AC side
Conclusion• Zero-sequence current induced by non-transposed coupled transmission lines has been investigated
• I60 induced by non-transposed transmission lines will cause noticeable impact on converter transformer
• Proposed CCC HVDC and band-stop LC filter methods are verified by simulation
SIMULATION VERIFICATION
Hybrid AC/DC Transmission Topology
I60 Influence On Transformer Saturation And Blocking Methods
• Original case
CHALLENGES
Complex line coupling characteristics
Cost effective blocking methods
a2 a1
b2 b1
c2 c1
* *
* *
* *
a
b
c
n
ia
ib
icin
GW15.7m
GW2
C1
C2 C3
C4
C5 C6
7.6m
8.2m
0.0m
4.6m
7.6m
19.2m4.4m
8.2m×4.5m
1
31.3944m
Ground Sag: 10m
Conductor Sag: 10m
Tower Line Sag
2
1
2 3
• CCC HVDC • Band-stop LC Filter
• 322 km non-transposed lines: I60 is around 2% for the original case and 0% for CCC HVDC and Band-stop LC filter cases
• CCC HVDC • Band-stop LC Filter
• Noticeable impact on converter transformer: I60 > 1%
• Blocking methods: CCC HVDC and LC filter
• I60 circulating loopI60 Induced by Non-transposed Lines
• Zig-zag transformer • Tower Structure
• DC current component on AC by I60
AC Bus Transmission Line AC Bus
CB1
CB3
CBx CBy CB2
CB4
CB5 CB6
AC Bus AC Bus
CB7 CBx CBy CB8
115 kV
115 / 280 kV
115 / 280 kV
280 / 115 kV
280 / 115 kV
115 kV
180 kV
1 kA
Non-transposed
coupled
I1_1
I2_1I1_2
I2_2
I1_3
I2_3I1_4
I2_4
Idc1
Idc2
Idc3
Idc4
Zig-zag
Zig-zag
Zig-zag
Zig-zag
• Non-transposed transmission lines
cause unbalanced line to ground and
line to line capacitances
• Unbalanced currents with I60 are
induced with balanced voltage
applying on unbalanced capacitances
• I60 increases with the line length
LCC Station LCC Station
CB1
CB3
CBx CBy CB2
CB4
CB5 CB6
CB7 CBx CBy CB8
Non-transposed
coupled
LCC Station LCC Station
CB1
CB3
CBx CBy CB2
CB4
CB5 CB6
CB7 CBx CBy CB8
14uF0.5H
14uF
14uF
14uF
Non-transposed
coupled
• DC component on AC side: 𝐼𝑎_𝑑𝑐 =1
2𝜋 02𝜋𝐼𝑎 𝑑𝜃 =
6
𝜋𝐼60 cos 𝛼 − 𝛽
𝐼b_𝑑𝑐 =6
𝜋𝐼60 cos 𝛼 − 𝛽 − 2𝜋/3 𝐼c_𝑑𝑐 =
6
𝜋𝐼60 cos 𝛼 − 𝛽 + 2𝜋/3
Transmission Line
I60
I0
Ia,b,c_dc
Transmission Line
I60
I0
Ia,b,c_dc
θ
I
0
0I
θ
θ
V
α
0
β
