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