5 Holboell Transients in Offshore HVDC-systemsV4

8/12/2019 5 Holboell Transients in Offshore HVDC-systemsV4

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Transients in Offshore HVDC-systems

Anders Geisler Eriksen and Joachim Holbll

Technical University of Denmark

17thOctober 2011

Offshore HVDC grids present and future - Workshop


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2 DTU Electrical Engineering, Technical University of Denmark

Transients in offshore HVDC systems,an approach

Point-to point systems

Switching and fault situations, Classical approach, Low frequent models

Multiterminal systems

Offshore/onshore

Challenges

Discussion, Focus?


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The components of a point-to point system

AC Circuit breaker

AC - Filter bus

Converter bus

Voltage Source Converter

DC-Bus

DC-Circuit breaker

DC-Cables

Converter transformer

AC-grid


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The system in question

Bipolar point-to-point HVDC-system

Converter technology based on Voltage Source Converters(VSC)

Offshore system cable-based system

HVDC-circuit breaker has been implemented

The transformer configuration of the VSC-stations are star/delta-configurations

Power frequency range (=models) on AC side


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Multi-terminal HVDC-systems

Although the HVDC-system about to be presented is a point-to-pointconnection, the emphasis of this project will be on transients in multi-terminal HVDC-system.

However only very little attention has been given to this area of researchat this point. Therefore, it will be interesting to see how the known

transients, from point-to-point systems affect multi-terminal systems.


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AC-breaker

Transients due to load swirching orclearing of AC-faults[1]

Transients arise when dischargedcapacitors placed near theconverter are charged after fault

clearing [1]

This transient could affect thesystem even more seriously ifresonance exist between the filtercapacitor and the grid[1]

AC Circuit breaker

AC - Filter bus

Converter bus

Voltage SourceConverter

DC-Bus

DC-Circuit breaker

DC-Cables


AC-grid


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AC-filter bus

Line-to-ground fault, overcurrenthandled by phase reactors,initiates blocking of both rectiferand inverter, voltage of adjacentlines increase by 1.73[1]

Line-to-line fault, overvoltage butno overcurrent[1]

AC Circuit breaker

AC - Filter bus

Converter bus


DC-Bus

DC-Circuit breaker

DC-Cables


AC-grid

[1]


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Converter bus 1

Line-to-ground fault results indischarge of DC-capacitor causingovercurrent through the VSC,rapid rise of current in the affectedphase[1]

AC Circuit breaker

AC - Filter bus

Converter bus


DC-Bus

DC-Circuit breaker

DC-Cables


AC-grid

[1]


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Converter bus 2

Line-to-line fault, no overvoltage phase reactors and convertertransformer will be exposed toshort-circuit current[1]

AC Circuit breaker

AC - Filter bus

Converter bus


DC-Bus

DC-Circuit breaker

DC-Cables


AC-grid

[1]


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Voltage source converters 2

Blocking of the inverter due toground fault causes overvoltage,as the rectifier keeps on feedingpower, until it senses theovervoltage[1]

In multiterminal systems when afault on a line occurs, the VSCs ateither end will be blocking, hencethe fault current will be drawn

from the capacitors of the adjacent

VSCs[4]

AC Circuit breaker

AC - Filter bus

Converter bus


DC-Bus

DC-Circuit breaker

DC-Cables

AC-grid


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DC bus fault

Not a fault necessarily occuring atthe bus itself, but generally faultsoccuring between the converterand the DC bus[5]

Ground fault occuring close to the

DC-capacitor, most serious fault ifDC-capacitor already charged tothe arrester protective level[1]

AC Circuit breaker

AC - Filter bus

Converter bus


DC-Bus

DC-Circuit breaker

DC-Cables


AC-grid


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Cable faults 1

Positive-line-to-ground fault[4]

Negative-line-to-ground fault[4]

In either of the above cases the

voltage of the unaffected pole willincrease to 2 pu.[4]

Line-to-Line fault [4]

Fault current dependent upon the

location of the fault, since thecurrent is dependent upon theinductance of the cable which isproportional to the length of thecable[4]

AC Circuit breaker

AC - Filter bus

Converter bus


DC-Bus

DC-Circuit breaker

DC-Cables


AC-grid


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Point-to-point HVDC-system


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The complete picture?

Low frequency range

Faults and fault current breaking can cause fast transients

Component models change to broad band models

Example

Transformer:

Another challenge: VSC Multi terminal HVDC system


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Another challenge: VSC Multi-terminal HVDC-system(Here Kriegers flak)


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Challenges, concluding

Transient events point-to-point vs. Multiterminal

Offshore: long cables, but no lightning

Broad band models

DC breaker models

Time domain simulations

Verification difficult so far


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References 2

[7] Tang, Lianxiang; Ooi, Boon-Teck; Loacting and Isolating DC Faults inMulti-Terminal DC systems; pp. 1877-1884; IEEE transactions on PowerDelivery, Vol. 22, No. 3, July 2007

[8] Yang, Jie; Zheng, Jianchao; Tang, Guangfu; He, Zhiyuan;Characteristics and Recovery Performance of VSC-HVDC DC TransmissionLine Fault

[9] An Analysis of Offshore Grid Connection at Kriegers Flak in the BalticSea. Joint Pre-feasibility Study By Energinet.dk, Svenska Kraftnt,Vattenfall Europe Transmission, May 2009

5 Holboell Transients in Offshore HVDC-systemsV4

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