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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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4 DTU Electrical Engineering, Technical University of Denmark
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
Converter transformer
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
Voltage SourceConverter
DC-Bus
DC-Circuit breaker
DC-Cables
Converter transformer
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
Voltage SourceConverter
DC-Bus
DC-Circuit breaker
DC-Cables
Converter transformer
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
Voltage SourceConverter
DC-Bus
DC-Circuit breaker
DC-Cables
Converter transformer
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
Voltage SourceConverter
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
Voltage SourceConverter
DC-Bus
DC-Circuit breaker
DC-Cables
Converter transformer
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
Voltage SourceConverter
DC-Bus
DC-Circuit breaker
DC-Cables
Converter transformer
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