A Mixed LCC-VSC Topology for Unidirectional Power Transmission

GradCon 2014
University of Manitoba
Department of Electrical & Computer Engineering
A Mixed LCC-VSC Topology for Unidirectional Power Transmission in Point
to Point and DC Grid Applications
Steven Howell
Dept. Electrical & Computer Engineering
University of Manitoba
[email protected]
Newly emerging Voltage Source Converter (VSC) technology eliminates some of the technical
issues of conventional HVDC schemes, but at additional financial cost. VSC technology also has
its own set of technical issues that are not prevalent in conventional HVDC transmission
technology. Foremost among these are higher switching losses, more complex control schemes,
lower ratings, and, crucially, and inability to clear DC faults. The inability to clear DC faults
using conventional dc fault clearing methods lengthen the overall fault clearing time because DC
breakers do not currently exist at transmission level ratings. Therefore, existing AC breakers are
required to clear the fault on the system’s AC side once the AC current reaches a zero value. This
protection operation is intolerably slow for VSC-based HVDC systems. These long clearing times
can reduce export revenues, produce load shedding, and damage equipment. HVDC systems that
utilize overhead transmission as the transmission medium also experience more faults than
HVDC systems with underground cables, which increases the need for fast clearing times.
Conventional LCC-HVDC systems employ a method to clear DC faults known as Force Retard.
Once a DC fault is detected, both the rectifier and inverter sides of a LCC-HVDC link assume
inverter mode of operation, which quickly reduces the current in the DC system to zero and deenergizes the line. Force Retard can clear and restart a DC system in tens of milliseconds.
However, VSC-based HVDC schemes cannot use the Force Retard method because the VSC
topology contains a free-wheeling diode to protect the power electronic switches utilized in VSC
schemes. This diode permits current to flow from the AC system into a DC fault. This
presentation discusses one possible way to combine both LCC-HVDC and VSC-HVDC
technologies to perform fast DC fault clearing at the expense of being limited to unidirectional
power flow.
ADVISOR: Dr. Filizadeh & Dr. Gole
Proceedings of the 2014 Graduate Students Conference, GRADCON 2014
Winnipeg, MB, Canada; October 17, 2014
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