IEEE Power & Energy Magazine - January/February 2014 - 45

By Michael Bahrman
and Per-Erik Bjorklund

The very firsT commercial high-volTage dc
(hvdc) transmission system, commissioned in 1954, had blackstart capability. This system links the island of gotland in the Baltic
sea with the swedish mainland. gotland had very few generating
units, and the hvdc system was required to start up and deliver
power without any generation operating on gotland. Because the
hvdc stations used line-commutated converters (lccs), synchronous condensers were necessary to start up the dc system with a
dead receiving network. This complicated the start-up sequence for
system restoration.
The introduction of voltage-sourced converters (vscs), with
ratings suitable for transmission, has simplified the black-start
sequence. synchronous condensers are not required for operation or for starting vsc-based hvdc transmission. interestingly
enough, the first commercial vsc system, commissioned in
1999 to deliver power from wind generation, is also on gotland.
conventional hvdc transmission complements ac transmission for certain well-established applications. These include longdistance bulk power transmission, long submarine cable crossings,
and asynchronous interconnections. The controllability of hvdc
can help mitigate loop flow, prevent cascading outages, and damp
system oscillations. vsc technology has broadened the hvdc
application base for use in relatively weak systems, as an outlet
for nontraditional renewable generation, and for long underground
connections with extruded cables. operation with weaker ac system interconnection is possible due to the improved voltage stability with vscs. vsc systems can better ride through under voltage
swings while reducing their severity by providing dynamic support. vscs are self-commutated and can operate indefinitely at
zero-power or very low-power transfers. These distinct capabilities
enable vsc systems to be used for black starts and to become a
more vital component of the system restoration process.

©Artville, llC.

Black Starts with VSC-Based HVdc

january/february 2014

The active power control in vsc-based hvdc transmission is
assigned to regulate the dc power flow at one station and the dc
voltage at the other station. active power and reactive power can
be controlled independently while still maintaining a constant dc
transmission voltage. reactive power control at each station can
be selected to regulate either the reactive power flow or the ac
voltage. references can be set within predetermined limits by the
system operator. during normal converter operation with a strong
or relatively strong ac network connection, converters can operate
in their standard control mode. in standard mode, the converter
controls are synchronized to the ac bus voltage via a phase-locked
loop (Pll); the respective real and reactive current orders are calculated in the d-q reference frame and controlled by ac vector current control as the inner most and fastest control loop.
Because vscs are self-commutated, they do not necessarily require a stiff ac voltage source for stable operation. during black-start conditions, however, the ac network is passive,
and no generation is connected. The vsc must then effectively
Digital Object Identifier 10.1109/MPE.2013.2285592
Date of publication: 17 December 2013

ieee power & energy magazine

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2014