IEEE Power & Energy Magazine - January/February 2014 - 46

The introduction of voltage-sourced converters,
with ratings suitable for transmission, has simplified
the black-start sequence.
indefinitely with little or no power or current flow. during
such periods, there is no issue with sustained discontinuous
current as there is with lccs. There is therefore no need to
rush to pick up load with the vsc. system restoration can be
performed in a careful and structured manner, with proper
coordination among transmission service providers, system
operators, and load-serving entities.
if the dc link interconnects two asynchronous networks, the
boundary between them is clearly defined. The dc link serves
as a "firewall" between the two grids so a blackout in one
network will not affect the other. The interconnection may be
lost, but the outage cannot propagate across the asynchronous
boundary. if, however, the dc link is embedded in a network,
the locus of system separation may not be readily predictable.
This suggests use of a remedial action scheme (ras) or special protection system (sPs) to limit the extent of the blackout.
The ras or sPs could be implemented by sensing out-of-step
conditions, impending voltage collapse, under frequency, or
circuit overloads. such a scheme could be designed to ensure a
well-defined separation boundary bisecting the dc link so that
one terminal remains energized and available for black start.
alternatively, the vsc-based dc link could be connected to a
designated power plant with black-start capability in another
restoration zone for an extended black-start range.
The same approach could perhaps be said to apply to ac
lines. The advantage with the vsc-based dc link, however, is
that it can control the ac system voltage and provide dynamic
reactive power reserve during the system restoration process.
The dc link can pick up a dead bus or a live bus without any need
for synchro-check or synchronization. furthermore, power flow
is controlled. The hvdc connection is therefore not subject to
overload or large swings in synchronizing power that could jeopardize stable
operation of the assisting grid or generator during restoration. The vsc dc
link is also not subject to uncontrolled
reactive power flows when a large voltage differential appears across its terminals. This distinction can be quite
IGBT Valves
CB and Preinsertion
beneficial when reenergizing long ac
Transformer
Resistor
lines and large transformer banks, for
cold load pickup, and for synchronizaFilter
tion of generation.
figure 1 depicts a vsc-based
hvdc
converter station with a capacdc Switchyard
ity on the order of 1,000 mW. Typical reactive power capability for a
figure 1. A VSC HVdc station layout, 1,000 MW, ± 320 kV (640 kV).

operate as a large uninterruptible power supply. Under such
conditions, a different fundamental control mode must be
used. This mode controls the amplitude, phase, and frequency
of the ac voltage so it matches preset reference values. it can
be referred to as "phasor control mode." The converter then
operates in a manner similar to a synchronous machine but
without physical inertia. This mode could also be called "synchronous machine-emulating mode." variations of phasor
control mode are used for black starts, for operation with a
very weak ac network connection, for operation with isolated
wind parks, and during quasi-islanded or islanded conditions
with relatively little or no generation online. during islanded
operation the dc terminal independently controls both the ac
voltage and the frequency of the islanded network to set reference points, i.e., it provides voltage and frequency control.
in the event of a blackout of the ac grid at one end of the
dc link, the station batteries ensure that the control and protection systems continue to function for some period of time,
e.g., 30 min, on stored energy. The station may be stopped
(blocked) and isolated safely during that time, after which
there may no longer be any source of auxiliary power. a
small diesel generator would then start up to supply enough
auxiliary power to keep the batteries charged and provide
stand by power to the converter valve cooling system. With
the dc link energized from the remote end in dc voltage control, the dc capacitors at the local end are charged and the
converter can be started (deblocked) in black-start mode. The
converter station is then controlling its ac output voltage and
frequency, thereby restoring station service power for its own
auxiliary loads such as cooling pumps and fans. The diesel
generator is then no longer needed. vscs can be operated

ieee power & energy magazine

january/february 2014

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