IEEE Power & Energy Magazine - July/August 2019 - 57

Equipment and Configurations for
Connection of HVdc Network Elements

HVdc Technology
HVdc is a well-established technology for grid reinforcements
that avoids some of the disadvantages associated with ac transmission upgrades (increases ac system fault level, requirements for reactive power, and synchronized ac networks) by
transmitting electric energy over dc circuits. It has been used
commercially for more than 60 years, mainly to transmit bulk
power over large distances and connect asynchronous ac systems, and it is a favorable choice when long (undersea) cable
connections are needed.
HVdc transmission utilizes power-electronic converters to
transform ac to dc and dc to ac. The converters utilize either
thyristor or insulated-gate bipolar transistor (IGBT) powerelectronic switches. Thyristors are used in line-commutated
converters (LCCs), the traditional HVdc technology, while
IGBTs are used in voltage-source converters (VSCs). While
both technologies can provide fast control of active power
and power transmission using overhead line or cable systems,
there are differences between the two. On the one hand, LCCs
are a more mature, cost-effective, and efficient technology
that is available for very large power ratings (on the order of
10 GW). Alternatively, VSC HVdc applications offer several
advantages, including a smaller overall footprint, fast activepower reversal capability, flexible and independent control of
reactive power, easier integration into weak power systems,
and use of extruded cables, for instance, based on cross-linked
polyethylene (XLPE).
Since the turn of the century, HVdc has seen a revival.
In countries such as China, India, and Brazil, LCC HVdc
installations transport bulk power over long distances, fulfilling a need caused by fast economic growth. At the same
time, the development of VSC technology led to new applications that include using submarine cables to interconnect
oil platforms and offshore wind farms with onshore facilities and underground cable and overhead line applications to
provide land-based links.
Recent advancements in VSC technology have enabled
the development of an HVdc grid with three or more converters connected in parallel. Several such systems have
been developed or are under construction. Whereas pointto-point systems are typically developed as projects using
an engineering-procurement construction procedure, future
HVdc grids will, in many cases, be developed gradually,
requiring interfaces among projects from different vendors.
Although LCC HVdc grids or hybrid VSC/LCC HVdc grids
july/august 2019

are possible, this article focuses on VSC HVdc grids, their
substation layout, and how they differ from their ac counterparts. HVdc substations consist of a dc switchyard connecting multiple lines or cables and, possibly, one or more
converter stations. As with ac systems, devices can be categorized as primary equipment, which carries the power,
and secondary equipment, which provides interfacing
and control.

Primary Equipment
In a future dc substation, the primary equipment could consist of various components for power transfer, protection,
and switching capabilities for system reconfiguration. Prospective components include ac/dc converters, circuit breakers and other switchgear, cables, lines and their terminations, means of discharging cables, and equipment to control
power flow, as shown in Figure 1.

AC/DC Converters
Converting ac to dc is an essential requirement for the power
system of the future, enabling the connection of HVdc systems to existing ac transmission networks for bulk energy
transfer. A VSC station consists of the VSC valve hall, in
which the electronic equipment is placed, as well as phase
inductors and measurement equipment. The ac transformers
and switchyard are typically placed outside the VSC area of
the substation.
Various VSC technologies can be used for ac/dc conversion in multiterminal systems, which can be roughly classified into two main types: dc-fault feeding (for example,
a half-bridge modular multilevel converter) and dc-fault
blocking (for example, a full-bridge modular multilevel
converter). Both types can control the dc-side current during normal operation; however, they can be differentiated by
their dc-fault response.
The dc-fault response by the converter can have a significant influence on the design of other primary equipment
within the HVdc substation. Following a dc fault, dc voltage
collapse rapidly propagates across the network; the current
rises quickly as the rate of rise is limited by the otherwise
relatively low inductance, as opposed to the relatively high
impedance in an ac grid. Consequently, the power-electronic
devices in affected fault-feeding converters are switched
off (blocked), protecting them from damage without stopping fault-current infeed from the ac grid. These topologies,
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