IEEE Power & Energy Magazine - July/August 2019 - 62

to ensure interoperability between vendor equipment. Similar
standards, such as IEC 61850 for digital substations, IEC 61869
for instrument transformers, and IEC 60834 for teleprotection,
could also be adapted for HVdc applications. However, the
required communication speed and channel bandwidth are
likely to be higher for HVdc grids compared to ac grids.
The protection and control architecture of a future dc substation can, from a communication perspective, adopt a similar structure as defined by IEC 61850 (Figure 2). Communication can be optimally designed for the whole dc substation
using technologies such as fast Ethernet. The required performance of the communication for ac grids is 80 samples/cycle
and 3 ms for sampled values and generic object-oriented
substation event message, respectively. This requirement for
HVdc grids should be on the order of 100 kSamples/s and
hundreds of microseconds to fulfil the requirement of fast
control and protection.
Communication between remote dc substations includes
dispatching, control signals, and teleprotection signals. Digital communication can be accomplished using fiber-optic
channels. Control set points and parameters at the dispatching level require the least bandwidth and communication
speed. Communication for teleprotection, and, in particular,
line-differential algorithms, requires the highest bandwidth
and communication speed.

Digital DC Substation
Existing ac substations are undergoing a transformation to
digital substations employing IEC 61850 standards in their
protection and control architecture. Future dc substations
can also employ a similar design principle, digitizing the
primary signals (such as current and voltage measurements),
switchgear-position status and control, and sending digital
signals to the protection and control devices using fast communication channels. An example is shown in Figure 2.

DC Substation Design Considerations
Although there are more than 100 HVdc systems in operation
and multiterminal HVdc systems have been built, the concept
of dc substations is not well established. A substation in a VSC
HVdc grid is different from an ac substation or VSC HVdc
point-to-point connection. A dc substation contains the bays
of incoming cables, transmission lines, and converter stations.
The bays include line and cable terminations, switchgear,
measurement equipment, and other secondary equipment for
protection and control. Within the substation, the bays are connected to busbars in a particular arrangement.
DC substation design may differ considerably from ac
substation design, given the increased dimensions and cost
of dc-side fault-clearing equipment and different options for
protection and configuration and grounding of the HVdc grid.
These aspects influence dc substation design because they
affect the balance between cost and expected availability as
well as considerations for the future growth of the HVdc grid.
We now briefly discuss factors affecting substation design
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choices for HVdc grids, including configuration, grounding,
protection philosophies, and technology.

HVdc Configuration and Grounding
The configuration of an HVdc grid influences the substation
layout because it establishes the number of bays per connected circuit. DC substations must accommodate a single
pole or two poles for each circuit, with the addition of a possible ground connection and metallic return, as illustrated in
Figure 3. The case of one connected bay per circuit results
in the asymmetric monopolar configuration [Figure 3(a)]. In
this case, the current flows through one pole and returns via
the ground. This option is often not permitted for environmental reasons. Two bays per connected circuit causes either
an asymmetric monopolar configuration with metallic return
[Figure 3(a)], which is low-impedance grounded, or a symmetric monopolar configuration [Figure 3(b)], where the dc
side is ungrounded or high-impedance grounded. Three bays
per connected circuit brings about a bipolar configuration
where one of the conductors is low-impedance grounded and
acts as a metallic return conductor [Figure 3(c)].
The grounding of the HVdc grid influences dc substation
design by determining the expected fault current and overvoltage levels and, hence, the required substation equipment ratings. When an asymmetric monopolar or bipolar
configuration is used, the voltages during the fault remain
limited, yet the prospective fault current is high. In systems
with symmetric monopolar configurations, the overvoltage
on the healthy pole could reach up to two times the nominal
voltage during faults in the absence of overvoltage protection, whereas the steady-state fault current would be zero.
The peak fault current and peak voltage that can occur
during dc short circuit faults depend on the speed of fault
clearing and fault-current-limiting equipment used within
the HVdc grid. These aspects enter the cost equation of the
substation as a whole. A tradeoff must be made between dcfault current or overvoltage-level reduction and the cost of
fault-clearing equipment.

HVdc-Protection Philosophies
The amount of protection equipment required in a dc substation depends on the protection philosophy of the HVdc
grid. To avoid a large number of potentially expensive HVdc
circuit breakers, protection philosophies for HVdc grids may
differ from the traditional ac system-protection approach. In
ac systems, protection zones are set for each component (line,
transformer, bus, and so forth) to protect the system selectively. The consequence of this approach is that, in ac systems, circuit breakers are typically placed such that they can
independently interrupt currents in each line terminal. This
may not be the best approach for HVdc grids. An HVdc grid
can adopt a nonselective, partially selective, or fully selective
protection philosophy, which, upon the occurrence of a fault,
clears the entire grid, a subpart, or only the faulted component, respectively.
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IEEE Power & Energy Magazine - July/August 2019

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