IEEE Power & Energy Magazine - July/August 2019 - 60

HVdc circuit breakers are likely to have more intelligent functions,
such as self-diagnostics, leading to potential coordinated protection
through the communication of breaker failures to nearby IEDs.

Design procedures from the insulation system used in
HVac bushings cannot be directly applied to HVdc bushings due to differences in electric-field distributions between
ac and dc systems. Additionally, various connected components in dc substations employ different types of bushings.
For example, the design of wall bushings, which connect the
valve hall with other primary components, is different from
that of the bushings used in HVdc circuit breakers.

Secondary Equipment
Secondary equipment in dc substations can be expected to
require more computational power and communications
than in ac substations, given the greater complexity of the
many control and protection functions needed to operate an
HVdc grid.

Measurement Devices
Conventional instrument transformers based on electromagnetic-induction technology are the most widely used primary
sensors in ac grids. The bandwidth of such transformers is usually limited to a few kilohertz, as, beyond these frequencies,
the conversion ratio of these devices is nonlinear due to resonances and capacitive couplings. For HVdc applications, nonconventional instrument transformers (NCITs) must be used.
Zero-flux current sensors, combined shunt and Rogowski
coils, and fiber-optical current sensors can be used to measure currents in HVdc grids. Resistive-capacitive (RC) voltage-divider technology is most commonly adopted for HVdc
voltage measurement. The future trends of NCITs for HVdc
grids include compact and possibly integrated options through
modern technologies, such as RC dividers for gas-insulated
switchgear (GIS) and integrated optic sensors. Compared to
ac substations, a larger number of current and voltage instrument transformers are likely to be used in dc substations. This
is because these substations contain more components that
must be controlled or protected, such as dc/dc converters, dc
lines, inductors, and HVdc circuit breakers. In a digital substation, instrument transformers are interfaced with merging
units, which digitize the measured quantities and send the
sampled values to control and protection devices via peer-topeer communication or multicasting to multiple subscribers.

Control
HVdc grid control ensures stable operation by controlling
the dc voltage, which presents itself as a global parameter
within the grid. Many dc-voltage control approaches are
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ieee power & energy magazine

expected to be used for HVdc grids, which can be divided
into centralized and distributed approaches. In the centralized approach, one converter responsible for the control of
direct voltage acts as a dc slack bus, while the other converters regulate active power. This approach is not suitable
for larger HVdc grids because dependence on a single slack
converter could lead to reliability issues. In the distributed
approach, all the converters in the HVdc grid control share
the active power balance by employing a voltage-droop
scheme. However, this requires coordination among substation controllers and HVdc grid controllers. The distributed
approach is more appropriate for larger HVdc grids but can
also be used for smaller HVdc grids if needed.
HVdc substation controllers are relatively fast regulators
that include primary voltage-power control in each converter
station in the HVdc grid. This control may take the form of
voltage-droop characteristics and, thereby, enable automatic
power sharing among several converters without depending on external communication. This approach is similar
to power-frequency-droop control in ac grids. The substation controllers have time constants on the order of a few
to tens of milliseconds and, in turn, require high-bandwidth
communication channels. The substation controllers might
consist of pole- or station-level controllers responsible for
setting the active power, reactive power, and ac or dc voltage
orders to the outer controllers of each converter.
The HVdc grid controllers are higher-level equipment;
this equipment includes the secondary and tertiary control. The secondary control is responsible for correcting the
active-power set points after a contingency, with a response
time of a few seconds. The HVdc grid tertiary control is
responsible for optimal power flow that considers the state
and requirements of the combined ac and dc grids. These
controls might be implemented as a master at a single physical location at any one time or as a distributed spread across
different physical locations. The HVdc grid controller provides set points to the local substation controllers. These
controllers generally have slower response times, tens to
hundreds of milliseconds or longer, and require relatively
low communication channel bandwidth.
The control signals and set points are communicated
to the converters via dedicated communication channels,
which might allow operator interactions from one central
location or from multiple locations within the system during steady-state operations, or from automatic set point or
control-parameter modifications during transients. The
july/august 2019

IEEE Power & Energy Magazine - July/August 2019

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