IEEE Power & Energy Magazine - July/August 2019 - 59

thus, require other equipment to interrupt the fault current.
By contrast, fault-blocking converters do not need to rely on
other equipment to interrupt fault currents, as they have the
capability to either block or control the current contribution
through the converter.

Circuit Breakers and Switchgear
Within a dc substation, there are several requirements for
switching, which can be grouped according to functionality:
grounding, disconnecting, current commutation, and current
interruption. Fault-current interruption devices at the dc side
(HVdc circuit breakers) are essential to achieve selective
protection of multiterminal HVdc systems by isolating only
the faulty element.
The requirements for interrupting a dc current lead to
challenges with HVdc circuit breakers not observed in ac
systems. The absence of naturally recurring current-zero
crossings requires the HVdc circuit breaker to drive the dcfault current toward zero and absorb the energy that remains
in the system at the moment the circuit breaker opens. To
perform these functions, an HVdc circuit breaker typically
makes use of parallel paths with auxiliary circuits to enable
current commutation and energy absorption. In addition,
high switching speeds require fast mechanical operation or
power-electronic switches in the circuit. Recently proposed
HVdc circuit breakers can operate within 2-10 ms, an order
of magnitude faster than an equivalent ac device.
The dc-fault current can reach destructive values within milliseconds and exceed the HVdc circuit breaker's interruption
capability. Therefore, series fault-current limiters (for instance,
a line inductor) may be required to limit the rate of rise of fault
current. Such inductors, for conducting a dc current, are air
cored, physically large, and require additional space to avoid
interference with neighboring equipment.
Apart from dc fault-current-interruption devices, several
other important HVdc switching applications operate 10-100
times slower than a typical dc fault-current interruption.
Applications include grounding a cable, disconnecting a converter, reconfiguring a dc substation, and isolating a faulty element (for example, following fault-current interruption using
fault-blocking converters). For these operations, disconnector,
grounding, and transfer switches can be used.

Energy Dissipation
There is often a requirement for discharging cables and overhead lines, either for safety when taking a line out of service for maintenance, for dissipating excess energy during a
momentary fault (for example, from a wind farm, for which
power cannot be instantaneously decreased), or for balancing
unequal pole voltages in a symmetric monopolar network.
For planned maintenance, a grounding switch can be utilized, which could take seconds to discharge the line and
reopen. This is acceptable given that this operation has no
stringent time constraints. In the case of pole balancing, however, there is a requirement to resume power flow as quickly
july/august 2019

as possible (in the post dc-fault case) or continue normal
operation (for balancing in nonfault conditions). Different
solutions based on power electronics or nonlinear resistors
have been suggested to meet these demands.

DC Power-Flow Control Devices
Controlling the power flow enables the use of any power system at its full capacity. The topology and configuration of
the passive network and the voltage at each node determines
power flow unless active devices are applied. While flexible
ac transmission systems already dynamically control power
flow in ac grids, there are currently no commercially available products for HVdc grids.
In an HVdc grid, the voltage difference and resistance
between two nodes determine the power flow through a transmission line. Therefore, the power flow can be controlled by
adding resistance (although incurring additional losses would
be unattractive) or modifying the node voltages at each end
of a line. On a meshed system, for which each node voltage
affects the power flow on multiple lines, it is not possible to
maximize the power flow on all lines, resulting in some lines
inevitably being underutilized. Some proposed solutions insert
a controllable voltage in series with a line, with the absorbed
energy transferred to an adjacent line using an equivalent but
opposing voltage. The voltage requirement would be of similar
magnitude to the voltage drop across the line, small compared
with the pole-to-ground voltage.
DC power-flow control devices are still in a conceptual
phase as they will influence losses and system reliability
and could be expensive. Within the substation, these devices
might consume considerable space and add to the complexity of system design and operation.

DC/DC Converters
Transferring energy between different dc voltages requires a
different solution compared to the ac transformer, which provides effective passive-voltage transformation. While there
are no HVdc dc/dc converters in operation today, there could
be benefits in connecting HVdc systems of different voltages.
An example would be increasing operational flexibility by
interconnecting networks that were built at different times.
DC/DC converters could either be directly connected or provide galvanic isolation when using an intermediary ac stage.
Furthermore, they would also have the ability to control
power flow. A dc/dc converter might also be able to prevent a
fault from propagating across the network, although this may
come at additional costs.

Bushings
Bushings, which are insulation devices, allow the HV conductor to pass safely through the grounded tank and isolate
energized primary components and substation buswork
from the ground. A bushing consists of a conductor surrounded by insulation material, which is typically made
from porcelain.
ieee power & energy magazine

IEEE Power & Energy Magazine - July/August 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2019