IEEE Power & Energy Magazine - May/June 2019 - 99

Subtransmission HVdc systems will likely
provide more flexible solutions than alternative
advanced transmission technologies.

be easier to obtain than those required for expansion of the
main transmission network. Urban grid subtransmission networks may already have existing connections with external system subtransmission networks. in some cases, there might be
opportunities to implement an hVdc feeder on an existing
corridor. it is also possible to interconnect more environmentally friendly external generation plants with the urban subtransmission networks, bypassing transmission corridors with
limited transfer capacity and qualifying the generation plants
as in-city capacity.
an urban power grid is required to provide reliable supply
to all of its customers under design n-1 and n-1-1 contingencies. with the existing grid infrastructure, the power transfer capability between load area subtransmission networks
is typically limited, and, in some cases, the subtransmission
networks of adjacent load areas are not interconnected under
normal operating conditions due to loop flow or short circuit
current issues. enhanced interarea power transfer capability by hVdc interties can increase urban grid efficiency and
reliability because these load areas typically have connections to relatively independent external sources. in some load
areas, the subtransmission networks must operate primarily
as radial configurations due to system constraints. enabling
meshed operating configurations by interconnecting critical
substations with hVdc interties can improve the reliability
and asset utilization of the load area networks.
subtransmission hVdc systems will likely provide more
flexible solutions than alternative advanced transmission technologies, especially for expansion cases that need to address
multiple system capacity enhancement and operational flexibility issues. Vsc-hVdc links can provide integrated system benefits, including flexible power flow control, dynamic voltage
support, emergency power support, and black-start capability.
table 1 shows a scenario-based comparison of technology options for urban network enhancement and modernization. expansion scenarios are arranged by the complexity of
system issues, from simple expansion to multiple enhancement needs. conventional expansion options include adding
new circuits or upgrading existing ones, installing passive
or dynamic reactive power-compensation devices at critical
substations, and installing Psts for providing network power
flow control. advanced expansion technologies include hts
cables for high-capacity power delivery and grid resiliency
enhancement, VFts or sts to enable flexible power flow
control, and Vsc-hVdc systems to address multiple system
expansion needs.
may/june 2019

conventional expansion options can cost-effectively address
simple expansion needs, such as circuit overload issues,
local voltage support, and steady-state power flow control
improvement. For the expansion scenarios requiring more
flexible power flow control, a VFt or st becomes more
attractive than a conventional Pst. hts cables might make
a case for increasing power delivery capability of critical
network circuits or improving meshed grid resiliency without increasing short circuit availabilities to unacceptable
levels (due to the inherent fault-blocking capability of hts).
Vsc-hVdc-based solutions are the most flexible advanced
options to address multiple system issues. note that detailed
system-planning studies are always necessary to determine
optimized expansion solutions, which could be a combination of conventional and advanced technologies.

Symmetric Monopole

Bipole With Metallic Return

Voltage Range
±50-±150 kV
Power Range
100-600 MW

Converter
Transformer

figure 5. The representative operating configurations and
voltage and power ranges of point-to-point or back-to-back
subtransmission HVdc systems. Symmetric monopole configurations are more likely for urban power grid applications. The
corresponding HVac network voltage ranges are 60-230 kV.
ieee power & energy magazine

99



IEEE Power & Energy Magazine - May/June 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2019

Contents
IEEE Power & Energy Magazine - May/June 2019 - Cover1
IEEE Power & Energy Magazine - May/June 2019 - Cover2
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