IEEE Power & Energy Magazine - May/June 2019 - 95

Power demand in large or mega cities is increasing as the world
urbanizes. metropolitan transmission networks are continuously upgrading to meet the
growing demand for power and to replace old-style generation with local renewable
generation and power supplies from cleaner external sources. the cost of land and its
scarcity make it difficult to secure new rights-of-way for traditional-style transmission lines. as power transmission levels increase, the risk of exceeding the short circuit capability of existing switchgear equipment as well as other network components
becomes a major constraint to the expansion of power networks. strategies to enhance
urban transmission networks must address all of these issues and prioritize solutions
that may be easily located within urban boundaries and have short lead times from
decision to commercial operation. advanced transmission technologies can effectively
increase the capacity, efficiency, and utilization of existing infrastructure and improve
grid resiliency under extreme contingencies or catastrophic events. these technologies
have been considered and implemented by electric utilities for urban grid enhancement
and modernization.

©istocphoto.com/Zapp2photo

General Structure of Urban Power Grids
Figure 1 is a general structure of metropolitan power grids. this structure comprises extra
high-voltage (ehV) main transmission networks, hV subtransmission networks, transmission substations, primary distribution substations, and local power-generating plants.
an urban power grid is typically divided into multiple load areas (also known as load
zones or load districts), which are specified portions of the grid designated by electric
utilities for convenience in system planning and operation. these load areas may have different demand characteristics, degrees of reliability of service, and power quality requirements. the ehV main transmission networks, typically in the voltage range of 345-
500 kV, transmit bulk power from external sources to and between the transmission substations. the hV subtransmission networks, typically in the voltage range 60-230 kV, are
mainly for power delivery from the transmission substations to primary distribution substations. in some metropolitan power grids, the main transmission networks form a single- or double-ring structure. the subtransmission networks are usually mesh connected,
but system constraints may necessitate operating them as radial systems. local generation
still constitutes a significant portion of total power supply in many urban power grids.
many of these units are aged, less-efficient thermal units and dispatched as reliability
must-run units to provide operating reserves and voltage support functions. For economic
reasons and environmental constraints, most of these aged thermal units will likely be phased
out in the near future.

Urban Power Grid Expansion Needs and Challenges
Urban power grids evolved with population and economic growth over decades and are
now facing many expansion and operational challenges to meet further demand growth
and the associated (more stringent)
reliability requirements. Fast demand
growth has caused power grids to
operate closer to their capacity limits.
inadequate power delivery capacity
from external sources into urban load
centers and between adjacent load
areas has caused frequent network congestion, resulting in increased reliance on local generation at higher production costs. to
maintain acceptable levels of short circuit currents, the looped ehV main transmission
and meshed hV subtransmission networks may have to operate under split and radial
configurations, resulting in reduced flexibility and reliability. in the situations without
adequate voltage and reactive power support from local generation, the urban power grid
might be exposed to the risks of voltage collapse and blackouts when the system experiences extreme events.

Using High-Voltage dc for
Enhancement and Modernization

may/june 2019

ieee power & energy magazine

IEEE Power & Energy Magazine - May/June 2019

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