IEEE Power & Energy Magazine - July/August 2019 - 38

The inertia is inherent in a synchronous condenser
because it is a synchronous rotating machine operating
as a motor with no mechanical load.
thermal plants in the coastal load center area were retired, and the replacement generation is fundamentally very different. It is more decentralized, is variable
and nondispatchable, and has much smaller long- and
short-term reactive power capabilities.
✔ The generation resource mix shift resulted in much
higher power flows from the east and depressed system voltages in the load center, especially during
peak load conditions. The majority of new resource
installations are large solar and wind farms in the area
around EC and IV (Figure 2, green oval).
✔ Shutting down 2,200 MW of the San Onofre Nuclear
Generating Station (SONGS) further worsened the
SDG&E's northern voltage profile. Without SONGS,
overall transmission imports and flows increased, system inertia was reduced, and dynamic reactive power
support with a range of 1,100/-800 Mvar was lost.
✔ The nature of renewable resources further challenges the system with high output during low-demand
periods. This produces some high-voltage pockets
during periods when dispatchable generation is often not available to provide operating flexibility and
voltage control.
For years, SDG&E has assessed the planning and operation
of its power grid based on 1) the risk of losing its nuclear power
plant and the retirement of old thermal power plants, 2) system
needs in meeting reliability challenges under California's RPS
goal, and 3) the effects of electric power industry technology
development. SDG&E has also been working with the California Independent System Operator (CAISO) and neighboring
transmission owners to assess scenarios and alternatives for
meeting system requirements in various regional assessments.
All of these technical assessments consistently illustrate the
need for additional reactive power resources.

Needs Assessment
Detrimental Impact of
SONGS Decommissioning
SONGS, the 2,200-MW and 1,100/-800-Mvar nuclear power
plant, was electrically located at a critical point between the
SDG&E bulk transmission system and its northern neighboring system. An unexpected early shutdown of the plant created capacity shortfalls for meeting load requirements, and
the huge loss of dynamic reactive resource support caused
voltage criteria violations in the highly populated area in the
northern neighboring system.
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ieee power & energy magazine

Additionally, power no longer flowed from SDG&E's
northern interties into its system. Instead, power flowed
from SDG&E's eastern interties, through the San Diego
basin, and then northward across SDG&E's northern interties. This flow pattern caused more losses than previous flow
patterns and further depressed the SDG&E voltage profile,
especially in the northern portion of its system.

Various Technologies Considered
and the Preferred Option
Long-term transmission planning assessments consistently
identified the need for additional reactive sources with the
following characteristics:
✔ output ranges to inject reactive power during peak
load hours and to absorb it during light load hours
✔ provision of smooth voltage profiles to regulate voltage in response to changing outputs of variable energy
resources.
With these requirements, SDG&E narrowed down its
product/technology search to dynamic reactive resources to
be deployed in several critical system locations. The technologies considered include static var compensators (SVCs),
static synchronous compensators (STATCOMs), and synchronous condensers.
SDG&E operates its grid with a ±100-Mvar, three-level
STATCOM, which had cooling system issues and resulted
in SDG&E discrediting, to some extent, the use of this technology for needed future system improvements. There were
space limitations at the substations being considered for new
dynamic reactive resources, which was a critical and determining factor for selecting the technology for dynamic reactive resources. Because synchronous condensers require
less space, they were selected as the preferred technology
over SVCs, which possibly require harmonic filter circuits.
Although utility-scale solar installations have the ability to
provide Mvar support, they were deemed unable to meet the
needs of the transmission system for this study.
Another important factor in selecting synchronous condensers was the addition of inertia to help maintain system rotor angle and frequency stability, which exhibited
degraded performance as a result of the retirement of conventional generation power plants. The inertia is inherent in
a synchronous condenser because it is a synchronous rotating machine operating as a motor with no mechanical load.
SDG&E submitted a proposal in CAISO's 2010-2011
annual grid-planning cycle to add several dynamic reactive
devices throughout its bulk transmission system. CAISO
july/august 2019

IEEE Power & Energy Magazine - July/August 2019

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