IEEE Power & Energy Magazine - November/December 2014 - 57

Most natural gas plants have the flexibility
to quickly change their dispatch and can manage the variability
of renewable energy units.
supply can thus affect the security and the economics of
electric power delivery.
Power system expansion planning is not based on firm
commitments from customer loads but rather on load growth
estimates. To balance this lack of commitment, the transmission grid is designed as a flexible system with multiple parallel
paths where facilities are interconnected to withstand multiple
contingencies. Natural gas pipeline network planning is less
flexible because industry practice is to expand pipeline capacity based on firm customer contracts. Pipeline owners do not
charge existing natural gas customers for the cost of new capacity. As a result, pipeline capacity closely matches firm customer
requirements. In essence, pipelines know the exact location
of customers that have firm rights and have contracts in place
that describe exactly how much firm capacity each customer
may call upon. For this reason, there will be little or no excess
pipeline capacity available if pipelines' firm customers use their
full capabilities. Unlike power transmission systems, pipelines
often do not have contingency planning standards. There is
some redundancy in the system, however, because it utilizes
side-by-side or loop pipelines in cases of forced or scheduled
outages. Nevertheless, these options do not lead to any dramatic
increases in natural gas flows.
The natural-gas-fired generators participating in electricity markets rely heavily on pipeline capacity releases
and interruptible services. Most natural-gas-fired generating units do not currently commit to firm natural gas services because the generators mostly serve midlevel to peak
demands. This trend is expected to change globally with the
pending retirements of many coal-fired units, thus forcing
natural gas generating units to serve base loads. Current
energy profiles, however, make it difficult for natural-gasfired units to commit to firm pipeline capacity.
Natural-gas-fired generating units offer flexible modes
of operation that help support variable renewable energy
resources, such as wind and solar. Wind energy has increased
globally over the last ten years, and this trend is expected
to continue. Although forecasting models are supported by
sophisticated monitoring equipment, the industry is still far
from predicting accurately when particular wind generators
will go online and how long they will produce energy. Electric storage, a seemingly reasonable solution to the variability of renewable generation, has yet to provide a large-scale
and cost-effective solution. Even hydropower-rich regions
are constrained in their capacity to balance variable power
generation because run-of-river projects have limitations on
their dispatchability.
november/december 2014

Concerns about the interdependence of natural gas and
electricity infrastructures are more acute in certain regions
of the world than others. In some jurisdictions, like Turkey,
this coordination is still a novel concept. For others, it has
already been examined, solutions have been implemented,
and adjustments have been made as the need for natural gas
supply increases. Worldwide, regions that are expanding both
their use of natural gas generators and renewable generation
are the ones most concerned with misalignments between the
natural gas and electric sectors.
The interdependence of the electric power transmission
grid and the natural gas pipeline network requires that certain issues be taken into account. The most important one
is the need for coordinated planning of the electric power
transmission grid and natural gas pipeline networks. Security and reliability studies of the electric power system in
Turkey, for instance, should take into account contingency
scenarios pertinent to the security and reliability of the natural gas pipeline network. These include:
✔✔ limitations on natural gas imports (socioeconomic
and geopolitical constraints)
✔✔ technical constraints imposed on natural gas supply,
such as forced outages of natural gas pipelines and
loss of pressure in the natural gas pipeline network
due to consumption by higher-priority residential
loads during the winter months.
In the worst conditions-e.g., during winter peak load
periods-prioritized electric load shedding is necessary when
balancing natural gas supply and demand, particularly when
the natural gas pressure is low due to significant increases in
residential consumption. Shedding large electric loads that are
electrically remote from natural gas power plants could be one
priority, for example, because the natural gas power plants could
suffer a capacity decrease due to loss of natural gas pressure, and
the load could be subject to electric network constraints (such
as voltage and thermal constraints). Such contingency studies
require the development of a methodology for solving shortand mid-term security-constrained unit commitment (SCUC)
that considers the effects of the natural gas pipeline network
and its interdependency with the highly complex electric power
transmission system. The solution of a long-term SCUC problem, which often spans several months to a year, may consider
multiple long-term fuel and emission constraints in addition to
operating constraints embedded in the short-term SCUC.
The factors listed below should be modeled in long-term
studies that integrate the SCUC model with natural gas pipeline network constraints. The integrated model can then be
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2014