POWER May 2016 - 33

DISTRIBUTED ENERGY RESOURCES
How Does the North American Electric Grid Work?
The North American electrical grid is an interconnected
network of power lines and
associated equipment for moving electricity
from suppliers to consumers. It is made
up of generation, transmission, and distribution.
The generation piece should be
familiar to POWER's audience, but the distinction
between transmission and distribution
may not be quite as well known. In
general, transmission lines are high-voltage
lines with ratings of 69 kV or greater that
are used to move power from generating
facilities to substations. Distribution lines
are the lower-voltage wires that deliver
power to most end users.
The continental U.S. and Canada are divided
into four electricity interconnections:
the Eastern Interconnection, the Western
Interconnection, the Quebec Interconnection,
and the Electric Reliability Council
of Texas (ERCOT). Although the boundary
dividing the Eastern and Western Interconnections
is far from straight, it essentially
runs between Alberta and Saskatchewan,
and then cuts over to the eastern borders
of Wyoming, Colorado, and New Mexico, before
ending inside Mexico.
Wholesale electricity markets are generally
broken into two types. One type has
organized competitive markets operated by
independent system operators (ISOs) or regional
transmission organizations (RTOs).
While there are differences between ISOs and
RTOs, it is beyond the scope of this article
to differentiate between the two. We will use
the more general term ISO to represent both.
The other type of wholesale electricity market
consists of regulated bilateral markets.
There are nine ISO/RTO service areas in
North America (Table 1). The ISO manages
the transmission system while individual
owners and operators of generation submit
However, rooftop solar does change the
demand curves used by CAISO in its dayahead
market, which commits non-DER
resources to generate power to meet the expected
demand, while also ensuring transmission
capacity is available for its delivery.
CAISO said it only sees the impact of rooftop
solar when clouds block the sun and all of
that demand suddenly shows up on its forecast
curves.
To help address the challenge, CAISO
proposed tariff revisions in March. The
proposal-submitted to the Federal EnMay
2016 | POWER
offers to supply electricity on a competitive
basis. This arrangement allows generation,
transmission, and retail service to
be owned by separate entities within the
region. In regulated bilateral markets, vertically
integrated electric utilities control
the transmission system and choose which
generation is dispatched.
Large control centers-or balancing authorities-are
staffed to monitor and match
the supply and demand of electricity in the
various transmission control areas that make
up the North American grid. As of October 1,
2015, there were 75 of these nerve centers
in the U.S. and Canada continuously finetuning
system parameters.
Operators at some of these locations use
advanced forecasting tools and geospatial
maps with multiple layers of data shown on
video screens up to 80 feet wide to do their
jobs (shown in the opening photo). They
utilize wind and solar availability patterns
to help predict renewable production and
issue dispatch instructions to generators
in their power pools to balance supply and
demand. When power needs to be moved
( " wheeled " ) from one region to another,
they coordinate with adjacent control centers
to make that happen.
Sophisticated computers are constantly
taking the pulse of the grid, providing a
continually updated picture so operators
can manage resources and react to emergencies-all
of which allows the North
American power grid to reliably deliver 830
GW to 334 million people 24/7/365.
Table 1. ISOs and RTOs. Independent system operators (ISOs) and regional transmission
organizations (RTOs) wield more than 85% of North America's power generation and
serve more than two-thirds of its population. Source: ISO/RTO Council
ISO/RTO name
Alberta Electric System
Operator
California Independent
System Operator
Electric Reliability Council
of Texas
Independent Electricity
System Operator
ISO New England
Midcontinent Independent
System Operator
New York Independent
System Operator
PJM Interconnection
Southwest Power Pool
States and provinces at
least partially served
AB
CA/NV
TX
ON
CT/MA/ME/NH/RI/VT
AR/IA/IL/IN/KE/LA/MB/MI/
MN/ MO/MS/MT/ND/SD/
TX/WI
NY
DC/DE/IL/IN/KE/MD/MI/NJ/
NC/OH/PA/TE/VA/WV
AR/KS/LA/MO/NE/NM/OK/TX
ergy Regulatory Commission (see sidebar
" Know Your Acronyms: FERC, ERO,
and NERC " )-would establish an initial
framework to enable aggregations of DERs
connected to distribution systems within
CAISO's balancing authority area (Figure
2) to participate in its energy and ancillary
services markets. For instance, the framework
would facilitate wholesale market
participation by microgrids interconnected
to distribution systems, third-party aggregators
operating DERs, or a utility distribution
company operating those resources.
www.powermag.com
Generation
capacity
(MW)
14,568
57,124
84,000
35,858
32,000
205,759
37,978
183,604
58,982
Miles of
transmission
lines
16,155
26,000
40,530
18,641
8,130
65,250
11,056
62,556
50,575
Millions of
customers
served
3.7
30
23
13.7
14
48
19.5
61
15
CAISO believes the change would add
operational flexibility to its system, but it
stressed that the revisions are only a first
step. As it gains operational experience
with the DER aggregations, it expects to
be better able to consider future refinements
and enhancements. If approved, the
revisions could go into effect as early as
June 3, 2016.
Another effort that CAISO has under way
to lower barriers and enhance the ability of
transmission grid-connected energy storage
and DERs to participate in the ISO market
33
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POWER May 2016

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