WESTERN INTERCONNECTION
TRANSMISSION PATH FLOW STUDY
1998 thru 2005
September 2007
WECC – Transmission Expansion Planning Policy Committee
(TEPPC)
Historical Analysis WG
TABLE OF CONTENTS
I. INTRODUCTION ------------------------------------------------------------------------------------ 3
II. PATH FLOW METHODOLOGY --------------------------------------------------------------- 4
a. Paths Analyzed -------------------------------------------------------------------------------- 4
b. Analysis Methodology and Indices ------------------------------------------------------- 5
c. Path OTC Assumptions --------------------------------------------------------------------- 6
III. PATH FLOW ANALYSIS AND RESULTS
Summary Graphs – 75% and 90% Indices --------------------------------------------------10
Summary Maps ------------------------------------------------------------------------------------10
Individual Path Results – Seasonal 75% Indices 1998 thru 2005----------------------11
Individual Path Results – Path Flow Frequency Distributions 2004-05 ---------------11
IV. TREND ANALYSIS AND RESULTS
1998 through 2005 --------------------------------------------------------------------------------17
V. OBSERVATIONS AND CONCLUSIONS -------------------------------------------------- 22
APPENDIX – included as a separate EXCEL file
A. Path Summary Charts –
a. % Time Exceeding 75% of OTC
b. % Time Exceeding 90% of OTC
B. Individual Path Results – Seasonal 75% Indices 1998 thru 2005
C. Individual Path Results – Path Flow Frequency Distribution 2004-05
D. Tabulated Analysis Results
2
SECTION I
INTRODUCTION
The analysis of actual historical power flow data for the transmission system in the
Western Interconnection provides an indication of how marketers and load serving
entities have utilized the transmission system to market energy and serve load. This
information is also useful in the analysis and identification of potential future areas of
congestion and for verifying model representation for power flow and production costing
analysis. Although the flow is a useful indicator, this may not reflect the schedules on a
path, and cannot reflect high value uses of transmission to allow access to operating
reserves, forced outage reserves or market options.
This work is the latest in a continuing effort to analyze utilization of the western
Interconnection transmission system. The WECC Transmission Expansion Planning
Policy Committee’s Historical Analysis WG performed the analysis in this report. Similar
analysis was performed previously by the three western Regional Transmission
Associations (RTA) as documented in the latest Biennial Transmission Plan, dated July
2000 and by the Seams Steering Group – Western Interconnection, in their report dated
February 2003. The time period covered by the analysis in this report is from Winter
1998-1999 through Summer 2005 with emphasis on the 2004-05 time period.
Results of this analysis were provided to the US Department of Energy by the Western
Congestion Assessment TF in May 2006, to assist with the US DOE study of
transmission congestion in the Western Interconnection and Eastern Interconnections.
The analysis was performed for 28 transmission paths, representing most of the major
transmission paths in the western interconnection.
The analysis utilized real time hourly power flow and operating transfer capability data
submitted by path operators and archived in the WECC EHV Data Pool database. Most
data in the EHV Data Pool database is complete. In some cases, the real time path
operating transfer capability was not reported and assumptions had to be made for this
analysis, based upon published path transfer capabilities. The OTC assumptions for
each path are noted in this report. Hourly path flow data for Canada to NW and the
Pacific HVDC Intertie was also obtained from the Bonneville Power Administration, BC
Hydro and the Alberta Electric System Operator (AESO).
To facilitate comparison among the paths, a utilization indicator was calculated. The
same indicator was used in the RTA’s Biennial Transmission Plan report. This indicator
is calculated as the percentage of time the path exceeds 75% of its operating transfer
capability. The 75% level was chosen as an indication of a path that may be considered
heavily utilized. A 90% probability level was also calculated for the 2004-05 time period.
These indices were developed for purposes of this analysis and have no basis in terms
of an accepted industry standard or practice. These indices were used by the US DOE
as a congestion measure in their August 2006 Congestion Report. The magnitude of the
indicators is not necessarily an indication that there is congestion, or an inability to meet
the needs of transmission users, on the path. In the western interconnection, paths are
3
designed to be loaded to 100% of their operating transfer capability and withstand a
credible outage without violating reliability standards.
SECTION II
PATH FLOW METHODOLOGY
Actual archived MW power flow data for the major transmission paths in the Western
Interconnection were analyzed from winter 1998-99 through summer 2005, using data
from the Western Electricity Coordinating Council (WECC) EHV Data Pool database.
Analysis of the data was performed in late 2005 and early 2006 and therefore does not
include the analysis of 2006 or later data. Future updates will include an analysis of
2006 and 2007 data. Results are presented in this report by season and by individual
path. Information from prior reports was used for analysis prior to April 2004.
The data analyzed consisted only of actual hourly MW flows. Hourly ATC and schedule
data was not available and therefore was not included in this analysis. Analysis of ATC
and schedule data is required to assess path congestion and whether paths are heavily
scheduled or whether contract rights are being scheduled. A path may experience low
actual flows depending upon the magnitude of its hourly schedules; yet it may be
congested to the market if the path is fully reserved on a firm basis (zero firm ATC).
The purpose of the analysis was to determine the historical utilization of the major
transmission paths in the Western Interconnection. It should not be concluded from this
analysis that it is either necessary or economical to take any corrective actions for the
loading levels reported. The results may, however, provide information for identifying
paths for further study regarding the potential benefits and costs of increasing their path
capacity.
Actual flows were calculated on a per unit basis, referenced to the paths Operating
Transfer Capability (OTC). Table I identifies the OTC used for each path. Hourly OTC
data was used unless it was not available.
Results are presented by seasons, defined as:
Winter – November thru March
Spring – April and May
Summer – June thru October
Paths Analyzed
Data for the following paths were analyzed. Path names and path numbers are from the
WECC Project Rating Catalog. A map showing the geographic location of the individual
paths is included in Figure 2. A list of the lines making up each path may be found in the
WECC Path Rating Catalog.
4
WECC Path #
1
3
8
14
15
17
18
19
20
22
26
30
31
34
35
36
45
47
48
49
50
51
65
66
WECC Path Name
Alberta - BC
Northwest – Canada
Montana to Northwest
Idaho to Northwest
Midway – Los Banos
Borah West
Idaho – Montana
Bridger West
Path C
Southwest of Four Corners
Northern – Southern California
TOT 1A
TOT 2A
TOT 2B
TOT 2C
TOT 3
SDG&E - CFE
Southern New Mexico
Northern New Mexico
East of Colorado River (EOR)
Cholla – Pinnacle Peak
Southern Navajo
Pacific DC Intertie (PDCI)
COI
Analysis Methodology and Indices
The WSCC EHV Data Pool database was used for the frequency distribution analysis.
The analysis period for the frequency distribution analysis was from April 2004 thru
October 2005. Yearly trend analysis utilized data from the 2000 Biennial Transmission
Plan, from the February 2003 report as well as updated information for the April 2004
through October 2005 time period.
Table I identifies the OTC assumption for each path.
Frequency Distribution
The percentage of time a path exceeded a given percentage of its OTC was
calculated and presented as a frequency distribution plot for each transmission
path, using the hourly MW flow data in the EHV Data Pool database. Plots for
the individual paths are presented in the Appendix.
Percentage of time Exceeding 75% and 90% of OTC
Beginning with the 2000 Biennial Transmission Plan, a utilization indicator was
developed. It is defined as the percentage of time over the season that the path
loading exceeds 75% of the path OTC. Based upon WECC reliability criteria, a
path may be loaded up to its OTC level and be able to withstand various outage
contingencies without violating reliability criteria. Experience has shown,
5
however, that loadings above approximately 75% of OTC may be associated with
paths considered to be heavily utilized from a marketing or commercial use
viewpoint. The 75% value is not based upon any industry standard or guide,
and was chosen primarily to establish a figure of merit for ease of loading
comparison between paths.
The derivation of the “Percentage of time exceeding 75% of OTC” value is
graphically illustrated in Figure 1. Previous reports also reported the peak
loading for each path, however this was not done in this report. The “definition”
of peak loading used in previous analysis is also shown in Figure 1.
OTC Assumptions
Flow analysis is presented as a percentage or per unit of path Operating Transfer
Capability (OTC). Hourly OTC values are reported in the EHV Data Pool database.
Those reporting path OTC are supposed to calculate the OTC each hour, adjusting
reported values for changing operating conditions. This is not always the case,
however. In some cases, no OTC values are reported by the path operator and a zero
value is included in the database. In some cases, an OTC value is reported each hour,
however the value is the same for each hour and is not varied as operating conditions
change. In some cases, the OTC data is reported correctly in which case the reported
OTC value does vary in magnitude, indicating that the path operator is adjusting the path
capability for changing operating conditions.
Table I summarizes the OTC value used in this analysis. If a value is reported by the
path operator, whether it is adjusted for changing operating conditions or not, this value
is used. If no value is reported each hour and a zero appears in the database, the
WECC path rating or the value set by the WECC OTC policy group is used.
Figure 1
% of Time
exceeding
x axis
% of time
exceeding
75% of OTC
value
1%
75%
% of OTC
Equivalent to the
99 percentile
point
Calculated
99%
probability
peak value
6
Max.
flow
TABLE 1
OTC Values used in 2004-05 Path Flow Study
Path
Number
Path Name
OTC Value
Comments
(Source of OTC values)
1
Alberta to BC
0 to 735 MW EW (Ave
= 224 MW)
0 to 715 MW WE (Ave
= 572 MW)
(See OTC probability
graph)
Hourly OTC from AESO,
Hourly flows from BCH
3
Canada to NW
0 to 3150 MW NS
250 to 2950 MW SN
Hourly OTC from BPA
Hourly flows from BPA
8
Montana to NW
2200 MW E to W
Constant
14
Idaho to NW
2400 MW E to W,
1200 MW W to E
Constant
15
Path 15
2050 to 5400 MW
Posted and variable
17
Borah West
2307 MW
Constant
18
Montana to Idaho
77-377 MW
Posted and variable
19
Bridger West
371 to 2200 MW
Posted and variable
20
Path C
300 to 950 MW
Posted and variable
22
SW of 4 Corners
837 to 2325 MW
Posted and variable
26
N to S California
3000 Max SN, 3135
MW Max NS
Posted and variable
30
TOT 1A
228 to 650 MW
Posted and variable
31
TOT 2A
218 to 676 MW
Posted and variable
34
TOT 2B
394 to 850 MW
Posted and variable
35
TOT 2C
139 to 300 MW
Posted and variable
36
TOT 3
450 to 1557 MW
Posted and variable
7
45
CAISO - CFE
800 MW SN
408 MW NS
Constant
Posted, but unchanging
47
S. New Mexico
Max to 1038 MW
Posted and variable
48
N. New Mexico
Max to 1946 MW
Posted and variable
49
E of River
2556 to 7550 MW
Posted and variable
50
Cholla – Pinnacle
Peak
1200 MW
Constant
51
Southern Navajo
815 to 2264 MW
Posted and variable
65
DC Intertie
0 to 3100 MW
Hourly OTC from BPA & WECC
66
COI
-3675 to +4800 MW
Posted and variable
NOTES:
“Posted and variable” – means the OTC value was posted hourly in the WECC EHV
Data Pool database and is a varying quantity
“Constant” – means the hourly OTC used in this analysis was a constant, unchanging
value and was not posted in the WECC EHV Data Pool
Geographic location of the WECC Paths included in this analysis are shown in Figure 2.
A description of the WECC Paths is contained in the WECC Path Rating Catalog.
8
9
SECTION III
PATH FLOW ANALYSIS AND RESULTS
Summary Graphs – 75% and 90% Indices
Figure 3 and 4 show in bar chart form the path loadings using the 75% and 90%
indices. Results are presented in order of magnitude, from highest to least.
Figure 3 (75% indices) covers the period 1998 thru 2005. Figure 4 (90% indices)
covers the period 2004-05. Results for years prior to 2004-05 draw on analysis
performed in those prior years. The 90% indices was not used consistently prior
to 2004-05, hence the 90% analysis is only presented for the 2004-05 time
period.
For each path, the highest seasonal loading for the seasons analyzed is
presented. Therefore, the three seasonal bars for each path may not be from the
same year. For example, path 19 has the highest winter loading in 2000-01, the
highest spring loading in 2001 and the highest summer loading in 2000. These
are the values that are plotted for path 19.
A line is drawn on the graph at the 50% and 25% levels. This was done to group
the paths into loading ranges as an approximate measure of use, such as high,
medium and low. This grouping was also used in presenting results on the
Summary Path Loading Maps described below.
Summary Maps
Geographic path maps with path loading grouped into the following categories
are shown in Figures 5, 6, and 7 for the 75% indices. Loadings are grouped on
the maps into the following three categories:
•
Paths with loadings greater than 75% of OTC occurring more than
50% of the time during a season
•
Paths with loadings greater than 75% of OTC occurring between
25% and 50% of the time during a season
•
Paths with loadings greater than 75% of OTC occurring less than
25% of the time during a season.
Separate maps are presented for each season showing the loading for the period
from 1998 through 2005.
10
Individual Path Results – Seasonal 75% Indices 1998 thru 2005
The percentage of time each path exceeds 75% of the path OTC is plotted in the
bar charts in the Appendix for each individual path from 1998 thru 2005, for the
following seasons:
Winter – 1998-99, 2000-01, 2001-02, and 2004-05
Spring – 1999, 2001, 2002, 2004 and 2005
Summer – 1999, 2000, 2001, 2004 and 2005
These results draw on analysis performed in prior years. All seasons have not
been analyzed in previous years, hence the missing seasons.
This represents the percentage of time during the season that the loading on the
path exceeded 75% of the path transfer capability. For example, a path with a
transfer capability of 1000 MW and a 30% calculated value for spring 2002
means that the path exceeded 750 MW (75% of OTC) for 439 hours (30% of a
total of 1464 hours) during the months of April and May 2002. In some cases, a
zero is reported for a path. This means that the path did not exceed 75% of the
path OTC during the season. A path could have been operated at 74% of the
path OTC for the entire season and the calculated value (for the % of time it
exceeded 75% of OTC) would be zero.
Individual Path Results – Path Flow Frequency Distributions 2004-05
Frequency distribution plots for each path are shown in the Appendix for the
Spring 2004 through Summer 2005 timeframe.
11
Figure 3
Path Ranking
Maximum Seasonal Values from 1999 through 2005
% of Time Path Actual Flow exceeds 75% of Path OTC
100
Bridger West
90
Cholla - Pinnacle Pk
Represents the highest
Seasonal Loading for
each Path, from 1999
thru 2005
% Time Path Flow Exceeds 75% of OTC
80
SW of 4C
S. New Mex.
70
TOT 1A (CO to Utah)
60
TOT 3 (WY to CO)
W
Sp
Su
50% of Time
50
40
30
25% of Time
20
10
0
19
50
22
47
30
36
3
8
48
35
17
66
65
15
WECC Path Number
12
31
1
26
45
49
18
20
34
51
14
Figure 4
Reliability Congestion Index - RCI (90%)
Maximum Seasonal Values from Spring 2004 through Summer 2005
% of Time Path Actual Flow Exceeds 90% of Path OTC
25
Ranking is based upon the
Maximum 90% value for the
five seasons between Spring
2004 and Summer 2005
Bridger West
RCI (90%) - % Time > 90% of Path OTC
20
NW to Canada (StoN)
RCI (90%) is the percentage of
time the physical flow exceeds
90% of the path OTC
Alberta to BC
15
Pacific DC Intertie
COI
90%
TOT 1a (CO to Utah
10
SW of 4Corners
5
0
19
3
1
65
66
30
22
36
45
15
35
18
31
48
Path Number
13
26
20
14
17
47
8
50
49
34
51
Figure 5
14
Figure 6
15
Figure 7
16
SECTION IV
TREND ANALYSIS
Path data has been analyzed for approximately a 7 year time period, from Winter 199899 through Summer 2005. An attempt was made to determine if there are any apparent
path loading trends over this period of time. For example, is there a tendency for paths
to be utilized or loaded more heavily now than earlier? Or are paths being utilized less
heavily? Or is there no apparent trend? A number of factors could cause path loading
trends to occur, such as increased or decreased line construction, changes in load
growth, changes in generation type usage. Other factors may cause more random
annual and seasonal variations, but less likely to affect longer-term trends, such as
changes in hydro conditions, weather and temperature changes, etc.
A very simple approach was taken to get a macroscopic view of the utilization of the
entire interconnection to see if a trend existed. The method used was to simply sum the
number of paths in the interconnection whose flows exceeded 75% of the path OTC for
each season, from 1998 through 2005. Two summations were made to reflect the
amount of time the paths exceeded the 75% value. Both summations are plotted in the
associated figures below. The two were:
a) summation of the number of paths that exceeded 75% OTC greater than 50% of
the time within each season
b) summation of the number of paths that exceeded 75% OTC greater than 25% of
the time within each season
These sums were then plotted graphically in chronological order, from the Winter of
1998-99 through the Summer of 2005, to see if any visual trend was apparent. Results
are plotted in Figure 8.
The same data is replotted in Figures 9, 10 and 11 for the separate Winter, Spring and
Summer seasons to remove the seasonal variations that are included in Figure 8.
Recognizing the limited amount of data in the data sample (limited number of paths and
years) and the simplified, quasi-quantified trending method used, it does appear that
there may be a slight trend to lower utilization on a macro system basis with time for the
years analyzed (between 1998 and 2005). This is more apparent in Figures 9, 10 and
11 where trends for each season are shown. This is particularly true for the summation
for “greater than 50% of the time”. It should be emphasized that this analysis is on a
macro total system basis and that individual areas of the system will exhibit different
results.
Other trending techniques should be investigated in the future to more accurately
quantify these trends.
17
Figure 8
Path Utilization Trend
1998 through 2005
Number of WECC Paths Exceeding 75% of their Path OTC, >25% and >50% of Time
10
9
8
Number of Paths
7
6
25% Level
50% Level
5
4
3
2
1
0
W98-99
Sp 99
Su 99
Su 00
W00-01
Sp 01
Su 01
W01-02
Season
18
Sp 02
Sp 04
Su 04 W04-05
Sp 05
Su 05
Figure 9
Path Utilization Trend - Winter Season
1998 through 2005
6
Number of Paths >75% of path OTC
5
4
>25% of time
>50% of time
3
2
1
0
W98-99
W00-01
W01-02
W04-05
W98-99
Season
19
W00-01
W01-02
W04-05
Figure 10
Path Utilization Trend - Spring Season
1999 thru 2005
8
7
Number of Paths >75% of path OTC
6
5
>25% of time
>50% of time
4
3
2
1
0
Sp99
Sp01
Sp02
Sp04
Sp05
Sp99
Season
20
Sp01
Sp02
Sp04
Sp05
Figure 11
Path Utilization Trend - Summer Season
1999 through 2005
10
9
Number of paths >75% path OTC
8
7
6
>25% of time
>50% of time
5
4
3
2
1
0
Su99
Su00
Su01
Su04
Su05
Su99
Season
21
Su00
Su01
Su04
Su05
SECTION V
OBSERVATIONS AND CONCLUSIONS
The following observations may be drawn from the analysis:
1. The following paths had at least one season over the study period, in which the
seasonal loading exceeded 75% of OTC 50% of the time or greater: (These may
be considered the more heavily utilized paths relative to their operating transfer
capability. This by itself is not an indication that these are the most commercially
congested paths. These are also not the most heavily loaded paths in terms of
the magnitude of MW loading)
Path 19 – Bridger West
Path 50 – Cholla – Pinnacle Peak
Path 22 – Southwest of 4 Corners
Path 47 – Southern New Mexico
Path 30 – TOT 1A - (Colorado to Utah)
Path 36 – TOT 3 - (Wyoming to Colorado)
Path 27 – IPP DC Line (was not analyzed in this report, however it does meet
this criteria based upon previous studies.
2. Paths with the highest loadings relative to their transfer capabilities are primarily
located in the Rocky Mountain and Desert Southwest regions (Wyoming,
Colorado, Arizona and New Mexico).
3. Future analysis should include analysis of commercial data (ATC and schedules)
and correlation of observed hourly power flows with reported ATC and
schedules.
4. Based upon the data analyzed in this report, it appears that total system loading
relative to total system capacity, may be decreasing slightly, however additional
trend analysis should be performed to confirm this finding. It should be
recognized that even if this is occurring, individual areas of the system might
simultaneously be experiencing increasing transmission loadings and increasing
transmission congestion.
5. As the loadings reflect power flow and not the schedules, or other contractual
uses of transmission that may not result in a flow (e.g. contracting transmission
rights for forced outage reserves that are seldom used), it should not be
concluded from this analysis that there was, or was not, significant congestion
(defined as the inability to obtain transmission capacity when needed) on a path.
Congestion analysis should include analysis of hourly ATC and hourly schedule
data. In addition, no attempt was made to calculate the economic benefits of
increasing the transfer capability. Additional studies would be needed to
22
determine if increasing the transfer capability of a path would bring economic
benefits.
23