[WESTERN INTERCONNECTION]{.underline}

[TRANSMISSION PATH FLOW STUDY]{.underline}

February 2003

Planning Work Group

Seams Steering Group -- Western Interconnection

This report was prepared in cooperation with the Western Electricity
Coordinating Council and the Committee on Regional Electric Power
Cooperation

[TABLE OF CONTENTS]{.underline}

######## I. INTRODUCTION 2 

######## II. PATH FLOW ANALYSIS 4 

> a\. Paths Analyzed 4
>
> b\. Analysis Methodology 5
>
> c\. OTC Assumptions 6
>
> d\. Analysis Results 9

**III. OBSERVATIONS AND
CONCLUSIONS** - - - - - - - - - - - - - - - - - - - - - - - - - - - 12

###### 

###### 

###### APPENDICES

A.  Path Summary Charts -- Peak loading and % Time Exceeding 75% of OTC

B.  Maps of Path Locations, Flows by Season and Bi-Directional Paths

C.  Summer and Winter Path Flow Comparison Charts by Region

D.  Individual Path Flow Distribution Plots -- Northwest Region

E.  Individual Path Flow Distribution Plots -- Rocky Mountain Region

F.  Individual Path Flow Distribution Plots -- Southwest Region

G.  Individual Path Flow Distribution Plots -- California Region

H.  Data Summary Tables

    a.  OTC Assumptions

    b.  \% of Time \> 75% of OTC by Season and Path

    c.  Peak Loading Summary by Season and Path

        [SECTION I]{.underline}

        [INTRODUCTION]{.underline}

        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.

        This work is the latest in a continuing effort building to
        analyze utilization of the western Interconnection transmission
        system. Similar analysis was performed by the three western
        Regional Transmission Associations (RTA) as documented in the
        latest Biennial Transmission Plan, dated July 2000. Historical
        data analyzed and included in the August 2001 Western Governor's
        transmission report was also incorporated. The time period
        covered by the analysis in this report is from Winter 1998-1999
        through Spring 2002.

        The analysis was performed for 33 transmission paths,
        representing all 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 Western Electricity Coordinating Council's 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 were made based upon
        published path transfer capabilities. These assumptions are
        noted in this report.

        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 over the season reported. The 75%
        level was chosen as an indication of a path that may be
        considered heavily utilized. This figure was developed for
        purposes of this report and has no basis in terms of an accepted
        industry standard or practice. The magnitude of the indicator 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 designed to be loaded
        to 100% of their operating transfer capability and withstand a
        credible outage without violating reliability standards.

        A second loading indicator presented in this report, is the peak
        loading during the season. This indicator does not include a
        time factor as does the 75% indicator.

        The work in this report was sponsored by the Seams Steering
        Group -- Western Interconnection (SSG-WI), which is supported by
        RTO West, the California ISO and WestConnect. Data used in the
        analysis was provided by the Western Electricity Coordinating
        Council. This effort is a product of the SSG-WI's Planning Work
        Group. Participants in the Work Group include RTO members,
        marketers, generators, renewable resource interests, non-RTO
        participating entities, representatives of various western state
        agencies and representatives of the Western Interstate Energy
        Board.

[SECTION II]{.underline}

[PATH FLOW ANALYSIS]{.underline}

Actual archived MW power flow data for the major transmission paths in
the Western Interconnection were analyzed from winter 2000-01 through
spring 2002, using data from the Western Electricity Coordinating
Council (WECC) EHV Data Pool database. Results are presented in this
report season and by individual path.

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. Posted OTC was used unless it was not available.

This report presents the analysis results for the 3 seasons studied.

# 

# [Paths Analyzed]{.underline}

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.

**[WECC Path \#]{.underline}** **[WECC Path Name]{.underline}**

3.  Northwest -- Canada

> 4 West of Cascades -- North
>
> 5 West of Cascades -- South

6 West of Hatwai

8 Montana to Northwest

14 Idaho to Northwest

15 Midway -- Los Banos

16 Idaho -- Sierra

17 Borah West

18 Idaho -- Montana

19 Bridger West

20 Path C

22 Southwest of Four Corners

23 Four Corners 345/500 Kv Qualified Path

24 PG&E -- SPP

26 Northern -- Southern California

27 Intermountain Power Project DC Line

30 TOT 1A

31 TOT 2A

32 Pavant -- Gonder 230 Kv

34 TOT 2B

35 TOT 2C

36 TOT 3

45 SDG&E - CFE

46 West of Colorado River (WOR)

47. Southern New Mexico

48. Northern New Mexico

49 East of Colorado River (EOR)

50 Cholla -- Pinnacle Peak

51 Southern Navajo

65 Pacific DC Intertie (PDCI)

66. COI

> 73 North of John Day

###### [Analysis Methodology]{.underline}

The WSCC EHV Data Pool database was used for the frequency distribution
analysis. The analysis period for the frequency distribution analysis
was from November 2000 through May 2002. Data from the 2000 Biennial
Transmission Plan was incorporated to compare "peak" and "75% of OTC"
values for the period from winter 98-99 through spring 2002. Data is
grouped by month into the following seasons:

> Winter - November 1 through March 31
>
> Spring - April 1 through May 31
>
> Summer - June 1through October 31

Table I summarizes for each transmission path, the years that were
analyzed for the three seasons over the period winter 98-99 through
spring 02. Table I also 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 Appendices D
> through G.
>
> **Percentage of time Exceeding 75% 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. In this
report, the utilization indicator was calculated for winter, spring and
summer seasons. 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, however, that loadings above approximately 75% of OTC may be
associated with paths considered to be "congested" 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.

> **Peak Seasonal Loading**
>
> Peak path loading is calculated as the 99 percent probability loading.
> This means that at this value of path flow, 99% of the hourly loading
> samples are below this value and 1% of the loading samples exceed this
> value. This definition for peak loading was utilized in the 2000
> Biennial Transmission Plan. It was felt that the absolute maximum
> hourly value should not be used as this could be associated with
> potentially erroneous data and a 99% value would be more
> representative of a peak value.

The derivation of the "Percentage of time exceeding 75% of OTC" and the
"Peak Loading" values are graphically illustrated in Figure 1.

# [OTC Assumptions]{.underline}

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**

![](media/image318.wmf)

# [Analysis Results]{.underline}

# Path Loading Summary Charts -- Table II and Table III

The percentage of time each path exceeds 75% of the path OTC is
tabulated in Table II for the following seasons:

> Winter -- 1998-99, 2000-01 and 2001-02
>
> Spring -- 1999, 2001, and 2002
>
> Summer -- 1999, 2000, and 2001

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.

Seasonal peak loadings are tabulated in Table III. The Peak value
reported is the 99% probability value. The 99% value is used to avoid or
decrease the likelihood of including erroneously high peak values. Peak
values are reported for the following seasons:

> Winter -- 1998-99, 2000-01 and 2001-02
>
> Spring -- 1999, 2001, 2002
>
> Summer -- 1999, 2000, 2001

In some cases, no data was available for analysis during the period
considered. This is indicated in the table as Not Available (N/A)

# Path Loading Summary Bar Chart -- 75% of OTC

Appendix A shows graphically the paths with the highest average loadings
over a season. Results are presented in order of magnitude, from highest
to least.

> For each path, only the highest seasonal loading for the three seasons
> analyzed is presented. 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. The geographic maps in Appendix B also group the paths into these
> same loading groupings.

# Path Loading Summary Bar Chart -- Peak 

Appendix A also shows graphically the paths with the highest peak
loadings. Results are presented in order of magnitude, from highest to
least.

For each path, only the highest seasonal loading over the three seasons
analyzed is plotted. For example, path 19 has the highest winter loading
in 2000-01, the highest spring loading in 2001 and the highest summer
loading in 2001. These are the values that were plotted for Path 19.

The data showed a few paths exceeding their OTC limit for their peak
flows. In actual operation, path operators make every effort to keep the
flows below the OTC limit in real time. It is possible that the data for
the OTC limit could have been calculated for a slightly different hour
that the posted flow data, depending upon how frequently the OTC value
is updated. In some cases, the OTC limit may be calculated for use
during the day ahead preschedule period and the actual OTC limit during
real time may have been different. If the real time OTC value were
higher, this could result in a flow level exceeding the reported OTC
value. In the future, the calculation and posting of OTC limits in the
EHV Data Pool Database should be reviewed and a standard methodology
adopted.

# Geographic Path Maps - Path Loading Groupings

Geographic path maps with path loading grouped into the following
categories are shown in Appendix B. Loadings are grouped 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.

Two maps are presented for each season showing the loading for the most
current season and for the highest loading over the past 3 years.

The first figure in Appendix B illustrates which paths have
unidirectional flows and which have bi-directional flows. This is
described further in the section below, "Frequency Distribution".

# Seasonal Summary -- by Region and Path

Appendix C shows the summer and winter frequency distribution plots by
Region (Pacific Northwest, Rocky Mountain, Southwest and California).
Winter plots represent the combined winter loadings for the 2000-01 and
2001-02 winter seasons. Summer plots represent the combined summer
loadings for the Summer 2000 and 2001 seasons. These plots were
calculated by totaling the hours for the two summer and winter seasons
and calculating a frequency distribution plot for these total hours.

> These plots show graphically which paths in each region are the most
> heavily utilized during summer and winter, relative to the path OTC
> over the two year period.

# Frequency Distribution 

Appendices D through G show the frequency distribution plots for each
path by Region. Frequency distribution plots are presented for the total
years analyzed and for the most recent season. On each plot, the peak
and % of time exceeding 75% of OTC values are tabulated for each season
analyzed.

Transfer Limits on the frequency distribution plots are the
non-simultaneous limit as published in the 2000 WECC Path Rating
Catalog.

> For some paths, power flows occurred in both directions during the
> season. The flows were considered bi-directional if the flows in
> either direction occurred greater than approximately 10% of the time.
> For example if a path had flows N to S 95% of the time and flows in
> the S to N direction 5% of the time, the path was considered to be
> unidirectional for purposes of this analysis. If the flows in one
> direction were 85% of the time and 15% of the time in the other
> direction, the path was considered to be bi-directional and a
> bi-directional plot was made. For the bi-directional plots, the
> tabulated 75% values on the plot represent the total time the path
> exceeded 75% of OTC in both flow directions.
>
> The first figure in Appendix B indicates which paths have
> unidirectional and which have bi-directional flows. For the
> bi-directional paths, the predominate direction of flow by season is
> noted.

# [SECTION III]{.underline}

**[OBSERVATIONS AND CONCLUSIONS]{.underline}**

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 27 - IPP DC Line
>
> 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)

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.  The two most heavily loaded paths, West of Bridger and the IPP DC
    Line, are transmission paths with high load factors dedicated to the
    integration of generating plants in Wyoming (Jim Bridger) and Utah
    (Intermountain Power Project).

> 4\. For use in future analysis, improvements should be made in the
> data reporting procedures for data to be included in the WECC EHV Data
> Pool database. One area that should be reviewed is the calculation and
> reporting of OTC limits.

From this analysis, it cannot be concluded whether there was significant
congestion (defined as the inability to obtain transmission capacity
when needed) on a path nor whether there are economic benefits to
increasing the transfer capability of any of the paths analyzed.

# 

# 

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