Purpose of WECC Basecases
Overview
A power system is composed of numerous discrete elements such as generators, transmission lines,
transformers, motors, etc. In order to study the behavior and operation of existing and future planned
facilities under both normal and abnormal operating conditions, it's necessary to be able to adequately
model it. Through multiple committees and subcommittees, WECC utilizes a robust model development
and validation process to ensure that power system equipment is represented accurately.
WECC's base cases are computer models of existing or projected power system conditions for a specific
point in time. These base cases include both steady state and dynamic data, and contain very large
amounts of data necessary to model power system behavior.
The System Review Work Group (SRWG), under the direction of the Technical Studies Subcommittee
(TSS) has been tasked with compiling a data bank of solved near- and longer-term power flow base cases
with associated stability data for selected study years and seasons, as outlined in the SRWG Charter.
The cases to be developed each year are outlined in the Base Case Compilation Schedule and consist of
a standard set of cases which cover the full range of operating and planning scenarios needed to
complete transmission planning studies as well as several scenario cases, as requested by SRWG, TSS
and Planning Coordination Committee (PCC) members. Current and past Base Case Compilation
Schedules can be found on the SRWG committee website at:
https://www.wecc.biz/PCC/Pages/SRWG.aspx under the Base Case Compilation Schedules dropdown
menu.
Purpose of WECC Basecases
The purpose of providing WECC-approved basecases to WECC membership is to ensure coordination
between members in developing a case representative of current or anticipated system topology and
conditions, as required by the applicable NERC Reliability Standards. These cases are intended for use in
effectively operating the current system and studying system adequacy in the future.
Posted basecases for the operating horizon represent realistic, reasonable conditions WECC-wide for
specific seasons and time periods. As such, it is recommended that operating cases be supported by
actual system data, such as historical peak and light load levels and generation patterns, and
demonstrated interchange flows for the conditions and time period being studied while taking into
account anticipated/forecasted growth rates. Entities should submit data representative of the season,
conditions and time period being modeled while acknowledging that the data may not represent
stressed conditions for their particular system/facilities. Long-term planned outages should not be
modeled in the operating cases unless the outage will extend for the duration of the season being
represented. Engineering judgement should be used when outage schedules are anticipated to change.
Entities should follow their normal outage coordination processes for modeling short-term outages in
seasonal and next-day operating studies.
Posted basecases for the planning horizon represent the extremes of expected conditions within WECC,
which may include coincident peak or light load conditions across WECC with a reasonable generation
dispatch. This allows entities to plan for the most extreme heavy or light load conditions that could be
experienced within the interconnection and ensures that entities can meet the requirements outlined in
the NERC Transmission Planning and Performance Requirements. While the season and time period
being represented by a case will be specified in the case description sheet, each data submitter should
use discretion when submitting data (i.e. load levels and interchanges) for a planning case to ensure that
the case is accurately representing their stressed system conditions.
In addition to operating and planning cases, scenario cases are developed each year based on requests
from WECC members. These scenario cases often consist of an abnormal system dispatch to stress a
particular area of the system. Care should be used when performing studies using a scenario case to
fully understand what the case is representing. The case description sheet may contain some of this
information, though it is best to contact WECC staff if questions arise.
Development of Basecases
There are a number of factors to be considered when submitting basecase data. The Data Preparation
Manual (DPM) is the best source for determining the data that is required for a complete basecase. The
most current approved DPM can be found on the SRWG committee website at:
https://www.wecc.biz/PCC/Pages/SRWG.aspx under the Data Preparation Manual dropdown menu.
Violations of the general guidelines below will also be identified in the RepLog and the Case Checks,
which are distributed with every Review Case. The RepLog identifies data that does not conform to the
requirements of the DPM or other WECC criteria (i.e. N-0 branch overloads, missing ratings, use of
unapproved models, etc.). Case Check routines are run on both steady-state and dynamic data to
identify data that is suspected of being incorrect, but which still meets the requirements of the DPM and
WECC criteria. Some examples of data identified in the Case Check routine include generators
connected to the system with zero output, a Load_Long_ID that does not match the non-conforming
load flag, generators with Pmin greater than Pmax and generator dynamic models with parameters that
are suspect. It is recommended that these tools be utilized to verify the completeness and accuracy of
the case data being submitted. Some general guidelines to ensure case completeness and accuracy are
included below.
Loads
For typical heavy load operating or planning cases, a 1-in-21 load forecast should be used to determine
loading in a case. For entities that do not prepare a load forecast for the season being represented, such
as a summer peaking utility that does not generate a winter forecast, it is at the discretion of the entity
preparing the basecase submittal to determine a load level that would be most appropriate for their
system. Methods for developing an appropriate load level may include scaling historical load levels up
1
A 1-in-2 forecast means:
-that there is an equal probability of peak load falling below the forecast value or exceeding the forecast value.
(CAISO)
-Expected peak load based on a specific design temperature where that design temperature is the average
temperature experienced during a peak event. (PSE)
by a forecasted load growth factor or scaling the forecast load levels by some known scaling factor.
Determining whether to use the previous seasons forecast or the next seasons forecast will be up to the
discretion of the individual entities developing the case load profile with the intent to most accurately
represent what the anticipated load level will be for the year and season being studied.
When scaling case load to the levels requested in the case description sheet, ensure that the
Load_Long_ID is populated correctly and the non-conforming load flag is appropriate for the assigned
Load_Long_ID. Industrial loads, agricultural pump loads and station service should have the nonconforming load flag set to “No” or “1” to indicate that the load is not scalable. All residential,
commercial, and mixed load types should have the non-conforming load flag set to “Yes” or “0” to
indicate that the load is scalable. Instructions for populating the Load_Long_ID can be found at
https://www.wecc.biz/Reliability/LID_Instructions_2012-4-10.pdf.
Additional items that may be flagged by the Rep log or Case Check tools are loads with Mvar greater
than MW. While this can be true for some industrial loads, it is not typical and should be verified using
actual load data when encountered.
Generation
Turbine type should be populated for each unit modeled in the case. The ‘Turbine Type’ field will
include either a number or a two letter code corresponding to the type of turbine being represented in
the case depending on the software platform being used. Specifics of the turbine types available in each
software platform can be found in the associated user’s manuals. When converting between programs,
a mapping of turbine type numbers and two letter codes is included below. While this does not
encompass all of the turbine types available, specific questions on the conversion of turbine types
between programs can be directed to the software vendors.
Turbine Type (PowerWorld/GE PSLF)
Unknown/Unknown
Combined Cycle Steam Part/Reheat Steam
Combined Cycle Generic/Steam in
Combined Cycle (Separate Shaft)
Compressed Air Storage/Other
Combined Cycle Single Shaft/Single Shaft
Combined Cycle
Combined Cycle Combustion Turbine
Part/Steam Cross Compound Unit
Represents DC Ties/DC Tie
Fuel Cell/Other
Gas Turbine/Industrial GT (Single Shaft)
Hydro/Hydro
Internal Combustion/Diesel – Non Turbo
Charged
Turbine Type Two
Letter Code
UN
CA
CC
Turbine Type
Number
0
2
4
CE
CS
99
13
CT
3
DC
FC
GT
HY
IC
40
99
11
5
6
Internal Combustion Turbo Charged/Diesel
Turbo Charged
Jet Engine/Aero Derivative GT
Motor/Pump
ST – Boiling Water Nuclear Reactor/Other
ST - Graphite Nuclear Reactor/Other
ST – High Temperature Gas Nuclear
Reactor/Other
ST – Pressurized Water Nuclear
Reactor/Other
Other/Other
Hydro Pumped Storage/Hydro
Photovoltaic/Photovoltaic
Synchronous Condenser/Synchronous
Condenser (No Turbine)
Steam Turbine/Non-Reheat Steam
Wind Turbine/Type 1 Wind Turbine
IT
7
JE
MP
NB
NG
NH
12
41
99
99
99
NP
99
OT
PS
PV
SC
99
5
31
14
ST
WT
1
21
When determining, a number of factors need to be considered including turbine type (for example,
steam units generation dispatch should not be in service without the associated combustion units),
ensuring Pgen is not greater than Pmax, and Pgen is not greater than the MW cap value included in the
dynamic model. If a generator is modeled out of service, the generator step-up transformer should be
opened. Generators should not remain connected to the system with zero real power output.
Generation levels should be typical for the season and time period being represented. When
determining generation real and reactive power limits and generation dispatch, consideration should be
made for seasonal limitations including but not limited to ambient temperature, typical seasonal water
levels and capacity factors.
Branches
WECC performs an analysis on every operating case using auto-generated N-1 contingencies for lines
and transformers 200 kV and above and for all generators. The expectation is that there are no
violations of the rating associated with the season being represented for an N-0 or N-1 condition. An
exception can be made for contingencies with associated RAS, operating procedures or protection
schemes that will mitigate the violation and for auto-inserted contingencies that do not represent
breaker to breaker operation of a line. In the event that a violation is reported, WECC staff will reach
out to the associated Area Coordinator or data submitter for the facility in question to resolve the issue.
It is recommended that data submitters run their N-1 contingencies on the cases before submitting data
to ensure data accuracy.
Planned Projects
Due to variability in annual budgeting processes and construction schedules, inclusion of planned
projects in basecases, particularly in the long-term planning horizon can be challenging. Planned
projects have various commitment levels based on the project sponsor. Not all planned projects will be
included in a case based on the individual project sponsor’s level of commitment.
Use of the Basecases
Once basecases have passed the review stage and WECC has compiled the final case, the case is posted
to the WECC website for use by members. The naming convention used for posted basecases is:
It should be recognized that, while the posted basecases are intended to represent realistic WECC-wide
conditions in the operating horizon and stressed WECC-wide conditions in the planning horizon, each
individual entity may or may not be experiencing peak or light load conditions during WECC-wide
stressed conditions. Consequently, it should be expected that basecases, as posted by WECC will
require adjustments before they are appropriate for use in operations and planning studies. While
every effort should be made to ensure that interchanges, load levels, and generation patterns provide
an effective starting point for studies, modifications will be needed to represent the conditions required
by each individual entity to effectively study their system.
When tuning a basecase for study purposes there are a number of considerations to be made. While
any value can be adjusted to achieve the desired conditions, when changing values outside of the
member system it can be easier to adjust generation and interchange levels than load levels. When
adjusting generation patterns, consideration should be made for turbine types and generator limits as
discussed above. While adjusting interchange flows can generally be accomplished by scaling
generation between areas, it is recommended that the interchange owner be contacted to ensure that
these adjustments are being done in accordance with the owner’s standard practices.
When preparing cases for dynamic simulations, initialization errors can be encountered outside of the
member system. The first step would be to contact the data owner to determine if a corrected model is
available. If a corrected model is not available, initialization errors can be mitigated by adjusting the
initial conditions of the load flow (MW dispatch, voltage schedules, etc.) or by commenting out the
associated models in the dynamics file. If the generators are located in the member system, then the
model should be corrected to ensure initialization. Every dynamic simulation should begin with a ‘No
Disturbance’ simulation to verify that a flat response is achieved.