System Stressing Methodology
Path Operator Implementation Task Force
January 25, 2016
Objective of this Methodology
The Objective of this system stressing methodology is to either identify instability risks or to rule them
out for the Operating Horizon studies. Henceforth:
1. If instability risks are identified, then there is a need to establish stability limits (which may
include implementing real-time stability limit calculators), and/or to establish Operating Plans
to address those instability risks.
2. If instability risks are ruled out, then subsequent reliability analyses might exclude stability
analyses for the Operating Horizon, provided system conditions are comparable to those
represented in prior studies.
If instability risks can be ruled out for a given interface or load area, then subsequent reliability
analyses – i.e., Operational Planning Analyses (OPA) and Real-time Assessments (RTA) – can be
confined to steady state Contingency analysis of actual or expected conditions, and it can safely be
stated that the system can be reliably operated by ensuring acceptable pre- and post-Contingency
performance with regard to Facility Ratings and steady-state voltage limits.
Background – Differing Objectives for System Stressing
Transfer analyses that stress the power system are performed to determine the pre- and postContingency reliability issues that can be encountered as transfers increase into a load area or across a
transmission interface. How far the system is stressed as part of transfer analyses depends on the
purposes and objectives of the analysis.
If the purpose of the transfer analyses is to determine Transfer Capability (TC) or Total Transfer
Capability (TTC), the system generally needs to be stressed only to the point where the first reliability
limitation is encountered (with an applicable margin). In principle, TCs are generally determined by
stressing the system until either of the following reliability constraints are encountered:
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In the pre-Contingency state, flows exceed normal Facility Ratings, voltages fall outside normal
voltage limits, or instability occurs (i.e., the system is stressed to the point of unacceptable precontingency performance with regard to thermal, steady-state voltage, or instability
constraints).
In the post-Contingency state, flows exceed emergency Facility Ratings, voltages fall outside
emergency voltage limits, or instability occurs (i.e., the system is stressed to the point of
unacceptable post-contingency performance with regard to thermal, steady-state voltage, or
instability constraints).
WESTERN ELECTRICITY COORDINATING COUNCIL
155 North 400 West, Suite 200
Salt Lake City, Utah 84103-1114
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Most Paths in the West are either thermally limited or steady state voltage limited, as opposed to
transient stability or voltage stability limited. For these Paths, transfer analyses have shown that the
first reliability limitation encountered are post-Contingency exceedances of emergency Facility Ratings
or emergency voltage limits. For example, when stressing a Path, transfer analyses indicate that at a
certain level of transfer, a P1 Contingency results in exceedance of another Facility’s emergency Facility
Rating. Similarly, these transfer analyses may indicate that at a certain level of transfer, a P1
Contingency results in voltage at a bus falling outside its emergency voltage limit.
While TC studies do not require that the system be stressed appreciably beyond the point of
encountering the first reliability limitation, the same cannot be said for transfer analyses that are
performed for purposes of determining instability risks. In order to adequately determine whether
instability risks practically exist for a given transmission interface or load area, the system must be
stressed beyond the point where thermal or voltage limitations are encountered. The question is, how
far does the system need to be stressed before instability risks can be ruled out? This methodology
intends to answer this question.
For purposes of the POITF and Peak’s SOL Methodology, the objective of stressing the system is to
determine whether or not instability risks practically exist for a particular transmission interface or load
area. Transfer analyses for purposes of determining TC or TTC is outside the scope of the POITF and
the RC’s SOL Methodology.
Source and Sink Depletion in Transfer Analysis
Source Depletion – When stressing a transmission interface, in some cases it is possible to deplete the
source generation in the simulation before any reliability issue is encountered. If the source
generation is depleted, and there is no more source generation that can be used to serve as a source
for simulation, then it can be concluded that there is no way to realistically simulate any additional
transfers.
Similarly, if the source is depleted before either the nose of a PV or VQ curve is reached, before
transient instability occurs, or before cascading takes place (per the cascading test outlined in Peak’s
SOL Methodology for the Operations Horizon), then it can be concluded that no stability risks
practically exist for the interface and there is no reliability need to establish a stability limit for the
interface or load area. Different methodologies will be used (as further discussed below) for
transmission interfaces where source generation cannot be depleted in the simulation.
Sink Depletion – When stressing an interface into a load area, it is possible to shut down all generation
internal to the load area (i.e., serve the load with ~100% imports) before any pre- or post-Contingency
reliability issues are encountered. If the generation internal to the sink load area is depleted and the
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sink’s load is maximized, then it can be concluded that there is no way to simulate any additional
imports into the area.
Similarly, if the generation internal to the sink load area is depleted and load is maximized either
before the nose of a PV or VQ curve is reached, before transient instability occurs, or before cascading
takes place (per the cascading test outlined in Peak’s SOL Methodology for the Operations Horizon),
then it can be concluded that no stability limits practically exist for the load area and there is no
reliability need to establish a stability limit for the load area.
Entities need to model the expected minimum generation commitment in the load sink area at the
expected maximum import level.
Stressing Requirements to Determine Instability Risks
If either of the limitations described in the Source Depletion or Sink Depletion sections does not exist,
the system must be stressed until simulations result in one of the following conditions:
1. Post-contingency flow on a Facility reaches the lesser of the following before instability occurs:
a) 125% of its highest applicable Facility Rating(this is typically equal to 125% of highest
Emergency Rating), or
b) The Facility’s loadability as determined by NERC Standard PRC-023, Transmission Relay
Loadability
This analysis assumes that pre- and post-Contingency flows are below applicable Facility Ratings
prior to the transfer analysis. If no instability occurs before the lesser of a) or b) is encountered,
then it can be concluded that no instability risks practically exist and thus there is no reliability
need for establishing stability limits or stability-related Operating Plans.
2. Simulated flow on the interface or into the load area reaches X% of historical maximum flows that
have occurred within the last Z years. If no instability occurs before flows reach X% of these
historical maximums, then it can be concluded that no instability risks practically exist for the
interface or load area and thus there is no reliability need for establishing stability limits or
stability-related Operating Plans.
3. The nose of a PV or VQ curve is reached 1 either under pre-contingency conditions or upon
occurrence of a credible Contingency. This condition indicates the presence of an instability risk
and thus the need to establish a voltage stability limit or to otherwise manage the instability risk via
an Operating Plan.
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If the nose is not reached and different solving techniques do not result in a solution, then the last solved solution
determines the stability limit.
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Transient instability occurs in response to a credible Contingency (team needs to define/describe what
transient instability means and does not mean). This condition indicates the presence of an instability
risk and thus the need to establish a transient stability limit or to otherwise manage the instability risk
via an Operating Plan.
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