Download WECC-0100 TPL-001-WECC-CRT

M. Papic (November 12th, 2013)
Definitions for Cascading Outages and other Terms Related to Cascading
1. The list of terminologies in the area of cascading used by industry and scientific
community follows:

Event: Any occurrence that has a significant impact to power system state (element outage, load/transfer
change, operational decision, etc.)

Extreme event: event resulting in the removal of two or more BES elements that likely Cascade: Two or
more dependent or separate events being an unplanned loss and/or failure of elements on the BES that results in the
loss of multiple facilities not common to a single zone of protection. The events occur simultaneously or in close time
proximity. Extreme events can be the result of widespread cascading elements initiated by equipment failure,
lightning strike, foreign intrusion, human error, the environment, contamination, sabotage, vandalism or fire or
multiple element outages initiated by regional disaster events (e.g., hurricane or volcano) on transmission
infrastructure.

Contingency: A set of events occurring within a short duration.

High-Impact, Low-Frequency (HILF) Events: is a class of improbable events with the potential to
significantly affect the reliability of the BES and could cause long-term, catastrophic damage to the bulk power
system. The probability and magnitude of these events’ occurrence is uncertain but can result in large consequences.

Cascading failure: is a failure in a system of interconnected parts in which the failure of a part
can trigger the failure of successive parts. Such a failure may happen in many types of systems, including
power transmission, computer networking, finance and bridges. Cascading failures usually begin when
one part of the system fails. When this happens, nearby nodes must then take up the slack for the failed
component. This overloads these nodes, causing them to fail as well, prompting additional nodes to fail
in a vicious circle [1].

Cascading failure: is common in power grids when one of the elements fails (completely or
partially) and shifts its load to nearby elements in the system. Those nearby elements are then pushed
beyond their capacity so they become overloaded and shift their load onto other elements. Cascading
failure is a common effect seen in high voltage systems, where a single point of failure (SPF) on a fully
loaded or slightly overloaded system results in a sudden spike across all nodes of the system. This surge
current can induce the already overloaded nodes into failure, setting off more overloads and thereby
taking down the entire system in a very short time [1].

Cascading: is the uncontrolled successive loss of system elements triggered by an incident at any
location. Cascading results in widespread electric service interruption that cannot be restrained from sequentially
spreading beyond an area predetermined by studies [2].

Cascading Outages: the uncontrolled successive loss of system elements triggered by an incident at any
location. Cascading results in widespread electric service interruption that cannot be restrained from sequentially
spreading beyond an area predetermined by studies (NERC Definition from Glossary Terms) [2]

Cascading Outages: the uncontrolled loss of any system facilities or load, whether because of thermal
overload, voltage collapse, or loss of synchronism, except those occurring as a result of fault isolation [3]

Cascading Failure: is a Sequence of Dependent Failures of Individual Components that successively
weakness the Power System (IEEE CAMS WG Definition) [4]

A cascading outage: is the event in which an abnormal cascading processes develop in power systems,
when every disturbance triggers a next one, and so on, result in blackouts, i.e. mass-scale tripping of generating
sources and disruption of electricity supply over large areas.

A cascading outage: is a sequence of events in which an initial disturbance, or set of disturbances,
triggers a sequence of one or more dependent component outages

Cascading: occurs when a BES element is interrupted due to any Disturbance and the original power flow
across the element is shifted to other BES elements causing those BES elements to exceed their capacity and
interrupt, shifting power flow to other BES elements causing those BES elements to exceed their capacity and
interrupt. A Cascading event will continue until sufficient load is consequentially shed due to the BES element
interruptions such that remaining in-service BES elements are supplying power within their rated capacity (NERC
Report on ALR Metrics)

Cascade: voltage collapse or system instability creating uncontrolled separation. An example is a
geomagnetic Disturbances (GMD) resulting in the failure of multiple BES elements (e.g. transformers) and voltage
collapse. HILF events include coordinated cyber, physical, or blended attacks; pandemic illness; and geomagnetic
Disturbances (GMD), Electromagnetic Pulse (EMP) and extreme weather events.

Reliable Operation of the Bulk Electric System (BES): is the operating the elements of the BES within
equipment and electric system thermal, voltage, and stability limits so that instability, uncontrolled separation, or
cascading failures of such system will not occur as a result of a sudden disturbance, including a cyber security
incident, or unanticipated failure of system elements

Adverse Reliability Impact: is the impact of an event that results in Bulk Electric System instability or
Cascading [3].

Voltage Collapse (NERC Report on ALR Metrics): conditions threaten when transmission lines are
delivering large quantities of power, over long distances with limited local reactive power supply. A voltage collapse
is characterized by a progressive decline of transmission system voltage over a period of time, usually accompanied
by a progressive escalation of electric system load.

Interconnection Reliability Operating Limit (IROL): The value (such as MW, MVar, Amperes, Frequency or
Volts) derived from, or a subset of the System Operating Limits, which if exceeded, could expose a widespread
area of the Bulk Electric System to instability, uncontrolled separation(s) or cascading outages.

NERC Adequate Level of Reliability (ALR): The BES will achieve an ALR when it possesses following
characteristics:
o Controlled to stay within acceptable limits during normal conditions
o Performs acceptably after credible contingencies
o Limits the impact and scope of instability and cascading outages when they occur
o Protects Facilities from unacceptable damage by operating within ratings
o Restored promptly if integrity is lost
o Able to supply the adequate electric power and energy requirements
From WECC-RC SOL Methodology
A. Impact Containment and IROL Load Impact [5]
a.
When studies in the Operations Horizon indicate pre- or post-contingency instances of instability,
uncontrolled separation, or Cascading, a potential IROL condition is present. The presence of an
actual IROL will be determined based on (a) whether or not impact containment is demonstrated and
(b) the level of load impact. TOPs and the RC shall consider impact containment to be adequately
demonstrated when all the following are accomplished:
b.
b.
Impacted area is predefined by studies
c.
Cascading is restrained from sequentially spreading beyond the impacted area
d.
If the impacted area involves more than one TOP, studies have been coordinated and all concerns
resolved
e.
Operating plans, processes and procedures have been developed to ensure adequate containment
within the impacted area.
f.
The TOP has provided the RC and impacted TOPs with appropriate documentation demonstrating
containment.
TOPs in coordination with the RC shall identify SOLs that qualify as IROLs to prevent uncontrolled
interruption of load equal to or greater than 1000 MW in the WECC RC Area. This threshold represents an
upper bound for load loss regardless of demonstrated containment, but excludes load loss due to
intended RAS/SPS actions. Unless adequate containment can be demonstrated, uncontrolled load
interruption less than 1000 MW shall point to presence of an SOL that qualifies as an IROL.1 [5]
2. Propagation of Cascading Outages
a.
Overloaded transmission lines that subsequently contact vegetation
b.
Overcurrent/ undervoltage conditions triggering distance relay actions
c.
Hidden failures or inappropriate settings in protection devices
d.
Voltage collapse
e.
Insufficient reactive power resources
f.
Stalled motors triggered by low voltages or off-nominal frequency
g.
Generator rotor dynamic instability
h.
Small signal instability
i.
Over /under excitation in generators
j.
Over/under speed in generators
k.
Operator or maintenance personnel error
l.
Computer or software errors /failures
m. Errors in operational procedures
3. Generic Scenario of a Cascading Event
1
This requirement is not intended to allow the RC to declare IROLs for unanticipated Real-time operating
conditions.
0 – System State Before Cascading Event
1 – Contingency Conditions
2 – Triggering Events
3 – Power Flow Surges, Voltage problems, Overloads
4 – Protection System Trips Lines, Transformers, Generators (Insecure System State)
5 – System Separation, Instability and Voltage Collapse
6 – Blackout Event
4.
1.
2.
3.
4.
5.
References:
Wkipedia: [Online] http://en.wikipedia.org/wiki/Cascading_failure
Glossary of Terms Used in NERC Reliability Standards, Updated October 30, 2013
NERC, Evaluation of Criteria, Methods and Practices Used for System Design, Planning and Analysis
Response to NERC Blackout Recommendation 13c, 2005
IEEE PES CAMS Task Force on Understanding, Prediction, Mitigation and Restoration of Cascading Failures
“Initial Review of methods for cascading failure analysis in electric power transmission systems” in Proc.
IEEE PES General Meeting, Pittsburg, Jul. 2008, pp. 1-6.
WECC Reliability Coordination System Operating Limits Methodology for the Operations Horizon, WECC,
October 2013