V&R Energy: Potential Cascading Modes (PCM) - Prediction of Cascading Outages

Fully integrated into POM, PCM inherits the speed, flexibility and robustness of the POM application, PCM is used to analyze the cascading outages as PCM-2 both the steady-state and transient stability phenomena.

PCM capabilities include:

Prediction of the cascading outages by quickly identifying the initiating events and possible cascading chains, and accurately modeling the protection relays and optimizing relay settings.
If possible, determining preventive actions to halt the cascading, such as under-voltage load shedding, under-frequency load shedding, generator re-dispatch or using other active and reactive sources available in the power system network.
Application of the effective islanding techniques, including under-frequency load shedding.
Ranking of the cascading outages (e.g., quantifying their impact);
Visualization of the initiating events, their spread, severity and control actions to prevent the cascades, in order to improve the situational awareness of a utility’s/ISO’s operators.

Use of PCM

PCM is utilized by the utilities’/ISOs’ in their NERC-compliance studies to assess the system performance following the extreme events. PCM is also incorporated as part of the extensive N-1-1 AC contingency analysis.

Performing Sequential Contingency Analysis in PCM: Steady-State Analysis

During the course of the PCM analysis, all of the overloaded branches are identified, but only those which are overloaded above the user-specified branch tripping threshold are automatically tripped for the purpose of simulating the operation of the protection schemes. A contingency which causes an overload above the branch tripping threshold is referred to as an initiating event, or Tier 0, and the overloaded branches form the following tier, Tier 1, etc.

Therefore, PCM performs the sequential contingency analysis while monitoring the thermal overloads and steady-state stability violations, and identifies tiers in the cascading chain. Following an initiating event, the branches overloaded above the user-specified branch tripping threshold are consecutively tripped until one of the following events occurs:

System fails to solve due to voltage instability;
Loss of load exceeds a user-specified threshold value;
Islanding with imbalance of load and/or generation within an island
A thermal violation is alleviated or drops below the line tripping threshold value.

Thus, a cascading outage is a successive loss of the system elements which causes stability violation, large loss of load, or islanding.

As result of this process, potential cascading modes (PCM) in the power system network are identified.

The PCM analysis results yield the following conclusions:

The weakest N-1 contingencies forming the N-2 initiating events
The severity of cascading outages in terms of the number of tiers in each cascade, the number of overloaded elements on each tier and the severity of these overloads.

A “Cluster” Approach to the Analysis of Cascading Outages

It is practically impossible to apply all of the N-k contingency combinations in a bulk power system. A “cluster” approach has the capability to quickly identify those possible initiating events that may lead to cascading outages and automatically determine the possible cascading chains. It identifies potential cascading modes due to thermal overloads.

A power system network is represented as number of "clusters" (e.g., groups) that are connected to the network via “critical” lines (e.g., cutsets). That is the electrical division of the power system. A cutset which connects the clusters with a large generator output or load is determined. If one of the “critical” lines (e.g., initiating events) within the cluster or connecting two clusters is outaged, that may cause large overloads on other line(s). If an overloaded line(s) is switched off as a system protection measure, that may lead to cascading.

After clusters are formed and cutsets are identified, the heuristic rules are used to select lines and transformers as initiating events.

Branches are selected from the entire network, and not from a particular control area (or control areas). That allows the user to identify those initiating events which may be located outside of the utility’s or ISO’s footprint but cascade into theirs.

Prevention of the Cascading Outages

If utilized together with OPM, PCM can be used to prevent the cascading outages by optimizing the existing available controls in the electric transmission network. the optimal remedial actions are applied first, after the occurrence of an initiating event, and then at each cascading Tier, until the cascading event is mitigated.

When PCM is combined with OPM, the obtained results will provide the Planning and Operations engineers with proper alternative options to prevent the spread of the cascading outages:

Identifying initiating events (N-1 and/or N-2 contingencies) that cause the overloads above the branch tripping threshold.
Determining the remedial actions in order to alleviate the overloads, and apply those remedial actions in order to bring the flow on the overloaded branches below the branch tripping threshold.
If the remedial actions identified in item (2) bring the flow on the overloaded branches below the branch tripping threshold (e.g. are successful), the cascading stops.
If the remedial actions identified in item (2) decrease the overloads, but do not bring the flow on the overloaded branches below the branch tripping threshold, the overloaded lines are tripped, and the next Tier in cascading is formed.

Since the overload is decreased as a result of using the remedial actions, the effect of this tripping is expected to be less than if the mitigation measures are not utilized.

Repeating steps (2) - (4) at each cascading Tier.
In the event a cascading outage cannot be prevented, identifying the remedial actions needed to mitigate the consequences of the blackout (e.g., steady-state stability violation).

Mitigation of the Cascading Outages

PCM also allows the Planning and Operations engineers to determine the remedial actions in order to alleviate voltage collapse after cascading had happened. Thus, the consequences of a cascading outage after all cascading tiers occur are mitigated.

PCM also allows for the ranking of the cascading outages based on a minimum amount of load curtailment which is needed to alleviate the steady-state stability violations caused by the cascading events.

The amount of load curtailment which is necessary to alleviate the steady-state stability violations after each cascading outage is computed. Then, the cascading outages are ranked based on the amount that is needed to be curtailed in order to alleviate those steady-state stability violations.

Thus, the amount of unserved load, in addition to the number of tiers in the cascading chain, the values of overloads, and amount of load and generation lost as a result of the cascading outage, may be served as an important component in the algorithm for ranking cascading outages.

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