Critical Clearing Time (CCT) – Automate Computation of Critical Clearing Time
The critical clearing time (CCT) feature of transient stability (TS) performs massive analysis to automatically determine CCT. It automates CCT computations and eliminates manual, time-consuming, cut-and-try calculations. It utilizes time-domain simulation to compute CCT under different fault conditions.
CCT is one of the key characteristics of TS. CCT of a fault is the maximum allowable value of the clearing time for which the system remains stable. It defines the TS margin at a given substation and is used as a measure of a power system TS The larger the CCT is, the more time the power system has available to clear the fault; and, thus, the more secure the system. If a fault is cleared within an established CCT, the power system is considered to be stable.
Need for automated CCT analysis
Time-domain simulation (e.g., numerical integration of differential equations) allows users to compute the accurate value of the CCT, but it is a time-consuming process. For each fault, repeated time-domain simulations are required for different values of the clearing time until a generator loses synchronism. Planning groups perform extensive stability studies for the protection group to ensure that there will be no adverse consequences on the BES by running multiple scenarios.
CCT tool speeds up the process and results in considerable time savings from hours to minutes. Each stability study performed using Siemens PTI PSS/E software for the specific system topology and protection scheme can take 1–3 hours, depending on the complexity of the study 1. With CCT software, it can be done in minutes.
CCT computation
Three scenarios are available in the CCT module:
- Bus Fault Scenario
- Transformer Fault Scenario
- Branch Fault Scenario
CCT may be automatically determined for three-phase and unbalanced faults at locations that are automatically identified by the program, or at user-defined locations.
1 S. Kolluri, M. Li, A. Lazo, P. Yu, M.Y. Vaiman and M.M. Vaiman, “Automated Critical Clearing Time Calculation for Analyzing Faults at Entergy”, IEEE PES T&D Conference & Exposition, 2016, Dallas, Texas.