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Transient Recovery Voltage Studies

Background

Transient recovery voltage (TRV) is the voltage across the terminals of a pole of circuit breaker following current zero when interrupting faults. TRV waveshapes can be oscillatory, exponential, cosine-exponential or combinations of these forms. TRVs due to short-line faults (SLFs) are characterized by triangular-shaped waveshapes and a very steep initial rate-of-rise.

Due to the concern for excessive TRVs during circuit breaker operations, utilities may require engineering studies to evaluate proposed substation designs, as well as the impact on nearby utility equipment. These studies evaluate the TRV concerns and possible solutions, such as adding capacitive devices to protect against the harmful transients that may damage the surrounding equipment or power system.

The analysis of high-frequency TRVs frequently requires the use of sophisticated digital simulation tools. Simulations provide a convenient means to characterize transient events, determine resulting problems, and evaluate possible mitigation alternatives. Occasionally, they are performed in conjunction with system monitoring for verification of models and identification of important power system problems. The complexity of the models required for the simulations generally depends on the system characteristics and the transient phenomena under investigation.

Electrotek utilizes the PSCAD program for TRV analysis. This program can be used for the analysis of circuit switching operations, capacitor switching, lightning transients, and transients associated with the operation of power electronic equipment. The output from each simulation cases is plotted against the circuit breaker's withstand capability using routines developed in MATLAB (example shown below).

Study Methodology

The TRV evaluation for various fault conditions is based on the methods provided in IEEE Std. C37.06, IEEE Std. C37.04, and IEEE Std. C37.011. This involves analysis of the most severe conditions, including the clearing of a three-phase ungrounded symmetrical fault at the circuit breaker terminal when the system voltage is at a maximum and short-line faults.

The TRV study considers normal cases where the system operates with all circuit breakers and lines in-service and various contingencies where only one circuit breaker is available to clear a fault. For both of these conditions, three-phase ungrounded and single-line-to-ground faults are evaluated.

According to IEEE Std. 37.011, the most severe oscillatory or exponential recovery voltages tend to occur across the first pole to open of a circuit breaker interrupting a three-phase ungrounded symmetrical fault at its terminal when the system voltage is at a maximum. When the TRV performance meets the withstand criteria when subjected to the fault condition mentioned above, a short-line fault evaluation is not necessary. This is due to the fact that short-line fault TRV capability is higher than that of a three-phase ungrounded fault.

Model Development and Verification

The model development process includes steps for data collection, data approximation, data simplification and model verification.

The TRV system model is based on short-circuit data that consists of positive and zero sequence impedances. The study area includes the substation and the adjacent system. All transmission lines are represented with a frequency dependent line model to account for traveling wave phenomena. The boundary of the study area is represented with equivalent sources and transfer impedances such that the electrical representation of the study area at 60 Hz is nearly identical to the original system. The accuracy of the transient model is verified by comparing three-phase and single-line-to-ground fault currents at all of the buses. The extent of the simulation model is determined during the initial stage of the study.

Circuit breaker TRV withstand capabilities (example shown below) are based on ANSI C37.06 and IEEE C37.04.

TRV Withstand Capability
TRV Withstand Capability

Capacitance values for substation equipment are determining using data provided by the utility and from Annex B of IEEE Std. C37.011. Generally, three equivalent capacitance values (minimum, maximum, and average) are determined and simulations are completed for each of these conditions.

Simulation Results

A TRV evaluation is conducted for the most severe operating conditions, including both three-phase ungrounded faults at the circuit breaker terminal and short-line faults. The study considers both normal cases where the system operates with all circuit breakers and lines in service and contingency cases where the only one circuit breaker is available to clear the fault.

The figure below shows an example of a simulation result for a three-phase ungrounded fault clearing case with an additional capacitance added at the line terminal. The graph includes the TRV simulated using PSCAD and an overlay of the withstand capability based on the IEEE standards.

TRV Withstand Capability - Added Capacitance
TRV Withstand Capability - Effect of Added Capacitance at Line Terminal

Electrotek's TRV study includes an evaluation of the TRV performance for various circuit breaker operations for circuit breakers on a utility power system. The evaluation is conducted for the most severe operating conditions, including clearing both three-phase ungrounded faults at the circuit breaker terminal and short-line faults. In addition, the TRV evaluation considers both normal cases where the system operates with all circuit breakers and lines in service and contingency cases where only one circuit breaker is available to clear a fault.


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