FTFR

AcronymDefinition
FTFRFirst Time Fix Rate (maintenance)
References in periodicals archive ?
Double-Phase to Ground Internal FTFR. As can be seen from Figure 2(b), the measured currents at M side and N side are [[??].sub.Ma] = [[??].sub.Mfa], [[??].sub.Na] = [[??].sub.Nfa], [[??].sub.Mb] = [[??].sub.Mfb], [[??].sub.Nb] = [[??].sub.Nfb], and [[??].sub.Mc] = -[[??].sub.Nc] when the internal double-phase to ground FTFR (A-phase to ground, B-phase to ground, e.g.) takes place.
Three-Phase to Ground Internal FTFR. As can be seen from Figure 2(c), the three-phase to ground internal FTFR can be analyzed by the 0-frame component of the measured currents when the fault resistance values or the fault positions of A-phase, B-phase, and C-phase are different.
Phase-Phase to Ground Internal FTFR. As can be seen from Figure 2(d), the measured currents at M side and N side are [[??].sub.Ma] = [[??].sub.Mfab], [[??].sub.Mb] = [[??].sub.Mfab], [[??].sub.Na] = [[??].sub.Nfab], [[??].sub.Nb] = -[[??].sub.Nfab], and [[??].sub.Mc] = -[[??].sub.Nc] when the internal phase to phase FTFR (A-phase to B-phase, e.g.) takes place.
As the above analysis, [mathematical expression not reproducible] in all types of internal FTFR, while [mathematical expression not reproducible] in external faults and normal operation.
Therefore, thresholds [mathematical expression not reproducible] are necessary to avoid maloperation during external faults and to ensure the high sensitivity during internal FTFR (the sensitivity coefficients should be [mathematical expression not reproducible] 1.2), and the practical protection criteria can be expressed as
However, this can reduce the sensitivity of the protection, which may refuse protection action in several cases when the internal FTFR takes place.
where [K.sub.r] = 1.2~13, [K.sub.[alpha]] = 1.1~1.5 in general, [[??].sub.N0] is the 0-frame component of the current, [I.sub.Nf] is the magnitude of the d-frame component of the current, and [mathematical expression not reproducible] is the ratio value of three-phase voltages and three-phase currents measured at N side during internal FTFR cases or external faults cases.
Firstly, (13) is used to identify the internal FTFR. If the output is 1, then the protection action is done when the criteria are satisfied.
It can be summarized that [mathematical expression not reproducible] is used for single-phase to ground internal FTFR and double-phase to ground internal FTFR in two-phase operation, and [mathematical expression not reproducible] is used for phase-phase to ground internal FTFR in two-phase operation, and [mathematical expression not reproducible] is used for single-phase to ground internal FTFR in single-phase operation.
According to (12) and (22) when the internal FTFR takes place, the above ratio includes neither [[theta].sub.0]-[[phi].sub.1] nor [[theta].sub.0]-[[phi].sub.2]; therefore, the changes of [[theta].sub.0]-[[phi].sub.1] and [[theta].sub.0]-[[phi].sub.2] do not influence the proposed protection scheme.
The existing protection schemes perform well for the solid faults in microgrid, and the performance for FTFR is poor.
Due to the error of current transformers, the values of [mathematical expression not reproducible] undergo a little change in fact when internal FTFR takes place.