Extended Summary 本文は pp.484–490 Influence of a Circuit Breaker’s Grading Capacitor on Controlled Transformer Switching Yves Corrodi Non-member (Mitsubishi Electric Corporation, [email protected]) Kenji Kamei Member (Mitsubishi Electric Corporation, [email protected]) Haruhiko Kohyama Member (Mitsubishi Electric Corporation, [email protected]) Hiroki Ito Member (Mitsubishi Electric Corporation, [email protected]) Keywords: transformer, residual flux, grading capacitor, controlled switching The residual flux, “locked in” a transformer core after a no loaded transformer deenergization, not only depends on its specifications but also on the equipments as a circuit breaker’s grading capacitor and its chopping level. Regarding the simplified, no-loaded trans- former model in Fig. 1, a power flow from the source trough the grading capacitor is enabled that affects the residual flux. In case of a no-loaded, single-phase transformer I S as well as I P can be consid- ered to be zero. In that reason the current I summates the currents of the overall transformer capacitance (C T ), transformer loss (R) and the overall inductive transformer reaction at the deenergization in- stant (L). Based on phasor analysis the magnitude of the integrated volt- age v(t) (compare Fig. 1) is calculated in Eq. (1)—The integral cor- responds to the induced magnetic flux within the transformer core after a no loaded transformer deenergization. Considering typical values for C T , R and L an almost linear dependency can be observed and therefore micro oscillations around the “locked in” residual flux level will become larger for an increased grading capacitance. v(t)dt = C G C G + C T - R ω 2 · LR 2 + 1 Rω 2 · V 0 ω ······················· (1) The calculation of the residual flux is slightly more complex, be- cause it is assumed that the value depends on the transient volt- age during the deenergization instant. In the full paper it could be calculated that the residual flux will decrease in case of a larger grading capacitor. Both phenomena could be measured for 50kVA, 6.6 kV:200 V, single-phase transformer systems (Fig. 2 and Fig. 3): For a larger grading capacitor, the residual flux decreases and the micro oscillations around this level increases. In that reason controlled transformer switching will depend on the circuit breaker’s grading capacitor, because the optimal Fig. 1. Equivalent circuit of a simplified, no-loaded transformer model Fig. 2. Residual flux measurement: Deenergization of a 50 kVA, 6.6 kV:200V, single-phase transformer at 145 degree of the measured voltage-Grading capacitors from 450 pF to 2600 pF Fig. 3. Micro hysteresis measurements: Deenergization of a 50 kVA, 6.6 kV:200V, single-phase transformer at 145 degree of the measured voltage-Grading capacitors from 450 pF to 2600 pF re-energization target for an independent pole operated (IPO) as well as for a three-gang operated (3GO) transformer systems is eval- uated regarding the residual flux level. Considering the measure- ments in Figs. 2 and 3, the difference at the same instant after the transformer de-energization (at 0.4 s) between the maximal appear- ing magnetic flux value nΦ (in case of a 450 pF grading capacitor) and the minimal residual flux value (in case of the 2600 pF grading capacitor) is 0.48 p.u. Therefore the optimal re-energization target will be different for each grading capacitor. –2–