Extended Summary 本文は pp.484–490

Influence of a Circuit Breaker’s Grading Capacitor onControlled Transformer Switching

Yves Corrodi Non-member (Mitsubishi Electric Corporation, Yves.Corrodi@dx.MitsubishiElectric.co.jp)

Kenji Kamei Member (Mitsubishi Electric Corporation, Kamei.Kenji@ce.MitsubishiElectric.co.jp)

Haruhiko Kohyama Member (Mitsubishi Electric Corporation, Koyama.Haruhiko@dx.MitsubishiElectric.co.jp)

Hiroki Ito Member (Mitsubishi Electric Corporation, Ito.Hiroki@aj.MitsubishiElectric.co.jp)

Keywords: transformer, residual flux, grading capacitor, controlled switching

The residual flux, “locked in” a transformer core after a no loadedtransformer deenergization, not only depends on its specificationsbut also on the equipments as a circuit breaker’s grading capacitorand its chopping level. Regarding the simplified, no-loaded trans-former model in Fig. 1, a power flow from the source trough thegrading capacitor is enabled that affects the residual flux. In case ofa no-loaded, single-phase transformer IS as well as IP can be consid-ered to be zero. In that reason the current I summates the currentsof the overall transformer capacitance (CT ), transformer loss (R) andthe 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 coreafter a no loaded transformer deenergization. Considering typicalvalues for CT , R and L an almost linear dependency can be observedand therefore micro oscillations around the “locked in” residual fluxlevel will become larger for an increased grading capacitance.∣∣∣∣∣

∫v(t)dt

∣∣∣∣∣ = CG√(CG +CT − R

ω2 · LR

)2

+

(1

Rω

)2· V0

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 becalculated that the residual flux will decrease in case of a largergrading capacitor. Both phenomena could be measured for 50 kVA,6.6 kV:200 V, single-phase transformer systems (Fig. 2 and Fig. 3):For a larger grading capacitor, the residual flux decreases and themicro oscillations around this level increases.

In that reason controlled transformer switching will dependon the circuit breaker’s grading capacitor, because the optimal

Fig. 1. Equivalent circuit of a simplified, no-loadedtransformer model

Fig. 2. Residual flux measurement: Deenergization ofa 50 kVA, 6.6 kV:200 V, single-phase transformer at 145degree of the measured voltage-Grading capacitors from450 pF to 2600 pF

Fig. 3. Micro hysteresis measurements: Deenergizationof a 50 kVA, 6.6 kV:200 V, single-phase transformer at145 degree of the measured voltage-Grading capacitorsfrom 450 pF to 2600 pF

re-energization target for an independent pole operated (IPO) aswell 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 thetransformer 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 gradingcapacitor) is 0.48 p.u. Therefore the optimal re-energization targetwill be different for each grading capacitor.

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