Effects of medium voltage potential transformer load on ferroresonance initiation Miroslav Novák Faculty of Mechatronics, Informatics and Interdisciplinary Studies Technical University of Liberec +420 48 535 3626 e-* miroslav.novak@tul.cz

Ferroresonance RLC resonance L – voltage transformer (saturable) C – parasitic capacitances (CB, side-running lines) R – circuit losses (VT`s load, VT`s primary R, line resistance) low losses circuits Nonlinear L Wide range or resonance freq.  not possible to tune out Bifurcations DS .. bus bar disconnector CD .. circuit breaker Ccb .. circuit breaker snubber PT .. potential transformer U1 Cs Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec

Hazards Ferroresonance Transformer endanger factors: Results Current spikes through saturated VT’s non-harmonic voltage voltage surges at transients Transformer endanger factors: Over-voltage Over-temperature Vibrations/forces from high peak currents Results Winding insulation failure Explosion Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec

Measurement Programmable power source 2 kVA Bank of capacitors 1-373 mF (0.02-7.7 nF) Standard MV potential transformer VTS 25 - modified Primary voltage U1 22000/√3 V Secondary voltage U2 100/√3 Secondary turns n2 84/42/21/10 - Nominal frequency f 50 Hz Nominal power  Pn 30 VA Extreme power Pe 500 Core type (29.8 kg) Q 13070 Measured on secondary side – R is equivalent, Ls 20 % lower. Results corresponds with primary tests. VT’s – no primary and secondary taps Voltage gradually inc to initiate resonance and than dec untie it ceases. Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec

Effect of VT’s working flux and load R 11 % of Pn Bw  shifts always res. region for small Cs Load  shifts for small Cs System voltage – potentially risky Earth fault - risky no load R 34 % of Pn Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec

Sub-harmonic resonance & Effect of core construction Core overlapping = ½ of cross-section = earlier saturation Always resonate at system voltage but low current and thermal load Higher Cs = sub-harmonic resonance but low current and thermal load Overlapped core Subharmonic resonance – unstable on transition between fundamental and 3th sub – not enough power supply capability. C core Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec

Dissipated power Over-temperature can be estimate form energy passing thought Cs The thermal loading should be simply estimated from Cs stored energy when Uc is limited by transformer saturation. Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec

Initiation sensitivity Simulates the circuit breaker disconnection Recovery voltage of circuit breaker is simulated by DC voltage of power supply. Results for VTS25 – load is good prevention – soaks charge from Cs. Number of cases and minimal observed starting recovery voltage decreases with load. Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec

Initiation sensitivity Higher Bw = more risky Always resonate region intersect system voltage VT’s load work with coordination to nominal working flux density. Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec

Conclusion System voltage is in risky region of VT’s Dangerous configuration – VTS25 + Cs 0.8-10 nF + load < 0.3 Pn Decreasing the working flux density helps but makes higher over-voltage Overlapped core – always resonate, but better conditions Ferroresonance prevention: Try to restrict operation of switched off and ungrounded VT’s Use VT’s with low working flux and load or overlapped core Dynamic load / smart load devices Structural parameter obtained from model parameters minimization (Matlab function fminsearch) Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec

Thanks you for attention Supported by company KMB systems, s. r. o. within the grant “Innovative vouchers 2011-2012” of Liberec Region Effects of medium voltage potential Miroslav Novák EPE 2012 transformer load on ferroresonance initiation TU of Liberec