1. Effects of medium voltage potential transformer load on ferroresonance initiation
Miroslav Novák
Faculty of Mechatronics, Informatics and Interdisciplinary Studies
Technical University of Liberec
e-*

2. 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

3. 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

4. Measurement Programmable power source 2 kVA Bank of capacitors
1-373 mF ( 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. Conclusion System voltage is in risky region of VT’s
Dangerous configuration – VTS25 + Cs 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

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