Q: How does an SR circuit work for HV testing, and why does it amplify voltage?
A: At resonance: ωL = 1/(ωC). The only remaining impedance is circuit resistance R. V_out = V_in × Q. This voltage amplification is the core of SR HV testing.
Circuit: VFD → Exciter TX → Reactor L → Load C (specimen)
The math:
• At resonance: X_L = X_C → total reactance = 0. Q = X_L / R = (1/R)√(L/C)
• V_out = V_in × Q. P_in = P_out / Q.
Real-world example - 10 km 110 kV XLPE cable:
• C = 2 μF, target V = 128 kV. Reactor L = 50 H. f_res = 15.9 Hz.
• Q = 50 → exciter needs only 128/50 = 2.56 kV.
• Equivalent output: 2,048 kVA. Actual input: ~6.5 kW (from a small generator).
Three conditions for resonance:
① Tuning: ω² = 1/(L·C). ② Stability: ±0.5 Hz. ③ Purity: THD < 5% (tank filters harmonics).
Key advantage: Eliminates Ferranti effect (series inductor compensates load capacitance).