I often see some equipment measure the impedance of a 'black box' with symmetric frequencies (the black box may be a complex high order network, but not only a simple one order). Such as if we want to know the target circuits impedance under utility frequency, but in order to minimize the utility frequency interference, they may measure it at fu+5 and fu-5, then use their average as a result. I think there may exist some 'approximation'. Any one can give the math in detail?

  • \$\begingroup\$ give links to specs of "some equip" \$\endgroup\$ Feb 18, 2017 at 18:02
  • \$\begingroup\$ Such as overhead power transmission line parameter measurement equipments. \$\endgroup\$
    – diverger
    Feb 19, 2017 at 1:33

1 Answer 1


I think you are referring SFRA line impedance analyzer which can measure from mHz to MHz.

It works on the basic principle like a Network Impedance Analyzer except with HV power lines using capacitive voltage couplers. I think it uses a constant current swept frequency sourcing with sophisticated signal processing.

It may also include Partial Discharge impulse sensing detectors.

Search Doble and Omicron equipment.

Value $50k ~ $150k


Effective maintenance test techniques and diagnostic measurements to improve the performance and reliability of Power System Transformers. (Assets worth $0.5m ~ $5m)


• Turns ratio, vector group and excitation current measurement

• Static winding resistance measurement

• Dynamic winding resistance measurement to test the On-Load Tap Changer (OLTC)

• Sweep Frequency Response Analysis (SFRA) measurement

• Frequency dependent capacitance and dissipation factor measurement

• Di-electric Response Analysis

• Partial Discharge (PD) measurement

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