Measure the the impedance of a 'black box' under two symmetric frequencies
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

Purpose

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

Uses

• 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