It is a matter of Physics and Power. A GIS busbar is essentially a massive, high-voltage coaxial capacitor.
The Problem: In AC systems, a capacitor requires a charging current. The longer the GIS busbar, the more current it "sucks" from the source.
The math: Testing a 500kV GIS might require several Amperes of current. A traditional transformer capable of providing 500kV and 2A would weigh over 10 tons and require a massive power supply.
The Solution: Series Resonance. By adding an adjustable reactor (L) that matches the GIS capacitance (C), the reactor "provides" the charging current, and the power source only has to cover the tiny losses from internal resistance. This allows us to use a portable, 25kg power supply to test a multi-ton GIS. When the test frequency is in harmony with the inherent frequencies of inductor L and capacitor C, the energy exchange between them reaches its peak. All you need to provide is a small amount of power to counteract the resistance loss, and the voltage can be amplified by 30 to 80 times. And the reactor can be connected in series (for voltage increase) or in parallel (for current increase) according to the voltage requirements. The 60kg module can be easily lifted onto the wind turbine platform by two people, completely solving the problem of transportation in the field.