Variable-Frequency Series Resonant Test Systems
AC withstand testing for cable and GIS without the reactive power bill
After a 220 kV cable circuit is jointed and terminated, somebody has to prove the installation. DC will not do it on extruded insulation, and a power-frequency source would have to supply the full charging current of the run. A variable-frequency series resonant system tunes its reactor bank against the cable's own capacitance and produces rated voltage from a supply that covers only the losses.
Rated capacity runs from 300 kVA to 30,000 kVA, rated voltage from 200 kV to 2,000 kV and rated high voltage current from 1 A to 100 A per system. The converter sweeps 20 to 300 Hz at 0.1 Hz resolution with frequency instability held to 0.05%, so the circuit finds resonance on anything from a lumped capacitance to a long submarine cable run. Quality factor is 50 or better and the sine wave is held within 1% distortion, which keeps a partial discharge measurement usable during the withstand test. Temperature rise stays within 65 K after 60 minutes at rated operation and audible noise within 65 dB.
Why it matters
A quality factor of 50 or better means the supply makes up only the losses, so a 30,000 kVA test load runs from a 380 V feed instead of a dedicated substation.
The 20 to 300 Hz range covers 110, 220 and 500 kV cable, gas-insulated equipment to 1,000 kV, and power transformers held between 45 and 65 Hz.
Sine distortion within 1% keeps the source out of the partial discharge record, so withstand and PD run in a single energization.
The series-parallel reactor arrangement handles a lumped-capacitance object and a long cable run without buying a second system.
On-site sets to 520 kV and 500 A connect in series for voltage or in parallel for current, and trailer-mounted options put the same method on a commissioning site.
Ratings
Rated capacity: 300 kVA to 30,000 kVA
Rated voltage: 200 kV to 2,000 kV
Rated HV current: 1 A to 100 A per system
Output frequency: 20–300 Hz
Quality factor: Q ≥ 50
Waveform: Sine, distortion ≤1%
Frequency resolution: 0.1 Hz; instability ≤0.05%
Temperature rise: ≤65 K after 60 minutes at rated operation
Noise: ≤65 dB
Supply: 380 V, 50/60 Hz
Reactor arrangement: Series-parallel combination covering both lumped-capacitance objects and long cable runs
Extended range
On-site and mobile versions: Systems to 520 kV and 500 A: 110 kV / 95 A, 160 kV / 50 A, 230 kV / 69.5 A and 260 kV / 85 A reactors, connectable in parallel to double current or in series to double voltage; trailer-mounted options
System components: Frequency converter with selectable sine or square-wave excitation, excitation transformer, series-parallel HV reactors, protection damping inductance, capacitive voltage divider, compensation capacitor and optional power isolation transformer
Standards
IEEE Std 4
IEEE 400.1 / 400.2
IEC 60060-1
IEC 60840 / IEC 62067 (cable after-installation testing)
Applications
After-installation AC withstand testing of 110 / 220 / 500 kV XLPE cable
GIS and GIL commissioning tests up to 1,000 kV
HV coupling capacitors and CVTs
Power transformer testing between 45 and 65 Hz
Cable factory routine testing including submarine cable
Questions engineers ask
Can one system test both a long cable circuit and a GIS bus? Yes. The reactors are arranged in series-parallel combinations, which is what lets the same set resonate against a few hundred picofarads of gas-insulated bus and against the far larger capacitance of installed cable. Frequency is swept from 20 to 300 Hz at 0.1 Hz resolution to find the resonance point for each object.
What frequency is used for power transformer testing? Power transformers are tested between 45 and 65 Hz, close to normal excitation for the core. The converter reaches that band within its 20 to 300 Hz range, with frequency instability held to 0.05%. Cable and gas-insulated objects are normally tested higher in the band, where a smaller reactor reaches rated voltage.
