HUAC-22/22-10kV
10 kV Variable Frequency Series Resonant Test System
AC withstand testing of 10 kV cables, switchgear and transformers, tuning to resonance anywhere from 30 to 300 Hz.
IEC 60060-1DL/T 849.6
Buying guide · AC withstand testing
Series Resonant Test Systems
Variable frequency AC hipot systems from 10 kV to 500 kV for cables, GIS, transformers and generators. How resonance testing works, how to size a system for your cables, and which HUAC model fits.
- 10 kV to 500 kV, up to 1500 kVA
- 20/30 – 300 Hz automatic tuning
- Runs from a 220 V or 380 V site supply
- Modular reactors, series or parallel
9 models
Choose a system
HUAC-30kV/35kV
HUAC Variable Frequency Series Resonant Test System for 10 kV & 35 kV Cable
Variable frequency series resonant test system, 30–300 Hz, with multi-stage reactors for AC withstand tests on 10 kV and 35 kV cables, transformers and GIS.
IEC 60060-1DL/T 849.6
HUAC-22/22-35kV
35 kV Variable Frequency Series Resonant Test System
AC withstand testing of 35 kV cables, transformers and switchgear, tuning to resonance anywhere from 30 to 300 Hz.
IEC 60060-1DL/T 849.6
HUAC-XB
Variable Frequency Series Resonant Test System, XB Series
XB series resonant test system, 20–300 Hz, Q > 50, distortion < 1 %, units under 40 kg, for AC withstand tests on cables, transformers, GIS and generators.
HUAC-22/22-75kVA/75kV
75 kVA / 75 kV Variable Frequency Series Resonant Test System
75 kVA resonant test system for AC withstand and partial discharge tests on cables, transformers, GIS and generators, with 25 kV / 3 A or 75 kV / 1 A output.
IEC 60060-1DL/T 849.6
HUAC-22/22-216kVA/108kV
216 kVA / 108 kV Variable Frequency Series Resonant Test System
216 kVA series resonant test system with up to 108 kV output for AC withstand tests on cables, transformers, GIS and generators, tuning from 30 to 300 Hz.
IEC 60060-1DL/T 849.6
HUAC-22/22-110kV/220kV
110 kV / 220 kV Variable Frequency Series Resonant Test System
AC withstand testing of 110 kV and 220 kV cables, transformers, GIS and transmission equipment, with modular reactors tuning to resonance from 30 to 300 Hz.
IEC 60060-1DL/T 849.6
HUAC-810kVA/270kV/27kV
810 kVA / 270 kV Variable Frequency Series Resonant Test System
810 kVA series resonant test system with two output levels, 270 kV / 3 A or 27 kV / 30 A, for AC withstand tests on cables, transformers, GIS and generators.
IEC 60060-1DL/T 849.6
HUAC-1500kVA/500kV
1500 kVA / 500 kV Variable Frequency Series Resonant Test System
1500 kVA series resonant test system with up to 500 kV output for AC withstand tests on GIS, switchgear and transformers up to 220 kV, in the lab or on site.
IEC 60060-1DL/T 849.6
How to size a system
Three numbers decide the size of a resonant system: the test voltage, the capacitance of the test object and the lowest frequency you will accept.
- Test voltage and duration. Take them from your specification: for cables, the after-installation AC test in IEC 60502-2, IEC 60840, IEC 62067 or a national code; for GIS and transformers, the relevant equipment standard or the purchaser's test plan.
- Capacitance. For a cable, multiply the capacitance per kilometre from the cable datasheet by the length. Medium-voltage XLPE cables are typically in the range of a few tenths of a microfarad per kilometre, depending on cross-section and insulation thickness. For GIS and transformers, use the value from the manufacturer or an earlier measurement.
- Test current. I = 2π · f · C · U. The lower the frequency, the lower the current, so size at the lowest frequency your standard accepts.
- Test power. S = U · I. Choose a system whose rated current at the required voltage is at least the test current, with some margin.
Worked example. A 3 km circuit of medium-voltage cable with 0.35 µF/km per phase has C ≈ 1.05 µF per phase. The specification asks for 17.4 kV. At 30 Hz:
- I = 2π × 30 × 1.05 µF × 17.4 kV ≈ 3.4 A
- S = 17.4 kV × 3.4 A ≈ 60 kVA
At 50 Hz the same test would need about 5.7 A and 100 kVA, which is why resonant systems are tuned towards the lower end of the frequency range. In this example, the 25 kV / 3 A level of the 75 kVA system covers up to about 0.9 µF at 17.4 kV and 30 Hz, roughly 2.6 km of this cable per phase; the full 3 km would need parallel reactors or a larger system. Send us the cable data and the test voltage and we will check the configuration for you.
What to send for a quotation
- Test objects: cable, GIS, transformer, generator, and their voltage class.
- For cables: cross-section, insulation type, the longest length you need to test, and the capacitance per kilometre if known.
- Test voltage and duration from your specification, and the standard it comes from.
- Available site supply: single-phase 220 V or three-phase 380 V, and its capacity.
- Transport limits: vehicle, lifting equipment and the heaviest unit you can handle.
For the test itself, see our step-by-step guide to the series resonant AC withstand test.
Read the full guide: how it works, resonance vs test transformer, model table, applicationsShow less
What a series resonant test system does
A series resonant test system generates a high AC test voltage for withstand tests on equipment that is mostly capacitive: power cables, gas-insulated switchgear (GIS), transformers, generators and bushings. Instead of a large power-frequency test transformer, it uses a high-voltage reactor in series with the capacitance of the test object. When the frequency is tuned so that the reactor's inductive reactance cancels the object's capacitive reactance, the circuit resonates and the voltage across the test object rises to Q times the excitation voltage.
The resonant frequency is
f = 1 / (2π √(L · C))
where L is the inductance of the reactor and C is the capacitance of the test object plus the voltage divider. A variable-frequency power source finds this frequency automatically, so the same system can test a short busbar section or several kilometres of cable, as long as the resulting frequency stays within the accepted range, typically 20 or 30 Hz to 300 Hz.
Why resonance instead of a test transformer
| Series resonant test system | Power-frequency test transformer | |
|---|---|---|
| Power drawn from the supply | Roughly the test power divided by Q, plus losses | The full reactive power of the test object |
| Weight per kVA of test power | Low; modular reactors that can be carried and stacked | High; one heavy transformer and regulator |
| Waveform | Near-perfect sine, the resonant circuit filters harmonics | Depends on the supply and regulator |
| At flashover | Resonance collapses at once; little energy reaches the fault | The supply keeps feeding the arc until the breaker trips |
| Frequency | 20/30 – 300 Hz, set by the test object | 50 or 60 Hz |
| Best for | Long cables, GIS, large capacitive loads, on-site work | Small capacitive loads, factory and laboratory tests |
Because the supply only has to cover the losses of the circuit, a series resonant system tests loads of hundreds of kVA from an ordinary 220 V or 380 V site supply. That is why it is the standard choice for after-installation tests on high-voltage cables and GIS. For small objects, a power-frequency test transformer or a cascade AC hipot test set is often simpler.
The HUAC range
All HUAC systems use a variable-frequency power source, an excitation transformer, modular high-voltage reactors that can be connected in series or parallel, and a capacitive voltage divider that also serves as compensation capacitance for small test objects.
| Model | Rating | Typical test objects |
|---|---|---|
| HUAC-22/22-10kV | 10 kV class, 30 – 300 Hz | 10 kV cables, switchgear, distribution transformers |
| HUAC-30kV/35kV | 10 kV and 35 kV cable, 30 – 300 Hz | Medium-voltage cables, transformers, GIS |
| HUAC-22/22-35kV | 35 kV class, 30 – 300 Hz | 35 kV cables, transformers, switchgear |
| HUAC-XB series | 20 – 300 Hz, system Q > 50, units under 40 kg | Cables, transformers, GIS, generators |
| HUAC-22/22-75kVA/75kV | 75 kVA: 25 kV / 3 A or 75 kV / 1 A | Cables, transformers, GIS, generators; partial discharge tests |
| HUAC-22/22-216kVA/108kV | 216 kVA, 108 kV / 2 A | High-voltage cables, transformers, GIS, generators |
| HUAC-22/22-110kV/220kV | 110 kV and 220 kV class | 110 kV and 220 kV cables, GIS, transmission equipment |
| HUAC-810kVA/270kV/27kV | 810 kVA: 270 kV / 3 A or 27 kV / 30 A | High-voltage cables, long medium-voltage cables, GIS |
| HUAC-1500kVA/500kV | 1500 kVA, up to 500 kV | GIS, switchgear and transformers up to 220 kV, laboratories |
What it is used for
- Power cables: after-installation AC withstand tests on medium-voltage and high-voltage XLPE cables, the largest application. See also our comparison of VLF and series resonant testing.
- GIS and switchgear: on-site AC tests after assembly, where the low capacitance gives a high frequency and a high Q.
- Power transformers: applied-voltage withstand tests on windings.
- Generators: AC withstand tests on stator windings, which have a lower Q than cables.
- Partial discharge tests: the clean sine output suits PD measurements when combined with a PD detector and coupling capacitor.
Frequently asked questions
What frequency does a series resonant test system use?
The frequency is set by the reactor inductance and the test object capacitance, and the system tunes to it automatically. HUAC systems cover 30 to 300 Hz, and the XB series 20 to 300 Hz. Cable standards such as IEC 60840 and IEC 62067 accept AC test voltages from 20 to 300 Hz for after-installation tests.
How long a cable can one system test?
It depends on the test voltage, the cable capacitance per kilometre and the system's rated current. Calculate the test current with I = 2π · f · C · U at the lowest accepted frequency and compare it with the rated current of the system at that voltage. Reactors connected in parallel increase the current and therefore the length that can be tested.
What is the quality factor Q and why does it matter?
Q is the ratio of the test voltage to the excitation voltage at resonance, and also the ratio of the reactive test power to the real power the system draws. A high Q means a small power source and a small site supply can test a large load. Q depends on the test object: it is typically higher for GIS and cables than for generators.
Why connect reactors in series or in parallel?
Reactors in series give a higher test voltage; reactors in parallel give a higher current for larger capacitance, such as long cables. Modular reactors let one system cover several voltage classes and cable lengths.
What happens if the test object breaks down?
A breakdown detunes the circuit, the resonance collapses and the voltage drops at once, so little energy is released into the fault. The control system also detects the flashover and switches the output off.
Series resonant or VLF for cable testing?
Both are accepted for medium-voltage cables in many specifications. A series resonant system tests at a frequency close to service conditions and is required for many high-voltage cable acceptance tests; VLF equipment is lighter for medium-voltage work. Our VLF vs series resonant guide compares them in detail.
Related guides
VLF vs. series resonant testing: which suits your cable?
How cable length, capacitance and site power decide the withstand method.
Series resonant AC withstand test: step-by-step procedure for cables and GIS
Preparation, connection, tuning, holding the test voltage and discharging the cable, plus what to do when the system will not tune or trips during the test.
ASEAN Power Grid in 2026: new funding, new interconnections and what they mean for testing
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