Resonant AC testing is used for generator stators because the stator’s large phase-to-ground capacitance makes a power-frequency AC test impractical with a direct source: the charging current and reactive power requirement at 50 or 60 Hz exceed what most sites can supply.
A resonant system tunes a reactor to the stator capacitance so that the reactive current circulates in the resonant circuit and the supply provides only the active losses. This allows a power-frequency-like AC proof of the stator insulation after rewinds, major repairs or at the intervals defined by the machine’s test programme.
Why Stator Capacitance Makes AC Testing Hard
A generator stator is a large capacitor. The windings, their insulation and the grounded core form a phase-to-ground capacitance that can be a microfarad or more on a large machine, and at power frequency that capacitance draws a charging current proportional to the test voltage. The reactive power needed for a meaningful AC test can reach hundreds of kilovolt-amperes or more, which is far beyond what a portable direct test transformer can supply from a typical station service supply. The problem is not the voltage; it is the reactive current that the voltage demands from a capacitive load.
The consequence is that a direct power-frequency AC test of a large stator is impractical on site with conventional portable sources, and the machine owner is left with a choice between sources that can deliver the required alternating stress with a practical supply demand. This is the reason resonant systems exist for machine testing, and it is why the machine standards and recommended practices describe resonant energisation for stator AC proof testing.
How a Resonant Source Solves the Power Problem
A resonant source connects a tunable reactor in series with the stator capacitance and adjusts the frequency or the inductance until the circuit is at resonance. At resonance, the inductive reactance cancels the capacitive reactance, the voltage across the stator can be many times the supply voltage, and the large reactive current circulates between the reactor and the stator rather than flowing through the supply. The supply then provides only the active losses of the circuit, which are a small fraction of the reactive power the stator would otherwise demand. The quality factor, or Q, expresses this benefit: a higher Q means the resonant circuit develops the test voltage from a smaller supply.
The practical result is that a resonant system can apply a power-frequency AC proof to a stator from a supply that a direct source could never use for the same load. The system also behaves benignly at breakdown: a fault in the insulation detunes or collapses the resonant voltage rather than sustaining a high fault current into the failure, which reduces the damage to the stator when a defect is found.
Open-Core and Variable-Frequency Resonant Approaches
Resonant systems for generator stators come in two main families. Open-core resonant transformers use a gapped or open magnetic core whose inductance is adjusted to tune the circuit, and they have a long history in hydro-generator stator testing, where the large capacitance of the stator is matched by tuning the core. Variable-frequency resonant systems tune the circuit by adjusting the frequency of a solid-state source until resonance is reached with a fixed reactor, and they are flexible across a range of load capacitances. The choice between the families depends on the machine size, the test frequency requirement, the site supply and the equipment available.
Both approaches achieve the same electrical purpose: power-frequency or near-power-frequency AC stress on the stator with a practical supply demand. The test voltage and duration come from the machine’s test standard or the manufacturer’s instructions, and the frequency at which the test is applied is part of the specification, so the resonant system must be capable of operating at the required frequency with the actual stator capacitance.
Test Setup, Frequency and Compensation
The setup begins with the stator capacitance, which is taken from the machine data or measured, and the test voltage and frequency from the governing document. The resonant system is tuned to the working point, and the test is applied with the voltage raised gradually while the operator watches the behaviour. The compensation provided by the reactor means the site supply carries only the losses, but the operator still confirms that the supply is adequate for the active power, the control systems and any auxiliary loads.
The stator must be isolated from the machine’s connected systems, including the excitation and any connected switchgear, and the test area must be controlled for the duration. The stored energy in the resonant circuit and the stator is part of the safety plan, and the discharge and verification steps follow the same discipline as any high voltage AC test. The setup record includes the machine identification, the capacitance used for tuning, the frequency, the test voltage and the duration.
Safety and Protection During Stator Testing
Testing a generator stator involves large stored energy and a test object that is expensive and difficult to repair, so the safety and protection plan is built for the specific machine. The stator windings are isolated and verified at zero energy before the test, the area is controlled, and the resonant system’s protection is set so that a breakdown collapses the test voltage quickly. The test team agrees the stop conditions and the response to a failure before the voltage is applied, because the reaction to a stator failure during a proof test is not the moment to improvise.
The machine’s own protection and the test circuit protection are separate, and the test plan should state which protections are in service and which are disabled for the test. The discharge behaviour of the stator and the resonant system is verified before any connection is changed, and the temporary grounds are controlled so that the machine returns to service in a known state. The safety detail is covered by the high voltage testing safety article in this series; the application here is a large capacitive machine that stores significant energy.
When to Use Resonant Testing for Generators
| Situation | Resonant AC testing? | Why |
|---|---|---|
| After stator rewind or major repair | Yes, where the specification requires an AC proof | Proves the repaired insulation with power-frequency-like stress. |
| Routine stator insulation proof at intervals | Yes, where the programme requires it | Provides the AC proof that the large capacitance demands. |
| Diagnostic condition assessment | No, diagnostics first | Use IR, PI, tan delta or PD to build the condition picture. |
| Small machines with modest capacitance | Direct source may suffice | The power problem scales with capacitance. |
Use the table to frame the decision, then read the machine’s test standard and the manufacturer’s instructions for the voltage, duration and method. The resonant system is the enabling technology for large-machine AC proof testing, but it does not replace the condition diagnostics that decide when a proof is appropriate.
The stator test programme also benefits from a recorded example of the sizing logic. A large hydro-generator stator with a phase-to-ground capacitance of 0.8 microfarads, tested at a few tens of kilovolts at power frequency, demands a charging current of the order of several amperes and a reactive power requirement of hundreds of kilovolt-amperes from a direct source. A resonant system with a quality factor in the tens reduces the supply demand to a few percent of that figure, which is the difference between a test that can run from the station auxiliary supply and one that requires an external power source. The figures are illustrative and depend on the actual machine data and test level, but they show why the resonant choice is an engineering calculation rather than a preference. The record should include the capacitance used, the test voltage and frequency, the calculated reactive power and the actual supply demand, so that the next test can be planned from the same verified numbers and the equipment can be sized with confidence.
Frequently Asked Questions
Why is the reactive power a problem for stator testing?
The stator is a large capacitor, and at power frequency its charging current demands reactive power proportional to the test voltage and the capacitance. On a large machine that requirement can reach hundreds of kilovolt-amperes, beyond what a portable direct source can supply from a typical site supply.
What is an open-core resonant transformer?
An open-core resonant transformer uses a gapped or open magnetic core whose inductance is adjusted to resonate with the stator capacitance. It has a long history in hydro-generator stator testing, where tuning the core matches the large capacitance of the machine.
When should a generator stator be tested after a rewind?
After a stator rewind or major repair, the machine’s test programme typically requires an AC proof of the repaired insulation, and resonant AC testing is the practical way to apply power-frequency-like stress to the large capacitance. The voltage and duration come from the machine standard and the manufacturer’s instructions.
For AC withstand and resonant test equipment, see the high voltage insulation and withstand testing page.