Testing high-voltage insulation requires a controlled source of electrical stress. For equipment with substantial capacitance, producing the necessary AC test voltage with a conventional test transformer can require a large power supply and heavy equipment.
A resonant test set offers another approach. By using the electrical relationship between inductance and capacitance, it generates high AC voltage while reducing the power required from the external supply. This makes resonant systems useful for certain insulation withstand tests in both factory and field environments.
Understanding Electrical Resonance
Capacitance is the ability to store energy in an electric field. High-voltage equipment has capacitance because its conductive parts are separated by insulation. In a power cable, for example, the conductor and metallic screen are separated by insulating material, creating a capacitive load.
Inductance stores energy in a magnetic field. A resonant test system introduces inductance through a component called a reactor.
When the reactor’s inductance and the test circuit’s capacitance are matched at a particular AC frequency, the circuit reaches resonance. Energy transfers between the electric field of the capacitance and the magnetic field of the reactor.
In a series resonant system, the inductive and capacitive reactances balance each other. This allows a high voltage to develop across the equipment being tested while the external power source primarily supplies the energy needed to overcome system losses.
The Major Components of a Resonant Test Set
Although designs vary, a typical series resonant test system includes several components that work together to generate, control, and measure the test voltage.
Power Supply and Controls
The power supply provides the energy needed to operate the system. Controls regulate the test output, monitor operating conditions, and manage protective functions.
In a variable-frequency system, a frequency converter adjusts the AC frequency to establish resonance with the connected equipment. Other systems operate at a fixed frequency and adjust the reactor’s inductance instead.
Excitation Transformer
An excitation transformer supplies the voltage that drives the resonant circuit. It provides an initial voltage step-up and electrical isolation between the supply and the high-voltage circuit.
The excitation transformer works with the reactor and capacitive load to produce the required test voltage.
High-Voltage Reactor
The reactor supplies the inductance needed for resonance. Its rating and configuration must suit the test voltage, current, frequency, and capacitance of the connected equipment.
Some systems use multiple reactor modules arranged to meet different testing requirements.
Equipment Under Test
The equipment being tested supplies much of the circuit’s capacitance. Its electrical characteristics influence the frequency or reactor adjustment needed to achieve resonance.
The total circuit capacitance also includes contributions from measuring equipment and other connected components.
Voltage Measurement System
A high-voltage divider reduces the test voltage to a level that measuring instruments can safely read. This allows the operator and control system to monitor the voltage applied to the equipment.
Where the test specification calls for partial discharge measurements, additional measuring components can be incorporated into a suitable test arrangement.
How High AC Test Voltage Is Generated
Before testing, the system is configured for the equipment and the specified test requirements. The resonant condition is established by adjusting the frequency or reactor inductance, depending on the system design.
Once resonance is achieved, the excitation level is increased under controlled conditions. In a series resonant circuit, the voltage across the capacitive equipment can become much higher than the excitation voltage because of resonant voltage magnification.
The system then maintains the required test voltage for the specified duration while monitoring the test conditions. Test voltage, frequency, and duration must follow the applicable procedure and equipment requirements.
Resonance does not create energy. It allows energy to circulate efficiently within the test circuit, reducing the amount the external supply must continuously provide.
Why Resonant Testing Is Useful for Capacitive Equipment
Under AC voltage, capacitive equipment draws charging current. That current increases as capacitance, frequency, or applied voltage increases.
For long power cables and other substantial capacitive loads, a conventional AC source may therefore need a high apparent-power rating. A resonant arrangement compensates for the capacitive load with inductance, reducing the burden on the input supply.
Depending on the application and system design, this can offer practical advantages:
- Lower input power requirements: The supply primarily covers losses in the resonant circuit.
- Practical field deployment: Modular systems can make certain high-voltage tests more manageable at the jobsite.
- Adaptable configurations: Reactor arrangements and tuning can accommodate different loads within the system’s operating limits.
- Controlled AC insulation testing: The system provides the voltage needed for an applicable withstand test.
These advantages do not mean every resonant test set can test every asset. The system must be suitable for the equipment’s capacitance and the required voltage, current, frequency, and test duration.
What Does a Resonant Test Tell You?
A resonant test set is a source of test voltage. The information obtained depends on the test performed and the measurements included.
An AC withstand test evaluates whether insulation sustains a specified voltage for a defined period without breakdown. When partial discharge measurement is included, it can provide additional information about localized electrical activity within the insulation system.
A successful withstand test does not establish the equipment’s remaining service life or rule out every possible defect. Results should be considered alongside the asset’s history, inspection findings, and other appropriate diagnostic tests.
Applying Resonant Testing to Your Project
Selecting the right test method requires an understanding of both the equipment and the purpose of the evaluation. Installation, commissioning, and repair projects may have different requirements, and the test arrangement must reflect those differences.
MVA brings transformer field experience and electrical diagnostic expertise to project planning. Our team can review your equipment information and testing specifications to determine the appropriate scope and confirm available capabilities.
Contact MVA to discuss your high-voltage testing project or request a quote. We also offer short- and long-term resonant test system rentals, including our RTD100-1000X. Explore our rental options and contact us for availability and pricing.
