Solid-state transformers (SSTs), also known as power electronic transformers, use power electronics and high-frequency isolation to convert and control electrical power. Compared with conventional line-frequency transformers, SSTs offer greater flexibility in power conversion but introduce new challenges in insulation design.
One of the most important reliability concerns is partial discharge (PD). As SST voltage levels increase from 10kV to 35kV, controlling partial discharge becomes significantly more difficult, particularly in high-frequency switching systems.
What Is Partial Discharge in an SST?
Partial discharge is a localized electrical breakdown within or along an insulation system that doesn’t completely bridge the insulation between conductors. It can originate from tiny air voids in epoxy resin, gaps between insulation layers, sharp conductor edges, or interfaces between different insulating materials.
Think of insulation as a rubber sheet containing a tiny air bubble. Under a strong electric field, a small discharge may occur inside the bubble. Each discharge gradually damages the surrounding material. Over time, the damage can develop into electrical treeing, eventually leading to insulation breakdown.
In an SST, this process is particularly important because the system uses high-frequency switching waveforms rather than only conventional sinusoidal voltage.
Fast switching edges, high (dv/dt), and repetitive voltage pulses can influence partial discharge inception and insulation aging. In SiC-based power converters, switching slew rates can reach several to tens of kV/μs, depending on the device and circuit design.
Repeated electrical stress may progressively weaken the insulation, even when the equipment initially passes functional and load tests.
Why Is Partial Discharge More Challenging in SSTs?
Three factors make PD control particularly important in SST designs.
- High-frequency voltage stress: Repetitive switching pulses can trigger discharges under conditions that differ from those of conventional AC insulation tests.
Local electric-field concentration: Sharp edges, winding ends and material interfaces can create localized field concentrations, increasing insulation stress.
Long-term insulation degradation: Repeated discharges can damage polymer materials, promote electrical treeing and eventually cause insulation failure.
The process may develop over a long period, so a short-duration test does not necessarily demonstrate long-term insulation reliability.
Two important parameters are used to characterize partial discharge:
- PDIV (Partial Discharge Inception Voltage): The voltage at which detectable partial discharge begins under specified test conditions.
PDEV (Partial Discharge Extinction Voltage): The voltage at which partial discharge stops as the voltage decreases.
For SST applications, these values must be evaluated using test waveforms and conditions representative of the actual operating environment. Repetitive-pulse PD testing can be particularly relevant for high-frequency converter insulation.
10kV vs. 35kV SST: What Are the Main Design Challenges?
Moving from a 10kV to a 35kV AC input system is not simply a matter of increasing insulation thickness. The higher voltage changes the requirements for insulation coordination, power devices, magnetic components and system protection.
The following comparison assumes a 10kV AC input system with a DC link in the approximate ±7–8kV range, versus a 35kV AC input system with a DC link around ±24kV. Actual voltage architecture depends on the converter topology.
| Design Area | 10kV SST | 35kV SST |
|---|---|---|
| Insulation and Partial Discharge | Insulation design and void control remain important, with risks generally easier to manage. | Higher electric-field stress makes insulation defects, material interfaces and field concentration more challenging. |
| Power Devices | Device selection and voltage sharing may be more manageable. | Higher DC-link voltage may require series-connected devices or a different converter topology. |
| Medium-Frequency Transformer | Insulation and shielding requirements may be less demanding. | More complex winding insulation, field grading and parasitic capacitance management are required. |
| Protection and Measurement | More established sensing and protection components may be available. | High-voltage sensing, insulation monitoring and fast fault protection become more demanding. |
| Thermal Management and Manufacturing | Manufacturing consistency and thermal control remain essential. | Insulation defects, curing variations and thermal-mechanical stress can have greater consequences. |
Insulation and field grading
At 35kV, local electric-field concentrations around winding ends, shielding boundaries and material interfaces become a major concern. Simply adding insulation thickness may increase material stress, manufacturing complexity and the risk of voids or delamination.
More advanced field grading, shielding and insulation-process control may therefore be necessary.
Power devices and voltage sharing
A higher DC link voltage may require multiple semiconductor devices in series, depending on device ratings and converter topology. Series-connected devices introduce static and dynamic voltage-sharing challenges, particularly under fast switching conditions.
Medium-frequency transformer design
SSTs rely on medium-frequency transformers for galvanic isolation and power transfer. At higher voltage levels, winding insulation, interwinding shielding and parasitic capacitance require greater attention.
Poorly managed parasitic capacitance can increase displacement current, common-mode noise and electromagnetic interference (EMI).
Testing and manufacturing consistency
Reliable 35kV SST production requires careful control of insulation materials, winding processes, vacuum impregnation or encapsulation, and curing conditions.
Partial discharge testing must also account for the relevant voltage waveform and switching characteristics. External corona and measurement noise can complicate testing at higher voltage levels.
How Can SST Designers Reduce Partial Discharge Risk?
Reducing PD risk requires a coordinated approach rather than relying on one insulation material or a single high-voltage test.
Key measures include:
- Improve insulation quality: Minimize voids, delamination and defects during winding, encapsulation and curing.
- Control electric fields: Optimize conductor geometry, winding ends, shielding and field grading.
- Evaluate realistic switching waveforms: Assess PD behavior under relevant repetitive-pulse conditions, not only conventional AC voltage.
- Manage parasitic effects: Consider winding capacitance, common-mode currents, grounding and EMI during system design.
- Verify long-term reliability: Combine appropriate electrical tests with thermal cycling and other application-relevant qualification procedures.
Some 35kV SST architectures use modular multilevel or cascaded converter arrangements to distribute voltage stress across multiple submodules. This can help manage individual module voltage requirements, but it also introduces additional control, balancing and integration challenges.
The appropriate architecture depends on system requirements, device availability, insulation coordination and overall cost.
Designing for SST Reliability
Partial discharge is a critical consideration in SST insulation design because its effects can accumulate long before a complete insulation failure occurs. At 35kV, the challenge extends beyond dielectric strength to include electric-field management, switching behavior, power-device coordination, transformer design and manufacturing consistency.
For engineers and equipment developers, understanding these interactions is essential when evaluating SST reliability and choosing suitable electrical components.
ALLBLUE supports B2B customers with sourcing and procurement of electrical and electronic components. For power conversion applications, component selection should always be based on the actual voltage, current, switching conditions, insulation requirements and operating environment.