With the rapid development of new energy grid integration, AI data center expansion, and smart grids, traditional power-frequency transformers can no longer meet the demands of modern power systems. Solid State Transformers (SSTs), featuring compact size, high efficiency, power quality regulation, and bidirectional energy flow, are emerging as key equipment for next-generation power grids.
However, as operating frequency surges from 50 Hz to 10–30 kHz and above, while voltage levels remain medium-to-high voltage (10 kV and above) and volumes shrink drastically, insulation systems face unprecedented challenges from coupled electro thermal mechanical multi-physics fields. Reliable insulation design has become a critical bottleneck restricting the development of high-power-density SSTs.
An SST is not simply an upgrade of conventional transformers, but a power conversion system integrating power electronics and high-frequency magnetic coupling. Its typical structure consists of three core units:
l Input Stage (Rectifier Unit)Converts medium-voltage AC to DC and withstands the system’s highest voltage. Insulation must endure lightning impulse (e.g., 95 kV BIL) and switching overvoltages.
l Isolation Stage (High-Frequency Transformer Unit)The core of the SST, comprising high-frequency transformers and power semiconductors (primarily SiC MOSFETs), operating at 10–100 kHz to achieve electrical isolation and voltage conversion. This is the focal area for insulation design, requiring highly reliable insulation between turns, layers, and primary secondary windings, while simultaneously withstanding high-frequency electrical stress, thermal stress, and mechanical vibration.
l Output Stage (Inverter Unit)Converts DC to low-voltage AC or DC for load applications.
The high-frequency transformer is where insulation challenges are most concentrated. Its typical structure includes:
l Magnetic Core: Commonly nanocrystalline or ferrite materials with sharp edges, requiring protection by silicone or polyimide sleeves.
l Windings: Configured as Litz wire, copper foil, or PCB windings.
l Insulation System: Turn insulation mostly uses polyimide film, with overall encapsulation via epoxy resin casting to form a composite insulation structure.In SSTs, insulation materials not only provide electrical isolation but also directly participate in thermal management and mechanical support, with their performance directly determining the power density and service life of the entire unit.
Insulation materials used in SSTs are predominantly solid, mainly including:
l Polyimide Film – Core material for turn-to-turn insulation.Heat resistance class reaches Class H (180 °C) and above, with electrical strength exceeding 200 kV/mm. It is often combined with epoxy resin to form composite insulation.
l Epoxy Resin – Main casting insulation material.Cures and encapsulates windings to provide insulation, mechanical support, and heat dissipation. With relatively low thermal conductivity (<0.2 W/(m·K)), it is typically modified with fillers such as boron nitride and alumina for performance enhancement.
l Encapsulation Silicone Gel – Local insulation protection.Used for edge protection of nanocrystalline cores. It features high elasticity and thermal stability, enabling partial-discharge-free insulation at 18 kV RMS.
l Polypropylene Sheet – Novel insulation structure.Exhibits low dielectric loss and high breakdown field strength. Combined with encapsulants, it supports partial-discharge-free operation at 30 kV RMS.
Insulating fiber materials (e.g., aramid paper) and PCB substrates (FR4) are also applied in SSTs.
As SSTs move from laboratories to commercialization, their development trends impose stringent demands on insulation materials:
3.1 Low Loss and High Partial Discharge Resistance under High-Frequency Electrical Stress
At kHz-level frequencies, dielectric loss increases exponentially, and the destructive effect of partial discharge intensifies sharply. Studies show that above 10 kHz, partial discharge in multi-layer polyimide insulation exhibits a “rabbit-ear” discharge pattern, with significant increases in discharge magnitude and frequency.
Requirements: Insulation materials must feature lower high-frequency dielectric dissipation factor and enable partial-discharge-free design. In engineering applications, graded insulation and electric-field shielding rings can limit partial discharge below 5 pC.
3.2 Improved Thermal Class in High-Temperature Environments
SST heat flux density can reach 15 W/cm³. Elevated temperatures reduce partial discharge inception voltage and amplify discharge magnitude, forming an electro thermal positive feedback loop.
Requirements: Thermal class of insulation materials must advance from traditional Class B (130 °C) and Class F (155 °C) to Class H (180 °C), Class N (200 °C), or even higher.
3.3 Interface Compatibility under Multi-Physics Fields
SSTs widely adopt multi-layer composite insulation structures of “polyimide + epoxy resin”. Differences in dielectric constant and thermal expansion coefficient between dissimilar materials make interfaces prone to partial discharge. Failure of multi-layer polyimide insulation proceeds in three stages: electrical tree propagation, electrical tree breakdown, and point pre-breakdown.
Requirements: Nano-Al₂O₃ composite coatings via ion exchange, or dopamine-grafted nano-boron nitride modified epoxy resin, can effectively suppress interface charge accumulation and enhance interfacial compatibility.
3.4 Thermal Conductivity in Compact Spaces
Insulation materials in SSTs also serve a thermal management function. Inadequate heat dissipation accelerates insulation aging.
Requirements: Modification with boron nitride, alumina, and other fillers raises the thermal conductivity of composite materials to above 0.8 W/(m·K), significantly improving heat dissipation.
3.5 Process Compatibility and Mechanical Reliability
Complex winding structures create small gaps, and equipment is subjected to high-frequency vibration and thermal cycling stress during operation.
Requirements: Encapsulants must have low viscosity for void-free potting, and sufficient mechanical strength after curing to withstand high-frequency vibration and thermal cycling stress.
The performance and reliability of solid state Transformers largely depend on insulation technology, a key bottleneck restricting advancement. From polyimide films and epoxy resins to nano-modified composite materials and interface engineering, every breakthrough in insulation technology drives SSTs toward higher power density, higher voltage levels, and greater reliability.
When SSTs handle megawatt-scale power in compact enclosures, these precisely engineered insulation materials silently safeguard the safety and reliability of power conversion under harsh conditions of high temperature, high frequency, and high voltage.
With the rapid development of new energy grid integration, AI data center expansion, and smart grids, traditional power-frequency transformers can no longer meet the demands of modern power systems. Solid State Transformers (SSTs), featuring compact size, high efficiency, power quality regulation, and bidirectional energy flow, are emerging as key equipment for next-generation power grids. However, as operating frequency surges from 50 Hz to 10–30 kHz and above, while voltage levels remain medium-to-high voltage (10 kV and above) and volumes shrink drastically, insulation systems face unprecedented challenges from coupled electro thermal mechanical multi-physics fields. Reliable insulation design has become a critical bottleneck restricting the development of high-power-density SSTs.
Epoxy Board exhibits excellent performance, featuring high temperature resistance, corrosion resistance, high mechanical strength, good machinability, low water absorption, and outstanding insulation and electrical properties. There are various grades of Epoxy Board, including 3240, G11, FR‑4, G10, and others.
As a professional engaged in electrical accessory processing for many years, we have handled a large number of custom Epoxy Laminates orders. In practice, many users simply treat this material as an “insulation board,” without fully realizing that it combines excellent electrical insulation, high mechanical strength, and strong corrosion resistance, making it widely used across multiple industries.
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