Phase-shifting transformer

I. Working Principle
The phase-shifting transformer is a specialized multi-winding power transformer operating on the law of electromagnetic induction. By employing specific winding connection configurations—such as zig-zag, extended delta, or multi-section phase-shifting arrangements—it introduces precise, predetermined phase displacement angles among the secondary output voltage groups. While a standard utility grid maintains a fixed 0° baseline phase difference, a phase-shifting transformer generates multiple sets of AC voltages with staggered phase angles.
In high-voltage, high-power variable frequency drive (VFD) and rectification systems, these phase-shifted secondary voltages supply multiple rectifier bridges to achieve multipulse rectification. By superimposing multiple phase-displaced pulsating waveforms, harmonic currents on the primary input side are mutually cancelled, drastically mitigating grid harmonic pollution and improving input power factor.

II. Key Performance Characteristics

Major Advantages:

Superior Harmonic Suppression: Leveraging multi-pulse phase-shifting rectification, it effectively eliminates characteristic harmonics (such as the 5th, 7th, 11th, and 13th) without requiring bulky, high-capacity passive filters, significantly improving grid power quality in compliance with IEEE 519 standards.

High-Power Conversion Compatibility: Seamlessly accommodates 12-pulse, 18-pulse, 24-pulse, or higher rectification topologies, with single-unit capacities ranging from hundreds to several thousand kVA.

Optimized Grid Operation: Effectively mitigates reactive power surges, reducing additional thermal losses and aging inflicted by rectifying equipment on upstream distribution transformers and switchgear.

High Integration: Features a compact layout that allows seamless cabinet integration with high-frequency inverter power cells or rectifier cubicles, ensuring high operational stability and reliability.

Inherent Limitations:

Complex Manufacturing: The presence of multiple secondary windings and complex phase-angle geometries makes design and fabrication significantly more demanding than standard distribution transformers, resulting in higher initial capital costs.

Strict Insulation & Dimensional Constraints: Multiple winding leads and complex tap arrangements demand rigorous insulation design, leading to a larger footprint and heavier overall weight.

Load-Dependent Efficiency: Optimized for harmonic cancellation under rated conditions; harmonic mitigation performance may degrade when the system operates under prolonged light-load or no-load states.

III. Application Scenarios

High-Voltage Variable Frequency Drive (VFD) Systems: Serving as the mandatory phase-shifting rectifier transformer for high-voltage VFDs (such as cascaded H-bridge multilevel inverters) driving mine hoists, large industrial fans, boiler feed pumps, and rolling mills.

High-Power Industrial DC Power Supplies: Utilized in aluminum electrolysis, electroplating, green hydrogen production via water electrolysis, DC metallurgical smelting, and large-scale battery energy storage testing equipment.

Rail Transit & Heavy Machinery: Powering heavy-duty converter units for shield tunneling machines (TBMs) and traction substation rectification systems.

Renewable Energy & Energy Storage: Front-end rectification units for high-power grid-forming energy storage power conversion systems (PCS) and centralized photovoltaic solar inverters.

Industrial Heating & Metallurgy: Feeding large-scale industrial electric arc furnaces (EAFs) and medium-frequency induction heating power supplies.

The core engineering value of a phase-shifting transformer lies in achieving multipulse rectification through spatial phase displacement to suppress harmonics at the source. It stands as an indispensable key auxiliary equipment for modern high-power power-electronic AC-DC conversion and drive systems.

Quality Assurance & Core Process：

1. High-Permeability Grain-Oriented Electrical Steel

We utilize Hi-B high-permeability grain-oriented (HPGO) electrical steel—the same grade specified by TBEA and CHINT—as the transformer core material, with standard nominal thicknesses of 0.23 mm and 0.27 mm. Core laminations are manufactured using high-precision cutting equipment to guarantee a burr height of ≤0.02 mm. The assembly incorporates a precision step-lap lamination structure, maintaining optimal clamping pressure without imparting excessive mechanical stress to the electrical steel, combining high-B steel characteristics with an optimized magnetic flux density.

2. High-Conductivity Winding Conductors & Core-Coil Assembly

High-conductivity electrolytic copper or aluminum conductors are selected based on transformer capacity, voltage class, and loss requirements. Incoming materials undergo stringent quality inspections for dimensions, electrical resistance, surface quality, and traceability. During the winding process, conductor tension, winding height, layer insulation, and conductor positioning are precisely controlled to ensure resistance consistency and mechanical stability, thereby minimizing load losses and temperature rise during operation.

During core-coil active part assembly, coil height, axial clamping force, radial support, and spacer block positioning are precisely controlled. Proper mechanical clamping prevents coil displacement during transport, normal operation, and short-circuit conditions. Meanwhile, clamping forces are strictly regulated to avoid excessive stress on conductors and insulation materials. Our mechanical design accounts for both radial and axial electromagnetic forces, safeguarding short-circuit withstand capability and long-term dimensional stability.

3. Advanced Insulation System (Oil-Immersed & Dry-Type)

For oil-immersed transformers, we utilize high-grade insulation materials, including Kraft paper, thermally upgraded paper, pressboard, and transformer-grade laminated board. For dry-type transformers, we employ DMD, NMN, Nomex, fiberglass, and epoxy resin systems. Insulation thickness, overlap width, oil duct dimensions, electrical clearances, and mechanical positioning are strictly regulated. All insulation materials are stored under climate-controlled conditions to minimize moisture absorption and contamination, ensuring stable dielectric strength and long-term insulation reliability.

4. Controlled Vacuum Drying

We apply a controlled drying curve regulated by temperature, vacuum level, and duration parameters, rather than relying on fixed cycle times. The process comprises controlled pre-heating, evacuation, primary drying, and final soak phases, with continuous temperature and vacuum monitoring and logging. End-point determination is based on validated process criteria and moisture limits. This process minimizes residual moisture in cellulose insulation, maximizing dielectric strength, retarding insulation aging, and extending overall service life.

5. Vacuum Epoxy Resin Casting

For cast-resin dry-type transformers, we utilize controlled vacuum casting technology. Resin-to-hardener ratio, material temperature, vacuum level, mixing time, pouring conditions, and curing temperatures are strictly regulated. Vacuum degassing minimizes micro-voids prior to pouring, while controlled curing ensures stable mechanical and thermal properties. Finished coils undergo thorough visual and electrical inspections, including routine partial discharge (PD) testing. This process eliminates internal voids, ensuring reliable insulation performance throughout the transformer's service life.

6. Clean Assembly & Torque Control

Final assembly takes place in a controlled environment to minimize contamination from dust, metallic particles, and foreign objects. Critical components are inspected for cleanliness, dimensional accuracy, and insulation condition. Electrical clearances, lead routing, grounding connections, and mechanical supports are rigorously controlled throughout assembly. Critical bolted joints are tightened to specified torque values using calibrated equipment and fully logged. Prior to sealing the tank or enclosure, a thorough internal inspection ensures no tools, loose parts, or foreign material remain.

7. 100% Factory Routine Testing

Every transformer undergoes 100% routine testing prior to dispatch, complying with applicable IEC standards, approved technical specifications, and customer requirements. Standard test routines include winding DC resistance, voltage ratio, vector group verification, no-load loss and current, load loss and short-circuit impedance, applied voltage withstand, and induced overvoltage withstand tests. Special tests—such as partial discharge (PD), dissipation factor (tan δ), or sweep frequency response analysis (SFRA)—are conducted upon request. Each unit is assigned a unique serial number linked to its material records, manufacturing data, and final test reports, ensuring end-to-end traceability.