In metering-class high-voltage voltage transformers (PT/VT) rated at 35kV and below, the adoption of multi-step laminated cores—with a cross-section approximating a circle—replacing conventional rectangular or square cores, is a comprehensive optimization grounded in electromagnetics, geometry, and insulation material characteristics.
Taking the Rockwill 11kV/33kV Outdoor Combined Metering Unit RBM Series, widely applied in international distribution networks, as an example—its outstanding metering accuracy and reliability are built precisely upon this core design philosophy.
Rockwill RBM Series 11kV/33kV Outdoor Combined Metering Unit
For metering transformers, the most critical requirements are high accuracy and high stability. The multi-step laminated core offers the following significant advantages:
1. Significantly Enhanced Metering Accuracy (The Core Advantage)
Metering voltage transformers typically demand very high accuracy classes. Transformer errors (ratio error and phase displacement) primarily originate from voltage drops caused by winding internal impedance.
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Reduced Conductor Length: Under the same effective core cross-sectional area, the multi-step laminated core has a shorter perimeter than a square core. This means that when winding the same number of turns, the mean length per turn (MLT) of the copper wire is shorter.
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Reduced Internal Impedance: The reduced copper consumption directly lowers the DC resistance and leakage reactance of the winding. In the Rockwill RBM Series Outdoor Combined Metering Unit, this low-impedance, high-quality magnetic circuit design ensures that the secondary-side voltage accurately reflects the primary-side voltage in proportion—providing a highly precise, zero-attenuation data source for the fully integrated energy meter.
2. Optimized Magnetic Flux Distribution, Reduced Core Loss and Waveform Distortion
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Avoidance of Local Saturation: Sharp corners of square cores tend to concentrate magnetic flux, causing localized saturation. The multi-step laminated core, approximating a circle, ensures a more uniform magnetic field distribution.
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True Waveform Transmission: Uniform flux distribution reduces the non-linear excitation current caused by saturation, ensuring that the secondary output voltage waveform remains undistorted.
The internal combined transformer of the Rockwill RBM Series consists of 2 or 3 single-phase voltage transformers and current transformers precisely configured. This pure, harmonic-distortion-free physical characteristic is critical for the integrated electronic metering unit, while also effectively reducing overall no-load losses.
Schematic Diagram of Multi-Step Laminated Iron Core
3. Improved Insulation Performance for Long-Term Operational Stability
Whether in dry-type or oil-immersed equipment, the structural compatibility between the core and windings significantly impacts insulation life.
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Elimination of Sharp-Edge Electric Field Concentration: Windings are typically cylindrical. If a square core is used, the four sharp corners create extreme electric field concentration, easily triggering breakdown at insulation weak points.
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Reduced Partial Discharge: The multi-step laminated core, paired with cylindrical windings, ensures extremely uniform inter-layer and inter-section insulation distances, significantly improving internal electric field distribution.
This electric field optimization design works in synergy with the No. 25 special transformer oil insulation medium used in the Rockwill RBM Series Outdoor Combined Metering Unit. Combined with the robust structure where primary and secondary terminals are mounted on a single panel through high-quality porcelain bushings, this builds an excellent insulation defense—ensuring outstanding insulation performance and long-term operational stability even in harsh outdoor environments.
4. Improved Space Factor for High Integration and Lightweight Design
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High Fill Factor: A cylindrical winding wound around a stepped core achieves significantly higher space utilization (fill factor) than a square core.
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More Compact Dimensions: Pole-mounted outdoor equipment has stringent requirements for size and suspension weight. A major innovation of the Rockwill 11kV/33kV RBM Series is the high integration of the combined transformer and the energy meter into a single unit. The high space factor of the multi-step laminated core effectively reduces the core component dimensions without compromising electromagnetic performance—significantly shrinking the overall tank size and weight, and greatly reducing pole-mounting difficulty and civil installation costs.
Rockwill RBM Series Outdoor High Voltage pole-mounted metering unit
5. Enhanced Short-Circuit Withstand Capability and Mechanical Robustness
During short-circuit events in the power grid, windings are subjected to enormous electrodynamic forces. A cylindrical winding experiences uniform radial tension, and when wound on a stepped core with high conformability, the mechanical structure is exceptionally robust. In contrast, square windings are highly prone to deformation under short-circuit conditions. This high mechanical stability ensures that outdoor equipment maintains its original high metering accuracy even after withstanding severe grid disturbances such as short-circuit faults.
Conclusion
The use of multi-step laminated cores in metering transformers rated at 35kV and below—by reducing copper winding length, optimizing electric field distribution, and minimizing dimensions—perfectly aligns with the modern development trends of high-voltage metering equipment: high accuracy, high insulation integrity, and compact design.
The Rockwill Outdoor Combined Metering Unit RBM Series exemplifies the engineering translation of these underlying electromagnetic advantages into product competitiveness—delivering an accurate, compact, and highly reliable integrated solution for medium-voltage distribution networks.
Edited From:Garca
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