Three-phase Combined Transformer

Updated: 2026-08-05

Overview

The three-phase combined current and voltage transformer is a specialized instrument transformer that consolidates the functions of separate current transformers (CTs) and voltage transformers (VTs) into a unified device. Primarily deployed in medium-voltage switchgear (typically 10kV-35kV systems), it serves as a critical component for accurate energy measurement and protective relaying. By combining both measurement functions, this device significantly reduces the physical footprint required in switchgear installations compared to using discrete transformers. Manufacturers often design these units with cast resin insulation for outdoor durability or gas-insulated configurations for compact indoor applications, making them versatile for different installation environments.

Structure and Working Principle

Structurally, the device contains three voltage transformer cores and three current transformer cores arranged in a single enclosure. The voltage transformer section typically uses a wound-type design with primary windings rated for system voltage (e.g., 10kV or 35kV), while the current transformer section employs either wound-primary or bar-primary designs based on current ratings. The working principle follows electromagnetic induction - the voltage transformer steps down line voltage to standardized outputs (usually 110V or 100V), while the current transformer reduces line current to either 5A or 1A secondary values. Advanced models incorporate anti-saturation features and temperature compensation to maintain accuracy across varying load conditions and environmental factors.

Key Features

Modern three-phase combined transformers offer several technical advantages. They achieve high accuracy across wide load ranges (often meeting 0.2S class for metering applications), with phase angle errors minimized for precise power measurement. The integrated design provides inherent phase matching between voltage and current signals, reducing measurement uncertainties. Durability features include partial discharge-free construction, UV-resistant materials for outdoor units, and corrosion-resistant terminals. Many models now include digital-ready outputs with test terminals for easy commissioning and maintenance. The compact form factor (typically 30-50% smaller than separate units) makes them particularly valuable for space-constrained substation retrofits.

Application Areas

These transformers are predominantly used in electrical distribution networks, appearing in utility substations, industrial plant switchyards, and commercial building power systems. They serve as the measurement foundation for revenue-grade electricity meters, providing the standardized inputs required by electronic multifunction meters. In protection applications, they feed protective relays with scaled-down signals for overcurrent, earth fault, and directional protection schemes. Some specialized variants are deployed in renewable energy installations, particularly in solar farm collector substations and wind turbine step-up systems, where space optimization is critical.

Maintenance and Precautions

Proper maintenance ensures long-term accuracy and reliability. Annual visual inspections should check for insulation cracks, terminal corrosion, and moisture ingress. Dielectric testing every 3-5 years verifies insulation integrity, while accuracy verification should align with local metering regulations (typically 5-10 year intervals). Critical precautions include ensuring the secondary circuits are never open-circuited (for CT sections) or short-circuited (for VT sections) during operation. Installation must follow manufacturer-recommended clearances, and all grounding connections should be verified. When de-energized, both primary and secondary terminals should be discharged before any maintenance work begins.

B2B Procurement Guide

When procuring these transformers for commercial projects, specify the exact system parameters: primary voltage/kV, primary current/A, accuracy classes for both VT and CT sections, and required burdens. Consider future expansion needs - some models allow for additional core taps or test windings. Evaluate manufacturers based on type test reports (including temperature rise and short-circuit tests), industry certifications (such as KEMA or CESI), and local grid compliance requirements. For large orders, request factory acceptance testing (FAT) to verify performance before shipment. Lead times for custom configurations typically range from 8-12 weeks, so plan procurement accordingly for project timelines.