Introduction
Rectifier transformers are specialized power conversion devices designed to transform alternating current (AC) into direct current (DC) for industrial applications requiring stable and efficient DC power. These transformers play a critical role in electrochemical processes such as aluminum and chlor-alkali production, electric traction systems (e.g., railways and mining locomotives), and various industrial manufacturing operations.
Available in configurations such as bridge or interphase (double-star) designs, rectifier transformers can be engineered as 6-pulse or 12-pulse units, with support for both diode and thyristor technologies. For high-current applications like aluminum smelting, advanced phase-shifting techniques enable systems with up to 60 pulses, ensuring smooth and efficient power delivery.
A key focus in rectifier transformer design is harmonics mitigation, achieved through technologies like self-saturable reactors to minimize distortion and optimize performance. Manufacturers tailor these transformers to meet specific client requirements, ensuring reliability, energy efficiency, and compliance with industry standards. With robust construction and adaptive voltage regulation, rectifier transformers serve as a cornerstone of modern industrial DC power systems.
Construction
Example Drawings
Applications
- Electrochemical Industry
- Application: Used in aluminum electrolysis, chlor-alkali production (e.g. sodium hydroxide, chlorine), and metal smelting (e.g. copper, zinc).
- Function: Provides high-current, low-voltage DC power to ensure stable operation of electrolytic cells.
- Features: Must withstand corrosive environments, with output current reaching tens of thousands of amperes.
- Traction DC Power Supply
- Scenario: Mining locomotives, urban rail transit (subways, trams), electrified railways.
- Function: Supplies DC power (e.g. 600V/1500V/3000V systems) for traction motors.
- Features: Requires high reliability and dynamic response to frequent start-stop cycles and load variations.
- Drive System DC Power Supply
- Application: Powers rolling mills, mine hoists, ship propulsion systems and other high-torque DC motors.
- Function: Enables speed regulation through controlled rectification.
- Features: Delivers smooth DC output with minimized harmonic distortion.
- HVDC Transmission Power Supply
- Scenario: Long-distance power transmission, submarine cables, grid interconnections.
- Function: Performs AC-DC-AC conversion at converter stations with thyristors/IGBTs.
- Features: Ultra-high voltage (±800kV+), demanding special insulation and cooling solutions.
- Electroplating/Electromachining DC Supply
- Application: Chrome/nickel plating, electrochemical machining, anodizing.
- Function: Provides precise low-voltage (6-12V), high-current (hundreds-to-thousands amps) DC.
- Features: Requires extremely stable current with minimal ripple for uniform coating.
- Excitation DC Power Supply
- Scenario: Synchronous generator/motor excitation systems.
- Function: Delivers controllable DC to rotor windings for power factor regulation.
- Features: Must respond rapidly to grid disturbances (e.g. forced excitation during faults).
- Charging DC Power Supply
- Application: EV fast chargers, battery charging (lead-acid/Li-ion).
- Function: Converts AC to battery-compatible DC (400V-1000V).
- Features: Incorporates CC-CV charging algorithms with protection mechanisms.
- Electrostatic Precipitator DC Supply
- Scenario: Flue gas treatment in power plants, cement/steel mills.
- Function: Generates high-voltage DC (40-100kV) to charge dust particles.
- Features: Automatic voltage adjustment based on dust concentration, explosion-proof design.
Classification
1.Classification by Purpose
Rectifier transformers are classified by purpose into 8 main types as mentioned above.
2.Classification by Voltage Regulation Method
(1) Non-excitation voltage regulation rectifier transformers.
(2) On-load tap-changer rectifier transformers:
- Single-active-part on-load tap-changer rectifier transformers with unequal voltage steps;
- Dual-active-part on-load tap-changer rectifier transformers with equal voltage steps;
- Three-active-part on-load tap-changer rectifier transformers with autotransformer voltage regulation;
- Series-transformer on-load tap-changer rectifier transformers (i.e., with “figure-8” shaped low-voltage windings).
3.Classification by Rectifier Circuit Form
(1)Three-phase bridge rectifier transformers;
(2)Double-anti-star rectifier transformers with balancing reactors;
(3)Double-anti-star three-phase five-limb rectifier transformers.
The above three types can be further divided into six-pulse, nine-pulse, twelve-pulse, and eighteen-pulse rectifier transformers based on the equivalent pulse number.
4.Classification by Active Part Installation Method
(1) Active part with connected tank cover rectifier transformers
(2) Bell-jar type rectifier transformers
- Full bell-jar type rectifier transformers, whose structural form is similar to that of large power transformers.
- Half bell-jar type rectifier transformers, commonly used in medium and large-sized rectifier transformers with on-load voltage regulation and side outlet.
- Three-section bell-jar type rectifier transformers. For large-scale rectifier transformers with complex structures, a three-section bell-jar design is adopted to facilitate maintenance, cleaning, and disassembled transportation.
5.Classification by Core Structure Form
(1) Conjugate core rectifier transformers.
(2) Multi-core separated rectifier transformers.
6.Other classification methods
There are other classification methods, such as classification by phase number into single-phase and three-phase; by cooling medium into dry-type, oil-immersed; and by cooling method into ONAN, ONAF, OFWF, OFAF, ODWF, etc.
Differences Between Rectifier Transformers and Power Transformers
Rectifier transformers and power transformers differ significantly in function, design requirements, and application scenarios, particularly in terms of equivalent phase number (pulse number), output current calculation, terminology, voltage regulation purpose, and range. Below is a detailed comparison:
1. Equivalent Phase Number (Pulse Number) Requirements
Rectifier Transformer
- Core Function: Provides multi-phase AC input to rectifier systems (e.g., thyristor/diode bridges) to reduce DC output ripple.
- Pulse Number Design: Achieves multi-pulse rectification (e.g., 12-pulse, 24-pulse) via phase-shifted secondary windings (e.g., 30°, 15°). For example:
12-pulse requires two secondary windings (star + delta) with a 30° phase shift.
Higher pulse numbers further reduce harmonics for industrial DC power (e.g., electrolysis, electroplating).
- Harmonic Suppression: Multi-pulse designs reduce grid-side harmonic currents (e.g., 12-pulse eliminates 5th and 7th harmonics).
Power Transformer
- Standard Design: Typically three-phase (6-pulse) without phase-shifting, directly supplying AC loads or grids.
- Harmonic Handling: If harmonics are generated by loads, external filters are required; the transformer itself does not suppress harmonics via winding shifts.
2. Output Current Calculation Methods
Rectifier Transformer
- Valve-Side Current: Calculated based on DC load current (
) and rectifier circuit type. For example:
Three-phase bridge rectifier: Valve-side RMS current 
.
Overlap angle consideration: Actual current increases slightly due to commutation, requiring correction factors.
- Grid-Side Current: More complex due to rectifier power factor and harmonics.
Power Transformer
- Standard Calculation: Output current derived directly from load power (S) and voltage (U):
(three-phase).
- Load Characteristics: Current waveform is sinusoidal; no rectifier-related non-idealities.
3. Terminology Differences
Rectifier Transformer
- Valve-Side Voltage/Current: Secondary side connects to rectifier valves (e.g., thyristors), hence “valve-side”; primary side is “grid-side.”
- DC-Related Terms: Such as “equivalent DC voltage,” “ripple factor.”
Power Transformer
- Standard Terms: Primary side called “high-voltage side,” secondary side “low-voltage side.”
- AC Focus: Rated voltage, short-circuit impedance; no “valve-side” concept.
4. Voltage Regulation Purpose and Range
Rectifier Transformer
- Purpose:
Adapt to process needs (e.g., staged voltage adjustment in aluminum electrolysis).
Compensate for DC-side voltage drop (e.g., line losses under high current).
- Methods:
On-Load Tap Changer (OLTC): Frequent adjustments (e.g., ±10% range, 1.25% per step).
Phase-Shift Regulation: Adjusts winding taps to control DC output voltage.
Power Transformer
- Purpose:
Maintain grid voltage stability (e.g., ±5% range).
Adapt to seasonal load changes, rarely requiring frequent adjustments.
- Methods:
Off-Circuit Tap Changer: De-energized adjustments (e.g., ±2×2.5%).
OLTC: Used in critical substations but with fewer tap changes.
SCOTECH: Mastering Industrial Electrical Challenges
High – Precision Current Regulation
In electronic manufacturing processes, precise current control is imperative, with accuracy requirements often reaching the micro – ampere level. As the trend of product miniaturization progresses, the allowable deviation range for current in our production lines continues to narrow. To uphold consistent product quality, SCOTECH is committed to refining current regulation at every stage of the production process.
Our R & D team has conducted in – depth research into semiconductor – based current control technologies. We have mastered the subtleties of:
- Maintaining sub – milliampere current stability in parallel circuit configurations, thus ensuring uniform power delivery to delicate components.
- Managing thermal conditions under precise current loads, preventing the formation of micro – thermal gradients that could impair component performance.
- Specialized design protocols have been formulated for key components:
- Integrating high – precision current sensors into PCB (Printed Circuit Board) layouts to enable real – time monitoring.
- Employing low – resistance, high – reliability interconnects on the component side to preserve current integrity.
Custom – Designed Filters for Noise Mitigation
In contemporary industrial environments, electrical noise emanating from diverse sources (such as variable – frequency drives and wireless devices) penetrates production lines, disrupting sensitive electronic processes. Rectifiers and power conversion units are common sources of noise harmonics (including 3rd, 5th, and 7th orders, among others).
SCOTECH’s engineers collaborate closely with electronics specialists to map noise profiles and incorporate mitigation strategies into system designs. During the solution – devising process, we take into account noise – induced signal degradation and develop countermeasures:
- Utilizing filter windings composed of multi – layered, shielded copper coils to block noise propagation.
- Optimizing the geometries of grounding systems to establish low – impedance paths for diverting noise currents.
- Lining critical equipment enclosures with ferromagnetic shielding materials to attenuate external noise interference.
Flexible Signal – Shaping Configurations
For the construction of smart, adaptive production systems with diverse signal requirements, SCOTECH offers versatile signal – shaping solutions. These solutions enable the dynamic adjustment of signal waveforms to align with component testing and manufacturing needs.
- In analog – to – digital conversion setups, a 15° phase offset between dual – channel input signals can be achieved through customized op – amp (operational amplifier) circuit topologies, facilitating precise signal differentiation.
- Signal – shaping across multiple production modules can be implemented via PLC (Programmable Logic Controller) – driven phase adjustment or dedicated signal – conditioning modules featuring adjustable RC (resistor – capacitor) networks. This modular approach ensures that all production – line signal interfaces within a facility adhere to a unified and scalable design framework.
Tests
Routine tests
1. Measurement of winding direct resistance
2. Measurement of voltage ratio and check of phase displacement
3. Check of Voltage Ratio and Vector Group
4. Measurement of Impedance Voltage and Load Losses
5. Measurement of short-circuit impedance
6. Measurement of no-load loss and no-load current
7. Dielectric routine tests
8. Applied Voltage Test
9.Induced voltage test
9. Oil leakage test
Type tests
Temperature rise test
Lightning impulse withstand voltage test
Switching impulse withstand test (if required for HV side)
Determination of sound level
Measurement of no-load current harmonics
Special tests
Partial discharge measurements
Zero-sequence impedance measurement
Capacitance and dissipation factor (tan δ) measurement
Dielectric frequency response analysis (FRA)
Winding hot-spot temperature rise measurement
Load rejection test
Short-circuit withstand test
Seismic qualification test
Gas analysis of insulation (DGA)
* Any of the special test can be arranged on special requirement of customer.
Test report
• Complete IEC-compliant reports with optional FAT video or witness testing
