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Views: 0 Author: Site Editor Publish Time: 2025-12-23 Origin: Site
In modern power systems, whether it's the massive turbo-generators in thermal power plants, hydro-generators in hydropower stations, or synchronous motors driving industrial processes, the core essence lies in the precise control of the rotor's magnetic field. The Excitation Transformer is precisely the key piece of equipment that provides the accurate and reliable "energy source" for this core essence. It does not deliver power directly to the grid, yet it fundamentally determines the operational performance, efficiency of the synchronous machine, and the stability level of the entire power network.
The rotor of a synchronous machine requires direct current (DC) to establish a powerful magnetic field (i.e., "excitation"). The excitation system is responsible for providing and regulating this DC current. The excitation transformer has three core missions within this system:
Voltage Matching & Electrical Isolation: It takes the relatively high AC voltage from the generator terminals or station service bus (e.g., 10kV, 15.75kV), safely steps it down to a lower level suitable for the downstream thyristor rectifier (typically several hundred to a thousand volts), and provides necessary electrical isolation to ensure the safety of the control circuits.
Providing Rectifier Power Source: It supplies a stable and appropriate AC input power source to the downstream thyristor rectifier cabinet, forming the energy foundation for converting AC to the DC required by the rotor.
Supporting Field Forcing & Fast Response: When grid faults like short circuits cause a sudden voltage dip, the system needs to instantaneously (within milliseconds) and drastically increase the excitation voltage and current (i.e., "field forcing" or "strong excitation") to fully support the grid voltage and maintain synchronous stability. The excitation transformer must possess the capability to provide several times its rated value for short-term overloads, serving as the energy pillar for the field forcing function.
In short, the excitation transformer is the dedicated energy conversion and adaptation hub connecting the main power system with the precise rotor field control circuit.
The operating environment and purpose of excitation transformers result in significant design differences compared to standard distribution or power transformers:
Feature | Excitation Transformer | Standard Distribution Transformer |
Load Nature | Non-linear, Impact Load (thyristor rectifier), with severe current waveform distortion and very high harmonic content (especially 5th, 7th, 11th). | Primarily linear loads, with current waveforms close to sinusoidal. |
Operating Condition | Continuously endures additional heating and electromagnetic forces from harmonic currents; must frequently handle rapid load changes due to system regulation and field forcing demands. | Load is relatively stable, with slow variations. |
Design Focus | High short-circuit impedance, strong resistance to harmonic overheating, excellent mechanical strength and overload capacity. | Optimized for efficiency (low losses), meeting standard temperature rise and lifespan requirements. |
Electrical Performance | Impedance voltage is typically high (can reach 8%-20%) to limit short-circuit current and protect the thyristors. | Impedance voltage is lower (typically 4%-8%) to minimize voltage drop. |
Core Challenges Focus On:
Harmonic Overheating: Special designs and materials (e.g., transposed conductors, reduced eddy current losses) are essential to counteract overheating in windings and structural parts caused by high-frequency harmonics.
High Electromagnetic Forces: High short-circuit impedance and frequent current surges demand windings with extremely high mechanical clamping and short-circuit withstand capability.
Fast Transient Response: The design must ensure that magnetic field energy can be built up rapidly upon a field forcing command, and the magnetic core must not saturate.
The creation of a high-performance excitation transformer depends on the following key technical considerations:
Precise Design of Impedance Voltage:
Role: Limits the peak fault current during a rectifier bridge arm short circuit, protecting the expensive thyristor elements; influences the transient response speed of the excitation system.
Consideration: Requires finding the optimal balance between "limiting short-circuit current" and "avoiding impact on system response"; this is the primary core design parameter.
Reinforced Winding & Insulation System:
Winding: Often employs epoxy resin cast (dry-type) or oil-immersed construction. Dry-type offers fire/explosion prevention and easier maintenance, suitable for indoor or demanding environments. Oil-immersed provides superior heat dissipation and stronger overload capability, commonly used for large units.
Insulation: Must withstand the impact of high-frequency harmonic voltages; insulation material class and structural design must have ample margin.
Selection of Cooling Method:
Dry-type: Commonly Natural Air (AN) or Forced Air (AF) cooling.
Oil-immersed: Oil Natural Air Natural (ONAN) or Oil Forced Air Forced (OFAF). For large units, OFAF provides stronger cooling capacity to meet temperature rise requirements during field forcing.
Matching of Vector Group:
Must precisely match the pulse number of the rectifier bridge (e.g., 6-pulse, 12-pulse). For example, to form a 12-pulse rectifier for reduced harmonics, a three-winding transformer or a combination of two transformers (with extended delta and wye connections) is often used to generate two three-phase supplies with a 30-degree phase shift.
Key Selection Parameters Checklist:
Rated Capacity (kVA, must consider field forcing capacity)
Primary/Secondary Rated Voltage (kV / V)
Impedance Voltage (%)
Vector Group (e.g., D, y11, d0, etc.)
Insulation Class & Cooling Method
Installation Environment (Indoor/Outdoor, altitude, ambient temperature)
Large Thermal/Hydro/Nuclear Generator Units: Serves as the core power supply equipment for static excitation systems, the standard configuration for modern mainstream generators.
Pumped Storage Power Plant Units: Units operate in both generator (generating) and motor (pumping) modes, placing extremely high demands on the bidirectional adaptability and reliability of their excitation transformers.
Large Industrial Synchronous Motors: Used to drive heavy equipment like compressors, fans, and pumps, providing reactive power compensation and stabilizing the plant's internal grid.
Marine Electric Propulsion Systems: Provides stable excitation for propulsion synchronous motors, adapted to harsh maritime environments.
Although the excitation transformer remains hidden beside the generator or in the switchgear room, its performance directly affects whether a synchronous machine can generate stable voltage, maintain synchronism during grid disturbances, and the dynamic stability of the entire power system. It is the power-amplifying execution unit that converts the precise control signals from the automatic voltage regulator into powerful magnetic field force.
