In modern power systems, the 35kV main transformer plays a crucial role as the core equipment connecting medium-voltage distribution systems with high-voltage transmission networks. It not only performs voltage level conversion but also ensures the stability and safety of power transmission. This article provides a comprehensive explanation of the structure, working principle, and technical advantages of 35kV main transformers, helping engineers gain deeper insights into this essential equipment.
1. What Is a 35kV Main Transformer?
A 35kV main transformer is a medium-to-high voltage transformer used in 35kV systems. It is commonly applied in urban and rural substations, industrial parks, wind and solar power stations, and more. Its main function is to convert high-voltage power into medium-voltage levels suitable for end users or further step-down transformation, while also providing isolation and voltage regulation capabilities.
2. Core Structure of a 35kV Main Transformer
35kV main transformers are typically classified as either dry-type transformers or oil-immersed transformers based on their cooling methods. However, the core structure remains largely the same, including the following key components:
Core
The magnetic core is the main part of the transformer’s magnetic circuit. It is usually made of laminated high-quality silicon steel sheets to enhance magnetic permeability and reduce core loss. Most 35kV transformers adopt a three-phase, three-limb or five-limb structure, which helps reduce magnetic flux leakage and improve efficiency.
Windings
The windings include high-voltage and low-voltage coils, typically made of copper or aluminum conductors, wrapped with insulation material. The high-voltage side connects to the 35kV grid, while the low-voltage side usually outputs 10kV or 6kV. The winding arrangement (e.g., concentric or interleaved) directly affects the transformer's short-circuit strength and cooling performance.
Insulation System
A robust insulation design is key to long-term stable operation. Dry-type transformers usually use epoxy resin casting, while oil-immersed types rely on transformer oil and laminated insulation paper for both electrical insulation and thermal management.
Tank and Cooling System (For Oil-Immersed Transformers)
Oil-immersed transformers typically come with corrugated tanks or radiators to facilitate oil circulation and heat dissipation. Advanced models may feature forced oil-air cooling systems (OFAF) for enhanced thermal performance.
5. Tap Changer
To adapt to grid voltage fluctuations, main transformers are often equipped with on-load tap changers (OLTC), which allow adjustment of the high-voltage side while the transformer is energized to maintain stable output.
6. Protection Devices
Common protection features include temperature monitoring, oil level sensors, gas relays (such as Buchholz relays), and overload protection, ensuring safe operation at all times.
3. Working Principle of a 35kV Main Transformer
The transformer operates based on the principle of electromagnetic induction. When alternating current flows through the primary winding, it creates alternating magnetic flux, which is transmitted through the core to the secondary winding, where it induces a corresponding voltage.
The voltage conversion ratio is determined by the turns ratio between the primary and secondary windings:
U1 / U2 = N1 / N2
Where U represents voltage and N represents the number of turns. The turns ratio is fixed, so output voltage varies proportionally with input voltage. By adjusting N1 using a tap changer, small voltage adjustments can be made.
4. Typical Application Scenarios
35kV main transformers are commonly used in the following scenarios:
First, in urban main substations, serving as key nodes for power distribution.
Second, in wind and solar power step-up stations for renewable energy integration.
Third, in large industrial load centers such as metallurgy, aluminum smelting, and steel manufacturing plants.
Fourth, in railway electrification substations to provide power to electric trains.
Finally, in petrochemical and high-demand power parks where reliability is critical.
5. Selection and Maintenance Tips
When selecting a 35kV main transformer, consider key parameters such as capacity (typically 1000kVA to 10000kVA), load type, cooling method (AN/AF or ONAN/OFAF), insulation class, and impedance voltage. During operation, regularly monitor winding temperature, oil quality, insulation resistance, and partial discharge levels to ensure optimal performance.
As a vital hub in power transmission and distribution systems, the scientific structure and reliable operation of 35kV main transformers are essential for ensuring grid safety and efficiency. Whether in the design phase or during long-term maintenance, understanding its structure and working principle is crucial to maximizing lifespan and performance.
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