Procuring a power transformer is a significant capital investment that impacts operational efficiency, reliability, and total cost of ownership for decades. With numerous technical parameters and models available, specifying the right unit can be complex. A clear, actionable checklist is essential for buyers to communicate requirements effectively and evaluate suppliers accurately.

As a leading manufacturer with over a decade of experience and an annual production capacity exceeding 10,000,000 kVA, Xinda Transformer has supplied reliable transformers for diverse applications, from the State Grid to overseas industrial projects. Based on our extensive engineering and manufacturing expertise, we have compiled this essential checklist focusing on five critical specifications that form the core of any transformer procurement decision.

1. Rated Power (kVA/MVA) and Voltage Ratio (Primary/Secondary Voltage)

This is the fundamental starting point, defining the transformer’s core job: handling a specific amount of power at defined voltage levels.

• Rated Power (kVA/MVA):​ This is the apparent power the transformer is designed to continuously deliver without exceeding its temperature limits. It must be carefully calculated based on the connected load, factoring in present demand and future growth. Undersizing leads to overload and failure, while oversizing results in inefficient operation at low loads. Our portfolio, as listed on our websites, covers a comprehensive range from distribution units like the 10KV S11-M series​ to large power transformers like the 220KV Oil-immersed Power Transformer.
• Voltage Ratio:​ This specifies the input (primary) and output (secondary) voltage levels. Common distribution ratios include 10kV/0.4kV, 35kV/10.5kV, while transmission applications may require 110kV/35kV or 220kV/110kV. The correct ratio is vital for seamless grid or equipment integration. The availability of On-Load Tap Changers (OLTC), as featured in our SZ20 series, allows for voltage adjustment under load to compensate for grid fluctuations.

Procurement Question:​ What is your facility’s present and projected load? What are your specific primary (incoming) and secondary (outgoing) system voltages?

2. Performance Benchmark: Efficiency Level & Loss Values (No-Load & Load Losses)

In an era of high energy costs and sustainability goals, efficiency is paramount. This specification directly determines operational (energy) costs over the transformer’s 20-30+ year lifespan.

• Efficiency Standards:​ Transformers are classified by efficiency levels such as S9, S11, S13, S20, with higher numbers indicating superior efficiency and lower losses. Mandatory standards like China’s GB 20052​ and international norms drive the market towards higher tiers.
• Loss Values (W or kW):​ These are the concrete metrics:
  • No-Load Loss (P0):​ The constant loss when the transformer is energized but not supplying load. It depends on core design and material. Advanced designs like our S13-M.RL Series Three-Phase Wound Core Transformer​ and S(B)H15-M Amorphous Alloy Transformers​ offer significantly lower no-load losses.
  • Load Loss (Pk):​ The loss that varies with the square of the load, primarily from winding resistance. Optimized design and materials minimize this.

Procurement Question:​ What is the guaranteed no-load and load loss (in Watts) at the specified efficiency level (e.g., S20)? Can you provide a Total Owning Cost (TOC) analysis comparing different efficiency models?

3. System Stability: Impedance Voltage (Uk%)

Often overlooked by non-specialists, the impedance voltage is crucial for system protection and parallel operation.

• Definition:​ It represents the percentage of rated voltage required to circulate full-load current with the secondary winding short-circuited. It is a inherent design characteristic.
• Impact:​ A higher Uk%​ limits fault current, reducing stress on downstream equipment but can cause larger voltage drops under load. A lower Uk%​ provides better voltage regulation but allows higher fault currents. The value must be specified to match the system’s short-circuit capacity and protection coordination. Standard values are typically between 4% and 10% for distribution transformers.

Procurement Question:​ What is the designed impedance voltage percentage? Is it suitable for my system’s available fault current and voltage drop requirements?

4. Construction & Duty: Cooling Method and Insulation Class

These specifications determine the transformer’s physical design, installation environment, and overload capability.

• Cooling Method (e.g., ONAN, ONAF):​ Defined by IEC standards, it describes the cooling medium and circulation method.
  • ONAN (Oil Natural Air Natural):​ Common for smaller oil-immersed units, relying on natural oil and air convection.
  • ONAF (Oil Natural Air Forced):​ Uses fans to force air over radiators, increasing capacity, common in larger units like our 110KV Power Transformers.
  • For dry-type transformers, common designations are AN/AF​ (Air Natural/Air Forced).
• Insulation Class/Temperature Rise:​ This defines the maximum allowable temperature rise of the windings over ambient temperature (e.g., 55K, 60K, 65K for oil-immersed; 80K, 100K, 120K, 150K, 185K for dry-type like our SC(B)10 epoxy resin cast​ or SG(B)10 non-encapsulated​ series). A higher insulation class (e.g., Class H, 180°C) provides a greater thermal margin and potential overload capacity.

Procurement Question:​ What is the cooling designation and insulation temperature rise? Does this suit my installation environment (indoor/outdoor, ambient temperature, ventilation)?

5. Application-Specific & Special Requirements

This final category tailors the standard product to your unique operational needs, transforming it from an off-the-shelf item into a tailored solution.

• Special Tests:​ Beyond routine tests, you may require Type Tests​ (e.g., temperature rise, lightning impulse) or Special Tests​ (e.g., zero-sequence impedance, sound level). Our capability to develop a 66kV special test transformer​ demonstrates our expertise in meeting non-standard testing requirements.
• Environmental Conditions:​ Specifications for seismic withstand, corrosive atmosphere (C4, C5-M), altitude (>1000m), extreme temperatures, or special coatings must be defined.
• Accessories & Smart Features:​ Requirements for Online Monitoring Systems​ (DGA, temperature, partial discharge), protective devices (Buchholz relay, pressure relief device), or remote communication interfaces (IEC 61850).
• Standards & Certifications:​ Required compliance standards (IEC, IEEE, ANSI, GB) and specific certifications.

Procurement Question:​ Are there any special application conditions, mandatory tests, smart features, or certifications required for this project?

Conclusion: From Checklist to Confident Procurement

A precise technical specification sheet incorporating these five critical areas forms the foundation for obtaining accurate quotations, ensuring fair bid comparisons, and ultimately receiving a transformer that delivers decades of reliable, efficient service.

At Xinda Transformer, we leverage our complete in-house manufacturing chain​ (from core cutting and winding to final assembly and testing), technology-driven R&D​ (supported by 1 invention patent and 14 utility model patents, and partnerships with institutions like Shandong University), and proven project execution​ (evidenced by units like the S11-63000/110​ supplied to Shenxian Shuangyuan Electric Power) to translate your specifications into a reliable asset. Our diverse portfolio, from standard Oil-immersed​ and Dry-type Transformers​ to specialized Amorphous Alloy​ and Prefabricated Substations, ensures we have the right solution.

Ready to specify your next transformer with confidence?