What are the Five Key Metrics for Evaluating Transformer ?

Five Key Metrics for Evaluating Transformer Quality, based on international standards (IEC 60076) and industry engineering practices, primarily focus on transformer performance efficiency, safety reliability, and operational stability. Below is a systematic explanation of the five core metrics:

1. Rated Capacity

Refers to the apparent power that a transformer can continuously deliver over extended periods under rated voltage, rated frequency, and specified cooling conditions, measured in kVA or MVA.

Function: Determines the transformer's load-bearing capacity and serves as the primary basis for selection.

Selection Principle: Based on total load power (considering power factor and simultaneous factor), with a 15%–25% margin reserved to avoid overloading and heating (“small horse pulling a big cart”) or idle loss waste (“big horse pulling a small cart”).

Engineering Application: 35kV transformers typically range from 1 to 40 MVA, while photovoltaic step-up stations commonly use 400 to 1000 kVA units.

Product Parameter Examples: Deyan Electric's 35kV transformer has a rated capacity of 160 kVA; Xiangtong Electric's dry-type transformer is rated at 400 kVA.

2. No-Load Loss

Also known as iron loss, this refers to the active power loss when the transformer's secondary side is open-circuited and the primary side is subjected to rated voltage. It primarily consists of core hysteresis and eddy current losses.

Influencing Factors: Core material (e.g., cold-rolled grain-oriented silicon steel sheets), laminating process, magnetic flux density design.

Energy Efficiency Significance: A core metric for evaluating transformer energy efficiency. Ultra-premium efficiency products have no-load losses ≤ 0.6W/kg.

Typical Values: A 160kVA oil-immersed transformer has no-load losses around 280W; high-efficiency models can drop below 200W.

3. Load Loss

Also known as copper loss, this refers to the active power loss generated by winding resistance under rated current, proportional to the square of the load current.

Influencing Factors:

Conductor material (copper outperforms aluminum), cross-sectional area, and connection process.

Evaluation Value:

Directly impacts operating costs; lower losses yield better long-term energy efficiency.

Typical Values:

A 160kVA transformer has load losses around 2820W, while high-efficiency models can be controlled below 2500W.

Product parameter example: Deyan Electric transformer load loss is 2820W

4. Short-circuit Impedance (%)

Refers to the percentage of rated voltage required to apply rated current to the primary side when the secondary side is short-circuited, reflecting the magnitude of the transformer's internal resistance.

Functions:

Limits short-circuit current (higher impedance reduces short-circuit current);

Affects voltage regulation rate (higher impedance causes greater voltage drop during load fluctuations).

Typical Range:

Distribution transformers: 4%–6% (balancing stability and voltage drop);

Industrial heavy-duty (e.g., arc furnaces): 8%–15%;

When multiple units operate in parallel, impedance deviation must be ≤±0.5%.

Product Examples:

Huarui Electric: 4.5% impedance voltage

Xiangtong Electric: 4.0% impedance voltage

5. Insulation Performance

Covers insulation strength and aging status between windings and between windings and ground, forming the foundation for safe operation.

Core Testing Items:

Insulation Resistance: Measured with megohmmeter, reflects insulation dryness and aging degree;

Power Frequency Withstand Voltage: Withstands specified voltage (e.g., 75kV for 35kV windings) for 1 minute without breakdown;

Partial Discharge (PD): ≤3pC is optimal; exceeding this threshold indicates insulation degradation;

Insulation Class: Class A (105°C), Class F (155°C), Class H (180°C), determining temperature rise limits.

Standard Basis: Both GB/T 6451 and IEC 60076 mandatorily specify insulation withstand voltage and partial discharge limits.

Product Parameter Example: Jinghengjin-manufactured transformers have insulation level LI200AC85; Jinyu Electric's insulation class is H.

Supplementary Notes: Other Critical Auxiliary Indicators

Though not among the “Five Core Parameters,” these remain essential in engineering practice:

Temperature Rise: Top oil temperature rise ≤60K, winding temperature rise ≤65K (IEC 60076-2);

Connection Group‌: Dyn11 is the mainstream for distribution, suppressing third-harmonic distortion and ensuring stable operation under three-phase unbalanced loads;

Cooling Method‌: Oil-immersed natural cooling (ONAN) and dry-type air cooling (AN) affect heat dissipation efficiency and environmental applicability;