I. Core Calculation Formulas

　　Basic Impedance Formula

　　Calculate transformer impedance based on the short-circuit impedance percentage:

　　ZT=uk%×SN.TU2N2

　　Where:

　　ZT = Transformer impedance (Ω);

　　uk% = Short-circuit impedance percentage (rated value from nameplate or measured value);

　　U2N = Secondary-side rated voltage (kV);

　　SN.T = Rated capacity (kVA).

　　Resistance and Reactance Decomposition

　　Calculate resistance using short-circuit loss:

　　RT=ΔPN×SN.T2U2N2

　　Calculate reactance using the vector relationship between impedance and resistance:

　　XT=ZT2−RT2

　　Where ΔPN = Short-circuit loss (kW).

　　II. Key Parameter Acquisition

　　Short-Circuit Impedance Percentage (uk%)

　　Prioritize the value stated on the transformer nameplate;

　　If unavailable, measure via short-circuit test: Apply a low voltage to the primary side to achieve rated current on the secondary side, then measure the ratio of input voltage to rated voltage.

　　Rated Capacity and Voltage

　　Rated capacity SN.T is directly obtained from the nameplate;

　　Secondary-side rated voltage U2N is determined based on system voltage levels (e.g., 6kV, 10kV).

　　III. Voltage Level Conversion

　　High-Voltage Side and Low-Voltage Side Impedance Conversion

　　When the system voltage differs from the transformer's rated voltage, convert impedance using the square ratio of voltages:

　　ZActual=(UActualURated)2×ZRated

　　Example: Converting impedance from a 6000V rated value to a 690V system:

　　Z690=(6906000)2×Z6000IV. Zero-Sequence Impedance Calculation (Special Scenarios)

　　Impact of Winding Structure

　　Three-phase five-limb core structures with D,yn11 winding configurations suppress third harmonics. Their zero-sequence magnetic flux paths differ from positive-sequence paths;

　　Calculate zero-sequence reactance using finite element methods or the Stanford-MIT method, considering winding spacing and core environment.

　　Design of Zero-Sequence Protection Components

　　The zero-sequence reactance value directly influences the selection of zero-sequence protection components, ensuring they can withstand the system's maximum zero-sequence current.

　　V. Engineering Application Considerations

　　Short-Circuit Current Calculation

　　Use impedance values to calculate the secondary-side short-circuit current of the transformer:

　　Isc=3×(ZT+ZLine)U2N

　　Requires comprehensive analysis incorporating line impedance.

　　Parallel Operation Requirements

　　When multiple transformers operate in parallel, the impedance voltage deviation should be ≤±10% to avoid uneven load distribution.

　　Compliance with Standards and Specifications

　　Mining transformers must comply with GB/T 1094 "Power Transformers" standards;

　　Typical impedance voltage values: 4%-5.5% for 6-10kV class, 6.5%-8% for 35kV class.