Short circuit impedance of power transformers and its significance

Short circuit impedance of power transformers and its significance

1、 Definition of short-circuit impedance

The short-circuit impedance of a power transformer, also known as impedance voltage, refers to the voltage applied to the primary winding when the rated current flows through the primary winding during a short circuit (steady state) of the transformer's secondary winding. The ratio of this voltage to the rated voltage is usually expressed as a percentage, i.e. \ (uz=(\ frac {Uz} {U1n}) \ times100 \%). The short-circuit impedance is actually an equivalent impedance value of the transformer winding, which includes two parts: resistance and reactance.

2、 The significance of short-circuit impedance

1. Maintain the smooth operation of the power system: The short-circuit impedance parameters of transformers are affected by various factors, such as internal resistance, operating frequency, and ambient temperature. The essence of conducting short-circuit impedance testing is to ensure the ideal output power generated by the transformer under various normal operating conditions. When a short circuit occurs on the low-voltage side of a transformer, the magnitude of the short-circuit impedance will greatly determine the value of the short-circuit current, thereby directly affecting the overall stability of the power grid operation and preventing the further expansion of short-circuit accidents.

2. Reflecting the integrity and insulation performance of transformer windings: By accurately measuring parameters such as current and voltage involved in the short-circuit test process, the short-circuit impedance value can be calculated. The significant variation of this parameter can indicate possible deformation or aging of insulation materials in transformer windings, and therefore plays an important role in early identification of transformer winding defects and faults.

3. Important steps in factory inspection and operation maintenance: Short circuit impedance testing is an essential step in the factory inspection, routine maintenance, preventive testing, and handover testing of power transformers. This aims to ensure the optimal level of safety and reliability in the operation of power transformers.

4. Impact on parallel operation and load distribution of transformers: When multiple transformers operate in parallel, if their short-circuit impedances are not consistent, it will cause circulating current. In addition, the impedance voltage of the transformer can also affect load distribution, as the load is distributed inversely proportional to the rated capacity of the transformer.

5. Impact on transformer design and manufacturing: The percentage of short-circuit impedance of a transformer is an important parameter that reflects the internal impedance of the transformer. In the manufacturing process of transformers, it is necessary to consider the impact of short-circuit impedance on transformer performance. For example, increasing the impedance voltage of a transformer can reduce short-circuit current, but it has higher requirements for the manufacturing process; At the same time, an increase in impedance voltage can also lead to an increase in the amplitude of voltage changes on the load side of the transformer when the load changes, affecting the stability of the voltage.

In summary, the short-circuit impedance of power transformers is an important performance indicator, which not only relates to the safe and stable operation of transformers, but also affects multiple aspects such as transformer design, manufacturing, testing, and parallel operation.