Application of Transformer Capacity and Its Impedance Voltage Relationship

   I. Introduction

　　The rated capacity of a transformer and its corresponding impedance voltage are subject to relevant requirements in GB1094.1, GB1094.5, and GB6451, which are mandatory standards. The impedance voltage values measured by transformer manufacturers at the time of factory delivery are all within the allowable deviations specified by the national standards.

　　Most cities in China supply power to users via 10KV power sources, which are stepped down to 380V (220V) by 10KV transformers (distribution transformers) to supply user loads. Therefore, the number of transformers in each city is essentially these distribution transformers. When load tests were conducted on these distribution transformers in a certain city, it was found that the impedance voltage deviations of a small portion of transformers exceeded the allowable range (the allowable deviation for distribution transformers is ≤±10%), especially for transformers of some small and medium-sized enterprises located in remote areas.Transformer ratio group tester is suitablefor testing the ratio and phase difference of special transformers and for measurement work on rectifier transformers with phase shifting.

　　During data analysis, it was found that most of the measured impedance voltage values were lower than expected, which was not accidental. Through further testing, it was discovered that the rated capacity on the transformer nameplate differed from the actual capacity, and in most cases, the actual capacity was one level higher. For example, a transformer with a nameplate capacity of 400KVA actually had a capacity of 500KVA. During the load test, 400KVA was input as a known quantity into the tester, but the actual capacity was 500KVA, resulting in a lower measured impedance voltage. This situation is difficult to detect without a load test (distribution transformers are not required to undergo load tests during handover tests).

　　These enterprise users are mostly large industrial users, so this directly results in a reduction in basic electricity charges, meaning the power supply department collects less electricity fees. In response to this situation, based on the corresponding relationship between the rated capacity and impedance voltage of transformers, a simple and portable transformer parameter tester can be used at the test site to conduct load tests on transformers. By analyzing the measured impedance voltage values, it is possible to preliminarily determine whether the nameplate capacity matches the actual capacity. Further tests (such as the direct load method) are needed to determine the actual output capacity of the transformer. This method is simple and easy to implement, and can be promoted and applied in power supply departments and power installation enterprises. It can be used to inspect distribution transformers in operation and to strictly enforce the handover test standards during installation, thereby recovering a portion of electricity fees for the power supply department and the state, resulting in good economic benefits.

　　II. Physical Significance and Measurement of Impedance Voltage

　　1. Physical Significance of Impedance Voltage

　　Impedance voltage is the percentage ratio of the voltage applied to the primary winding (short-circuit voltage) to the rated voltage when the secondary winding is short-circuited and the primary voltage is gradually increased until the short-circuit current in the secondary winding reaches the rated current. The impedance voltage Uk (%) is an important economic indicator related to transformer cost, efficiency, and operation, and is one of the main parameter bases for transformer condition diagnosis.

　　For transformers of the same capacity, a lower impedance voltage means lower cost, higher efficiency, and cheaper price. Additionally, the voltage drop and voltage regulation rate during operation are also smaller, making voltage quality easier to control and ensure. Therefore, from the perspective of grid operation, a lower impedance voltage is preferable. However, from the perspective of limiting short-circuit current, a higher impedance voltage is preferable to prevent electrical equipment (such as circuit breakers, disconnectors, cables, etc.) from being damaged by short-circuit currents during operation. The national standards specify the corresponding impedance voltage values for transformers of different capacities, but large-capacity transformers and substation transformers are not within the scope of this article. This article focuses on the large number of 10KV (and below) user transformers.

　　2. Measurement of Impedance Voltage

　　In actual field conditions, impedance voltage can be measured by conducting a load (short-circuit) test on the transformer using a transformer parameter tester. The load test must be performed at the rated frequency (sinusoidal waveform) and with the rated current applied to the windings. Generally, the primary winding of the transformer is selected as the test winding, and the secondary side (high-current side) is artificially short-circuited. When an AC voltage of rated frequency is applied to the primary side (at the rated voltage tap) so that the current in the transformer windings reaches the rated value, the applied voltage and power are measured. Note that the cross-sectional area of the connecting plate (cable) used for the secondary short-circuit connection must be sufficiently large, not less than the cross-sectional area of the transformer conductor, and its length should be as short as possible to prevent measurement inaccuracies caused by high resistance in the connecting plate.

　　The measured voltage as a percentage of the rated voltage of the energized winding is the impedance voltage. The measured active power converted to the value at rated temperature is the load (short-circuit) loss, which is also a very important parameter but is not within the scope of this article.

　　When using a transformer parameter tester for measurement, the transformer capacity is treated as a known quantity, and typically the rated capacity from the nameplate of the transformer under test is input into the tester.

　　III. Relationship between Impedance Voltage and Transformer Capacity

　　The impedance voltage of small and medium-capacity transformers is specified in GB1094.1, GB1094.5, and GB6451. Among them, the corresponding relationship between the rated capacity and impedance voltage of 10KV voltage class transformers is summarized in Table 1.

　　Table 1 Relationship between transformer capacity and impedance voltage

　　When the impedance voltage value is <10%, the allowable deviation is ±10%. The relationship between impedance voltage and transformer capacity