In the operation of the substation, the main purpose of the selection and application of station transformers is to ensure that the output voltages of the station transformers and backup transformers in the substation connected to the grid are consistent, so as to provide a guarantee for the safe and stable operation of the power system.
Combined with the current transformer types used in power system substation stations, Dyn and Yyn connection types are the main ones. And the zero-sequence impedance of the transformer of the first connection type is much lower than that of the second connection type.
Among them, the single-phase short-circuit current in the operation of the transformer will increase and change, and the difference between the three-phase short-circuit current and the three-phase short-circuit current will be significantly reduced.
This means that in the station transformer system using the first connection type, the protection action sensitivity of the system protection equipment under the problem of single-phase short-circuit faults is greatly improved, and at the same time, the system protection equipment and the circuit feeder line cross-section selection and coordination are greater.
In addition, the simplified setting and application of the system protection structure and form can also be realized, without the need for separate settings and application of single-phase short-circuit protection. In the high-voltage side, overcurrent protection of the power system and the low-voltage side single-phase short-circuit protection, both have more advantages.
Dyn type connection transformer equipment, its internal delta winding arrangement creates a current path for the third harmonic current or zero sequences current in the transformer connection line, and at the same time makes the phase voltage in the line closer to the sine wave The voltage change form has a more prominent effect and effect on the improvement of the line voltage waveform quality.
In addition, in this type of connection type transformer equipment, if there is a three-phase load imbalance on the low-voltage side of the transformer connection line, the floating displacement of the low-voltage side neutral point in the transformer equipment will not change, thereby supplying power to the grid. The stability of it has a greater guarantee.
Therefore, in actual design and application, D, yn type connection transformers are the main choice for substation station transformers.
In addition, the impedance calculation of the D, yn type connecting transformer in the operation of the substation is generally determined based on the current-carrying capacity of the low-voltage electrical appliances in the substation system under short-circuit faults. According to this situation, the impedance of the transformer is used in the substation station. In the selection and design applications, ordinary transformers designed with standard impedance are generally used as transformer equipment for substation stations for selection and application.
Take a transformer for a 220kv substation station as an example. The transformer is a dry-type transformer with forced air cooling and a protective enclosure.
After determining its technical parameters, including the operating frequency of the transformer, no-load current, insulation grade, withstand voltage, connection method, operating environment, etc., the capacity of the transformer can be calculated according to the comprehensive situation of each load in the substation Determine, and select the application in the design of the substation.
Power Transformer and Distribution Transformer