The Calculation of Parallel Operation Conditions And Load Distribution of Transformers Is Explained Very Clearly And Thoroughly

What is parallel operation of transformers? Presumably most electrical people are very clear. But there are still some electricians who are still ignorant. The simple understanding of parallel operation of transformers is that if a transformer has insufficient capacity, it can be operated in parallel (parallel operation) with other transformers to increase the power supply capacity. However, the transformers cannot simply run in parallel, and must meet three conditions at the same time: ① the same transformation ratio; ② the same connection group (that is, the same phase); ③ the same short-circuit impedance.

Its meaning is that when one transformer fails, other transformers running in parallel can continue to operate to ensure the power consumption of important users; or when the transformer needs to be repaired, the standby transformer can be connected in parallel, and then the transformer to be repaired can be powered off. Maintenance can not only ensure the planned maintenance of the transformer, but also ensure that the power supply is not interrupted and improve the reliability of the power supply. In addition, due to the strong seasonality of the electricity load, some transformers can be withdrawn from operation in the light-loaded vehicle season, which can not only reduce the no-load loss of the transformer, improve the efficiency, but also reduce the reactive excitation current and improve the power factor of the grid. Improve the safety and economy of system operation. Mr. Di, the online teaching of electrical design, the following article will share with you the calculation of parallel operation conditions and load distribution of transformers, hoping to be helpful to the majority of electrical personnel.

(1) Conditions for parallel operation of transformers:
1. Equal ratio
Different transformer ratios and different secondary voltages will also generate circulating currents in the secondary windings, which will occupy the capacity of the transformer and increase the loss of the transformer. The difference should not exceed ±0.5% at most.

2. The serial number of the connection group must be the same
Different wiring groups will cause a voltage difference in the secondary winding of the parallel transformer, and a circulating current will be generated inside the secondary side of the transformer.

3. The capacity ratio of the two transformers should not exceed 3:1
Transformers with different capacities have different short-circuit voltages, unbalanced load distribution, and uneconomical operation.

4. The short-circuit voltage is the same
The description about the same short-circuit voltage requirement is actually very close, because the test value often has a certain deviation from the design theoretical value, and the test value written on the nameplate is the actual value.
If the short-circuit voltage difference is too large, it will lead to overload phenomenon when the short-circuit voltage is small. It is recommended that the allowable difference generally does not exceed 10%. As to why, see the load distribution calculation for parallel operation of transformers at the end of the article.

(2) The impedance voltage and short-circuit voltage of the transformer:

1) The impedance voltage of the transformer
The impedance voltage percentage of a transformer consists of the reactive voltage drop and the resistive voltage drop. The value is equal to the impedance percentage of the transformer, indicating the size of the impedance in the transformer. The impedance voltage percentage indicates the magnitude of the impedance voltage drop of the transformer itself when the transformer is operating at full load (rated load).

It is of decisive significance to the magnitude of the short-circuit current that will be generated when the transformer is short-circuited on the secondary side. It is also of great significance to the manufacturing price of the transformer and the parallel operation of the transformer. It is also important to consider the thermal stability and dynamic stability of the short-circuit current and the setting of the relay protection. in accordance with. This value complies with national standards in transformer design.
The magnitude of the impedance voltage percentage is related to the capacity of the transformer. Generally, the larger the capacity of the transformer, the greater the short-circuit impedance (generally). For power transformers produced in my country, the impedance voltage percentage is generally in the range of 4% to 24%.

2) The short-circuit voltage of the transformer
The percentage of short-circuit voltage of a transformer is the voltage when one side of the transformer is short-circuited and the other side is supplied with rated current, and this voltage accounts for a percentage of its rated voltage. In fact, this voltage is the voltage drop of the leakage reactance on the live side and the short-circuit side of the transformer at the rated current. The larger the reactance of the transformer of the same capacity, the greater the percentage of short-circuit voltage. The same current passes through, the transformer with large reactance will produce greater voltage loss, so the rate of change of reactance of the transformer with a greater percentage of short-circuit voltage is also greater. . So the percentage of short-circuit voltage = the percentage of impedance voltage (sometimes referred to as the percentage of short-circuit impedance)

(3) Selection of transformer short-circuit impedance:

The short-circuit impedance of the transformer has its own advantages and disadvantages. If you choose a larger one, when the load end of the transformer is short-circuited, the short-circuit current will be smaller, the short-circuit force on the transformer will be smaller, and the damage will be relatively smaller. But usually the line voltage drop will increase, the line loss will increase, the heat generation will increase, and sometimes the tap switch cannot even be adjusted, so that the equipment cannot obtain the appropriate voltage, thus affecting the normal operation of the equipment.

On the other hand, if the short-circuit impedance is large, the geometric size of the product will increase relatively, that is, the material will increase, and the manufacturing cost will increase. If it is too small, the short-circuit current will be large, and the short-circuit force on the transformer will be large. In order to prevent damage to the equipment, the short-circuit capacity must be increased in the selection of equipment, which is not economical.
Therefore, when selecting the value of the short-circuit impedance of the transformer, it is necessary to consider comprehensively, comprehensively, and comprehensively. Important thing to say 3 times because I don’t understand

(3) Calculation of load distribution for parallel operation of transformers:
Under the premise that the transformers are allowed to run in parallel, when the transformers are running in parallel (usually when multiple circuits are put into parallel operation of the transformers at the same time), what is the relationship between the load distribution of each transformer? Of course, it is not the transformation ratio, because the transformation ratios of the transformers running in parallel are the same. , which is the capacity? Short circuit impedance? Rated current? No-load current? Iron loss?

Under normal circumstances, in fact, the load distribution value of the parallel operation of the transformer is only related to the short-circuit impedance of the transformer (the short-circuit impedance is generally proportional to the transformer capacity), and the load distribution value of the parallel operation of the transformer is inversely proportional to the short-circuit impedance of the transformer.