Transformer parallel connection

Same impedance, same wiring configuration, same turns ratio; it is best to use the same brand and capacity. Otherwise, energy will be wasted, and the transformer may get damaged as a result. . . I’m not aware of anything more complex than that; I’ll wait for an expert to answer@

Conditions for parallel operation of transformers: Transformers are important electrical devices in power grids. Due to their long periods of continuous operation, in order to ensure their safe and economical operation as well as to enhance the reliability and flexibility of power supply, two or more transformers are usually operated in parallel. Parallel operation of transformers involves connecting the primary windings of two or more transformers in parallel to a busbar at the same voltage, while their secondary windings are connected in parallel to another busbar at a different voltage. Its meaning is that when one transformer fails, the other transformers operating in parallel can continue to function, thereby ensuring power supply for important users ； Or, when a transformer needs maintenance, a spare transformer can be connected in parallel first, after which the transformer under maintenance can be powered off for repairs. This approach ensures that the transformer can be maintained as planned while also preventing any disruption to power supply, thereby improving the reliability of power delivery. Furthermore, due to the strong seasonal variation in electricity demand, some transformers can be taken out of service during periods of low demand. This not only reduces the no-load losses of the transformers and improves efficiency, but also decreases the reactive excitation current, thereby improving the power factor of the power grid and enhancing the economic viability of the system. The ideal operating condition for parallel operation of transformers is that, once the transformers are connected in parallel but before any load is applied, there should be no circulating current between them ； When loaded simultaneously, the various transformers are able to distribute the load reasonably, that is, they should share the load in proportion to their respective capacities. Therefore, to achieve optimal operation, the following conditions must be met when transformers are operated in parallel: (1) The voltage ratio (turns ratio) of each transformer should be the same; (2) The impedance voltage of each transformer should be equal; (3) The wiring configuration of each transformer should be the same. The following analyzes the adverse consequences that occur when one of the conditions for parallel operation of transformers is not met: (1) Parallel operation of transformers with different voltage ratios: Since the principles behind three-phase transformers and single-phase transformers are the same, for ease of analysis, we will use the example of two single-phase transformers operating in parallel. Since the primary voltages of the two transformers are equal, but their voltage ratios are not, the induced electromotive forces in the secondary windings are also unequal, resulting in a potential difference ΔE. Under the action of △E, a circulating current IC appears in the secondary winding. When the rated capacities of the two transformers are equal, that is, SNI=SNII. The circulating current is given by: IC = ΔE / (ZdI + ZdII), where ZdI represents the internal impedance of the first transformer, and ZdII represents the internal impedance of the second transformer. If Zd is expressed in terms of the impedance voltage UZK, then Zd = UZK * UN / 100IN, where UN represents the rated voltage in volts and IN represents the rated current in amperes. When the rated capacities of the two transformers are not equal, that is, SNI ≠ SNII, the circulating current IC is given by: IC = á * II / (UZKI + UZKII), where UZKI represents the impedance voltage of the first transformer, UZKII represents the impedance voltage of the second transformer, INI < INII, and á represents the percentage difference in secondary voltages. II represents the load current on the secondary side of transformer I. From the above analysis, it can be seen that under load conditions, due to the presence of the circulating current Ic, the current in the windings of the transformer with a smaller turns ratio increases, while the current in the windings of the transformer with a larger turns ratio decreases. As a result, transformers operating in parallel cannot share the load in proportion to their capacity. If the total load current on the busbar is I (I = INI + INII), then when transformer I operates at full load, transformer II operates under partial load ； If transformer II operates at full load, then transformer I operates under overload. It can be seen that when transformers with unequal turns ratios are operated in parallel, due to the presence of circulating current Ic, the transformers cannot operate at full load, resulting in the total capacity not being utilized fully. Furthermore, since the circulating current in the transformer is not the load current, it still occupies the transformer’s capacity, thereby reducing the output power and increasing losses. When the turns ratio difference is large, it may disrupt the normal operation of the transformer and even cause damage to it. To prevent excessive circulating current Ic resulting from a large difference in turns ratio, which could affect the proper operation of paralleled transformers, it is specified that the difference in turns ratio should not exceed 0.5%. (2) Parallel operation of transformers with different impedance voltages: Since the load distribution among transformers is proportional to their rated capacity and inversely proportional to their impedance voltages. In other words, when transformers are operated in parallel, if their impedance voltages differ, the load is not distributed proportionally to their rated capacities. The current carried by each transformer in parallel is inversely proportional to its impedance voltage, that is, II/III = UZKII/UZKI or UZKIIIII = UZKIIIII. Suppose two transformers are operating in parallel, with capacities of SNI and SNII respectively, and impedance voltages of UZI and UZII. Then the load on each transformer can be calculated using the following formulas: SI = * (SNI/UZKI) and SII = * (SNII/UZKII). Thus, S△I/SII = (SNI * UZKII) / (SNII * UZKI). From this analysis, it can be seen that when two transformers with different impedance voltages operate in parallel, the transformer with the higher impedance voltage carries a smaller load; when this transformer is operating at full capacity, the transformer with the lower impedance voltage will operate under overload conditions. It is not permissible for transformers to operate under prolonged overload conditions; therefore, only those transformers with high impedance voltage can be operated at underload. This limits the total output power, increases energy losses, and as a result, it is not possible to ensure the efficient operation of the transformers. Therefore, to prevent severe uneven distribution of the load current in parallel transformers due to large differences in impedance voltage, which would prevent the transformers from achieving their full capacity, it is specified that the impedance voltage differences must not exceed 10%. (III) Parallel operation of transformers with different connection groups: The connection group of a transformer indicates the relationship between the high-voltage and low-voltage sides, and it is generally represented using the clock method. When the voltage ratios of parallel transformers are equal and their impedance voltages are also equal, but their connection groups differ, it means that there is a phase angle difference α and a voltage difference ΔU between the secondary voltages of the two transformers. Due to this voltage difference, a circulating current Ic is generated: Ic = ΔE / (ZdI + ZdII). If the angle α represents the angle between the line voltages of transformers with different winding connections, and Zd is denoted by UZK, then the circulating current can be expressed as follows: Ic = 2U1sin(α/2) / (ZdI + ZdII) = 200sin(α/2) / … If In1 = In2 = In and UZK1 = UZK2 = UZK, then the above formula becomes Ic = 100sin(α/2) / UZK. Here, In and UZK can be replaced by the rated current and impedance voltage of either transformer. Assuming that the turns ratios of the two transformers are equal and their impedance voltages are also equal, with their connection types being Y/Y0-12 and Y/△-11 respectively, it can be seen from these connection types that when á=360°–330°=30°, UZK%=(5–6)%Ic=100sin(á/2)/UZK; hence IC=(4–5)In, meaning that the circulating current is 4–5 times the rated current. Analysis shows that when two transformers with different connection types are operated in parallel, the circulating current can sometimes be as large as the rated current. In such cases, neither the differential protection nor the instantaneous current overload protection will trigger to shut down the transformers, and if the overcurrent protection fails to act in time, it can lead to overheating of the transformer windings, or even their destruction. From the above analysis, it can be seen that if the voltage ratios (turn ratios) are different, parallel operation of the two transformers will generate circulating currents, affecting the output of the transformers. If the percent impedances are not equal, the load carried by the transformers cannot be distributed proportionally to their capacity; the transformer with a lower impedance carries a greater load, while the one with a higher impedance carries a smaller load, which also affects the output of the transformers. When transformers are operated in parallel, it is common for the voltage ratio (turns ratio) and percent impedance to not be exactly the same; the impedance value of the transformers can be adjusted by changing their tap positions. If the third condition is not met, it will cause a circulating current equivalent to a short circuit, which may even burn out the transformer ； Therefore, transformers with different wiring groups cannot operate in parallel. Under normal circumstances, if transformers with different winding connections are to be operated in parallel, methods such as swapping the names of the phases and the starting and ending points must be employed, depending on the differences in their winding connections, in order to make the winding connections identical and thus enable parallel operation. Based on operational experience, when two transformers are connected in parallel, their capacity ratio should not exceed 3:1. Because transformers of different capacities have high impedance values, the load distribution is extremely uneven ； From an operational perspective as well, when the operating mode changes, during maintenance, or in the event of an outage due to an accident, transformers with small capacity will not be able to serve as a backup.

1. The short-circuit impedance is the same. 2. The wiring connection type is the same. 3. The capacity ratio of the two transformers operating in parallel is greater than 1:3. 4. They come from the same power source

Transformer parallel operation: In this mode of operation, the primary and secondary sides of two or more transformers are connected to each other through the same busbars at terminals with the same polarity; this is what is known as parallel operation of transformers. 　　Conditions that must be met for parallel operation of transformers: 1. The transformer connection groups must be the same. 　　2. The transformation ratios of the transformers are the same (a difference of ±0.5% is allowed), which means that their rated voltages are equal. 　　These two conditions ensure that no circulating current exists in the windings of the transformer when it is unloaded. The presence of such circulating currents can affect the proper utilization of the transformer’s capacity; if the circulating current is several times the rated current, it can even damage the transformer. 　　3. The short-circuit voltages of the transformers must be equal (with a permissible difference of ±10%); this condition ensures that the load distribution is proportional to the capacity. 　　4. The capacity ratio of parallel transformers should not exceed 3:1, which helps to prevent large differences in their short-circuit voltage values. 　　Two power transformers manufactured in the same factory and of the same model must meet the following conditions in order to be operated in parallel: they must have the same voltage level, the same wiring configuration, and the same impedance voltage. These are the essential requirements for parallel operation of two power transformers.

1. The short-circuit impedance is the same. 2. The wiring connection type is the same. 3. The capacity ratio of the two transformers operating in parallel is greater than 1:3. 4. They come from the same power source