Parallel Operating Conditions of Power Transformers

The most ideal operating situation for parallel operation of power transformers is: when the transformers have been paralleled, but there is no load, there should be no circulating current between the transformers; at the same time, the transformers can distribute the load reasonably after the load is loaded. Share the load according to their respective capacity ratios. Therefore, in order to achieve the ideal operating conditions, the following conditions must be met when the transformers are operated in parallel:

1. The voltage ratio (transformation ratio) of each transformer should be the same;

2. The impedance voltage of each transformer should be equal;

3. The wiring group of each transformer should be the same.

The following analyzes the undesirable consequences when a certain condition in the parallel operation of the transformer is not met: 

Ⅰ. Parallel operation of transformers with different voltage ratios (transformation ratios):

Since the principles of a three-phase transformer and a single-phase transformer are the same, in order to facilitate analysis, two single-phase transformers are operated in parallel as an example for analysis. Since the primary voltages of the two transformers are equal and the voltage ratios are not equal, the induced potentials in the secondary windings are also not equal, and a potential difference Δe appears. Under the action of △e, circulating current ic appears in the secondary winding. When the rated capacity of the two transformers are equal, that is, sni=snii. The circulating current is:

ic=△e/(zdi＋zdii) where zdi——represents the internal impedance of the first transformer zdii——represents the internal impedance of the second transformer. If zd is represented by impedance voltage uzk, then zd=uzk*un/100in Where un represents the rated voltage (v), in represents the rated current (a) when the rated capacity of the two transformers are not equal, that is, sni≠snii, the circulating current ic is: ic=α*ii/[uzki＋(uzkii/β)" Where: uzki——represents the impedance voltage of the first transformer uzkii——represents the impedance voltage of the second transformer ini＜iniiα——the secondary voltage difference expressed as a percentage ii——the secondary load current of the transformer i

According to the above analysis, it can be known that in the case of a load, due to the existence of the circulating current ic, the current of the transformer winding with a small transformation ratio is increased, and the current of the transformer winding with a large transformation ratio is reduced. As a result, the parallel operation of transformers cannot share the load in proportion to the capacity. For example, when the total load current of the bus is i (i=ini+inii), if the transformer i runs at full load, then the transformer ii runs under load; if the transformer ii runs at full load, the transformer i runs over load. It can be seen that when transformers with unequal transformation ratios are operated in parallel, due to the existence of circulating current ic, the transformer cannot carry the full load, so that the total capacity cannot be fully utilized.

And because the circulating current of the transformer is not the load current, but it occupies the capacity of the transformer, so the output power is reduced and the loss is increased. When the transformation ratio differs greatly, the normal operation of the transformer may be damaged, or even the transformer may be damaged. In order to avoid the excessive circulating current ic caused by the excessive phase difference of the transformation ratio, which will affect the normal operation of the parallel transformer, it is stipulated that the phase difference of the transformation ratio should not be greater than 0.5.

Ⅱ. Parallel operation of transformers when the impedance voltage is unequal: 

Because the load distribution between transformers is proportional to its rated capacity and inversely proportional to the impedance voltage. That is to say, when the transformers are operated in parallel, if the impedance voltage is different, the load is not distributed in proportion to the rated capacity, and the current carried by the parallel transformer is inversely proportional to the impedance voltage, that is, ii/iii=uzkii/uzki or uzkiiii=uzkiiiii, Assuming that two transformers are operated in parallel, their capacity is sni, snii, and the impedance voltage is uzi, uzii, then the load of each transformer is calculated as follows:

si=［(snisnii)/(sni/uzkisnii/uzkii)》＊(sni/uzki)

sii=［(snisnii)/(sni/uzkisnii/uzkii)》＊(snii/uzkii)

That is, s△i/sii=(sni＊uzkii)/(snii＊uzki)

According to the above analysis, when two transformers with different impedance voltages are operated in parallel, the distributed load with a larger impedance voltage is small. When this transformer is fully loaded, the other transformer with a smaller impedance voltage will be overloaded. Long-term overload operation of the transformer is not allowed. Therefore, only the transformer with a large impedance voltage can be operated under load. This limits the total output power and increases the energy loss, which cannot guarantee the economic operation of the transformer. Therefore, in order to avoid that the load current of parallel transformers is seriously unevenly distributed due to the large difference in impedance voltage, which affects the capacity of the transformer and cannot be fully utilized, it is stipulated that the impedance voltage cannot be different by 10.

Ⅲ. Parallel operation of transformers with different wiring groups:

The wiring group of the transformer reflects the corresponding relationship between the high and low side voltages, and is generally represented by the clock method. When the voltage ratio of the parallel transformer is equal, the impedance voltage is equal, and the wiring group is different, it means that there is a phase angle difference α and a voltage difference △u between the secondary voltages of the two transformers, and a circulating current ic is generated under the action of the voltage difference. :

ic=△e/(zdizdii)

If the angle α is used to represent the angle between the line voltages of transformers with different winding groups, and zd is represented by uzk, the circulating current can be represented by the following formula:

ic=2u1sin(α/2)/(zdizdii)=200sin(α/2)/［uzk1/in1uzk2/in2》

If in1=in2=in and uzk1=uzk2=uzk, the above formula becomes:

ic=100sin(α/2)/uzk

In the formula, in, uzk can use any transformer rated current and impedance voltage.

Assuming that two transformers have equal transformation ratios and equal impedance voltages, and their wiring groups are y/y0-12 and y/△-11, it can be known from the wiring groups that when α=360°-330°=30°, uzk=(5～6)ic=100sin(α/2)/uzk gives ic=(4～5)in, which is 4～5 times the rated current when circulating current. Analysis shows that two transformers with different wiring groups are in parallel The circulating current caused by operation is sometimes equal to the rated current, but neither the differential protection nor the current quick-break protection can trip. When the overcurrent protection fails to trip in time, it will cause the transformer winding to overheat and even burn out.

From the above analysis, it can be seen that if the voltage ratio (transformation ratio) is not the same, the parallel operation of two transformers will produce circulating current, which will affect the output of the transformer. If the percent impedance is not equal, the load on the transformer cannot be distributed in proportion to the capacity of the transformer. The load on the transformer with small impedance is large, and the load on the transformer with large impedance is small, which also affects the output of the transformer. Parallel operation of transformers often encounters situations where the voltage ratio (transformation ratio) and percent impedance are not exactly the same. You can adjust the impedance value of the transformer by changing the transformer tap. If the third condition is not met, it will cause a circulating current equivalent to a short circuit, and even burn the transformer; therefore, transformers with different wiring groups cannot be operated in parallel. Under normal circumstances, if transformers with different wiring groups need to be operated in parallel, according to the differences in wiring groups, methods such as different names, start and end exchanges should be adopted to change the wiring of the transformer into the same wiring group. To run in parallel.

According to operating experience, the capacity ratio of two transformers should not exceed 3:1 in parallel. Because transformers of different capacities have large impedance values, the load distribution is extremely unbalanced; at the same time, from the perspective of operation, when the operation mode changes, overhauls, and accidental power outages, small-capacity transformers will not serve as a backup.