Transformer short-circuit impedance, also known as impedance voltage, is defined in the transformer industry as follows: when the secondary winding of the transformer is short-circuit (steady state), the voltage applied by the rated current of the primary winding is called impedance voltage Uz. Uz is usually expressed as a percentage of the rated voltage, i.e. uz= (Uz/U1n)*100%
When the transformer is fully loaded, the short-circuit impedance has a certain influence on the output voltage of the secondary side. The short-circuit impedance is small, the voltage drop is small, the short-circuit impedance is large and the voltage drop is large. When the transformer load is short-circuit, the short-circuit impedance is small, the short-circuit current is large, and the transformer bears large electric power. The short-circuit impedance is large, the short-circuit current is small, and the transformer bears less electric power.
(1) Transformers with different voltage ratios operate in parallel:
Because the principle of three-phase transformer and single-phase transformer is the same, in order to facilitate analysis, two single-phase transformers parallel operation is taken as an example to analyze. Because the primary voltage of the two transformers is equal and the voltage ratio is not equal, the induced potential in the secondary winding is not equal, so the potential difference (E) appears. Under the action of Delta E, circulating current IC appears in the secondary winding. When the rated capacity of two transformers is equal, that is, SNI = SNII. The circulating current is:
ZdI in Formula 1 - Indicates 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
In the formula, UN denotes rated voltage (V), IN denotes rated current (A)
When the rated capacities of the two transformers are not equal, that is, SNI_SNII, the circulating current IC is:
IC = II/[UZKI+ (UZKII/?)]
UZKI - Indicates the impedance voltage of the first transformer
UZKII -- Represents the impedance voltage of the second transformer
- Secondary voltage difference expressed in percentage
II--Subside Load Current of Transformer I
According to the above analysis, under the condition of load, the current of transformer windings with smaller variable ratio increases and the current of transformer windings with larger variable ratio decreases due to the existence of circulating current IC. In this way, transformers operating in parallel can not share load in proportion to capacity. If the total load current of bus is I (I = INI + INII), if transformer I runs at full load, transformer II runs under load; if transformer II runs at full load, transformer I runs under load. It can be seen that when transformers with different conversion ratios are running in parallel, the total capacity of transformers can not be fully utilized because of the existence of circulating current IC.
Because the circulating current of transformer is not load current, but it occupies the capacity of transformer, so it reduces the output power and increases the loss. When the ratio difference is very large, it may damage the normal operation of transformer, or even damage the transformer. In order to avoid the circulating current Ic caused by too large phase difference of transformer ratio affecting the normal operation of parallel transformers, it is stipulated that the phase difference of transformer ratio should not be greater than 0.5%.
(2) Transformers with unequal impedance and voltage operate in parallel:
Because the load distribution among transformers is proportional to its rated capacity and inversely proportional to the impedance voltage. That is to say, when the transformer runs in parallel, if the impedance voltage is different, its load is not proportional to the rated capacity. The current carried by the parallel transformer is inversely proportional to the impedance voltage, i.e. II/III = UZKII/UZKI or UZKIIII = UZKIIIII. Two transformers operate in parallel with the capacity of SNI, SNII and the impedance voltage of UZI and UZII. Then the load of each transformer is calculated according to the following formula:
That is S Delta I/SII= (SNI*UZKII)/(SNII*UZKI)
According to the above analysis, when two transformers with different impedance voltages run side by side, the distribution load with large impedance voltage is small, and when this transformer is full load, the other transformer with small impedance voltage will be overloaded. Transformer long-term overload operation is not allowed, therefore, only the transformer with high impedance voltage can operate under-load, which limits the total output power and increases the energy loss, so it can not guarantee the economic operation of the transformer. Therefore, in order to avoid the excessive difference of impedance voltage, the load current of parallel transformers is seriously distributed unevenly, which affects the full capacity of transformers and stipulates that the impedance voltage should not be 10% different.
(3) Transformers of different wiring groups operate in parallel:
The connection group of transformer reflects the corresponding relationship between high and low side voltage, which is generally expressed by clock method. When the voltage ratio of parallel transformers is equal, the impedance voltage is equal, and the connection groups are different, it means that the secondary voltage of the two transformers has phase angle difference and voltage difference (U). Under the action of voltage difference, the circulating current Ic is generated.
If the angle is expressed as the angle between transformer line voltages of different windings, and Zd is expressed as UZK, the circulating current can be expressed as follows:
If In1 = In2 = In, UZK1 = UZK2 = UZK, the upper expression becomes
Ic = 100sin (_/2)/UZK
In and UZK can be used for rated current and impedance voltage of any transformer.
Assuming that two transformers have the same conversion ratio and impedance voltage, and their connection groups are Y/Y0-12 and Y/Delta-11, respectively, it can be seen from the connection groups that when=360-330 degrees=30 degrees, UZK%=(5-6)% Ic=100sin(/2)/UZK, IC=(4-5) In is obtained, i.e., the rated current is 4-5 times that of the circulating current. Analysis shows that two transformers with different connection groups have different IC=(4-5) In. The circulating current caused by parallel operation is sometimes equivalent to the rated current, but its differential protection and current quick-break protection can not operate tripping, while overcurrent protection can not.