Impact on distribution transformers

    ( A ) three-phase unbalanced load will increase transformer losses :

    Transformer load loss and load loss including loss . Under normal circumstances the operating voltage transformer basically unchanged, ie no-load loss is a constant. While the load loss of the operation of the transformer with the load change, and the square of the load current

    Proportional . When the three-phase unbalanced load operation, the transformer load loss can be viewed as three single-phase transformer load loss of .

We know from mathematical theorem : Suppose that a, b, c 3 numbers are greater than or equal to zero , then a + b + c ≥ 33 √ abc.

When a = b = c , the algebraic sum a + b + c to obtain the minimum of : a + b + c = 33 √ abc.

    Therefore, we can assume that the three-phase transformer losses were : Qa = Ia2 R, Qb = Ib2 R, Qc = Ic2 R, the formula Ia, Ib, Ic , respectively, the phase current transformer secondary load , R is the phase resistance of the transformer . The loss of expression of the transformer

    Following formula:

    Qa + Qb + Qc ≥ 33 √ [ (Ia2 R) (Ib2 R) (Ic2 R) ]

    It can be seen , the transformer in the case of constant load , when Ia = Ib = Ic , i.e., balanced three-phase load , the transformer loss is minimized .

    The transformer loss:

When the transformer three phase operation, ie Ia = Ib = Ic = I when , Qa + Qb + Qc = 3I2R;

When the transformer is operating at maximum unbalance , i.e. Ia = 3I, Ib = Ic = 0时, Qa = (3I) 2R = 9I2R = 3 (3I2R);

I.e., the maximum variation of unbalance to balance the loss of three times .

    ( 2 ) three-phase unbalanced load may cause serious consequences transformer burned :

    The heavy phase current imbalance is too large ( increased to three times ) , excessive overloading may cause overheating of windings and transformer oil . Winding overheating, insulation aging accelerate ; transformer oil overheating, resulting in deterioration of oil quality , quickly lower the transformer insulation

    Performance and reduce the transformer life ( the temperature is increased 8 ℃, life will be reduced by half ) , and even burned windings.

    ( 3 ) three-phase unbalanced load operation will result in zero sequence current transformer is too large , the local metal temperature increased:

Under the three-phase unbalanced load operation of the transformer, would have a zero-sequence current, while internal transformer zero sequence current exists, zero-sequence generated in the core flux , these zero- sequence flux in the transformer tank wall will or other metal member constituting the

     Loop. But does not consider the distribution transformer design elements for the magnetic component of these metals , the resulting hysteresis and eddy current heating of these components , resulting in partial metal transformer abnormal temperature rise , transformer operation will lead to serious accident