The Harm of Three-phase Current Unbalance in Transformer
1. Influence on distribution transformers
(1) The three-phase load imbalance of power transformer will increase the loss of transformer: the loss of transformer includes no-load loss and load loss. Under normal conditions, the transformer operating voltage is basically unchanged, that is, the no-load loss is a constant. The load loss varies with the transformer operating load and is proportional to the square of the load current. When the three-phase load is unbalanced, the transformer load loss can be regarded as the sum of the three single-phase transformer load loss.
We know from the mathematical theorem: assuming that the Numbers a, b and c are all greater than or equal to 0, then a+b+c≥33√abc.
When a=b=c, a+b+c gets the minimum value
Therefore, we can assume that the three-phase loss of the transformer is: Qa = Ia2 R, Qb= Ib2 R, Qc = Ic2 R. In the formula, Ia, Ib and Ic are the phase current of secondary load of the transformer respectively, and R is the phase resistance of the transformer. The loss expression of the transformer is as follows:
Qa+Qb+Qc≥33√［（Ia2 R）（Ib2 R）（Ic2 R）］
It can be seen that under the condition of constant load, when Ia=Ib=Ic, that is, when the three-phase load reaches balance, the loss of transformer is the minimum.
When the transformer is in three-phase balanced operation, i.e. Ia=Ib=Ic=I, Qa+Qb+Qc=3I2R;
When the transformer operates in the maximum imbalance, i.e. Ia=3I, Ib=Ic = 0, Qa=(3I)2R=9I2R=3(3I2R);
That is, the transformer loss at maximum imbalance is 3 times that at equilibrium.
(2) Three-phase load imbalance of power transformer may cause serious consequences of transformer burning: excessive heavy load phase current (increased by 3 times) and excessive overload in the above unbalanced condition may cause overheating of windings and transformer oil. Winding overheat, insulation aging accelerated; Transformer oil overheats, causing oil degradation, rapidly reducing the insulation performance of the transformer, reducing the service life of the transformer (for every 8℃ increase in temperature, the service life will be reduced by half), and even burning out the winding.
(3) Unbalanced load operation of three-phase power transformer will cause excessive zero sequence current of transformer and increase temperature rise of local metal parts: Under the unbalanced operation of three-phase load, the transformer will inevitably generate zero sequence current, and the existence of zero sequence current in the transformer will generate zero sequence flux in the iron core, and these zero sequence flux will form a loop in the transformer tank wall or other metal components. However, when the distribution transformer is designed, it is not considered that these metal components are magnetic conductive parts, so the hysteresis and eddy current loss caused by this will make these parts hot, resulting in abnormal temperature rise of local metal parts of the transformer, which will lead to transformer operation accidents in serious cases.
(4) Some of the phase load looks similar, the current of each phase is similar, but the neutral line current is very large, even more than the maximum phase current, which is caused by the different nature of the three-phase load. For example, UA=UB=UC=220V, IA=IB=4A, IC=3.2A, IN= 4.2a for A three-phase four-wire power supply line.To verify IN the values of the measured phase of each phase load | Φ A | = | | Φ B = 40 °, Φ = 0 ° C, then one of ZA and ZB is inductance and the other is capacitive. Let ZA be inductance, ZB be capacitive.
IN =｜IA＋IB＋IC｜=4.3（A）The theoretical calculation and instrument measurement results are basically in agreement, which indicates that the large neutral line current is caused by the different properties of three-phase loads.
2. Influence on high-voltage lines
(1) Increase the loss of high-voltage line: When the three-phase load on the low-voltage side is balanced, the high-voltage side is also balanced at 6 ~ 10kV. Let the current in each phase of the high-voltage line be I, its power loss is: ΔP1 = 3I2R
The three-phase load imbalance of the low-voltage grid will be reflected to the high-voltage side. In the case of maximum imbalance, the corresponding high-voltage phase is 1.5I, and the other two phases are 0.75I, power loss is: ΔP2 = 2（0.75I）2R＋（1.5I）2R = 3.375I2R =1.125（3I2R）That is, the power loss on the high-voltage line increased by 12.5%.
(2) Increase the times of trips of high-voltage lines and reduce the service life of switchgear equipment: We know that the high voltage line overcurrent fault accounts for a large proportion.
(3) The imbalance of three-phase load in the low-voltage power network may cause too much current in a certain phase of high voltage, which will cause overcurrent tripping and power failure accident. At the same time, frequent tripping of switching equipment in substation will reduce the service life.
3. Impact on distribution panel and low-voltage line
(1) Unbalanced three-phase load will increase the line loss: three phase four wire system power supply circuit, distributing the load average on the three-phase, set for each phase of the current I, neutral line current is zero, the power loss is: ΔP1 = 3I2R
At the maximum imbalance, that is, one phase is 3I, the other two phases are zero, and the neutral line current is also 3I, power loss is: ΔP2 = 2（3I)2R = 18I2R = 6（3I2R); That is, the energy loss at maximum imbalance is six times that at balance.
(2) The imbalance of three-phase load may cause line breaking and switchgear burning: When the above imbalance, the heavy load phase current is too large (3 times), and the overload is too much. Since the calorimetric value Q = 0.24I2Rt, the current increases by 3 times, and the calorimetric value increases by 9 times, which may cause the linear rise of the temperature of the phase conductor and lead to the burning
4. Impact on users
Unbalanced three-phase load will inevitably increase the voltage drop in the line and reduce the power quality, which will affect the normal use of other equipment of the company or cause the transformer to burn down, the line to burn down, the switch equipment to burn down, affect the company's other power supply, bring inconvenience and economic loss. A large number of low-voltage electrical appliances may also be burnt when the neutral line is broken.