The general national standard stipulates that the unbalance rate of three-phase resistance line is not more than 2%, and that of phase is not more than 4%

3 hazards of unbalanced three-phase load

1.1 impact on distribution transformer

(1) Unbalanced three-phase load will increase the loss of transformer:

Transformer losses include no-load losses and load losses. Under normal conditions, the operating voltage of the transformer is basically unchanged, that is, the no-load loss is a constant. However, the load loss varies with the operating load of the transformer and is proportional to the square of the load current. When the three-phase load is unbalanced, the load loss of the transformer can be regarded as the sum of the load losses of three single-phase transformers.

From the mathematical theorem, we know that if the number of a, B and C is greater than or equal to zero, then a+b+c ≥ 33 √ ABC.

When a=b=c, algebra and a+b+c get the minimum value: a+b+c=33 √ ABC.

Therefore, we can assume that the three-phase losses of the transformer are: QA=ia2r, QB=ib2r, QC=ic2r, where IA, IB, IC are the secondary load phase current of the transformer, and R is the phase resistance of the transformer. Then the loss expression of 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 of transformer, when ia=ib=ic, that is, when the three-phase load reaches balance, the loss of transformer is the minimum.

Then transformer loss:

When the transformer operates in three-phase balance, i.e. ia=ib=ic=i, Qa＋Qb＋Qc=3I2R；

When the transformer operates at the maximum unbalance, i.e. ia=3i, ib=ic=0, Qa=(3I)2R=9I2R=3(3I2R)；

That is, the variation loss at maximum unbalance is three times that at equilibrium.

(2) Unbalanced three-phase load may cause serious consequences of transformer burning:

When the above unbalance occurs, the heavy load phase current is too large (increased by 3 times), and the overload is too much, which may cause overheating of windings and transformer oil. Winding overheating, insulation aging accelerated; Overheating of transformer oil will cause deterioration of oil quality, rapidly reduce the insulation performance of transformer, reduce the service life of transformer (the service life will be reduced by half for every 8 ℃ rise in temperature), and even burn the windings.

(3) Unbalanced operation of three-phase load 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 magnetic flux in the iron core, which will form a circuit in the oil tank wall or other metal components of the transformer. However, these metal components are not considered as magnetic conductive components in the design of distribution transformers, so the resulting hysteresis and eddy current losses will heat these components, resulting in abnormal temperature rise of local metal components of transformers, which will lead to transformer operation accidents in serious cases.

1.2 impact on high voltage lines

(1) Increase high voltage line loss:

When the three-phase load at the low-voltage side is balanced, the 6~10kv High-voltage side is also balanced. Set the current of each phase of the high-voltage line as I, and its power loss as Δ P1=3i2r

The three-phase load imbalance of low-voltage power grid will be reflected on the high-voltage side. At the maximum imbalance, the corresponding phase of high-voltage is 1.5i, and the other two phases are 0.75 I. the 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 tripping times of high-voltage lines and reduce the service life of switchgear:

We know that the over-current fault of high-voltage line accounts for a considerable proportion, and the reason is that the current is too large. The three-phase load imbalance of low-voltage power grid may cause excessive current of a high-voltage phase, which may cause overcurrent tripping and power failure of high-voltage line, and cause large-scale power failure. At the same time, frequent tripping of switchgear in substation will reduce the service life.

1.3 impact on distribution panel and low voltage line

(1) Unbalanced three-phase load will increase line loss:

The three-phase four wire power supply line distributes the load evenly to three phases, and sets the current of each phase as I, the neutral line current as zero, and its power loss as Δ P1=3i2r

When the maximum unbalance occurs, that is, one phase is 3I, the other two phases are zero, the neutral line current is 3I, and the power loss is:

ΔP2 = 2（3I)2R = 18I2R = 6（3I2R)；

That is, the power loss at the maximum unbalance is six times that at the balance. In other words, if 1200 kwh is lost per month at the maximum unbalance, only 200 kwh is lost at the balance. Therefore, it can be seen that the loss reduction potential of adjusting the three-phase load.

(2) Unbalanced three-phase load may cause serious consequences of burning out lines and switchgear:

The heavy load phase current is too large (increased by 3 times) and overload is too much when the above imbalance occurs. Since the calorific value Q=0.24i2rt, when the current increases by 3 times, the calorific value increases by 9 times, which may cause the temperature of the phase conductor to rise linearly and burn out. And because the cross-section of the neutral conductor should generally be 50% of the phase conductor cross-section, but when selecting, some are often too small, and the quality of the connector is not good, so that the conductor resistance increases. The probability of neutral wire burnout is higher.

Similarly, on the distribution panel, the switch heavy load phase is burnt out, and the contactor heavy load phase is burnt out, thus the whole machine is damaged and other serious consequences are caused.

1.4 impact on power supply enterprises

The direct management of power supply enterprises to households and the large loss of low-voltage power grid will reduce the economic benefits of power supply enterprises, and even cause loss operation of power supply enterprises. The rural electricians contracted for the line loss in the substation area, and the bonuses of the rural electricians were withheld due to high line loss, and even the wages could not be obtained, which would inevitably affect the mood of the rural electricians, ranging from negative work to illegal crime in order to get money.

The burning out of transformers, lines and switchgear, on the one hand, increases the cost of power supply for power supply enterprises, on the other hand, power outage maintenance and purchase replacement lead to long-term power outage and less power supply, which not only reduces the economic benefits of power supply enterprises, but also affects the reputation of power supply enterprises.

1.5 impact on users

Unbalanced three-phase load and abnormal weight of one or two phases will increase the voltage drop in the line, reduce the power quality and affect the use of electrical appliances by users.

The burning of transformers, lines and switchgear will affect the power supply of users, which may cause inconvenience or economic losses, such as the death of breeding animals and plants caused by power failure or punishment for failure to supply goods according to the contract. The burning out of neutral line may also cause a large number of low-voltage electrical appliances of users to be burned.