Harm of unbalance of three-phase load in distribution transformer

This paper analyzes the harm and cause of unbalance of three-phase load of distribution transformer, and puts forward some measures to solve the unbalance.  

November 29, 2021
Harm of unbalance of three-phase load in distribution transformer

I. Harm of unbalance of three-phase load in distribution transformer

1. Line loss increase: the load loss of the distribution transformer changes with the load current of the transformer, and is proportional to the square of the load current. In the case of the transformer conveying the same capacity, the three-phase load is unbalanced, and the active loss increases.

In addition, there will be power loss on the wire. The greater the imbalance, the greater the line loss.

2. Increase the power loss of the distribution transformer: the distribution transformer is the main power supply equipment of the low-voltage power grid. When it runs under the unbalanced three-phase load, the distribution transformer loss will increase. Because the power loss of the distribution changes with the degree of unbalance of the load.

3. Reduced output of distribution transformer: when distribution transformer is designed, its winding structure is designed according to load balance operating condition, and its winding performance is basically the same, and the rated capacity of each phase is equal.

The maximum allowable output of distribution transformer is limited by the rated capacity of each phase. If the distribution transformer is operated under unbalanced three-phase load, the phase with light load will have surplus capacity, thus reducing the output of the distribution transformer.

The degree of output reduction is related to the unbalance degree of three-phase load. The larger the unbalanced three-phase load is, the more the output of the distribution transformer is reduced. Therefore, when the three-phase load is unbalanced, the output capacity of the transformer cannot reach the rated value, and its spare capacity is reduced accordingly, and the overload capacity is also reduced.

If the distribution transformer runs under overload condition, it is easy to cause the distribution transformer to heat up, and even cause the distribution transformer to burn out in serious cases.

4. Distribution transformer produces zero sequence current: distribution transformer runs under unbalanced three-phase load condition, will produce zero sequence current, the current will change with the degree of three-phase load imbalance, the greater the imbalance degree, the greater the zero sequence current.

When the zero sequence current passes through the steel member, it will produce hysteresis and eddy current loss, which will make the local temperature of the steel member of the distribution change rise and heat up, and the winding insulation will accelerate aging due to overheating, leading to the reduction of equipment life.

5. Affecting the safe operation of electrical equipment: The distribution transformer is designed according to the operation condition of three-phase load balance, and the resistance, leakage reactance and excitation impedance of each phase winding are basically the same.

When the transformer runs in three-phase load balance, its three-phase current is basically equal, and the voltage drop of each phase inside the transformer is basically the same, then the three-phase voltage output of the transformer is also balanced.

If the distribution transformer is operated in three-phase load imbalance, the output current of each phase is not equal, and the internal three-phase voltage drop of the distribution transformer is not equal, which will lead to three-phase imbalance of the output voltage of the distribution transformer.

At the same time, the distribution transformer is operated in unbalanced three-phase load, three-phase output current is different, and the neutral line will have current through. Thus the neutral line produces impedance voltage drop, resulting in neutral point drift, resulting in the phase voltage changes, seriously endangering the safe operation of electrical equipment.

6. Reduced efficiency of the motor: the distribution transformer runs under the unbalanced three-phase load condition, which will cause the unbalanced three-phase output voltage. When the unbalanced voltage is input to the motor, the negative sequence voltage produces a rotating magnetic field opposite to the rotating magnetic field generated by the positive sequence voltage, acting as a brake.

But because the positive magnetic field is much stronger than the negative magnetic field, the motor still rotates in the direction of the positive magnetic field. However, due to the braking effect of negative sequence magnetic field, the output power of the motor will be reduced, and the efficiency of the motor will be reduced.

At the same time, the temperature rise and reactive power loss of the motor will increase with the unbalance of three-phase voltage. Therefore, it is very uneconomical and unsafe for the motor to operate under the condition of unbalanced three-phase voltage.

II. Causes of unbalanced three-phase load

(1) Insufficient understanding of the importance of three-phase load balance. Management personnel in the management did not strictly according to the rules and regulations to do, not to carry out the assessment requirements.

(2) a large number of single-phase electrical equipment. In recent years, a large number of high-grade, high-power single-phase electrical appliances have entered the common home. When the power consumption of single-phase load increases greatly and the probability of simultaneous use is not consistent, the unbalance degree of three-phase load of low-voltage power grid may increase.

(3) As the management personnel are not familiar with the changing rules and distribution of three-phase load in the platform area, the application of new single-phase users, especially large single-phase equipment, cannot be distributed in a balanced manner according to three-phase load.

(4) Temporary power consumption and seasonal power consumption increase, such as summer, winter, holidays, each user's power consumption increase is inconsistent, resulting in three-phase load imbalance.

III. Improvement measures

1. Solutions to three-phase voltage imbalance of power grid caused by asymmetric load:

(1) The asymmetric load is dispersed to different power supply points to reduce the problem of serious excess of imbalance caused by centralized connection.

(2) The method of cross exchange and equality is used to make the asymmetric load reasonably distributed to each phase and try to make it balanced.

(3) Increase the short-circuit capacity of the load access point, such as changing the network or improving the power supply voltage level to improve the ability of the system to bear unbalanced load.

2. Strengthen management

(1) Organize special personnel to draw transformer network diagram and load distribution diagram every year, make the relevant data such as the number of electricity users on each phase and the model of electricity meter into a convenient and easy to check table, and check whether there are missing or new users, and timely update combined with the load changes.

(2) Assign the person with a clamp meter, and carry out a load test at least once a month to check the unbalanced three-phase load.

(3) For temporary power consumption and seasonal power consumption, management personnel are required to be familiar with the basic situation of users, installation sites, changes in power consumption, and then timely adjust according to the situation.

(4) Apply for new single-phase equipment to use electricity, do a good job of load power distribution, as far as possible to evenly distribute to the three-phase circuit.

3. Adjust three-phase unbalanced load to achieve "four balance"

"Four balance" is the balance of measuring points, the balance of branches, the balance of main lines and transformer low-voltage outlet side balance.

Among the four balances, the key is the measurement point and the balance of branches, the average user electricity consumption can be used as the basis for adjustment, the electricity consumption is roughly the same as a class, respectively evenly adjusted to the three-phase.

4. Connect the three-phase line to the load point simultaneously

In order to obtain the symmetry of three-phase load, three-phase lines should be introduced into the load point at the same time because the loss is significantly reduced when three-phase lines are introduced into the load point at the same time.

Expand the distribution area of three-phase and four-wire system as far as possible and reduce the length of single-phase power supply main line. The access line should be introduced from u, V and W phases on the same pole as far as possible. And the load of the three groups of single-phase line should be balanced as far as possible.

5. Reasonably design the power grid renovation scheme

In order to achieve three-phase load balance after transformation, it is necessary to design the power network transformation scheme reasonably. Before design, it is necessary to understand the law of load change and load distribution, carry out on-site investigation, master the load distribution, and draw the load distribution wiring diagram.

Wiring should be carried out strictly according to the principle of three-phase load balance, and three-phase and four-wire should be penetrated into each important load center as far as possible.

Source: Windows on Power

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