Three-phase imbalance, explain the causes and solutions

Three-phase imbalance is an important indicator of power quality. Although there are many factors that affect the power system, the normal imbalance is mostly caused by three-phase components, line parameters or load asymmetry. Since the factors of three-phase load are uncertain, the three-phase voltage and current at the power supply point are prone to imbalance, which will cause line loss. Not only that, it will also have an adverse effect on the motor at the power supply point, endangering the normal operation of the motor.

Causes of three-phase imbalance in N1 distribution network

1. Unreasonable distribution of three-phase load.

Many workers who install meters and connect electricity do not have professional knowledge of three-phase load balance. Therefore, when connecting electricity, they do not pay attention to controlling the three-phase load balance. They just blindly and arbitrarily connect the circuit and install the meter, which causes the imbalance of three-phase load to a great extent.

Secondly, most of the circuits in our country are a mixture of power and lighting, so when using single-phase electrical equipment, the efficiency of electricity use will be reduced. Such a difference further aggravates the imbalance of the three-phase load of the distribution transformer.

2. The power load keeps changing.

The reasons for unstable power load include frequent demolitions, meter relocations or an increase in power users;

The instability of temporary and seasonal electricity consumption. The uncertainty and non-concentration in total amount and time make the electricity load change with the actual situation.

3. Weakening of monitoring of distribution transformer loads.

In the management of distribution network, the management issues in three-phase load distribution are often ignored. In the detection of distribution network, the three-phase load of distribution transformer is not regularly detected and adjusted.

In addition, there are many other factors that cause three-phase imbalance, such as the influence of the line and the unequal three-phase load torque.

Hazards of N2 three-phase imbalance

1. Increase the power loss of the line

In a three-phase four-wire power supply network, when current passes through the line conductor, power loss will inevitably occur due to the existence of impedance, and the loss is proportional to the square of the current passing through.

When the low-voltage power grid is powered by a three-phase four-wire system, it is inevitable that the three-phase load will be unbalanced due to the presence of single-phase load.

When the three-phase load is unbalanced, current will flow through the neutral line, which will cause loss not only in the phase line but also in the neutral line, thus increasing the loss of the power grid line.

2. Increase the power loss of distribution transformer

The distribution transformer is the main power supply equipment of the low-voltage power grid. When it operates under the condition of unbalanced three-phase load, the loss of the distribution transformer will increase, because the power loss of the distribution transformer varies with the unbalanced degree of the load.

3. Distribution transformer output reduction

When designing a distribution transformer, its winding structure is designed according to the load balance operating condition, its winding performance is basically the same, and the rated capacity of each phase is equal. The maximum allowable output of the distribution transformer is limited by the rated capacity of each phase.

If the distribution transformer is operating under the condition of unbalanced three-phase load, the phase with light load will have surplus capacity, which will reduce the output of the distribution transformer. The degree of output reduction is related to the unbalance of the three-phase load.

The greater the three-phase load imbalance, the greater the reduction in distribution transformer output.

Therefore, when the distribution transformer is operated under unbalanced three-phase load, its output capacity cannot reach the rated value, its spare capacity is reduced accordingly, and its overload capacity is also reduced. If the distribution transformer is operated under overload conditions, it is very easy to cause the distribution transformer to heat up, and in severe cases, it may even cause the distribution transformer to burn out.

4. Distribution transformer generates zero-sequence current

When the distribution transformer is operated under the condition of three-phase load imbalance, zero-sequence current will be generated. This current will change with the degree of three-phase load imbalance. The greater the imbalance, the greater the zero-sequence current. If there is zero-sequence current in the distribution transformer in operation, zero-sequence magnetic flux will be generated in its core.

(There is no zero-sequence current on the high-voltage side) This forces the zero-sequence magnetic flux to pass only through the oil tank wall and steel components. The magnetic permeability of the steel components is low. When the zero-sequence current passes through the steel components, hysteresis and eddy current losses will be generated, causing the local temperature of the steel components of the distribution transformer to rise and generate heat.

The winding insulation of the distribution transformer will age faster due to overheating, resulting in a reduction in the life of the equipment. At the same time, the existence of zero-sequence current will also increase the loss of the distribution transformer.

5. Affect the safe operation of electrical equipment

The distribution transformer is designed according to the three-phase load balance operation condition, and the resistance, leakage reactance and excitation impedance of each phase winding are basically the same. When the distribution transformer operates under the three-phase load balance, its three-phase current is basically equal, and the voltage drop of each phase inside the distribution transformer is basically the same, so the three-phase voltage output by the distribution transformer is also balanced.

If the distribution transformer operates when the three-phase load is unbalanced, the output current of each phase will be unequal, and the three-phase voltage drop inside the distribution transformer will be unequal, which will inevitably lead to unbalanced three-phase output voltage of the distribution transformer.

At the same time, when the distribution transformer operates when the three-phase load is unbalanced, the three-phase output currents are different, and current will flow through the neutral line.

As a result, an impedance voltage drop occurs in the neutral line, which causes the neutral point to drift and causes the phase voltages of each phase to change.

The voltage of the phase with heavy load decreases, while the voltage of the phase with light load increases.

When the voltage is unbalanced, it is easy to burn out the user's electrical equipment connected to the phase with high voltage, while the user's electrical equipment connected to the phase with low voltage may not be usable. Therefore, when the three-phase load is unbalanced, it will seriously endanger the safe operation of the electrical equipment.

6. Reduced motor efficiency

When the distribution transformer operates under the condition of unbalanced three-phase load, the output voltage will be unbalanced in three phases. Since the unbalanced voltage has three voltage components: positive sequence, negative sequence, and zero sequence, when this unbalanced voltage is input into the motor, the rotating magnetic field generated by the negative sequence voltage is opposite to the rotating magnetic field generated by the positive sequence voltage, which plays a braking role. However, since the positive sequence magnetic field is much stronger than the negative sequence magnetic field, the motor still rotates in the direction of the positive sequence magnetic field.

However, due to the braking effect of the negative sequence magnetic field, the motor output power will inevitably decrease, resulting in a decrease in motor efficiency. At the same time, the motor temperature rise and reactive power loss will also increase with the imbalance of the three-phase voltage. Therefore, it is very uneconomical and unsafe for the motor to operate under the condition of unbalanced three-phase voltage.

N3 Technology for improving three-phase imbalance in distribution network

1. Pay attention to the reasonable distribution of three-phase load

When it comes to the distribution of three-phase loads, power workers should carefully collect and record relevant data in their actual work so as to be able to predict the power load to a certain extent.

Secondly, better three-phase balanced distribution can be achieved by installing balancing devices.

2. Methods for controlling unbalanced current in three-phase load

According to the compensation principle of unbalanced current susceptance, at any certain moment, there is mainly a three-phase ungrounded unbalanced load, then each phase load among them can be connected in parallel with the same resistor and capacitor.

Therefore, under the guidance of the unbalanced current management susceptance compensation theory, the equivalents of different properties can be analyzed to determine the reactive compensation amount between phases and relative to ground.

When the distribution transformer needs to compensate for unbalanced current, the following principles should be met.

First, it is necessary to note that current management should have two aspects, one is to compensate for the power factor, the other is to adjust the three-phase current imbalance. These two together determine the reactive power required for compensation.

Secondly, in actual engineering construction, a fully capacitive management method should be adopted to distinguish it from inductive compensation to avoid serious over-compensation.

The third point is that it is necessary to consider that the load will change over time. Based on this characteristic, the compensation amount should also be appropriately adjusted according to the change in load.

The fourth point is the limitation on the number of switching on and off of the device switches and compensation equipment. The optimization plan for the whole day should be strategically managed during the design.

In short, when setting the proportional regulation coefficient, it is necessary to consider both the power factor limitation and the over-compensation limitation.

3. Add detection and adjustment of three-phase load

It is also very necessary to regularly conduct three-phase load testing. After the three-phase loads are reasonably distributed and controlled, the relevant departments should conduct testing.

The balance of electricity cannot be absolute, but can only be relatively balanced. In the actual detection work, each department should use the balance measurement indicators formulated by the state and relevant departments as a standard, record and analyze the test results professionally, and conduct regular inspections on the load current of each phase in order to promptly discover some three-phase imbalance conditions.

When a potential safety hazard is found during the test, adjustments and modifications should be made in a timely manner. For parts where no problems were found during the test, vigilance should also be heightened. After the test, not only data needs to be collated and analyzed, but also timely feedback should be provided.

The feedback here mainly refers to the adjustments that need to be made to the three phases based on the test results, as well as the prediction of the possibility of using new technologies in the three phases. Through reasonable testing and in-depth analysis of the test results, we can avoid the occurrence of imbalance to the greatest extent and reduce the occurrence of power accidents.

Solutions to the three-phase voltage imbalance of the power grid caused by asymmetric load

1. Distribute asymmetric loads to different power supply points to reduce the problem of severe imbalance caused by centralized connection.

2. Use methods such as cross-phase switching to reasonably distribute the asymmetric load to each phase and try to balance it.

3. Increase the short-circuit capacity of the load access point, such as changing the network or increasing the power supply voltage level to improve the system's ability to withstand unbalanced loads.

4. Install a balancing device.

The above technical measures to solve the harm of three-phase voltage or current imbalance to the power grid and power quality are briefly listed. Which specific measure should be taken is more reasonable and effective, and it should be determined and implemented after technical and economic comparison based on actual conditions.

In low voltage three-phase four-wire urban residential and rural power supply systems

Since most electricity users use single-phase loads or a mixture of single-phase and three-phase loads, and the load sizes and electricity usage times are different, the unbalanced current between the three phases in the power grid is an objective existence, and this unbalanced power consumption is irregular and cannot be predicted in advance. This leads to long-term unbalanced three-phase loads in the low-voltage power supply system. For the unbalanced three-phase current, the power sector has almost no effective solution except to distribute the load as reasonably as possible.