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Details of three-phase unbalance reasons and hazards

Author: Site Editor     Publish Time: 2022-09-19      Origin: Site

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Three-phase unbalance refers to the inconsistency of the three-phase current (or voltage) amplitudes in the power system, and the amplitude difference exceeds the specified range. Due to the unbalanced load added by each phase power supply, it belongs to the fundamental wave load configuration problem. Three-phase unbalance is an important indicator of power quality. If the three-phase unbalance exceeds the range that the distribution network can bear, the safe operation of the overall power system will be affected.

Unbalanced current in three phase system causes

1. Disconnection fault

If one phase is disconnected but not grounded, or if one phase of the circuit breaker and isolating switch is not connected, the fuse of the voltage transformer is blown, which will cause the three-phase parameters to be asymmetrical. When one phase of the line of the previous voltage level is disconnected, the voltage of the next voltage level shows that the voltages of the three phases are reduced, one of which is lower and the other two phases are higher but the voltage values of the two are close. When the line at this level is disconnected, the phase voltage of the disconnected line is zero, and the phase voltage of the unbroken line is still the phase voltage.

2. Ground fault

When one phase of the line is disconnected and the single phase is grounded, although the three-phase voltage is unbalanced, the voltage value does not change after grounding. Single-phase grounding is divided into metallic grounding and non-metallic grounding. For metallic grounding, the voltage of the faulty phase is zero or close to zero, and the voltage of the non-faulty phase increases by 1.732 times and remains unchanged; for non-metallic grounding, the voltage of the grounding phase is not zero but reduced to a certain value, and the other two phases increase less than 1.732 times higher.

3. Reasons for resonance

There are two types of three-phase voltage imbalance caused by resonance:

1) Fundamental resonance

The fundamental frequency resonance is similar to single-phase grounding, that is, the voltage of one phase decreases and the voltage of the other two phases increases. It is difficult to find the fault point when looking for the cause of the fault. At this time, you can check the special user. If it is not the grounding cause, it may be caused by resonance. .

2) Frequency division resonance

The other is the frequency division resonance or high frequency resonance, which is characterized by the simultaneous rise of the three-phase voltage.

In addition, it should also be noted that when the airdrop busbar cuts off part of the line or the single-phase grounding fault disappears, if a grounding signal occurs and the voltage of one, two or three phases exceeds the line voltage, the pointer of the voltmeter hits the head and moves slowly at the same time, or The three-phase voltage rises in turn to exceed the line voltage. In this case, it is generally caused by resonance.power distribution device for sale -XiChiElectric

4. Unreasonable distribution of three-phase loads

Many workers who install meters and connect electricity do not have professional knowledge and concept of three-phase load balance, so they do not pay attention to controlling three-phase load balance when connecting the power, but just blindly and randomly connect and install the circuit. table, which to a large extent caused the unbalance of the three-phase load. Secondly, most circuits in our country are combined with power and lighting, so when single-phase electrical equipment is used, the efficiency of electricity consumption will be reduced, and this difference further exacerbates the inconsistency of the three-phase load of distribution transformers. balance condition.

5. Constantly changing electricity load

The reasons for the instability of electricity load include frequent demolition, relocation of meters or the increase of electricity users; the instability of temporary and seasonal electricity consumption. In this way, the uncertainty and inconcentration in the total amount and time make the load of electricity consumption also have to change with the actual situation.

6. Weakening of monitoring of distribution transformer loads

In the management of distribution network, the management problems in three-phase load distribution are often ignored. In the detection of the distribution network, the three-phase load of the distribution transformer has not been regularly detected and adjusted. In addition, there are many factors that cause the phenomenon of three-phase unbalance, such as the influence of the line and the unphased load moment of the three-phase and so on.

What are the hazards of unbalanced current in three phase system?

1. Increase the power loss of the line

In the three-phase four-wire power supply network, when the current passes through the line wires, there will be power loss due to the existence of impedance, and the loss is proportional to the square of the passing current. When the low-voltage power grid is powered by a three-phase four-wire system, due to the existence of a single-phase load, it is inevitable that the three-phase load will be unbalanced. When the three-phase load is running unbalanced, the neutral line has current flowing through it. In this way, not only the phase line is lost, but also the neutral line is lost, thereby increasing the loss of the grid line.

2. Increase the power loss of distribution transformers

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

3. The output of the distribution transformer is reduced

When the distribution transformer is designed, its winding structure is designed according to the load balance operation condition, the 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 operated under the unbalanced three-phase load condition, the light-loaded one phase has surplus capacity, which reduces the output of the distribution transformer. The degree of output reduction is related to the unbalance of the three-phase load. The larger the three-phase load unbalance is, the more the distribution transformer output is reduced.

For this reason, when the distribution transformer runs when the three-phase load is unbalanced, its output capacity cannot reach the rated value, its spare capacity is also reduced accordingly, and the overload capacity is also reduced. If the distribution transformer operates under overload conditions, it will easily cause the distribution transformer to heat up, and even cause the distribution transformer to burn out in severe cases.

4. The distribution transformer produces zero-sequence current

When the distribution transformer operates under the unbalanced three-phase load, zero-sequence current will be generated. This current will change with the unbalanced degree of the three-phase load. The greater the unbalanced degree, 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 iron core. (There is no zero-sequence current on the high-voltage side) This forces the zero-sequence magnetic flux to only pass through the tank wall and the steel member as a channel, while the magnetic permeability of the steel member is low. When the zero-sequence current passes through the steel member, hysteresis and eddy current will be generated. loss, so that the local temperature of the steel components of the distribution changes increases and generates heat. The winding insulation of the distribution transformer is rapidly aged 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 distribution transformer.

5. Affect the safe operation of electrical equipment

The distribution transformer is designed according to the three-phase load balance operating conditions, and the resistance, leakage reactance and excitation impedance of each phase winding are basically the same. When the distribution transformer operates in three-phase load balance, its three-phase currents are 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 is not equal, and the three-phase voltage drop inside the distribution transformer is not equal, which will inevitably lead to the three-phase unbalance of the output voltage of the distribution transformer.

At the same time, the distribution transformer operates when the three-phase load is unbalanced, the three-phase output current is different, and the neutral line will have current passing through. As a result, the neutral line produces an impedance voltage drop, which causes the neutral point to drift and causes the voltage of each phase to change. The voltage of one phase with heavy load decreases, while the voltage of one phase with light load increases. Supplying power under the condition of voltage imbalance, that is, it is easy to cause the user's electrical equipment with a high-voltage one-phase connection to burn out, while the user's electrical equipment with a low-voltage one-phase connection may be unusable. Therefore, when the three-phase load is unbalanced, the safe operation of the electrical equipment will be seriously endangered.

6. Reduced motor efficiency

The distribution transformer operates under the unbalanced three-phase load condition, which will cause the three-phase unbalance of the output voltage. Since the unbalanced voltage has three voltage components: positive sequence, negative sequence and zero sequence, when this unbalanced voltage is input to 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 acts as a brake. effect. But 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 output power of the motor will be reduced, thereby reducing the efficiency of the motor. At the same time, the temperature rise and reactive power loss of the motor will also increase with the unbalance of the three-phase voltage. Therefore, it is very uneconomical and unsafe to operate the motor under the condition of three-phase voltage imbalance.



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