Solution to three-phase voltage imbalance of power grid

There are many reasons for the three-phase voltage imbalance, such as single-phase grounding, disconnection resonance, etc., the operation manager can only process it correctly if it is correctly distinguished.

I. Broken wire fault If one phase is disconnected but not grounded, or the circuit breaker and the isolating switch are not connected, the voltage transformer fuse is blown and the three-phase parameters are asymmetrical. When the previous voltage level line is disconnected, the voltage of the next voltage level shows that the three phase voltages are reduced, one of the phases is lower, and the other two phases are higher but the voltage values ​​of the two are close. When the line of this stage is disconnected, the voltage of the disconnected phase is zero, and the voltage of the unbroken phase is still the phase voltage.

2. Ground fault When the line is disconnected and single-phase grounded, although the three-phase voltage is unbalanced, the voltage value after grounding does not change. Single-phase grounding is divided into metallic grounding and non-metallic grounding. Metal grounding, fault phase voltage is zero or close to zero, non-fault phase voltage rises 1.732 times, and lasts forever; non-metallic grounding, ground phase voltage is not zero but decreases to a certain value, the other two phases rise It is less than 1.732 times higher.

Causes of resonance With the rapid development of industry, the nonlinear power load increases a lot, and some loads not only generate harmonics, but also cause fluctuations and flicker of the supply voltage, and even cause three-phase voltage imbalance.

One is the fundamental frequency resonance, the characteristic is similar to single-phase grounding, that is, the voltage of one phase is lowered, and the voltage of the other two phases is increased. It is difficult to find the fault point when looking for the cause of the fault. At this time, the special user can be checked. If it is not grounding, it may be Caused by resonance.

The other is frequency-divided resonance or high-frequency resonance, characterized by a simultaneous increase in three-phase voltage.

In addition, it should be noted that when the air-drop busbar cut-off part line or single-phase ground fault disappears, if a grounding signal occurs, and the one-phase, two-phase or three-phase voltage exceeds the line voltage, the voltmeter pointer hits the head and moves slowly at the same time, or The three-phase voltage alternately rises above the line voltage. In this case, it is generally caused by resonance.

Harm to the transformer. In the production and living power, when the three-phase load is unbalanced, the transformer is in an asymmetrical operating state. Increased transformer losses (including no-load losses and load losses). According to the transformer operating regulations, the neutral current of the transformer in operation shall not exceed 25% of the rated current of the low-voltage side of the transformer. In addition, the unbalanced operation of the three-phase load will cause the zero-sequence current of the transformer to be too large, and the temperature rise of the local metal parts may even cause the transformer to burn out.

The impact on electrical equipment. The occurrence of a three-phase voltage imbalance will result in several times the current imbalance. The reverse torque is increased in the induction motor, so that the temperature of the motor rises, the efficiency decreases, the energy consumption increases, vibration occurs, and output loss is affected. The imbalance between the phases can lead to shortened service life of the electrical equipment, accelerate the frequency of equipment component replacement, and increase the cost of equipment maintenance. The circuit breaker allows the current margin to decrease, and overload and short circuit are likely to occur when the load changes or alternates. An excessively large unbalanced current flows into the neutral line, causing the neutral line to thicken.

The effect on line loss. Three-phase four-wire system connection method, when the three-phase load balances, the line loss is the smallest; when the one-phase load is heavy, the two-phase load is light, the line loss increment is small; when the one-phase load is heavy, the one-phase load is light, When the load of the third phase is the average load, the line loss increment is large; when the phase load is light, and the load of the two phases is heavy, the line loss increment is the largest. When the three-phase load is unbalanced, the current imbalance is greater and the line loss increment is larger regardless of the load distribution.

First, the main hazards caused by three-phase voltage or current imbalance:

1. When the rotating motor runs in an asymmetrical state, the rotor will generate additional loss and heat, which will cause the whole or partial temperature rise of the motor. In addition, the additional torque generated by the reverse magnetic field will cause the motor to vibrate. For the generator, a series of higher harmonics are also formed in the stator.

2, causing a variety of protection with the negative sequence component as the starting component to malfunction, directly threatening the grid operation.

3. Unbalanced voltage causes non-characteristic harmonics in silicon rectifier equipment.

4. For generators and transformers, when the three-phase load is unbalanced, if the maximum phase current is controlled as the rated value, the other two phases cannot be fully loaded, so the equipment utilization rate is reduced. Otherwise, if the rated capacity is to be maintained, The one-phase overload caused by the load is large, and the magnetic circuit imbalance causes the waveform distortion and the additional loss of the equipment to increase.

Second, the solution to the three-phase voltage imbalance caused by the asymmetric load can be taken:

1. Distribute the asymmetric load to different power supply points to reduce the problem that the imbalance is seriously exceeded due to the concentrated connection.

2. Use the cross-change equal method to distribute the asymmetric load to each phase reasonably and try to balance it.

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

1. Pay attention to the planning work of the low-voltage distribution network, strengthen the communication with the local government planning departments, and avoid the disorderly construction of the distribution network , especially to avoid the situation of headaches and pains in the low-voltage distribution network. In the construction and transformation of the distribution network, the low-voltage station area shall be properly partitioned and divided, and the distribution points shall be as close as possible to the load center to avoid fan-type power supply and bypass power supply. The construction of the distribution network shall follow the "small capacity and multiple distribution points". , short-radius" distribution site selection principle.

2. In areas where low-voltage three-phase four-wire power supply is used, it is necessary to actively strive to use a 3-core or 4-core cable or a low-voltage bundled conductor to supply power to the user terminal in the conditional distribution area , so that it can be used in low-voltage line construction. To avoid the occurrence of partial phase of the three-phase load to the greatest extent, and to do the low-voltage loading work at the same time, the distribution of the three-phase electric meter in the three phases of A, B and C should be as uniform as possible, so as to avoid the single-phase electricity being only connected to one phase or On both phases, the load is out of phase at the end of the line.

Third, the multi-point grounding is adopted in the zero line of the low-voltage distribution network to reduce the zero-line power loss . At present, due to the unbalanced distribution of the three-phase load, the current appears in the neutral line. According to the regulations, the neutral current should not exceed 25% of the phase current. In actual operation, the neutral wire has a thin section and the resistance value is the same length. The phase line is large, and the zero line current is too large, which will cause a certain proportion of power loss on the wire. Therefore, it is recommended to use multi-point grounding in the common main neutral line of the low-voltage distribution network to reduce the zero-line power loss and avoid the load imbalance. The voltage generated by the neutral current seriously endangers personal safety, and through multi-point grounding, the broken wire breakage caused by heat and the like is reduced, so that the phase voltage used by the user rises and the household appliance is damaged. In addition, for the problem of zero line loss, in the current low-voltage cable, the cross-section of the neutral line is 1/2 of the phase line, and the large resistance value causes the loss of the neutral line to increase when the three-phase load is unbalanced. Appropriately increase the wire cross section of the neutral wire, for example, a five-core cable, one core wire for each phase and two core wires for the neutral wire.

4. Actively promote single-phase power supply in power supply areas where single-phase load accounts for a large proportion. At present, most of the load electrical appliances in urban residential quarters use single-phase electricity. Since the line load is mostly power and lighting, the electrical equipment is used at a low rate, so that the low-voltage three-phase load is not in actual operation. The balance is even greater. In addition, from the current rural electricity consumption situation, in many rural areas of underdeveloped and underdeveloped areas, there are problems such as small per capita electricity consumption, scattered living, and long power supply lines. For these areas, users can be considered to be more dispersed and used. The electric load is mainly based on lighting and the load is not large. The single-phase transformer is used to supply power to reduce losses and build funds. At present, the single-phase transformer loss is 15%-20% lower than that of the same-capacity three-phase transformer. Some manufacturers produce single-phase transformers on the low-voltage side that can draw 380V and 220V voltage levels. At the same time, in some areas, multiple units have been used. The pilot of phase-change power supply to three-phase loads provides a broader space for the use of single-phase power supplies.

5. Actively carry out the actual measurement and adjustment of transformer load. The measured load of the distribution transformer seems to be simple, but there are some points to be noted in the actual work. First, the actual measurement work cannot simply measure the phase current of the three-phase lead-out line of the low-voltage side A, B, and C, and measure zero. The current on the line, or the zero line (row) to ground voltage, so that the three-phase load imbalance can be better compared, and the actual measurement work should extend to the end and branch ends of the low-voltage distribution line, so It is possible to further discover the location of the unbalanced load and determine the load point. Third, the load measurement work must be carried out regularly or irregularly, especially when large user loads are put into operation and during peak load periods, the number of actual measurements is increased. By timely measuring the low-voltage outlet line and the low-voltage line current close to the user end, it is convenient to accurately understand the operation of the equipment and to make a balanced and reasonable distribution of the load.