There are many reasons for the unbalance of the three-phase voltage in the factory, such as: single-phase grounding, disconnection resonance, etc. The operation management personnel can only deal with it quickly if they are correctly distinguished.
1. Disconnection fault
If one phase is disconnected but not grounded, or the circuit breaker and isolating switch are not connected, the fuse of the voltage transformer will cause the three-phase parameter asymmetry. When one phase of the line of the previous voltage level is disconnected, the voltage of the next voltage level is shown as the voltage 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 of 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 single-phase 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 is increased 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 rise Less than 1.732 times higher.
Three, resonance reason
With the rapid development of industry, non-linear power loads have increased significantly. Some loads not only generate harmonics, but also cause fluctuations and flicker in the supply voltage, and even cause three-phase voltage imbalance.
1. There are two types of three-phase voltage imbalance caused by resonance:
One is the fundamental frequency resonance, which is similar to single-phase grounding, that is, the voltage of one phase decreases and the voltage of the other two phases increase. It is not easy to find the fault point when searching for the cause of the fault. At this time, you can check special users. If it is not the cause of grounding, it may be Caused by resonance.
The other is frequency division resonance or high frequency resonance, which is characterized by the simultaneous increase of three-phase voltage.
In addition, it should be noted that when the airdrop bus is cut off part of the line or the single-phase ground fault disappears, if a ground signal occurs and the one-phase, two-phase or three-phase voltage exceeds the line voltage, the voltmeter pointer hits the end 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.
2, the hazards and effects of three-phase imbalance:
(1) The harm to the transformer.
In the production and living electricity, when the three-phase load is unbalanced, the transformer is in asymmetrical operation state. Increase the loss of the transformer (including no-load loss and load loss). According to the transformer operation regulations, the neutral current of the transformer in operation shall not exceed 25% of the rated current on the low-voltage side of the transformer. In addition, the unbalanced operation of the three-phase load will cause excessive zero-sequence current of the transformer, increase the temperature of local metal parts, and even cause the transformer to burn.
(2) Impact on electrical equipment.
The occurrence of three-phase voltage imbalance will lead to the occurrence of several times the current imbalance. Induce the reverse torque in the motor to increase, so that the temperature of the motor rises, the efficiency decreases, the energy consumption increases, vibration occurs, and the output loss is affected. The imbalance between the phases will shorten the service life of the electrical equipment, accelerate the replacement frequency of equipment components, and increase the cost of equipment maintenance. The circuit breaker allows the current margin to be reduced, and overload and short circuit are prone to occur when the load changes or alternates. Excessive unbalanced current flows into the neutral wire, causing the neutral wire to thicken.
(3) The influence on line loss.
The three-phase four-wire connection method, when the three-phase load is balanced, the line loss is the smallest; when the load of one phase is heavy and the load of two phases is light, the line loss increment is small; when the load of one phase is heavy, the load of one phase is light, When the load of the third phase is an average load, the increase in line loss is larger; when the load of one phase is light and the load of two phases is heavy, the increase in line loss is the largest. When the three-phase load is unbalanced, regardless of the load distribution, the greater the current unbalance, the greater the line loss increment.
3. The hazards and solutions of three-phase imbalance:
The main hazards caused by three-phase voltage or current unbalanced factors:
(1) Rotating motor running in an asymmetrical state will cause additional loss and heat generation of the rotor, which will cause the motor to heat up as a whole or partly. In addition, the additional torque generated by the reverse magnetic field will cause the motor to vibrate. For generators, a series of higher harmonics will also be formed in the stator.
(2) Causes the malfunction of various protections with negative sequence component as the starting element, which directly threatens the operation of the power grid.
(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 to the rated value, the remaining two phases cannot be fully loaded, so the equipment utilization rate decreases. On the contrary, if the rated capacity is to be maintained, the It will cause an overload of one phase with a large load, and the unbalance of the magnetic circuit will cause waveform distortion and additional equipment loss.
4. The solutions that can be taken to solve the three-phase voltage imbalance of the power grid caused by asymmetric load:
(1) Distribute the asymmetric load to different power supply points to reduce the problem of serious unbalanced excess caused by centralized connection.
(2) Use the cross-exchange equalization 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 power supply voltage level to improve the system's ability to withstand unbalanced loads.
(4) Use a three-phase split-adjustable fully automatic compensating regulator to solve the problem of three-phase voltage imbalance.
5. Several measures to solve the three-phase load imbalance:
(1) Attach importance to the planning of the low-voltage distribution network, strengthen the communication with local government planning and other departments, avoid disorderly construction of the distribution network, especially 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 should be reasonably divided and divided to supply power, and the distribution point should be as close to the load center as possible to avoid fan-shaped power supply and circuitous power supply. The construction of the distribution network should follow the "small capacity, multiple The principle of location selection for distribution transformers with “point layout, short radius”.
(2) In areas where low-voltage three-phase four-wire system is used for power supply, it is necessary to actively strive for the use of 3-core or 4-core cables or low-voltage clustered wires for power supply to the user end for the conditional distribution station area, so that construction on low-voltage lines To the greatest extent, avoid the occurrence of partial phase in the three-phase load. At the same time, do the low-voltage meter installation work. The single-phase meter is distributed as evenly as possible in the three-phase A, B, and C to avoid the occurrence of single-phase power only connected to one phase. Or on two phases, causing load bias at the end of the line.
(3) Multi-point grounding is used in the neutral line of the low-voltage distribution network to reduce the power loss of the neutral line. At present, due to the unbalanced distribution of the three-phase load, current appears in the neutral line. According to regulations, the neutral line current shall not exceed 25% of the phase line current. In actual operation, the resistance value of the neutral line is relatively thin due to the thinner section of the neutral line. If the phase line is large and the neutral current is too large, it will also cause a certain percentage of power loss on the wire. Therefore, it is recommended to use multiple grounding points in the public main neutral line of the low-voltage distribution network to reduce the power loss of the neutral line and avoid load imbalance. The voltage generated by the neutral line current seriously endangers personal safety, and multi-point grounding reduces the disconnection of the neutral line caused by heat and other reasons, which increases the phase voltage used by the user and damages the household appliances. In addition to the problem of zero line loss, in the current general low-voltage cable, the cross section of the zero line is 1/2 of the phase line, and the large resistance value causes the loss of the zero line to increase when the three-phase load is unbalanced. This can be considered Properly increase the conductor cross-section of the neutral wire, such as using a five-core cable, with 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 loads account for a large proportion. At present, most of the load electrical appliances in urban residential areas use single-phase electricity. Since the line loads are mostly power and lighting mixed loads, and the use of electrical equipment is low, the low-voltage three-phase load is not in actual operation. The balance is greater. In addition, judging from the current situation of electricity consumption in rural areas, in many underdeveloped and underdeveloped rural areas, there are problems such as low per capita electricity consumption, scattered residences, and long power supply lines. For these areas, it can be considered that the users are more dispersed and used. When the electric load is mainly lighting and the load is not large, a single-phase transformer is used for power supply to achieve the purpose of reducing losses and construction funds. At present, the loss of single-phase transformers is reduced by 15%-20% compared with three-phase transformers of the same capacity. The single-phase transformers produced by some manufacturers can lead to two voltage levels of 380V and 220V on the low-voltage side. At the same time, multiple single-phase transformers have been used in some areas. The pilot phase change to three-phase load power supply provides a broader space for the use of single-phase change power supply.
(5) Actively carry out actual measurement and adjustment of transformer load. The actual load measurement work of the distribution transformer seems simple, but there are several points to note in actual work. First, the actual measurement work cannot simply measure the phase current of the three-phase lead wires of the low-voltage side A, B, and C of the distribution transformer, and it must measure zero. The current on the line, or the voltage of the neutral line (row) to the ground can be measured to better compare the unbalance of the three-phase load. The second is that the actual measurement work should extend to the end and branch end of the low-voltage distribution line. It is possible to further discover the location of unbalanced load and determine the load adjustment point. The third is that the load measurement should 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 should be increased. , Through timely measurement of the distribution transformer low-voltage outgoing line and the low-voltage line current close to the user end, it is convenient to accurately understand the operation of the equipment and do a balanced and reasonable load distribution.