Three-phase imbalance is an important indicator of power quality. Although there are many factors that affect the power system, the unbalanced nature is mostly due to three-phase cloud components, line parameters or load asymmetry. Because the factors of three-phase load are not necessarily, the three-phase voltage and current of the power supply point are easily unbalanced and the circuit is lost. Not only that, it will also have an adverse effect on the motor on the power supply point, and endanger the normal operation of the motor. Therefore, if the three-phase imbalance exceeds the range that the distribution network can tolerate, the overall power system's safe operation will be affected. This article first introduced the causes of the three-phase imbalance, followed by the introduction of the impact of three-phase imbalance, and finally elaborated the measures to solve the three-phase imbalance, the specific follow Xiaobian together to find out.
1, broken line failure
If one phase is disconnected but not grounded, or the circuit breaker and disconnector are not connected in one phase, the fuse of the voltage transformer fuse causes all three-phase parameter asymmetry. When a voltage level line is disconnected in one phase, the voltage of the next voltage level shows that all three phase voltages are reduced, one phase is lower, and the other two phases are higher, but the voltages of the two are close. When the current line breaks, the disconnected phase voltage is zero, and the unbroken phase voltage is still the phase voltage.
2, resonance caused by three-phase voltage imbalance
(1) Fundamental resonance
Fundamental resonance, characteristic similar to single-phase grounding, that is, the voltage of one phase decreases and the voltages of the other two phases increase. It is not easy to find the fault point when searching for the cause of failure. At this time, special users can be checked. If it is not the grounding reason, it may be due to resonance. .
(2) Frequency division resonance
The other is frequency-divided resonance or high-frequency resonance, characterized by the simultaneous rise in three-phase voltage.
In addition, it is also important to note that if a ground fault signal occurs when one or more of the phase dropouts or the single-phase earth faults of an airdrop busbar are removed, and the voltage of one, two, or three phases exceeds the line voltage, the voltage meter pointer hits the head and moves slowly at the same time, or The three-phase voltage increases in turns over the line voltage. In this case, it is generally caused by resonance.
3, ground fault
When the line is disconnected and single-phase grounded, the three-phase voltage is unbalanced, but the voltage value does not change after the grounding. Single-phase grounding is classified into metallic grounding and non-metallic grounding. Metallic grounding, fault phase voltage is zero or near zero, non-fault phase voltage rises 1.732 times, and lasting; non-metallic grounding, ground phase voltage is not zero but reduced to a certain value, the other two phase rise Less than 1.732 times higher.
Causes of Resonance With the rapid development of industry, a large number of non-linear power loads increase, and some loads not only generate harmonics, but also cause fluctuations in power supply voltage and flicker, and even cause three-phase voltage imbalance.
4. Constant changes in electricity load
The reasons for the instability of the electricity load include the frequent demolition and relocation of the land II, increase in the number of shifting tables or electricity users, and the instability of temporary electricity and seasonal electricity consumption. In this way, the total amount and time uncertainty and decentralization make the power load also have to follow the actual situation.
5. Unreasonable distribution of three-phase load
Many of the workers who installed the power meter do not have the professional knowledge concept of three-phase load balancing. Therefore, they did not notice that the three-phase load balancing should be controlled when they were connected, but only blindly and randomly connected to the circuit. This, to a large extent, caused an imbalance in the three-phase load. Second, most of the circuits in China are mixed with power and lighting. Therefore, when using single-phase power equipment, the efficiency of power consumption will be reduced. This difference further exacerbates the three-phase load of distribution transformers. Balanced condition.
6, weakening the monitoring of distribution load
In the management of distribution networks, management issues in three-phase load distribution are often overlooked. 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 imbalance, such as the impact of the line and the three-phase load moment is not the same.
8, overloaded
The motor is in an overloaded operating state, especially when starting, the motor stator and rotor current increase heat. The time is slightly longer and the imbalance of the winding current is easily occurred. The overload is mainly manifested in:
(1) Belts, gears, and other transmission mechanisms are too tight or too loose.
(2) The coupling mechanism is skewed, and the drive mechanism is stuck with foreign matter.
(3) The lubricating oil is dry, the bearing is stuck, and the mechanical rust is dead (including the mechanical failure of the motor itself).
(4) If the voltage is too high or too low, the loss increases.
(5) Improper load matching, the rated power of the motor is less than the actual load.
9, improper operation, maintenance
Operators cannot regularly perform the inspection and maintenance of electrical equipment. This is a major factor that causes motor leakage and phase-to-phase operation and generates unbalanced currents. The improper operation and maintenance are mainly manifested in:
(1) Operation The installer will reverse the phase and neutral lines.
(2) The incoming line collides with the junction box and there is leakage current.
(3) The lack of phase caused by the connection switches, loose contacts, and oxidation.
(4) Frequent start, too long or too short starting time, causing the fuse to break.
(5) Long-term use, lack of maintenance, aging of the motor and degradation of local insulation.
1, increase the power loss of the line
In the three-phase four-wire power supply network, when the current passes through the line conductor, power loss will be generated due to the 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, the three-phase load imbalance is inevitable because of the presence of a single-phase load. When the three-phase load is unbalanced, there is current through the neutral line. This not only has a loss of phase, but also a loss of the neutral line, which increases the loss of the power line.
2. Increase the power loss of distribution transformers
The distribution transformer is the main power supply equipment for the low-voltage power grid. When it is operated under unbalanced three-phase load conditions, the distribution transformer loss will increase. Because the power loss of the distribution varies with the degree of load imbalance.
3. Zero-sequence current generated by distribution transformer
The distribution transformer operates under the three-phase load unbalanced condition and will generate a zero-sequence current, which will change with the degree of unbalance of the three-phase load. The greater the unbalance, the greater the zero-sequence current. If there is a zero-sequence current in the transformer during operation, zero-sequence flux will be generated in the iron core. (There is no zero-sequence current on the high-pressure side.) This forces the zero-sequence flux to only pass through the tank wall and the steel member as a passage. The steel member has a low permeability, and hysteresis and eddy currents are generated when the zero-sequence current passes through the steel member. Loss, so that the distribution of the temperature of the steel component to increase heat. The winding insulation of the transformer is accelerated due to overheating, resulting in a reduction in the life of the equipment. At the same time, the presence of zero-sequence current will also increase the loss of transformer.
4, the motor efficiency is reduced
The distribution transformer operates under unbalanced three-phase load conditions and will cause unbalanced three-phase output voltage. Because the unbalanced voltage has three voltage components of positive sequence, negative sequence, and zero sequence, when the unbalanced voltage is input to the motor, the negative sequence voltage generates a rotating magnetic field and the rotating magnetic field generated by the positive sequence voltage acts as a brake. effect.
1. Pay attention to planning and strengthen communication
Pay attention to the planning of low-voltage distribution network, strengthen the communication with the local government planning and other departments, avoid the disorderly distribution network construction, and especially avoid the situation in the low-voltage distribution network where there are headaches and headaches. During the construction and reconstruction of the power grid, reasonable subdivision power supply is provided to the low-voltage station area, and distribution distribution points are as close to the load center as possible to avoid fan-type power supply and circuitous power supply. The construction of the distribution network must follow the principle of “small capacity, multiple distribution points, and short "Radius" principle of location allocation.
2, for low voltage bundled wire power supply
In regions where low-voltage, three-phase, four-wire power supply is used, it is necessary to actively strive to use three-core or four-core cables for the conditional distribution station or use low-voltage bundled conductors to supply power to the subscribers, so that they can be used to the maximum extent in the construction of low-voltage lines. To avoid the occurrence of phase error in the three-phase load, and to perform low-voltage metering work, the distribution of the three-phase A, B and C phases of the single-phase power meter should be as uniform as possible to avoid the occurrence of single-phase power only in one phase or two phases. At the end of the line, it causes the load to be out of phase.
3, multi-point grounding, reducing the zero-line power loss
The use of multi-point grounding in the zero line of the low-voltage distribution network reduces the loss of zero-line power. At present, due to the unbalanced distribution of the three-phase load, a current occurs in the neutral line. According to the regulations, the neutral current must not exceed 25% of the phase current. In actual operation, since the cross section of the neutral wire is finer, the resistance value is longer than the same length. Large phase lines, excessive current in the zero line will also cause a certain percentage of power loss in the wire, so it is recommended to use a multi-point grounding in the common main zero line of the low voltage distribution network to reduce the zero line power loss and avoid the occurrence of load imbalance The voltage generated by the zero-line current seriously jeopardizes personal safety, and through the multi-point grounding, the zero-line breakage of the strand due to heat and other causes is reduced, and the phase voltage used by the user is increased to damage the household appliance. In addition, for the zero-line loss problem, 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 zero line loss to increase when the three-phase load is unbalanced. Appropriately increase the wire cross section of the zero line, for example, using a five-core cable, one core wire for each phase and two core wires for the zero wire.
4, install three-phase unbalanced intelligent regulator
Leading technology research and development team, for the three-phase imbalance problem, developed a three-phase unbalanced intelligent regulator - STUR. The product effectively solves this problem. The device has the function of adjusting the unbalanced active current while compensating the system for reactive power. The theoretical result is that the three-phase power factor can be compensated to 1, and the three-phase current is adjusted to balance. The practical application shows that the three-phase power factor can be compensated to more than 0.95, so that the unbalanced current is adjusted within 10% of the rated current of the transformer.
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