10
2022
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02
Distinguishing Judgment Method and Solution of Three-phase Voltage Unbalance
Author:
Disconnection fault If one phase is disconnected but not grounded, or one phase of circuit breaker and isolating switch is not connected, the fuse of voltage transformer will cause asymmetry of three-phase parameters. When one phase of the previous voltage level line is broken, the voltage of the next voltage level shows that the voltages of the three phases are all reduced, one of which is lower, the other two are higher, but the voltage values of the two are close. When the line of this stage is broken, the broken phase voltage is zero, and the unbroken phase voltage is still the phase voltage.
There are many reasons for the three-phase voltage imbalance, such as: single-phase grounding, broken line resonance, etc., the operation management personnel can quickly deal with it only by correctly distinguishing it.
1. disconnection fault If one phase is disconnected but not grounded, or one phase of circuit breaker or isolating switch is not connected, the fuse of voltage transformer will cause asymmetry of three-phase parameters. When one phase of the previous voltage level line is broken, the voltage of the next voltage level shows that the voltages of the three phases are all reduced, one of which is lower, the other two are higher, but the voltage values of the two are close. When the line of this level is broken, the broken phase voltage is zero, and the unbroken phase voltage is still the phase voltage.
2. grounding fault When the line is broken and single-phase grounding, although the three-phase voltage imbalance is caused, but the voltage value does not change after grounding. Single-phase grounding is divided into two kinds of metallic grounding and non-metallic grounding. For metallic grounding, the fault phase voltage is zero or close to zero, and the non-fault phase voltage is increased by 1.732 times and will remain unchanged. For non-metallic grounding, the ground phase voltage is not zero but reduced to a certain value, and the other two phases are increased by less than 1.732 times.
Three. With the rapid development of industry, non-linear power load increases greatly, some loads not only produce harmonics, but also cause power supply voltage fluctuations and flicker, and even cause three-phase voltage imbalance.
There are two kinds of three-phase voltage unbalance caused by resonance
One is the fundamental frequency resonance, which is similar to the single-phase grounding, I .e. the voltage of one phase decreases and the voltage of the other two phases increases. It is not easy to find the fault point when looking for the cause of the fault. At this time, special users can be checked. If it is not the grounding cause, it may be caused by 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 bus cuts off part of the line or the single-phase grounding fault disappears, if there is a grounding signal and 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 rises in turn to exceed the line voltage, which is generally caused by resonance.
Hazards and effects of three-phase unbalance
1. The harm to the transformer. In the production and living electricity, when the three-phase load is unbalanced, the transformer is in an asymmetrical operation state. The loss of the transformer increases (including no-load loss and load loss). According to the transformer operation regulations, the neutral line 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 transformer zero-sequence current to be too large, the temperature of the local metal parts will increase, 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. The reverse torque in the induced motor increases, so that the temperature of the motor rises, the efficiency decreases, the energy consumption increases, the vibration occurs, the output loss and so on. The imbalance between the phases will shorten the service life of the electrical equipment, accelerate the frequency of replacement 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 easy to occur when the load changes or alternates. Excessive unbalanced current flows into the neutral line, resulting in thickening of the neutral line.
3. Impact on line loss. In the three-phase four-wire system, when the three-phase load is balanced, the line loss is small; 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, and the load of the third phase is average load, the line loss increment is large; when the load of one phase is light and the load of two phases is heavy, the line loss increment is large. When the three-phase load is unbalanced, regardless of the load distribution, the greater the current imbalance, the greater the line loss increment.
Hazards and Solutions of Three-phase Unbalance
Main hazards 1. to unbalanced three-phase voltage or current:
1. The rotating motor running in an asymmetric state will cause additional loss and heat in the rotor, which will cause the overall or local heating of the motor. In addition, the additional torque generated by the reverse magnetic field will cause the motor to vibrate. For generators, a series of higher harmonics are also formed in the stator.
2, causing a variety of protection with negative sequence component as the starting element to malfunction, directly threatening 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 large-phase current is controlled to be the rated value, the remaining two phases cannot be fully loaded, thus the utilization rate of the equipment decreases. On the contrary, if the rated capacity is to be maintained, it will cause a large phase overload, and there will also be a magnetic circuit imbalance causing waveform distortion and an increase in additional loss of the equipment.
Solutions that can be taken to 2. grid three-phase voltage unbalance caused by asymmetric loads:
1, the asymmetric load distributed in different power supply points, in order to reduce the centralized connection caused by the serious problem of imbalance exceeding the standard.
2, the use of cross-exchange equal method to make the asymmetric load reasonable distribution to each phase, as far as possible to make it balanced.
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.
Several Measures to Solve Three-phase Load Unbalance
Pay 1. attention to the planning of low-voltage distribution network, strengthen communication with local government planning and other departments, avoid disorderly construction of distribution network, especially avoid the situation of headache and foot pain in low-voltage distribution network, carry out reasonable partition and slice power supply for low-voltage station area during the construction and transformation of distribution network, and arrange distribution points as close as possible to the load center to avoid fan-type power supply and circuitous power supply, the construction of distribution network should follow the principle of "small capacity, multi-point and short radius.
2., in areas where low-voltage three-phase four-wire power supply is adopted, it is necessary to actively strive to adopt 3-core or 4-core cables or use low-voltage cluster conductors to supply power to users in conditional distribution stations, so that phase deviation of three-phase load can be largely avoided in low-voltage line construction. At the same time, low-voltage meter installation should be done well. Single-phase meters should be distributed as evenly as possible in A, B and C phases, avoid the occurrence of single-phase electricity only connected to one phase or two phases, causing load deviation at the end of the line.
3., multi-point grounding is adopted in the neutral line of low-voltage distribution network to reduce the power loss of the neutral line. At present, due to the unbalanced distribution of three-phase load, the zero line current is caused. According to the regulations, the zero line current shall not exceed 25% of the phase line current. In actual operation, due to the fine cross section of the zero line conductor and the large resistance value compared with the phase line of the same length, excessive zero line current will also cause a certain proportion of power loss on the conductor. Therefore, it is recommended to adopt multi-point grounding in the common main zero line of low-voltage distribution network to reduce power loss, avoid the voltage generated by the zero line current due to load imbalance, which seriously endangers personal safety, and through multi-point grounding, the zero line broken strand broken caused by heat and other reasons is reduced, which makes the phase voltage used by users rise and damages household appliances. In addition, for 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 zero line loss to increase when the three-phase load is unbalanced. Therefore, it can be considered to appropriately increase the wire cross section of the zero line, for example, a five-core cable is adopted, with one core line for each phase and two core lines for the zero line.
4. single-phase load accounted for a large proportion of the power supply areas to actively promote single-phase variable power supply. At present, most of the load appliances in urban residential areas are single-phase electricity, because the line load is mostly power, lighting mixed load, and the use of electrical equipment at the same time rate is low, so that the low-voltage three-phase load in the actual operation of the imbalance is greater. In addition, judging from the current living power consumption in rural areas, there are many problems in rural areas in many underdeveloped and underdeveloped areas, such as small per capita power consumption, scattered residence and long power supply lines. For these areas, it can be considered that the users are scattered, the power load is mainly lighting, and the load is not large. Single-phase transformer power supply is adopted to reduce losses and construction funds. At present, the loss of single-phase transformers is 15% ~ 20% lower than that of three-phase transformers with the same capacity. Some single-phase transformers produced by manufacturers can lead to 380V and 220V voltage levels on the low voltage side. At the same time, pilot projects of using multiple single-phase transformers to supply power to three-phase loads have been carried out in some areas, providing a broader space for using single-phase transformers.
5. actively carry out the actual measurement and adjustment of transformer load. The load measurement of the distribution transformer seems simple, but there are several points to be noted in the actual work. First, the measurement work cannot simply measure the phase current of the three-phase outgoing lines A, B and C on the low-voltage side of the distribution transformer, but also measure the current on the zero line, or measure the zero line (row) voltage to ground, so as to better compare the imbalance of the three-phase load, second, the measurement work should be extended to the end and branch ends of the low-voltage distribution line, so that the location of unbalanced load can be further found and the load adjustment point can be determined. Third, the load measurement work should be carried out regularly or irregularly, especially during the operation of large user load and during peak load, the number of measurements should be increased, and the current of low-voltage outgoing lines close to the user end should be measured in time, it is convenient to accurately understand the operation of the equipment and make a balanced and reasonable distribution of the load.
Broken wire,Fault,Circuit Breaker,Isolating switch
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