Distinguishing and judging methods and solutions of three-phase voltage unbalance

2022-08-20


There are many reasons for three-phase voltage imbalance,such as single-phase grounding,broken line resonance,etc.the operation management personnel can quickly deal with them only by correctly distinguishing them.

1、Disconnection fault:if one phase is disconnected but not grounded,or one phase of the circuit breaker and disconnector is not connected,and the fuse of the voltage transformer is blown,the three-phase parameters will be asymmetric.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 three phases decreases,one of which is lower and the other two are higher,but the voltage values of the two are close.When the line at this level is disconnected,the voltage of the disconnected phase is zero,and the voltage of the non disconnected phase is still the phase voltage.
2、Grounding fault:when one phase of the line is disconnected and one phase is grounded,although the three-phase voltage is unbalanced,the voltage value does not change after grounding.Single phase grounding is divided into metal grounding and non-metal grounding.Metallic grounding,fault phase voltage is zero or near zero,non fault phase voltage increases by 1.732 times and remains unchanged;For non-metallic grounding,the voltage of the grounding phase is not zero but decreases to a certain value,and the other two phases rise less than 1.732 times.
3、Resonance reason:with the rapid development of industry,the nonlinear power load increases greatly.Some loads not only generate harmonics,but also cause the fluctuation and flicker of power supply voltage,and even cause the three-phase voltage imbalance.
There are two kinds of three-phase voltage imbalance caused by resonance
One is 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 increases.It is difficult to find the fault point when finding the fault cause.At this time,special users can be checked.If it is not the grounding cause,it may be caused by resonance.
The other is frequency division resonance or high-frequency resonance,which is characterized by three-phase voltage rising at the same time.
In addition,it should be noted that when the air drop bus cuts off part of the line or the single-phase grounding fault disappears,if there is a grounding signal and the voltage of one phase,two phases or three phases exceeds the line voltage,the voltmeter pointer will hit the head and move slowly at the same time,or the three-phase voltage will rise in turn to exceed the line voltage,which is generally caused by resonance.

Harm and influence of three-phase unbalance
1.Damage to transformer.When the three-phase load is unbalanced in the production and living electricity,the transformer will be in an asymmetric operation state.Resulting in increased loss of the transformer(including no-load loss and load loss).According to the transformer operation specification,the neutral line current of the transformer in operation shall not exceed 25%of the rated current at the low-voltage side of the transformer.In addition,unbalanced operation of 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 down.
2.Impact on electric equipment.The occurrence of three-phase voltage imbalance will lead to the occurrence of several times of current imbalance.The reverse torque in the induction motor increases,so that the temperature of the motor increases,the efficiency decreases,the energy consumption increases,the vibration occurs,and the output loss is affected.The imbalance between phases will shorten the service life of electrical equipment,accelerate the replacement frequency of equipment components,and increase the cost of equipment maintenance.The allowable current margin of the circuit breaker is 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 the thickening of the neutral line.
3.Influence on line loss.Three phase four wire connection mode,when the three-phase load is balanced,the line loss is small;When one phase load is heavy and two phase load is light,the line loss increment is small;When one phase load is heavy,one phase load is light,and the load of the third phase is the 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,the greater the current imbalance,the greater the line loss increment,regardless of the load distribution.
Harm and solution of three-phase unbalance
1、Main hazards caused by unbalanced three-phase voltage or current:
1.When the rotating electric machine operates in an asymmetric state,the rotor will generate additional loss and heat,which will cause the overall or local 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 will be formed in the stator.
2.The misoperation of various protections with negative sequence component as the starting element directly threatens the operation of power grid.
3.Unbalanced voltage causes non characteristic harmonics in silicon rectifier.
4.For the generator and transformer,when the three-phase load is unbalanced,if the large phase current is controlled to the rated value,the other two phases will not be fully loaded,so the equipment utilization rate will drop.On the contrary,if the rated capacity is maintained,the one phase with large load will be overloaded,and the unbalanced magnetic circuit will cause waveform distortion,and the additional loss of equipment will increase.
2、Solutions to three-phase voltage imbalance of power grid caused by asymmetric load:
1.The asymmetric load is connected to different power supply points to reduce the problem that the imbalance caused by centralized connection seriously exceeds the standard.
2.Use the method of cross exchange to make the asymmetrical load reasonably distributed to each phase and try to make it balanced.
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.
Several measures to solve three-phase load unbalance
1、Pay attention to the planning of the low-voltage distribution network,strengthen the communication with the local government planning and other departments,avoid the disorder of the distribution network construction,especially avoid the situation of treating the headache and the foot pain in the low-voltage distribution network.During the construction and transformation of the distribution network,the low-voltage station area shall be provided with reasonable power supply in sections and sections,and the distribution and transformation points shall be as close to the load center as possible to avoid fan power supply and circuitous power supply,The construction of distribution network shall follow the principle of"small capacity,multiple distribution points and short radius"for distribution and transformation site selection.
2、In areas where low-voltage three-phase four wire system is used for power supply,it is necessary to actively strive to use 3-core or 4-core cables or low-voltage bundled conductors to supply power to the user end in the distribution station area where conditions are met,so as to avoid the occurrence of phase deviation of three-phase load to a large extent in the construction of low-voltage lines.Meanwhile,it is necessary to do a good job in low-voltage meter installation,and the distribution of single-phase meters in three phases a,B and C is as uniform as possible,Avoid that single-phase power is only connected to one phase or two phases,resulting in load phase deviation at the end of the line.
3、Multipoint grounding is adopted for the zero line of low-voltage distribution network to reduce the power loss of the zero line.At present,due to the unbalanced distribution of three-phase loads,the zero line has current.According to the regulations,the zero line current shall not exceed 25%of the phase line current.In actual operation,due to the small cross section of the zero line conductor,the resistance value is larger than that of the phase line of the same length,and the 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 on the public main zero line of the low-voltage distribution network to reduce the power loss of the zero line,Avoid the voltage generated by the zero line current due to load imbalance,which seriously endangers personal safety.Moreover,through multi-point grounding,the zero line breaking due to heating and other reasons is reduced,which makes the phase voltage used by users rise and damages household appliances.In addition,for the loss of zero line,in the current general low-voltage cable,the section of zero line is 1/2 of the phase line,and the large resistance value causes the loss of zero line to increase when the three-phase load is unbalanced.Therefore,it can be considered to appropriately increase the conductor section of zero line,for example,five core cable is used,one core wire is used for each phase and two core wires are used for zero line.
4、We will actively promote single-phase transformer power supply in power supply areas where single-phase load accounts for a large proportion.At present,most of the load appliances in urban residential areas are single-phase power.Since the line loads are mostly mixed loads of power and lighting,and the use rate of electrical equipment is low,the imbalance of low-voltage three-phase loads in actual operation is greater.In addition,according to the current domestic power consumption in rural areas,there are many problems such as small per capita power consumption,scattered residence and long power supply lines in many underdeveloped and undeveloped areas.For these areas,the single-phase transformer can be used for power supply in order to reduce losses and construction funds,considering that the users are scattered,the power load is mainly lighting and the load is small.At present,the loss of single-phase transformer is 15%~20%less than that of three-phase transformer with the same capacity.The single-phase transformer produced by some manufacturers can lead out two voltage levels of 380V and 220V at the low-voltage side.At the same time,the pilot of using multiple single-phase transformers to supply power to three-phase loads has been carried out in some areas,providing a broader space for using single-phase transformers to supply power.
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 noticed in the actual work.First,the measurement cannot simply measure the phase current of the three-phase outgoing lines a,B and C at the low-voltage side of the distribution transformer,but also the current on the zero line or the voltage of the zero line(row)to the ground,so that the imbalance of the three-phase load can be better compared.Second,the measurement should extend to the end and branch end of the low-voltage distribution line,In this way,the occurrence place 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 large user load operation and peak load period,the number of measurement should be increased.By timely measuring the low-voltage outgoing line of the distribution transformer and the low-voltage line current close to the user,it is easy to accurately understand the operation of the equipment and do a balanced and reasonable load distribution.

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