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Distribution voltage three-phase imbalance

2020-06-30 15:28:19

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The three-phase load imbalance of low-voltage distribution network will not only increase the loss of transformers and lines, but also affect the safe operation of equipment, so it is of great practical significance to control the three-phase load imbalance. Firstly, this paper studies the mechanism and classification of load unbalance in low-voltage distribution network. Secondly, the principle and engineering applicability of several three-phase load imbalance treatment devices are compared. Finally, the engineering operation data of the new scheme based on intelligent phase selector switch are given. The operation data of 15 months show that this technology can solve the problem of low voltage load imbalance in distribution network, has the advantages of obvious effect, low cost and high reliability, and is suitable for popularization and application. 


1.1 introduction. 


Three-phase load imbalance in low-voltage distribution network is a long-term objective problem. Low-voltage distribution network usually adopts three-phase four-wire power supply mode, and most users are single-phase load or single-phase and three-phase mixed load. Whether it is the residential areas, office buildings of the urban distribution network, or the lighting, water pumps, motors and other loads of the rural power network, they all show a large discreteness of the spatio-temporal characteristics of electricity consumption. The three-phase load imbalance of low-voltage distribution network will increase the loss and affect the safe operation of equipment. With the large-scale access of distributed new energy, especially the access of 220V single-phase new energy, the problem of load imbalance in distribution network will be more prominent. 


1.2 Mechanism of low voltage load unbalance in distribution network. 


The asymmetric access of low voltage load between three phases in distribution network is the basic mechanism of imbalance. Further, according to the performance characteristics of three-phase load asymmetry, there are the following categories: 


Class I-imbalance in the cause of user wiring. 


Due to the lack of planning and management of the low-voltage station area, the distribution of the number of users between the three phases is uneven, or there is a great difference in electricity consumption among the users of each phase. This kind of performance is that the phase with large load is always large, and the phase with low load is always small, and the proportion of difference is basically unchanged at all times of the day. 


II class-temporal imbalance. 


II class-temporal imbalance. In the light load period during the day, the three-phase load is basically balanced, and the imbalance is serious in the peak load period at night. Or the situation where the factory three-phase production electricity and single-phase domestic electricity are mixed, during the day, the three-phase production electricity is mainly used, and the interphase current is more balanced. At night, during the peak period of domestic power consumption, the three-phase current is very different. 


III class-seasonal imbalance. 


III class-seasonal imbalance. Due to the change of the proportion of three-phase production electricity and single-phase domestic electricity in each season, and the uneven distribution of single-phase load in the three-phase. Typical such as irrigation in busy season, air conditioning in summer and so on. 


IV Class-Random imbalance. 


The current of each phase varies with time and has no regularity. This situation reflects the large fluctuation of the single-phase load, but the fluctuation is not synchronous in the three-phase.


   

Fig. 1 Classification of distribution low voltage imbalance


According to the unbalanced type of three-phase load in distribution network, the corresponding unbalanced control strategy and regulating device can be adopted. For the long-term regular and seasonal imbalance of Class I and IV, artificial or automatic intelligent equipment is used to change the load access periodically to balance the interphase load. For the imbalance of II and Class III random changes, it is appropriate to use automatic intelligent equipment to adjust the interphase load. 


1.3 Engineering applicability of several unbalanced treatment devices. 


In the State Grid Corporation's "Circular of the State Grid Operation and Inspection Department on carrying out the treatment of three-phase load imbalance in distribution stations" (Operation Inspection No. 3 (2017) 68), the following control measures are given: 


1. Take operation and maintenance control measures. 

2. Three-phase four-wire power supply into households to minimize two-wire power supply. 

3. 

When the power factor is greater than 0.85, the low voltage load automatic commutation device should be selected in combination with the construction and transformation of the distribution transformer area. 

4. 

For the distribution transformer area where the power factor is less than 0.85, one of the measures should be selected according to the actual situation of the distribution transformer area. 

a. 

The capacitor reactive power compensation device is installed or modified on the low-voltage side of the distribution transformer and the low-voltage load automatic commutation device is installed at the same time. 

b. 

For the distribution transformer station where there are few low-voltage branch lines and the load is mainly concentrated on a few branch lines, the mode of independent decentralized compensation of phase-to-phase reactive power compensation device is adopted. 

c. 

For the distribution transformer area where there are few low-voltage branch lines and the load is mainly concentrated on a few branch lines, the independent decentralized compensation mode of power quality comprehensive management device is adopted. 

d. 

The mode of "independent centralized compensation of power quality comprehensive control device" or "centralized compensation of power quality comprehensive management device + automatic commutation device of low voltage load" is adopted. 

e. 

Among them, changing the load access phase and evenly distributing the load capacity borne by each phase is the most direct and fundamental method. 

The load access switch can use thyristor, compound switch, intelligent phase selection switch and so on. 

The three-phase load unbalance control device based on the interphase span capacitor and the low-voltage static var generator (SVG) mainly realize the three-phase load impedance balance by dynamically compensating the network impedance.



   

Fig. 2 comparison of schematic diagram of unbalanced treatment device



Two mainstream technical solutions, (a) and (d), are selected and compared with the latest scheme (c). The comparison between typical application scenarios and engineering applicability is as follows:

表2 工程适用性对比

Through the above comparison, the load imbalance treatment technology of low-voltage distribution network based on intelligent phase selection switch uses the power electronic components of diodes, contactors and relays as power electronic switches, which simplifies the control circuit and has a short switching time between phases. Arc-free switching can greatly prolong the life of contactors and auxiliary relays and improve the reliability of equipment. Therefore, this scheme is selected to further evaluate the effect in the actual project. The operation effect of other schemes can be reported in the existing literature. 


1.4 Engineering application effect. 


A residential area in Yangquan City, Shanxi Province is selected as the demonstration application area. The station area is powered by 1-way 10kV, and the transformer in the station area is 630kVar, with a total of 372low voltage users. The low-voltage distribution network is powered by a 10kV transformer, and its low-voltage side is connected with a number of cable tap boxes, each of which has one three-phase input and multiple single-phase output. The low voltage load in the station area is respectively connected to the outlet side of the cable tap box. The structure of the power supply station is shown in figure 3:

   

Fig. 3 wiring diagram of the station area

Installing an intelligent phase selector in the cable tap box can improve the intelligent level, and only about 30% to 50% of the total intelligent cable tap box can achieve the treatment effect of load imbalance in the whole Taiwan area. According to the actual situation of the site, the intelligent phase selector switch of this project is installed at the tap box of the cable, and the load borne by each of the three phases is evenly distributed through the commutation operation. 


After the device is put into operation, a good regulation effect is obtained:


   

Fig. 4 switching process waveform

In figure 4, waveform 1 is the switching control command issued by the superior control, and waveform 2 is the load voltage during the commutation operation. 

The waveform shows that the intelligent phase selector switch can quickly realize the commutation switching operation in the 5ms, which can ensure the reliable and continuous power supply of the load. 


According to the data of the rural distribution network operation monitoring system of the State Grid, the evening peak hour at 19:00 on January 1, 2016 is selected as the representative time, and the operation data of the station area are shown in figure 5:

   

Fig. 5 data of operation monitoring system

At the same time, January 1, 2016 is selected as the representative day, and the imbalance of the whole day is shown in figure 6:


   

Fig. 6 all-day imbalance in the post-operation area.

Compare the data of Taiwan district representative days before and after operation, as shown in Table 3:

   

Table 3 comparison of data before and after throwing


At present, the device has been running steadily and safely for 15 months. According to the field operation data, the following benefits can be obtained by using the device: 


1.The quality of power supply is improved and the balance of three-phase load is ensured. The imbalance of the station area remains less than 10% for a long time during operation. 


2. Adjusting the three-phase unbalanced current can effectively reduce the loss of transformer and line. According to the data of the rural distribution network monitoring system of the State Grid Corporation, during the operation of the device, the line loss in the Taiwan area has been reduced by an average of 2.37%, and about 50,000 kilowatt-hours of electricity can be saved in the whole year, thus saving energy and reducing consumption. 


3. By adjusting the three-phase unbalanced current, the three-phase load is balanced, the equipment utilization rate of the transformer is improved, the probability of transformer and neutral line burnout due to overload is reduced, and the life of the equipment is prolonged. 


4. The automation level of distribution equipment is improved and the frequent manual commutation of power workers is avoided. 


1.5 conclusion. 


It is of great practical significance to control the unbalance of low-voltage three-phase load in distribution network. According to the treatment requirements, the basic principles and engineering applicability of three kinds of unbalanced treatment devices are compared. The engineering verification of one of the intelligent phase selector switches is given. The actual operation shows that the load imbalance control technology of low-voltage distribution network based on intelligent phase selector switch has the advantages of obvious effect, low cost and high reliability, and it is one of the effective methods to solve the problem of load imbalance of low-voltage distribution network.

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