Management of unbalanced operation of three-phase

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Management of unbalanced operation of three-phase load of distribution transformer

management of unbalanced operation of three-phase load of distribution transformer Jiang Tianfu Zhao Jingliang Nianzishan Power Supply Bureau of Qiqihar, Heilongjiang Province (161046) there are 193 distribution transformers in the urban area of Nianzishan Power Supply Bureau, with a total capacity of 25305kva In recent years, due to the unbalanced three-phase load of distribution transformers, there are many problems in operation, which are mainly manifested in: some transformers operate uneconomically, the transformer failure rate is high, individual contacts are frequently overheated and burned, and the voltage in individual stations changes greatly, burning user equipment. In 2001, Nianzishan Power Supply Bureau measured the load of all distribution transformers in the urban area. The results showed that 355 transformers had unqualified three-phase current imbalance, 155 transformers had imbalance of more than 255, and the highest reached 755 1. Influence of transformer load imbalance on the system 1. Increase line loss when the three-phase load of distribution transformer is unbalanced, the increase of line loss is manifested in two parts: first, increase the loss of distribution transformer; The second is to increase line loss

based on the increased loss of low-voltage line, it is analyzed according to three cases (three-phase imbalance is!): One phase load is heavy and one phase load is light, The third phase is the average load: the power loss on the line per unit length is: "1 the regulation stipulates that the imbalance degree r should not be greater than 205. After calculation, when r70.2,%71.11, that is, the line loss caused by three-phase imbalance is 73.67. It is estimated that the line loss caused by three-phase imbalance will increase by 805.76, and the calculated line loss will increase by 5 times.

in 2001, the statistics of line loss calculation of Nianzishan Power Supply Bureau showed that Under the influence of load current imbalance, the power loss of low-voltage line increases, and the loss of transformer itself increases, which increases the system loss

reduce the utilization rate of the transformer and threaten the safe operation. The rated capacity of the distribution transformer is designed according to the winding of each phase. When the distribution transformer operates under the unbalanced state of three-phase load, the phase with high load of the transformer often fails, such as lack of phase, overheating of contacts, deterioration of individual sealant pads, etc. In 2001 alone, due to the unbalanced load of the transformer, there was a power outage for repair, accounting for 735 of the total, resulting in the abnormal operation of the transformer for more than 100 hours

at the same time, the distribution transformer operates under the unbalanced state of three-phase load and generates zero sequence current at the low-voltage side. For the distribution transformer with y/y connection, there is no neutral line on the high-voltage side of the transformer, so it is impossible to bring convenience to the installation and pre-test on the high-voltage side. As long as the falling fuse is taken and the safety of China Academy is made, in case of lightning stroke, the lightning voltage breaks through the insulator and discharges to the ground, which can avoid the lightning arrester and transformer directly bearing too steep lightning current, and ensure the safety of the distribution transformer

in addition, the shorter the lightning arrester grounding down lead (i.e. the connection with the distribution transformer housing), the better. Because even if the grounding wire is 0.6m long, its inductance L is about 1mH, and the lightning wave steepness is 10ka/! S, the voltage drop on the grounding wire also reaches a large value of nearly 10kV. Its superposition with the residual voltage of lightning arrester on the insulation of distribution transformer will also greatly aggravate the damage. Therefore, for the high-voltage side, the lightning arrester should be installed at the lower end of the high-voltage drop fuse. This can not only reduce the length of the grounding lead, but also give the arrester full measures, which will not affect the operation of the high-voltage line; Secondly, when the lightning arrester is of poor quality and the discharge arc cannot be extinguished, the power frequency freewheeling will fuse the high-voltage drop fuse and the fuse tube will fall automatically, which can avoid the impact on the power supply of the high-voltage line and reduce the tripping rate of the line

6 the distribution transformer in the lightning strike area should be appropriately replaced. For lightning, the commanding point is just the zone most vulnerable to lightning strike or inductive discharge. Therefore, in addition to considering the load center, when configuring distribution transformers in mountainous areas, while handling low-voltage outgoing lines, we should try to avoid lightning strike areas, do not install distribution transformers at the commanding height, and take lightning protection measures

zero sequence current, the zero sequence magnetic flux generated by the zero sequence current on the low voltage side cannot be offset. Therefore, the zero sequence magnetic flux can only pass through the oil tank wall of the distribution transformer and the steel components. The hysteresis and eddy current generate heat in the steel components, resulting in lower heat dissipation conditions and higher temperature rise of the distribution transformer. In serious cases, the insulation of the transformer is damaged and the distribution transformer is burned

impact on electrical equipment when the three-phase load of the distribution transformer is unbalanced, the neutral point will shift, and the single-phase voltage may rise to the line voltage when the shift is serious

if the line grounding protection is not good, the voltage generated by the neutral line current will seriously endanger personal safety. At the same time, current imbalance will cause single-phase equipment to fail to use electricity normally, or overvoltage will burn user equipment

in 2001, Nianzishan Power Supply Bureau found that 56% of the abnormal voltage was caused by the unbalanced three-phase current of the transformer, among which 12 single-phase equipment of users were burnt out, with a loss of 4 70000 yuan

influence of unbalanced three-phase load of transformer on system voltage. When the transformer operates with unbalanced three-phase load, EPS board is a good thermal insulation material. Due to the different voltage drop of transformer winding, the outlet voltage is unbalanced, and the voltage at the user end has a large three-phase deviation, so the voltage quality cannot be guaranteed

2 there is not another problem that affects the three-phase load of the transformer. The reason for the balance cannot be ignored. According to the current situation of low-voltage distribution in our bureau and years of operation and management experience, the main reasons for the unbalanced operation of the three-phase load of the distribution transformer are: 2.1 there are weak links in management. Due to insufficient attention to the operation and management of the unbalanced three-phase load of the distribution transformer, there has been no assessment management method, The management of the three-phase load of the distribution transformer is blind and random, so that the operation and maintenance personnel relax the management of the three-phase load of the distribution transformer, resulting in the long-term operation of most distribution transformers in the extremely unbalanced state of the three-phase load

the influence of single-phase electrical equipment is mainly due to the mixed load of power and lighting. However, the simultaneous use rate of single-phase electrical equipment is low, and the horizontal power consumption difference of users is large, which often leads to the imbalance of three-phase load of distribution transformer, and increases the difficulty of management

the impact of unreasonable power pattern. The low-voltage power structure is weak, the operation time is long, and the transformation investment is not complete. The single-phase low-voltage line is the main frame problem in the substation area, which has not been effectively cured

secondly, most residential electricity is supplied by single-phase power supply, which extends disorderly when the load develops, resulting in the imbalance of three-phase current in the substation area and cannot be adjusted. For such a low-voltage network, we must invest a large amount of money, completely solve the low-voltage layout, and increase the coverage area of the low-voltage four wires, which will greatly improve the line loss, voltage quality, power supply reliability, power supply safety, etc

temporary power consumption and seasonal State Bureau of quality and technical supervision (3) 0 approved the implementation of movie sound, temporary power consumption and seasonal power consumption have a certain timeliness. After no charge for power capacity increase, in the production season of the greenhouse, single-phase water pumps are widely used, and the distribution is extremely scattered, so the power consumption time is not easy to grasp. At the same time, due to the failure to consider the distribution of its three-phase load in management, it is not able to monitor in time Adjusting the three-phase load of distribution transformer, its use and power failure have a great impact on the balance of three-phase load of distribution transformer, especially when the capacity of single-phase electrical equipment is large

influence of line fault due to improper operation and maintenance and management or external force damage, low-voltage conductor disconnection, transformer phase loss operation, untimely repair or temporary treatment on site may cause a phase to lose part of the load for a long time, which will make the distribution transformer operate in an unbalanced state

from the above analysis, for example, rice husk and straw can be ground into powder and added to the material. The factors affecting the unbalanced operation of three-phase load of distribution transformer are subjective and objective, both from the power supply department and users

3 measures to prevent transformer load imbalance: 1. Formulate the operation management system of transformer load imbalance, and assess the unbalanced degree of three-phase load of transformer quarterly based on the amount of electricity read by the transformer electric energy measurement assessment box. Set up special rewards and punishments for transformer load management at different levels

the load shall be measured at least once a month. Under special circumstances (such as peak load, when the load changes greatly, etc.), the number of measurements can be increased to know the load condition of the distribution transformer well, so as to provide accurate and reliable data for adjusting the load of the distribution transformer. The relevant management personnel of the power distribution discipline shall regularly or irregularly conduct supervisory measurement of the three-phase load condition of the distribution transformer, and master the first-hand materials as the basis for professional management and assessment

transform the electricity, increase the coverage density of low-voltage four wires, grasp the dynamic distribution of three-phase load, and reasonably design the electricity transformation scheme in combination with the urban transformation

the distribution transformer is set with a load center, and the power supply radius is not greater than, The three-phase four wire system is used for power supply of trunk lines and branch trunk lines. It is recommended that residents with more than 5 households do not use single-phase power supply. At the same time, the wiring diagram of load distribution in the substation area should be formulated to ensure that the power transformation access system of any user is limited by the three-phase load balance and avoid the randomness of transformation

strengthen the management of power supply and consumption to ensure the balance of transformer load. Power consumption and distribution should be closely coordinated. The users' temporary power consumption, seasonal power consumption and distribution transformer operators should be informed in time. Especially for single-phase equipment, after a year of adjustment, the imbalance has decreased significantly, and the ability of safe and economic operation has been significantly enhanced

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