Application of reactive power compensation in 10kV transmission line _ How much reactive power compensation is normal?

[Abstract] At present, most of the reactive power compensation devices for 10kV distribution lines widely used in China are a complete set of manual switching devices, which can only be switched at high load and at low load. They can't adjust automatically with the change of load and the demand for reactive power in time, let alone realize the comprehensive automatic adjustment of voltage and reactive power compensation. In view of this, this paper discusses the application of reactive power compensation in 10kV line.

[Keywords:] reactive power compensation 10kV capacity

[China Library Classification Number] TM [Document Identification Number] A [Contribution Number]1007-9416 (2010) 03-0130-01.

1 preface

In China, the urban and rural 10kV distribution lines are widely used in one-way radiation power supply with multi-branches and large trunk lines. These lines are characterized by low load rate, large seasonal fluctuation of load, low average load rate of distribution transformers, long power supply radius, high reactive power consumption, low power factor, large line loss and poor terminal voltage quality. Due to manual operation, reactive power may be under-compensated and over-compensated. Under-compensation will affect the power quality and increase the loss, while over-compensation, namely reactive power backward transmission, will endanger the stability and safety of system operation. In order to prevent overcompensation, the installed capacity is usually small, so it is in a state of insufficient compensation for a long time. Therefore, the accuracy of the whole group compensation is low. In addition, industrial and mining enterprises with changing power load and other occasions with changing reactive power demand need automatic regulating devices to realize dynamic voltage control and reactive power compensation. Decentralized compensation for 10KV distribution lines has the advantages of small investment, quick recovery, high compensation efficiency and convenient management and maintenance. It can significantly improve the operation performance of power lines, reduce power consumption and improve the voltage quality of power supply network, and is suitable for long distribution lines with low power factor and heavy load.

In this paper, the reactive power compensation system of 10kV line is designed.

Design of Reactive Power Compensation System for 2 10kV Transmission Line

2. 1 Determination of compensation point and compensation capacity

In order to find the optimal reactive power compensation capacity and location under operation constraints, the minimum annual cost is taken as the objective function, the power flow equation is taken as the equality constraint, and the operation constraints such as load voltage and compensation capacity are taken as the inequality constraint. The annual expenditure includes annual operation and maintenance expenses of compensation equipment, investment recovery, active loss of compensation capacitor and energy loss expenses paid for line loss of 10kV power grid after compensation. The total active loss consists of two parts: (1) due to the flow of active current and (2) due to the flow of reactive current. By installing a compensation capacitor on the line, the reactive current can be reduced, thus reducing the active loss caused by the flow of reactive current. The algorithm is suitable for all nodes in the network except the power supply node, and the candidate compensation nodes can be obtained according to the active line loss reduced by each node after compensating the optimal capacity. The system uses genetic algorithm to solve the compensation node and compensation capacity, and obtains the candidate compensation node, and the compensation point can only be selected on this node. However, these nodes may not be the best compensation points. Therefore, the system proposes a compensation algorithm based on non-nodes, that is, using the characteristics of parallel optimization of genetic algorithm, adding corresponding nodes (called non-nodes) in the upper and lower branches of each compensation node according to the position of telephone poles, taking the electrical distance between nodes and non-nodes as the control variable set, and then using genetic algorithm to find the best compensation position and compensation capacity. The example analysis shows that this "node-less" compensation method can further improve the voltage level and reduce the line loss without increasing the cost of reactive power compensation equipment.

2.2 Determination of compensation position

The selection of installation position of reactive power compensation device should conform to the principle of local reactive power balance, and the goal is to reduce the reactive current on the main line as much as possible. The formula for calculating the optimal installation position of different capacitor banks is as follows: Li = (2i/2n+ 1), where l is the Line length, n is the number of capacitor banks, li is the installation position of group I capacitors, and i =1... n According to practical experience, reactive power compensation cabinets are generally installed at two-thirds of the line load. Reasonable allocation of reactive power compensation capacity and selection of the best installation position of capacitors can improve voltage quality and reduce line loss. Generally speaking, the more power capacitors installed on distribution lines, the more obvious the loss reduction effect, but the workload of operation and maintenance also increases accordingly, and the investment cost of compensation equipment also increases. Therefore, in general, it is better to install 1 group capacitors on lines with relatively uniform load distribution, with no more than 2 groups at most. Because the shunt capacitor installed on the pole is far away from the substation, it is prone to engineering problems such as difficult protection configuration, high control cost, heavy maintenance workload and limited by objective conditions such as installation environment and space. Therefore, the optimal reactive power compensation on poles must be combined with the following practical engineering requirements: (1) The number of compensation points should be less, and single point compensation should be adopted in a distribution line instead of multi-point compensation; (2) The control mode is simple. Pole compensation does not set packet switching; (3) The compensation capacity should not be too large. Excessive compensation capacity will lead to overvoltage and overcompensation of distribution lines under light load; In addition, the space on the pole is limited, and too many capacitors are installed on the same pole, which is neither safe nor conducive to capacitor heat dissipation. The capacity of the line dispersion compensation capacitor bank should be controlled below 150kv; (4) The wiring should be simple. It is best to use only one capacitor device per phase to reduce the failure rate of the whole set of compensation equipment; (5) Protection methods should also be simplified. Drop fuse and zinc oxide arrester are mainly used for over-current and over-voltage protection respectively, and the rated current of fuse is selected according to 1.43 ~ 1.55 times of the rated current of capacitor bank; 150kvar or above, which shall be automatically controlled by the on-column circuit breaker or load switch.

2.3 Technical requirements for reactive power compensation

① Leakage specific distance: ≥24mm/kV. ② rated voltage: 10.5kV, maximum working voltage: 12kV. ③ Connection type: single-star type, with neutral point ungrounded. ④ All connections: copper bar or copper stranded wire (copper braided wire) and copper lead-out terminals. ⑤ Capacitor bank with self-discharge resistance: The residual voltage of the capacitor bank is. The discharge is completed within 10 minute. ⑤ Box: stainless steel. ⑦ Iron components: galvanized. ⑧ Switch: high-voltage vacuum contactor. Pet-name ruby line current transformer: LZKW- 10 open type. Attending capacitor protection: overcurrent, overcurrent interruption, overvoltage, undervoltage, zero sequence and capacitor. Measurement accuracy: voltage and current 0.5%. Control function: power factor control capacitor switch. Statistical function: daily maximum and minimum voltage, current, power factor, operation data before and after capacitor switching, and capacitor operation time. Remote communication and meter reading: RS485 interface, which realizes the download of current operation data, the download, modification and upload of set values, remote switching and the download of statistical records. During this period, short-range radio frequency wireless communication is adopted. Data management software: it has perfect functions of equipment management, communication, analysis and printing, including: downloading operation data: line voltage, line current, line reactive power, line power factor, three-phase capacitor current, capacitor cumulative action times, capacitor input time and system time; Download and upload set values or operating parameters; Remote switching; Download the operation data before and after capacitor switching; Download daily maximum and minimum voltage, current and power factor values and their occurrence time.

2.4 Management and maintenance

After the installation of reactive power compensation device, it is necessary to use notebook computer for dynamic management on site, call up historical data and check whether the device is automatically switched on and off according to the set method and parameters. It is also necessary to observe whether the power factor and current on the line change when the compensation device is switched from the substation, so as to achieve the expected effect. If it is not ideal, it is necessary to analyze the reasons, adjust the setting parameters, and even adjust the fixed compensation and self-compensation capacity. To do a good job in reactive power management, it is necessary to write detailed and clear responsibilities and realize closed-loop management. From the proposal of compensation device scheme to the technical requirements of equipment, selection and ordering, acceptance test, installation and construction, operation management, maintenance and repair, demolition and relocation, etc. Every link should be implemented to the relevant departments, and the responsibility should be implemented to people. (1) The operation of reactive power compensation device can be reflected in the monthly report of dispatching automation or substation operation. (2) Reactive power compensation device is one of the important electrical equipment, which should be checked once a month in combination with line operation. (3) Use infrared thermometer to measure the temperature of capacitor shell (especially in summer), and make measurement records. (4) The main bushing, capacitor bushing, insulator, drop fuse, lightning arrester and bracket of the compensation device should be cleaned and inspected in combination with the power failure prevention work in spring and autumn every year.

3 Conclusion

In a word, China's distribution network, especially rural power grid, has great energy-saving potential. Rational configuration of capacitors, making them intelligent and maintenance-free, is the key technology in this field. The installation of reactive power compensation device in distribution lines is a good way to reduce losses and save energy, with simple installation, quick effect, remarkable effect, advanced technology and obvious economic benefits. It is worth popularizing and applying.

[References]

[1] Yuan Shun. Reactive power optimization and reactive power compensation device for distribution network [M]. Beijing: China Electric Power Press, 2006.

[2] Liu. Practical technical manual of power supply and distribution [M]. Beijing: China Water Resources and Hydropower Press, 2006.