Voltage is one of the indicators to measure power quality. The quality of power grid operation voltage is a comprehensive reflection of the technical level of distribution network structure, reactive power supply configuration, reactive real-time control and operation management. It is also the embodiment of the coordination ability of power generation, supply and use. In the operation of distribution network, reactive power has a great impact on voltage quality. There is a strong coupling relationship between reactive power and voltage, and reactive power imperceptibly affects voltage quality. The deficiency or excess of reactive power will lead to the decrease or increase of the operating voltage of the distribution network. In extreme cases, it may also lead to the significant decrease of the operating voltage and the phenomenon of "voltage collapse".
On the premise of ensuring the safe operation of distribution network, the optimal control of reactive power and voltage from the perspective of the whole network can realize the reasonable switching of reactive power compensation capacitor and reduce the adjustment times of tap changer of main transformer, so as to realize the layered local balance of reactive power and ensure the qualified operation voltage of distribution network. Therefore, it is necessary to explore the optimal control mode of reactive power and voltage and make it perfect day by day, gradually standardize and form a set of effective management system, so as to issue the switching command of compensating reactive power capacitor in real time according to the demand of reactive load of distribution network, ensure the local balance of reactive power stratification, maintain the qualified operation voltage of distribution network, and realize the safe and economic operation of distribution network.
1、 Disadvantages of traditional reactive power management
Reactive power is a kind of resource, which is a necessary condition for establishing magnetic field and maintaining electromagnetic energy conversion. Reactive power, like active power, is an important part of AC energy. It goes hand in hand with each other and is indispensable. It always accompanies and takes care of the normal operation of electrical equipment. Provide support for the normal operation of the power system and escort the transmission of active power of the power grid.
In the operation of distribution network, there is a strong coupling relationship between reactive power and voltage. The change of reactive current has a much greater impact on the load voltage than the change of active current, and the operating voltage changes with the proportion of active and reactive power transmitted by the line. In order to ensure the qualified operation voltage of distribution network, it is necessary to monitor and manage the reactive voltage. However, the traditional reactive power and voltage management only improves the voltage quality by manually switching the compensation capacitor (or adjusting with the transformer tap changer) according to the change of operating voltage.
The traditional manual regulation and management of reactive power and voltage has many disadvantages: first, the power dispatching only collects the parameters according to the operation of the power grid, and issues the switching command of the compensation capacitor after statistical analysis, which is easy to occur after the compensation regulation, and the voltage fluctuation occurs from time to time; Second, the compensation capacitor is switched only by voltage fluctuation, which not only makes the reactive power compensation inaccurate, but also produces frequent staggered switching operations; Third, the manual switching of compensation capacitor is not only cumbersome and time-consuming, but also increases the workload of dispatching and operators.
With the progress of science and technology, many substations are manned or unattended. If reactive power and voltage are still manually regulated, operators must adjust the integrity of the power grid after receiving the dispatching instructions, and then adjust the integrity of the reactive power load after adjustment. This will not only produce staggered and frequent operation, but also cumbersome and time-consuming, and the voltage regulation effect is not good. In addition, the reactive power and voltage management adopts manual operation, which can not track the change of reactive load in time, and can not achieve real-time hierarchical and local balance of reactive power, so it is difficult to effectively improve the quality of operating voltage.
2、 Voltage quality and power loss
The transmission of electric energy from power source (power plant) to users needs to go through transmission, transformation, distribution and other links, and the distribution is at the end of the power system. In the operation of distribution network, due to the impedance of power supply network, when the current passes through lines and electrical equipment, it is bound to generate heat and cause power loss. The power loss in the operation of distribution network is related to the voltage level, network structure, conductor section, load nature, equipment performance, etc., especially the operation voltage offset has a great impact on the power loss. If the operating voltage decreases by 15%, the power loss will increase by 30%.
The influence of distribution network operating voltage offset on all kinds of electrical equipment is different. For asynchronous motor, it is very sensitive to operating voltage offset. When the motor and output power are constant, its stator current, power factor and efficiency change with the deviation of supply voltage. The torque of the asynchronous motor is directly proportional to the square of the input voltage. When the power supply voltage decreases, the torque of the motor will obviously decrease, resulting in the increase of slip rate, stator current, temperature rise, low operation efficiency and increased power loss. On the contrary, if the power supply voltage increases, the core magnetic flux density of inductive equipment such as iron core increases, but the core magnetic saturation causes the increase of excitation current, the increase of core loss, the heating of electrical equipment and the decline of operation efficiency. In short, the operation voltage quality of distribution network has a great impact on the power loss caused by the operation of electrical equipment, so effective measures should be taken to improve the operation voltage quality.
3、 Reactive power balance and voltage quality
All kinds of inductive equipment are running in the distribution network, and the amount of reactive power required is related to the quality of operating voltage. Characterizing the relationship between different operating voltages and the reactive power required by inductive equipment is called the reactive voltage static characteristics of electrical equipment.
The distribution network supplies power to users through multiple 10kV feeders. Each feeder is connected with many electrical equipment, and their reactive power voltage static characteristics are different. The reactive power is often sent to the reactive power distribution network at the same place as the reactive power feeder, and the reactive power is often sent to the reactive power distribution network. When the feeder operates under a certain voltage, the reactive power sent by the reactive power supply to the inductive load must be equal to the sum of the reactive power required by all inductive loads on the feeder under this voltage. That is, the qualification of feeder operating voltage at any time completely depends on the local balance of reactive power.
In order to realize the real-time and local balance of reactive power, we must give full play to the power dispatching function and master the voltage fluctuation and reactive load demand in feeder operation in real time. While increasing the detection of feeder reactive load, issue the switching command of compensation capacitor according to the reactive power required by the feeder to ensure that the reactive power meets the needs of inductive load on the feeder and maintain the local balance of reactive power, so as to reduce the reactive power flow. Only by accurately using the dispatching means and maintaining the local balance of reactive power on the feeder in real time, can the feeder operation voltage be qualified and the power supply quality be improved.
4、 Reactive power compensation and reactive power flow
The reactive power transmitted on the transmission and distribution lines in the power system is called reactive power flow. In the operation of distribution network, if the reactive power flow is greater, the network loss will also increase. The reactive power supply and inductive load on the distribution network feeder are often not installed in the same place. The reactive power required by inductive load needs to be sent from reactive power supply to inductive load through line. In the process of reactive power transmission, the voltage difference between the sending end and the receiving end of the line is bound to occur. If the line transmits more reactive power, the greater the voltage difference is. In order to realize the local balance of reactive power, it is necessary to operate the power dispatching function to scientifically and reasonably allocate the reactive load according to the change of operation voltage and reactive power flow of the distribution network in real time, so as to ensure the local balance of reactive power of the feeder under the given operation mode of the distribution network. Therefore, according to the operation status of the distribution network and the reactive power required by the inductive load on the feeder, the power dispatching shall issue the command of switching the compensation capacitor and regulating the tap changer of the main transformer in real time, so as to achieve the local balance of reactive power, reduce the reactive power flow and improve the quality of operating voltage.
In addition, in order to better realize the local balance of reactive power, reactive power compensation can also be implemented at the insufficient reactive power, which is also an effective measure to realize the local balance of reactive power and improve the voltage quality. To optimize reactive power compensation, we must adhere to the principle of "comprehensive planning, reasonable layout, hierarchical compensation and local balance", and scientifically and reasonably configure reactive power compensation equipment, which can not only reduce the pressure of reactive power compensation in the upper power grid, but also avoid increasing network loss due to reactive power transmission.
To optimize reactive power compensation, we should also adhere to the combination of decentralized local compensation and centralized compensation in substation, focusing on decentralized compensation; Power grid compensation is combined with user compensation, focusing on user compensation; High voltage compensation is combined with low voltage compensation, mainly loss reduction compensation. For the user's low-voltage motor or inductive equipment, it is mainly to implement random follow-up compensation, promote them to consciously install reactive power compensation capacitors, and promote their real-time operation through the supervision of reactive load, so as to improve the power factor and voltage quality. At the same time, during the operation of the distribution network, all reactive power compensation equipment in the network should be monitored and detected to avoid reactive power over compensation. If the user has excessive reactive power compensation, it will be counterproductive. Excessive reactive power compensation will cause the power factor to be too high and the reactive power will be sent back to the power grid. It will also increase the operating voltage of the power grid and increase the power loss.
With the increasing operation of nonlinear equipment in distribution network, its harmonic will also have an adverse impact on the quality of operating voltage. Especially for the users of transformers with capacity of 315KVA and above, in addition to strengthening the institutionalized management of reactive power compensation, it is also necessary to monitor and manage the harmonic source in real time to improve the voltage quality.
5、 Tap changer and voltage regulation
During the operation of the transformer, the output voltage can be adjusted by changing the gear of the tap changer, that is, changing the primary and secondary voltage of the transformer. Operating transformer tap changer for voltage regulation will not change the size of reactive power in the system, but only change the distribution of reactive power. If the tap changer is used for voltage regulation, a good voltage regulation effect can be achieved only on the premise that the reactive power of the system is abundant or basically balanced. If the reactive power of the system is insufficient and the switching tap changer is used for voltage regulation, the required reactive power will increase, resulting in a greater reactive power shortage, but the operating voltage will drop, which shows that when the reactive power of the system is insufficient, it is difficult to improve the voltage quality by switching tap changer.
Considering the whole network, the qualification of line operating voltage completely depends on the local balance of reactive power on the line. If the system is lack of reactive power, use the tap changer to adjust the voltage. When the voltage is low, first input the compensation capacitor to make the line reach the basic balance of reactive power. If the line voltage is still lower than the lower limit of the allowable offset value, adjust it with the tap changer of the main transformer to make the operating voltage meet the standard. Therefore, only when the line reactive power is basically balanced and the tap changer is used for voltage regulation, can a good voltage regulation effect be achieved and the line operating voltage be maintained to be qualified.
6、 Optimal control of reactive power and voltage
The power supply structure of distribution network is complex, and the power load changes greatly and dispersed. It is not easy to detect and control the load voltage offset of each user. However, from the perspective of whole network operation, only the optimal control of real-time reactive power and voltage can control the voltage at the receiving end within the allowable offset range and reduce the power loss of distribution network.
The optimal control modes of reactive power and voltage include decentralized control and centralized control.
Decentralized control means that reactive power compensation capacitors are installed on the 10kV bus of the substation, and the number of compensation point containers is determined only according to the operating voltage of the bus, so as to maintain the qualified operating voltage of the line. Good voltage regulation effect can be achieved only by the combined regulation of on load voltage regulation transformer and centralized reactive power compensation capacitor. This decentralized control mode only detects the bus voltage, switches the compensation capacitor and changes the tap changer gear to maintain the qualified line operating voltage. However, this control method can not automatically track the change of reactive load of power grid, and can not realize the optimization of reactive power and voltage of the whole network.
With the development and application of power electronics technology, using reactive power and voltage optimization control software, we can automatically track the change of reactive power load and optimize the reactive power and voltage, so as to realize the closed-loop control of reactive power and voltage and the real-time dynamic management of reactive power flow of the whole network. This centralized control mode can not only reduce the work intensity of power dispatching and operators, but also reasonably adjust the reactive power compensation capacitance and the tap changer of main transformer in real time, avoid the delay of reactive power compensation, maintain the real-time qualification of feeder operation voltage, and realize the safe and stable operation of distribution network.
The distribution network adopts the centralized control mode of reactive power and voltage, which uses the optimized centralized control software to automatically track the change of reactive load of the power grid and dynamically manage the reactive power compensation device and voltage regulation equipment in real time. Using multiple programming functions of the computer, various parameters are automatically carried out. After calculation and analysis, the command of switching compensation capacitor and tap changer position is issued, which is automatically executed by the terminal device to maintain the qualified operation voltage of the feeder. This centralized control mode can not only avoid voltage fluctuation caused by manual regulation of reactive power, but also realize less switching of compensation capacitor and regulation times of tap changer, timely achieve reactive power local balance, reduce reactive power flow, improve voltage quality, realize the optimization of reactive power and voltage of the whole network, and provide dynamic reactive power support for the safe operation of distribution network.
In the operation of distribution network, reactive power imperceptibly affects the voltage quality. Reactive power compensation is the basis to improve the voltage quality, and reactive power local balance is the condition to realize the qualified operation voltage. In the operation management of distribution network, we must strengthen reactive power management and innovate the control mode of reactive power and voltage. Relying on scientific and technological progress and using reactive power and voltage automatic control software, the switching and tap changer adjustment times of compensation capacitor can be reduced to a minimum, so as to achieve reactive power local balance, provide dynamic reactive power support for distribution network operation, and realize the economic operation of power grid.