The comprehensive device for arc suppression, harmonic suppression, line selection, and overvoltage protection is suitable for 6-35kV medium voltage power systems. This product is widely used in power systems where the neutral point is not grounded, the neutral point is grounded through arc suppression coils, or the neutral point is grounded through high resistance. It can limit various overvoltages in the above systems, effectively improving the operational safety and power supply reliability of the above systems.
1 Overview
For a long time, China's 6-35kV (including 66kV) power grid has mostly adopted an ungrounded neutral operation mode. When a single-phase grounding occurs in this type of power grid, the ground voltage of the non fault phase will increase to the line voltage, but the line voltage of the system will remain unchanged. Therefore, according to Chinese national standards, 6-35kV (66kV) power grids are allowed to operate with faults for a short period of time after a single-phase grounding fault occurs. Therefore, the ground insulation level of various electrical equipment in this type of power grid, such as transformers, voltage/current transformers, circuit breakers, lines and other primary equipment, should meet the requirement of long-term withstand line voltage without damage.
The traditional concept holds that the 6-35kV (including 66kV) power grid belongs to the medium and low voltage transformer distribution network. The absolute value of internal overvoltage in such power grids is not high, so the main factor that endangers the insulation safety level of the power grid is not internal overvoltage, but atmospheric overvoltage (i.e. lightning overvoltage). Therefore, the overvoltage protection measures taken for a long time are only to prevent atmospheric overvoltage from damaging equipment. The main technical measures are limited to the installation of various types of lightning arresters. The discharge voltage of the lightning arrester is more than 4 times the phase voltage, and it is designed to avoid internal overvoltage. Therefore, it is only effective in protecting against lightning damage and does not have any protective effect on internal overvoltage.
However, operational experience has shown that when this type of power grid develops to a certain scale, internal overvoltage, especially the arc grounding overvoltage generated during single-phase intermittent arc grounding and the ferromagnetic resonance overvoltage generated under special conditions, has become a major threat to the safe operation of such power grid equipment, with single-phase arc grounding overvoltage being the most severe.
With the large-scale technological transformation of urban and rural power grids in our country, the distribution networks in urban and rural areas must develop towards cable oriented development. The system's capacitance current to ground is gradually increasing, and the problem of arc grounding overvoltage is becoming more serious day and night. In order to solve the above problems, many power grids adopt resonant grounding method, which is to install arc suppression coils at the neutral point of the power grid. When the system experiences single-phase arc grounding, the inductive current generated by the arc suppression coils is used to compensate for the capacitive current at the fault point, reducing the residual current passing through the fault point and achieving natural arc extinguishing. Operating experience has shown that although arc suppression coils have a certain effect on suppressing intermittent arc grounding overvoltage, some problems with arc suppression coils have also been found during use.
Due to the diversity of power grid operation modes and the randomness of arc grounding points, it is indeed difficult for arc suppression coils to effectively compensate for capacitor current. Moreover, arc suppression coils only compensate for power frequency capacitor current, while the actual current passing through the grounding point not only includes power frequency capacitor current, but also contains a large amount of high-frequency current and resistive current. In severe cases, only high-frequency current and resistive current can maintain the continuous combustion of the arc.
When there is a non grounding fault in the power grid, such as disconnection, non full phase, or capacitance coupling of the same pole line, the asymmetric voltage of the power grid may increase, which may cause the automatic adjustment controller of the arc suppression coil to misjudge the grounding of the power grid and act. At this time, a high neutral point displacement voltage will be generated in the power grid, causing a significant increase in the voltage of one or two phases in the system, resulting in damage to other equipment in the power grid.
The arc suppression coil has a large volume, multiple components, high cost, a large installation area, and complex operation and maintenance.
As the power grid expands, the arc suppression coils also need to be replaced, which is not conducive to the long-term planning of the power grid.
At present, foreign countries adopt a neutral point direct grounding method for 6-35kV power grids, and a few regions in China also adopt a small resistance grounding method. Although it suppresses arc grounding overvoltage and overcomes the problems of arc suppression coils, it sacrifices the reliability of power supply to users. When a single-phase grounding occurs in this system, artificially increasing the short-circuit current causes the circuit breaker to operate, regardless of the nature and importance of the load, all faulty lines are cut off, and it is also impossible to distinguish between metallic or arc grounding. The metal grounding fault line, which does not pose a risk of arc grounding overvoltage, is also cut off, expanding the power outage range and time. Due to the increased fault current, arc grounding exacerbates the burning of the fault point.
2 Basic functions and characteristics
1. It can limit the atmospheric overvoltage and operating overvoltage of the system to a lower voltage level, ensuring the insulation safety of the power grid and electrical equipment.
The 2 devices have fast operation speed and can operate within 30ms-40ms. They can quickly eliminate intermittent arc and stable arc grounding faults, suppress arc grounding overvoltage, prevent accidents from further expanding, and reduce the accident tripping rate of the line.
3 can quickly and effectively eliminate resonance overvoltage in the system, prevent long-term resonance overvoltage from damaging the system insulation, and prevent resonance overvoltage from damaging the lightning arresters and small inductive loads installed in the power grid.
After the operation of the 4 devices, a 200A capacitor current is allowed to pass continuously for at least 2 hours. Users can handle the faulty line after completing the load transfer switching operation.
Being able to accurately locate single-phase grounding fault lines is of great significance in preventing further accidents and reducing the workload of operation and maintenance personnel.
From the working principle of the device, it can be seen that the mechanism of limiting overvoltage is independent of the magnitude of the capacitance current from the grid to the ground. Therefore, its protection performance does not change with changes in the operation mode of the grid. Both large and small grids can be used, and the expansion of the grid has no impact.
The voltage transformer in this device can provide system voltage signals to measuring instruments and relay protection devices, and can replace conventional PT cabinets.
8 can measure the single-phase grounding capacitance current of the system.
The 9 devices are simple in equipment, small in size, and easy to install and debug. They can be used in substations and are also suitable for high-voltage auxiliary power systems in power plants; Suitable for both new and old power station renovations.
