Automatic recloser: solution for single phase grounding of distribution line
In the 10kV distribution network, non-grounding, arc suppression coil grounding, and low resistance grounding coexist, and non-grounding and arc suppression coil grounding are most widely used. When a single-phase grounding fault occurs in a power grid with neutral grounded in this way, the current can only form a small current loop through the grounding capacitance or impedance, which is called a small current grounding system. After the single-phase grounding occurs in the small current grounding system, the fault phase to ground voltage decreases, and the phase voltage of the non-fault two phases increases, but the line voltage is still symmetrical, so the continuous power supply to the user is not affected, and the system can operate for 1 ~ 2H, which is also the advantage of the small current grounding system. However, it is very easy to cause breakdown to ground at the weak insulation of non-fault phase of the line, resulting in two-phase grounding short circuit. More serious is the personal electric shock accident and electrical fire accident caused by single-phase grounding, which endangers people’s lives and property safety.
At present, there are few switching devices that can accurately protect the single-phase grounding fault of the products running in the system. In case of grounding fault of the line, it shall be judged by the grounding line selection device of the substation. Because the grounding line selection device of the substation is far from the fault point and the accuracy of line selection is low, the problems of long time to find the grounding fault, difficulty finding the accident, and poor safety and reliability are caused, which seriously affects the reliability of power supply and may endanger the life and property safety of the people.
Typical case: forest fire caused by power failure caused by specific wind direction.
Why is it so difficult to detect small current grounding system?
- The grounding current is small, the resonant grounding residual current is less than 10a, and the high resistance fault has only 1A current; High resistance (resistance above 1K Ω) fault ratio is high.
- The fault location and time are different.
- The compensation current of arc suppression coil causes the zero sequence current of fault line to be smaller in amplitude and in the same direction than that of non-fault line.
- Intermittent grounding and unstable arc.
- The fault signal superimposed on the load current and the electromagnetic interference of the environment affect the accuracy of fault resolution
- The operation mode of the system is changeable.
- There are many uncertain factors such as changeable fault states.
Therefore, in order to improve the accuracy of single-phase grounding judgment of distribution networks, it is necessary to solve the problem of distribution equipment from the aspects of sensor accuracy, single-phase grounding algorithm, and anti-interference.
Automatic circuit recloser
Strong power electric automatic recloser (intelligent automatic circuit recloser) integrates 11 electronic sensors (4 currents and 7 voltages) with three-phase current, power supply measured voltage, load side voltage, zero sequence voltage, and zero sequence current, wherein the current reaches 0.5s level and the voltage reaches 0.5 level. The single-phase grounding algorithm uses the steady-state criterion and a variety of transient criteria for comprehensive judgment. The product has passed the RTDS simulation system test and the true type test of the China Electric Power Research Institute. The true type test has passed the high resistance 4K fault grounding and arc light grounding test. Moreover, the switch body has its own 20W power supply, lithium battery + super capacitor dual backup power supply, and the partial discharge of the whole machine is less than 10pc.
Automatic recloser applied to the 10kV distribution network overhead line to solve the equipment installation and commissioning in the large-scale construction and transformation of the distribution network, and effectively solve the problem of mismatch between primary and secondary equipment, and the lean level of line loss management will be continuously improved, so as to improve the operating efficiency and improve the reliability of power distribution.
| Item | Traditional Recloser | Strong power automatic recloser |
|---|---|---|
| FTU and relay protection function | Centralized or local fa | Differential protection, local or centralized FA, which can access fault sensor information |
| Single phase grounding fault judgment | Realize fault judgment (single point judgment) of partial single-phase grounding fault | Using Internet of things and regional data analysis to realize single-phase grounding point positioning |
| Power supply mode | Adopt Pt, or adopt the power taking sensor built in the switch pole + solar power taking mode | Only capacitive high-voltage power supply is adopted, and the primary and secondary power supply are separated to ensure the switch life of 20 years, and there is super capacitor and battery dual backup power supply |
| Switch structure | Traditional zw32 recloser structure | Optimized operation of energy storage mechanism and display |
| Service life | The recloser is more than 20 years, and some are less than 10 years (when DC is output from the energy extraction power supply) | The overall part of the recloser is more than 20 years, and the recloser controller and backup power supply are 8 years, replaceable |
