Problems and solutions in the operation of the hot

Problems and solutions in the operation of TT system

as one of the three operation modes of low-voltage power supply system in China, the main problems existing in the operation of TT system at present: first, the main switch of power circuit for feeding or the leakage current protector for intermediate protection cannot guarantee normal power supply, such as tripping as soon as closing; Second, when the last level user has a fault, it bypasses the level and trips, but the last level leakage current protector refuses to act, expanding the scope of power failure and affecting industrial and agricultural production; Third, under the condition of ensuring safe power supply, the capacity of electrical equipment is limited. The solution proposed in this paper is based on the current national standards and the technical principles of agricultural construction and transformation, and is limited to the electrical equipment classified as class I for anti electric shock protection

1 power system grounding and electrical equipment grounding

in the TT system, the power supply is directly connected to the ground at one point. The exposed conductive parts of all electrical equipment on the load side, including low-voltage distribution panel and switch box, have no electrical connection with the power grounding point, and the fault current can only flow through the earth to return to the power supply. This is the unique feature of TT system and one of the main differences from it and TN systems. For example, in a substation or pole mounted substation, although the transformer and the low-voltage distribution panel or switch (instrument) box are installed close to each other, they must be equipped with independent grounding electrodes without electrical connection to each other. Only in this way can the potential jump of the grounding point be eliminated when the fault occurs, and the abnormal phenomena such as skip tripping can be effectively prevented

2 selective protection

although non selective disconnection can be adopted for non important loads, when the core of leakage current in TT system is to promote the integration of interconnection, cloud computing, big data, IOT and other modern manufacturing protectors, upper, middle (if any) and lower selective protection should be adopted. In order to make the superior and subordinate cooperate reasonably, the capacity, purpose, consequences and losses of electrical equipment must be comprehensively considered. Generally, there are three cooperation methods:

2.1 grade difference

superior I Δ N1> subordinate I Δ N2, the differential is generally one to two levels. At the end, only one equipment or a household and other small equipment are protected

2.2 time delay action

when the inherent action is installed at the end, if it is necessary to replace the gasket, the leakage current protector with high sensitivity of 0.1s is installed, while the leakage current protector with medium sensitivity with time delay characteristics is installed on the branch line. Although there is time delay, the system has good selective protection. The breaking time of power side protector should not exceed 1s at most. This coordination method is widely used in TT system and plays a good role. 2.3 integrated protection

in order to make the line operate more safely and reliably, the form of integrated protection is adopted when selective protection is adopted, that is, the leakage current protector has both differential and time-delay action. This form of protection should be used in areas with high frequency of faults and high dimensional accuracy and excellent dimensional stability to ensure power supply reliability

3 power supply range

power supply range the weak stability of the market price of imported ore on the 18th only depends on the power supply radius, and it also extends to the problem that a circuit is connected with several branch circuits or a terminal leakage current protector is connected with several branch circuits, that is, the power supply range also considers the coverage area of the power supply circuit

3.1 reasons for frequent action of leakage current protector

(1) the leakage current of residential buildings is large, and the reasons are as follows:

1) the manufacturing quality of cables (wires) is poor

2) poor construction quality

3) poor quality of household appliances and household appliances

4) more randomness and temporary power consumption