The Study Of 10kV Grounding Resistors 's Thermal Stability

With the increasing urban power capacity and the wide use of cable, single-phase short-circuit capacitive current is going higher than ever before. In a urban power distribution network with cables, low resistance grounding mode are wide used because it can effectively reduce the over-voltage level, eliminate resonant over-voltage, etc. However, because of the feature of the grounding resistor, there are occasional burning accidents when there is a large current going through it, which is the main factor restricting its widespread using.In power distribution network, the resistors will heat when the system neutral point voltage offsets with a tens of amps current. It hardly causes damage of the devices because the current is not enough high to make such temperature rising. Then, when there occurs a fault of single-phase metallic grounding in the system, the resistors will extremely heat for the current of about 1000A. In that case, relaying devices will take action immediately to cut off the faulty part. So probability of the resistors'damage is still low. But when there occurs a non-metallic grounding fault or intermittent grounding fault, the fault current may be just 100 or several hundred amps which perhaps cannot lead the relay devices to cut off the fault. In that case, the heat will be accumulated in the resistors which can burn and damage the devices with a extremely high temperature. Thus, the grounding means of the power grid can be changed and the fault may spread. In most cases of resistors burning accidents are in such a situation. So far, there is still no detailed data to describe the relationship between the time and current in resistorsIn order to lower probability of the resistors burning damage, we need to study on the thermal stability of the resistors and work out the solution to protect the divices. Analyzing the structure, the operating condition, the application status and the thermal performance of the grounding resistors, I set a theory that the non-metallic grounding and intermittent grounding are the main causes of the burning damage of the grounding resistors. Testing the resistors and the monolithic resistor under different frequency current, we get the temperature– time curve by the data from infrared temperature measurement. Analyzing the data, we conclude the relationship among current, temperature and time working on ZDZ66-10-10 resistors and some operation suggestions as follows:1)This model of resistor can reach the thermal stable condition within less-than-385K temperature rising when the current is no higher than 100A. And the condition maintains faultlessly in 2 hours.2)When the current is higher than 100A, the testing model cannot reach the thermal stable condition within less-than-385K temperature rising, it can damage the device with long time operation. We get a current(100~300A)-time curve of the working resistor within less-than-385K temperature rising. Thus, we recommend protect the resistor appropriately according to current and working time. E.G., we can set a device to monitor low current, which can warn the staff to check the device or just cut off the fault automatically when the grounding current reach a certain value. We recommend the value as 120A maintaining 5s or accumulated 9 times with less-than-5s maintaining time.3)The long-time thermal stability can be improved by modifying the ventilation structure. It can be improved heavily indoor by using exposed shell or being fanned.