Field test and Research on the coordination of the

Field experimental research on the level difference coordination of circuit breakers and fuses

Abstract: This paper discusses the multi-level coordination of protective elements in the newly issued DC design code, puts forward suggestions on the configuration selection of DC system fuses and DC circuit breakers, and gives the field test results of typical level difference configuration of DC system

key words: DC system differential protection test at the same time before

with the continuous progress of China's power industry, the power system is developing towards ultra-high voltage and large capacity, providing control, protection, signal and operating power supply for these large capacity power equipment. The safe, reliable and economic operation of DC system must be raised to a new level. During normal operation, the DC system provides closing power for the circuit breaker and DC power for relay protection, automatic devices, communication, etc; In case of fault, especially in the case of AC power interruption, the DC system provides safe and reliable DC power for relay protection and automatic devices, circuit breaker closing and tripping, and emergency lighting, which is the basic guarantee for the correct action of relay protection, automatic devices, and circuit breakers in the power system. In the DC circuit, fuses and circuit breakers are the main protective elements for the overcurrent and short-circuit faults of the outgoing lines of the DC system, which can be used as the disconnection and isolation of the power supply network of the feeder circuit. Whether their selection and action value setting are appropriate and whether there is selective cooperation between the upper and lower levels of protection are directly related to whether the system fault can be limited to the minimum range, which is important to prevent system damage The expansion of the accident and the serious damage of the main equipment are very important. Therefore, strengthening the accuracy of fuse and circuit breaker selection and configuration is of great significance to improve the safety and reliability of power system operation

1. Main problems of differential coordination

due to the large amount of power supply and wide distribution of circuits in the DC system of the substation, there are often many branches in a DC network that need to be protected by circuit breakers or fuses, and they are often divided into three or four levels in series, which leads to the problem of correct selection of protection scheme and the coordination between the upper and lower levels of protection

1.1 mixed use of AC and DC circuit breakers

due to the different arc burning and arc extinguishing processes of AC and DC, the ability of AC and DC circuit breakers with the same rating to cut off DC power supply is not exactly the same. Replacing DC circuit breakers with AC circuit breakers or mixing AC and DC circuit breakers is one of the main reasons for protection over grade maloperation

the instantaneous action of the circuit breaker adopts the magnetic tripping principle. The criterion is the peak value of the passing current. The rated value of the circuit breaker is the effective value, while the peak value of the AC is higher than the effective value. Under the same fixed value, the actual rated value of the AC circuit breaker in the DC circuit is higher than the DC circuit breaker. In addition, because the arc extinguishing principle of AC circuit breaker is different from that of DC circuit breaker, the AC circuit breaker used in DC circuit cannot effectively and reliably extinguish the DC arc, which is easy to cause superior leapfrog action

1.2 fuse quality and parameter problems

the technical data of the fuse provided by various manufacturers are obtained during the product type test, and the breaking capacity of the verified fuse is made under the effective value of the periodic component of the AC power supply, and the melt action selection and coordination characteristic curve is also the AC ampere second characteristic curve. There is a certain gap between this and the actual situation when short-circuit fault occurs in the DC system of the substation. The level difference coordination provided by each fuse manufacturer and design manual is to determine the upper and lower level differences according to the same model and the same melt material, so as to ensure the selectivity. When there are different types of fuses in the circuit, the level difference coordination between fuses should be paid more attention. At the same time, there are many manufacturers of low-voltage electrical appliances at present, which can not fully guarantee the product quality. Therefore, even if the melt of the same manufacturer and the same model has certain dispersion in its parameters

1.3 the rated value differential between the upper and lower levels is improperly selected. The fuse adopts the principle of thermal effect, while the circuit breaker is a combination of magnetic effect and thermal effect. The ampere second characteristic curve is different, and the coordination differential is also different. For the differential coordination between circuit breakers and between circuit breakers and fuses, the coordination provisions between fuses should not be copied

2. Field test of differential configuration of fuses and DC circuit breakers

in order to adapt to the relevant provisions of the newly issued dl/t5044-2003 technical code for DC design of Electric Power Engineering (hereinafter referred to as the design code), verify whether several typical differential configuration schemes of circuit breakers and fuses in the DC system of substations meet the requirements of selective protection, and explore the differential coordination between DC circuit breakers The coordination of DC circuit breakers and fuses and the selection and configuration between their superiors and subordinates, the typical protection differential coordination scheme of some DC circuit breakers and fuses in the DC system of substations under the jurisdiction of Shijiazhuang power supply company was selected for field tests, and the three-stage DC circuit breakers with time-delay function were also tested and verified, and the coordination conditions for realizing selective protection were confirmed

2.1 the selection of short-circuit current is based on the maximum short-circuit current that may appear on the installation site of DC circuit breaker and fuse, and the test elements are installed in series for short-circuit test. In order to ensure that the test power flow from the government level is higher than the maximum short-circuit current that may occur on site, an appropriate margin is increased. The selection results of short-circuit current are:

a.300ah and 200ah battery packs. For the short-circuit fault of the closing feeder cable, the short-circuit current is 2000a

b.100ah storage battery, for the short-circuit fault of closing feeder cable, the short-circuit current is 1000A

c. the fuse at the end of the control and protection circuit or the outlet of the DC circuit breaker is short circuited, and the short-circuit current is 200A

2.2 test scheme and results after investigating the protective appliances installed and used in the DC system of the substation, according to the relevant provisions of the design specification, 12 test schemes of 4 categories, fuse fuse, circuit breaker, fuse breaker, are determined

2.3 result analysis

2.3.1 fuse fuse

design specification requirements: if the model of the installed fuse is the same as the material of each fuse, in order to ensure selectivity, it is necessary to make the difference between the rated current of the fuse elements of the adjacent two levels in the circuit at least two levels

the premise of this test is to select products of the same factory, model and batch. Because it has the same ampere second characteristics, the coordination of two-stage differential operates correctly within the range of 10 ~ 30 times of the upper level of short-circuit current, showing good coordination characteristics. Shijiazhuang power supply company has uniformly replaced the fuses of the DC circuit of the substation, which meets the site conditions for this test

2.3.2 circuit breaker fuse

design specification requirements: the next level of the circuit breaker can be equipped with a fuse as a protective appliance, and the rated current of the circuit breaker should be 4 times or more greater than the rated current of the fuse

the test result is consistent with the design specification

strain at a certain point of an elastic element ε It is in direct proportion to the force on the elastic element

but the premise of this test is that the short-circuit current is 8 ~ 9 times the rated current of the upper level, and it just enters the quick break zone of the upper circuit breaker. If the short-circuit current increases to a certain value, according to the test results in reference [2], the upper and lower levels will act at the same time, causing skipping. Therefore, in engineering applications, in addition to the rated current of the circuit breaker should be greater than 4 times and more than the rated current of the fuse, it should also be verified that the maximum short-circuit current should not exceed 10 times the rated current of the superior circuit breaker

2.3.3 fuse circuit breaker

this test condition is stricter than the design specification, and the rated current of the fuse is greater than 1.6 times the rated current of the circuit breaker. The test results are consistent with the design specifications

but the short-circuit current of this test is 12 ~ 13 times of the rated current of the upper fuse, so:

a. because the outlet fuse of the battery is selected according to the 1H discharge capacity of the battery and the first level is increased, its maximum short-circuit current is within this range, and it can cooperate with the next level circuit breaker, so it is not necessary to verify the maximum short-circuit current

b. if it is used at the lower level of the circuit, the fusing speed of the fuse accelerates with the increase of the short-circuit current, while the tripping speed of the circuit breaker is basically unchanged. When the short-circuit current is large to a certain extent, the two actions are close to each other, resulting in overstepping. This is also confirmed by the test results of reference [2]. Therefore, in engineering applications, in addition to the rated current of the circuit breaker should be greater than 4 times and more than the rated current of the fuse, the maximum short-circuit current should not exceed 10 ~ 12 times the rated current of the upper circuit breaker

2.3.4 DC circuit breaker - DC circuit breaker

2.3.4.1 two stage DC circuit breaker

the two-stage DC circuit breaker operates correctly and cooperates well under the condition that the short-circuit current is 8 ~ 10 times the rated current of the upper switch and the level difference of 4 ~ 5

however, if the short-circuit current reaches or exceeds 10 times the rated current of the upper circuit breaker, the upper and lower circuit breakers will enter the quick action zone and act at the same time, causing skip tripping

in addition, because the inherent action time of the molded case DC circuit breaker is higher than that of the miniature circuit breaker, the selective short-circuit current value of the cooperation of the upper level with the molded case circuit breaker and the lower level with the miniature circuit breaker is higher than that of the same type. According to the relevant domestic tests, the selective limit current extends to about 20 times the upper rated current

2.3.4.2 three section DC circuit breaker

three section DC circuit breaker, when the upper level is three section DC circuit breaker and the lower level is two-section or three section DC circuit breaker, the level difference is level 2, and it operates correctly when the short-circuit current is 25 ~ 40 times the rated current of the upper circuit breaker

the design specification does not specify the differential coordination of the three-stage DC circuit breaker. From the test results, because the short delay time of the upper circuit breaker is greater than the full breaking time of the lower circuit breaker, and the upper short delay can return, the small differential coordination can be realized without considering the influence of short-circuit current, and it can meet the requirements of the design specification for multi-level coordination in the design scheme of DC distribution panel

3. Suggestions on the selection of DC system protection scheme

3.1 fuse fuse fuse coordination

products of the same factory and model accumulated by the experience of designers of regular manufacturers should be selected, which can be easily used for the unified replacement of the original fuses in substations. However, for new stations, since fuses are self-contained, it is difficult to ensure that they are of the same manufacturer and model, especially the requirement of "the same material of each fuse element" required by the design specifications. Therefore, the level difference should be increased for fuse coordination of different manufacturers and models

3.2 the coordination between fuse and circuit breaker (two-stage type) and two-stage circuit breaker of the same type shall be carried out in accordance with the design specification, and the maximum short-circuit current shall not exceed 8 ~ 10 times of the rated current of the upper component

3.3 the coordination of different types of circuit breakers

should consider the inherent action time of the circuit breaker, and must ensure that the inherent action time of the upper circuit breaker is not less than that of the lower circuit breaker. It is recommended to adopt the cooperation of upper molded case circuit breaker and lower miniature circuit breaker

3.4 three stage circuit breaker

the use of three-stage circuit breaker can achieve small differential coordination, and does not need to consider the impact of short-circuit current, and can meet the requirements of multi-stage coordination in the design scheme of DC distribution panel in the design code

references

fan Ruifeng, shaoweixiang. Introduction to experimental research on differential coordination of air circuit breakers and fuses in DC systems ［ J ］. DC power supply, 2004, (2). (end)