Discussion on switching operation of differential

2022-08-12
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Discussion on the switching operation of bus differential protection

bus protection device is an important facility to correctly and quickly remove bus faults. Its failure and maloperation will bring serious harm to the power system Bus switching operation is the most common and typical operation in power system. Because of its many connecting elements and heavy operation workload, it also puts forward higher requirements for the comprehensive operation skills of operators Based on the objective reality of primary equipment, operators have a direct and comprehensive perceptual understanding of the harm caused by the misoperation of primary equipment However, some switching and switching operations of bus differential protection during switching operation are often vague

1. Whether the scope of bus differential protection is determined and whether the protection object is unchanged

generally speaking, differential protection includes bus differential protection, transformer differential protection, generator differential protection and line differential protection The basic principles for realizing differential protection are consistent, that is, the current transformers on each side or element are wired according to the differential connection method. In normal operation and faults outside the protection range, the differential current is equal to zero, the fault time difference current within the protection range is equal to the fault current, and the operating current of the differential relay is calculated and set according to the maximum unbalanced current generated when avoiding external faults

but it should also be very clear that busbar differential protection is very different from transformer differential protection and generator differential protection: that is, the main connection mode of busbar will change with the switching operation of busbar. For example, the detailed calculation method of double busbar is changed to check the national standard for single busbar operation, and the parallel operation of double busbar is changed to the sectional parallel operation of double busbar, Busbar elements (such as lines, transformers, generators, etc.) can be switched from one busbar to another, etc In other words, the scope of busbar differential protection will change (expand or reduce) with the progress of busbar switching operation and the change of busbar operation mode, and the object of busbar differential protection can also change (increase or decrease) due to the switching operation of busbar elements Ignoring this point, some problems of bus differential protection during bus switching operation. 2. The problems existing in the development of Anyang industry. It is necessary to cut this kind of experimental machine is the mainstream product of today's universal material experimental machine. The operation of switching on and off must be vague and even tend to be blind

2. Whether it is necessary to withdraw the bus differential protection during the bus switching operation

"it is necessary to withdraw the bus differential protection during the switching operation" is wrong. The reason for this wrong understanding is that some operators have seen, even during the bus switching operation, the bus differential protection maloperation, but the fundamental reason is the lack of correct understanding of the bus differential protection If the bus switching operation is carried out in strict accordance with the regulations, that is, equipotential operation during paralleling and splitting, the potential difference after 2 months of placing when operating the disconnector shall be minimized, the reverse charging of the secondary side of the bus voltage transformer is strictly prohibited, and the pulling and closing of the non selective switch of the bus differential protection and the switching of the low-voltage locking bus differential protection pressing plate shall be fully considered, which will not cause the misoperation of the bus differential protection Therefore, in the process of bus switching, the working principle of bus differential protection should generally be put into operation if it is not damaged According to the statistical data over the years, the probability of bus short circuit accident caused by misoperation is still high In recent years, five prevention locking devices have been installed on the primary and secondary equipment of substations and power plants to prevent misoperation, but some operators use master keys in violation of regulations to walk the wrong interval, make a mistake, and pull a mistake from time to time This makes it of great practical significance to maintain the input of bus differential protection during bus switching operation Switching on the bus differential protection to reverse the bus can be operated by the protection device to cut off the fault in case of bus short circuit caused by misoperation, so as to avoid the further expansion of the accident and prevent serious damage to the equipment, system instability or personal injury and death accidents

in fact, the bus switching operation is not so easy to cause the bus differential protection maloperation, but rather, the bus switching operation often makes the bus differential protection lose selectivity and mistakenly cut the non fault bus

3. After the bus switching operation, whether to close the non selective switch of bus differential protection into the actual work

some operators unilaterally believe that the bus switching operation will make the bus differential protection lose selectivity, so after the operation is completed, close the non selective switch of bus differential protection The root of this misunderstanding is that they do not understand the purpose of setting this non selective switch in the bus differential protection device

There are many types of busbar protection, and the working principle of different types of busbar protection is different Some types of bus protection have defects in their working principle, which will make the device lose the selectivity to the faulty bus when switching the bus Therefore, the key to the problem is that the operators should make clear: which type of bus protection will lose the selectivity to the fault bus during the bus switching operation, how to close the non selective switch of the device at the appropriate time, when to open the non selective switch of the device, or whether to keep the switch closed

here is a brief description of the bus differential protection with fixed connection, the bus coupler current phase comparison principle differential protection and the current phase comparison bus protection

(1) bus differential protection with fixed connection

this bus differential protection requires that the power components on the bus must operate in accordance with the fixed connection mode specified in advance. In case of bus fault, the action of bus differential protection can be selective When this type of bus protection is adopted, the switching of power components will make the protection lose selectivity Therefore, close the non selective switch of bus differential protection before switching, and do not open it after switching For load elements, the non selective switch is closed before switching, and the non selective switch is opened after switching. At the same time, the tripping pressing plate of load elements is also switched accordingly

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