High- and low-voltage switchgear are widely used and distributed switchgear in power systems. Despite significant improvements in their technical capabilities and safety performance, potential accidents still exist during field operation due to various factors. According to field statistics, 6-10kV switchgear accidents account for over 50% of all accidents involving switchgear of all voltage levels, posing a serious threat to the safe operation of power grids. Therefore, the safe and reliable operation of switchgear is a crucial aspect of the safe operation of power systems.The main factors affecting the safe and reliable operation of switchgear include insulation, mechanical properties, and current carrying capacity, which are discussed below.
Insulation
There are many factors that cause insulation accidents in switchgear. In addition to defects in the insulation structure and insulation material quality of the switchgear itself, there are also many other reasons such as the operating environment conditions of the switchgear (such as temperature, humidity and dirt), the structure of the distribution system, etc.
1.The insulation material is poor. The external insulation of the air-insulated switchgear uses the atmosphere as the main insulation, and the supporting insulation parts use porcelain materials and organic materials (such as epoxy resin, etc.). If the insulation material is of poor quality, it is easy to absorb moisture and the dielectric properties will decrease in a humid and condensing environment. In the condensation test, the insulation partition will have a strong brush discharge, causing local burning of the edge and eventually leading to flashover. Epoxy resin insulation has poor hydrophobicity and is not flame retardant. It is a polar medium. When it is humid and the surface is dirty, the surface conductivity will increase significantly. When the electric field strength reaches a certain value, local discharge will occur. Since epoxy resin insulation has poor electrical properties at high frequencies, local discharge will accelerate the deterioration of the insulation at that location until surface flashover occurs, causing an accident. The epoxy resin insulation partitions in the switch cabinet are connected and assembled using metal screws, which results in a large amount of floating potential and easily leads to dendritic discharge.
2.The insulation structure is unreasonable. If the air insulation distance between the conductors and the conductor to the ground is small, the impulse insulation level will not meet the standard. When there is a sharp electrode on the surface of the conductor, the impulse insulation level will decrease. Secondly, in the trolley-type switch cabinet, if the composite insulation size of the charged body, air gap, assembled insulation partition and grounding body used to reduce the size of the equipment is unreasonable, such as if the air gap is too small, the insulation level will decrease in harsh environments and cannot meet the environmental conditions of high humidity and severe pollution on site.
3.The creepage distance is too small. For components with a small surface creepage distance, the power frequency flashover voltage and impulse flashover voltage will drop significantly under condensation and dirty environmental conditions, and will not meet the specified withstand voltage requirements. If the surface creepage distance of the supporting insulators used in switches and busbars is small, the discharge voltage is relatively low under the condition of contamination and condensation, which makes it easy for external insulation creepage flashover and partial discharge to occur, causing electrical breakdown and short circuit accidents.
4.The operating environment is harsh. The switchgear is subjected to the effects of operating voltage, internal overvoltage and atmospheric overvoltage during operation. If the quality of the components is poor, when certain parameters of the power grid change, the operating conditions will be worsened. If there are pollutants and constant humidity, pollution flashover is easy to occur. Contamination not only affects the power frequency characteristics of the external insulation, but also affects the impulse discharge voltage. Under contamination conditions, the impulse withstand voltage of the external insulation will be greatly reduced. Relevant tests show that it can generally decrease by 30% to 40%. Therefore, the insulation strength of indoor switchgear should have sufficient margin. Humidity and condensation are also factors that cannot be ignored. When the external environment changes greatly or the temperature difference between day and night is too large, humid weather will affect the insulation performance of the equipment, resulting in an increase in leakage current, which will develop from partial discharge to creepage and finally develop into a flashover accident.
5.Partial discharge. Partial discharge refers to the discharge or breakdown in the local area of the insulating medium caused by uneven electric field distribution and excessive local electric field in the insulation structure. It may occur in the pores of solid insulation, bubbles of liquid insulation, or between insulation layers with different dielectric properties. If the electric field strength is higher than the specific value of the dielectric, it may also occur in liquid or solid insulation. Local discharge gradually develops and will continuously erode the insulating medium around it, and may eventually cause the failure of the entire insulation system. Therefore, local discharge is the main cause of insulation deterioration. It is also an important sign and manifestation of insulation deterioration. It is closely related to the deterioration and breakdown process of the insulating material and can effectively reflect the failure of the internal insulation of the power equipment.
Partial discharge is generally divided into internal discharge, surface discharge and corona. Partial discharge will not cause immediate failure of high-voltage system components. For example, the discharge between the inner conductor core and the cable shield may last for a long time, even several years, before the solid dielectric fails. Partial discharge will cause dendritic discharge in the solid dielectric. Due to the deterioration effect of dendritic discharge on the cable, the dielectric withstand voltage will be reduced. If it works under a strong electric field for a long time, the insulation withstand voltage will become lower and lower until it fails. Holes in dielectric materials, metallic contamination of dielectric materials, and interface protrusions of semiconductor dielectric materials can all cause dendritic discharges.
Mechanical Failures
Mechanical failures primarily occur in the operating mechanism, manifesting as circuit breakers refusing to open or close. Failure to open accounts for a significant proportion of switchgear failures. The main causes of circuit breaker failures are:
1. Stuck operating mechanism, primarily due to poor manufacturing quality, followed by improper installation and commissioning. Improper adjustment of the four-link operating mechanism, or excessive post-dead-centering of the connecting plate intermediate shaft, can also cause circuit breaker failures.
2. Component deformation, displacement, or damage. Component deformation or displacement can indicate design or material issues. Furthermore, failure to reset the trip mechanism after the switch is opened, failure to reset the bracket after actuation, a stuck opening electromagnet core, or vibration causing the opening core to bounce during the moment of opening can all contribute to circuit breaker failures.
3. Stuck opening core, faulty auxiliary switch. Poor quality and poor contact in the auxiliary switch can cause circuit breaker failure. The mechanical causes of circuit breaker failure to close are similar to those for failure to open. The electrical reasons are mainly the failure of the closing contactor of the electromagnetic operating mechanism, secondary wiring failure and low power supply voltage.
Current Carrying
The conductive circuit (current-carrying circuit) is also a common fault location within switchgear. The conductive circuit in fixed switchgear primarily consists of fixed connections between components. The reliability of these connections is determined by the conditions at the time of primary connection and is largely unaffected by operation, maintaining the original operating conditions. However, the conductive circuit in removable and drawer-type switchgear is not only affected by the reliability of the connection points between components, but also largely depends on the contact conditions of the primary isolating contacts during operation.
Unsmooth or uneven busbar surfaces, oily or oxidized surfaces, insufficient lap area, insufficient contact pressure, or failure to treat with a dedicated conductive paste can all increase contact resistance at busbar connection points, leading to heating. Insufficient contact surface or contact pressure between cables and switch components, or even the lack of a copper-aluminum transition between the aluminum busbar and the outgoing cable, can all cause heating at the cable joints.
Fatigue or rust and aging of the primary isolating contact finger compression springs can reduce their function, leading to insufficient contact pressure and contact between the fingers. This increases contact resistance, reduces the effective current-carrying cross-section, and causes heating. Furthermore, during operation, the contact finger pressure spring is subjected to long-term compression, generating heat under the action of current. This, combined with arc burns generated during the contact opening and closing process, can degrade the spring's elasticity over time. This in turn leads to insufficient pressure on the contact finger pressure spring, resulting in inadequate contact between the moving and static contacts. This exacerbates heating, causing increasingly severe contact heating and even the pressure spring to break, causing the contacts to scatter, resulting in poor contact and burns.
If the trolley operating mechanism's stroke is improper, the trolley cannot be fully advanced to the preset position, resulting in insufficient insertion depth between the moving and static contacts and insufficient contact surface, which can cause heating.
