Case analysis of circuit breakers
Years of on-site accident statistics on circuit breakers indicate that the main types of operational accidents are as follows:;
(1) Operation failure;
(2) Insulation fault;
(3) Poor opening and closing performance;
(4) Poor conductivity.
The causes of accidents can generally be roughly divided into two categories: technical reasons and work reasons. The so-called technical reasons refer to defects in the product itself or operation mode; The so-called work reasons refer to the worker's negligence that caused these defects. This section will analyze the reasons for these two aspects.
Analysis of Technical Causes of Accidents
（1） Operation failure
Operation failure manifests as circuit breaker dragging or misoperation. Due to the fact that the most basic and important function of high-voltage circuit breakers is to operate correctly and quickly cut off power grid faults. If the circuit breaker is dragged or mistakenly operated, it will pose a serious threat to the power grid, mainly: ①) expanding the scope of accident impact, which may cause only one circuit fault to expand to the entire busbar, or even power outages for the entire substation and factory; ② If the fault removal time is extended, it will affect the stable operation of the system and aggravate the degree of damage to the controlled equipment; ③ Causing non full phase operation. The result often leads to abnormal operation and oscillation of power grid protection, which can easily escalate into system accidents or large-scale power outages. For example. During the shutdown and cracking operation of a small generator in a certain power plant, the sleeve SW2-220 type low oil circuit breaker rod strength was insufficient and broke, and phase B was not disconnected, resulting in non full phase operation. Spark discharge occurred in the neutral point gap of the No. 4 main transformer, and the arc affected the 220kV east lower busbar, causing the differential protection of the busbar to act. The No. 2 generator and two lines tripped, and due to the influence of negative sequence current, the main insulation of the rotor magnetic poles of the generator was almost completely damaged.
The main reasons for operational failure are:
(1) Defects in the operating mechanism;
(2) Mechanical defects in the circuit breaker body;
(3) Operation (control) power supply defect.
The specific analysis is as follows.
1. Defective operating mechanism.
The operating mechanism includes electromagnetic mechanism, spring mechanism, and hydraulic mechanism
On site statistics indicate that defects in the operating mechanism are the main cause of operational failure, accounting for approximately 70%. The main cause of mechanical failures in electromagnetic and spring mechanisms is jamming and inflexibility. The jamming here may be due to the inflexibility of the original assembly adjustment, or it may be caused by poor maintenance. Another reason for the mechanical failure of the mechanism is improper adjustment of the locking buckle. The self tripping (tripping) of the circuit breaker during operation is mostly due to this type of reason. Loose or misaligned connection parts are mostly due to loose screws, improperly fastened pins, or defects in the original anti loosening structure. It is worth noting that loosening and displacement faults far outweigh component damage, indicating that preventing loosening is no less important than preventing component damage.
For hydraulic mechanisms, mechanical failures are mainly caused by poor sealing, so ensuring reliable sealing in high oil pressure areas is particularly important.
Accidents caused by electrical defects in institutions are mainly caused by defects in auxiliary switches and micro switches. The faults of auxiliary switches are mostly due to not switching, which often causes the operating coil to burn out. In addition, the malfunction is also caused by poor contact after switching, resulting in refusal to move. Micro switches mainly refer to interlocking and protective switches on hydraulic mechanisms, etc. According to the accident statistics of SW6 type circuit breakers, the microswitch failure accounts for about 50% of the electrical failures of their mechanisms. Except for defects in auxiliary switches and micro switches, the second circuit fault has the highest proportion of electrical defects in the mechanism. Attention should also be paid to these "supporting role and supporting role". For example.
(1) The SW6-220 circuit breakers of the main transformer in a 220kV substation of a certain power bureau accidentally tripped during operation, resulting in non full phase operation and serious consequences. The reason is that the outer skin of the lead wire of the C-phase mechanism's opening coil is worn out, and it discharges with the iron yoke window, forming a negative grounding of the DC system. Due to the malfunction of the insulation monitoring device in the substation, which could not be detected in a timely manner, the instrument team mistakenly touched the positive pole during operation, causing the DC two points to be grounded and causing the circuit breaker phase C to trip secretly.
(2) The SW6-220 type low oil circuit breaker in a certain substation was mistakenly connected to the secondary line during maintenance, resulting in the circuit breaker refusing to open when the fault occurred, leading to a power outage for the entire substation.
(3) The F4 auxiliary switch of the CY3 mechanism of the SW6-220I type low oil circuit breaker in a certain power plant seems to be disconnected due to poor manufacturing quality and insufficient contact elasticity. When there is a line fault, the circuit breaker cannot open correctly, causing the circuit breaker to fail and protect it. The 220kV busbar loses power, resulting in 155000 kW • h less power transmission and 75000 kW • h less power generation.
2. Mechanical defects in the circuit breaker body
The defects that cause the circuit breaker to malfunction are all mechanical defects. This includes damaged porcelain bottles, loose connection parts, damaged components, and foreign objects getting stuck. For example; The SW7-220 type low oil circuit breaker of a transformer in generator 3 of a certain power plant refused to close phase C during grid connection operation, causing non full phase operation and tripping of the 220kV busbar and line circuit breaker, resulting in a reduction of 400000 kW • h. The cause of the accident was that the circuit breaker had been operated 3600 times, with severe component wear, deformation of the straightening mechanism, and failure to conduct timely maintenance and replacement, ultimately leading to the accident.
The SW7-220 low oil circuit breaker has a special "late action" fault, which is due to the isolation of oil in the arc extinguishing chamber and the triangular box of the circuit breaker. In order to prevent oil leakage from the arc extinguishing chamber into the triangular box during operation, the dynamic seal of the conductive rod is generally adjusted very tightly. When the temperature rises in summer, the dynamic seal often holds the conductive rod. When the circuit breaker receives the opening command, the movement of the conductive rod needs to overcome this clamping force, often several tens to several hundred millimeters later to complete the opening action. For this "late movement" phenomenon, it is not easy to detect it only by checking the circuit breaker after the accident, and it can only be detected by looking at the fault recorder waveform. In order to avoid such accidents, the allowable range of the pulling force of the conductive rod has been specified in the maintenance process of SW7-220 low oil circuit breakers. As long as the maintenance process is carefully executed, there will be no "late action" accidents during operation.
3. Operation (control) power supply defect
The operational power supply defect of circuit breakers is also one of the three main causes of operational failure. Insufficient operating voltage is the most common defect in operating power supply defects. The reason for this is mostly due to the use of AC power supply after silicon rectification as the operating power supply in the power station. In the event of a system failure, the power supply voltage drops significantly, or although there is a battery pack, the voltage drop in the connection between the operating power supply and the circuit breaker is too large, resulting in the actual operating voltage being lower than the specified lower limit. For example, a substation experiences a fault in a distribution line, and the circuit breaker explodes when it coincides; The phase connection of the 44kV line in another substation was incorrect, and the circuit breaker exploded when it was closed and connected to the grid. These are all due to the fact that the power supply of the silicon rectifier is supplied by this substation, and when the line fails, the bus voltage decreases. Therefore, in 1982, the former Ministry of Water Resources and Electric Power formulated the Provisional Regulations on Operating Energy in Substations, which required that new substations should no longer use silicon rectification as the operating power source. It was suggested to promote the use of batteries and energy storage operating mechanisms, renovate and improve the operating power source of existing substations, and strengthen management.
（2） Insulation accidents
Circuit breaker insulation accidents can be divided into internal insulation accidents and external insulation accidents. The harm caused by internal insulation accidents is usually greater than that of external insulation.
1. Internal insulation accidents
Internal insulation accidents mainly include sleeve and current transformer accidents, which are mainly caused by water ingress and dampness; Secondly, there is deterioration in oil quality and insufficient oil quantity. It may also be caused by insulation quality issues with certain main insulation components. For example:
(1) The SW6-220 type low oil circuit breaker in a certain substation experienced an explosion in its B-phase north column under operating voltage, causing the complete shutdown of three large substations and the power outage of 28 medium-sized substations, resulting in an undertransmission of 60000 kW • h. The cause of the accident was water ingress into the aluminum cap and moisture on the insulation rod. In fact, in preventive tests, it was found that the oil withstand voltage was low (only 18.8kV/2.5mm), but the shutdown was not arranged in a timely manner, resulting in internal insulation flashover and circuit breaker explosion.
(2) A SW3-110G low oil circuit breaker in a certain substation discharged approximately 20kg of water during maintenance. Due to water ingress, insulation components are affected by moisture, causing flashover and even explosions in many cases. The province has experienced four explosion accidents in just one year. The main reason for water ingress is; The aluminum cap and cap structure are unreasonable or have sand holes and air holes; The installation process is not strict. The path of water ingress is generally from the screw entering the arc extinguishing chamber along the surface or from the hole pin at the top of the nozzle penetrating.
(3) The SW7-220 type low oil circuit breaker of a certain hydropower station suddenly exploded in phase B during operation. The main cause of the accident is due to moisture in the switch oil, which causes the insulation rod to become damp and the insulation strength to decrease. As a result, under normal voltage, the insulation rod may experience surface flashover, leading to the accident.
(4) Multiple incidents of insulation breakdown of epoxy resin insulated current transformers inside SW2-35 low oil circuit breakers have occurred, causing circuit breaker explosions. The main reason is poor quality of epoxy resin casting, with bubbles inside, causing partial discharge. Secondly, the unreasonable voltage distribution structure at the neck of the current transformer results in a relatively concentrated electric field at the neck.
By the way, it should be pointed out that water ingress into the circuit breaker not only affects its insulation performance, but may also lead to refusal to operate. For example, a certain SW4-110 type low oil circuit breaker in Anhui Province, due to a large amount of water entering the triangular box, formed ice in winter, resulting in the circuit breaker refusing to operate.
2. External insulation accidents
External insulation accidents are mainly caused by circuit breaker flashover and explosion accidents caused by pollution flashover and lightning strikes. The main reason for pollution flashover is that the leakage distance of the porcelain bottle is too small to be suitable for use in polluted areas; Secondly, there is oil leakage and leakage from the circuit breaker, which makes it easy for dirt to accumulate on the porcelain skirt and cause flashover. For example:
(1) The SW4-220 type low oil circuit breaker of a certain power plant experienced a pollution flashover during light rain due to dust accumulation in the oil leakage casing, resulting in a complete power outage accident in the 220kV substation.
(2) DW8 of a certain hydropower plant&# 0 The 35 type multi oil circuit breaker experienced an external insulation flashover accident due to lightning overvoltage.
（3） Performance of disconnection and closing
The task of opening and closing is the most rigorous test for circuit breakers. On site statistics show that due to severe opening and closing conditions, the probability of occurrence during operation is relatively low, so the proportion of circuit breaker opening and closing performance accidents is generally not high. The main cause of the vast majority of breaking and closing accidents is due to obvious mechanical defects in the circuit breaker, followed by lack of oil or oil quality not meeting the requirements. It is also due to the insufficient breaking capacity of the circuit breaker. But the former is more common because a considerable number of accidents occur during the opening and closing of small capacities, and even during the opening and closing of load currents. For example, in the secondary circuit breaker SW2-60G of the No. 1 main transformer in a certain substation, a ground wire closing accident occurred on the 63kV busbar. When the circuit breaker tripped and overlapped, the porcelain sleeves of phases B and C burst, and the well was sprayed with oil and caught fire. After calculation, the short-circuit capacity of the first disconnection was 1500MVA. After 4.5s of overlap, the breaking capacity was only 600MVA, far lower than the nameplate capacity of 2500MVA, so it does not belong to the accident of insufficient breaking capacity. This type of circuit breaker originally had a defect of upper cap fuel injection during normal operation. This circuit breaker was originally processed and refined by the operating unit, and it is questionable whether the size and position of the exhaust hole are correct, and whether it is blocked after assembly.
（4） Accident of poor conductivity
The proportion of accidents with poor conductivity in circuit breaker accidents is relatively small, due to: ① the actual load current of most circuit breakers is much lower than their rated value; ② The conductivity in a stationary state is easily guaranteed.
Analysis of on-site accident statistics shows that poor conductivity faults are mainly caused by mechanical defects. Among them: ① Poor contact. Including unclean contact surfaces, insufficient contact size and contact pressure; ② Falling off, jamming. If the copper tungsten contact falls off, etc.; ③ Loose screws at the contact point; ④ Soft connection breakage, etc.
Information source: Power Search