-6kV voltage level equipment commonly used oil switches and vacuum circuit breakers to control motors and furnace transformers and other production equipment. Such as the Anshan Iron and Steel Co., Ltd. to transform the new three-burning project to) vacuum circuit breakers to meet the large mixer and conveyor belt machine. The characteristics of this type of equipment in the production process are often frequent start-stop state of heavy load, resulting in serious wear and tear of the operating mechanism, accelerated deterioration of the oil switch oil quality, reduced vacuum circuit breaker vacuum, bellows mechanical fatigue, etc., resulting in the switch Reduced service life and increased failures. The author only counts the production and operation reports of Xinsansan from June 1999 to May 2000. There are a total of 88 switch failures and a total of 186 hours of production stoppages, resulting in direct economic losses of 1.766 million yuan for production shutdowns, burning of motors, and spare parts consumption.
1 oil switch, vacuum circuit breaker, vacuum contactor performance comparison 1.1 Operating mechanism The oil switch, vacuum switch operating mechanism are equipped with opening and closing and maintaining mechanism, and controlled by the opening and closing coil respectively. When the closing coil is energized and closed, the mechanism drives the auxiliary switch to power off, and the opening coil enters the working state; when the opening coil is energized and opened, the mechanism drives the auxiliary switch to self-power off, and the closing coil enters the room. Working status. The advantage of this mechanism is that the coil is powered on for a short period of time, and it can power off the maintenance control circuit while keeping the original switch in the same state. Disadvantages are complex mechanisms, short mechanical life, heavy weight, and need two sets of coils, and the electrician in the case of stopping the power supply, if you do not seriously confirm the switching state, and out of the trolley (or pull-up isolation switch), easy to mistake The operation resulted in personal and equipment injuries.
The vacuum contactor employs an electromagnetic operating mechanism, which is provided with a set of coils and is held electrically, that is, the coil is energized and closed, and the power is switched off. The disadvantage of this mechanism is that the coil operating circuit must be electrified for a long period of time, but the advantage is that the operating mechanism is simple. When the electrician stops operating the power supply, the electrician enters or exits the trolley (or pulls apart the isolation switch), which can not cause personal and equipment injury accidents, and thus the institution has more safety.
1.2 Insulation and arc extinguishing oil switch Insulation and arc extinguishing medium using No. 25 transformer oil as the switch, because the oil insulation and arc extinguishing chamber capacity is poor, the current zero crossing plus arc extinguishing effect can be used to extinguish the arc when the circuit is disconnected. Therefore, the volume of the arc extinguishing chamber of the oil switch is large, and the opening distance of the contacts is also large, and thus the weight is large. Vacuum circuit breakers and vacuum contactors use vacuum as the insulating medium. Residues generated by the arc are attenuated very quickly when the current is interrupted at the room temperature in the arc extinguishing chamber. Therefore, the contact gap insulation degree recovers quickly. Usually a cell gap insulation voltage rise rate up to 20kV/ps, is 15 times that of air medium, so under the same voltage and short-circuit breaking current conditions vacuum interrupter contact spacing can be designed to be smaller, 3 ~ 10kV The grade can be shortened to 3~12mm, which is only 1/15 of the oil circuit breaker stroke, so it has the characteristics of light weight and small volume compared with the oil switch.
1.3 Switch short-circuit current, rated current and operating current of the equipment used in the design of the pressure switch are mainly considered in two aspects: to meet the requirements under normal working conditions, mainly selected switch rated voltage, current is greater than or equal to the line voltage and Load rated current; to meet the requirements of short-circuit switching stability and thermal stability, mainly the selected switch rated short-circuit breaking current (or limit breaking capacity) is greater than the point of the line short-circuit current, both must be met at the same time. Table 1 lists the phase parameters of the 6kV power supply network of the Anshan Iron and Steel Co., Ltd. New Three-burner mixers. Table 2 and Table 3 list the main performance parameters of various switchgears.
Combination of the above parameters, while meeting the above requirements of the oil switch only Table 1 new three-burner mixer 6kV power supply network parameters equipment name motor capacity / kV rated voltage / kV full load current / A number of starts / times h1 switch feed / kV 6kV bus short circuit parameters Superior Protection Time/s Primary Mixer Secondary Mixer Table 2 Types of Switches Main Performance Parameters Vacuum Circuit Breaker Oil Switch Tube Vacuum Contactor Tube Parameters Rated Current/A Break Switch Segment Current/kA Limit Breaking Capacity Rated Voltage/kA Table 3 Various types of switch performance Item Item Oil switch Vacuum circuit breaker Vacuum contactor Rated voltage/kV rated current/A rated open circuit breaking current/kA limit breaking capacity/kA contact open distance/mm mechanical life secondary rated current breaking times times Dong Ag operating mechanism type electromagnetic operating mechanism characteristics after the closure of the coil power off after closing the coil off the power off the gate can use the rated current of 1000A SN10-ion type, vacuum circuit breaker can only use the rated current of 1250A ZKB10 / 1250/31.5-I vacuum tube, although this choice can meet the design and selection requirements, but at the same time it has another problem, that is, the full load current of the mixer only reaches the rated power of the switch. Flow 5.8%, even 3500kW, 6kV motor current is only close to 30% of the rated current, it seems that the traditional design method of using a circuit breaker to control the motor is not reasonable. The use of a vacuum contactor is not only light weight, simple and reliable operation, long service life, but also can fully and reasonably meet the requirements of the motor below 3500kW in terms of rated voltage, current, but can not meet the requirements of short-circuit breaking current, this article will be 2 Some problems and countermeasures in the application of pressure vacuum contactors 2.1 Transient overvoltage 2.1.1 Vacuum cut-off characteristics Vacuum as dielectric and arc extinguishing medium has strong arc extinguishing characteristics and dielectric strength recovery characteristics, when the switch contacts are When the frequency cycle breaks at a certain moment, during the time when the arc elongates during the remaining time of the half wave, the current suddenly cuts off before passing through the natural zero point due to the effect of vacuum gap extinction and insulation.
2.1.2 Overvoltage Generation and Suppression 2.1.2.1 Current cut-off The temporary cut-off of transient overvoltage currents will cause a drastic increase in the rate of change of the motor current, producing a pulse maximum voltage of 6 to 8 times the rated value. See the waveform.
2.1.2.2 Rekindling transient overvoltage Under normal circumstances, the current cutoff can always cause overvoltage, and the contact is broken before the current just passes the zero crossing. Because the gap is small, the voltage is close to the peak again, causing the high voltage generated by the breaking. This again causes gap insulation to re-hit the window, resulting in a reignition transient overvoltage. See the waveform.
2.1.2.3 Motor over-voltage distribution Under steady-state voltage conditions, the voltage distribution in the winding is uniform, and under transient conditions, the voltage will be unevenly distributed, and part of the windings will withstand higher voltages. When the wave crest propagates into the windings, it causes the power line nearby.åŒ withstand large pulse 篼 voltage, so it is necessary to raise the insulation level of the winding of the motor terminal winding. At the same time, the overvoltage generates the 开关 voltage at the beginning of the output line of the vacuum switch, which can seriously cause interphase or ground strike. Wear discharge.
2.1.2.4 Overvoltage suppression The presence of overvoltage in vacuum switches poses a great threat to the insulation of motors, cables and switchgears. Therefore, it is a primary research question to adopt effective measures to limit the voltage amplitudes in order to protect the electrical equipment from damage. Observe and can see that the waveform is irregular pulse voltage. It is characterized by large steepness, high amplitude, and short maintenance time. According to this feature and in combination with the previous analysis of overvoltage, a varistor is installed at the output end of the contactor. When the resulting transient overvoltage amplitude increases, the resistance is (17=CI°, a=0.05). The linear characteristic immediately decreases, and the discharge current drastically decreases when the voltage drops, and finally the resistance value returns to a high-impedance state. This process releases the magnetic energy in the motor and protects the insulation from excessive voltages.
3 Special considerations for relay protection problems 3.1 Implementation of overload protection The 12/10-type relay acts as an inverse-time overcurrent protection for the motor. Taking the new three-burner mixer as an example, the minimum operating value is lmin=60.75 octaves and dz2nn=99.9 amps, and the maximum operating current at maximum 10 times/setting is the dzlmsx=607.5A and 1 may be equal to 999A, and the maximum disconnection may be selected. The CKG2-400 type vacuum contactor with current up to 4500A cuts off the overload current. Overcurrent relay action time: Taz is set to 2s. 3.2 Implementation of short-circuit protection When a short circuit occurs in a certain part of the power distribution network, the network voltage is short-circuited The direction gradually decreases. At this moment, when the motor induced potential is lower than the supply voltage and other motor back potentials, the motor will feed the short circuit point and other motors with current. At the same time, the motor is subject to braking force, and all the motor speeds on the network will increase with short circuit time. Both are decreasing. When the voltage recovers, it starts and draws a lot of power to the grid at the same time. The longer the voltage recovery time and the greater the voltage drop, the more self-starting power is absorbed, the greater the threat to the power grid, and the short-circuit device. The damage caused by itself is also greater. Due to the poor ability of the vacuum contactor to withstand the short-circuit current impact, the primary consideration for selecting the vacuum contactor short-circuit protection element is: when rin occurs, the protection operation time limit is less than the upper limit time, and the protection element rated breaking current needs to be on the line. The D2dmax value at this point, the rated voltage of the protection element is greater than or equal to the line voltage. The WFNO or WKNO type fuses shown in Table 4 can be selected according to the above requirements according to the performance parameters of the helium pressure limiting current limiter.
Take the new three-burner mixer as an example: The current values ​​of the primary and secondary mixers during the start-up time are Y1=6nl=621A, Y2=612=1021A, respectively, and the corresponding arc front time is 7=2.7s, respectively. = 4S, due to the time when the fuser melt is 100A, zlmax current action time> 7, the secondary melter melter is 160A, current action time> 2. Table 4 fuse basic parameters Rated current / A rated voltage / kV rated breaking current / kA melt rated current / A In summary, the protection design idea is 1. 35n ~ 10x1.35n current, to ensure that the contactor to achieve "repetitive" protection .
When the current is more than 10X1.35n, the fuse can be "one-time" protected.
4 Conclusions The Angang Sintering Plant has applied vacuum contactor devices to the equipments of Xinsansha and Pellet 2 hammer crushers, air compressors and pressure belt conveyors since June 2000. The accident is zero, and it has the advantages of light weight, simple operation and maintenance, and safety and reliability. In particular, the service life is as long as 250,000 times (25 times the life of the vacuum circuit breaker), which provides a reliable guarantee for the long-term stable operation of the equipment. Therefore, the promotion and application of the helium pressure contactor will surely bring positive development for the equipment. influences.
Economic Benefits: 4 sets of mixers and hammer crushers use vacuum contactor cabinets to create benefits: WH=1.396 million mixers, MP crusher=37 million, WH+MP=17.767 million.
Mixer vacuum contactor trolley investment: =16800 (兀/Taiwan) x4 (Taiwan) = 67,200 Hammer Crusher Vacuum Contactor Handcart Investment: Hammer Crusher Vacuum Contactor Cabinet Investment: =29500 (兀/ Taiwan) 2 (Taiwan) = 59,000 兀 Equipment Total Investment: +Gp2 = 160,000 兀 The equipment investment payback period is: = 12 months x total equipment investment (10,000 yuan) / year efficiency (10,000 yuan) 篼 pressure FC switch The rational application of the device not only ensures the safe and stable operation of the power grid, but also solves the key problems of production, and at the same time, it also creates great benefits for the enterprise.
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