The analysis was carried out to find out the best device for absorbing over-voltage, and the RC resistance-capacitor protection device was tested by oscillating the 780KW coal-fired motor vacuum circuit breaker, which further confirmed that the RC resistance-capacitor protection was absorbed. The superiority of the overvoltage, through the parameter selection, applied tests on some of the motors, the effect is very good.
The vacuum circuit breaker has the advantages of simple structure, convenient maintenance, safety and reliability, and is suitable for frequent operation. Qilu Petrochemical Thermal Power Plant has 87 high-voltage motors equipped with ZN5-10 vacuum circuit breakers. Due to the strong arc-extinguishing ability of the vacuum circuit breaker, the high over-voltage is generated by the interception, which poses a serious threat to the insulation of the motor. The high-voltage motor operated by the vacuum circuit breaker has suffered multiple insulation breakdown accidents during the welding process, which poses a serious threat to safety production. In order to clarify the actual situation of the vacuum circuit breaker to meet the overvoltage generated by the high-voltage motor, so as to adopt the correct Protection measures. To this end, the over-voltage condition of the high-voltage motor of the ZN5-10 vacuum circuit breaker was recorded. The best over-voltage protection measures were found on the basis of the test, that is, the RC resistance-protection device was added. The voltage is absorbed and a good limit overvoltage effect is received.
2 Mechanism of generating overvoltage The vacuum circuit breaker inevitably generates overvoltage when it is connected to the induction motor. The generation of overvoltage is caused by the following aspects.
2.1. Cut-off over-voltage The so-called cut-off over-voltage is the arc-breaking phenomenon that occurs before the power-frequency current naturally crosses zero. Closure vacuum interrupter reasons occurred primarily at a certain current value, due to the fast diffusion rate of the arc column, metal vapor pressure and temperature of the sharp drop in the vicinity of the cathode spot, the evaporation diffusion of the metal particles arc column can not be maintained, resulting in an arc suddenly went out. Size depends largely on the chopping current contacts material, while the network related parameters.
Since the cutoff frequency is truncated when the current I and the voltage U, the motor is stored in the leakage inductance L + LI2 magnetic energy stored in the equivalent capacitance C + CU2 motor power. After closure occurs, capacitance, inductance loop will oscillate a high frequency, high oscillation voltage, i.e., current chopping. The above process can be derived from the overvoltage maximum oscillation frequency: 0.7 ~ 5 seen from the above equation, the size of the current chopping characteristic impedance and the motor closure Io z = / LTC related. A motor with a small breaking capacity has a low overvoltage due to a small intercept value Io. When a large-capacity motor with several thousand kilowatts is interrupted, the number of turns of the coil is small, and the cross section of the wire is large, so that the inductance L is small and the capacitance is large, so that the characteristic impedance Z is small and the overvoltage is also low. However, when a medium-capacity motor of several hundred kilowatts is interrupted, since both 1 and Z are large, the overvoltage is high. The capacity of the motor operated by the vacuum circuit breaker is 2,300 KW except for the water supply motor. The other motors are mostly between 310 and 800 KW. It is the case of heavy overvoltage and reliable protection measures must be taken.
2.2 Over-voltage vacuum circuit breaker caused by multiple re-ignition When the high-voltage induction motor is cut off, the arc is extinguished at the moment of current zero-crossing. At this time, the opening distance of the moving and static contacts is very small. After the current crosses zero, the voltage recovery speed exceeds the recovery speed of the medium strength between the moving and static contacts. After the arc is extinguished, the static and dynamic contacts are broken down again, and the arc is reignited, resulting in high Frequency oscillation, high frequency current occurs, the frequency is generally 105~106Hz. At this time, the capacitor inductance of the load side electromagnetically oscillates, generating a higher voltage, and the arc will not be extinguished until the recovery rate of the medium strength exceeds the recovery speed of the voltage.
The amplitude of the reignition overvoltage is very high, and the rise time of the overvoltage wave is tenths of a microsecond. The wave head is extremely steep, and the overvoltage will be concentrated on the windings of the first end of the motor, causing the insulation breakdown of the motor between turns. The insulation of the motor is extremely harmful.
According to the test data, when the wave head is below 1. 5us, the high-frequency re-ignition over-voltage is applied to the winding of the motor, so that the potential distribution between the turns is extremely uneven, and some of the turns are subjected to a very high voltage, and breakdown occurs first. Causes motor breakdown damage.
2.3 Overvoltage caused by pre-breakdown when the vacuum circuit breaker is closed When the vacuum circuit breaker is closed, due to the reduction of the contact gap, the medium of the contact gap cannot withstand the loop voltage, resulting in arc breakdown and high frequency oscillation. The current is extinguished at the first zero-crossing point of the high-frequency current. This will cause multiple breakdowns and will generate a transient over-voltage. The peak value generally does not exceed 3 times the system voltage, but the frequency is as high as 1MHz, and the wave head is extremely steep. Every time the motor starts, it will be very harmful to the motor. When the vacuum breaker breaks the current multiple times and forms a “whisker†on the contact surface, the earlier the pre-breakdown will occur, the more pre-breakdown will occur. , the harm is even greater. In addition, the overvoltage caused by the bounce of the vacuum circuit breaker contacts is also very harmful.
From the above analysis, it is known that the overvoltage generated during the start and stop of the motor, especially the steep wave front, the high frequency overvoltage wave, the damage to the motor, causing serious damage to the motor insulation, sometimes the insulation breakdown short circuit occurs. Burn out the motor.
3 Overvoltage protection measures For the overvoltage generated by vacuum circuit breaker operation, some protective measures have been studied at home and abroad. At present, zinc oxide surge arresters and RC protection RC absorption devices are mainly used.
3.1 Zinc Oxide Arrester The zinc oxide arrester is actually a kind of non-linear resistor. When the normal working voltage or over-voltage is not high, the arrester works in the high-resistance zone, that is, it has no effect on the process. When a high amplitude overvoltage occurs and exceeds the arrester When the initial operating voltage (voltage drop when passing 1 mA DC current), the operating point of the arrester moves to the low resistance region, absorbing the energy of the overvoltage, thereby suppressing the overvoltage.
According to the above process, the initial operating voltage Ulma of the arrester should satisfy: 1 ​​a power supply voltage head coefficient; 1.1 a dispersion coefficient between resistors; 1.05 - a resistance aging coefficient; if the motor insulation level takes a preventive withstand voltage test voltage U: 1 is Impact factor.
The arrester protection characteristic is usually expressed by the protection ratio K. The protection ratio K is the ratio of the maximum current that the arrester can pass under the overvoltage to the voltage drop at 1 mA. It can be seen that in order to make the arrester reliably protect the motor insulation, it must be ensured that the protection characteristics of the zinc oxide arrester currently produced in China cannot meet the above requirements, and some manufacturers give K=U106a/U1mA=1.3~1.4 the safety factor is too small. For the motor with frequent operation, considering the cumulative effect of insulation damage, the zinc oxide arrester is not very effective as the overvoltage protection of the motor. For example, although the factory is equipped with FY-6 zinc oxide arrester on the vacuum circuit breaker, the motor often occurs. Insulation breakdown accident.
3.2 RC absorption protection device RC RC overvoltage protection device is composed of capacitor and resistor series. Capacitor is a simple low-pass filter that absorbs transient surge voltage and reduces wavefront steepness. The series resistor has the function of reducing the inrush current. As long as it is properly designed, the impedance at the transient frequency is much lower than the voltage impedance, forming a voltage divider, and connecting it in parallel with the electrical equipment can effectively reduce The wave front steepness of the overcurrent is cut off, so it is especially suitable for overvoltage protection of the motor.
4 Test wiring, test results and overvoltage analysis 4.1 Test wiring Connected as shown, in the switch room of the coal mill motor, in the actual operation of the coal mill. The recording device is started by the trip signal of the circuit breaker, and the three-phase current is operated with less delay, and the current transformers of each phase are respectively connected to the three current inputs of the oscilloscope.
The three opposite ground voltages on the motor side are connected to the three voltage inputs of the oscilloscope by a three-component coaxial cable via a 3-meter long coaxial cable.
4.2 Test results and overvoltage analysis During the actual operation of the coal mill motor, the current and voltage waveforms were recorded when more than 20 trips were taken. The typical oscillogram is as shown in the test results. The vacuum breaker is used to cut the light load motor. The interception phenomenon and the corresponding interception overvoltage, due to the limited measurement data, can not accurately calculate the highest multiple of the overvoltage, but the average overvoltage multiple measured from the measured data is about 2.0 times, after processing according to the normal distribution The maximum overvoltage multiple is 2.8, which has exceeded the peak value of the motor preventive test voltage Umax=2. 6Uy If the cumulative effect of the motor insulation damage is taken into account, this overvoltage will pose a serious threat to the motor insulation, and protective measures must be taken.
In the above 780KW motor circuit, in the vacuum circuit breaker trolley, a set of RC protection and absorption devices with C=0.1 microfarad and R=200 ohms are installed on the motor side of the circuit breaker, and also during the actual operation of the motor. The current and voltage waveforms when the switch is closed multiple times, the typical oscillogram is as follows. In contrast, the interception value significantly reduces the overvoltage phenomenon. The measured results prove that the above-mentioned resistance-capacitor protection and absorption device works well, so it is decided to install the resistance-capacitor protection on the motor of the frequent coal mill.
5 The selection of the parameters of the RC protection absorption device shows that the three-phase circuit can be drawn into a single equal value circuit under different breaking conditions. In the figure, Ly is the leakage inductance of the motor, which is obtained by the short-circuit test result. In Table 1, C and the parameters of the selected RC absorption device.
When a phase is first broken (as shown), L=1 in the figure. The absorption device Co, Ro can be obtained according to the following formula: the voltage peak, ie: Io is the intercept value, and the main cut-off range of the vacuum circuit breaker is based on The product samples of Shanghai Electric Machinery Co., Ltd. are selected as follows. The vacuum circuit breaker of 440KW circuit is selected as GS6-1 type RC absorption device, C=200 ohms.
Motor capacity 310KW circuit vacuum circuit is selected GS6-2 type RC absorption device, C= ~400 ohms, but considering the change of Ro value is not very obvious, and in order to reduce the loss of normal operation, R=200 ohm.
Table 1C Calculated value Motor capacity (KW/) first disconnects one phase and then disconnects two phases 6 Conclusion The high-voltage 6kV motor with hundreds of kilowatts of capacity frequently operated by ZN5-10 vacuum circuit breaker will generate a shut-off overvoltage that is dangerous to motor insulation. The FY-6 zinc oxide arrester is used as the overvoltage protection of the motor. Since the safety factor between the protection characteristics of the arrester and the insulation level of the motor is too small and not reliable enough, it can be seen from the above analysis. Conventional zinc oxide surge arresters are low in value due to their rated voltage and continuous operating voltage. In some cases, they not only fail to protect the equipment but increase the accident of the power system. If the continuous operating voltage Uc of the zinc oxide arrester reaches the line voltage, the corresponding voltage protection value is greatly increased accordingly. Such a protection value cannot protect the insulation of the motor at all, and the series gap is increased, which only enhances the internal overvoltage capability with a long tolerance duration, and cannot solve the problem of the Ul00A=U phase.
In recent years, a zinc oxide surge arrester with a series gap four-star connection has appeared in China, which is described below.
5 Zinc oxide arrester with series gap four-star connection The wiring method of zinc oxide arrester with series gap four-star connection is shown.
Wiring diagram of three-phase four-star zinc oxide arrester This wiring makes Ul00A=u phase, the overvoltage between phase and phase is greatly reduced, and the protection performance is improved. Let's analyze its protection performance.
5.1 Interphase overvoltage protection: When an overvoltage occurs between any two phases of the three phases, two of the three protection units P1, P2, and P3 are discharged in series through their respective gaps, and the conduction limit of the zinc oxide valve is limited. Pressure; after the overvoltage disappears, the discharge gap is automatically restored due to the small leakage of the zinc oxide valve.
Therefore, the zinc oxide arrester with a series gap four-star connection well protects the insulation between the opposite phases of the high voltage motor.
5.2 Relative overvoltage protection: When an overvoltage occurs between any one of the three phases and the ground, the corresponding phase of the three protection components P1, P2, P3 and the grounding cell P4 are discharged in series through the respective gaps. The zinc oxide valve plate is turned on and the pressure is limited; after the overvoltage disappears, the discharge gap is automatically recovered due to the small leakage of the zinc oxide valve piece. Also, therefore, a zinc oxide arrester with a series gap four star connection well protects the insulation between the high voltage motor and the ground.
6 Conclusion In summary, the conventional zinc oxide arrester has a low value due to its rated voltage and continuous operating voltage. In some cases, it not only fails to protect the equipment but increases the accident of the power system. If the continuous operating voltage Uc of the zinc oxide arrester is increased to the line voltage, the relative voltage protection value is correspondingly increased. Such a protection value cannot at all keep the insulation of the motor at all.
The gap used in the four-star-connected zinc oxide arrester protects the zinc oxide valve plate and the gap from each other. The gap makes the charge rate in the zinc oxide valve piece zero, avoiding the aging problem of the zinc oxide valve piece, zinc oxide. Good nonlinear volt-ampere characteristics make the gap no-clip after discharge, no freewheeling, and the gap does not bear the task of arc-extinguishing, which improves the service life.
The zinc oxide arrester adopts a four-star connection method, which can greatly reduce the overvoltage between the opposite phases and improve the reliability of protection. Moreover, the relative phase and relative ground protection voltage values ​​are low, and the operating overvoltage can be reliably It is limited to the insulation allowable range of the protected equipment and is operated for a long time under single-phase grounding, intermittent arc grounding and resonant overvoltage. (Upper page 22) The effect of the protection is not ideal. Therefore, the accident of motor insulation breakdown still occurs many times.
In order to prevent the breakdown of the motor insulation, the above-mentioned vacuum circuit breaker should be replaced with a RC absorption device. After installing the RC protection on the motor side of the frequent pulverizer vacuum circuit breaker in 1996, the effect is very good, but due to the open circuit The structure of the device is limited, and it is difficult to retrofit.
In order to better protect the motor so that it is not damaged by overvoltage during operation, it is recommended to install a RC absorption protection device in the design when retrofitting and new high voltage 6kV motor vacuum circuit breakers.
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