Overvoltage and Protection Measures of Vacuum Transformer Breaking Furnace Transformer

And restrictions.

Foreword After the application of the vacuum switch to the metallurgical industry, especially the continuous improvement in recent years, it has been widely used because it has the following advantages: Short arcing time: vacuum is used as the insulating medium between the main contacts and arc extinguishing Medium, its arc extinguishing ability is very strong, after the contact is just separated, the current cuts off the current when the current is zero for the first time; The volume is small and the weight is light: it is only about one-third of the volume of the general oil switch, and its weight is only Less than one-half of the oil switch, the reason is that the vacuum has very strong dielectric strength and a small contact distance; long life, simple maintenance, low workload: especially suitable for frequent operation, such as the furnace transformer of metallurgical enterprises Control; The operating power is small, the noise and vibration during the operation are small; not only can cut off the power frequency, but also have a strong ability to break the high-frequency current; There is no oil in the empty switch, so there is no danger of explosion. Makes the working environment clean and does not endanger the staff and nearby explosives.

The main disadvantage of the vacuum switch is that the overvoltage is higher and the price is higher in some applications. When the vacuum transformer is used to break the transformer of the electric furnace, an overvoltage with a very large amplitude may be generated, which endangers the insulation between the main insulation and the turn and reduces the service life of the transformer.

1 Switching off the overvoltage switchgear of the no-load furnace transformer causes an operating overvoltage due to a sudden change in the state of the circuit when the circuit is opened and closed. Due to the high recovery speed of the vacuum medium after the arc, the vacuum switch has the ability to open and cut high-frequency currents. When it is breaking current, it may cause excessive voltage due to interception, or it may cause overvoltage due to re-ignition and cutting of high-frequency currents.

When the switch closes, the distance between the contacts is very small, and when the contact is about to occur, the strike-through voltage is generated and the bounce occurs due to the contact of the contacts. In the process of bounce, the cut-off and re-ignition occur again, which is the closing overvoltage. Visible closing over-voltage in essence is still caused by the cut-off and re-ignition cut-off current, so cut off the circuit over-voltage can be used as a representative of the operating over-voltage. Other switchgears also have similar overvoltage issues, except that the vacuum switch has a strong ability to cut off high-frequency currents. The general manager assistant and the head of the production department of Shuhuihuijin Co., Ltd. are engaged in production technology management and scientific research work.

Higher levels, in some cases more serious.

1.1 In the initial period of development of cut-off overvoltage vacuum switchover, overvoltage was once very serious, because tungsten and molybdenum with very high shutoff levels were used as the contact material at that time. However, the current interception level of contact material is very low and it is no longer what Problem. h 1.1.1 Causes of interception of overvoltage I, in the inductive loop (see), current cut-off, current in L is /*, when the current in vacuum switch S suddenly drops from /. to zero, inductance i The current in the transformer cannot be mutated and continues to charge the capacitor C, causing the capacitor voltage to rise.

From the point of view of energy conversion, the equivalent circuit of the inductive loop has a certain energy stored in the inductor and the capacitor when the interception occurs. The stored energy in the inductor i and the capacitor c is: JTfl/2/02L, ITC=l/2 ( *m2cs2wC, these energy are all converted into capacitance energy, which is the maximum voltage that may appear on the load side. I Ce2*1/2 speak+1/2 (*n2cos2wC0 When coso=l, the maximum voltage is the visible cutoff overvoltage is the inductance The stored energy is transferred to the distributed capacitance, and the voltage waveforms before and after the shutoff are as shown.

For no-load transformers, the characteristic impedance of the loop is large. About tens of thousands of euros, even if /. very small, 1 ~ 2A on the 10kV line / 02i / the maximum voltage can be reduced to eraV; that is, the amplitude is proportional to the product of the cut-off current / load-side characteristic impedance.

1.1.2 Cut off the over-voltage of the no-load transformer Cut off the no-load transformer to cut off the excitation current of the transformer. Because the excitation current is not large, it is easy to cut off. The no-load transformer is quite a large inductor and its characteristic impedance is large. Theoretically, it needs to be generated. Very high over-voltage cut-off, but in fact due to energy transfer to consume a part of the voltage, the contact gap in the recovery voltage rise during the breakdown of the overvoltage suppression effect, therefore, the actual value of the over-voltage is smaller than the theoretical value Many, only about 40% of the theoretical value.

The cable-to-transformer connection between the vacuum switch and the transformer is less than the over-voltage generated by the connection of the overhead lines. Because the cable has a large distributed capacitance, the load-side wave impedance W decreases.

The relationship between the capacity of the transformer and the over-voltage of the interception is: the larger the capacity, the smaller the amplitude of the over-voltage. The reason is that the large excitation current is large, and the shutoff value may be higher. On the other hand, the capacity is large, the magnetizing inductance is reduced, and the distributed capacitance is large. Therefore, the characteristic impedance sharply decreases with the increase of the exciting current, and the product of the characteristic impedance and the exciting current becomes smaller, so the overshoot voltage value is small.

1.2 Voltage rise caused by multiple reburning 1.2.1 Causes of voltage rise The cause of reignition, the root cause of voltage rise is the vacuum switch has the ability to break high-frequency current, when the contact is just in the current Before the zero point is separated in a short time, when the contact is just separated, the current quickly crosses zero and the vacuum arc extinguishes. At this time, the gap between the contacts is very small, and the recovery voltage cannot withstand the heavy breakdown. The current flowing through the contact gap during the breakdown process contains high-frequency components. If the amplitude of the high-frequency component is greater than the instantaneous value of the power-frequency current, the high-frequency current zero will appear, the high-frequency current zero will be cut off, and the back-loop capacitance will be cut off. Inductors may experience high-frequency oscillations that generate high voltages. This voltage once again breaks through the contact gap and once again cuts off the current at the high-frequency current zero point, resulting in a higher voltage. The breakdown occurs repeatedly and the voltage increases continuously. As the number of breakdowns increases, the energy stored in the load circuit will increase. If the breakdown occurs after a certain breakdown, the stored energy is transferred to the capacitor C, ie, a high overvoltage is generated (equivalent circuit). see).

The frequency breakdown is not repeated indefinitely because the contact distance is increased during the repeated breakdown, and the withstand voltage level is also increased. When the withstand voltage reaches a certain value, the current in the inductor is Smaller than before, the storage energy of the load cell is also reduced, and finally the load-side overvoltage is less than the withstand voltage between the contacts, then the heavy breakdown is no longer over the voltage multiple 5MVA furnace transformer, no protection 5MVA furnace transformer, RC Protection of 3MVA furnace transformers, ZnO protection of 3MVA furnace transformers, RC protection of 1.5MVA furnace transformers, ZnO protection maps, preparation of 隈 隈 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 洧 如 如 如 如 如 如 如 如Can be limited to 3U+ or less.

2.1 Parallel RC Reactor and Dissipation Devices When a 3MVA furnace transformer was opened during a test, it was found that re-ignition overvoltage occurred. It is not sufficient to add shunt capacitors to this overvoltage, and appropriate resistors must be placed in the capacitors. Attenuation, to prevent the occurrence of frequency oscillation, because the series resistance, the high-frequency current is quickly attenuated, limiting its amplitude, can not form a zero frequency current, plus a capacitor can reduce the role of surge front steepness.

According to the load device impact pressure level selection.

Relevant data indicate that the choice of resistors for the transformer load is (see): 100 ft. R increases losses over the assembly and smaller capacitors can be installed in the capacitors because the bus inductance is very small, so c is very high, up to 1 MHz. The conditions under which the fuel is generated are generally not likely to cause reignition. It must be possible for the following conditions to occur in order: the contacts must be separated shortly before the current zero. The current in the work must be disconnected at zero and no significant cut-off occurs.

During reignition, the circuit generates high-current high-current, and high-frequency current can occur. The high-frequency ripple must be greater than the instantaneous power-frequency value.

The amplitude and frequency of the high-frequency current must be within the range of vacuum switch disconnection. The occurrence of re-ignition circuit is not much, but in some occasions it is very easy to happen. If the starting current of the motor is cut off, when the charge of the electric furnace falls, when the electrode of the electric furnace touches the bottom of the furnace, re-ignition may occur when the transformer overload current is cut off. The voltage level rises and does not occur easily under normal load. When the vacuum switch cuts off the 3MVA electric furnace transformer, a 3 watts JS level rise equivalent circuit occurs and the voltage level rise terminates. Since the overvoltage amplitude is limited by the withstand voltage of the contact, it cannot be very high, but its ripple rate is high. The oscillation is in the Cs-1-rSC loop. The corner frequency is: The first half of the figure is high frequency. Rekindled, the latter part of the frequency is equivalent to the current carrying voltage frequency.

2 Limitation measures for operating overvoltage As mentioned before, the use of a vacuum switch to break the transformer of the no-load furnace will result in an operating overvoltage with a high amplitude and steepness, which is a great hazard to the main insulation and the inter-turn insulation.

In order to ensure the safe operation and normal working life of the electric furnace transformer, protective measures must be taken. The RC resistance absorption device and the zinc oxide arrester protection device were used in the tests of the three electric furnace transformers and received good results. It is the over-voltage probability curve of the open-circuit electric transformer when the vacuum switch is installed and protected.

S6R-CK device connection diagram is easy to operate. In addition, the oscillogram of the overvoltage generated in the transformer for breaking the 15MVA electric furnace of the RC unit 0.1 installed with Guilin's production line 0.1 is shown in Fig. 10. The overshoot voltage is only 1.56U+ in the ten breakings. Oscilloscope 9 (15000 KVA> 22 Zinc Arrester General Arrester consists of discharge gaps and silicon carbide nonlinear resistors, while the Zinc Oxide Arrester eliminates the discharge gap, using only ZnO nonlinear resistors. It is better than the former. Linearity, its volt-ampere characteristics are: /=4 production, zinc oxide a=20~60, silicon carbide ot=6~7, after over-voltage, the resistance reduction time is: silicon carbide needs a few tenths of micro Seconds, and zinc oxide is only a few nanoseconds, so the zinc oxide arrester can limit the very steep overvoltage amplitude of the wave head, and can not reduce the steepness of the overvoltage.

For the cut-off overvoltage, protection with ZriO arrester is very effective, also has some effect on the level of overvoltage rise, but the level of over-voltage steepening is too steep, it should try to slow the steepness, so it is not to protect the transformer insulation between transformers is not The big one is to use a zinc oxide arrester to protect the measured overvoltage oscillogram of the 15MVA furnace transformer.

3 Existence of Liaoyang International Boron Alloys Co., Ltd. 98 installed in all electric furnace transformers zno ray 9 over-voltage oscillogram (15000KVA vacuum switch power supply circuits are used to limit the operation of over-voltage protection measures, mostly osmium oxide arrester In the case of infrequent operation, the operation effect is good. No abnormal situation has occurred for many years. However, in the near future, the 10kV busbar indirect phenomenon often occurs on the upper power supply side. In the initial stage, it was mistakenly considered as the distribution cable grounding. * Immediately ground the zinc oxide arrester of the furnace (10kV, 2.25MVA) after the overvoltage action. After the replacement of the new zinc oxide surge arrester, the momentary grounding condition is not relieved. Because the installation position is limited, the zinc oxide arrester is removed and replaced with a protective capacitor. Grounding did not appear.

This event suggests that the protective capacitor will perform better when the vacuum switch operation is particularly frequent.

4 Conclusion 4.1 Take effective measures in time When vacuum transformer is used to break the transformer of the electric furnace, it may produce a very large amplitude (up to 71), and the steepness of the crucible is over-voltage, which is very large for the main insulation and inter-turn insulation of the transformer. Hazards, if no effective measures are taken, will greatly reduce the working life of the transformer.

42 Limiting over-voltage measures It is better to use RC device, which has obvious effect on the current-carrying over-voltage and re-ignition over-voltage.

4*3 The installation of RC equipment or zinc oxide arrester installation is as close as possible to the electric furnace transformer.

According to foreign experience: It is best to use the RC resistance absorption device and the zinc oxide surge arrester in double protection at the same time, and the effect is very good.

This article is a superficial understanding of the lesson in my study and practice. It is inevitably shortcomings or even mistakes and mistakes. I hope my colleagues will criticize and correct me. I am grateful.

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