"Technology" on-load tap-changer structure principle, accident cause analysis and preventive measures

Summary:

The current situation of the transformer on-load tap-changer accident is analyzed, and the preventive measures for reducing the accident of the on-load tap-changer are proposed. The reasonable selection of the on-load tap-changer is discussed to ensure the smoothness of the cracker. Safe and economical operation.

Chapter One

introduction

The primary side of the transformer of the 1·1 is connected to the power grid. Since the voltage of the grid system fluctuates due to various reasons, the secondary voltage of the transformer also fluctuates correspondingly, which affects the normal operation of the power equipment. Therefore, the transformer needs to have a certain voltage regulation capability to adapt to the needs of the power grid operation and power equipment. The working principle of transformer voltage regulation is to change the number of turns of the winding, that is, to change the voltage ratio of the first and second sides.

According to the voltage fluctuation condition or the load to the voltage requirement, the secondary side voltage of the coil 匝 value is adjusted to meet the load demand, and the method of adjusting the number of coil turns on the coil is generally adopted. The taps are generally on the high-voltage side coils because the high-voltage coils on the outside of the low-voltage coils are convenient, and the taps are convenient, and the insulation treatment is also simple. In addition, the current on the high voltage side of the transformer is small, and the cross section of the lead wire and the tap changer and the like can be made smaller, saving metal materials.

"Technology" on-load tap-changer structure principle, accident cause analysis and preventive measures

The basic principle and structure of the 1·2 on-load tap-changer. The on-load tap-changer is an electrical device that can change the tap position in an excited state. The basic principle of on-load tap-changer voltage regulation is to change the effective number of turns after switching off several taps in the transformer windings and switching from one tap to the other without interrupting the load current. That is to change the voltage ratio of the transformer, thereby achieving the purpose of voltage regulation. Therefore, during the operation of the on-load tap-changer, one must ensure the continuity of the load current; the second is to have good arc-breaking performance in the switching tapping action.

The 1·3 on-load tap-changer must use a resistor to achieve a transition during the change tap to limit the circulation during its transition. A resistive combined on-load tap-changer is usually used. In actual work, the structure of the resistor current limiting on-load tap-changer can be divided into three parts, namely, a switch, a selector switch, and an operation structure. Which part of these problems will directly affect the normal operation of the transformer.

Chapter two

Transformer voltage regulation and the main part of the switch

The voltage regulation mode of the 2.1 transformer can be divided into non-excitation voltage regulation and on-load voltage regulation.

2·1·1 non-excitation voltage regulating tap changer:

The non-excited voltage regulating tap changer is used to change the tapping of the primary side or secondary side coil when the primary side and the secondary side of the transformer are disconnected from the power supply network, and change the effective number of turns to perform stepwise voltage regulation. ;

2·1·2 on-load tap-changer

The on-load tap-changer is used to change the tapping of the primary or secondary coils, change the effective number of turns, and perform step-by-step voltage regulation.

The 2·1·3 on-load tap-changer is composed of a diverter switch, a selector switch and an operating mechanism.

The switch is specifically responsible for switching the load current, and its action is quickly completed by a fast mechanism according to a certain procedure. The selector switch is a portion in which the taps to be immediately adjacent to each other are switched on in the tapping order and bear the continuous load. The switch and the selector switch, which are generally referred to as the switch body, are generally installed in the oil tank of the transformer, and the switch generates an arc when the load current is switched, which deteriorates the oil quality. It must therefore be housed in a separate insulation tube to isolate it from the oil in the transformer tank.

The 2·1·4 operating mechanism is the power source for the action. It also has the necessary limits, safety interlocks, position indications, counting and attachments to the signal generator. The electric operating mechanism vertical shaft gear, motor shaft belt, vertical shaft and horizontal shaft are connected to the switch body. Usually the operating mechanism is mounted on the wall outside the transformer tank.

The 2·1·5 resistive on-load tap changer has two types of composite type and combined type according to the combination of its switching switch and selector switch. The composite switch body, the changeover switch and the selection switch are integrated into one body, which is also called a selection switch, and both the switch and the switch contact (auxiliary contact) are selected. The following is an example of the application of the on-load tap changer in our company to understand the voltage regulation level of the on-load tap changer and the current development status.

Chapter III technical parameters, work process. Test, test index, test data

3.·1 main parameters of the process

3·1·1 cracking transformer

The cracking transformer model is HSZ9-3000/10 rated capacity 3000KVA, primary voltage 10KV, secondary voltage 160V-315V, Y, △ wiring, 14-stage on-load tap-changer, 3∮, 50HZ, Ynd11,

3·1·2 on-load tap-changer

The on-load tap-changer is SYXZ3-35/200-8, three-phase, maximum rated current 200A, 8-level voltage regulation, and secondary voltage regulation range of 160V-315V.

3·1·3 on-load tap-changer basic data

a. rated voltage 35KV;

b. The maximum rated through current is 200A;

c. The maximum rated voltage is 600V;

d. Pressure regulation level 8 and below;

Main performance indicators of 3·1·4 on-load tap changer

a. The contact pressure of the working contact is 40±5N;

b. The contact resistance of each contact point of the working contact is not more than 500u;

c. The excessive mode is a double resistance transition;

d. The electrical life of the switch is not less than 50,000 times;

e. The mechanical life of the switch is not less than 50,000 times;

The 3·1·5 switch can withstand the following voltage test for 1 min in the transformer oil;

a. Switch to ground 95Kv;

b. Switch phase between 85Kv;

c. 45Kv between any contacts;

d. Auxiliary circuit 2Kv;

3`1`5 on-load tap changer controller model YTK-S-10/380V, 50HZ

The fourth chapter is the switch accident, on-site and test conditions, accident analysis and technical measures.

4·1 accident

4·1·1 March 17, 2002

Sodium cyanide 5# solution transformer on-load tap changer in 14 files, primary current 90A, secondary voltage 300V, secondary current 5000A, cracking transformer room sends out a large explosion sound when the accident occurs, oil from the transformer indoor Squirting, cracking transformer substation with high-voltage switchgear quick-break protection action (setting current value 2.5 times rated current, time 0 seconds) cut off the power supply, while the company's part of the low-voltage power system is affected.

4·1·2 August 20, 2002

Sodium cyanide 5# solution transformer on-load tap changer in 13 files, primary current 80A, secondary voltage 285V, secondary current 4500A, other accidents are the same as 4/1. Several similar accidents occurred in the future.

Site and test situation after the accident

4·2·1 The accident of March 17, 2002.

1) The anti-film (setting value 2*105 Pa) on the cover of the on-load tap-changer is blown up. The 5# solution has a large black oil on the top of the transformer and the surrounding walls and the ground. There is no abnormality in measuring the DC resistance of the transformer coil, and there is no abnormality in the low-voltage insulation resistance between the phases on the sides, the relative ground, and the high voltage.

2) Check the hanging core of the on-load tap-changer, the gear transmission mechanism is normal, the working position of the indicator switch and the tap changer is normal, and it is consistent with the position of the position indicator, no mechanism is stuck or malfunctions.

3) It is found that the static and dynamic contacts are severely ablated, and the three-phase transition resistances are all blown.

4) Analysis of the oil quality of the transformer main body, everything is normal. Therefore, it is determined that the transient resistance of the on-load tap changer is completely blown.

4·2·2 August 20, 2002

1) The anti-film whole block on the on-load tap-changer cover is blown up, and there is a large black oil on the top of the transformer and the surrounding walls and the ground. There is no abnormality in measuring the DC resistance of the transformer coil, and there is no abnormality in the low-voltage insulation resistance between the phases on the sides, the relative ground, and the high voltage.

2) Check the hanging core of the on-load tap-changer, the gear transmission mechanism is normal, the working position of the indicator switch and the tap changer is normal, and it is consistent with the position of the position indicator, no mechanism is stuck or malfunctions.

3) When pumping oil, it is found that the main oil level drops at the same time. It is found that the arc contact and auxiliary contact of the three-phase moving contact group are severely ablated, the transition resistance is all blown, and the tapping of the oil chamber of the switch oil chamber is performed. The three-phase static arc contact at the position is severely ablated and the insulation barrel is burned through.

4·3 Analysis of the cause of the accident

Through several on-site inspections and analysis of the on-load tap-changer accidents, the main causes of the accident are as follows;

1) The oil quality of the switch deteriorates and the inner and outer surfaces of the insulating cylinder of the switch are rough, so that a large amount of free carbon accumulates on these parts of the switch, so that a serious discharge and insulation breakdown occur, causing a phase-to-phase short circuit.

2) Our company's cracking transformer has an on-load tap changer, which uses a resistance transition during the change tapping process.

The transition resistance is burned due to improper selection and long-time high current, causing a switch accident.

3) Blind copy switch manufacturer's switch instruction manual recommended maintenance time gap, delay switch oil change, core inspection and cleaning time, resulting in excessive free carbon generated during switching operation accumulated on the switch components , the overall insulation performance of the switch is reduced, which is easy to cause short circuit of the internal electrical equipment of the switch.

4·3 Measures to prevent switch accidents

4·3·2 In order to ensure the quality of the transformer oil in the switch oil compartment during operation, combined with the actual operating conditions of the switch, timely oil filtering, oil change and core inspection are performed. When the number of switching times reaches 2000, the oil should be filtered to remove the free carbon, oxide, moisture and acid in the oil in time to ensure that the insulating transformer oil meets the operating standards.

4·3·3 When the number of switching of the switch reaches 3500-4500 times, the core should be inspected to remove the oil on the switch, and

Replace the qualified transformer oil (disabled regenerating oil) to ensure the insulation performance of the switch and effectively prevent the switch accident.

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