Accidents caused by box transformer and wiring from box transformer to frequency converter

At present, many new wind farms are located on high mountains with dense forests. In case of unit burning accident, it will not only bring huge losses to people and property, but also burn the surging forest sea in an instant and turn the green home into black scorched earth. Therefore, it is of great significance to take effective measures to prevent the burning accident of wind turbine.

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In the process of design, production, installation or maintenance of wind turbine units, some defects of the units themselves may lead to unit collapse and burning accidents. At the same time, accidents may also occur when there are problems in the design, production, installation or maintenance of its auxiliary equipment. The following is an analysis of several cases of wind turbine burning accidents caused by box transformer and connection failure from box transformer to frequency converter, so as to summarize the preventive measures for similar accidents. Wind turbine burning case

1、 The circuit breaker at the low voltage side of box transformer does not have automatic tripping function

In 2013, the grid connected switch of a doubly fed 1.5MW unit in a wind farm could not be normally disconnected from the power grid during "low wind cut-out", that is, the generator stator could not be separated from the power grid. During shutdown, the rotating speed of the impeller decreases continuously, the rotating magnetic field of the stator and the slip rate of the rotor increase continuously, and the induced electromotive potential generated on the generator rotor breaks down and shortens the IGBT and low-voltage through module. The huge heat also melts and burns the insulating skin of the rotor wiring of the frequency converter until the rotor wiring at the frequency converter is open (after the accident, it was further confirmed when checking the frequency converter).

When the rotor wiring of the generator is open circuit, the impedance on the stator of the generator is smaller, the current flowing through the stator is larger, the heating power consumption of the stator coil increases continuously, and the heating increases sharply. The operation data shows that the heating power consumption quickly reaches 1.8mw. Under the condition of load overcurrent of the box transformer, the circuit breaker at the low voltage side of the box transformer does not have automatic tripping function (as shown in Figure 1) It cannot be disconnected, so the stator temperature rises rapidly.

The large amount of heat generated by the generator stator burns the combustibles in the generator and its accessories, such as the lubricating oil in the generator bearing, the connecting cable, the lubricating oil pump on the generator, etc. 3 minutes after the unit executed the shutdown command, the unit tripped due to the fusing of the high voltage fuse at the 35kV side of the box transformer, and the unit was powered off. However, due to the power failure too late, all the units were burned out at last.

The failure of the circuit breaker on the low-voltage side of the box transformer matched with the unit to have tripping function is the key factor of this accident. As early as 2011, a unit of the wind farm was repaired, the frequency converter was burned due to the misoperation of the grid connected switch. The on-site personnel manually disconnected the circuit breaker on the low-voltage side of the box transformer in time, which avoided the further expansion of the accident and the occurrence of unit burning accident. Unfortunately , the circuit breaker at the low-voltage side of the box transformer did not have automatic tripping, which did not attract enough attention. This hidden danger was not eliminated until 2013, which eventually led to the unit burning accident.

2、 The wiring cable is short circuited, the circuit breaker at the low-voltage side of the box transformer cannot trip normally, and the high-voltage side cannot fuse normally

In 2015, a 1.5MW doubly fed unit of a wind farm with frequency converter arranged at the tower base. After 4 years of operation, the unit appeared "transient power grid" alarm during operation, and then reported "frequency converter fault" after 1 second Shutdown. After the accident, during the investigation, it was found that during the incident, the wiring cable from the box transformer to the frequency converter was short circuited, resulting in serious arcing and carbonization, as shown in Figure 2. The high and low voltage sides of the box transformer could not be powered off normally due to faults, resulting in the rapid expansion and spread of the fire, resulting in the burning of the lighting, communication and control of the tower foundation and the power supply cables of the engine room and tower, and all the way down The upper combustion to the engine room eventually led to the unit burning accident.

During the installation of the unit, the wiring cable from the emergency unit box transformer to the frequency converter adopts a main cable, with three wiring cables inside. The cable has large diameter, high strength and difficult wiring. Therefore, during the wiring, the phases of the cables cross each other. When the cable passes through the wiring port of the frequency converter platform, it is not constructed according to the specifications. The cable insulation layer is squeezed with the steel frame of the frequency converter platform Seriously, after the unit is put into operation, it is caused by personnel walking on the frequency converter platform or the operating parts on the frequency converter when the unit is running (e.g. fan) The cable insulation layer was gradually rubbed, scratched and damaged due to the slight movement of the platform. During the unit maintenance, the incoming cable of box transformer was not inspected, resulting in the damage of cable insulation layer, short circuit to ground and serious arc ignition.

According to the design requirements, the electrical protection between the frequency converter and the box transformer is realized by the circuit breaker at the low-voltage side of the box transformer. However, it cannot be disconnected due to failure in the event of an accident; the striker type high-voltage fuse of the box transformer is reversed when the unit is installed. Therefore, when the high-voltage fuse is melted, its trigger device cannot make the release of the high-voltage circuit breaker act, resulting in instability at the high-voltage side of the box transformer The circuit breaker at the low voltage side and the fuse at the high voltage side of the box transformer are shown in Figure 3 and Figure 4. Due to the serious short circuit of the accident unit, the accident unit causes the 35kV circuit oscillation, the main transformer line protection is opened, and the accident unit is also powered off.

The lighting, communication, control and power supply cables under the inverter platform burn due to long-term short circuit to the ground. They burn to the engine room after more than 5 hours from the tower foundation until the whole unit is burned.

3、 Accidents caused by box transformer failure

On February 26, 2010, when the unit of a wind farm in Inner Mongolia was commissioned to reverse power transmission, due to the failure of the circuit breaker at the low-voltage side of the box transformer, the staff tried to repair the box transformer. Due to the failure of the box transformer, the 35kV high voltage rushed to the low-voltage side. The staff misoperated when using the screwdriver and touched the high voltage, resulting in the amputation of the operator by electric shock. In 2008, when the unit was installed in a wind farm, the safety was improved When the installer tightened the connecting screw of the connecting cable between the frequency converter and the box transformer in the tower, the installer was injured by electric shock. At this time, although the high-voltage side of the box transformer has been powered on, there should be no power in the tower because the circuit breaker at the low-voltage side of the box transformer has not been closed. After the accident, it did not attract enough attention and the cause was not checked. It was generated during the static commissioning of the unit Now the inverter grid connection cabinet of the unit has been completely burned. After inspection, it was found that the accident was caused by the internal wiring error of the box transformer.

Cause analysis of major accident of unit

The collapse and burning accidents of wind turbines are related to the design, production, installation and maintenance quality of ancillary facilities of wind turbines, as well as the management of wind power enterprises and the technical level and sense of responsibility of on-site personnel in operation and maintenance.

1、 Failure to give full play to the protective effect of box transformer on the unit for some reasons:

The box transformer in the wind farm is not only to provide operating conditions for the unit and transmit the power generated by the unit to the power grid; at the same time, when the load of the box transformer (wind turbine unit) has overcurrent and short circuit, it will trip and power off through the circuit breaker at the low-voltage side of the box transformer, or the fuse and circuit breaker at the high-voltage side will trip and power off, so as to protect the equipment and personal safety.

It can be seen from the previous case analysis that the circuit breaker at the low-voltage side of the box transformer does not have automatic tripping function, or the circuit breaker at the low-voltage side cannot trip in time due to fault, and the high-voltage side cannot fuse and open normally, the unit will be burned under certain conditions.

For the fire accident caused by electric fire, first cut off the power supply to avoid the further expansion of the accident. The design concept of wind turbine is to operate independently without supervision. In case of overcurrent and short-circuit fault of the unit, the box transformer shall automatically cut off the power according to the established protection setting. Therefore, the box transformer plays an important role in preventing the unit burning accident and preventing the expansion of the accident For example, the box transformer is located at the tower base, the frequency converter is arranged on the engine room, and the wiring cable from the box transformer to the frequency converter is longer. When the possibility of cable damage, short circuit and arcing is greater, the automatic overcurrent and power-off protection of the box transformer will become more important.

2、 The quality problems of production, installation and maintenance of box transformer are not easy to find and easy to ignore:

During the normal operation of the unit, on the one hand, it is difficult for people to find the production and installation quality problems and potential safety hazards of the box transformer, for example, the circuit breaker at the low-voltage side of the box transformer cannot trip automatically when overcurrent occurs, and the high-voltage side cannot fuse or break normally, which cannot be found without special test; on the other hand, people generally pay more attention to the faults of the unit itself, but pay less attention to the box transformer Less. During equipment maintenance, many wind farms will not carry out professional inspection and maintenance on the box transformer. The maintenance and inspection of the connecting cable from the box transformer to the frequency converter is even blank.

In order to reduce the cost, if the wind farm does not carefully investigate the quality of circuit breakers and key components, and only purchases the box transformer in the form of winning the bid at a low price, the final result may not be worth the loss. From the practice of the wind farm, the grid connected switch of the frequency converter cannot be disconnected during the operation of the unit, and the wiring cable between the box transformer and the frequency converter has a serious short circuit to the ground , the probability of occurrence is very low. However, judging from the above-mentioned cases of wind turbine unit burning, there are many problems in the box transformer of the wind farm. It is also confirmed from the inspection of some box transformers of the wind farm that the high-voltage side of the box transformer can not be fused or disconnected normally, and the over-current of the circuit breaker at the low-voltage side can not be tripped automatically. Some automatic tripping mechanisms at the low-voltage side of the box transformer are not wired, and the high-voltage side can not be tripped automatically Faults such as no installation and reverse installation of voltage side fuse often occur. Some wind farm box transformers violate the power design code in line design.

Therefore, once the box transformer protection action is required and the power supply is cut off, the failure probability of the box transformer is very high, resulting in the expansion of failure and accident loss, and even the occurrence of unit burning accident.

Preventive measures for box transformer:

The wind turbine burning accident will cause huge losses. If the root cause of the accident is found out and effective measures are taken after the accident, good results can be achieved to avoid the recurrence of the unit burning accident. The specific preventive measures are as follows:

When purchasing and installing box transformer in wind farm, the function and quality of box transformer shall be guaranteed. Ensure that the circuit breaker at low-voltage side of box transformer has due protection function, the selection and matching of fuse at high-voltage side of box transformer, the correct installation mode of fuse, etc.; ensure the cable quality and installation quality of connecting cable from box transformer to frequency converter, and carry out necessary insulation protection; the neutral wire at low-voltage side of box transformer shall be grounded as required, Perfect the zero sequence protection at the low-voltage side; reasonably set the parameters of the box transformer. The overcurrent setting value of the box transformer circuit breaker shall be equal to the rated power value of the unit. If it is too large, it is not conducive to the protection equipment; if it is too small, it is not conducive to the operation of the unit.

Do a good job in regular maintenance of box transformer, such as parameter setting inspection and automatic opening function test of circuit breaker at low voltage side; reliable installation of fuse at high voltage side and inspection of tripping mechanism.

epilogue

When preventing the above fire accidents, we must do a good job in the quality control and related maintenance of the box transformer; pay attention to every detail during the daily unit operation and maintenance; eliminate all possible potential safety hazards; grasp the key points and some losses, so as to reduce the long-term power consumption cost of the unit.