10 kv Oil Filled Transformer
Fault Diagnosis and Analysis of 10 kv Oil Filled Transformer 10 kv Oil filled transformer
Short-circuit faults are common in wind turbine step-up transformers and are one of the main causes of threats to the safe operation of wind farms. The sources of short-circuit faults in wind turbine step-up transformer are summarized, mainly short-circuit in low voltage chamber, short-circuit in wind turbine system and short-circuit in transformer, and the causes of short-circuit faults are analyzed, and corresponding preventive and improvement measures are proposed according to different short-circuit faults.
As an important part of wind power generation, wind turbine step-up transformer is also widely used.
The harsh operating environment of wind turbine farms, the complexity of wind turbine systems, the large number of components, and the complicated operating conditions cause many faults, such as large load fluctuation range, circuit breaker rejection, generator and inverter failure.
These reasons lead to the wind turbine step-up transformer short-circuit fault probability is significantly higher than the ordinary civil Pad mounted transformer, sometimes burned accident, affecting the safe operation of wind power plants.
In this paper, combined with the long-term experience of wind turbine step-up transformer design and repair, we systematically summarize the sources and causes of short-circuit failure of wind turbine step-up transformer, and give corresponding preventive measures to improve the operational reliability of wind turbine step-up transformer.
Wind turbine step-up transformer short-circuit fault causes, according to the different sources can be divided into low-voltage room short-circuit, wind turbine system short-circuit, transformer short-circuit.
The low-voltage side of wind turbine step-up transformer is simpler than that of civil pad mounted transformer, and the main circuit only has main circuit breaker, no feeder cabinet and capacitor cabinet.
Some manufacturers lack experience in the design and production of such low-voltage cabinets at the early stage, and design wind turbine step-up transformer according to the specifications of low-voltage cabinets for civil distribution. The main reasons for short-circuit fault are as follows.
Wind farms are mostly located in mountainous areas, Gobi beaches, grasslands and other open areas, wind turbines and blades suffer high probability of lightning strikes.
Although the fan position is much higher than Pad mounted transformer, Pad mounted transformer will not be subject to direct lightning, but Pad mounted transformer is closer to the lightning strike point, lightning current from the fan through the cable transmission to Pad mounted transformer, the lightning current in the low-voltage line can reach 5 ~ 10kA.
In addition, the wind turbine needs to cut the circuit breaker according to the wind speed, the current is suddenly cut off, the motor windings and other inductive components in time to release the energy, which raises the line voltage.
The probability of overvoltage on the low-voltage side of wind turbine step-up transformer is significantly higher than that of civil distribution Pad mounted transformer, so it is easy to damage electrical components due to induction lightning and potential rise.
If the electrical distance is too small, it is easy to cause phase to phase discharge, insulation breakdown to ground and other faults.
A wind turbine step-up transformer low-voltage copper row is too small due to processing deviation, and the copper row discharges and burns the low-voltage cabinet under overvoltage, as shown in Figure 1.
Pad mounted transformer low-voltage side and the connection cable between the fan belongs to the fast channel of lightning current, in order to make the Pad mounted transformer from lightning and system overvoltage damage, the low-voltage side of the Pad mounted transformer needs to be configured with lightning protection components, usually surge protector (SPD).
However, some design units and Pad mounted transformer manufacturers lack experience in the selection of SPD and the matching protection fuse, and the component selection is unreasonable, resulting in the lightning protection components and matching fuse instead of becoming one of the sources of failure.
SPD tripping device can not cut off the lightning current and frequency continuity, losing the role of protection of the low-voltage side of the Pad mounted transformer, resulting in a short circuit.
If the SPD upper end of the open fuse holder, the arc will spread to the upper end of the fuse, the fuse will lose its protective role, resulting in low-voltage cabinet burn.
(3) Condensation and temperature rise exceeds the standard. Some areas are prone to sandstorms and blizzards, which are extremely harmful to the insulation of high and low voltage chambers. According to the actual situation, the operation and maintenance units of wind power farms require upgrading the protection level of wind turbine step-up transformer, eliminating louvers and top cover heat dissipation holes, and adopting a fully sealed system for the silo body.
In recent years, wind power generation has continued to develop in some mountainous areas, and the design unit still uses the fully sealed structure of the wind turbine step-up transformer in some mountainous areas.
However, the temperature and air humidity in the south are significantly higher than those in the north, and the use of a fully sealed box is not conducive to heat discharge and ventilation of the box, which will easily lead to the temperature rise of the Pad mounted transformer, and the temperature difference between the box and the air inside the box is so great that even if the relative humidity of the air inside the box is very low (less than 50% RH), condensation will occur on the inner wall of the box.
Wind turbine step-up transformer low-voltage circuit lack of thermal relay protection and insulation monitoring, copper row temperature rise exceeds the standard will trigger thermal breakdown of insulation, serious condensation leads to insulation capacity decline in short circuit.
Wind turbines are affected by wind speed is volatile and fluctuating, and the prediction is difficult, the impact on the fan system.
In addition, the fan unit working environment is harsh, windstorm, sand, dust, freezing, high temperature and other factors make the operational reliability and service life of the unit has been seriously tested, the impact on the Pad mounted transformer can not be ignored.
Some Pad mounted transformer low voltage circuit breakers fail to trip due to unreasonable setting of the rectification value, and the high voltage fuse does not operate due to the mismatch of action curve and other reasons.
Pad mounted transformer high and low voltage protection are out of action, the short circuit fault of the fan system is not controlled, resulting in serious accidents of Pad mounted transformer and fan burning. The main short-circuit faults are as follows.
In the fan system, the converter changes the AC power with fluctuating frequency into AC power that meets the requirements of the grid, for which components such as IG-BT, DC capacitor, AC capacitor and filter reactor need to be configured. Under the action of overvoltage and local high temperature, there is breakdown of these components and the current is rapidly raised in a short time, burning the components and causing short circuit.
According to the wind power and the demand of the grid, the wind turbine unit can start, regulate, stop and turn on and off the load through the control device in time. Switching components are frequently switched on and off, bringing a high failure rate, such as contactor sticking, circuit breaker refusing to move, arcing short circuit, etc., causing line short circuit in serious cases.
Fan due to its complex operating environment, such as fan vibration, aging insulation materials and poor heat dissipation, the probability of short circuit greatly increased. The stator winding of the permanent magnet direct-drive fan is exposed to the air and vulnerable to salt spray, moisture and mold, with the risk of inter-turn short circuit.
The rotor windings of doubly-fed asynchronous fans need to output electrical energy through collector rings and carbon brushes. As high-speed sliding contact parts, improper maintenance causes collector ring fires, and serious fires will lead to short circuits of collector rings and carbon brushes.
The wind turbine is connected to the power grid through the transformer, and the electrical energy quality has the characteristics of power fluctuation, voltage flicker, high dv/dt value, etc. Meanwhile, the connection cable between the wind turbine and transformer is mostly in the form of direct burial, which has the hidden danger of cable short circuit and affects the safe operation of the transformer.
In addition, the combined Pad mounted transformer has load switch and high voltage fuse inside the tank, which will increase the failure rate of the transformer with additional components.
Transformer is the core component of wind turbine step-up transformer, once short-circuit fault occurs, it will bring serious damage, the common causes of short-circuit fault are as follows.
Pad mounted transformer low-voltage side and wind turbine system there are more short-circuit fault, the transformer needs to have the ability to resist short-circuit.
At present, the wind turbine transformer high-voltage coil more than the use of two cylindrical coil structure, when the wind turbine step-up transformer external short circuit, under the action of the spoke short circuit force, low-voltage winding to the core compression, high-voltage winding outward expansion, the main airway gap, high-voltage inner coil lost the inner wall support, easy to be disrupted by the axial short circuit turns arrangement, when the high-voltage winding internal short circuit is inevitable. Multiple accident anatomy, also indicates that the high-voltage inner layer belongs to the weak point of short circuit.
Influenced by the unstable wind speed, the fan unit operation produces voltage fluctuations and flicker, frequency overvoltage amplitude up to 1.1 times the rated voltage.
Wind turbine converters use IGBTs as switching elements. The fast switching frequency brings a high voltage rise rate (dv/dt).
When the rate of change of the shock voltage wave (equivalent to a high frequency) projected to the transformer winding, the longitudinal insulation caused by adverse effects, such as corona discharge, local discharge exceeds the standard.
Wind power combined Pad mounted transformer using plug-in full range fuse, with two functions of cutting off the overload current and short-circuit current.
But the fuse is temperature-sensitive components, the action curve is affected by the ambient temperature, usually fuse placed in the upper position of the oil tank.
Transformer oil temperature can be as low as the freezing point in winter when no-load operation, and in summer when full-load operation, the temperature inside the round dry cylinder of the high-voltage current-limiting fuse will reach 100 ℃.
For this reason, the fuse often misfuse, refusing to fuse and other failures, and even cause the fuse to blow and transformer internal short circuit.
The causes of short-circuit faults in wind turbine step-up transformer are different from those in power transformer and civil distribution Pad mounted transformer. Suitable preventive measures need to be formulated according to the actual situation, both to improve the short-circuit resistance strength of the product itself and to reduce the probability of external short-circuit faults by improving the structure and reasonable layout and reducing the risk factors.
The following short-circuit fault sources, according to the corresponding preventive measures.
The main reason for the short circuit fault in the low-voltage room is the low level of structural insulation and the wrong choice of components and protection level, so the following preventive measures will help to reduce the short circuit in the low-voltage room.
The inter-phase and ground distances of the copper row need to be higher than the national standard, the copper row set of high-quality heat-shrinkable tube, sharp corners and bolts exposed wrapped insulation protection, effectively blocking the generation of electric arc.
Control lines need to be far away from the copper row, circuit breaker heat sink hole, to avoid high equipment temperature leads to the failure of the control circuit to expand the fault.
Conventional civil power distribution components can not meet the requirements of wind power generation environment, need to use the wind power generation special components developed for wind power generation working conditions, Pad mounted transformer engineers should carefully select components, and strengthen communication and exchange with component engineers.
Reasonable arrangement of components, isolation of failure-prone components, such as surge protectors and fuses, to avoid the failure of such components cause a short circuit in the copper row and control lines.
Southern temperature and high air humidity, should not choose a fully sealed shell, the need to set up louvers and fans to strengthen the ventilation and cooling capacity.
Northern regions more blizzards and sand invasion, it is appropriate to choose a fully sealed shell, such as low-voltage room temperature rise can not meet the requirements, the need to increase the self-hanging louvers and fans.
Wind turbine system short circuit for wind turbine step-up transformer is an external fault, first of all, operation and maintenance units should strengthen inspection and management, short-circuit hidden trouble spots in advance; secondly, Pad mounted transformer should be able to cut off the fault line in time to eliminate the impact of external short circuit on Pad mounted transformer.
Since the wind turbine step-up transformer low-voltage frame circuit breaker instantaneous action setting value is not unified norms, some wind farms Pad mounted transformer along with the distribution Pad mounted transformer setting value, resulting in the setting value is too large, the circuit breaker can not be tripped in time.
The current wind turbine adopts non-shock grid connection, the power rise curve is gentle, and the box transformer is no-load closing from the high voltage side, so the wind turbine step-up transformer does not need to consider the low voltage side closing inrush current, grid connection inrush current and motor starting current.
Pad mounted transformer low-voltage side of the high current will only come from the short-circuit current and low-voltage crossing current, subject to the converter’s over-current capability and low penetration control strategy, usually the low penetration current of permanent magnet synchronous wind turbine does not exceed 2.0 p.u., and the low penetration current of doubly-fed asynchronous wind turbine is between 3.0 and 5.0 p.u..
For this reason, the instantaneous setting value of the wind power box transformer low-voltage frame disconnector can be set to 5.0p.u., so that the external short-circuit fault can be removed in time to prevent the accident from spreading and causing a larger-scale short circuit.
Usually the external short circuit of the transformer is an indirect trigger, and the weak short circuit resistance of the transformer itself is the main reason for transformer burnout. While preventing the external short circuit, the transformer body must be strengthened to resist short circuit.
(1) Improve the mechanical strength of the winding.
When a short circuit occurs, the inner side and end of the high-voltage winding is the most stressful, easy to produce deformation, so the coil should choose a circular structure, this structure has good yield resistance characteristics.
In the inner side of the high-voltage laminated winding, the stability of the axially stacked enameled round wire is poor, so the coil should use flat wire, which can improve the stability of the inner side of the winding.
In addition, increase the hardness of the wire, the reasonable choice of wire winding tension, layer insulation between the use of heat-curing dotted paper can enhance the mechanical strength of the winding.
(2) Ensure the safety-turn balance.
Wind power transformer high-voltage winding mostly adopts two-layer coil structure, between the two windings and the end of the winding, the high and low voltage turns balance is poor, the magnetic force line is distorted, the axial short circuit force is too large, the design should control the high and low voltage coil reactance height difference to ensure the safety turns balance.
In addition, low-voltage coils mostly use copper foil, high-voltage coils have end insulation and turn insulation, after the body dries, copper foil does not shrink, high-voltage end insulation and turn insulation will shrink, causing the center point of high-voltage and low-voltage coils to shift, the design should consider the shrinkage of end insulation and turn insulation and give the corresponding margin.
(3) Optimize the lead structure.
Pad mounted transformer tank internal space is small, many components, lead structure affects the safe operation of the transformer.
Reasonable component layout is crucial to the quality of the leads, and complex structures should be avoided to ensure the insulation strength and electrical distance of the leads.
From the high-voltage casing into the device body of the connecting line, should get clamped, the overall easy to operate and check; low-voltage leads should have sufficient mechanical strength, clamped solid and reliable, to avoid the copper row in the short circuit electric force under the action of the copper row between or copper row to ground short circuit.
(4) improve the layout of the fuse.
In the combined Pad mounted transformer, the plug-in full range fuse is placed inside the tank, which has certain defects.
With the different seasons and Pad mounted transformer load, the oil temperature changes significantly, resulting in fuse misfuse or refusal to fuse;.
Secondly, the failure of the fuse itself can cause the body to burn; in addition, the fuse increases the complexity of the connection inside the box, and the fuse is prone to discharge failure between the fuse and the lead, the clamp and the oil tank.
Replace the structure of oil-immersed load switch plus inserted fuse with vacuum load switch plus ordinary current limiting fuse, placed in the high voltage chamber of Pad mounted transformer, the temperature change inside the chamber is less than the oil temperature, which can reduce the probability of fuse misfuse and rejection.
The fuse explosion and discharge will not cause short circuit in the body of the device, which can eliminate the phenomenon of fuse leakage, and can greatly simplify the complexity of the high voltage leads inside the tank, reduce the space inside the tank, and reduce the amount of oil used.
Due to the harsh operating environment and complex working conditions of wind power plants, the failure rate of wind turbine step-up transformer is significantly higher than that of civil distribution Pad mounted transformer, and various short-circuit faults seriously affect the safe operation of wind power plants.
Each link of the wind power system is connected to each other and affects each other. Pad mounted transformer designers need to understand the operating characteristics of the whole system, strengthen the communication with each link, and improve the short-circuit resistance of the pad mounted transformer at the same time, they also need to cooperate with the upstream and downstream manufacturers of the system to jointly reduce the sources of short-circuit faults and reduce the probability of occurrence.
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