1 introduction
Under certain applied voltage, local and repeated breakdown and extinction phenomena may occur inside the transformer body.
The partial discharge occurring in one or a few small space, can discharge the Tuo is small, the existence of partial discharge does not affect the short time of transformer insulation strength = but under running voltage as a transformer partial discharge electricity phenomenon is found in the insulation, the weak discharge and the resulting some adverse effect, can damage the insulation slowly, and finally lead to the whole insulation breakdown the harmfulness of this phenomenon has been gradually known by people.
In order to improve the reliability of transformer operation, part 3 of the gb1094.3-2003 power transformer, which was implemented on January 1, 2004, "insulated water level, insulated test and external insulated air gap" stipulated = 72.5kv and transformers with rated capacity of 10 0o0kVA and above and U > 72.5kv should be measured by partial discharge if there is no other agreement.
Through partial discharge measurement, it can verify whether the design structure of transformer is reasonable, whether the process level and production environment meet the requirements, whether the transformer has defects and so on.
2. Cause analysis of excessive partial discharge under high voltage
The high voltage bushing of a 110kV power transformer is imported from Germany.
The structure of plug - in cable terminal is different from that of oil-paper capacitive bushing, and the connection mode of the outlet line of the body is different.
Before the trial production of two SZIO 1 40,000/1 10 transformers, due to lack of experience, the partial discharge in the high-voltage short-time induction withstand voltage test (ACSD) failed to meet the requirements below l00pC stipulated in the contract, and the extinguishing voltage was 58kV, while the other tests all passed at one time.
In recent years, our company has designed and manufactured nearly 100 1l0kV oil-paper capacition-type casing power transformers, and each transformer has its high voltage partial discharge below l00pC. Therefore, it is necessary to analyze the design structure and manufacturing process of the transformer high voltage outlet, find out the causes of excessive high voltage partial discharge, and solve them:
2.1 Reasons for excessive PARTIAL discharge under high voltage To find out the reasons for excessive partial discharge under high voltage, it is necessary to determine the possible location of the partial discharge under high voltage.
We first adopt ultrasonic positioning method to detect, but did not signal collected, capacitive bushing instead of plug is used after the paper type cable terminal test, partial discharge are two products high pressure under l00pC, can rule out the problem of body and then to plug type cable terminal block and matching test cable individually test, partial discharge in about 5 PCS, can be ignored:
Partial discharge through the above test can determine high pressure exceeds the problems occurred during the installation plug type cable terminal block of elevated bridge inside we open up, check the internal structure, found no trace discharge, and combining with the design drawings and test of partial discharge, after analysis are mainly caused by the following reasons.
2.1.1 The lead connector connected to the plug-and-plug cable terminal seat has a pressure equaling ball at the end of the tip discharge oil-paper capacitance bushing. After the lead cable is wrapped into a certain taper, it penetrates into the sleeve tube. The surface of the pressure equating ball is smooth, without sharp Angle, and the electric field is uniform, which generally does not cause partial discharge.
The end of the plug seat is a cylindrical metal conductor, which is connected with the high-voltage lead connection by bolts. The connection connection is made of copper material, and each edge is not rounded or has a small radius of chamfering.
The head of the hexagon head bolt connected with the socket is exposed outside the connector.
The bolt head has flanged edges and pointed corners, and the electric field intensity is concentrated at the flanged edges and pointed corners of the wiring head and bolt head.
When the electric field intensity reaches a certain value, the transformer oil on its surface becomes dissociated, generating polar charge and partial discharge.
2.1.2 The size of transformer lift seat and cable box is too small, the cross-section of lead wire lift seat and cable box along the surface of wire clip is square, the length and width of inner wall are too small, and each side of high voltage lead wire is covered with 20RAM thick insulation.
Lead to cable box clamp plate along the surface of creepage distance is able to meet the requirements of power frequency voltage and lightning impulse test, but there were two cable box wire clamp clamping lead, its composed of insulating board hot-pressing, vulnerable to pollution in the process of processing and storage, the dielectric strength of these areas are much lower than the normal state, under the effect of strong electric field, the formation of creeping discharge, increasing power bureau department.
The shape of the ground part directly affects the insulation distance.
The cross section of the lifting seat and cable box is square, with many edges and sharp corners, which is difficult to be eradicated. It belongs to the extremely uneven electric field from the point to the point.
When the electric field intensity is concentrated to a certain degree, it is easy to cause partial discharge.
2.1.3 Vacuum immersion oil for high pressure phase incomplete transformer adopts traditional ding technique. When the oil level is 200mm ~ 300mm from the top of the oil tank, the oil filling stops.
Because the high voltage lead is higher than the top of the oil tank, and the lead has thicker outer insulation and harder cable, it cannot be bent and placed below the box cover 200mm ~ 300mm, so a part of the high voltage lead cannot be vacuumized with oil, and the wire clip in the elevated seat is also stored in the same problem.
Thus, there may be gas (generally air) in the lead insulation and wire clip in the hV cable box. Therefore, the electric field intensity of the gas inside the insulation exceeds its allowed electric field intensity, causing gas discharge.
3 Solutions
Based on the above analysis, we have made major changes in the structure.
3.1 Remake the high-voltage connection head, change the connection bolt and remake the high-voltage connection head. All the edges are processed into R5 rounded corners, and the surface is polished smoothly, and no sharp corner burrs are allowed.
The wiring head and the socket are connected with hexangular cylinder head screw. The screw head sinks into the wiring head slot and the wiring head is used to shield the tip of the screw head to prevent the discharge of the tip.
The edge of the contact disk of the plug socket is smaller than the contact disk of the plug socket, and the contact disk of the connector socket and the plug socket is slightly larger than the conductive disk of the plug socket, so that the contact disk of the plug socket can shield the sharp corner and the edge of the conductive disk of the plug socket.
3_2 Improved high voltage lift seat and cable box
The section of the high voltage lift seat and cable box is round and the diameter is enlarged. The cable box is overarched at right angles. The inside of the lift seat and cable box is polished smoothly and no sharp burr is allowed.
After the hole is opened at the welding place between the raised seat and the oil tank, the hole circumference shall be ground smooth and the flute edge shall be ground into the rounded corner of R5.
3.3 Improve the structure of the wire clip in the high voltage cable box
Under the condition that the lead is firmly clamped, the number of wire clamps should be reduced as far as possible to reduce the discharge path.
We adopted the clamping structure as shown in FIG. 2, which not only ensured the clamping strength, but also reduced the number of wire clamping plating.
3.4 Special vacuum immersion process for transformers We added a transition tank between the separation tank and the transformer on the basis of traditional vacuum immersion.
When the transformer is filled with oil, the transformer oil will not enter the separation tank. When all the insulation parts in the high-voltage cable box are immersed in oil, stop the oil filling. The operation process of other vacuum oil immersion processes remains unchanged.
This ensures that the insulation in the cable box and the wire clamp are completely vacuumized.
3.5 Pay attention to the transformer cleaning during the repair process, we shall pay attention to keep the transformer clean and cover the transformer body with plastic cloth to prevent dust and other foreign objects from polluting the transformer body in the production environment.
Before the transformer is hooked up to the oil tank, flush the body with qualified transformer oil to ensure the body is clean.
In order to test the feasibility of the above measures, a product was first put into a spallage and treated according to the above measures. The partial discharge measured in the test dropped below 100pC, which preliminarily proved the correctness of the solution.
So the other one and the rest of the force of I1 transformer in strict accordance with the above method for restructuring and production, local discharge of electricity are qualified.
It can be seen that the improvement measures adopted by J 2 to the local excessive discharge of such transformer have achieved the expected effect.
4 conclusion
(1) The design structure and process of the transformer directly affect the size of the partial discharge of the transformer. If there is a concentration of local electric field intensity in the transformer, especially sharp Angle and burr on the conductive body, and the electric field intensity reaches a certain value, the transformer oil on its surface will be separated and the partial discharge will be generated.
(2) if the insulation or in gas insulated parts, under the effect of a certain voltage, the insulation of internal gas field strength more than the allowed under the electric field strength in the transformer partial discharge standard (3) for electrode "cutting-edge" caused by partial discharge exceeds bid, can use fillet or the way such as shielding, increases the radius of the pointed end electrode, in order to reduce the electric field intensity on the surface of the electrode and reduce the partial discharge.
(4) In order to ensure that all insulating parts are immersed in transformer oil and avoid gas discharge in insulating materials, leading to partial discharge failure, an appropriate method of vacuum oil immersion must be adopted.