Transformer iron core correct grounding method and multi-point grounding fault diagnosis method
The transformer is the core equipment in the power system and plays a key role in the power supply and distribution enterprise. Once the transformer fails, it will greatly affect the safe and stable operation of the system. This article will be a common fault of the transformer - the multi-point grounding of the iron core. Speaking in detail from the three aspects of diagnosis, analysis, and elimination, the reader will eventually learn a little about transformer knowledge. High quality-- our nozzle tips are constructed from high quality stainless steel and brass. Pressure Washer Nozzle,Car Washing Nozzles,Metal Jet Lance Nozzle,High Pressure Washer Spray Nozzle Ningbo Senmiao Trading Co., Ltd. , https://www.senmiaowasher.com
Transformer core multi-point ground fault will directly lead to overheating of the transformer core, burning the coil. Because the iron core is grounded at multiple points, a circulation will be formed at these points, resulting in local overheating. The long-term operation will cause the iron core to heat, so the transformer can only be grounded at one point. The multi-point grounding of the iron core is divided into two types, one is multi-point grounding of core instability, and the other is multi-point grounding of core stability. It is necessary to study the multipoint grounding of the transformer core.
From the defects of the Harbin Electric Power Bureau's internal faults and discoveries in recent years, basically there are two aspects: First, the DC resistance of the transformer coil is unqualified, and the transformer core is grounded at multiple points.
So many hidden dangers bring unsafe factors to the stable operation of the system, which requires us to carry out a comprehensive inspection, diagnosis, discovery and timely handling of this failure.
Correctly grounding the transformer core In the normal operation of the transformer, the electric field formed between the charged winding and the lead wire and the fuel tank is an inhomogeneous electric field, and the iron core and other metal components are in the electric field. There is a parasitic capacitance between the high voltage winding and the low voltage winding, between the low voltage winding and the iron core, and between the iron core and the earth (transformer fuel tank). The charged winding will cause the core to generate a certain potential through the coupling action of the parasitic capacitance. It is usually called floating potential. Due to the different locations of the iron core and other metal components, the floating potential is also different. When the potential difference between the two points reaches the insulation capable of breaking through, the spark discharge occurs. This discharge is intermittent, the two points after the discharge of the same potential; but the discharge immediately stopped, and then generate a potential difference, and then discharge ... intermittent discharge of the results of the transformer oil decomposition, long-term, and gradually make the transformer solid insulation damage, It is obviously not allowed to cause accidents. To avoid this from happening, the national standard stipulates that transformer cores and larger metal parts should be reliably grounded through fuel tanks. For transformers 20000 kVA and above, their cores should be led from the upper part of the fuel tank through a bushing and be reliably grounded. The specific approach is to reliably connect the transformer core to the main grid system of the substation. In this way, the parasitic capacitance between the iron core and the ground is shorted so that the iron core is at a zero potential. At this time, only the parasitic capacitance current of the live winding to the iron core flows through the ground wire. For a three-phase transformer, since the three-phase structure is basically symmetric and the three-phase voltage is symmetrical, the sum of the three-phase winding's capacitive current to the iron core is almost equal to zero.
At present, the method of grounding a copper sheet between iron core silicon steel sheets is widely used. Although there is an insulating film between each piece, the entire core is still considered to be grounded. The resistance value of the two ends of the core can be measured. This resistance is generally very small, only a few ohms to several tens of ohms. It can be regarded as a path in the high-voltage electric field, so the core needs only one point of grounding.
Usually, in order to ensure that the core is grounded, the transformer has four core grounding methods as follows:
1 When there is a tie bar or pull plate between the upper and lower clamps and it is not insulated, the grounding copper piece is connected to the upper clamp piece, and then the upper clamp piece is grounded via the hanging core screw;
2 When the upper and lower clamps are not insulated, the grounding copper is grounded from the lower clamp via the grounding screw;
3 When the upper and lower clamps are insulated, insert a grounding copper plate to connect the clamps at the symmetry positions of the upper and lower iron yokes. From the upper clamps to the lower clamps and then to the grounding, the purpose of symmetrically locating the grounding plate is to Avoid grounding the core at two points;
4 When using a grounding sleeve, the core is connected to the grounding sleeve via the grounding plate to the upper clamp.
Types of Core Failure There are several types of core failures:
1 Iron core touches the shell and touches the clip. After the installation and overhauling are completed, due to negligence of the staff, the studs (positioning) used for transportation on the top lid of the fuel tank are not turned over or removed, causing the iron core to collide with the tank shell; the iron core paper sheet touches the core Column; iron core under clips and iron yoke between the cardboard off; thermometer seat cover too long and the clip or iron yoke, the core column collided.
2 threaded core steel seat cover too long and silicon steel short.
3 There are metal foreign objects in the tank, making the silicon steel sheet partially short-circuit.
4 The core insulation is damp or damaged. Sludge and moisture are deposited at the bottom of the box. The insulation resistance is reduced, and the insulation of the clips is damp or damaged. As a result, the iron core is grounded at multiple points.
5 The bearing of the submersible pump is worn out. The metal powder enters the oil tank and accumulates at the bottom. Under the action of electromagnetic attraction, the bridge is formed, and the lower iron yoke is connected with the stepping feet or the bottom of the box to cause multi-point grounding.
Poor operation and maintenance, no overhaul or overhaul.
Diagnosis methods for multi-point earthing faults of transformer cores There are two general methods for diagnosing multi-point earth faults of transformer cores: gas chromatographic analysis and electrical methods. They are respectively elaborated:
Gas chromatographic analysis This method is currently the most effective method for diagnosing multi-point grounding of large power transformer cores. The results of chromatographic analysis of dissolved gases in transformer oils in which many multi-point ground faults occur in the iron core indicate that the chromatographic results of transformers usually have the following characteristics:
1. The total hydrocarbon content is high and often exceeds the attention value (150ppm) stipulated in the “Preventive Test Regulations for Electric Power Equipment†DLT/596-1996. The composition of its constituent gas content decreases in the order of C2H4-CH4-C2H6-C2H2 even if it is oil The gas content of the characteristic gas in the medium does not reach the attention value, and it also follows the above decreasing law.
2. C2H4 is the main characteristic gas of iron core multi-point grounding fault. If we calculate the ratio of hydrocarbon constituents to total hydrocarbons (C1+C2), we can obtain the ratio of hydrocarbons to total hydrocarbons when the transformer core is grounded at multiple points. The ratio of C2H4 is the highest.
3. The rate of production of total hydrocarbons often exceeds the value specified in the DLT/596-1996 Regulations for Electrical Equipment Preventive Tests (sealed 0.5 mL/h), in which the production rate of ethylene is in a sharp upward trend.
4. When using the IEC three-ratio method, the characteristic gas ratio code is generally 0, 2, and 2.
5. The estimated fault point temperature is generally higher than 700°C and lower than 1000°C.
If the chromatographic analysis shows the above characteristics, and tries to prove that the bare metal is not overheated due to poor contact of the tap changer and the failure of the submersible pump, at the same time, if the measured core insulation resistance is zero or significantly lower than before the operation, it can basically Judgment for the occurrence of transformer core multi-point grounding fault. Of course, the gas chromatograph analysis can make the main transformer without power outage, and regular oil sample analysis can be used.
Electrical method 1 Electrically charged test and analysis: Use a clamp ammeter to measure the ground return current of the iron core. If the power transformer is in operation, use a clamp ammeter to measure whether the lead is located on the ground lead of the outer core of the transformer core. There is current. "Detection test for power equipment" DLT/596-1996 stipulates that the grounding current of the transformer core during operation is not more than 0.1A. If the iron core is grounded at multiple points and the circulating current is large, the current flowing through the core ground wire may be obviously. Increased, some up to several amperes or even tens of amperes. At present, the Bureau has not yet monitored this project. Most of the main transformers in our bureau have grounded the iron core outside the transformer so that it can be tested in this area in the future.
2 Power failure electrical test analysis method, the contents and methods of the multi-point grounding electrical test of the transformer that may occur after a power failure are as follows: (1) Correctly measure the DC resistance of the windings at each level, if each data does not exceed the standard, and each Compared with the previous test data between the phases, there is no significant deviation and the law of change is basically the same, which can eliminate the fault part in the electrical circuit (such as bad contact of the tap switch, high pressure lead loosening, lead wire on both ends of the bushing conductive rod Poor contact, etc.). (2) To further verify whether it is a multi-point grounding of the iron core, disconnect the grounding wire and test the insulation resistance of the iron core with a 2500 V megohmmeter, and finally determine whether the iron core is grounded more than two points.
Therefore, regular overhaul of transformers is very necessary, and it is imperative to strengthen the strength of transformer insulation projects. Chromatographic analysis of oil in oils is a good way to monitor faults. We must vigorously promote online monitoring of transformer chromatographs. The chromatographic analysis of main transformers can be easily performed at the office. Under control, eliminating this failure is also accurate and rapid.
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