Please ask the gods to discuss these issues at the bottom of the article. Welcome everyone to actively discuss:
First, step down to change the low-voltage side bus single-phase grounding protection action
This situation has been discussed in related discussions. It will not be discussed here. It is only summarized as a conclusion, and relevant conclusions are attached.
Second, step-down grounding problem:
1. The buck-change core and the clamp are grounded together with the ground of the transformer shell; (whether it meets the specification)
2. The iron core and the clamp ground share a lead wire grounding, and the iron core and the clamp ground are separately grounded through the lead wire.
The transformer core and the clamp are respectively led out of the casing by a small casing and then grounded. In normal operation, there are no differences between the two situations. When the main transformer has internal clamps and cores grounded or iron core multi-point grounding, there will be differences:
1) After the main transformer occurs, the core and the clamp are grounded through the wire or high resistance ground. If the main transformer core and the clamp ground share a grounding lead, the main transformer will leak magnetic due to the operation of the main transformer. - The outer core and the clip piece form a loop I,. However, this circulation does not flow into the earth through external leads. Therefore, there is a defect that the external lead cannot measure a large ground current.
2) After the main transformer occurs, the iron core and the clamp are grounded through the wire or high impedance ground. If the main transformer core and the clamp are grounded separately through the lead wire, they will be at the ground point of the core-iron core grounding point. —Circuit current is formed in the clamp circuit. This current passes through the external leads. Therefore, it is easy to measure the increased ground current at the external ground lead monitoring point, and the A and B monitoring point currents are the same.
3) In the case of a multi-point grounding of the iron core of the main transformer, since the clamp and the ground cannot form a conductive loop, the current can not be monitored at the monitoring point A, and the iron core can be in the core—earth lead—earth - The other ground point of the core forms a loop, so a large ground current can be measured at the B monitoring point.
Therefore, using this grounding method can further distinguish the internal ground faults in the main transformer and provide a reliable basis for our judgment of defects.
3, step-down shell without direct lead wire grounding, but through the grounding of the base, whether to meet the specification. (#3 buck change).
The step-down shell is grounded for protective grounding. When the transformer is insulated, the shell is close to ground potential to avoid personal shock. According to the requirements of Article 5 of GB50148-2010 4.12.1, the transformer body should be grounded at two points (some information shows that one point can be grounded, is it correct?). According to the requirements of the regulations, the welding between the transformer base and the basic steel plate cannot be regarded as an effective grounding, and can only be used as a means to prevent the transformer from shifting during operation. My station #3 step-down shell ground is grounded by sharing a grounding point with the core and the clips, but not grounded through the reserved grounding point. Is it in compliance with the technical specifications? Whether the grounding point reserved on the transformer body should be grounded directly with the grounding flat iron and the main grounding net. (The figure below shows the grounding point of the #3 main transformer in my station. It shares a ground with the iron core and the clamp. The shell only has this one grounding point. The rest of the grounding points are the grounding of the base.)
Third, the switching problem of the cooling fan power supply of the strong oil circulation transformer
The substation has a regular work of switching the power supply of the transformer cooling fan every month. When this periodical work is performed, it is required that the relative load be minimized during the shift shift. We know that the transformer's heavy gas protection is a serious fault inside the transformer body, and the oil flow rate in the gas relay is greater than 1.0~1.4 m/s, that is, the oil flow impinges on the baffle reed contact and closes, sending a “heavy gas†action signal and issuing a trip pulse. , Or for the composite type gas relay (FJ type) with upper and lower open cups and baffles, when the transformer has serious oil leakage and the oil level is reduced, first open the cup to expose the oil surface, send a "light gas" signal; then open the cup After the oil level is exposed, a “heavy gas†action signal is issued and a trip pulse is issued, and the transformer high and low side switches are opened to protect the transformer.
The influencing factors of the internal flow state of the gas relay are: head and flow of the oil pump; height of the oil pillow and height of the gas relay; arrangement of cooler inlet and outlet oil channels; arrangement of internal winding heat dissipation oil channels; Fault nature; operating in the submersible pump; maintenance and repair operations are reasonable.
My station #4 step-down submersible pump and radiator are connected and symmetrically arranged in a female tube type. When the cooling power is switched, four pumps are stopped at the same time and then started simultaneously. During this time, there will be large oil flow. Shock may cause heavy gas transformer protection actions. According to relevant data, there are cases where the main transformer of the transformer station caused transformer heavy gas protection actions due to the simultaneous start and stop of the submersible pump. I did not have the #4 step-down oil flow impact test data and required relevant test proofs. (I station has made several power switch, no problem occurred)
However, in order to prevent the occurrence of unknown accidents, it is still necessary to formulate relevant operating regulations. Example: A strong oil circulating transformer should be symmetrically put into the corresponding number of coolers. The setting of coolers working, auxiliary, and standby should be in accordance with the manufacturer's regulations. When starting or stopping the coolers according to the load or oil temperature, the selected coolers should be operated symmetrically. When the cooling device power is lost, restore the power supply should be symmetry interval (interval time not less than 2 min) into the corresponding number of coolers.
Question: Is it necessary to formulate appropriate operating specifications when switching cooling power supplies?
Fourth, 35KVC bus A, B, C phase voltage difference is larger
1, Zoufangfang 35KVC section bus Ua = 20.15KV, Ub = 20.77KV, Uc = 19.83KV, the highest minimum phase difference of nearly 1000V.
Uab=35.06KV, Ubc=35.04KV, Uca=35.09KV, 3Uo=1.67KV, the grounding variation screen shows that the displacement voltage is 597.7V.
Because my station 35KV system is a neutral arc-suppression coil grounding system, many theoretical data are needed to verify this problem, so I will not discuss it. The gods who are interested can follow me to verify the data.
1. Analysis of possible causes: The three-phase load on the line is not balanced. After looking at the three-phase current load curve of the low-voltage side of the transformer, it is found that the C-phase current is always lower than the A and B phases, which is inconsistent with the Uc phase voltage, so the reason for the unbalanced three-phase load is excluded.
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