Analysis on the operation of electrical equipment in substations under two recent extreme cold weathers

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During the peak winter season in recent years, the temperature in the province has been relatively low. Among them, there have been two extremely cold weather in the province from December 28 to 31, 2020 and January 5 to 8, 2021. During this period, Jiangsu The power grid load hit a new high in winter and a new high in the year. This paper summarizes and analyzes the operation of electrical equipment in substations under the double influence of two extreme cold weathers and high loads, uses abnormal signals and defects of equipment as data sources, excavates possible equipment quality problems and weak links in management, and summarizes coping experience. Analyze deficiencies.

1 The weather and load conditions of the whole province

1.1 Snowfall and sudden drop in temperature

During the two extreme cold weather periods, the temperature in the whole province dropped sharply. During the first extreme cold weather period, all regions in the province experienced snowfall, and during the second extreme cold weather period, local snowfall occurred in the province. The lowest temperature in the whole province drops by 10~12℃, and the lowest temperature point is from the evening of January 7th to the morning of January 8th. Among them, the Huaibei area is -13℃-12℃, the Jianghuai area is -10~-9℃, and the southern Jiangsu area is -7℃. , The whole province is frozen. The average daily maximum temperature in each region of the province is 15°C, and the average minimum temperature is -8°C.

1.2 Grid load hits a new high

2021-01-07T19:15, the real-time load of Jiangsu Power Grid reached a new high, with an instantaneous transmission power of 117,016Mw, exceeding the peak load of 115,211Mw during the summer peak in 2020. Figure 1 shows the dispatching power consumption curve of the province from December 28, 2020 to January 8, 2021, and Figure 2 shows the DC transmission power curve in the province. The DC transmission power reaches 20:00 on January 7. The peak value is 18493Mw.

Analysis on the operation of electrical equipment in substations under two recent extreme cold weathers
The number of overloaded main transformers increases. A total of 160 main transformers were overloaded during the two extreme cold weather periods (calculated once a day), and the overloaded main transformer reached 41 on January 7.

2 Overview of electrical equipment alarms and defects during the two extreme cold weather periods

After analyzing and comparing all the alarm data, it is found that the abnormal alarms of the substation equipment under the jurisdiction of the company during the extremely cold weather are mainly alarms of low sF6 pressure, abnormal pressure of the oil pump, abnormal oil level, N2 leakage, abnormal cooling system of the main transformer and heater failure alarms Signal. The six types of signals were sent out a total of 2326 times during the extremely cold weather, involving 83 devices. Compared with before the extreme cold weather, during the extreme cold weather, the signal related to low sF6 air pressure increased by nearly 13 times, the signal of abnormal oil level increased by nearly 61 times, the signal related to oil pump suppression increased by nearly 22 times, the signal related to N2 leakage increased by nearly 10 times, and the main transformer More than 600 cooling system signals have been added, and more than 100 heater failure-related signals have been added.

During the last two extreme cold weather periods, a total of 11 defects caused by cold waves were found in the substation equipment under the jurisdiction of the company, including 6 cases of oil leakage (gas) defects, 2 cases of abnormal low oil level, 2 cases of pillar insulator fracture, and surge arrester tilt 1 up.

3 Analysis of main alarms and defects of electrical equipment

3.1 SF2 pressure alarm analysis

During the extremely cold weather, 287 times of sF6 pressure alarm signals occurred in the porcelain column equipment, among which the 35kV switches produced by Siemens (Hangzhou) produced the most sF6 low pressure alarms in the porcelain column switches. From the perspective of equipment operation years, equipment that has been in operation for more than 11 years accounts for 88%. For equipment that has been in operation for a long time, its flanges and sealing rings are aging to varying degrees. The main reason for the sF6 low air pressure alarm is that the sealing ring will shrink in extremely cold weather, resulting in poor sealing and air leakage.

Case 1: The 3530 switch sF6 low air pressure alarm of the No. 3 main transformer of Tai × Bianfa, after 11 minutes, the switch sF6 low pressure switch closes and locks, the first group/second group control circuit disconnection and other signals are issued. On-site inspection switch B phase gas chamber pressure is 0.55MPa (rated value 0.7MPa, alarm value 0.62MPa, blocking value 0.6MPa), A, C phase pressure is 0.72MPa. The infrared precision leak detector was used to detect that there was an air leakage point at the seal of the lower flange of the switch B-phase interrupter, and the disassembly inspection of the switch found that there was a crack at the crimping place between the lower flange of the B-phase interrupter and the sealing ring, resulting in air leakage.

sF6 meter problems can also cause false alarms. The double-layer metal temperature compensation capability of the double-metal compensated sF6 gas density relay has a certain linear variation range. When the temperature exceeds the linear variation range under low temperature conditions, the compensation capability will be insufficient, resulting in errors in the readings of the sF6 density meter. In serious cases, it will cause the switch to send sF6 low pressure alarm or blocking signal by mistake.

3.2 Signals related to abnormal oil level

The signals related to abnormal oil level mainly include “×× low resistance/main transformer/station transformer oil level abnormal alarm”, including main transformer, low resistance, station transformer, etc., and a total of 22 devices sent alarms during the two extreme cold weather periods 55 articles. Among them, the BKsJ-60000/35 type reactor of Beijing Electric Power Equipment General Factory sent the most letters, 14 times. After analysis, the main reason is that the size of this type of low-resistance product oil pillow is too small, 800×2500, and the rated effective capacity accounts for 6.4% of the total oil volume, which does not meet the design requirements of 10%, resulting in low oil conservatories in severe cold weather. It is easy to generate high oil level alarm in high temperature weather after refueling.

Case 2: The oil level of No. 1 main transformer No. 2 of Qin × substation was abnormal, and the oil level of the oil conservator was checked on site and the oil level was lower than the alarm value. No. 1 main transformer No. 2 has no obvious oil leakage at low resistance. It is judged that the capacity of this type of oil conservator in Beijing Electric Power Equipment General Factory is small (does not meet the design requirement of 10%). After exiting operation, the ambient temperature drops sharply, the insulating oil in the low-resistance body and the oil conservator shrinks, reaching the low oil level non-electricity protection fixed value alarm threshold, and a low oil level alarm signal is issued.

3.3 Switch Mechanism Alarm Signal

There are a total of 263 alarm signals for the switch mechanism, all of which are hydraulic mechanism signals, including 228 oil pump pressure overtime signals, 34 N2 leakage alarm signals, and 1 low oil level alarm signal. Among them, Siemens’ switching mechanism sent the most letters. The switch of the hydraulic mechanism is prone to changes in oil pressure and N2 pressure when the temperature drops suddenly, resulting in abnormal situations.

Case 3: Wu × variable 2L30 switch alarm for low oil level. On December 31, Wu × substation 2L30 switch alarmed for low oil level. After on-site inspection, it was found that the oil level indication of the switch hydraulic mechanism was lower than the lower limit. After the maintenance professional came to the scene to replenish oil with electricity, the alarm returned and the defect was eliminated.

3.9 Heater fault-related signals

A total of 350 heater failure signals were sent out, and there were 22 sending devices. The 3AP3FI-550 type equipment of Siemens (Hangzhou) High Voltage Switchgear Co., Ltd. sent the most letters, accounting for 43.71%. Check that the action current setting of the heater is too small. After adjusting the action current, the signal returns and the equipment returns to normal operation.

Case 4: The 5053 switch heater failure alarm of Jiong××changjinjin line was sent 153 times, and it was checked that the setting of the operating current of the heater was too small. 3.5 Oil leakage from oil-filled equipment

In extremely cold weather, oil-filled equipment (especially 35kV reactors) is prone to oil leakage. The main reason is that the operating conditions of the reactor are relatively special. The switching operation in severe cold weather will cause the reactor to undergo a large temperature change. The quality is uneven, and after a period of operation, it ages and hardens, which cannot meet the sealing requirements, resulting in oil leakage defects.

Case 5: Three × substation 220kV Baowu 2w74 line voltage transformer A phase oil seepage. On January 18, a professional inspection found oil leakage from the upper part of the lower section of the 2w74 line voltage transformer phase A capacitor unit. The disassembly analysis found that there were two leakage points in the vacuum oil injection pipe and the connecting pipe inside the capacitor in the lower section, and oil droplets could be seen at the pipeline bolts, and the copper rust of the bolts also dripped to the top of the expander along with the oil droplets. The pressure transformer has been put into operation for 12 years. In the previous month, the operating environment of the equipment was relatively harsh and the temperature was extremely low.

3.6 Equipment tilted and broken

In extremely cold weather, severe weather conditions such as low temperature and strong wind are likely to cause defects such as inclination, cracks or even breakage of the connecting flange of the lightning rod, the base of the arrester, and the insulator of the capacitor support. The main reasons are as follows: (1) Some lightning rod flange bolts do not adopt the double cap and double pad structure, which increases the risk of bolt loosening; (2) Some lightning arrester bases are made of small-diameter steel pipes, connected by fillet welds, and welded without reinforcement support. This design leads to low flexural strength of the transition section, especially the weak strength of the weld fusion line: (3) Water accumulation occurs on the base of the arrester or the support due to poor sealing or unreasonable setting of drainage holes. Damage caused by freezing and expansion: (4) Some capacitor manufacturers’ ceramic parts are made of poor materials, which become brittle when exposed to cold weather, and have inconsistent expansion coefficients at the junction with the metal, causing the ceramic parts to shatter due to cold.

Case 6: The lightning rod in the southeast corner of 3×× transformer 220kV is tilted. After inspection, it was found that there were a total of 6 fastening bolts at the flange connection of the lightning rod, 3 of which fell off, and 2 of the remaining 3 bolts were deformed, causing the lightning rod to tilt to the west.

4 Countermeasures

(1) Professional inspections should be carried out before extremely cold weather, focusing on whether the pressure indication of porcelain column circuit breakers and combined electrical appliances is within the normal range. For the equipment that has been found to have air leakage points, it should focus on follow-up inspection, and if it is close to the alarm value, it should be considered to replenish air in advance [2]. For the newly discovered sF6 low pressure warning equipment, leak detection should be carried out in time, and the equipment with leakage points should be repaired in conjunction with the power outage plan.

(2) In response to the small volume of low-resistance oil storage tanks of Beijing Electric Power Equipment General Factory, the manufacturer has issued an implementation plan for replacing large-capacity oil storage tanks, and the treatment work will be completed in succession.

(3) Conducted an interview with Siyuan Electric Co., Ltd., which has many low-resistance leakage oil failures in recent years, and clarified that the cause of the failure was that the acrylic gasket used in the company’s early days (before 2016) had batch material defects and could not be used. Meet the long-term use requirements of the reactor. Before the extreme cold weather comes, the company organizes special low-resistance inspections to judge whether there is a low oil level alarm and oil leakage situation one by one. At present, Siyuan Company has been organized to formulate a detailed plan for the treatment of low anti-leakage oil. For products that have not replaced the gaskets before 2016, all the gaskets on the upper part of the box cover will be replaced gradually according to the priority.

(4) For oil-immersed rheology and pressure change, it is necessary to do a good job of professional inspection in extreme weather, focusing on inspecting porcelain sleeves, secondary junction boxes, valves, flanges, and rheological heads when the temperature difference between cold and hot is large Check whether there is leakage in other parts, and deal with abnormalities in time.

(5) Carry out special investigations on the problem of tilted exposure of structural lightning rods. Through the use of telescopes, reviewing drone inspection videos and photos, etc., carry out hidden danger investigations of independent lightning rods and structural lightning rods, focusing on checking whether the body of the lightning rod is tilted or cracked, and whether the flange connection is rusted, deformed, cracked, or loose. Whether the flange connection bolts use “double caps and double pads”, whether the bottom of the steel pipe lightning rod is provided with drainage holes, etc.

Analysis on the operation of electrical equipment in substations under two recent extreme cold weathers

(6) In view of defects such as cracks or even fractures in the arrester base and capacitor support insulators, the first is to do a special inspection of substation equipment in extremely cold weather, paying attention to whether there are cracks in the capacitor support insulator, the corrosion of the arrester base, the structural support and the corresponding main If the equipment is tilted, the problem should be dealt with in a timely manner: second, the small-diameter transitional base of the lightning arrester without reinforcing ribs should be repaired and welded in time to increase the mechanical strength; If there is any problem or the steel plate is cracked, warped, or deformed, take timely measures such as drainage and reinforcement.

5 Conclusion

Through the big data analysis of equipment operation during the cold wave, the characteristics and potential problems of equipment operation under specific climatic conditions can be found, and based on this, relevant management regulations on the special work of cold and frost protection for primary substation equipment in winter can be formulated, and differentiated tracking control measures can be implemented. , to ensure the safe operation of the equipment during the cold wave.

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