Main Factors Affecting the Safe Operation of China's Power Grids and Situation Analysis

From August to September 2003, the catastrophic power outages in the United States, Canada, and the United Kingdom, and the large-scale power outages in 25 cities in and around Moscow on May 25, 2005, showed that There are advanced control systems and advanced operating applications, but aging and obsolete power transmission and transformation equipment still cause the grid to work in a dangerous state. The statistical results of China from 1999 to 2004 show that not only the complex climate is one of the main factors causing large-scale blackouts in China's power grids, but also the grid accidents caused by the failure of power transmission and transformation equipment are increasing year by year. The main hidden danger factors of China's power grid security. However, the current analysis and research on the existing problems of the power grid emphasizes the reliability of the grid's own operation, and the in-depth study on the safety technology to prevent large-scale trip accidents caused by complex climate environment and equipment failure has not caused enough. Pay attention to it. To this end, this paper focuses on the analysis of the continuation of the complex climate environment and equipment failure threats to China's power grid security, and then emphasizes the urgency of researching and establishing the first line of defense against large-scale power outages.

1 Analysis of the main factors affecting the security of China's power grid

The causes of power grid security are complex and varied, such as complex climate environment, insulation aging of power equipment, natural disasters, illegal operation, and external damage. In the 2004 power grid accident, there were 14 natural disasters, 29 equipment failures, 6 personnel liabilities, and 5 other causes. From the classification comparison of fault causes in the past 4 years (Fig. 1) [1], it can be seen that natural disasters and equipment failures are the main ones, but through continuous research to find safety technical measures to reduce accident rates, and other causes It is accidental.

1.1 Complex climate environment

Natural disasters are one of the main factors that have historically jeopardized the safe operation of power grids, including irresistible factors such as lightning, typhoon and mudslide. Here, only the complex climatic environmental factors that can greatly reduce the accident rate can be found by strengthening scientific research. .

Generally, the so-called complex climate environment mainly includes different combinations of factors such as low air pressure (high altitude), pollution, snow, acid rain, acid mist, and high humidity. The existing research results show that in non-high altitude areas, the insulator string is mostly caused by pollution, but the acid wet deposition has increased the frequency of pollution flashover in the past 10 years. When the pH of acid wet deposition is ≤3~4 At the high altitude, the total effect of the three parameters of air pressure, air density and humidity, the discharge voltage of the air gap and the flashover voltage of the clean insulator, The altitude is increased and decreased (Fig. 3); the flashover voltage of the insulator string decreases with altitude (Fig. 4); not only the high-altitude and high-humidity cold areas often cause ice flashover, wire dancing and reversing accidents, and Non-high-altitude areas with a height of more than 150m on the top of the mountain, micro-topography at the foot of the mountain, and microclimate conditions that cause the transmission line to icing, will also cause a large-scale ice flash trip accident (Figure 5); various ice-covered insulators exist during the ice-melting period. The minimum flashover voltage, if the insulator is contaminated before icing, the acidity of the acid deposition in the icing zone is greater (pH less than 5.6), and the probability of triggering a flashover trip is greater (Fig. 6);

In 1990 and 2001, in the large-scale "soil flash" accidents such as Northeast China, North China, and Henan Power Grid, snow and acid mist were the direct factors inducing flashover; from December 2004 to February 2005, the Central China Power Grid was covered. Ice and snow have caused the transmission line to trip 97 times, and four 500kV lines in Hunan have tripped. The measured conductivity of ice coating on the site is up to 300μs/cm2, and 80% of the ice flashing tower is in the pollution area of ​​Grade 3 and above. From the analysis of three large-scale blackout accidents in the past 10 years, it can be seen that the climatic factors that reduce the electrical strength of the outer insulation are various, and the acid wet deposition and ice coating are the main reasons for directly inducing “soil flash”. Therefore, the overall reliability level of overhead lines in 2004 was lower than that in 2003, especially the climatic factors became the first factor leading to the unplanned outage of 500kV overhead lines [2].

In order to improve the accident rate of large-scale power outage caused by the large-scale reduction of the complex climatic environment and the decrease of the electrical insulation strength of the external insulation, it is necessary to systematically carry out research on the external insulation discharge mechanism and electrical characteristics in a complex climatic environment, for reasonable insulation selection and anti-flashover tripping. The choice of technical measures for accidents provides theoretical and technical support.

1.2 Device failure

According to statistics, accidents caused by power transmission and transformation equipment in China have been high, from 18 in 1999, 22 in 2000, 29 in 2001, 22 in 2002, 27 in 2003, and 29 in 2004. Half or more of all grid failures. In the "220kV and above power plants and substations all stop" faults that occurred the most in 2004, there were 21 failures caused by equipment failures, accounting for 70% of all such failures [1]. In the 12 major types of power transmission and transformation equipment [2], the forced power outage rate of 220 and 330kV transformers increased in 2004, the overall reliability level was lower than that in 2003, and the 220, 500kV transformers were unplanned. The main reason for the shipment is that the quality of the product is not high, especially the coil of the main part. Although the overall reliability level of the circuit breaker is higher than that in 2003, the important reason for the unplanned shutdown of the circuit breaker is that the quality of the 220kV class is not high, and the first reason for the 330, 500kV class is the aging of the components.

At present, the characteristics of power transmission and transformation equipment failures are as follows: 1 equipment service time is too long, equipment aging is serious, various rated operational indicators are seriously reduced, can not be replaced in time, causing some equipment to "have disease" operation; 2 some equipment in the design process Or inherent defects in the process of manufacturing, installation and commissioning, leaving a safety hazard, causing the grid to malfunction under certain conditions; 3 long-term full load or even overload operation, tight maintenance time, often leaving a safety hazard to the equipment.

In order to greatly reduce the threats caused by the failure of the equipment itself to the safe operation of the power grid, in addition to improving the manufacturing level and maintenance level, it is necessary to actively carry out research and development and application of equipment status online monitoring and fault diagnosis technology, in order to achieve reliability. The state's state maintenance provides decision support.

2 The threat of external insulation in complex climates will continue

China's mountainous hills account for about 43%, and the plateau accounts for 26%. The mountains and plateaus with an altitude of over 1km exceed 2/3 of the country's total area. 94% of China's coal-fired power resources are distributed in the high altitude area of ​​1 to 3 km north of the Dabie-Kunlun Mountain, and 85% of the hydropower resources are also located in high altitude areas such as Yunnan, Guizhou, Sichuan, Yunnan, Shaanxi, Gansu, Ningxia and Xin. Therefore, the ultra-high-voltage AC-DC lines sent by West-East Power Transmission must pass through high-altitude areas. For example, the highest altitude of Guizhou section of Guiguang ±500kV DC line is more than 1.6km, from Xiaowan to Guangdong, Xiangjiaba to Huazhong and East China. The DC super-high line has an altitude of up to 3km.

It is generally believed that the influence of atmospheric parameters on the electrical insulation of external insulation includes three parameters of air pressure, temperature and humidity. The level of air pressure depends mainly on the altitude and is basically linear. Although the temperature and humidity are related to various factors, in a certain area, the temperature still decreases with the increase of altitude, and the absolute humidity and altitude generally decrease exponentially. Although most countries and IEC recommend the relative air density and absolute humidity to characterize and study the influence of atmospheric parameters on the external insulation discharge voltage, the existing standards are only for the external insulation selection below 4000m. In addition to atmospheric parameters, China's power grid is also threatened by three major climate factors.

2.1 Air pollution has a tendency to increase

The pollution of the atmospheric environment mainly comes from poverty pollution, modern pollution and greenhouse gas emissions. Directly threatening grid security is regional environmental pollution, namely poverty pollution and modern pollution. Although China has adopted a variety of anti-pollution measures, the changes in air pollution in the country are still small, especially in urban air pollution.

In 2004, sulfur dioxide emissions were 22.549 million tons, soot emissions were 10.95 million tons, industrial dust emissions were 9.548 million tons, and sulfur dioxide and soot emissions were still rising (Table 1).

Although the overall urban air quality in 2004 did not change much in 2003, the proportion of air quality reaching the secondary standard city is declining. Among the 342 cities monitored in 2004, there were 141 cities with air quality level 3, up to 41.2%, an increase of 9.7 percentage points over 2003; 69 cities inferior to level 3 still account for 20.2%; The population accounts for 33.1% of the statistical urban population, which is 3.3 percentage points lower than that of 2003. The carbon dioxide and particulate matter of the main pollutants in the air of large and very large cities with a population of more than one million exceed the highest standards, and the air quality standards are low (Figure 8). .

Particulate matter is still the primary pollutant affecting air quality, with 46.8% of urban particulate matter exceeding the secondary standard, an increase of 1.2 percentage points over 2003 (Figure 9). In 2004, the cities with heavy pollutants were mainly distributed in Shanxi, Inner Mongolia, Liaoning, Henan, Hunan, Sichuan and the northwestern provinces (autonomous regions).

In 2004, cities with severe sulfur dioxide pollutants were mainly distributed in Shanxi, Hebei, Henan, Hunan, Hubei, Yunnan, Inner Mongolia, Gansu, Guizhou, Guangxi, Sichuan, Chongqing and other provinces, autonomous regions and municipalities. Cities with an annual average concentration reaching the national secondary standard (0.06mg/m3) accounted for 74.3%, which was the same as in 2003; cities exceeding the national third-level standard (0.10mg/m3) accounted for 9.1% of the statistical cities, which was lower than that in 2003. Percentage points (Figure 10).

In the sulfur dioxide pollution control area, cities with annual average concentration of sulfur dioxide reaching the secondary standard accounted for 40.6%, and cities exceeding the secondary standard accounted for 59.4%, of which 19 cities exceeded the third-class standard, accounting for 29.7%. In the acid rain control area, cities with an annual average concentration of sulfur dioxide reaching the secondary standard accounted for 69.4%, and cities exceeding the third-level standard accounted for 7.2% (Figure 11).

Among the 113 key cities for air pollution control, the air quality of 51 cities is 3, accounting for 45.1%; the air quality of 29 cities is inferior to the third level, accounting for 25.7%. Among the 47 national key environmental protection cities, air quality in 21 cities is Grade III; 6 cities are inferior to Grade 3 standards and air pollution is serious (Figure 12).

2.2 Acidic wet deposition is intensified

The proportion of cities with acid rain in 2004 increased by 2.1 percentage points compared with 2003; the proportion of cities with annual average pH ≤ 5.6 increased by 4 percentage points, of which the proportion of cities with pH less than 4.5 increased by 2 percentage points; the frequency of acid rain exceeded 80%. The proportion of cities in % increased by 1.6 percentage points (Figures 13, 14). The proportion of cities with low annual average pH and high acid rain frequency increased compared with 2003, indicating that the acid rain pollution in 2004 was worse than that in 2003.

It can be seen from Tables 2 and 3 that in 112 cities with acid rain control area, the annual average pH value of precipitation ranges from 3.05 (Jishou City, Hunan Province) to 7.26 (Cangzhou City, Hunan Province), among which 101 cities have acid rain, accounting for 90.2%: precipitation There are 83 cities with an average annual pH less than or equal to 5.6, accounting for 74.1%, an increase of 3.4 percentage points over 2003; the proportion of cities with an annual average pH of less than 4.5 increased by 6.4 percentage points, Shaoguan in Guangdong and Changsha in Hunan. Changde and Jishou have an average annual pH of less than 4.0. There were 67 cities with an acid rain frequency greater than 40%, accounting for 59.8%, an increase of 6.1 percentage points over 2003. The acid rain pollution area in the acid rain control area is basically stable, but the pollution level is further aggravated.

In 2004, the average annual precipitation pH value of less than 5.6 (acid rain) was mainly distributed in Central China, Southwest China, East China and South China: the acid rain area in Central China was the most polluted, and the annual average pH value of precipitation (≤5.6) was 58.3% for acid rain. The proportion of cities with acid rain frequency greater than 80% is 21.4%; Hunan and Jiangxi are the most serious areas of acid rain pollution in Huazhong acid rain area; the acid rain area in South China is mainly distributed in the southeast of Guangdong and the eastern part of Guangxi in the Pearl River Delta, the annual average pH value of precipitation. The proportion of cities with less than 5.6 is 58.9%. Compared with 2003, acid rain pollution is aggravated; the southwest acid rain area is centered on Yibin, Nanchong, Zunyi and Chongqing in Guizhou, and the proportion of cities with annual average pH less than 5.6 is 49.0%. The East China acid rain area has a wide distribution, covering the southern part of Jiangsu Province, Zhejiang Province, Fujian Coastal Area and Shanghai. The proportion of cities with high acid rain frequency (≥80%) and high acidity precipitation (pH ≤5.6) is second only to Huazhong acid rain. Districts, 21.0% and 14.6% respectively; Beijing, Tianjin, Qinhuangdao and Chengde in the north, Houma in Shanxi, Dalian, Dandong, Jinzhou, Fuxin, Tieling, Hulu in Liaoning

In 2004, the average annual pH range of precipitation in 527 cities (counties) nationwide was 3.05 (Jishou City, Hunan Province) ~ 8.20 (Jiayuguan City, Gansu Province). There were 298 cities with acid rain, accounting for 56.5% of the statistical cities. There are 218 cities with an average annual precipitation pH less than 5.6, accounting for 41.4% of the statistical cities, including Changsha, Changde and Jishou of Hunan Province, Shaoguan of Guangdong Province, and the annual average pH of Gao'an precipitation in Jiangxi Province is less than 4.0, and the acidity of precipitation is strong; More than 40% of the cities accounted for 30.1% of the statistical cities, including Changde in Hunan Province, Dexing in Jiangxi, Lishui, Anji in Zhejiang, and the frequency of acid rain in Kaihua was 100%.

2.3 Ice-covered snow causes line trip accidents to occur

China is one of the countries with severe ice coating. The average number of ice-covered days in alpine regions is more than 40-60 days. For example, the upper reaches of the Yellow River, the Jinsha River Valley, and the mountainous areas adjacent to Sichuan and Chongqing, the thickness of ice on transmission lines is generally 20~. 40mm, some as high as 80~100mm. Guizhou, Yunnan, Sichuan, Chongqing and other areas with high altitudes in the west are mainly haze (density less than 0.6g/cm3), while in Hunan, Hubei, Shanxi, Henan and East China, the rainy area (density 0.87~0.92g/ Cm3) is dominant; due to the influence of topography and meteorological conditions, mixed rafts with a long duration (density 0.67~0.878g/cm3) often appear at the top of the mountain and at the foot of the mountain at altitudes above 150m in high altitude areas and plain areas.

Since 1954 in China, ice and snow have caused ice-flashing trips in the insulators of transmission lines, wire dancing and bar-breaking accidents. In February 2000, the Kunming area caused more than 140 trips due to ice and snow; in December 2001, two times of the 500kV Geshuang II back occurred, the B phase caused grounding caused by ice and snow; in February 2003, 500kV On the Yangzhun line, two-phase tripping accidents caused by ice flashes occurred twice; from December 2004 to February 2005, the thickness of ice on the local lines of 500kV lines in parts of Central China and East China was as high as 100mm, and a large range of ice flashes appeared. Tripping, wire dancing and reversing accidents. From the changing climate of the country, snow and ice will still be one of the main causes of power grid accidents.

3 The failure rate of the device itself will remain high.

In recent years, with the acceleration of power construction and the reform of the power system, various types of power equipment at home and abroad have been widely used in power grids, and the manufacturing level of equipment has been uneven. Some equipments have inherent defects in the process of manufacturing, installation and commissioning, and the grid coverage area is large. It is difficult to overhaul and inspect some equipment with poor operating environment, and the initial failures cannot be eliminated in time. Due to the large investment in equipment, some old power transmission and transformation equipment cannot be replaced, not only the insulation has been degraded, but also the insulation between the old and new equipment is improperly combined, which further aggravates the insulation of the equipment. The harsh climate and full-load disease operation also accelerate the reduction of equipment insulation levels. In short, from the current situation of China's power transmission and transformation equipment, the hidden dangers caused by its own faults to the safe operation of the power grid will continue.

Under the situation that the failure rate of power transmission and transformation equipment itself is still high, it is necessary to continue to promote the state monitoring technology that combines online monitoring with necessary offline tests. At the same time, it is necessary to systematically and deeply study new technologies for on-line monitoring and fault diagnosis of power transmission and transformation equipment, continuously summarize and analyze a large amount of diagnostic data accumulated by equipment state diagnosis, and formulate fault diagnosis standards and use of various power transmission and transformation equipment. Guideline, based on this, realizes on-line evaluation of the operating state of power transmission and transformation equipment and online prediction of remaining life, and establishes a state maintenance system, which can greatly improve the safety and reliability of power grid operation.

4 Conclusion

Statistics on power grid accidents at home and abroad have shown that complex climate environments and equipment failures are the main causes of large-scale power outages in power grids. Therefore, through systematic research on the external insulation discharge mechanism and electrical characteristics in complex environments, effective safety measures can be taken to greatly reduce the accident rate of large-scale power outages caused by the decrease of the electrical insulation strength of the external insulation of power transmission and transformation equipment caused by complex environments. . Through the systematic research on the insulation aging mechanism and on-line monitoring and fault diagnosis technology of power transmission and transformation equipment, it is possible to develop a highly stable and highly intelligent equipment status online monitoring and fault diagnosis system, which is a reliability-centered state. Maintenance technology provides technical support.

In short, in the situation of China's unique climatic environment and equipment failure rate, starting from power transmission and transformation equipment, research and solve the two main causes of grid accidents, and establish the first road to prevent large-scale power outages in power grids. Line of defense is very necessary, especially for the planned construction of UHV AC and DC transmission lines is a challenging topic.