Источник: Transmission&Distribution world
http://tdworld.com/mag/power_electric_reliability_people/index.html
The objective of most municipal and provincial city utilities is to create a first-class enterprise to satisfy the basic principle that "an electric power industry of the people is for the people." This means maximizing system reliability to meet the demands of developing the national economy and improving the population's standard of living.
The investment required to improve system reliability is massive compared with the resulting increase in revenue. Consider Beijing, a utility that annually sells approximately 25 billion kWh. The investment of Ntdollars 2220 million (USdollars27 million) in its distribution system produced an increase of 0.85 percent in system reliability. This improved reliability will increase the annual revenue by an estimated Ntdollars7.7 million (USdollars0.93 million), or 3.5 percent of the capital invested. Hence, it will take 29 years to recover the investment.
However, unless certain fundamental issues are addressed, it will be difficult for the electricity industry to satisfy the market economy. Those issues include the following: * China is a developing country, but the development of the economy in various areas is unbalanced. What should the level of service reliability be for different types of consumers? * There is a need to establish a tariff structure for electrical energy that is directly related to the different levels of system reliability or that offers greater consumer choice. As China faces the tremendous challenge of raising its reliability standards to levels found in industrially developed countries, it must analyze the reliability of the power-supply system for each main city. Such an analysis will determine the system's capability of meeting demand and measure the effectiveness of capital investment. Historically, there are three principal causes of poor system reliability: * Shortage of generating capacity. A survey of system reliability in China's cities revealed that two-thirds experienced a shortage of power at times of peak demand, increasing the number and duration of supply interruptions. * Major construction programs. Road and bridge construction also made a significant impact on system performance in some provinces as circuits were decommissioned for site safety reasons. For example, the consumers in Taiyuan experienced interruptions exceeding 3000 hr in the first six months of 1997 during construction of nine roads and a bridge. * Aging plants and inadequate maintenance. System faults caused more than one-third of the reported supply interruptions. No investigations were made to determine the prime cause, but the combination of aging plants and irregular maintenance was considered the most likely reason.
The Electricity Reliability Management Center (ERMC) of China's Electricity Council monitors and analyzes the performance of power-supply networks at various voltage levels annually for four municipalities (directly under the central government) and 26 provincial capitals (autonomous regions). The monitoring is based on the 10-kV system, since this is the distribution voltage supplying the majority of China's consumers. The reliability performance statistics are considered accurate even though the quality and accuracy of the data collection management systems used by each of the contributing power bureaus in this large country may differ.
Review of 1998 10-kV System Performance for 30 Cities
The State Power Corp. (SPC) of China still assigns target values for system reliability, especially for a 10-kV system, and only those supply networks with a performance exceeding 99.9 percent are regarded as first-class enterprises. Therefore, particular attention has been given to the reliability of 10-kV systems by power-supplying utilities in order to meet target values and quality standards for this highly prestigious honor.
Before 1998, lack of system capacity had a major impact on reliability. But in 1998, as a result of increased infrastructure investment, a significant improvement in average reliability was recorded. Average 10-kV system reliability increased to 99.82 percent, and the average interruption time per consumer was reduced to 15.86 hr. Among the 30 city networks monitored, there were 18 cities in which reliability was not influenced by insufficient capacity, and eight cities that had system reliability higher than 99.9 percent.
In Table 1, which shows 1998 system reliability, Jinan recorded the highest reliability (99.97 percent), and Urumqi displayed the lowest (99.21 percent). Twenty-three cities improved reliability between 1997 and 1998, with Xi'an having the greatest growth (0.72 percent).
Insufficient system capacity still has a large impact on system reliability in each city, increasing the total interruption time per consumer. This can be seen when the reliability statistics are represented excluding the outages due to insufficient system capacity.
Guangzhou had the greatest improvement (0.19 percent). Even though no outages occurred in 1997 and 1998 from insufficient capacity, system reliability continued to improve. The typical extra-high voltage, high-voltage and 10-kV systems used in China show the transmission and distribution system supplying the urban area of Weifang in 1998.
SPCs: First-Class Power-Supply Enterprises
The SPC also has a first-class enterprise standard that excludes outages caused by insufficient capacity (for example, greater than 99.6 percent for cities that sell in excess of 4 billion kWh per annum). The electricity sales in all 30 cities exceed this value, but only six cities achieved the standard of first-class power-supply enterprises by achieving both target system reliability values in 1998. These cities were Yinchuan, Jinan, Harbin, Guangzhou, Shijiazhuan and Changchun.
The survey confirmed that from 1997 to 1998 there were seven major cities (Beijing, Hohhot, Nanjing, Hefei, Guiyang, Urumqi and Nanning) in which overall system reliability declined. After excluding outages due to insufficient system capacity, these seven cities, plus Chongqing, recorded a decline in reliability. Interruptions caused by city power-network reformation and large-scale municipal construction in 1998 were primarily blamed for the decline.
System Performance Improvements in 1998
One of the main influences on significant improvements in supply reliability is the removal of power consumption limitation in most cities due to the increased availability of electrical energy in 1998. Though 18 cities have system capacity sufficient for consumer demand, this is the first time since these 30 cities were surveyed that more than 50 percent were able to supply the demand at all times. Although 12 cities were suffering from insufficiency of energy source in 1998, Urumqi was the only city where reliability was reduced (by 0.224 percent) due to consumption limitation.
The power-supply utilities in China have successively strengthened the management of reliability in recent years. They are establishing management strategies for supply reliability and emphasizing the need to control planned maintenance interruptions, increase the volume of repairs on live equipment and introduce condition-based maintenance disciplines. Also, they are actively seeking new techniques and methods for fault location to improve the accuracy and speed of this process. Fault-management techniques will reduce the influence of fault outages and effectively enhance supply reliability.
Programs to reform and reconstruct power networks in China's main cities are the other keys to sustained improvements in supply reliability. Cities that initiatedprograms a few years ago are now reaping the benefits. Statistics confirm improved supply reliability, which has prompted more cities to promote similar programs. Need for more construction of bulk grid substations has been addressed to ensure city distribution networks are supplied with electrical energy sufficient to meet demand.
Cities started these programs by first considering the weakest links: * The installation of more distribution substations and transformers shortened the extended overloaded 10-kV feeders. * Inadequate and aging bare conductors were replaced with insulated conductors. * Underground cables with larger cross-sectional areas have been installed in downtowns, and cables have replaced overhead circuits routed through trees. * The configuration of distribution networks was gradually refurbished from radial to open-ring main mode. As a result, the electricity distribution network is more flexible, offering load transfer facilities to limit consumer interruptions for planned maintenance and limiting the network and consumers affected by fault outages. * In the city network reformation program, more equipment has been installed that requires less or no maintenance, such as vacuum switchgear, synthetic insulators and ZnO arrestors. All facilities designed to lessen the circuit outage time for planned maintenance may be significantly reduced. * The installation of fault locators, together with automatic devices to identify and isolate faults, have greatly reduced fault-location time. Consumer outages caused by system faults are alleviated, enhancing supply reliability.
Conclusion
Supply reliability in the majority of the cities surveyed has risen year after year, but a large discrepancy still exists between the system reliability of China's cities with that recorded for industrially developed countries: Japan 99.995 percent (interruption time/consumer=9 min/annum) France 99.82 percent (interruption time/consumer=94 min/annum) UK 99.985 percent (interruption time/consumer=77 min/annum) USA 99.99 percent (interruption time/consumer=58 min/annum)
These statistics indicate an investment shortage in the past that is now being addressed. But, China faces a familiar dilemma in deciding a level of system security that can be serviced by the country's financial resources.
However, with the countrywide Reformation and Reconstruction of City Networks Program, all power-supply utilities in China's main cities are investing in projects to reconstruct their networks and further improve system reliability. The management objective is to achieve system reliability standards of 99.9 percent in urban areas and 99.99 percent for downtown areas as soon as possible.
Consideration also must be given to the wide use of tariffs designed to encourage load management. This would reduce system peak demands and increase the use of the existing generating capacity-a practice that is well developed in the form of demand-side management in industrialized countries. Thus, establishing energy tariff structures that more accurately reflect the reliability and quality of the power supply should be seen as a longer-term objective.