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CIGRE论文我国联网经验37_305_1996[1]

CIGRé 1996 : 37-305O

ABSTRACTS

The growth of installed generating capacity, system expansion and interconnection are faced with chal-lenging problems. Certainly the problems in China and those in most industrialized countries are very different, but by sharing and comparing these com-mon points of the problems, a series of reliability criteria suitble to national condition have been made in leading power system toward better devel-opment. This paper presents system planning and operation experiences and development of system interconnections, especially in the past 15 years. Some main points of the criteria and main differ-ence to those in industrialized countries are out-lined. Based on the statistics of bulk system instability disturbances for years the paper describes how the criteria are necessary and appli-cable. The paper introduces several important regional system interconnection projects under construction or planning including the power trans-mission scheme of the Three-Gorges hydro power project.

1.INTRODUCTION

In the past 15 years, the power industry in China has grown rapidly. By the end of 1994, the total installed generating capacity amounted to 199,000 MW, and electricity generation amounted to 909 TWh. After four provincial systems in South China and Hongkong systems have been interconnected six regional power systems, including ?ve systems exceeding 20,000 MW in installed generating capacity, cover 24 of the country's 31 provinces (municipalities or autonomous regions). The com-mission of a ± 500 kV HVDC transmission line interconnecting two large regional systems with a total capacity of 40,000 MW has ended the era of pure AC transmission in China. Some facts of power industry in China are shown in the Fig.1.

Regional Systems: EC, NC, NEC, CC, NWC,

CIGRE论文我国联网经验37_305_1996[1]

SCR.

Independent Provincial systems: SD, SC, FJ, HN, XJ, XZ, TW.

Capacity (MW) Length of Lines

(km, 500 or 330 kV)

EC 31673 2414

NC 27146 1597

NEC 26534 1652

CC 27602 1826

NWC 11483 5680 (330 kV) SCR 30577 2292

Fig.1 The existing power networks distribution in China in 1994

POWER SYSTEM PLANNING OVERVIEW AND DEVELOPMENT OF

SYSTEM INTERCONNECTIONS

IN CHINA — EXPERIENCES AND CRITERIA

LEI XIAOMENG HUANG WANYONG ZHANG FENGXIANG ZENG QINGYU National Power Control Center CIGRé China National Committee

CHINA

With the concentration of load to the east, coal mine deposits and hydro resources in remote areas, particularly in the north and west respectively, a large amount coal transportation and power trans-mission has been necessarily needed. Hydro power is predominant in the regions of Central, Northwest China and the provinces of Sichuan and Yunnan, while the thermal power predominant in North China. The transmission system for Three-Gorges hydro power station will make interties among Central, East China regional systems and Sichun system by the ?rst decades of next century. At present, the power systems are operated by hierarchical control organizations consisting of 6 regional, 25 provincial system control centers and 250 district control centers. In the mean time, the National Power Control Center (NPCC) has also been set up to manage and control the power inter-changes among regions and some big generating stations transmitting power cross regions like the Three-Gorges hydro power station and Yongcheng thermal generating station to be introduced below. By summarizing these experiences in system plan-ning, operation and development history some important criteria have been made to guide the sys-tem planning, operation and interconnection.

The general plan for the year 2000 has been made by Ministry of Electric Power. In accordance with the plan 45,000 MW generating capacity will be installed during 1995-1997, 60,000 MW installed in 1998-2000 and the total installed capacity nationwide will have reached 300,000 MW by the year 2000.

2. POWER SYSTEM PLANNING AND OPERA-TION PRACTICES OVERVIEW

In the 1970s, instability problems become severe because of weakly con?gured 220 kV systems and shortage of capital investment in transmission net-works. There were no adequate criteria to guide power system planning and operation. Since the Conference on Power System Stability in 1981, a series of technical policies and guidelines, such as "Power System Stability Criteria", "Power System Technical Criteria" (shortened to "Stability Crite-ria" and "Technical Criteria" hereafter) and "Fre-quency and Voltage Control Criteria" issued by Ministry of Electric Power of P.R.China, worked out in succession, have played an important role in leading power system toward better development. The "Stability Criteria" was issued in 1981 to solve the serious instability problems.

2.1 Instability Incidents Statistics

According to statistics, 22.2 times of instability disturbances occured each year in 1970's on aver-age. In accordance with the criteria transmission networks were enhanced. A lot of countermea-sures, including load shedding, units tripping, sys-tem splitting and so on, were adopted in the regional and provincial networks. Therefore, the times of occurance of instability incidents were obviously reduced (Fig.2). The total times were 60 corresponding up 6 times each year on average in 1980's against 22.2 times each year in 1970's

CIGRE论文我国联网经验37_305_1996[1]

Times Years

Fig.2 The statistics of systems instability

incidents in China

2.2 Some Important Criteria Of System Planning And Operation

(1). China's power system reliability standards The "Stability Criteria" and "Technical Critetia" are the main reliability criteria established accord-ing to conditions existing in China. The "Stability Criteria" refers mainly to synchronous stability, frequency stability and voltage stability. In regard to various kinds of short circuits and single and multiple contingent faults, the system stability sdandard is classi?ed into 3 levels, called "3 defense lines".

The ?rst defense line refers to single contingent faults, such as instantaneous single phase ground-ing faults, and some other faults during which

operation can be maintained stable under current condition, the system operation shall be stable and in normal service.

The second defense line refers to the more severe single contingent faults, such as 3-phase fault, under which the system operation shall be kept sta-ble by taking certain technical measures, including shedding part of the load.

The so called third defense line, or last defense line, refers to the most severe non-single contin-gent faults (such as multiple contingent faults, mal-operation of protective relay or operational failure of circuit breaker). Under this condition, the sys-tem operation may likely lose stability. Certain pre-ventive measures shall be taken to limit the disturbance within the pre-estimated controllable range to prevent fault cascading and system col-lapse.

The "Technical Criteria" divides the power net-work into receiving, sending subsystems and tie-lines and stipulates different security standards according to their importance and technical and economical conditions, Among which the receiv-ing end construction is stressed, i.e.:

Under normal operating conditions, should any severe single contingent fault (including 3-phase short circuit) occur on receiving subsystem, normal service shall be maintained without loss of load. (2). Voltage control standards

The essentials of "Voltage Control Guide" are that the reactive power, both the capacitive and the inductive, should be compensated on the hierarchi-cal and local basis. It means that a large amount of reactive power ?ows between networks of different voltage levels and areas is not allowed.

The hierarchical and regional reactive power bal-ance can be implemented by installing enough capacitive and inductive compensators.

The permissible voltage deviation:

Sending side: 500 kV 0--10%

220 kV 0--10%

35-110 kV -3--+7%

(3). Frequency control standards

The permissible frequency deviation:

50 Hz ± 0.5 Hz to the small systems with

3,000 MW and below 50 Hz ± 0.2 Hz to the big systems above

3,000 MW

Underfrequency load shedding:

The starting value of the ?rst step: 49.1 Hz

The difference between steps: about 0.25 Hz The allowable sustained time (under 47 Hz): °?5S Reserve capacity standard:

Spinning reserve: 2-5% of peak load Emergency reserve: 10% of peak load Maintenance reserve: 8-15% of peak load

The exact value is determined according to the sys-tem conditions such as system capacity, maximum capacity of single units, composition of hydro and thermal generating units and so on.

Load following capability:

Thermal Units (Coal): 20-35% of rated power Thermal Units (Oil): 45% of rated Power

All the three criteria mentioned above have been implemented in system Planning, operation and interconnection nationwide.

2.3 Comparison Between China And Industrialized Countries regarding The Power Network Intensity China has vast territory and her power networks have wide coverage compared to some industrial-ized countries with relatively smaller territory and higher load density. Under the same installed gen-erating capacity, China power networks need more capital investment for transmission and substation constructions than others. The following compari-son, based on the same basic conditions while tak-ing into account the aforementioned issues, results in a clearer conclusion.

(1). The indices and conditions adopted in the com-parison

A. Comparison between French Power Network and East China Network, whose industries are more developed and the load density is higher regions in China.

B. The indices adopted for comparison:

Density of installed capacity =

system capacity/Region's (country's) territory area Transmission loading index =

system capacity/total length of 220 kV C. The comparison re?ects mainly the relationship between transmission investment and power source construction; therefore, the 500 and 400 kV can be converted to the equivalent length of 220 kV lines

based on their capital investment. According to the data provided some materials, the converting fac-tors for 500 and 400KV lines are 2.205 and 1.773 respectively.

(2). Results of comparison

A. The comparison of time of the commissioning of ?rst EHV lines (500 or 400 kV)

The ?rst French 400 kV line was commissioned in 1975 while the density of installed capacity was 29.64KW/km; the ?rst 500 kV line in East China power network was commissioned in 1987 while the density of installed capacity was 55.56KW/km. This implies that the ?rst 500 kV in the East China power network was commissioned later.

B. The comparison of transmission lines loading This comparison is based roughly on the same den-sity of installed capacity, for example, the density during 1959-1967 in France was about the same as the period of 1981-1989 in East China, while the line loading of East China power network as shown in the Fig.3 was far higher than that in France.

In general, the standard is lower than that of most industrial countries, for instance, NERC stipulate that the system shall be in normal service under severe conditions such as 3-phase fault, but it has better coordination between safety and economy. according to 1980's nationwide statistics of system disturbances the light disturbances like single phase faults shared 85% above of the total and the others occured much less. Consequently, the sys-tems can ensure normal service to a very great extent.

59 60 61 62 63 64 65 66 67

CIGRE论文我国联网经验37_305_1996[1]

Fig.3 The loading indexes of East China and

French power network 3. REGIONAL INTERCONNECTIONS

CIGRE论文我国联网经验37_305_1996[1]

OVERVIEW AND DEVELOPMENT

3.1 Regional Interconnection Overview

To promote regional interconnection, make the bet-ter utilization of energy resources and get more bene?t from load diversity are important functions of NPCC.

In the early stage the regional interconnections were characterized by weak and low tie-line capac-ity and operated in the way of energy exchanges. The several AC interconnection projects (Fig.4) were not successfully operated because of instabil-ity and electromechanic oscillation problems.

Fig.4 The exist regional interties

By summarizing experiences of regional intercon-nections the decision of a ± 500 kV, 1,200 MW HVDC tie-line connecting Central and East China regional systems was made and it was put into operation in 1989.

(1) A test of Central and North China regional sys-tem interconnection

In order to get performance of regional system interconnection the NPCC conducted the test in 1982. Based on detailed system analysis the fault preventive measures had been carefully prepared and the 2 systems with the capacity of 7,500 MW and 4,500 MW each were interconnected by an exist 110 kV line. An electromechanic oscillation which reached 30 MW of maximum took place. The system analysis and test data shown that the tie-line transfer limits was 200 MW correspond to 4% of the Central China system's capacity.

(2) The system interconnection of Northwest and Southwest (Sichuan and Guizhou) regional sys-tems.

A hydro power station, Bikou, with three 100 MW units, located on the boundary of the 2 systems with capacity of 4,000 MW each were built to transmit power to the 2 systems through two 220 kV lines (Fig.4). 40-50 MW power ?uctuation fre-quently happened at the beginning of the intercon-nection. To avoid the instability incident effect to each systems overload tripping was adopted. Because of tie-line control dif?culty the line were frequently tripped. After installation of simple TBC the the trips were signi?cantly alleviated from 110 times in 1984 to 30 times in 1986. Along with the expansion of the 2 systems the tie-line control got so dif?cult that the 2 systems had to be sepa-rated.

(3) Development of South China interconnected system

Guangdong was connected to Hongkong system by double 66 kV lines in 1979 and gradually installed 132 kV lines up to 6 in 1980's and ?nally con-nected by four 400 kV lines in 1993. Before the construction of 400 kV tie-lines low frequency ocsillations arose with 80-90MW of maximum.

A 465 km long, 220 kV, Guangxi-Guangdong tie-line was come into service in 1985. The transmis-sion power was limited to 150 MW but the capac-ity had not been utilized for the same reason with Northwest and Southwest interconnection men-tioned above and shut down after two years opera-tion.

The South China system including 4 provincial systems and HongKong system had been com-pletely interconneced by a 1000 km, 500 kV line untill 1993. One of the 4 systems, Guizhou was splitted from Southwest region. The west three provinces are energy rich areas and Guangdong are lack of energy. Since 1993 energy exchanges reached to 10 TWh, net energy transmission to Guangdong 4.5 TWh and maximum power 1,000 MW. Signi?cant economic results have been gained from the intertie but the long distance and a large amount power transmission caused 3 times of instability faults. The instability preventive mea-sures have been elaborately equiped to cope with the problem afterwards. The interconnection inten-sity of the west 3 energy rich provinces will be enhanced by 1000 km approximate long, 1,800 MW, ± 500 kV HVDC line to send power to Guangdong. (4) Central and East China HVDC link project The 1,045.7 km, 1,200 MW and ± 500 kV HVDC transmission project was fully completed in 1990 and directly controlled by NPCC. The capability of the project has not been completely utilized because of shortage of power supply and electricity tariff problem, consequently, the recorded maxi-mum power transfer reached 450 MW. Neverthe-less, capacity will be fully utilized to transmit power from the Three-Gorges hydro power station to East China.

3.2 Further Projected Regional Systems Interties

The Fig.5 shows the regional interties till 2010. The projected interties associated with related gen-erating projects are presented below.

CIGRE论文我国联网经验37_305_1996[1]

Fig.5 The regional interties till 2010

(1). The transmission project from Yangcheng gen-erating station to East China

The transmission project and Yangcheng power station are jountly invested by some domestis power companies, local governments and foreign investors. The power station with six 350MW ther-mal units, geographically located in North China, will be commercially operated in 2000.

For the technical, economic and organizational problems the two regional systems will be still operated separately so the power station without any connections to North China system will directly transmit power to East China through 700 km long and 500 kV lines. The lines are divided by 2 switching stations into 3 sections, or the interme-diate section with 2 lines in parallel equipped with 40% line reactance of series compensator and the other two sections with 3 lines in parallel each.

The possibility, HVDC back-to-back intertie con-necting the two systems at Yangcheng power sta-tion, will still exist along with the system expansion.

(2). Northwest China and Sichuan intertie

The 1,200 MW, 600 km HVDC transmission scheme has been decided to send power from the boundary thermal power station, Boji, with capac-ity of 1,800 MW, to the densely loaded area of Sichuan system. The reason for HVDC solution is mainly regarded as weak AC intertie related prob-lems after Sichuan is grouped into Central China by 2 long lines from Three-Gorges hydro power station. The power transmission to Northwest China will be implemented by ?ve 330 kV lines.

(3). The transmission scheme of Three-Gorges hydro power Station

From 2004 to 2010, the 26 generating units of the world largest station, located in Central China, with capacity of 18,200 MW will be put into operation one after another to transmit power to Central, East China regional systems and Sichuan system. The values of transmission capac-ity used in designing transmission system are 7,200 MW to East China, 2,000-2,500 MW to Sichuan system and the remained power to Central China itself. The 18 units will be operated normally in 4 separated groups (8,6,6,6 units each) with junction breakers in between. The con?guration of the sta-tion shown in the Fig.6.

Fig.6 The con?guration of Three-Gorges hydro

power station

A lot of studies have been undertaken to make comparison between AC and HVDC solution. The studies demostrate that AC scheme operating with the existing ± 500 kV HVDC line in parallel will cause these problems such as:

*Complicated instability preventive measures.

*Instability problem existed when loss of existing HVDC line and three-phase fault occured in some AC lines.

*More AC lines needed.

*Series compensation needed.

*Complicated electromechanic osillation modes. *Dif?cult tie-line power control.

On the basis of comprehensive study and technical and economic comparison for Central and East China interconnection the HVDC solution has been made. The two systems will be intercon-nected by two 1,000 km approximate long, 3,000 MW HVDC interties from the station and the exist 1,200 MW HVDC intertie in addition at that time. Sichuan system will be connected into Central China by two 500 km long, 500 kV AC lines. Some system study shows very serious electromechanic oscillation exsited after 3-phase fault at sending point because of so weak interties comparing to 28,850 MW of Sichuan system and 69,680 MW of Central China system in 2010. More detailed stud-ies are needed to choose adequate measures to improve system stability.

4. PROSPECT OF REGIONAL INTERCONNEC-TIONS FOR 2020

Generally speaking, the motives of regional inter-connections are outlined in the following aspects: utilization of load diversity, capacity transaction, and energy transmission. The regional interconnec-tions will be based on the equal right and bene?t sharing principles. For the reason of serious imbal-ance of loads and energy resources distribution among regions as mentioned above, the incentives to regional interconnections in China are character-ized by long distance, a large amount of power transmission from the west to the east and energy exchanges by utilization of load diversity and capacity transaction between the north and the south. Based on long-term power sysem expansion planning, the Northwest and Northeast China could keep balance between generation and consumption by 2020 approximately; seriously energy lacked regions in the east need a large amount power

CIGRE论文我国联网经验37_305_1996[1]

importation. The only concept widely accepted is

that certainly the hydro energy aboundant south-west region will export electricity to East China, Guangdong(GD) province and small amount to Central China. According to feasibility study of hydro power exportation from Sichuan and Yunnan to EC, CC, GD systems, superior economic results are certainly existed comparaing with the solution of building thermal power stations within the regions. Sichuan province, one of the richest area in hydro power resources in China, owns 91,000 MW hydro power potential and shared 26.8% of national total.

Associated with construction of Three-Gorges hydro power station, Central, East China and Sichuan systems will form a huge Mid-China sys-tem with the capacity of 200,000 MW; the hydro energy rich southwest provinces will transmit more electricity to the fast developed coastal provinces in the southeast to expand South China system; Northwest China could be connected to thermal power predominant North China to make the better utilization of thermal and hydro power energy.

In the reformation of electric power industry, the market oriented structure are most recommended, consequently, it is expected that the newly founded generating companies and distribution companies will make electricity transactions through the national owned transmission grids.

5. SUMMARY

5.1 The reasons why the existed regional intercon-nections have not been successful are mainly:

*Weak AC intertie related problems such as insta-bility, electromechanic oscillation (low frequency oscillation, random load ?ow ?uctuation), dif?-culty of tie-line control etc.

*Interconnection agreements not be fully imple-mented.

*Energy exchange pricing problems.

.

5.2 Centralized by Mid-China system, the three huge electric power systems will be formed in northern, southern and middle parts of China and could be separated by HVDC lines in between. The main missions the Ministry of Electric Power is facing are how to realize long distance and a vast amount of power transmission from the west to the east and how to solve the related technical, eco-nomic and organizational problems. Following the mission, the Ministry of Electric Power are orga-nizing to draft some planning and operating guide to promote regional interconnections and keep sta-ble and economic operation of interconnected sys-tems.

5.3 Planning and operational experience has shown that the safety standards stipulated in both "Stabil-ity Criteria" and "Technical Criteria" conform well with technical and economical performance and national conditions. Thus, they are signi?cant for the healthy development of power grids. Despite being well below the level of industrialized coun-tries, power system development in China can still ensure safety and stable system operation if the two criteria are met.

REFERENCES

(1) "Guiding Principle For Power System Stabil-ity" (In Chinese), Ministry of Electric Power of P.R.China, 1981.

(2) "Guiding Principle For Power System Technol-ogy" (In Cninese), Ibid, 1984.

(3) "Comparison of Cost and Bene?ts For AC and DC Transmission", J.P.Stovall etul, ORNL-6204, 1987.