【正文】 近幾年，無論是發(fā)達國家還是發(fā)展中國家，由于經(jīng)濟和環(huán)境保護的壓力，大部分提供電能的地區(qū)都遠離消耗電能的地區(qū)，這樣一來負荷中心就出現(xiàn)了。一般來說，負荷中心可以這樣描述：；。電力撤銷管制制度加劇了這一特點；；；除此之外，在許多國家負荷中心可能還有以下特點：；；，有時百分比非常高，比如北京。電壓穩(wěn)定評估可以分為離線評估和在線評估兩種，分別用在計劃好的和運營中的電力系統(tǒng)。本文以一個確定的負荷中心—北京電力系統(tǒng)BPS—為目標，一個兼顧實際要求的靜態(tài)電壓穩(wěn)定評估機制已經(jīng)建立，以幫助北京電力系統(tǒng)的計劃和運營。110KV及以上的系統(tǒng)共有5822Km的路和電纜，傳輸網(wǎng)絡運行在110kV, 220kV和500kV。2005年北京電力系統(tǒng)的負荷高峰是10,650 MW，其中3/4的電力不得不依靠遠處的能源基地，而內部發(fā)電廠僅能滿足1/4的電力需求。220kV和500kV分別當做鄰近和遠距離系統(tǒng)的通道。，由電力潮流的方向可知，北京，連同唐山和天津可以認為是一個負荷中心。、山西、河北的電力供應，同時向天津和唐山提供電力。靜態(tài)VSA的目的之一就是識別和分類設備故障，這些故障會明顯減少電力系統(tǒng)的安全幅度。PV 和 VQ曲線可以用一個優(yōu)秀的電力潮流計算程序畫出，該程序可以恰當?shù)靥幚聿⒙?lián)電容組、發(fā)電機組和其他調節(jié)裝置。參考文獻[6]、[7]表明這種這種計算的準確性是可取的。因此，為了完成評估任務本文使用VQ曲線。本文中，從PV曲線得到的系統(tǒng)平均裕度采用和WSCC同樣的標準，比如，當單條傳輸線路或一臺發(fā)電機跳閘時，平均裕度曲線不能低于5%，%，三者或更多組合時不能低于0。其中為當系統(tǒng)處于正?；A條件時QV曲線的最低無功功率，為 當系統(tǒng)出現(xiàn)意外事故時QV曲線的最低無功功率。在線研究和離線研究應該考慮不同水平下的不確定性，所以VSA的標準可能不同。如何區(qū)分兆瓦裕度和兆乏裕度更適用哪個指數(shù)是困難的，因為電力系統(tǒng)可以從不同的角度來描述。A. 傳輸界面意外事故評估對一個負荷中心來說，傳輸界面意外事故的評估是必不可少的，因為來自其他系統(tǒng)的大量電能通過該界面進入。如果界面線路在意外事故發(fā)生前處于重負荷條件下，應該給予更多的關注。當我們感興趣的僅僅是單個或兩個界面意外事故發(fā)生的情況時，更多用于意外事故選擇和篩選的復雜算法式是沒有必要的。例如，當預測高峰電量等于9500 MW時，Table I中列出了單個界面線路事故的VSA結果。意外事故通過兆瓦裕度分級，比如第五行，第一等級的意外事故有最小的兆瓦裕度。對一個負荷中心，如果界面發(fā)送端和接受端的相角相差很大，無功功率就不能通過界面?zhèn)魉?，即使兩者的電壓等級明顯不同。就像傳輸界面的意外事故評估一樣，負荷中心系統(tǒng)評估的完成可以只考慮單一當?shù)匕l(fā)電機組故障中斷的情況。因為有些機組的容量很小可以忽略，所以這里只給出了最嚴重的10種情況。意外事故通過兆瓦裕度分級，比如機組Ji GX，第一等級的事故有最小的兆瓦裕度。系統(tǒng)狀態(tài)x定義為x = (x1, x2,…,xn )，系統(tǒng)在狀態(tài)空間S有24 個不同狀態(tài)。那么 (2)每個元素狀態(tài)單獨設定。例如， 對于有20條傳輸線路和20個機組的組合，狀態(tài)空間會有240種狀態(tài)，這個數(shù)值大約，是無法計算的。在這種方法中，系統(tǒng)狀態(tài)是在重疊故障中斷的上升序列中生成的。大多數(shù)情況下，電力系統(tǒng)中三個重疊故障的水平就會達到這一限制。2. Transmission interfaces have heavy burden. This is because most of load centers are big city and it is difficult to build new corridors due to environmental protection. Power deregulation will deteriorate this feature。4. Load character takes on constant PQ feature which is insensitive to the change of voltage and frequency。6. Numerous MSCs and MSRs are used to regulate bus voltage。t defined the criteria for VSA. In this paper, the MW margin of system which is obtained from a PV curve adopts the same criteria used by WSCC [9], ., when single transmission line or a generator trips, the MW margin curve cannot be less than 5%, and % for any bination of two elements, and 0 for any bination of three or more elements.According to the QV curve, an index used to measure the reactive power reserve margin of bus i after a contingency is defined as:: the lowest reactive power of QV curve when system is under the base condition.: the lowest reactive power of QV curve when system is under the contingency condition.The Mvar margin of the system can be defined as:KQ = max（KlQ, K2Q,… ,KiQ） i = 1, 2 ,…, nn : monitored bus number.To this index, the adopted criteria in our practical studies are less than 30% for single contingency, 70% for double contingencies and 100% for three or more contingencies.Uncertainty for different levels should be considered in online and offline studies, so the criteria for VSA can be different. The above methods and criteria are adopted for online VSA in this paper.It is difficult to distinguish which index is better between MW margin and Mvar margin, because power systems are described from different angle of view. Based on the suggestions proposed in [2], the MW margin is adopted as the main index to assess the voltage stability for a load center while the Mvar margin is taken as the secondary index to aid the assessment.IV. VOLTAGE STABILITY ASSESSMENT FOR LOAD CENTERA. Assessment for transmission interface contingenciesThe assessment for the transmission interface contingencies are indispensable for a load center, because great amount of demand has to be imported through the interface from the other systems. When an important interface line trips, power will transfer to other interface lines, which increase the transmission power of other elements and decrease the transmission capacity of the whole interface, consequently the MW margin of the load center system will be reduced. If the interface lines are under stress conditions before a contingency, the study deserves more attention.For a specified load center, generally the interface linked to outer systems consists of finite lines up to several decades。t violate the MW margin and Mvar margin criteria. The contingency ranks according to the MW margin. The first contingency, . the line L5, has the smallest MW margin.B. Assessment for local generator contingenciesFrom another point of view, voltage instability can be considered to be that the supply of power system cannot meet the demand of consumers (under static or dynamic conditions), especially the deficient reactive reserve may be vital to power system voltage instability [10]. For a load center, reactive power cannot be transmitted through the interface if the angle is very different at the sending and receiving terminals of the interface, even though the voltage magnitude is obviously different [11].Thus, as important reactive sources, the status of local generators will exert distinct effects on VSA for a load center.Just like the assessment for transmission interface contingencies, the assessment for the load center system can be pleted when only considering single local generator outage contingencies. When the forecasted peak load equals to 9500 MW, under single generator contingencies the VSA results for BPS are listed in Table II. Because the capacities for some generators are very small and can be neglected, only the ten severest contingency are presented here.From Table II we can see that the system under NI local generator contingency cases doesn