【正文】 t violate both the MW margin and Mvar margin criteria. The contingency ranks according to the MW margin. The first contingency, . the generator Ji GX, has the smallest MW margin.C. Assessment for contingency binationsLet n be the number of local generators plus transmission elements in a load center system. Each element i , is represented by a random variable ξ iI， i =1, 2, ..., n, and di is submitted to the distribution: (1)To a local generator or an interface line, 0 state means that it is out of service and 1 state means that it is in service with rated capacity.The system state x will be defined as x = (x1, x2,…,xn ) . The system state will have 24 distinct states in the state space S.Let Pr{X = x} be the probability associated with a state X. X is a random vector. Then (2)where element states are assumed independent.When the state space consists of more than 20 elements, the putation burden is huge, even impossible. For example, to the binations of 20 transmission lines and 20 generators, there are 240 states in the state space. The number is approximately , therefore it is impossible to pute.A state space truncation method used in [12][13] is adopted to reduce the scale of the state space. In this approach, system states are generated in an increasing order of overlapping outages. If the probability of generated states exceeds a specified level (usually 95%), the remaining states are neglected. In most cases, a level corresponding to three overlapping outages satisfies this constraint for power system. 。 therefore the calculation burden for detailed N1 even N2 contingencies is acceptable in the contingency screening to the interface. When what we are interested in are only the single or double interface contingency cases, more plicated algorithms for contingency selection and screening are needless.There are 20 transmission lines through the interface linking BPS to other systems. When the forecasted peak load equals to 9500 MW, as an example, the VSA results for the single interface line contingencies are listed in Table I.From Table I we can see that the system under all NI contingency cases doesn39。7. Cable lines are used widely in HV and MV networks and sometimes the percentage is very high, for example, in Beijing.By reason of the above facts, the load center area is more susceptible to voltage instability, so the VSA is of great importance for the power system in the load center. The VSA can be divided into offline and online assessments which deal with planning and operating aspects of power system respectively. The theories and methods on VSA have been summarized in detail in [2][4].In this paper, aiming at a specified load center, Beijing Power System (BPS), a static VSA framework considering practical request was built to aid the planning and operation of BPS.II. BEIJING POWER SYSTEM DESCRIPTION BPS is the most important part of North China Grid (NCG) and one of the biggest load centers in china. At and above 110kV BPS has over 5,822 km of lines and cables. The transmission network operates at 110kV, 220kV and 500kV and 220kV networks form the backbone of the transmission network while the 110kV network acts as a subtransmission network.The peak load of PS is 10,650 MW in 2005, 3/4 of which has to rely on faraway energy bases and only 1/4 of which is met with internal power plants. The interface between BPS and its neighboring systems consists of 20 transmission lines which include 14 500kV circuits and 6 220kV circuits. 220kV and 500kV circuits are respectively used as the channels of vicinal and remote systems.The BPS owns 42 local generation units and the total capacity is approximately 4629 MW. The single generator capacity is relatively small to the regional demand.Fig. 1 is the sketch map of BPS and its neighboring systems. According to the direction of power flow, Beijing, together with Tangshan and Tianjin, can be thought as a load center. However in this paper we only take BPS as a research object. From we can see BPS receives remote power supply from Neimeng, Shanxi and Hebei while supplies energy for Tianjin and Tangshan. In fact, the supply for Tangshan and Tianjin is very small.III. THE INDICES AND CRITERIAOne of the purposes of the static VSA is to identify and rank the equipment outages which reduce the security margin of power systems obviously. PV and VQ curve are generally adopted for the assessment of proximity to voltage instability [5]. PV and QV curve can be drawn by an excellent power flow calculation program which can deal with the actions of shunt capacities, generators and other regulating devices properly. Essentially the PV curve method regards the singular point of power flow Jacobian matrix as the collapse point of power system. References [6] and [7] indicate the calculation precision is acceptable. Though a PV curve is quite useful in assessing transfer or loading limits, it cannot identify the location of shortage of reactive and its cause. Therefore the VQ curve is employed to implement the task in this paper.Reference [8] is a guide on security and stability for operation and planning of power system in China. However it hasn39。In addition, in some countries load centers can possess features as follow:5. In MV distribution network large numbers of OLTC[OnLoad Tap Changer]exist。3. Local power plants cannot satisfy the local power demand。附錄2 文獻(xiàn)翻譯——原文Study on Static Voltage Stability Assessment for