It is well known that power system is a complex network consisting of numerous equipment like generators, transformers, transmission lines, circuit breakers etc. Failure of any of these equipment during its operation harms the reliability of the system, hence leading to outages. Thus one of the major agenda of power system planning and its operation is to study the effect of outages in terms of its severity. Installation of redundant generation capacity or transmission lines is essential in order to make the system run even when any of its components fail [1]. In steady state security assessment of a power system, it is important to predict the line flows and bus voltages for different operating conditions and network topologies of a power system [2, 3]. But, power system being dynamic in nature, does not guarantee that it will be 100 % reliable. Further, such arrangement may not be cost-effective. Hence, a detailed security assessment is essential to deal with the possible failures in the system, its consequences and its remedial actions. This assessment is known as power system security assessment. Power system security involves system monitoring where the real time parameters of the system are monitored by using the telemetry systems or by the SCADA (Supervisory Control and Data Acquisition) systems. It then involves the most important function of contingency analysis where the simulation is being carried out on the list of credible outage cases, so as to give the operators an indication of what might happen to the power system in an event of unscheduled equipment outage. This analysis forewarns the system operator, and allows deciding some remedial action before the outage event. For a power system to be secure, it must have continuity in supply without loss of load. For this, security analysis is performed to develop various control strategies to guarantee the avoidance and survival of emergency conditions and to operate the system at lowest cost. Whenever the pre-specified operating limits of the power system gets violated, the system is said to be in emergency condition. These violations of the limits result from the contingencies occurring in the system. Thus, an important part of the security analysis revolves around the power system to withstand the effect of contingencies. The system security assessment process is carried out by calculating the system’s operating limits in the pre- contingency and post contingency operating states at an operation control centre or at the Energy Management System (EMS) of the utility company. The contingency analysis is time consuming as it involves the computation of complete AC load flow calculations following every possible outage events like outages occurring at various generators and transmission lines. This makes the list of various contingency cases very lengthy and the process very tedious. In order to mitigate the above problem, automatic contingency screening approach is being adopted which identifies and ranks only those outages which actually cause the limit violation on power flow or voltages in the lines. The contingencies are screened according to the severity index or performance index where a higher value of these indices denotes a higher degree of severity.

Unpredictable condition in the power system is known as contingency. The operation and control of power systems in real time require that a number of supervision functions be executed in the Control Center. First of all, the current operating condition of the system (commonly referred to as base case) is obtained through state estimation from real time measurements and data base information. Once the base case is known, the security analysis function is carried out. It is well known that, security analysis is a very demanding task as far as computational effort is concerned. In case violations are detected for the base case, corrective control actions should be enforced so as to eliminate them. Afterwards, the impact of the occurrence of contingencies should be evaluated. This process, usually called contingency analysis, aims to detect post-contingency operational limits violations. Power systems are operated so that overloads do not occur either in real-time or under any statistically likely contingency. This is often called maintaining system “security” Simulator is equipped with tools for analyzing contingencies in an automatic fashion Contingencies can consist of several actions or elements. Simple example: - outage of a single transmission line. Complex: - outage of several lines, a number of generators, and the closure of a normally open one. The contingency analysis is required to operate the power system in such a way that power is delivered reliably. Within the constraints placed on the system operation by reliability considerations, the system will be operated most economically. The primary purpose of maintaining power system security is to keep power system operation under stable condition, such that the single line failure does not lead to cascade tripping and overall blackout. Any piece of equipment in the system can fail either due to internal causes or due to external causes such as lightning strikes, objects hitting the transmission towers or human errors in setting relays. As a consequence of above disturbances, single failure lead to cascaded tripping and overall blackout.

Two major types of failure events

• Transmission line outages.
• Generation unit failures.

Transmission line failures cause changes in the line flows and voltages on the transmission equipment remain connected in the system. Therefore the analysis of transmission line failures requires methods to predict these line flows and voltages, so as to be sure they are within their limits. Generation failure can also cause line flows and voltages to change in the transmission system with the addition of dynamic problems involving system frequency and generator output. Therefore, given failure causes serious problems in both the configuration of existing system and its operation [4]. Contingency evaluation for power system using artificial neural network is presented in [5]. Contingency evaluation for planning and operation is presented in [6].

A secured system is one which has the ability to undergo a set of disturbances without getting into an emergency condition. In other words, a normal operating condition of a power system is considered to be secured if there is neither any occurrence of overloading of any equipment nor transient instability due to a set of credible contingencies. The secured condition satisfies not only the loading and operating constraints, but also a third set of constraints known as “Security constraints”. Security constraints include the loading and operating constraints for each of the credible contingencies and can be expressed as

S(X, U, P) ≤ 0

If the above security constraints are not satisfied, the system becomes insecured and operating condition is known as “ALERT STATE”. Contingency evaluation is needed to determine whether the prevailing normal operating condition is secured or not. The Contingency evaluation methods differ from load flow solution methods with respect to accuracy and speed. The requirement of contingency evaluation methods is that they should be fast and relatively approximate. Because the control actions have to be taken fast by the power system operators. Hence, the method of contingency evaluation should be fast and less accurate. In order to achieve this, the method comes with some valid approximations. The Contingency evaluation is carried out and the power system operators are provided with the information like over loaded lines and under loaded lines and the lines which are at the verge of crossing limits. Among several contingencies that occur in the power system, it is not possible to evaluate all the contingencies to know whether the system is secured or not. For this purpose, contingencies are ranked. This can be decided from the probability of their occurrence within the next short period. In most of the power systems contingencies like

• Outage of generating unit.
• Single line outage.
• Double line outage.
• Generator and single line outage.
• Generator and double line outage.

Any of the problems which occur in the power system can cause serious trouble, so to avoid this we go for a contingency analysis. The major factor which affects the power system security is both internal and external causes.

The calculations made with the help of network sensitivity factors for contingency analysis are faster, but there are many power systems in which voltage magnitudes are the critical factors in assessing contingencies. The method gives rapid analysis of the MW flows in the system, but it cannot give information about MVAR flows and bus voltages. In systems where VAR flows predominate, such as underground cables, an analysis of only the MW flows will not be adequate to indicate overloads. Hence the method of contingency analysis using AC power flow is preferred as it gives the information about MVAR flows and bus voltages in the system. When AC power flow is to be used to study each contingency case, the speed of solution for estimating the MW and MVAR flows for the contingency cases are important. If the solution of post contingency state comes late, the purpose of contingency analysis fails. The method using AC power flow will determine the overloads and voltage limit violations accurately. It does suffer a drawback that the time such a program takes to execute might be too long. If the list of outages have several thousand entries, then the total time to test for all of the outages can be too long. However, the AC power flow program for contingency analysis by the Fast Decoupled Power Flow (FDLF) provides a fast solution to the contingency analysis since it has the advantage of matrix alteration formula that can be incorporated and can be used to simulate the problem of contingencies involving transmission line outages without re inverting the system Jacobian matrix for all iterations. Hence to model the contingency analysis problem, the AC power flow method using FDLF method has been extensively chosen. Table 1 shows the Load Data and Table 2 shows the Line Data.

Mi-Power is a highly interactive, user-friendly windowsbased Power System Analysis package. It includes a set of modules for performing a wide range of power system design and analysis study. Mi-Power features include a top notch windows GUI with centralized database. Steady state, transient and electro-magnetic transient analysis can be performed with utmost accuracy and tolerance. Table 3 shows the Generator data and Table 4 shows the Load level.

The IEEE 6-Bus System is shown in Figure 1.
System Data:
MVA Base=100 MVA
System Frequency = 60 Hz
Bus Nominal Voltage =11KV
Bus Maximum Voltage = 11.5 KV
Bus Minimum Voltage = 10.4 KV

In this section, the results of contingency analysis problem using FDLF method implemented in mi-power software for different loading conditions are presented. Table-5 gives Variations of real and reactive power generations and losses for different loading conditions. Table-6 gives Variations of bus voltages at different buses for different loading conditions. Figure 2 represents variations of bus voltages at buses 5 & 6 for different loading conditions.

Figure 1. IEEE 6-Bus System

Table 1. Load Data

Table 2. Line Data

Table 3. Generator Data

Table 4. Load Level and Loads at Each Bus

Table 5. Variations of real and reactive power generations and losses for different loading conditions

Table 6. Variations of bus voltages at different buses for different loading conditions

Figure 2. Variation of bus voltages at Bus 5 & Bus 6 under line 3-4 outage case for different loading conditions

Thus from the above case study and results obtained from mi-power software for contingency analysis with single line outage, cases have been done for different loading conditions. It concludes that the MW and MVAR generation and losses are increased with variation of load. Also, bus voltages at bus 5 & 6 decrease with increase in load, which will be helpful in identifying the over loading condition of transmission line and identify the severity level of the system & weak cluster of system.