10-04-2017, 09:20 PM
Power system security is the ability of power system to survive in looming
disturbances conditions (contingencies), without hampering to the safety, reliability and
customer service. Power system security refers to strength or robustness of any system to
imminent disturbances and it depends on system operating conditions and also on contingent
probable conditions. When the system is of insufficient security, it gets exposed to
catastrophic and system failure. So it is of paramount importance in having a safe, reliable,
continuous and economic operating condition of power system.
Power system security is the probability of system s operating conditions which should
remain within the tolerable ranges. This aspect plays an important role in point of view its
operations and planning. Following are few points which make the power reliability and
safety over a long run. Firstly, power system should be properly designed with taking security
as main concern. Secondly, regular monitoring during operation, maintaining with acceptable
ranges is second most concern. Thirdly, good engineering is required to achieve these goals
which mainly rely on the usage of tools used for power system analysis. The changes that are
occurring in the environment have finely tuned the requirement of power system security
analysis and its assessments and has also changed the analysis tools of power system.
Power system consists of numerous of electrical based devices and is a complex network in
itself. And failure of any of these devices during operating condition hampers the continuity
of operation, security, safety and thus leads to outages, thus influencing security of power
system. Thus power system security is an important part of power system. The most
important aspect is evaluation of contingency, which leads to bus limit violations,
transmission line overloads during the operating conditions. Critical contingencies must be
identified firstly and fast to ensure secure, reliable and continuous operation.
As a major and important part of power system security , operational engineers need to study
the effect of outages and contingency on power system in terms of severity. Power flow or
load flow are important part of this analysis. They are required to have proper operation,
scheduling, and controlling of existing and operating power system and also on proper
planning for future expansion.
Contingency selection or contingency screening is a process in which probable and potential
critical contingencies are identified for which it requires consideration of each line or
generator outage. This method is very time consuming as it does not suit real time
requirement, as real time systems are large systems and requires lot of time for computation.
To solve this problem a number of algorithms have been developed which are classified into
two methods. One of those method is the Performance Index (PI) which is based on
calculation the PI values and ranking them accordingly to quantify the severity for each case.
The other method is based on approximate power flow which is used to eliminate those
critical contingencies. This method is known as screening method.
There are number of methods for evaluation of contingency of power system. AC load flow
and mathematical calculations are mostly used in most of the methods.
For contingency screening several methods have been developed. The most widely used
method for calculation of the performance index is based on the conventional method known
as Newton Raphson load flow program [7]. The modern power system is a complex network
and due to its complexity and large scale networks, contingency analysis should be powerful
and computation should be fast. The most important point for contingency or outage is that
all possible outages does not affect overload in lines and transformers, and does not affect
voltage drops in different nodes of system. Therefore it is not required to consider all possible
outages for computer simulation purpose. It is necessary to specify the outages which can
cause the most of overloads and voltage drops in the system for contingency screening. Such
critical and potential contingencies should be quickly identified for further evaluation process
in detailed manner. thus contingency selection is defined as the process of identifying these
critical contingencies. Thus contingency selection/screening or contingency ranking is
projected so in order to rank those outages which will violate the normal operating condition.
Contingency selection methods are based on Performance Index (PI), that may represent a
line overloading or bus voltage drop limit violation. Then sorting of performance index is
done in such a way contingencies are ranked according to their severity. In these last few
years, a lot of work has been done in this part which consists of selection of the potential
contingencies cases by using ranking methods or screening methods. Bounding methods ,
Distribution methods , Expert and new method for contingency selection , Neural
Network and other latest mathematical techniques have been used in the indirect
calculation of MW flow violation ranking. The recent developments using Artificial Neural
Network have brought lot of advancement in the speed of contingency screening.