A New and Comprehensive Classification of Fault Related Operations in Power System
Generally, operating procedures associated with faults in the power system can be classified as follows:
1-Pre-fault stage :
Operations in this stage aimed to preventing of faults and so interruptions in the power system. In this way, power system’s reliability will increase. So this operations is called reliability centered maintenance (RCM). This type of operation itself is classified into three categories : preventive maintenance (PM), predictive and preventive maintenance (PPM ) and condition monitoring (CM).
2-Concurrent with fault stage:
At this stage, the fault must be detected and isolated from the power system. This operation is done automatically by the protection system.
3- Before fault locating :
At this stage, the approximate range of fault locations will be determined. This stage is called power system step restoration. In this stage, operators by re-closing of some circuit breakers in power system and monitoring that they trip again or not, can estimate a more accurate range of fault locations.
Even in some cases, because of transient faults, the whole of system may be restorated (fault natural clearance). Sometimes auto-reclosers and sectionalizers is used to do this step automatically.
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Comparison of the reliability of low voltage and medium voltage networks
In all books written about the reliability of power systems, it is explicitly stated that the reliability of medium voltage networks is lower than similar (overhead/underground) low voltage networks. In the other words, the failure rate in the unit length of a medium voltage network (in a definite period) is more than the similar low voltage network. However, these books does not explains why this is so.
One of the main reasons that cause to decreasing of medium voltage networks’ reliability, is the electric field intensity. Two important phenomena are the main source of faults in the overhead and underground networks respectively, fault path carbonization and water treeing (polymers reaction with water in XLPE cables).
Figure 1 : Water treeing in a XLPE cable joint
It is worth noting that these two phenomena can form only in the presence of voltages higher than 5 kV. In the other words, the voltage gradient plays both the activator and catalyst roles in water treeing and fault path carbonization mechanisms. For this reason, in low voltage networks, the occurrence probability of water treeing and fault path carbonization (the most important factors that tends to network aging and fault occurrence in MV networks) is almost zero.
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Eigenvalues and Eigenvectors
The effective analysis of any of complex dynamic system requires understanding the concepts of “eigenvalues and eigenvectors” . These concepts generally are introduced during linear algebra course in university. Innovative and beautiful presenting of these concepts at “MIT’s Open Course of Linear Algebra” at the following address (specially lectures 21 and 22) will build deeper and more subtle attitude which is necessary to the engineering courses such as Power Systems Dynamics .
http://ocw.mit.edu/courses/mathematics/18-06-linear-algebra-spring-2010