Power system

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.

Right Choosing Of Film Capacitors for Outdoor Installation (Pole Mounted Capacitors)

Dissipation factor of capacitors  is a function of temperature and frequency (of supply voltage). In the other words, capacitors’ losses vary with temperature and frequency. Dissipation factor of capacitors has three additive components. The first and second components are related to changing of capacitors’ parallel and series (ESR) resistances due to temperature. Since both of these resistances increase with temperature, so these two components always are positive. The third component is related to intrinsic characteristic of dielectric material. Dielectric dissipation factor may increase, decrease or even remain constant with increasing temperature. So, cleverly choosing of film capacitors may decrease the probability of capacitor overheating and extends its overall life.

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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 .



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