## Power system

## 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.

### (continue reading…)

## 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