Farhad Yazdi

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.

Reliability is a branch of power quality which investigates the impact of  “Electricity Interruptions” on customers. Interruption is an index of power quality which has two categories: short-term and long-term. In the “short-term interruptions”, indices of  momentary interruptions (with a duration between 30 cycles up to 3 seconds) and temporary interruptions (continued 3 seconds to 1 minute) and in the “long-term interruptions” category, the index of sustained interruptions (with continuity over than 1 minute) is defined. It must be mentioned that interruption refers to the reducing of voltage below the 10% of rated voltage.

In discussing the reliability of a power system, the dependent indices of interruption (eg. SAIFI, CAIDI, SAIDI, …), which evaluate the number of customers which is affected by the interruption and the duration of  interruption is studied.