Power Quality

Discourse about the challenges of technology development in three examples

Although the dominant view to technology is optimism, but there are some evidences that casts doubt it. Here are three examples that demonstrate this point :

1- Changing the models (curves) of  supply – demand: Studies show that use of the CFLs in the UK has led to increased power consumption in lighting sector. This situation can be explained by economic theory of  “William Stanley Jevons” . According to this theory, technological change leads to changes in the supply and demand curves. In other words, the demand curve of  lumen (lighting flux) changes due to the cheaper price of the lumen so that the final consumption increases. Thus, the electric utilities which switch from incandescent lamps to CFLs and LEDs lighting technology to reduce power consumption in the coming years, will be quite disappointed.

2- Changes in construction presuppositions: About two month ago, when the IEEE-519 2014 standard was published, electrical equipment manufacturers and  power system operators were amazed; because the limit of THDV increased from 5% to 8% to respond the development of renewable generation technologies. Thus a large variations in the design and manufacturing of electrical equipment in order to improve the electromagnetic compatibility, expected. Also it can be predict that meters manufacturers in the worldwide create more pressure on IC manufacturers to change their energy measurement model based on IEEE  1459–2010 standard. This standard follows the instantaneous power theory of Alexander Eiegeles Emanuel instead of  the of  Bodeanu’s power theory. With this action of IEEE power quality work group, it can be expected that the limit of THDV  increase to 20% until 2020. With such amount of harmonic distortion, the overall philosophy of the design and operation of power systems,  in the demand, generation, transmission and distribution sides will change.

3-Information overload: Smart grids is based on data more than anything else. The role of  the data in engineering systems can be analogized with the role of vitamins in biological systems. Excessive data, beyond the management capability of these systems, transforms to the load of system and prevents the flexible performance of it. The cost of building and maintaining data centers, as well as cyber security for information systems is very high and so the lack of value-added production makes these systems useless overload. Thus, the development of smart grids without having an effective strategy in benefiting from value-added  (such as improving utilization  and increase the life of electrical equipment), may tends to electric utilities bankruptcy.

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.

Relationship Between Reliability and Power Quality

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.

Inter-harmonics, A Modern Source of Flickers

Inter-harmonics, specially first and second order inter-harmonics, in modulation with fundamental frequency, 2nd order harmonic, or also (with lower probability) higher order harmonics, can produce intensive flicker.

One of the main sources of inter-harmonics is VSD (Variable Speed Drive). Specially in places where lighting loads are powered by the UPS, due to the low short circuit  level of  DC bus,  inter-harmonics have a high potential to generate flicker.

Based on “William Stanley Jevons’  theory” , this problem also can be theorized as follows:

1 -Increasing electricity consumption prices leads to using more and more energy saving technologies such as VSDs.

2 – Using VSDs leads to increasing of inter-harmonics and also flicker.

3 – Flicker leads to Increased eye diseases.

4 – Increased eye diseases  leads to increased treatment costs.

So costs will be balanced again near the previous point.

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