Page 219 - Hybrid-Renewable Energy Systems in Microgrids

Smart grid and power quality issues 199

Rapid voltage changes cause flickers [21]. The rapid voltage changes occur by the
switching action of the on load tap-changer (OLTC) action. The magnitude of rapid
voltage changes depends upon the number of steps in a tap-changer. The value of
2%–2.5% is mentioned for tap-changer [22,23].

3.2 Short duration under voltages

Short duration under voltages refer to the voltage sags caused in the distribution sys-
tem. The study in [22] shows that the equipment trip due to this kind of PQ issue is
negligible, and therefore it is considered as not quite dangerous. This is related to the
studies done for a short scale of time. The study is made with a very limited data avail-
able and therefore requires further analysis with respect to longer monitoring periods.

3.3 Harmonic distortions

The total harmonic distortion (THD) values of current and voltage tend to change fol-
lowing a reconfiguration. The mix of loads due to reconfiguration may contribute in
the increase of decrease of the THD values. The reconfiguration may also lead to in-
crease in transformer energization number, which contributes to the second harmonic
distortion. Transients are likely to occur due to transformer energization due to the
change of load towards lower resistance [24]. Using smart meters of high harmonic
energy and distributed filtering provides minimum network resistance for the funda-
mental harmonic. Measurement of high harmonic energy and settlement of legal is-
sues related to payment for this energy should encourage power supply companies and
consumers to take technical and organizational measures to decrease high harmonic
level in the network. These measures should lead to the improvement of the supply
network voltage waveform, which will enhance the efficiency of electric energy use
by consumers [25].

3.4 Switching transients

Use of switched capacitor banks lead to switching transients. Also multiple capacitor
banks lead to back-to-back switching transients, which contribute to higher oscillation
frequency. This leads to the further amplification of the oscillations. The magnitude
of the capacitor energizing transient is directly related to the damping available in the
network. Studies related to the limits of damping are required for switching transients
in a distribution network [26].

3.5 Power quality concerns related to demand side management
The major PQ concerns due to the implementation of DSM schemes in smart grid
[27] involve four entities as follows: (1) change in the characteristics of the grid due
to changes in the loads connected and disconnected contributing to the increased level
of harmonics and voltage variations, (2) the load recovery period after DSM based
load reconnection is high causing higher levels of harmonic emission, (3) frequent