Page 218 - Hybrid-Renewable Energy Systems in Microgrids

198 Hybrid-Renewable Energy Systems in Microgrids

harmonics. The control of harmonic currents drawn by a load converter, or the voltage ripple
produced by a DER converter is achieved using electro magnetic interference (EMI) filters,
which are usually placed at the connection with the DC bus.
2. the capacitance of EMI filters can draw a substantial inrush current when a load connected to
the DC system is switched on, causing voltage oscillations at the point of connection of the
load. When they exceed the operating voltage range, some loads may be switched off [16].
3. faults occurring on DC systems are supplied through DER converters and limited by the
converters’ power ratings and associated protection/control circuits. To increase fault cur-
rents to the values that will trip the system protection, the EMI filter's capacitance needs to
be increased, which will then imply higher inrush currents.
4. the choice of an appropriate grounding configuration for a DC distribution system has an
impact on the PQ and safety, particularly during faults. The type of grounding will determine
the path of current for a ground fault and impact the level of fault current, including electric
shock hazard of a person in contact with affected fault circuit [17].

3 Potential impact on power quality

The following topics discuss about the major PQ issues, which have significant PQ
effects in smart grids [18]. The PQ disturbances are categorized into those originated
within the microgrids and outside the microgrids. The microgrids can operate in Is-
landed or stand-alone condition during momentary disturbances such as the voltage
sag or voltage dip condition. This sudden change also contributes to some distur-
bances although DC microgrids can act in a more stable condition during this kind of
disturbance. The modern flexible AC transmission systems (FACTS) and resilient AC
distribution systems (RACDS) technology help in the transition of the transmission
networks and distribution networks to Smart grids respectively [19]. Modernizing the
transmission and distribution networks ensures smooth transition to the smart grid
environment. The RACDS enable increased penetration of renewable energy sources
in the distribution systems and also mitigate the PQ issues to improve the reliability
of power supply to critical loads. A microgrid can be essentially called upon as one
RACDS. Microgrids improve resiliency and controllability, and the interface power
electronics are classified as controllable RACDS devices.

3.1 Slow and rapid voltage variations

The voltage profile of a grid can be classified into two conditions: slow voltage varia-
tions and rapid voltage variations. These voltage variations or perturbations in voltage
level are concerned to the transient due to connection and disconnection of genera-
tors to the grid. The distributed generators contribute to the perturbation of levels of
voltages in the grid. Rapid voltage changes occur, when active power is injected to
the grid [20]. Also voltage level changes, when power is injected in the middle of the
line. Conservation voltage reduction (CVR) is applied for active power is utilized to
reduces the supply voltage to reduce the electricity consumption of the low voltage
consumers. The voltage range is expected to be maintained between 0.94 and 1.1 pu.
CVR helps in keeping the voltage level of the distribution network as low as possible.