Page 119 - Hybrid-Renewable Energy Systems in Microgrids

An overview of control techniques and technical challenge for inverters in micro grid 103

3.1 Centralized method

There are two different approaches to control the electronic inverter based microgrids
in this circumstance. The first is the centralized microgrid based on the communica-
tion links. For instance it offers another breed hybrid distributed network based power
control scheme [26]. This plan contains distributed power controllers that have been
mounted on every DG unit to guarantee exact following of the improved set focuses
appointed by the central energy management unit (EMU) [27]. The deliberate normal
power is sent to the EMU to compute the offer of every based on the real time optimi-
zation criteria [28]. In this way, a low data transmission correspondence connection
can be utilized. The proposed structure has good tolerance to correspondence delays.
Different methods such as master-slave, centralized load sharing, average load shar-
ing, current limitation control and circulating chain control had been developed pro-
posed to offer good voltage regulation and power-sharing accuracy but unlike droop
methods, the output voltage is closer to the nominal value [29,30].

3.2 Decentralized method

In the decentralized control strategy, each distributed energy source (generation or
capacity) works freely utilizing measured local signals. Subsequently, no source is
the reference and all sources are at a solitary control level. So removing or including
a source makes no aggravation in other source operations [31–33]. This technique
requires all resources to be dispatchable. Nondispatchable resources can be changed
to dispatchable ones by including an energy storage component. There is no require-
ment for the correspondence system and the absence of a reference source builds the
reliability of the system. These techniques manage the output voltage and frequency
based on the active and reactive powers delivered by the inverter [34,35]. Subse-
quently, better reliability and flexibility can be accomplished in the physical area of
DG units. Here, the agent-based decentralized control method, sliding-mode voltage
control and droop controls, which are more common for decentralized methods.

3.3 Hierarchical control
Hierarchical control is recently used to standardize the performance and functions of
microgrids. It has three different levels such as grid level, management level and field
level [36–38]. Grid level control is also called as tertiary control, management level
and field level are also called as secondary control and primary control. The main
objectives of the tertiary control are the optimal operation in both modes like capaci-
tance and inductance and power flow control in the grid-connected mode [39]. The
main objectives of the secondary control are to restore the microgrid voltage and fre-
quency, enhancement of power quality and synchronization between the microgrid and
main network. The main objectives of the primary control are to stabilize the voltage