Page 40 - Hybrid-Renewable Energy Systems in Microgrids

24 Hybrid-Renewable Energy Systems in Microgrids

association with renewable energy sources. Application of these converters provides
an opportunity to achieve a sophisticated control over the microgrid operation [5]. In
addition, a suitable architecture for combined operation of microsource, converter,
and loads is necessary to achieve reliable and economical operation [6]. Section 2
of this chapter presents a review of different microgrid architectures discussed and
tried for reliable and efficient microgrid operation. Section 4 discusses various control
schemes devised for desired microgrid operation.

2 Microgrid architecture

Since the very beginning, it has been an issue of debate regarding the choice of
AC or DC as system of electrical power transfer. Initially, DC system was adopted
and used intensively for electric power supply. However, transmission power losses
and significant voltage drops along the DC line forced the researchers to search an
alternative. The development of transformer and AC transmission by L. Gaulard and
J.D. Gibbs certainly came as a relief, and AC power system was adopted quickly.
Another support to AC power system was given by Nicola Tesla when he developed
the polyphase system. A polyphase system gave a high-reliability component to AC
power network, and subsequently AC system dominated over DC system at the end
of the 19th century.
New incoming renewable-based energy sources have been identified as genera-
tors of clean electrical energy and therefore are being encouraged worldwide over
the conventional generators. The increasing penetration of small-sized renewable
energy sources into existing grid has created new sort of challenges for power engi-
neers. One of the most significant challenges is that most of these renewable energy
sources generate either DC output or variable frequency, or else variable voltage AC
output. Presence of these sources creates a new challenge to maintain the stability
of the existing grid. Therefore a suitable architecture is required so as to manage
the power flow within the different microsources and with grid. Three main archi-
tectures have been suggested in literature as a possible solution, namely, AC, DC,
and AC–DC hybrid microgrid. Fig. 2.1 presents electrical equivalent circuit of a
parallel converter microgrid system. With the help of a suitable droop control along
with voltage and current control, load on microgrid can be shared in a desired ratio.
Because load and generation both are intermittent in nature, therefore a mathemati-
cal analysis of dynamics of microgrid will be helpful in developing a suitable con-
trol system.

3 Mathematical analysis of microgrid structure

Dynamics of a parallel converter microgrid structure as shown in Fig. 2.1 is given as
follows:
p denotes differential function (d/dt) and i is for specific converter.
Dynamics of individual converter in d-q axis reference system is presented as,