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356 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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sources such as photovoltaic or wind power in microgrid are fluctuating, the stability and reliability
are a big concern for a microgrid system. Reliability is the essential requirement for applications
such as data centers, ships, aircrafts, and hospitals [5–9]. The resilience of the microgrid can be
achieved through careful design of architectures, controls, protection, etc.

14.1 HIGHLY RELIABLE MICROGRID

A microgrid for high-surety power called super uninterruptible power supply (UPS) is proposed to
improve the reliability of the microgrid [10–13]. In traditional UPS, only utility is used as primary
power source in grid normal condition. Here besides the grid, natural gas pipeline is introduced as
the second energy source, which is complementary to the grid. Actually, the utility and natural gas
pipeline are complementary and redundant energy supplies due to their competition. By leveraging
these two independent infrastructures, utility and natural gas pipeline, the power system availability
is significantly increased. Second, two independent storages are introduced such as battery and fuel
cell. Since both sources and storage components have redundancy, power conversion circuits may
become the weak point in the complete power system. Therefore, reliable power conversion cir-
cuit is introduced into the microgrid. Third, multiple energy sources, multiple storages, and highly
reliable power conversion circuit work together to guarantee uninterruptible power supply for the
critical load. Figure 14.1 shows the typical structure of Super UPS.

14.2 ARCHITECTURES
There are a variety of architectures for the microgrid, such as star architecture, multiple-star archi-
tecture, and ring architecture. In order to get the best architecture for Super UPS, evaluation methods
for the microgrid are needed to evaluate the performance of different architectures according to
certain criteria. The criteria include system energy efficiency, reliability, and economy.

14.2.1 Star Architectures

The first star architecture is load center architecture. In this architecture, the load is selected as the
center and all the other sources are connected with it through power converters. Obviously, the cen-
tral load has the most accessibility to all energy sources. Therefore, the power to the load is highly
guaranteed. Figure 14.2a shows a load-centered architecture, which is very suitable as a highly reli-
able power supply system.
The second star architecture is DC bus center architecture, shown in Figure 14.2b. All the sources
and loads are connected to the central DC bus. Since DC has no frequency and phase, the DC bus

Main sources Load

Grid Power
hub

Natural gas
pipeline Storage
Electricity
Gas engine
Highly
reliable Lithium
PV PCS battery
H 2
O 2 Hydrogen
H 2
Fuel cell

FIGURE 14.1 The concept of Super UPS.