IEEE Power Electronics Magazine - September 2019 - 40

has been gaining popularity since it can harness the advantages of both ac and dc microgrids and be more compatible. It can directly supply the dc and ac loads, eliminating
many unnecessary intermediate power transformations
Overview of Hybrid Microgrids
[3]. Hence, it is of great significance to enlarge the scale
With the development of more advanced control techniques
of clean electricity generation with renewable energy. In
for power electronic devices, ac microgrids are appearing
2010, the Illinois Institute of Technology (IIT) in the United
with increasing power generation capacity, helping to solve
States installed on campus a hybrid ac/dc microgrid. The dc
electric power supply problems in many remote areas. In
subgrid is mainly composed of PVs. The dc bus is connected
2010, an islanded ac microgrid offering 50-kW wind power
to a centralized small-scale energy storage system (ESS) to
and including 1.04-MWp photovoltaics (PVs), a 1,220-kW
directly supply the dc loads. The ac subgrid is composed of
diesel generator, and 2,000-kWh energy storage was put into
wind power, PVs, and an ESS. The ac bus, which has a voltoperation in Dong'ao Island, China. The microgrid is also
age level of 4.16 kV, can be connected to the utility grid in
equipped with intelligent local and remote energy control
grid-connected mode or disconnected to operate in island
systems to integrate the clean energy management and stamode. In 2014, Nanrui Company completed the PV, ESS, and
ble load operation. This project, managed by Singyes Solar
Heat Integrated Hybrid AC/DC Microgrid Demonstration
Company, is the first islanded microgrid project successfully
Project in Hebei Electric Power Technology Park, China.
and commercially operating in China. In 2011, Singapore
The project includes a 190-kW PV system and a 250-kWh
and Tianjin's Eco-City Smart Grid Demonstration Project
lithium iron phosphate ESS, a 100-kWh supercapacitor ESS,
was activated. Its maximum load is expected to reach
a power quality monitor and management system, and elec587.5 MW in 2020. Meanwhile, two 220-kV substations and
tric vehicle charging piles.
four 110-kV substations are being planned regionally. In
Significant results have been achieved in promoting the
2014, Aalborg University in Denmark and North China Elecdiversification of microgrids. In 2017, the national hybrid ac/dc
tric Power University began working together to develop an
microgrid demonstration project in Zhejiang, China, started
intelligent dc microgrid project with the aim of promoting
operting on a trial basis. The distributed power supply has a
the development and application of dc-distributed sources
total installed capacity of 3.92 GW. The dc bus is connected to
in a planned residential and industrial zone [1]. Bosch has
many dc subgrids with multivoltage levels, and the different
also installed several dc microgrids in such locations as
dc bus bars are interconnected by solid-state dc breakers.
Charlotte, North Carolina, to track several key metrics,
BICs are needed in hybrid ac/dc microgrids to connect
including annual grid energy, system energy efficiency, and
various ac/dc microgrids with different voltage/frequency
PV utilization fraction, for several commercial building
(VF) levels [3]-[5]. The main functions of BICs are twofold.
types under a variety of operating schedules [2].
In grid-tied mode, BICs must guarantee that the hybrid
In the past few years, with the increasing penetration
microgrid can perform bidirectional power interaction with
of dc renewable energy, the concept of hybrid microgrids
the utility grid. This means that when there
is a short-term grounded fault, the hybrid
microgrid can provide sufficient reactive
power and voltage support to help meet
the low-voltage ride-through requirement
Utility Grid
[3]. Meanwhile, in islanded mode, BICs
Wind
DG1
need to flexibly coordinate the power flow
DG3
Energy
Turbine
Storage
among various subgrids to help maintain
the ac bus VF and dc bus voltage stability.
This means that when the power imbalBIC
ance occurs in one subgrid, the others are
PV
DG2
DG4
PV
obliged to provide the necessary power
support to ensure the power supply of all
of the important loads in the hybrid system.
BIC
Therefore, highly reliable BIC control techEnergy
Energy
DGi
DGj
niques are essential for stable operation of
Storage
Storage
hybrid ac/dc microgrids.
BICs are usually based on power elecac Microgrid
dc Microgrid
BIC
tronic devices, which are very flexible
ac Load
dc Load
and easily controlled with multiple application functions. A topology with a BICbased connection subsystem is shown in
FIG 1 A diagram showing a hybrid microgrid topology with BICs as a connection
Figure 1. To achieve the aforementioned
subsystem. DG: distributed generation.
and dc subgrids. This article focuses on the commonly
applied control techniques for hybrid microgrids.

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