1. search using google as well as looking through

1.   Introduction

A microgrid is a system which consists of minimum one energy source, a power conversion system, control and energy storage systems. On the other hand, Hybrid energy storage systems(HESS) provide two or more energy storage systems for optimal results. In this literature review I aim to give an overview regarding the sizing, architecture and control of HESS applied in micro-grids.

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2.   Research Methods


At the beginning of my research I started by looking at the basic information to get familiar with idea of microgrids. As the topic of microgrids is too broad, I kept the scope of my search to understand the basic architecture and working. As I moved on towards HESS systems I noticed that the HESS is very modern approach towards energy storage and still under development. As a result, I referred the information from scientific research papers on science direct and IEEE. Considering the scope of this literature review I formulated following sub questions on the topic:

·         What are the current storage systems used in microgrids?

·         Why Energy storages are required? How does HESS implementation befit microgrids?

·         What are the different architectures used for HESS implementations in microgrids?

·         How does sizing affects the microgrids?

·         What benefits does HESS provide in terms of sizing in microgrids?

·         What are the control strategies involved?

 The main research method I opted to answer these question is search using google as well as looking through books concerning microgrids technology.

3.   Main body of text 3.1         Energy storage systems in microgrids

Micro grids are usually operated in a stochastic circumstance and under a dynamic load conditions. Thus, there is a need for energy storage systems which can work with the two operation modes for micro grids which are mainly grid-connected mode (When connected to main grid) and isolated mode (When disconnected from main grid). Different forms of energy storages are used based on the use scenario of the microgrids:

·         Pumped Hydro Storage(PHS)

·         Compressed air energy storage(CAES)

·         Battery energy storage system(BESS)

·         Hydrogen-based energy storage systems

·         Flywheel energy storage systems(FESS)

·         Superconducting magnetic energy storage (SMES)

The basic layout of microgrid can be given with the following diagram.

Figure 1:Layout of Microgrid Implementation

Storage systems with a discharge time of several hours are used for energy management in which energy can be shifted over longer timescales (CAES, FESS, and SMES). Battery energy storage systems are most widely applied because of their ease of availability, advancement in sizing, cost and fast response times as opposed to above mentioned technology.

3.2         Hybrid Energy Storage Systems in Microgrids

As stated there are many storage technologies available but none of them singlehandedly satisfy the requirements of the microgrids applications. The storage system for microgrids must have a high-power density to face fast power variations, and at the same time it must have a high energy density to give autonomy in operation. For this reason, it is necessary to associate more than one storage technology also known as Hybrid Energy Storage Systems (HESS).

3.3         Sizing in microgrids 


3.4         Architectures and Control strategies for HESS used in microgrids

HESS includes combinations of energy storage technologies which provide the characteristics of both, a high energy density as well as high power density with the added advantages of high life-cycle and high efficiency.

This chapter presents comparison of classical controller implementations in HESS systems based on the different topologies. The control methods mainly make use of a low pass filter and PI controllers to regulate the different currents and voltages. Even though the implementation is different, the main objective of the control algorithm is to divide the power variations of the system into two parts depending on the frequency, that is the low frequency part and a high frequency part which are absorbed by suitable storage technologies used in conjunction.

1)      Parallel DC/DC converters topology and control algorithm:

This architecture is based on the use of parallel bidirectional DC/DC converters to control the power flow of each storage device. Each storage device can be independently controlled, which gives flexibility to the control algorithm. As the microgrid can work connected or disconnected from the main weak grid, the control algorithm is adapted to each operation mode. When the microgrid is operating in grid-disconnected mode, the inverter fixes the AC side voltage/frequency and when the microgrid works connected, the power is taken directly from the grid. The transition between both control modes is smoothed restarting the PIs of the grid-disconnected mode with the last value of the grid-connected mode as the Initial Condition value.





Figure 2:Parallel DC/DC Converter Topology


Figure 3:Control Algorithum for Parallel DC/DC

2)      Floating topology and control algorithm:

In this topology the DC/DC converter of the battery is eliminated and the battery is directly connected to the DC bus.