Page 47 - Energize May 2022

TECHNICAL

How to optimise energy storage

in a microgrid

INFORMATION FROM MICROGRID KNOWLEDGE

icrogrids are receiving much attention currently because
of their ability to provide higher energy surety, quality
Mand security while also providing sustainability and energy
efficiency. A microgrid is defined as a network of loads and local
generations (distributed generations), as well as energy storage
systems. Mainly, the presence of energy storage systems enables
microgrids to regulate their voltage and frequency while the system
operates in various modes, such as when a microgrid is islanded
from the utility grid. Further, due to the increasing penetration
of renewable energy, which brings power intermittency into the Figure 1: The configuration of the microgrid studied in this article
microgrid, maintaining the system at a stable mode of operation is
an issue. Four types of power sources were added: a 250 kW solar PV
Various researchers are focusing on power management and plant, two 750 kW wind turbines, a 1,5 MVA natural gas generator,
control strategies of energy storage systems within microgrids, to and two 250 kW, 500 kWh zinc-bromide energy storage elements.
improve a system’s stability. However, in addition to the controls, The modelling and capacity design for these sources have been
the physical placement of energy storage elements in the system presented in [7–9]. 7-9
must be studied. A few papers discuss this issue from different A system collapse case had been observed and discussed in
1-6
perspectives. This article follows up on the work of a paper which previous papers, which signifies the issue of placement and sizing of
9
studied the generation capacity sizing to ensure power quality. energy storage systems. Wrong placement of energy storage not only
increases the overall microgrid costs but also the risks of low voltage
Novel technique and/or high voltage issues which might damage other devices in the
In this article, a technique is introduced to find the best location for microgrid.
an energy storage system in a microgrid, to support the voltage and Most microgrids are built at low and medium voltage levels,
frequency profile. A voltage sensitivity study is conducted, and four where the distribution lines have significant impedances. Charging
sensitivity indexes derived from Jacobian matrix are proposed and and discharging the energy storage in some instances may create
adopted to determine the candidate locations for energy storage voltage instability. In addition, the capability of the energy storage
devices. to support the voltage and frequency in the microgrid can be
Various scenarios and cases are discussed, simulated and undermined. The strategic storage location and size also determine
compared for all the selected candidates in the proposed microgrid. the threshold for renewable energy penetration, and the so-called
12
Finally, the utility power quality indexes are calculated for various stable operation (voltages and frequency) of a system.
cases using the model developed in papers [7–9] to evaluate Having an oversized storage connected at a randomly selected
7-9
and compare for our system. The details and constraints of the bus within a considerably large sized microgrid will not intrinsically
components, including generations, loads, energy storage and have the best impact in terms of stability on the rest of the buses.
controls, have been considered in the models. The energy storage system must help supply the critical loads in
the case of a shortage from the utility grid, but it has to be located
Problem statement properly to regulate both active/reactive power without an adverse
A microgrid system based on IEEE 34 bus with high renewable impact on the voltage or frequency. The proper placement also
energy penetration is proposed and studied in [7–9]. Figure 1 enables the system to alleviate the voltage problems at various buses
7-9
shows the configuration of the microgrid. The original system is with smaller current flow from the storage element.
60 Hz, 24,9 kV, 12 MVA with different fixed loads connected to
the utility main at bus 800 and no distributed generations (DG) Proposed technique
on the system. In order to match the properties of the system The voltage sensitivity study is widely used in power system analysis
10
with a microgrid under construction at Fort Sill, Oklahoma, USA, to identify the relationships between voltage magnitude and active/
the nominal voltage of the system is changed to 12 kV and other reactive power at a particular bus. These relationships are usually
components of the system, including loads and line impedances, depicted in so-called P-V and Q-V curves. In a transmission system, a
have been scaled accordingly. Q-V curve is commonly adopted and studied to evaluate the impact

energize | May 2022 | 45