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Almousawi & Aldair |3
(a) To solve the disadvantages of HESS passive connection,
DC-DC converters are interfaced between ESS and DC link
(b) to actively manage power flow from and to ESS [24]. Only
one of the two ESS components is actively regulated in a
(c) semi-active HESS configuration. When only the BESS is
Fig. 1: HESS for (a) Passive structure (b) Semi-active directly linked to the DC link, the SC will work along with a
greater voltage range, significantly enhancing volumetric
structure (c) Active structure. efficiency. The battery's direct link also ensures stable Bus
The passive link is the least expensive and simplest HESS voltages [25]. The direct connection of the BESS, on the
structure. Also, reduce transient current under load variation, other hand, unavoidably exposes the BESS to a rapidly rising
minimize internal losses, and maximize the peak power [22]. charging/discharging current, which reduces its life span
The output voltage of the SC and BESS must be the same. [26].
When dealing with an unregulated power flow, the BESS When only the SC is connected to the DC-link [27], the
may have rapid fluctuations in charging/discharging currents BESS current can be kept in a relatively softer way despite
in the event of sudden power requirements. It will result in power demand fluctuations, and the voltage output is not
BESS functioning degeneration and a significant loss in a required to fit the DC link voltage, enabling for more
lifetime. As a result, SC's power handling potential is efficient and flexible battery sizing and configuration [28].
underutilized because the voltage change of the battery The volumetric efficiency of the battery, on the other hand,
terminal is low, and the SC will not operate at its full SOC is low since most systems do not let the SC run in its wide
range, resulting in poor volumetric efficiency [23]. SOC range. SC's linear charge/discharge feature also creates
significant variation in the DC link, which can lead to poor
power quality and system stability. To keep the DC link
voltage generally stable, the SC must be exceedingly large,
which is not cost-effective.
Bidirectional DC-DC converters manage the power flow
between BESS & SC in active HESS. A properly planned
control technique improves DC bus stability and cycle life to
further improve the HESS's flexibility and performance [26].
DC-DC converters, for example, can adjust the Bus voltage
to preserve Bus voltage stability. Also, the SC can be
programmed to respond to high-frequency surge power, but
the BESS with a high energy density is designed to meet low-
frequency power transfer. The separating of the BESS and
the SC enables both elements to work at a wider range of
SOC, considerably enhancing the volumetric efficiency of
the HESS. The total efficiency of the HESS will decrease as
the number of power converters increases because of power
losses in the converters.
III. PROPOSED SYSTEM STRUCTURE
Fig. (2) describes an islanded microgrid PV system
consisting of a PV panel, BESS, and SC configurations. A
unidirectional boost converter ties the PV panel to the DC
link. By using the P&O algorithm as an MPPT algorithm
[29]. The unidirectional boost converter is used to extract the
maximum power from the PV array by regulating the PV
panel terminal's input voltage. Instead of employing a single
huge capacitor for the DC link, each converter terminal is
coupled to a small filter capacitance as demonstrated (C!",
C!#, and C!$).
The power efficiency is improved by employing the new
topology depicted in Fig. (2) because of using one converter
to charge BESS or SC and reducing the unnecessary power
losses. This arrangement employs four separate DC-
DC converters for the BESS-SC charging/discharging
scenario. Due to a mismatch between load and generation,
HESS is employed to ensure a constant output voltage level
("%&). When the load exceeds a production, "%& drops from
its set point. As a result, HESS will discharge to meet the
