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Batteries and Ultracapacitors for Electric Power Systems with Renewable Energy Sources 333

overcharging and over discharging that could seriously damage cells, reduce useful lifetime, and
even cause fires and explosions. Therefore, an effective battery cell balancing system, which main-
tains the SOC of the cells at the same level, is an important feature of any BMS.
Cell balancing systems are either passive or active. According to the typical passive balancing
methods, the extra energy of an imbalanced cell is released by increasing the cell body temperature,
a technique that is useful especially for small battery packs with low voltage [59]. This technique
is relatively straightforward and inexpensive to implement, but its applicability is mostly limited to
cells that do not damage severely due to overcharging [60].
The active balancing technique utilizes an active circuit to distribute as evenly as possible the
energy among the cells [57]. Active balancing techniques are employed in several different ways
including shunting and shuttling and energy converting method. From the energy flow point of view,
active balancing methods comprise dissipative and non-dissipative methods [61]. The extra energy
in dissipative methods is wasted as heat across a resistor, while in nondissipative techniques, the
excess energy is distributed among the string cells, leading to a higher system efficiency.

13.5 ENERGY STORAGE INTERFACE SYSTEMS
Power electronics converters are required in order to ensure the connection between the AC grid
and electric energy storage devices such as batteries and ultracapacitors. These converters, which
are the basis of energy storage interface systems, operate bidirectionally and they are of the AC/DC
and DC/DC types.

13.5.1 AC/DC Converters

AC/DC power converters may employ one or multiple phases, with the three-phase versions being
the most popular for AC grid connection. Based on the circuit topology, bidirectional AC/DC con-
verters may be categorized into boost, buck, buck–boost, multilevel, and multipulse types.
Bidirectional boost-type converters, which may have different topologies (see Figure 13.15),
have been typically employed in applications like energy storage and line interactive UPS systems.
A conventional configuration is based on a three-leg, six-switch (controllable switches, each con-
nected in parallel with a freewheeling diode) H-bridge voltage source converter for which PWM-
based closed-loop control is the most common technique for DC link voltage regulation at close to
unity power factor. Alternative topologies for bidirectional boost converters were also proposed in
a four-switch configuration, which is typically employed for variable speed induction motor drives
[62], and in a four-wire arrangement that is used to reduce the DC link voltage ripple and balance
the supply currents [63–65].
Some of the most common bidirectional buck-type converters are shown in Figure 13.16 and
include a six-switch configuration with gate turn-off thyristors (GTOs) for higher power and insu-
lated gate bipolar transistors (IGBTs) for lower power ratings. The four-leg topology is advanta-
geous for reducing the DC link voltage ripple and for balancing the currents in case of an unbalanced
supply voltage [66, 67].
Bidirectional buck–boost converters are employed in cases where the DC link voltage is substantially
variable around an average value. Configurations reported include cascades of buck and boost convert-
ers or matrix converters, as shown in Figure 13.17. From a performance point of view, matrix converters
are advantageous because they are capable of operation as bidirectional buck or boost converters and
because the higher switching frequency reduces the size of the input and output filters [68, 69].
For higher voltages and higher power, bidirectional multilevel power converters have been pro-
posed, including multilevel diode-clamped [70, 71] (Figure 13.18), flying capacitor [71], and cas-
caded topologies. The multilevel and multipulse types of high-power converters provide better AC
quality with lower total harmonic distortion (THD) and electromagnetic inference (EMI) noise and
a higher power factor [72].