Page 352 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

Batteries and Ultracapacitors for Electric Power Systems with Renewable Energy Sources 339

+ +

L
C 2
V in C 1 V out
1:n
– –

FIGURE 13.26 A bidirectional DC/DC converter with DAB topology with voltage-fed FB.

+ +
L R
C C
V in 1 C R 2 V out
1:n

– –

FIGURE 13.27 Isolated and bidirectional DC/DC SRC topology.

The dual active bridge (DAB) topology, which consists of two voltage-fed FBs as illustrated in
Figure 13.26, is also widely used for bidirectional isolated converters. This solution is particularly
attractive for ESS because it has the capability to connect two DC link sources. The configuration
is advantageous because the number of switches is lower than that for voltage-fed and current-
fed FB topology and ZVS is achievable for all switches at both sides. The voltage switch stress is
lower than that for current-fed FBs and a high efficiency is expected [92, 93]. The narrow voltage
range for optimal operation is a disadvantage of the DAB topology. Overall, voltage-fed topologies
are preferred over current-fed ones due to their reliability and converter cost [85]. Other converter
topologies that have been derived from the DAB topology are presented in [85–93].
Another topology for a bidirectional isolated converter can be obtained by adding a capacitor in
series to the leakage inductance of the transformer, which corresponds to a series resonant converter
(SRC) as shown in Figure 13.27 [94, 95]. In this case, ZVS is achievable in a wide range of voltages
and loads with the exception of light load conditions [87].

13.6 UTILITY-LEVEL STORAGE SYSTEMS

ESSs are evolving as a viable technical and economical solution for improving both short-term and
long-term utility operations. Increased peak power demand, limited generation capacity, variabil-
ity of renewable energy generation, and transmission and distribution issues can be addressed, in
principle, with ESSs. Different types of ESSs can be employed for various utility-level applications,
such as voltage and current regulation, power quality management, load leveling, and peak power
shaving, as explained in the following.

13.6.1 Ancillary Services
ESSs can improve the operation of ancillary services. Due to the bidirectional power flow capability, ESSs
can absorb or inject active and reactive power from and into the grid, respectively, in order to regulate the
voltage and frequency and provide higher quality and smoother power. The stored energy can be used to
manage the power quality in the short term and to provide increased reliability for grid in the long term.