Page 19 - PEN eBook May 2023

SMART ENERGY SMART ENERGY

Figure 4: Role of power electronics systems in energy storage and distribution 1
Figure 6: Conventional single-stage PCS (Source: Muller)
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aging. The weakest cell can be a fail path for a system include higher costs, as well as increased power
and can be a reliability bottleneck. Hence, this simple conversion loss.
single-stage PCS is used only for 600-V voltages or
lower. Multi-level inverters within each stage are commonly
used. These have several advantages over traditional
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A multi-stage PCS, shown in Figure 7, offers two-level inverters, including:
advantages of breaking the DC-link voltage constraint.
Some advantages of a multi-stage PCS are: ▶ Reduced harmonic distortion
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▶ Higher voltages in the first stage, which can ▶ Lower switching losses
leverage the benefits of higher-voltage SiC
devices and multi-level inverter topologies ▶ Lower PWM switching frequency

▶ Improved DC voltage stability, allowing for ▶ Increased power rating
a reduced DC-link capacitor and improved
lifetime ▶ Ability to use lower-voltage–rated devices for a
higher-voltage application
Some disadvantages of the multi-stage approach

Figure 5: 2020 DOE/OE TRAC program solid state power substation roadmap (Source: U.S. Department of Energy 2020 TRAC
report)

Increasing the working voltage is one of the best A conventional PCS solution typically consists of a
routes to increasing power density. A battery-cell single-stage inverter, shown in Figure 6.
voltage is fixed by the cell chemistry. Hence, several
cells are stacked in a series combination to create In this case, the DC-link voltage sets the constraint,
modules, which can then be series-connected to as it needs to be greater than the peak AC voltage with
create a rack. Racks can then be parallel-connected some margin. Although this single-stage PCS is cheap,
for capacity to create an individual system. A power it suffers from disadvantages of lack of scalability.
control system (PCS) controls this system and presents Within series-connected battery cells, there is variation
a voltage to the DC-link. in cell voltage and current, and this also changes with Figure 7: Multi-stage PCS (Source: Muller)
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