PhD Thesis - Cranfield University

Chapter 5
than power management. The decisions of energy management are accordingly termed as
the ‘strategy’ of the system that takes places within an Energy Management Shell (EMS).
The energy management strategy processes the slower dynamic parameters such as the
battery and ultracapacitor SoC, the vehicle kinetic energy and the operating mode of the
vehicle. The EMS then provides periodic manipulation of a set of control parameters to the
next stage of the PEMS process in order to implement a given strategy.
The next stage in the hierarchical process is the power management. With the prescription
for action stipulated by the EMS strategy, the power management process determines
power-split decisions for the multiple energy storage system. These power management
decisions are termed as the ‘policy’ of the system that is executed within a Power
Management Shell (PMS). The PMS is tasked to adhere to a given policy under constraints
that are periodically altered by the EMS strategy. Since the PMS handles the continuous
power split between the multiple energy systems, its iteration rate is several magnitudes
higher than the EMS. Both the EMS and PMS structure forms a method to loosely separate
the control of ‘energy’ and ‘power’.
For a dual energy storage system comprising of batteries and ultracapacitors, determining the
instantaneous power split ratio between the ultracapacitor and battery whilst regulating the
DC bus voltage is the power management problem. Maximising the battery SoC, keeping the
ultracapacitors at the optimum SoC as well as maximising the useable energy of both systems
is the energy management problem. The two objectives of managing power as well as energy
cannot be completely decoupled. However, it is valid to classify the power management
problem as a fast decision making problem and energy management as a slower decision
making problem, both of which share some common control variables.
The final stage in the modular structure is the Power Electronics Shell (PES), which is
responsible for the actual power blending of multiple energy sources using power spit ratios
determined by the PMS. The PES decomposes the reference power trajectories into
appropriate control switching functions, which is then fed to a power electronics interface.
Voltage and current regulation as well as system protection takes places within this process
shell.
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