IEEE Electrification Magazine - December 2019 - 14

TECHNOLOGY LEADERS

generators, provide proper dynamics
and transients during load variations,
and guarantee system stability. The
control structure can be divided into
the high- and low-level controllers;
high-level control includes a power-
management system (PMS) that is
responsible for load sharing and pro-
tecting the system. In addition, the
high-level control may include an
EMS, which is designed to further
optimize energy efficiency as well as
system reliability and the lifetime of
the battery and fuel cell. This control
level is also known as a power-and-
energy management system (PEMS).
The low level is associated with the
converter controllers and includes
power/current and voltage controllers.
It could also include inner and outer
loops. The inner loop is normally a
current controller, while the outer one
regulates the voltage or power and
generates the reference current for
the inner-loop control of the convert-
ers. The inner loop or current control-
ler then follows the current reference
to ensure the required power supply.
The battery-management system
(BMS) is an essential part of the high-
level control that is responsible for
regulating and monitoring the bat-
tery. It provides a battery-power refer-
ence according to requests from the
PMS while ensuring the proper SoC
and cell balancing. In addition, it
identifies and protects against under-
voltages, overvoltages, overcurrents,
and thermal faults. Based on the
energy provided by the carriers, the
PEMS generates the power references
for the low-level control and BMS. A
supervisory control selects the opti-
mal energy carrier for the ship's oper-
ating mode. Given the power that
is generated and absorbed in the

Voltage +
Reference

Error
-

PI
(Voltage)

system, the PMS provides references
to the converter controllers at the
interfaces with the energy carriers. It
identifies the optimal number of
active energy carriers to supply the
load demand and operates them
accordingly. It also ensures that there
is proper load sharing between the
energy carriers and monitors the sys-
tem to prevent blackouts.

Low-Level Control for the
Power Converters
Proportional-Integral Controllers
The controllers' voltage and current
output can be managed with pro-
portional-integral (PI) controllers,
which are commonly used and em-
ploy a simple algorithm. In general,
a converter's objective is to control
the output current or output volt-
age. Based on the objective, different
control loops can be designed with
the PWM block. In such a case, a
converter is considered to be a VSI.
The PWM block consists of the com-
parator and logic and drive circuitry.
The amplitude- and frequency-mod-
ulation ratios of the control wave
and carrier wave decide the output
magnitude and frequency. However,
the carrier-wave frequency is nor-
mally kept constant to have a steady
switching frequency, and hence, the
desired output frequency is achieved
by using the sinusoidal control-sig-
nal frequency. In addition, the selec-
tion of the modulation ratios affects
the output harmonics, so careful
selections based on the applications
and device ratings have to be made.
Based on the objectives, the cur-
rent- and voltage-mode control can
be selected. Current-mode control is
mostly used in servo drives where

Current +
Reference

Error
-

Figure 10. The converters' cascaded voltage and current-control loop.

14

I E E E E l e c t r i f i cati o n M agaz ine / DECEMBER 2019

PI
(Current)

Vc

the motor-current control is crucial.
Its benefits include a fast response
and an easy load-sharing approach
between power suppliers. However,
the possible insertion of noise into
the control loop, resulting in poor
voltage regulation, is a drawback. Tol-
erance-band and fixed-frequency
control are the most commonly used
methods in current-mode control.
The voltage-control loop is also a reg-
ularly employed control methodolo-
gy. Some of its benefits include stable
modulation, better cross-regulation
for multiple outputs, and simpler cir-
cuits. However, it suffers from a slow
response and a variable loop gain.
The cascaded (voltage and cur-
rent) control loop, consisting of PI
controllers, is another widely used
method for converter management.
The capacitor voltage is governed by
the voltage controller, whereas the
inductor current is manipulated by
the current controller. The inner cur-
rent loop is also used to control the
torque, current, and harmonic com-
pensation in ac machines. The inner
control (current) loop's bandwidth
should be higher than that of the
outer (voltage) control loop to ensure
the dynamic performance. A simpli-
fied block diagram of cascaded volt-
age-current control is shown in
Figure 10. The PI controller takes the
voltage error to generate the current
reference. The current error is then
fed to the PI (current) controller,
which generates an output (Vc). The
PWM block takes the Vc as an input
and creates the converter switching
signal (d). The voltage reference sig-
nal can be a constant value in the
case of a single converter. Howev-
er, when more than one converter is
connected in parallel, voltage

PWM

d

Converter

Output



IEEE Electrification Magazine - December 2019

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