IEEE Electrification Magazine - June 2015 - 28

experimental Test Setup of a Small-Scaled,
Real-Time Power-Hardware-in-the-Loop
Simulation

Frequency
Compensation Controller
AFE and BESS
Energy Controller

BESS SOC
Controller
(a)
Voltage
Compensation Controller

AFE Reactive
Power Controller

Power Factor
Controller
(b)

Figure 8. The AFE rectifier control strategies of (a) active and
(b) reactive power.

Because the majority of the power is consumed by the propulsion motor, the configuration in Figure 7 may provide a
better BESS operating efficiency.
The power control of a directly connected BESS can be
achieved because the other components connected to the
BESS firmly determine their respective power flow. The
AFE converter determines the power
flow between the PCC and dc-link.
The propulsion inverter determines
the power flow between the propulsion motor and the dc-link. Hence,
the mismatched power inevitably
flows to the BESS. The propulsion
inverter maneuvers the ship, so it
consumes or regenerates active
power only to secure the maneuverability regardless of the other ship IPS
components' status. Therefore, indirect control of the BESS is accomplished by the AFE converter. The
active and reactive power control
strategies of the AFE converter are
shown in Figure 8. The AFE converter
transfers active power from the PCC
to the propulsion inverter most of the time, keeping the
BESS SOC in a certain range. Since there is normally no
need of reactive power flow, it keeps the power factor as
unity. In abnormal situations such as generator trip or
faults, the PCC frequency or voltage can deviate from its
recommended ranges. The frequency and voltage compensators counteract this abnormal PCC behavior, suppressing transients and keeping both the voltage and frequency within tolerable ranges by controlling the active
and reactive power flows. There would be dead bands in
the compensators not to disturb the governor (GVR) and
automatic voltage regulators (AVRs) of generators in normal situations.

As a ship IPS construction requires a considerable amount of
time, cost, space, etc., a preceding IPS simulation is highly recommended. However, the ship IPS complexity makes it difficult to model and simulate properly. The typical IPS structure,
as shown in Figure 2, contains engine generators with associated controllers, switchboards, ship service loads, propulsion
loads with BESSs and pulsewidth modulation (PWM) converters, future mission loads, and system faults. The typical simulation model in Figure 2 with the aforementioned BESS configuration, as depicted in Figure 7, is shown in Figure 9. The model
has simplified the ship service load to constant resistor and
inductor (RL) loads because the RL load variations are slow
enough to be handled with classical engine generators and
associated controllers: governors and AVRs. The propulsion
system is modeled as the ideal current source, which neglects
the switching and control dynamics of converters to reduce
the simulation time. The BESS is modeled as simple resistor,
inductor, and capacitor and dc voltage source circuits whose
physical limitations are ignored. The future mission load,
which is in the form of the pulse load, is modeled as the current sink interfaced with Y-Y, Δ transformer, and diode rectifiers. This simulation model can be useful for system configuration design levels such as when deciding
the component capacity, placement and
wiring, and steady-state analysis.
As the simplified model has bounds
related to the detail characteristics of an
actual system, more detailed models of
each component should be considered.
The detailed propulsion system model
includes an AFE converter, a BESS of
hundreds of submodules, a PWM inverter, a propulsion motor, and ship hydrodynamics. The propulsion system
includes ship hydrodynamics and Robinson curve, as shown in Figure 10. The
Ship Hydrodynamics Model and Propulsion Motor Plant are mechanical models
and propulsion motors related to their
shape, structure, etc. The Robinson curve
is a two-dimensional input function according to the propeller
shape, which describes the relationship among the propeller
(fixed with propulsion motor shaft) speed, ship speed, and ship
thrust. The ship speed/thrust controller and motor speed controller belong to the control strategy of the propulsion inverter,
which can vary as the control strategy changes. The ship speed
follows its commanded value with a typical time constant of
several seconds to tens of seconds. The PWM inverter and the
AFE converter usually have a few kilohertz switching frequency. Thus, a proper time step of simulation should be less than
10 μs. This short time step and large time constant of the ship
hydrodynamic system causes stiff system problems since simulations should last more than tens of seconds for a few

The AFE converter
transfers active
power from the PCC
to the propulsion
inverter most of the
time, keeping the
BESS SOC in a
certain range.

I E E E E l e c t r i f i c ati o n M agaz ine / j un e 2015

Table of Contents for the Digital Edition of IEEE Electrification Magazine - June 2015