Troy V. Nguyen, Donald C. Simon, Northrop Grumman Corporation – Electronic Systems,

Naval & Marine Systems Division, Charlottesville, Virginia USA

An Approach to Modeling and Analysis of Ship Propulsion

Electric Drives

ABSTRACT

Design of high-performance electric propulsion

systems requires thorough dynamic analysis of

both the electric power and propulsion systems.

This paper presents a simple but effective

method to model the relevant components in the

electric and propulsion plants to facilitate the

integration and analysis of the entire system for

the purpose of predicting the overall system

performance. The objective is to illustrate an

approach to integrate the electric and propulsion

subsystems in a modeling and simulation

environment that can potentially expose

alternatives in component design.

By way of examples, the paper addresses

modeling, analysis, and validation of electric

motor propulsion systems as well as hybrid

systems that consist of a gas turbine engine and

an electric motor driving the same propeller

shaft. An instance of the hybrid propulsion

systems is the LHD 8 propulsion design

discussed in a recent conference publication by

the NAVSEA engineering staff. A specific issue

in the hybrid propulsion system is the starting of

a shaft on electric drive and then clutching in the

gas turbine engine. This scenario exists in the

system due to differences in operational speed

range and performance dynamics of the gas

turbine and the electric motor. Using the

resulting system model, the examples in the

paper will demonstrate the behavior of the

subsystems based on this particular scenario.

The dynamic models under consideration are

representative of the actual systems and are

sufficiently robust for use in a preliminary

design analysis of electric propulsion drives. An

added benefit of the models is the ability to

examine different performance aspects of the

power distribution system, e.g. sizing of

electronic filters in the motor drive, as a function

of propeller shaft acceleration. The method

presented in this paper is in general applicable to

both conventional and hybrid electric propulsion

systems. Furthermore, appropriate use of the

models and the analysis tool can facilitate

integration study as the power system is

integrated with the propulsion aspects allowing

evaluation of the power plant capability and

operational requirements against the ship‟s hull

dynamics.

INTRODUCTION Advances in power electronics have contributed

to the rapid growth in utilization of electric drive

technology in commercial and naval marine

industries. Evolution of external, podded

electric propulsors has been a major impetus to

the application of all-electric propulsion system

in new commercial ships and currently some

naval vessels. These technologies coupled with

commercial availability of high power, variable

frequency converter drives enabled the evolution

to all-electric ships in which propulsion and

auxiliary systems are powered by common

electric power sources through a unified

architecture (NRAC 2002).

In traditional ship designs, up to 90% of

installed power (electrical and mechanical) is

dedicated solely to propulsion and not available

for operating other power consuming such as

weapon and sensor systems (NRAC 2002). As

the US and international Navies move toward

electric propulsion and ultimately all-electric

warship, there is the need to redirect electric

power from ship power/propulsion to support

high-power weapon and sensor systems in order

to realize the full war fighting capabilities. For

this to occur what is required is common power

system architecture that is both flexible and

robust to permit dynamic allocation of large

amounts of power from ship‟s service and

propulsion systems to advanced electric

weapons, sensors and countermeasures (NRAC

2002).

To achieve the all-electric ship objective, system

designers need to understand the relationship

among different power-consuming ship systems

and their interfaces to the electric power

generation, distribution and management

subsystems. The design can be better assessed

with mathematical models of the systems

developed for analysis so that the impact of one

system on another can be readily analyzed.

In this paper we focus on a hybrid electric

propulsion configuration where either the gas

turbine or electric motor drives the propeller

through a reduction gear and clutch

arrangement. Electric propulsion and auxiliary

systems include the electric propulsion motors,

motor controllers, and the auxiliary systems that

provide services to the ship and its systems. Our

primary objective is to present an approach for

modeling and analysis of these hybrid electric

propulsion system components to facilitate

model reuse and to minimize the effort for

model validation for different system

configuration. The basic sequence of events in

model development is:

Determine a suitable mathematical

description of the problem. The description

should be physically realistic and fit for the

application purpose. This means that the

model should reproduce the required physics

at the required fidelity. In some instances, a

steady-state model is sufficient. In other

cases, a simple linear transfer function is

sufficient. More generally, more complex,

possibly non-linear, dynamic models are

required.

Define clearly the interfaces of the system

components. For example, a model of a

rotating machine can often be implemented

with any two of power, torque and speed as

either input or output. It is necessary to

select a convention for each component

before developing the model to keep avoid

mismatch of signal physical quantities

during the modeling process.

Identify the parameters that will govern the

detailed behavior of the component. While

it is not necessary to determine all

parameters values precisely before model

development, but it is useful to have a good

idea of what parameters will be required

and, if possible, have a rough idea of the

likely range within which the system

parameters will lie.

Treat a large, complex system in a

hierarchical manner. It is prudent to break

the system down into smaller, more

manageable sub-systems. For each sub-

system, follow the same development rules

as for the components.

HYBRID ELECTRIC

PROPULSION SYSTEMS

FIGURE 1 illustrates a representative hybrid

electric configuration for a twin-shaft ship

propulsion plant and its control systems. The

propulsion motors can be either DC or AC

motors, both of which will be discussed in the

subsequent sections. FIGURE 2 illustrates a

hybrid electric propulsion system design. In this

hybrid design, the primary propulsion device is

the gas turbine engine at high ship speeds. The

electric motor is an auxiliary driver that is

utilized for loitering speeds. For the analysis

purpose, it is assumed that the system is driven

at high speeds by the gas turbine and can be

driven by either the electric motor or the gas

turbine at low speeds but not by both. The

transition between the gas turbine and electric

motor is accommodated by the two-stage

reduction gear with self-synchronizing clutch for

each of the prime movers. The system

configuration in FIGURE 2 can also represent a

bi-directional hybrid drive where propulsion

motor acts as a generator (driven by the gas

turbine), and with appropriate power electronics

design, supplies electric power feeding into the

ship electrical plant bus.

FIGURE 1: Representative Ship Propulsion Plant Configuration

FIGURE 2: Propulsion Hybrid Electric Drive

It should be mentioned at this point that the

primary focus of this paper is to analyze the

hybrid drive system performance in the

“propulsion only” mode. This work will provide

a solid basis for carrying out additional

modeling and analysis of power transmission or

conversion of the ship power. The propulsion

and electric plant components shown in

FIGURE 1 and FIGURE 2 emphasize the

interconnection between the ship propulsion and

the power generation plant and the conversion

and distribution to other ship systems.

PROPULSION SYSTEM

COMPONENT MODELING

This section of the paper presents the

mathematical description of the various

components in the propulsion plant.

Propeller:

The propeller can be modeled in terms of torque

and thrust coefficients, both expressed as a

function of J′, the modified advance coefficient.

This relationship is expressed in FIGURE 3 and

the equations below. The propeller torque and

thrust are computed by interpolating steady-state

maps where torque and thrust coefficients model

consists of test data for a range of ahead and

astern values of ship speed and pitch ratio (for

ship with controllable pitch).

22'

),'(

),'(

nDV

VJ

PRJfK

PRJfK

a

a

q

t

Where,

Kt = Propeller Thrust Coefficient

Kq = Propeller Torque Coefficient

Va = Speed of advance (ft/sec)

n = Propeller Speed (Rev/sec)

D = Propeller Diameter (ft)

PR = Propeller Pitch Ratio (dimensionless)

J′ = Modified Advance coefficient

(dimensionless)

The curves are implemented as look -up tables

(LUT). The LUTs would normally be derived

from reduced scale tank tests. The modified

advance coefficient more easily allows

simulating cases where the propeller shaft is

locked (n = 0).

FIGURE 3: Propeller Model Block Diagram

Another way of representing the propeller is

simply using steady-state look-up tables relating

the shaft RPM to developed torque and thrust.

Another look-up table can be used to relate

developed power to ship speed through the

water. This alternative propeller model is then

described simply by two LUTs.

Line Shafting:

The model for the transmission assembly

account for the Main Reduction Gear and line

shafting characteristics that include the gear

ratios, drive train moments of inertia, and all

transmission losses.

In many cases, the shaft can be considered to be

point inertia. The shaft model is very simple:

Net torque acting on the shaft is the difference

between that supplied by the prime mover(s),

and the load presented by the propeller, plus any

losses represented by a viscous friction due to

the bearings and windage. This relationship is

illustrated in the block diagram in FIGURE 4

and the equations below.

Basic equation

dt

dNKTNKTT s

AsdLPM

TPM = Prime Mover Torque

TL = Propeller (Load) Torque

TA= Net torque

K = Total inertia (shaft, propeller)

Kd = Viscous friction coefficient

Ns = Shaft rotational speed

FIGURE 4: Line Shafting Model Block Diagram

For a long shaft, it may not be appropriate to

lump the entire inertia of the shaft into a single

point. Long shafts can be modeled with a

number of flexible elements. A distributed

approach can be taken by effectively

implementing a set of difference equations (in

the axial direction only therefore remaining a

one dimensional problem keeping the

complexity down). The complete shaft is then

treated as a number of elements.

Couplings:

Shaft couplings can be modeled as torsional

springs. Damping term should be included, as

one of the purposes of the couplings is to

attenuate vibration. A simple linear spring may

be sufficient, but if nonlinear behavior is

required (i.e. the spring stiffening as its angular

displacement increases), a more sophisticated

model may be necessary. The torsional spring is

parameterized by the „simple‟ linear stiffness,

and details of how the spring stiffens as it

exceeds its linear operating range. FIGURE 5

illustrates a detailed schematic of the

connections between the prime movers (gas

turbine & electric motor) to the gear and

propeller assemblies. Based on this schematic,

equations of motion can be written to describe

the coupling behavior between the various

components attached to the drive train (Parker

and Garvey, 1972).

Gas

Turbine

Electric

Motor

Im

Cc1 Ks

Cs

Cp

Igear2

Ig

∆Nm ∆Tm

∆Nt ∆Tt

Kc1

Kc2

Cc2

∆N1

∆N2

∆Np∆Ns

Igear3

Igear1

FIGURE 5: Power Transmission Block Diagram

∆Nm, ∆Nt = Motor, Turbine speed deviation

relative to rated speed

∆Tm, ∆Tt = Motor, Turbine torque deviation

relative to rated torque

Im, Ig = Motor, Turbine moment of inertia

Igear1, Igear2, Igear3 = Gear box moment of inertias

Kc1, Kc2 = Motor, Turbine coupling torsional

stiffness

Ks = Propeller shaft torsional stiffness

Cc1, Cc1 = Motor, Turbine coupling damping

coefficients

∆Ns = Propeller shaft speed deviation relative to

rated propeller speed

∆Np= Propeller speed deviation relative to rated

propeller speed

Cs = Propeller shaft coupling damping

coefficient

Cp = Propeller shaft damping coefficient

∆N1, ∆N2 = Coupling shaft speed deviation

relative to rated speed

Gearing:

Reduction Gear

The gearbox can be modeled as point inertia,

accelerated by the net torque, i.e. the difference

between the torque applied on the drive shaft

minus that the torque delivered by the driven

shaft. It is important to reflect the inertias and

torque through the gearbox – this means scaling

by the gear ratio. The gearbox can be

parameterized in terms of the gear ratio, and the

inertias of the input and output shafts.

Combining Reduction Gear

A two input combining gearbox can be modeled

by considering the two separate shafts. Each

half is modeled from the point of view of the

torque input to each shaft – there are three terms

in the torque balance (the two input torques, and

the one load torque).

Combining Reduction Gear with Clutch

This model considers a clutch on either or both

input shafts. For a single clutched shaft this

means that there are only two states the system

can be in – engaged, or disengaged. If a clutch

is modeled on the other input shaft too, then

there would be 4 possible states. As for the

single shaft gearbox, when engaged the dynamic

model looks the same as the gearbox model

without a clutch. When disengaged, the de-

clutched shaft has only its own inertia to

accelerate. The other shaft still transmit torque

to the load – the model of this part of the system

now looks like the gearbox model for the single

input shaft.

Ship Hull:

There are a variety of ways of modeling the

motion of the ship through the water. They all

amount to a model of net thrust, made up of

propulsive thrust from the propeller, and drag.

Total drag comprises a superposition of a

number of contributors.

Hull Resistance: Ship speed/resistance

relationship for both ahead and astern. The hull

resistance as a function of ship speed is typically

documented in lookup tables for various ship

conditions, e.g., deep or light condition:

)( ss VfR

Where Rs is the hull resistance and Vs is the

ship‟s speed.

The ship acceleration is proportional to the sum

of the effective thrust of the propeller, minus the

ship resistance:

)( sRTM

g

dt

dV

T is the propeller thrust, M is the ship

displacement, which includes an added entrained

water mass, and g is the gravitational constant.

Thrust Deduction and Wake Fractions: The

wake and thrust deduction factors describe the

interaction between the hull and the propellers.

These factors (for both ahead and astern

directions) are usually determined through self-

propulsion tests or from full-scale power trials of

similar ships.

Wake fraction:

s

as

V

VVw

Where Vs is the ship speed and Va is the relative

speed of the propeller.

Thrust deduction factor (1-t):

21

)1(TT

Rt s

Where t is the thrust-deduction coefficient.

This particular model uses the concept of

„effective power‟ to quantify the total drag. This

is the power required to drag the ship through

the water at a given speed. Any thrust from the

propulsion system that exceeds the effective

power is available to accelerate the ship. If there

is insufficient thrust to match the effective

power, then the ship must decelerate. The

model is parameterized by a few simple values,

the most important of which is the speed-to-

effective- power look-up table.

Gas Turbine and Controller:

Gas turbines are complicated systems and

require slightly more sophisticated models. A

simplified approach is to use a representative

model of the gas turbine with look-up tables that

characterize its performance as a function of

engine speed and load. Such models are specific

to a particular gas turbine, and it is likely that the

turbine manufacturer can provide the model (or

failing that, then the LUTs that characterize the

machine).

In the absence of such a LUT-based model, a

simple aero-thermal model utilizing constant

specific heats and ratio of specific heat (gamma)

values can be used. For this type of model, a

compressor map of pressure ratio, mass flow and

efficiency as functions of corrected compressor

speed (NGGR) are required. In addition, maps

of turbine flow rate as a function of pressure

ratio are necessary. The pressure ratio across the

turbine determines the temperature drop, which

in turn determines the work output. The

compressor, combustor and turbines are treated

as point components while the inter component

plenums are treated as volumes. Details of this

modeling technique are given in (Walsh and

Fletcher, 2004) and (Camporeale, Fortunato, and

Mastrovito. 2006). The turbine flow rate, in the

absence of a machine specific map, can be

estimated by utilizing a simple Stodola ellipse

function combined with the pressure drop across

the turbine. A typical ellipse function is given

in Chapter XVII of the publication

“Aerodynamic Design of Axial Flow

Compressors” (NASA, 1965).

The controller model contains a simple start

sequencing routine typical in gas turbine

applications. Upon receipt of a Start Command,

the start sequencer first engages the starter

motor. When ignition speed is reached, the start

sequencer opens the fuel shutoff valves (FSOV)

and energizes the igniters. A fixed amount of

fuel is fed into the engine. If the combustor

loading and fuel-air ratio (FAR) are within the

ignition zone, the model assumes combustion is

present and adds heat from the fuel. When self

sustaining speed is reached, the starter and

igniter are deactivated. Concurrently, a gas

generator speed Proportional-Integral-

Differential (PID) controller is activated and gas

generator speed demand is ramped up towards

idle speed. When idle speed is reached, the gas

generator speed demand input is handed over to

the external control system input. The controller

model also incorporates an emergency stop (E-

Stop) capability, where the FSOV are closed

immediately, and a normal stop (N-Stop)

capability, where the engine is brought to idle

speed for a specified cool down period before

FSOV closure. This last capability is needed

when investigating transitions between gas

turbine and electric propulsion motor modes.

For transients, the controller model should

include some sort of maximum and minimum

allowed fuel schedules. These limiting schedules

can be implemented as lookup tables of fuel

flow as functions of NGGR, compressor

discharge pressure (P3) and compressor

discharge temperature (T3) (Walsh and Fletcher,

2004). Anti-windup protection should be built

into the controller model to prevent the PID

controller from integrating past the fuel limits.

In addition, any protective functionality which

may impact the analysis should be included. A

typical protective functionality, which may

impact both transient and steady state analyses,

is the power turbine speed limiting.

The above proposed approach to modeling the

gas turbine provides a generic and indicative

model for a preliminary dynamic analysis. For

detailed study, it may be replaced with a

machine-specific version.

Electrical Motors:

All motor models are electro-mechanical in

nature and can be split into an electrical and a

mechanical part.

DC Motor & Drive

A DC motor can be represented by a very simple

model. For a DC motor, the electrical model is

just a series combination of resistance and

inductance, representing the motor armature.

The model also includes a current measurement

device, and a voltage source used to develop the

back electromotive force (EMF). The

magnitude of the back EMF is proportional to

the rotational speed of the machine. The

constant of proportionality is the motor constant.

The armature current measurement is required

because the torque developed by the machine is

proportional to the armature current. The

constant of proportionality is the torque

constant. This torque is available to accelerate

the machine rotor. Load and friction torques

oppose the acceleration.

DC motors are controlled by adjusting the

voltage applied to the armature. Motors usually

require some form of control. Constant speed

and constant power are two common control

modes. A standard approach with DC motors is

to use a cascade controller to achieve constant

speed operation. The first stage compares the

actual speed against a set value reference speed,

and outputs the corresponding desired armature

current. The second stage compares the desired

armature current against the actual armature

current, and adjusts the motor armature voltage

accordingly. Simple PI control is sufficient for

the speed controller. For the current controller,

a simple relay is all that is required.

AC Motor & Drive

The principle of modeling AC machines is

similar to that of DC machines where a basic set

of differential equations govern the combined

electro-mechanical behavior. The primary

difference is that AC machines require a 3-phase

(or multi-phase) power supply inputs.

Taking the synchronous machine model as an

example, these models are quite complex and

are therefore parameterized by a large number of

distinct parameters. There are a few variations

on the general theme of the synchronous

machine – for example, they come in two

„flavors‟ – salient pole and round rotor.

The models are however quite standard, and the

equipment suppliers should be able to provide

values for these parameters that are reasonably

accurate. Dynamic models for synchronous and

induction motors and motor drives are also

readily available in the published literature and

in a large number of textbooks.

Variable Speed Drive

If the propulsion system is to permit electric

drive over a range of frequencies, including very

low frequencies (i.e. for starting an initially

stationary propeller shaft) then a variable speed

drive (VSD) is required. This takes fixed

frequency AC in, and produces variable-

frequency AC out, to supply the motor.

As shown in FIGURE 6, a typical VSD consists

of two stages:

Rectification (rectifier) – the AC input

supply is converted to DC

Inversion (switch-mode converter) – the DC

is converted back to AC.

+

-

Controller

Motor

Power sourceswitch-modeConverter

capacitor

rectifier

FIGURE 6: Electric Drive Power Processing

The operation of the switch-mode converter is

manipulated by a feedback controller as shown

in FIGURE 6.

FIGURE 7 illustrates the concept of the

induction motor drive control for the LHD 8

propulsion auxiliary motor (Turso et.al, 2010,

reused with permission). The drive internal

processor implements a traditional field-

oriented, flux-vector control technique.

Measured 3-phase currents (A,B,C) are

transformed into a 2-phase reference frame of

direct and quadrature axes (D,Q), in which the

control algorithm functions. This transformation

allows the current producing torque to be

decoupled from the current producing the

magnetic field in the motor and greatly

simplifies the control implementation.

Additional coordinate transformation

calculations are required for each control

processing cycle and the accuracy of

transformations depends on accurate estimations

of the rotor field position. For control, the

magnetizing current (D-axis current) is held

approximately constant and the output torque is

altered by changing the torque current (Q-axis

current) commands to the VSD. These current

demands are converted into switching signals

with result in a change in motor input voltage –

with subsequent current drawn by the motor to

supply the required amount of torque to support

the motor operation. Measured current is fed

back to the controller after first being

transformed into the D-Q reference frame.

FIGURE 7: LHD 8 Auxiliary Propulsion Motor Drive Schematic (Turso, et al., 2010, reused with permission)

An Approach to Modeling the LHD

8 Propulsion System

The following section outlines an approach to

modeling the LHD 8 propulsion system based

on the system configuration described in Dalton

et.al. (2002). The paper was presented at an

unclassified conference in the UK in April 2002.

Configuration of the LHD 8 System

The power and propulsion system is CODLOG

– Combined Diesel Electric OR Gas. There is

no intention to run the electric motor in parallel

with the gas turbine.

The complete system comprises

The ship is 40,500-ton displacement,

844 feet in length.

Twin shaft CPP propellers.

One main LM2500+ gas turbine, rated

35,000 HP (26.1 MW) per shaft, at 3600

rpm. The gas generator is rated at a

pressure ratio of 23.3:1 and a mass flow

rate of 186 lbm/sec at a corrected gas

generator speed of 9586 RPM (Wadin,

Wolf, and Haase, 2004).

One electric Auxiliary Propulsion

Model, rated 5000 HP (3.7 MW) per

shaft, at 1800 RPM

One combining gearbox, two inputs, per

propeller shaft. Gear ratio 20:1 with the

gear ratio the same on the two input

shafts. Both input shafts are clutched

with overrunning clutches.

Two diesel generators per propulsion

motor rated 4 MW each.

One Variable Speed Drive (VSD) for

each electric propulsion motor. The

drive is isolated by a transformer from

the DGs that supply it.

Two additional diesel generators whose

primary role is to supply ship‟s service

load. These DGs can be connected to the

propulsion system if necessary.

Main power distribution bus consists of

port and starboard longitudinal 4160 V

AC bus.

Zonal electrical distribution system

(ZEDS) – local 4160/450 VAC

transformers.

Approach to Model Development

Before developing a model, it is necessary to

determine what useful output is to be extracted

from the model, and hence the appropriate level

of detail. Issues of concern in the context of the

LHD 8 model may include (Dalton et. al, 2002):

Steady-state powering under gas turbine

and diesel-electric propulsion.

Performance of the system under trail-

shaft and locked shaft operation.

Response of the system to severe, but

controlled, transient events such as crash

stop.

Response of the system to severe and

uncontrolled events such as tripping out

of a prime mover, emergence of a

propeller shaft from the water during

high sea state, short circuits in the

electrical distribution system.

Load sharing of diesel generators under

diesel-electric propulsion.

Transition from gas turbine to diesel-

electric propulsion and from diesel-

electric to gas turbine propulsion.

Starting a stationary shaft under diesel-

electric propulsion.

Synchronization of diesel generators as

additional generation capacity is brought

on line.

Balancing supply and demand using the

management system.

Handling the connection of the ship‟s

power distribution system to shore

supply.

The modeling aspects listed above is not

exhaustive and will require further analysis to

determine more precisely what is required from

the model. This will help establish the range of

scenarios that have to be simulated, and the

measurements that must be extracted from each

scenario. From this, we can determine the scope

of the model and the fidelity with which

individual components should be modeled.

ANALYSIS AND SIMULATION

The simulation model constructed for this paper

falls into the isolated system category. The hull

is modeled as a simple surge model in calm seas.

Both the port and starboard shafts are modeled

and thus can interact with each other via the hull

surge model. Propulsion prime mover start and

stop capability is provided for all prime movers.

Locked shaft, trail shaft and powered shaft

conditions can be simulated.

The gas turbines are modeled as simple aero-

thermal models. The interface between the gas

turbine simulation model and the control system

logic is via inputs of start, stop and gas generator

speed commands, and via model output

feedbacks of engine run status, clutch status and

propeller shaft speed. The control system

modulates its gas generator speed command in

order to attain the desired propeller shaft speed.

The Variable Speed Drive (VSD) is modeled as

a simple PI controller (with limiters) and a feed-

forward gain. During this study, it was found

that the typical bandwidth of a motor drive is

much higher than that of the remaining

propulsion subsystems and that it was not

necessary to use the detailed motor drive model

outlined in previous sections for this analysis.

The interface between the VSD and the control

system logic is via inputs of start, stop and

motor speed reference commands, and via model

output feedbacks of VSD run status, clutch

status and propeller shaft speed. The control

system is essentially “open loop” with regards to

shaft speed as it simply generates the motor

speed reference. The VSD model modulates the

generated torque in order to maintain motor

speed.

The gearbox model contains the descriptions of

the overrunning clutches and thus controls

which shaft dynamic model is used for each

prime mover.

FIGURES 8, 9, and 10 are top-level block

diagrams of the system models in Simulink®.

Two scenarios were chosen for this model. The

first scenario is a gas turbine start followed by a

crash- ahead transient then a crash astern

transient. The second scenario is a transition

from gas turbine operation to motor operation.

FIGURE 11 captures the gas turbine start, shaft

breakaway and acceleration to idle ( time 0 to

time 100 sec), a crash ahead maneuver starting

at time 150 sec, and a crash astern maneuver

starting at time 250 sec. The gas turbine

controller‟s power turbine speed limiting

function is activating during the crash astern

maneuver (after about time 300 sec), indicating

further refinement of the shaft speed control

algorithm is necessary.

FIGURE 12 illustrates the transition from gas

turbine operation to auxiliary control operation.

The ship is initially at 10 knots on gas turbine

propulsion when the Auxiliary Propulsion

System (APS) is ordered on-line at time 1000

sec. The control system responds to the APS on-

line order by ordering the gas turbine to normal

stop while simultaneously bringing the motor up

to speed. During the gas turbine cool-down

period (reduced here to 60 sec) both prime

movers are clutched into the reduction gear and

provide propulsion power. At the end of the gas

turbine cool-down period (time 1060 sec) the

fuel valves to the gas turbine are closed, rapidly

shutting down the engine and transferring all

propulsion load to the motor. Variations in

motor and propeller shaft speed during the

transition are evident.

FIGURE 8: The Propeller Shaft Simulation. This figure shows only one propeller shaft, the other side being

functionally identical.

FIGURE 9: The Gas Turbine and its Engine Control Unit (ECU).

FIGURE 10: Gas Turbine Model. This model was developed from (Walsh, et. al, 2002) and (Caporeale, et.

al., 2006)

FIGURE 11: This simulation run captures the gas turbine start, shaft breakaway and acceleration to idle (

time 0 to time 100 sec), a crash ahead maneuver starting at time 150 sec, and a crash astern maneuver

starting at time 250 sec. The gas turbine controller’s power turbine speed limiting function is activating

during the crash astern maneuver (after about time 300 sec), indicating further refinement of the shaft speed

control algorithm is necessary.

FIGURE 12: The ship is initially at 10 knots on gas turbine propulsion when the Auxiliary Propulsion System

(APS) is ordered on-line at time 1000 sec. The control system responds to the APS on-line order by ordering

the gas turbine to normal stop while simultaneously bringing the motor up to speed. During the gas turbine

cool-down period (reduced here to 60 sec) both prime movers are clutched into the reduction gear and

provide propulsion power. At the end of the gas turbine cool-down period (time 1060 sec) the fuel valves to

the gas turbine are closed, rapidly shutting down the engine and transferring all propulsion load to the

motor. Variations in motor and propeller shaft speed during the transition are evident.

HYBRID ELECTRIC DRIVE

INTEGRATION STUDY

An adequate simulation model can provide

useful information to steady state and transient

performance of the control system. Control

schedules, gains, set points and limits can be set

based on the predicted performance. Severe

transients such as crash back maneuvers may

reveal areas where the control system is

inadequate and requires redesign, or at least

retuning. It is far better to detect such issues as

early as possible in the design process, rather

than after control system deployment to the

actual platform. The simulation models above

were developed with this purpose in mind. The

next step is to enhance the model further to

include the models of electric power flows

between the propulsion plant and electric plant.

This will in effect allow the model to be used in

bi-directional hybrid drive system analysis.

Utilizing the basic building blocks presented

above, it will be relative easy to construct and

examine various system alternative

configurations. It is also feasible to build in

reconfiguration into a single model. However,

this would add to the complexity of the task. It

is typically better to start with models of fixed

configuration, and to add the flexibility to

reconfigure subsequently. It is recommended

that a step-wise approach be taken, i.e. looking

at two particular configurations, before

developing a fully flexible re-configurable

model. Using the LHD 8 system as an example,

the following approach should be considered.

Isolated System: Start with the simplest

configuration of an isolated system, where each

shaft is powered by either gas turbine or diesel

electric. When under electric drive, the two

diesels can be paralleled together, but not

connected to the rest of the power distribution

system. In this configuration, two separate

models are required:

The propulsion system model consists of gas

turbine, gearbox, electric propulsion motor,

variable speed drive and one or two diesel

generators. The output of the gearbox drives

the shaft, which in turn propels the ship. The

port and starboard sides of the system can be

considered to be identical.

The ZEDS comprises two diesel generators

supplying the main 4160 VAC distribution

bus on port and starboard sides, with

suitable transformers and hotel load

representing each zone. For this

configuration, two separate models will be

required, but the complexity of each model

is relatively low. If trailing shaft is to be

modeled, then it will be necessary to

explicitly model the port and starboard

shafts separately, but by definition with

trailing shaft there will be no requirement to

model the power and propulsion system that

supplies the trailing shaft, as it is shut-down.

However, there may be a requirement to

model the „picking up‟ of a trailing shaft

when transitioning from cruise to full

propulsion. This model will allow the

investigation of propulsion system behavior

without interaction with the rest of the

power system – issues that can be addressed

include steady-state propulsion, transition

from gas to diesel electric, load sharing

between the two DGs per motor, cold start

under electric drive, operation of the

variable speed drive.

Parallel System:

At the other end of the range of complexity is

the full paralleled system, with cross-connects

between the propulsion and distribution

networks. A model of this system would

comprise the two separate models discussed

above, connected together electrically. Also, the

propulsion system model would be „doubled up‟

to explicitly represent the port and starboard

sides. This model will allow the investigation of

issues concerned with interactions between the

propulsion and power generation systems. It

could include the effects of harmonic distortion,

short circuit behavior etc., if the VSD model has

sufficient fidelity to include such effects.

Variable Configuration System:

Finally, the paralleled model should be further

modified by addition of circuit breakers to make

it reconfigurable within the simulation. This will

allow transients such as paralleling of

generators, behavior under short circuit or other

fault conditions, to be investigated, as well as

the behavior of the platform management system

to be simulated in the presence of realistic

dynamic models of the plant under its control.

CONCLUSION

This paper presents a simplified and methodical

approach to developing component models of a

large ship propulsion system having an auxiliary

motor drive. The approach utilizes available

published information to model physical

behavior of various system components. The

paper uses the LHD 8 propulsion design

parameters in an example to illustrate the

modeling, analysis, and validation of hybrid

propulsion systems. The dynamic models are

representative of the actual systems and

determined to be sufficiently robust for use in a

preliminary design analysis.

The dynamic models presented above provide

the capability to quickly prototype a system

simulation for a first-cut response analysis. The

proposed technique illustrates an approach to

facilitate the integration the electric and

propulsion models that can provide insight into

system dynamic behaviors.

For the “propulsion only” mode, the models

described are sufficient to obtain reasonable

prediction of the steady-state and transient

response of the ship speeds when under gas

turbine driven mode, during transitioning, or

under electric motor driven mode. For the

motor and motor drive, it was determined that

the models can be greatly simplified because

their dynamics are so much faster than the rest

of the components in the system.

A potential enhancement to the proposed model

is to include capabilities to examine different

performance aspects of the power

generation/distribution system, e.g. sizing of

electronic filters in the motor drive, as a function

of propeller shaft acceleration, and the power

exchange between the propulsion and electric

plants. The goal should be to develop a

sufficiently robust analysis tool for use in

extensive integration study of hybrid electric

propulsion systems.

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AUTHORS’ BIBLIOGRAPHY ___________________________________

Troy V Nguyen, Ph.D, P.E., is an Advisory

Systems Engineer at Northrop Grumman –

Naval and Marine Systems Division in

Charlottesville, VA. For over 20 years, he has

been working in control design and analysis of

gas turbines, steam turbines, diesel engines, and

auxiliary systems for power generation and ship

propulsion applications. He received a BSME

and a MSME from the University of North

Dakota and a Ph.D. in Systems Engineering

from Colorado State University.

Donald C. Simon, is a Systems Engineer at

Northrop Grumman – Naval and Marine

Systems Division in Charlottesville, VA, where

he designed the propulsion control algorithms

for the LHD 8. He served in the U.S. Navy as a

nuclear propulsion qualified Surface warfare

Officer prior to joining Northrop Grumman. He

received a BSME from the University of

Virginia.