THE MECHATRONIC APPROACH
TO THE OPTIMAL CONTROL OF THE VSCF GENSET
.
Vladimír ŘEŘUCHA, Zdeněk KRUPKA, Jan LEUCHTER
Military Academy in Brno, Kounicova 65, 612 00 Brno, Czech Republic
Abstract: The new generation of electrical power generating sets (GS) is based on some new technologies.
Constant speed sets with combustion engines driving brushless field excited synchronous generators are
replaced by optimally controlled variable speed diesel engine, driving robust permanent magnet generator
(SGPM) equipped with power electronics frequency and voltage control (VSCF technology). The new 3rd GS
generation (GS3G) represents the sophisticated mechatronic system, consisting of mechanical parts,
electromechanical energy conversion part, power electronic converters for electrical energy transformation and
optimal control feedback system. Mechatronic approach to the theoretical analysis and synthesis, development
and implementation, using modern methods of mathematical modeling and simulation and optimal control
procedures, gives the chance to solve the GS3G problem successfully.
1. INTRODUCTION
Mobile autonomous (off-grid) power supply systems, known as electrical generating sets (GenSets, GS) usually consist of diesel engine-driven synchronous generator, where the output
voltage is controlled by means of exciting current controller and the output frequency is kept
constant by controlled constant speed of the engine.
GS are used as stand-by generators for backup power as primary power sources for a wide
range of applications such as mobile workshops, in building industry, in emergency and rescue
operations, off grid construction sites, farms and households. Quite indispensable are GS in
military use for off-grid supply of command posts, communication centers and appliances, field
hospitals and many weapon systems including sophisticated air-force and air defense systems.
New, so called 3rd generation of GS (GS3G), just under development, is based on new
technologies and principles. The Variable Speed-Constant Frequency (VSCF) technology is
used to reduce the fuel consumption and harmful emissions and to optimize the operation of
the whole GS system.
The synchronous generator with permanent magnets (SGPM) driven by optimally varying
speed supplies voltage and frequency proportional to the engine speed. Constant output
voltage and frequency is reached by means of power electronics voltage and frequency
converters. The optimum speed is controlled by means of engine feedback controller. The
simplified control diagram of EC3G can be seen in Fig. 1.
FUEL
UVAR
f VAR
n VAR
ENGINE
PMG
M VAR
XU
AC/DC
Xf
DC/AC
UCONST
f CONS T
u
CONTROLLER
Fig.1. The simplified control diagram of VSCF EC3G.
In comparison with the previous 1st and 2nd GS generations the new, 3rd GS generation
represents the sophisticated mechatronic system, consisting of mechanical parts,
electromechanical energy conversion part, power electronics converters for electrical energy
transformation and optimal control feedback system. Mechatronic approach to the theoretical
analysis and synthesis of the GS3G problem, the development and implementation of the
GS3G system, using modern methods of mathematical modeling and simulation, gives the
chance to solve the problem successfully.
The historical change from constant speed to optimally variable speed is based on the analysis
of GS loading regimes. Due to the usual low average load the engine operates at the
superfluous high constant speed (usually 3000 or 1500 RPM) in uneconomical and from the
point of view of the environment undesirable conditions.
2. THE GS3G MODEL MEASUREMENTS AND SIMULATIONS
In order to solve fundamental theoretical and research problems of GS3G it was decided to
build a physical model, consisting of the chosen driving diesel engine, synchronous generator
with permanent magnets, AC/DC, DC/DC and DC/AC converter, output filter and controller.
Some laboratory measurements were effected to find fundamental parameters and
characteristics of appropriate components. On the SGPM no load parameters and
characteristics, resistive load characteristics, the shape of output voltage, heating and short
circuit parameters were measured. Static and dynamic parameters and I/O characteristics were
measured and analyzed to prove the correct function of power electronics converters and the
output filter.
To get a general picture of the GS3G components and of the whole system static structure and
dynamical behavior, the measured parameters and characteristics have been used to build up of
the mathematical model for simulation of individual components and the whole the GS3G
system. The main objective of this simulation was to discover the most suitable configuration
and function of the control unit securing constant frequency, constant voltage and
corresponding quality of output electrical energy under the condition of optimal speed, minimal
fuel consumption and harmful emissions.
2.1. MODEL OF THE DRIVING ENGINE
The chosen driving engine is a single cylinder diesel with the rated power output Pn=7,6 kW at
a speed nn=3000 RPM. The engine model includes several partial models: the model of fuel
burning and exhaust gas expansion, passive mechanical resistance torque and forces,
dynamical torque and model of fuel consumption (fuel injection).
The definition and identification of the mathematical model is based on the physical analysis of
the above given phenomena (partial models). Fundamental differential equation of the engine
behavior is based on the well-known torque equation.
M E (d , )  M D  M R ()  M L (PL ) ,
where M E is the driving torque of the engine, M R and M L are resistant and load torque,
respectively, PL is the load power of GS,  is the angular velocity and d is a quantum of the
fuel injection. The detail analysis of individual partial models is out of the scope of this paper.
The engine represents the source of both kinematic movement and driving torque. Using the
approximative expression for M R and M E in form of 1-st order polynom, i.e. M R  r0  r1
and M E  m 0  m1d , the model of the loaded engine has the form of the 1-st order linear
differential equation
    K  d  K L (MS  M L ) ,
T
which corresponds to the standard control equation form (see [3]). Here
m
J
1
T  [s], K  1 [ mg 1s 1 ], K L  [ N1m 1s 1 ], M S  (r0  m 0 ) [Nm],
r1
r1
r1
where T is the time constant, K is the engine gain, K L is the load gain, M S is the effective
static torque and M L is the load torque. When assuming that the engine is controlled in
feedback by proportional controller described by equation
d  K R ( p  )  K R  e ,
where  p is the required angular velocity of the engine,  is the instantaneous velocity and e
is the control error. The programming scheme of the corresponding feedback control model is
given in Fig. 2.
KL / T
ωp
d
e

dom

K/T
KR
s-1
MS  ML
ω
1/T
Fig.2. The programming scheme of the GS3G feedback control
Resultant feed-back model has been defined and the system behavior has been simulated in
MATLAB – SIMULINK environment as shown in Fig. 3.
load torque ML
1
0
0.0009259
dynamic torque MD
ML
In1
Saturation
0.5
293.4
s
model of the
engine driving
torque
angular velocity
of engine om
!00
inic. velocity
30/pi
KR
Out1
reg. error
driving torque
5.333
driving torque
of engine M h
300
angular velocity
1
1/J
Out1
dynamic torque
In1
Gain3
2802
speed RPM n
model of the
engine resistance
torque
simout
To Workspace
5.332
engine resistance
torque Mom
required
angular velocity
of engine
dc
start
dc
graphics
Fig.3. The GS3G engine feedback model in SIMULINK environment.
The differential equation of the feedback system is in form of
    K C  P  K LC (M S  M L ) ,
TC 
and the important constants of the closed-loop system are
T
K  KR
KL
, KC 
, K LC 
,
1  K  KR
1 K  KR
1  K  KR
where TC is the time constant, K C is the engine gain, K LC is the load gain.
TC 
2.2. MODEL OF THE SYNCHRONOUS GENERATOR
The designed synchronous generator with permanent magnets (SGPM) has the following
parameters: 2p = 12, Pn = 4 kW, cos  = 0.8, nn = 3000 RPM, Un = 400/230 V, f = 300 Hz.
The variable output no load voltage changes within the range from 210 V to 420 V at the
speed varying from 1500 RPM to 3000 RPM (frequency 150 Hz to 300 Hz). The generator
can be integrated to the engine and arranged with an inner stator and outer rotor replacing at
once the engine flywheel as can be seen in Fig. 4.
Fig.4. The GS3G engine and SGPM arrangement
On the base of theoretical analysis and fundamental parameter measurements the simplified
mathematical model has been derived. The model of the generator could be expressed by the
equation of the generator power efficiency   (PL , ) that takes into account losses both in
magnetic and electric circuits of the generator
U 20  2 R ef PL
k e2 2  2 R ef PL
PL
.



PI U 20  R ef PL  p1U 0 PL k 2e 2  R ef PL  p1k e PL
The resultant equation for the model of the generator expressed in the term of the load torque
M L as a function of power output PL and instantaneous angular velocity  is
Pi
PL
PL U 02  R ef PL  p1U 0PL
P
R  p1k e
ML  

 [ L (1  PL 2 ef2
)] ,
2
  
U 0  2  R ef PL

k e   2  R ef PL
where PL is the generator output power, k e is the voltage coefficient, R ef is the effective
armature winding resistance, PI is the generator input power, U 0 is the generator no load
voltage and p1 is the identified coefficient.
The details of the generator model analysis are given in [3].
2.3. THE MODEL OF VOLTAGE AND FREQUENCY CONVERTER
To simplify the solution of the optimal control problem let us suppose, that the power losses in
the converter are negligible, then:
PGS = Po and ILGS = PGS/ UGS
where PGS is the GS (converter) power output, ILGS is the GS load current, UGS is the GS
output voltage.
The time constants corresponding to electrical and electromagnetic circuits are negligible in
comparison with the mechanical and electromechanical time constants of both the driving
engine and the generator. The moment of inertia of the generator is integrated in the dynamical
properties of the engine. The simplified functional scheme of the converter can be seen in
Fig.5. illustrating the whole schematics of GS3G structure.
CONTROL
UNIT
EGINE
CONTROL
UNIT
PROCESOR CONTROL UNIT
DISPLAY
FEED
BACK
+DC
U
SGPM
V
-DC
W
DC/DC
STEP UP
CHOPPER
+U D
U
C
U
V
V
FILTER
W
W
DIESEL
ENGINE
RECTIFIER
VOLTAGE STEP UP
CHOPPER
DC/AC CONVERTER
CONTROLLED
RECTIFIER
AC/DC STEP UP CONVERTER
FREQUENCY CONVER TER
Fig.5. The simplified functional scheme of GS3G
3. THE GS3G OPTIMAL CONTROL
The objective of the GS3G optimal control is to adjust such a speed (angular velocity) of the
engine and generator which corresponds to the required power output at optimum speed and
at the same time it ensures the minimum of fuel consumption.
From the GS3G system analysis it follows, that the appropriate individual requirements to the
control can be solved relatively independently. The control law can be fulfilled by three
separated subsystems creating fundamental components of the designed control structure:
- The required course of the transient process will be ensured by the feedback controller
- The static forced errors will be eliminated by the integral part of the controller or by means of
compensating couplings from error sources to the control error
- The minimum of fuel consumption will be ensured by the module of required speed, which
generates optimum angular velocity of the engine depending on the instantaneous load with
respect to the chosen optimization criteria.
One of the variants of GS3G control system structures including three above mentioned
control components is given in Fig. 6.
PL
Specific fuel consumption
ML
700
dM(ML)
M(PL,)
ωP(PL)
ωP
ekor
KR
u
600
SmEC
[g/kWh] 500
300
dω(ωP)
ω
Fig.6. The GS3G control system structure
 =314 rad/s
400
Popt
1000
2000
3000 4000
P [W]
5000
6000
7000
Fig.7. The fuel economy of GS3G.
This structure corresponds to the chosen control law, given by the relation
u  R (, PL )  K R {P (PL )  d [P (PL )]  d M [M L (PL , )]  } .
Here K R is the controller gain and d  , d M are correction values of steady-state errors of
angular velocity  and of load torque M L . As the most important part of the control law is
the model of the required angular velocity, i.e. P  P (PL ) . This equation is found as a result
of the minimizing procedure, where the quality criterion is the model of the GS fuel
consumption per hour S h or the GS specific consumption Sm  Sh / PL . Taking into account
identified models of the engine and of the generator we can derive the consumption model S h
in form
R p K 
0,9
0,9  d  PL 0,9
[PL (1  PL 2 ef2 1 e )  r1  2  (r0  m 0 ) ].

d 
ML


k e   2 R ef PL
The optimal angular velocity model has been derived as a solution of the equation


S h (PL , )  0
or
S m (PL , )  0 .


This model is applied in the region of angular velocity according to the activity of the
frequency converter. The resultant simulated relation between specific GS3G fuel consumption
and power output for constant angular velocity  =314 s-1 and for optimally controlled velocity
opt is in Fig.7. The results of simulations correspond to the expectation (see Fig. 3) and values
obtained by measurements and experiments performed on series of the 2nd generation of
generating sets in use [1], [4] and on the model of the 3rd generation VSCF generating set.
S h (PL , ) 
4. CONCLUSIONS
The mathematical analysis and simulation of static characteristics and dynamic behavior of the
GS3G VSCF structure proved, that the dynamical properties of the system enable the
formulation of control laws and thus to solve successfully the control requirements by the use
of minimum fuel consumption optimizing criterion.
The results of simulation have been partially proved by measurements on the realized physical
GS3G model, consisting of diesel engine, synchronous generator with permanent magnets,
voltage and frequency power electronic converter at different operating regimes.
One of serious problems not yet satisfactorily solved is the system behavior at high sudden
increasing of loads from low loads at low speed close to the idle run to high loads demanding
maximum speeds.
The physical and mathematical GS3G model will be further improved and precized to acquire
more parameters, characteristics and to get more experience for the practical development and
implementation of new generation of mobile electrical generating sets.
REFERENCES
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Autonomous Load-adaptive Adjustable-speed Generating System”.Proceedings of IEEE
applied Power Electronics Conference APEC’99, Dallas, Texas, March 1999, pp. 945–
950.
[2] Kurka, O., Leuchter, J.: “New Generation of Mobile Electrical Power Sources”.
Proceedings of International Conference on Electrical Machines ICEM’2000, Helsinki
University of Technology, Espoo, Finland, 28-30 August 2000, pp.
[3] Rerucha, V., Krupka, Z., Kurka,O.: “The Optimal Control of 3rd Generation Generating
Sets”. Research Study of the project ELCENTRALY, Military Academy in Brno, K301,
Brno, Czech Republic, 12/2000.
[4] Knitterscheidt, H.: “Neue Generation SEA für militärische Nutzung”. Proceedings of
BAKWVT Wehrtechnisches Symposium Moderne Elektrische Energietechnik”,
Mannheim, Germany, 1998, pp. 14.1-14.22.