Real time feedback control of synchronous

generator.

Slim Tnani and Gerard Champenois

University of Poitiers

LAII-ENSIP

Poitiers, France

Email: slim.tnani@univ-poitiers.fr

Email: gerard.champenois@univ-poitiers.fr

Emile MOUNI

Leroy Somer Motors

Angouleme France

Email: emile.mouni@leroysomer.com

Abstract—Presently, the most synchronous generator’s excitersare based on the use a reverse synchronous generator whichfeeds the revolving field cross a three-phase diodes bridge. Thesestructures are often controlled by a traditional ProportionalIntegral Derivative controller. In this paper, a new excitationstructure using a permanent magnet synchronous generator ispresented and the synchronous generator output voltage controlis achieved by using a predictive strategy. The proposed structureallows not only to improve signal quality but thanks to theinvolved controller, some other interesting effects such as timeresponse reduction are also obtained. An experimental valida-tion is provided to illustrate the proposed structure efficiencycompared to the classical approach.

I. INTRODUCTION

In the normal operating mode of a synchronous generator

(SG), a main field is needed to produce energy on its terminals.

To do that, the machine’s revolving field is fed by direct cur-

rent provided by several approaches called excitation structures

which bases have been formulated in 60’s [1], [2]. In [3] pure

derivative excitation type is presented. This method consists in

deriving the SG’s output voltage by using transformers. This

last three-phase voltage is rectified via a Silicon Controlled

Rectifier (SCR) bridge generally controlled by a Proportional

Integral Derivative (PID) regulator and allows to force the

main SG output voltage to follow a pre specified reference.

Another excitation technique is the compound which combines

the above structure with power current transformers on each

phase [3] and diodes bridge. Nevertheless, both these struc-

tures require a starter system. In order to solve this problem,

companies choose to use a reverse SG coupled to the main

SG as given in Fig.1.

GS

If

LVf

A

B

Exc

Iex

Fig. 1. Synchronous generator classical excitation scheme

In this case, one deals with current source which is

combined with diodes bridge. Then the rectified voltage

is used to feed the main SG excitation winding. In order

to supply the reverse SG, several strategies are used. The

first consists in deriving voltage from the main SG terminal

and is called shunt type. This structure presents a low

short-circuit/overload capability. This explains why it is often

associated to a booster system. The second one is based on the

use of auxiliary windings rightly positioned among the main

windings (AREP:Auxiliary Winding Regulation Excitation

Principle). Thanks to this strategy, a source of energy is

created and allows feeding the exciter with appropriate current

whatever in load operation or in short-circuit one. The last

type uses a permanent magnet synchronous generator [4], [5],

[6] which is placed on the same shaft that the main machine.

Note that all these structures generally use a traditional PID

controller to find the right excitation current and even if they

provide interesting results, they face to limitations as regards

the signal quality and the time response. These limitations are

partially due to the excitation nature (current source) which

presents a very low dynamics.

Loadvf

Voltage sensors

references

PMSG

Thyristors

SG

Bridge

Controller

Fig. 2. Proposed control structure of the synchronous generator

In order to improve the machine performances, one proposes

a new excitation structure based on the use of a voltage source.

In this case, the above reverse SG or the pure derivative

Proceedings of the 2011 International Conference on Power Engineering, Energy and Electrical Drives Torremolinos (Málaga), Spain. May 2011

978-1-4244-9843-7/11/$26.00 ©2011 IEEE

excitation (current source) is replaced by a permanent magnet

synchronous generator (voltage source) and the traditional

diodes bridge by a thyristors one which triggers are controlled

thanks to modern algorithm. One proposed a new excitation

structure given by Fig.2. The central problem in control is to

find a technically feasible way to act on the given process

so that it adheres, as closely as possible to some desired

behaviour. Thanks to the use of voltage source coupled with

thyristors bridge, no free wheel phenomenon is noticed and

any control law is immediately effective.

In section II, a modeling of the synchronous generator is

presented. The predictive control method [7], [8], [9], [10]

strategy is already used in power electronics domains [11],

[12], [13] and have proved its effectiveness. We will not

develop these control method theory in the paper. The ex-

perimental test bench presentation, algorithms implementation

and results are given in section III. In this section special tests

are performed to check the validity of the proposed structure

compared to the classical one and a qualitative analysis ends

the paper.

II. CLASSICAL MODELING OF THE SYNCHRONOUS

GENERATOR

It is well known that the synchronous generator can be

described in three axes frame as follows [14], [15], [16], [17]:

vabc = −rs.iabc +ddtΨabc

vf = rf .if + ddtΨf

0 = rD.iD + ddtΨD

0 = rQ.iQ + ddtΨQ

(1)

The study will be done in Park’s framework thanks to Park’smatrix defined below according to θe. In this expression the

factor

√

2

3ensure the conseravtion of the instantaneous power

in the new frame.

P (θe) =

√

2

3

(

cos(θe) cos(θe −2π3) cos(θe +

2π3)

− sin(θe) − sin(θe −2π3) − sin(θe +

2π3)

)

such as:

P (θe).vabc = vdq (2)

Then, the equation (1) becomes

vd = −rs.id +ddtΨd − ωe.Ψq

vq = −rs.iq +ddtΨq + ωe.Ψd

vf = rf .if + ddtΨf

0 = rD.iD + ddtΨD

0 = rQ.iQ + ddtΨQ

J ddtωm = Te − Tr

(3)

Tr depends on the external load.

Te is expressed according to the machine current as given by

[14]:

Te =3

2p(Ψdiq −Ψqid) (4)

In this work, ωm is supposed to be constant. Indeed, in

the test bench which is being achieved for validating the

algorithms, the synchronous generator is involved by a DC

motor controlled by a double thyristors bridge motor drive

(WNTC4075 from Alstom Company). Thanks to a right con-

figuration of this last one, the speed can be considered as

constant during all the tests. The constant speed assumption

allows us to generate a model which suits very well to the

involved control strategy. A classical state space modeling of

the machine, taking into account the rated load (r1, l1),as given

in Fig.3, is considered:

r1, l1

r1, l1 r1, l1

vf

Fig. 3. Synchronous generator scheme with inside load

In the following, the currents in the three resistances (r1) are

represented by ia1, ib1, ic1 and those in the three inductances

(l1) by ia2, ib2, ic2. Thanks to this scheme, the output voltage

can be written in Park’s frame as follows:(

vdvq

)

=

(

0 −ωel1ωel1 0

)

.

(

id2iq2

)

+

(

l1 00 l1

)

.d

dt

(

id2iq2

)

or(

vdvq

)

=

(

r1 00 r1

)

.

(

id1iq1

)

=

(

r1 00 r1

)

.

(

id − id2iq − iq2

)

with:

(

id1iq1

)

= P (θe).

ia1ib1ic1

;

(

id2iq2

)

= P (θe).

ia2ib2ic2

(5)

Thus, the partial equation of the synchronous machine in the

Park’s frame is given by equation (6).

From this equation, the model can be written in the state

space modeling shape as follows:

x = Ax+Bu

y = Cx+Du(7)

where:

A = −M−1.R is the state matrix,

B = M−1.(

0, 0, 0, 0, 1, 0, 0)T

is the control matrix,

C =

(

−r1, 0, r1, 0, 0, 0, 00,−r1, 0, r1, 0, 0, 0

)

is the observation matrix,

D =

(

00

)

is the direct transmission matrix,

x =(

id2, iq2, id, iq, if , iD, iQ)T

is the state vector, y =

0000vf00

=

r1 −l1ωe −r1 0 0 0 0l1ωe r1 0 −r1 0 0 0r1 0 −(rs + r1) lqωe 0 0 −ωemsQ

0 r1 −ldωe −(rs + r1) ωemsf ωemsD 00 0 0 0 rf 0 00 0 0 0 0 rD 00 0 0 0 0 0 rQ

.

id2iq2idiqifiDiQ

+

l1 0 0 0 0 0 00 l1 0 0 0 0 00 0 −ld 0 msf msD 00 0 0 −lq 0 0 msQ

0 0 −msf 0 lf mfD 00 0 −msD 0 mfD lD 00 0 0 −msQ 0 0 lQ

.d

dt

id2iq2idiqifiDiQ

= R.

id2iq2idiqifiDiQ

+M.d

dt

id2iq2idiqifiDiQ

(6)

(

vd, vq)T

is the output voltage and u = vf is the control

vector.

For numeric application, the table III is considered. The

conversion method from these data (time constants, reactances)

to those involved in the state matrices (inductances, mutual

inductances,...) uses the classical electrical scheme. The reader

is referred to [18], [19] for more details on the transformation

procedure.

III. REAL TIME APPLICATION

A. Experimental test bench presentation

Several ways can be used for experimental implementation

of the control algorithm [20], [21], [22]. In our case a DSpace

architecture [23] is considered due to its flexibility, accuracy

and easy to take control.

The figure 4 gives an overview of the test bench involving the

main components used during the tests.

Equipment

User

Process

dc motor PMSG SGLoad 1

RL

Load 2

RL or ASM

Tachometer Resolver

PD2

PD3

Source

3ph

50Hz

Emergency

control

speed

+ −

Reference

Ωref

Ωmes

Cond

PD3

Arcos

α

DAC

Algorithmcontrol

Control desk

referenceVoltageparameters

ControllersVisualizationvs, is, vf , if

Iex

Command

Measurement system

(VISION R©)

Cond Cond Cond

Cond

if vf

ADC

ADC

ADC

ADC

if

vf

Ωmes vs is vf if

output files

vs, is, vf , if et Ωmes

33

DSpace +PC

2

3

: digital signal

Legend

: Analogue signal

vs

is

: Power

vfif

vsis

is

3

r2ci

Parameters references

θ

6

Idcm 3

3 3

3 3

stop

(AU)

speed

Fig. 4. Experimental test bench scheme

As we can see on this scheme, the process is composed of

3 machines: the main SG, a permanent magnet synchronous

generator (PMSG) equipped with a resolver for mechanical

angle measurement and a DC motor involving the two first

machines at rated speed thanks to a motor drive. Resis-

tances/inductances load (RL) and Asynchronous motor (ASM)

are used to performed tests.

In order to allow the process to work porperly, equipment is

used. It consists in several electronics circuits (cond, DAC,

ADC, r2ci on Fig.4) which interface the analogue environ-

ment with the DSpace. For example the circuit r2ci allows

transforming the resolver output signal to coder data required

by the DSpace. Then, thanks to the information on the shaft

angle, all calculations can be done in Park framework using

controller parameters, voltage sensing and references.

From these calculations a firing angle is generated for the

thyristors bridge connected to the PMSG terminal and then the

main SG can be fed with the right excitation current depending

on the machine output voltage.

To perform tests such as impact load and shedding test, Load

1 and/or load 2 are used. During all the tests, a powerful

measurement instrument (VISION R©) is used to record data

in files.

Fig.5 depicts the global test bench with apparatus.

Fig. 5. Global experimental test bench

The main components of the experimental test bench can

be listed as follows

• The WNTC4075 thyristors converter is a motor drive

which controls the DC motor in order to maintain the

mechanical speed on the shaft to 1500rpm. Then the main

SG output voltage’s frequency is about 50Hz (the main

SG is a 4-poles machine).

• The passive variable load is composed by a 6KW

variable active power and a 6KVAR variable reactive

power and is connected to the main SG terminals.

• The Dspace 1104 and controldesk: they allow to im-

plement developed algorithms. The generator’ states are

given to the controldesk via Currents and voltages

sensors.

• The Vision: it is a 16 channels measurement instrument

allowing to record data during the tests.

The predictive controller has been implemented on

Matlab/SimulinkTM and downloaded in the DSpace 1104.

In the following section, several results will be given and

discussions on the validity of the excitation structures and

controllers will be done.

B. Load impact and shedding load tests

The main test, used to validate the proposed structure is

the load impact and shedding one. To do that the main

synchronous generator is involved at rated speed until it

reaches its rated state in terms of output voltage. Then a load

impact is performed. When the system reaches again its steady

state a shedding test is done to bring it back to its initial state.

In order to get a right comparison between the two structures,

the experimental conditions have been the same. The table I

gives the electric conditions before and during the load impact

test.

TABLE IEXPERIMENTAL CONDITIONS

P(kW) S(kVA) Q(kVAR) PF I(A)

Before 0.81 3.99 3.91 0.2 5.8

After 1.53 7.18 7.01 0.21 10.5

By considering this table, one can notice that this test

induces a current increase from 5.8 A to 10.5 A (roughly

the rated current as given in Table III) corresponding to a

50% increase of the rated apparent power. The two following

figures give the phase currents and voltages during load impact

and shedding test according to the involved control structure.

2 2.5 3 3.5 4 4.5 5

−1

−0.5

0

0.5

1

Phase voltage (p.u)

2 2.5 3 3.5 4 4.5 5−20

−10

0

10

20

Time(s)

phase current (A)

load impact shedding test

Fig. 6. Phase voltage and current with proposed structure

0.5 1 1.5 2 2.5 3 3.5

−1

−0.5

0

0.5

1

Phase voltage (p.u)

0.5 1 1.5 2 2.5 3 3.5−20

−10

0

10

20

Time(s)

phase current (A)

load impact shedding test

Fig. 7. Phase voltages and current with classical structure

The next figure denotes a comparison of phase-to-phase rms

voltage depending on the involved excitation structure.

0.5 1 1.5 2 2.5 3 3.5 40.7

0.8

0.9

1

1.1

Urms(p.u)

2 2.5 3 3.5 4 4.5 5 5.50.7

0.8

0.9

1

1.1

Time(s)

Urms(p.u)

Proposed structure

Classical structure

Fig. 8. Comparison between phase-to-phase rms voltage

During the load impact test, a little voltage drop is noticed

on Fig.6. Thanks to the predictive controller and the excitation

structure, this disturbance is rejected in about 120 millisec-

onds. As regards the output current, it rightly corresponds to

what we were supposed to get with this kind of test. When

the system reaches again its steady state in terms of output

voltage, a shedding test is performed to bring back the system

to its initial state. This induces a voltage overshoot which is

also quickly eliminated by the predictive controller.

The Fig.7 denotes the generator behaviour under the same

conditions as those presented by Fig.6. The voltage drop is

more important even if the overshoot is not very different.

When a load impact is performed, the system takes more

time to reach its steady state (roughly 300 milliseconds).

The current’ shape and magnitude are very similar to those

presented in Fig.6. By observing the voltage on the Fig.7, one

can notice a poor signal quality due to the excitation structure.

This is confirmed in Fig.8. In this last figure the superiority of

the proposed structure and control is checked as regards the

time response, voltage overshoot/drop and signal quality.

During the modeling process, one has assumed that the rota-

tion speed is considered as constant. Fig.9 allows to confirm

this assumption.

2 2.5 3 3.5 4 4.5 50.5

0.6

0.7

0.8

0.9

1

1.1

Time(s)

Mechanical speed (p.u)

Load impact Shedding test

Fig. 9. Mechanical speed during load impact and shedding test

This figure is recorded during the load impact and shedding

test and the data are given in p.u (1 p.u corresponds to

1500rpm). Thanks to the use of the motor drive, the speed

drop and overshoot can be considered are negligible during

the tests.

C. Results analysis

In the following table, Tr denotes the time response in

milliseconds which corresponds to the necessary time for the

system to reach again its steady state after a disturbance.

Or and Dr are respectively the relative voltage overshoot

and drop compared to the rated output voltage. In order to

examine the output signals quality, the voltage and current

THD are given respectively by THDv and THDc.

The Table II shows data during the load impact and shedding

test.

TABLE IIRESULTS FOR LOAD IMPACT AND SHEDDING TEST

Approaches Tr(ms) Or(%) Dr(%) THDc(%) THDv(%)Classical 300 4 12 1.12 5.41

Proposed 120 5 6 0.67 1.76

The new approach gives very satisfactory results compared

to the classical one. From time response to voltage THD, the

new method’s efficiency is noticeable. Both these tables allows

us to see that it is very interesting to use a voltage source

(PMSG) instead of current one (reverse SG) for synchronous

generator excitation. Thus, thanks to this structure, the output

voltage quality is strongly improved. The combination of such

a structure with a modern control strategy such as predictive

controller leads to interesting results as presented above.

IV. CONCLUSION

In this paper an original excitation structure for synchronous

generators is given. The particularity of such a structure is

based on the use of a voltage source (PMSG) combined

to thyristors bridge to feed the excitation winding instead

of the classical reverse SG combined with diodes bridge

which presents a low dynamics. To control this new structure,

one chooses a R S T type controller computed thanks to a

predictive strategy and using a classical synchronous gener-

ator model which integrates the rated load. The test bench

composed by electronics circuits, apparatus and a DSpace

1104 is presented and tests have been performed. The main

test presented in this paper is the load impact/shedding one.

The experimental results, comparing the proposed structure

and the classical one are presented. Thanks to these tests, one

has noticed that the new approach provides interesting results

as regards the signal distortion reduction and time response

improvement. These results show that improvements can be

obtained by right modification in the machine structure. The

next step of the study is the way to have a rotating thyristors

bridge and how it can be controlled by high frequency waves

for example. This will be an important innovation in electrical

machine performances improvement.

APPENDIX

Characteristics of the lsa371

The real time tests have been performed thanks to a syn-

chronous generator which parameters are given by Table III.

TABLE IIICHARACTERISTICS OF THE LSA371 4-POLES

Labels Values

Rated Power Sn (KVA) 7.5Stator resistance rs (Ω) 1.19Rotor resistance rf (Ω) 3.01Phase-to-phase rated voltage Urms (V) 400Direct synchronous reactance xd (p.u) 1.4Transverse synchronous reactance xq (p.u) 0.7

Open-circuit Transient time constant T′

do(ms) 522

Direct transient synchronous reactance x′

d (p.u) 0.099

Direct sub transient synchronous reactance x′′

d (p.u) 0.049

Direct transient time constant T′

d (ms) 40

Direct sub transient time constant T′′

d (ms) 3.7Armature time constant Ta (ms) 6

NOMENCLATURE

vabc machine three-phase voltage

vf main field voltage

iabc machine three-phase current

if main field current

id, iq stator direct and transverse currents

iD, iQ direct and transverse dampers currents

Ψabc stator total flux

ΨD direct dampers total flux

ΨQ transverse dampers total flux

Ψf main field total flux.

rs stator phase resistance

rD, rQ direct and transverse dampers resistances

rf main field resistance

r1, l1 load resistance and inductance

ld, lq stator direct and transverse main inductances

lf main field main inductance

lD, lQ direct and transverse damper main inductances

msf stator and main field mutual inductance

mfD main field and direct damper mutual inductance

msQ stator and transverse damper mutual inductance

msD stator and direct damper mutual inductance

ωe, θe electrical speed and position

2p machine number of poles

ωm mechanical speed

Te electromechanical torque

Tr load torque

J machine inertia

P active power

Q reactive power

S apparent power

I root mean square phase current

PF power factor

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