POZNAN UNIVERSITY OF TECHNOLOGY ACADEMIC JOURNALS No 95 Electrical Engineering 2018

DOI 10.21008/j.1897-0737.2018.95.0020

__________________________________________ * Maritime University of Szczecin

Maciej KOZAK*

APPLICATION OF SELF-EXCITED SYNCHRONOUS GENERATOR WITH INVERTER AS DC GRID VOLTAGE

SOURCE

Self-excited generators are the most popular voltage sources installed onboard of modern ships. Because of fuel savings on some specialized vessels (PSV, cable layers etc.) varying revolutions drives are used and this trend seems to increase. Very interest-ing issue is use of such type of alternator working with inverter acting as rectifier in DC grid system. As the control method most suitable to use is modified field oriented control known from induction machines. This control method involves decoupling of currents and control voltages to flux and torque components and keeping them in optimal (or-thogonal) condition. Theoretical background of inverter and synchronous generator adopted FOC control method along with experimental results obtained in laboratory of such system were presented. KEYWORDS: Self-excited generator, FOC control, DC grid, controlled voltage source.

1. PROPERTIES OF SELF-EXCTITED SYNCHRONOUS GENERATOR WITH VOLTAGE REGULATOR

The 7 kVA self-excited synchronous generator (SESG) driven by squirrel

cage motor was used for purpose of investigation. To obtain proper intermediate direct current voltage level electric generator has to be controlled in certain way. Because of self-excitation and use of the compound transformer feeding voltage regulator in synchronous generator there is no need to independent control of reactive current control so the field oriented control (FOC) algorithm was cho-sen. Compound transformer is very important part of investigated system be-cause it assures proper excitation current that is composed of vector sum of re-sulting phase voltages and currents

In Fig. 1, the scheme of a brushless self-excited synchronous generator with compound excitation system and a voltage regulator is presented. The excitation winding of the synchronous generator receives power from the rotating rectifier, which is powered by a secondary coil of the compound current transformer. The compound transformer contains current coil (coilI) and a voltage coil (coilU). The

210 summatioin this casource intsignal.

F

To ensditional cois provide

Fig. 2. In

The conominal oage regulregulation

on of the bothase the capato a current

Fig. 1. Scheme o

sure a properoil, which poed.

nfluence of rotat

ompound traone within thlator selects n is done by

h coils signalcitors. Compsource and

of automatic vo

r initial self-eowers the ex

tional speed on

ansformer is he range from

the redundadecreasing c

Maciej Kozak

ls is done bypound elemeshifts the ph

oltage regulator

excitation ofxcitation coil

n field current anwinding curren

designed to

m an idle to aant power abcurrent for m

k

y the compouent convertshase (by abo

with compoun

f the synchro(field windin

nd waveforms cnt

provide a han initial loabove the no

most of the o

und elementss coilU from out 90°) of th

nd transformer [

onous generang) through

close-up of rect

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which are a voltage

he voltage

2]

ator, an ad-a rectifier,

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ge than the matic volt-age, so the

me. Simpler

type of extional win

Fig. 3. Sch

All exvoltage reutilizes inpending oregulatorsspeed, thuFig. 3. wway.

Fig. 4. St

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xcitation uninding (coil0)

heme of voltage

xcitation direegulator intonformation oon load powes are designus the speed

while no-load

tartup voltage o

ication of self-

it is used in nor inductio

e regulator embden

ect current flo excitation of currents, ver factor rotoned for cont

change resud startup pro

of self-excited smagn

f-excited synch

investigatedon exciter wit

bedded in investnoted as SDR 1

lows throughwindings. Svoltages and

or current. Motinuous workults in voltagecess generat

synchronous genet generator P

hronous gener

d system. It dth rotating re

tigated system w82/6

h two slip riStill it is comd phase shiftost of synchrk with almoe changes. Ator’s voltage

nerator (SESG)PMSG

rator …

does not conectifier.

with compound

ings and brumpound exct to create pronous generost constant As it can be se changes in

) compared to p

211

ntain addi-

d capacitors

ushes from iter which

proper, de-rators with rotational

seen in the n nonlinear

permanent

212 Maciej Kozak

In contrary permanent magnet synchronous generators have more linear de-pendence between rotational speed and generated voltages, so PMSG’s slightly altered control techniques can be used for synchronous generators with com-pound voltage regulators. This property leads to conclusion that such type of generator can operate in limited range of rotational speed then PMSG.

1.1. Principle of synchronous generator operation

The classical control principle of the synchronous generators with independ-ent exciting winding is well known, considering the frequency and voltage con-trol by means of the active and reactive power adjustment. The active power is coming from the mechanical drive such as Diesel or gas combustion engines while the reactive power is commanded and controlled with the DC current exci-tation winding and voltage regulator. In most case the two control loops are op-erating separately each to others with use of RPM’s governor and voltage regu-lator. This kind of operation may be considered as a scalar control procedure, which disregards some phenomena, i.e. the coupling effect between electrical axis the synchronous generator [1]. The vector control is based on the field-orientation principle. It can be used as an AC induction motor drives control, but also for squirrel cage generator running. Because of its performance during tran-sient operation modes, it comes quite close to direct current machines. The mathematical background of the dynamic model and vector control of AC ma-chine is given by the space-phasor theory [1], [5]. Thanks to field oriented con-trol simplicity of DC machines control got into the AC motors and generators methods. The rotor flux oriented synchronous machine model is similar to a shunt excited direct current machine. It is suitable for the simulation of the syn-chronous generator operation, but the control will be realized with the field ori-ented model considering the resultant stator flux. This model leads to the analo-gy with the compensated DC machine, which allows the independent control of the two variables that produce the machine torque [1], [4], [5], [6]. In Fig. 5 there are shown the stator field oriented components of the stator current:

sqsds iii j (1)

while the armature coil flux equals to:

sqmqsdmdsssqsssdss iLiL j j (2)

where λs is the angular position of the resultant stator flux Ψs.

In genwhich detnegative dnegative, tive poweproduces componenphenomentriangle o[1]. In sucmeans actthe stator resultant smations (s

In ordtaken into

Appli

Fig. 5. Diagraand the sta

nerating modtermines thedue to the rdue to its de

er. The excitthe torque innt of the fielna in the syf the machinch a case, acttive and reaccurrent has tstator flux Ψsee Fig. 7).

1.2. Synch

er to have a o considerati

ication of self-

am of the synchator-field orient

de, the quade active powreversed actiemagnetizingting current n the air-gap.ld oriented e

ynchronous mne powers betive and reac

ctive stator cuto be orienteΨssqλ, realized

hronous gen

simpler conion regarding

f-excited synch

hronous generatted space phaso

drature comp

wer and dc inive energy flg character. has influenc. Because of exciting currmachine. Neecome also sctive power current comp

ed according d by a Clark

nerator FO

ntrol for SESg the produc

hronous gener

or with leadingors of the stator

ponent of thntermediate cflow. Flux deThis flux co

ce on an actf decoupled crent contribueglecting thesimilar to thacan be controonents. To cto the magne and Park (

OC control

SG some simced torque. T

rator …

g stator current, current [1]

he armature circuit voltaenoted as Ψorresponds toive compone

control the loutes to the me stator resisat of the statoolled indepen

control the poetizing direc(α-β and d-q

properties

mplification hThe load torq

213

flux Ψssdλ, age will be Ψssqλ is also o the reac-ent, which

ongitudinal magnetizing

stance, the or currents ndently by ower flow, ction of the q) transfor-

have to be que can be

214 controlledstrategy, tvector curequal to 9ity. The tois the one

where: p iAfter s

after a few

Fig. 6

From very simptorque and

2. INVE

Contro

drive appmediate v

In a mactive andprovides czero becalating blo

d by controllthe d axis current is align

90°. This is oorque equati derived in:

is a number osubstituting w simplificat

. Vector diagram

above equatple to impled machine cu

ESTIGATE

ol technique plications. Mvoltage contromanner of fied reactive cuconstant DC

ause of rotor ock (Machine

ling the torqurrent is kepned with the

one of the moon for a SES

eT p2

3

of pole pairsthe d-q curretions the torq

m for constant

eT

tion it can bement, just burrent.

ED SYSTEMTO T

is based onMain advantag

ol with meaneld oriented urrents. Acti

C link voltagcurrent creae equations

Maciej Kozak

que angle. Inpt zero for ae q axis in oost used contSG, taking in

dsqm LLi (

and ψm is fieents in equat

que value is t

torque control o

sqme iT p2

3

be easily obsby represent

M WITH GTHE INVER

n FOC methge of proposns of control control ther

ive current ie Udc value,

ating magnet– see Fig. 7

k

n the constanll of the opeorder to maitrol strategy nto account b

sdsqq iiL )

eld winding tion (3) as pthe one prese

of SESG in stea

served that tting the line

GENERATRTER

hod known fsed method active currene can be indisq control lowhile reacti

tic flux. In ad7) which can

nt torque angeration time,ntain the torbecause of itboth isd and i

flux. presented in ented in equa

ady state operat

the control pearization be

OR CONN

from inductiis possibilitynt loop. dependently oop in d-q cive current iddition, thern be useful

gle control while the rque angle ts simplic-isq currents

(3)

Fig. 6 and ation (4).

tion [6]

(4)

property is etween the

NECTED

on motors y of inter-

controlled coordinates isd is set to re is calcu-in case of

sensorlesswas utilizClark tran

Fig. 7.

The co

space vecphase quaes. For exexpressedplex plane

The α-nitudes ac

Anothethe stationthe rotor i

Appli

s control of zed to obtainnsformations

Vector control

ore of FOC ctor propertieantities as easxample, spacd by two-phae. Mathemati

-β componenccording to:

x

er very usefunary abc refeis called Park

ication of self-

generator. Inn rotational sp.

structure of the

is use of traes there’s possy to controlce vector xs ase magnitudically this re

sx x jx

nts of the spa

Re sx x

Im sx x

ful set of equerence framek transform.

f-excited synch

n the investipeed and sta

e self-excited syentation

ansformationssibility of pl constant varepresenting

des called xα lationship ca

2

3ax a

ace vector ca

2 1

3 2ax

2 3

3 2ax

uations transe to the d-q r

hronous gener

igated systemator field ang

ynchronous gen

s calculated rojection sin

alues of curreg aforementio

and xβ in than be written

2

b cax a x

an be calcula

1

2b cx x

3

2bx

forms stator reference fra

rator …

m incrementagle needed in

nerator with stat

in real-timenusoidal balaents, voltagesoned quantit

he real-imagin as:

ated from the

phase quantame which ro

215

al encoder n Park and

tor-field ori-

e. Using of anced three s and flux-ties can be inary com-

(5)

e abc mag-

(6)

(7)

tities from otates with

216

Equatitime just chine invneed for rknow valunous genefunctions indirect mor using m

2.1.

While ciple of mpacitors. Bis minimation.

Fig. 8. Cha

0

2

3

d

q

x

x

x

ions given into obtain vaerter and DCrotating framue of rotationerator in the and α-β pla

methods of cmachine state

Stand-alon

in so called maintaining cBecause of nal value of ge

ange of generat

cos cos

sin sin

1

2

n 4-8 are haalues of curreC link voltame angle θ cnal speed of steady state

ane voltages calculating roe simulators

ne work of tvolta

island operaconstant DCnonlinear fluxenerator RPM

tor phase currenunder re

Maciej Kozak

2s c

3

2n

3

1

2

ard coded inents and vol

age. To propcalculation. Tf the shaft. Inangle is calcand currents

otor positionand observer

the generatage control

ation synchro

C voltage on x current depM’s which e

nt due to speed esistive load of

k

2cos

3

2sin

3

1

2

nto VDSP++ltages neededper operationTo obtain ro

n this particuculated with s. In the liter

n angle by inrs [3].

tor in dc inl loop

onous generainverter’s in

pendency onensure proper

decrease (1500f 2,1 kW.

a

b

c

x

x

x

+ and executd for easy co

n of algorithmotor angle it ular system outilizing trigrature there

ntegration of

ntermediate

ator works inntermediate n rotational sr excitation a

0 min-1 down to

(8)

ted in real ontrol ma-m there is crucial to

of synchro-gonometric are shown

f the speed

e circuit

n the prin-circuit ca-peed there and opera-

o 900 min-1)

In preslike in thedepends omachine irotor anglload, the stand-aloning for shcal load aterm shou

With s

values of voltage erting moreWhen DCthe voltagble.

To pre

SESG invdiodes muportant wused in po

As it cbe fair ening diodeAlthough some adv

Appli

sented systeme generator won temporaryinverters conle calculationvoltage fre

ne operation hort time neeapplied to DCuld also chan

Fig. 9. Depend

significant eproportionalrror increasee significant C voltage signge overshoot

3. US

event circulaverter and aust be imple

while parallelower electroncan be seen inough but in s are placed the negativ

antages espe

ication of self-

m minimal sworking withy rotational ntrol algorithn and instanquency chan intermediat

eded by PI reC bus can va

nge.

dence between

rror value inl gain isq chaes above min

so the set vnal error dect is relatively

SE OF AU

ating currentanother convemented intoing generatonics circuits in Fig. 10 plpresented syin series wi

ve leg diode ecially while

f-excited synch

speed of opeh constant spspeed and t

hm. The connt inverter frnges to the e circuit vol

egulators to sary in wide r

proportional ga

n actual DC anges in the mnimal allowavoltage valuecreases propoy small and t

CTIONEE

ts flow throverter conneo the power or in DC powwith controlacing only oystem was uth both the padds lossesground fault

hronous gener

eration was speed the resuthis relationsntrol methodrequency cha

stable poinltage Udc chaset error valurange so the

ain term and DC

voltage whmanner showable level proe can be obtortional gain the oscillatio

ERING DIO

ough conduccted in paracircuit [5].

wer grid. Suled rectifiers

one diode on used another positive and s to the systt operation.

rator …

set to 800 RPulting electriship is includ is based onanges. With nt of operatianges with loue to zero. Tvalues of pr

C bus voltage e

hile transient wn in Fig. 9. oportional tetained in shoterm also de

ons are gettin

ODES

ting transistallel the aucThis is espe

uch systems as and power d

positive outapproach. Anegative out

tem this app

217

PM’s. Un-ical power ded in the

n real time increasing on. While oad apply-

The electri-roportional

error

processes When DC

erm is get-orter time. ecreases so ng negligi-

tors of the ctioneering ecially im-are widely diodes. tput would

Auctioneer-tput feeds.

proach has

218

Since

sion, comtion of theering diodownstreabuses andlead to hineeds to b

3.1. P

Auctio

rect voltacontrolledasynchronsen as it equipped nying devAnother gbatteries asynchroniof voltagesented sysin Fig. 10trols levelmaintaine

Fig. 10. Sch

symmetry ismmon mode ahe dual grounodes in the pam loads dud the asymmgh common

be managed b

arallel work

oneering diodage sources d rectifiers innous squirrelis shown in with dedica

vices reducesgreat benefit and work wizers becausees to the levstem is robus0 while parall of DC bus

ed voltage ch

heme of propose

desirable inand differentnd fault scenositive feed

uring a dual metry of aucti

mode circulby converter

k of the self

des allow palike DC-DCn DC grid. Il cage generaFig. 10. In

ated inverterss weight andof using DC

ith varying re connectingel of commost power distllel grid opevoltage. Be

hanges sligh

Maciej Kozak

ed system with

n systems thaial mode behnario leads tonly will miground faulioneering diolating currenr controls and

excited sync

arallel operatC converters In the investator (SCG) wsuch a grid

s but lack ofd size of shipC system worotational sp

g other DC son bus voltagtribution alg

eration worksecause of its htly accordin

k

use of auctione

at involve swhavior is impo following itigate voltagt of oppositeodes in the p

nts between Dd protections

chronous ge

tion of SESGor other typ

tigated systemwith its machall of the A

f transformerps electrical

orking in parpeed. Such ssources is bage. Another orithm. Gens as a so calpower (this

ng to electric

eering diodes

witching powportant. The conclusions

ge doubling se polarity onpositive feedDC/DC convs [6].

enerator in d

G along withpes of genem as other Dhine inverter

AC consumerrs and other system of ab

rallel is possiystem doe nased on slighgreat thing erator denotelled “master”is weak grid

cal load. Sy

wer conver-considera-

s. Auction-stresses on n opposite d only will verters that

dc grid

h other di-erators and DC source r was cho-rs must be accompa-

about 30%. ible use of not require ht changes about pre-ed as SCG ” and con-d) level of

ynchronous

generator active curgenerator ly unlimitalgorithm

In paraSESG anchange an

Fig.

Appli

in parallel mrrent isq and and inverter

ted number om.

Fig. 1

allel operatiod another so

nd such type

12 Process of p

ication of self-

mode worksin case of e

r decreases toof converters

11 Process of pa

on control algource in ordof control is

parallel operatio

f-excited synch

in power shexceeding thoo. This mets and genera

arallel connecte

gorithm can der to synchs shown in Fi

on SESG in powchange

hronous gener

haring modehis value volthod allows cators equippe

ed SESG load s

be set to dishronous geneig. 12.

wer sharing mod

rator …

e. There is liltage drops connection thed with curre

haring

stribute poweerator rotatio

de while rotatio

219

imit set on so load of heoretical-ent control

er between onal speed

onal speed

220 Maciej Kozak

As it can be observed that with rotational speed increase active current value drops because the amount of generated power is still the same. What is im-portant all characteristics can be customized for flawless work with digital ener-gy management system and digital governors to obtain optimal fuel consumption by prime movers [5].

4. CONCLUSIONS AND FURTHER WORK

Proposed system including variable speed generators in altered versions is

now tested onboard of few seagoing specialized vessels. Estimated fuel savings values are in range between 22% and 30% depending on the ships work condi-tions. As it was proven in laboratory test bench system is very stable and DC voltage variations does not exceed 5% in steady state and 10% in transient oper-ation. Further work will focus on integrating ultracapacitors with controlled DC-DC converter to presented system. Thanks to its properties, the system will be further developed towards further optimization and easy integration with exist-ing solutions.

BIBLIOGRAPHY

[1] Imecs M., Iov Incze I., Szabo C., Stator-Field Oriented Control of the Synchronous

Generator: Numerical Simulation, 2008 International Conference on Intelligent En-gineering Systems, ISSN 1543-9259, 2008.

[2] Djagarov N. F., Lazarov T., Investigation of Automatic Voltage Regulator for a Ship’s Synchronous Generator, European transactions on electrical power engi-neering 2016/33, pp. 16-21, 2016.

[3] Rashed M., MacConnell P.F.A., Stronach F., Acarnley P., Sensorless Indirect-Rotor-Field-Orientation Speed Control of a Permanent-Magnet Synchronous Motor With Stator-Resistance Estimation, IEEE Transactions on Industrial Electronics, Volume 54, Number 3, pp. 1664-1675, 2007.

[4] Kozak M., Bejger A., Gordon R., Control of squirrel-cage electric generators in parallel intermediate dc circuit connection, Zeszyty Naukowe Akademii Morskiej w Szczecinie, Volume 45, Number 117, pp. 17-22, 2016.

[5] Baktyono S. A Study of Field-Oriented Control of a Permanent Magnet Synchro-nous Generator and Hysteresis Current Control for Wind Turbine Application, The Ohio State University, 2006.

[6] Balog, R., Krein, P.T.,Bus Selection in Multibus DC Microgrids. IEEE Transac-tions on Power Electronics. Volume 26, Number 3, pp. 860–867, 2011.

(Received: 08.02.2018, revised: 15.03.2018)