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

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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 because 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

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210

summatio in this ca source int signal.

F

To ens ditional co is provide

Fig. 2. In

The co nominal o age regul regulation

on of the both ase the capa

to a current

Fig. 1. Scheme o

sure a proper oil, which po ed.

nfluence of rotat

ompound tra one within th lator selects n is done by

h coils signal citors. Comp

source and

of automatic vo

r initial self-e owers the ex

tional speed on

ansformer is he range from

the redunda decreasing c

Maciej Kozak

ls is done by pound eleme

shifts the ph

oltage regulator

excitation of xcitation coil

n field current an winding curren

designed to m an idle to a

ant power ab current for m

k

y the compou ent converts hase (by abo

with compoun

f the synchro (field windin

nd waveforms c nt

provide a h an initial loa bove the no most of the o

und elements s coil_U from out 90°) of th

nd transformer [

onous genera ng) through

close-up of rect

higher voltag ad. The autom ominal volta

peration tim

which are a voltage he voltage

2]

ator, an ad- a rectifier,

tified field

ge than the matic voltage, so the me. Simpler

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type of ex tional win

Fig. 3. Sch

All ex voltage re utilizes in pending o regulators speed, thu Fig. 3. w way.

Fig. 4. St

xcitation uni nding (coil₀)

heme of voltage

xcitation dire egulator into nformation o on load powe s are design us the speed while no-load

tartup voltage o

it is used in nor inductio

e regulator emb den

ect current fl o excitation of currents, v er factor roto ned for cont change resu d startup pro

of self-excited s magn

investigated on exciter wit

bedded in invest noted as SDR 1

lows through windings. S voltages and or current. Mo

tinuous work ults in voltage cess generat

synchronous ge net generator P

d system. It d th rotating re

tigated system w 82/6

h two slip ri Still it is com

d phase shift ost of synchr k with almo e changes. A tor’s voltage

nerator (SESG) PMSG

does not con ectifier.

with compound

ings and bru mpound exc

t to create p ronous gener ost constant As it can be s e changes in

) compared to p

ntain addi-

d capacitors

ushes from iter which proper, de-

rators with rotational seen in the n nonlinear

permanent

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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 compound 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 independent exciting winding is well known, considering the frequency and voltage control 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 excitation 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 regulator. 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 transient operation modes, it comes quite close to direct current machines. The mathematical background of the dynamic model and vector control of AC machine is given by the space-phasor theory [1], [5]. Thanks to field oriented control 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 synchronous generator operation, but the control will be realized with thefield oriented 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:



 sq

sd

s i i

i   j (1)

while the armature coil flux equals to:



 



ss  ssds j ssq s Lmdisd  jLmqisq (2) where λ_s is the angular position of the resultant stator flux Ψ_s.

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In gen which det negative d negative, tive powe produces componen phenomen triangle o [1]. In suc means act the stator resultant s mations (s

In ord taken into

Fig. 5. Diagra and the sta

nerating mod termines the due to the r due to its de er. The excit

the torque in nt of the fiel na in the sy f the machin ch a case, act tive and reac current has t stator flux Ψ see Fig. 7).

1.2. Synch

er to have a o considerati

am of the synch ator-field orient

de, the quad e active pow reversed acti emagnetizing ting current n the air-gap.

ld oriented e ynchronous m

ne powers be tive and reac ctive stator cu

to be oriente Ψ_ssqλ, realized

hronous gen

simpler con ion regarding

hronous generat ted space phaso

drature comp wer and dc in ive energy fl g character.

has influenc . Because of exciting curr

machine. Ne ecome also s ctive power c

urrent comp ed according d by a Clark

nerator FO

ntrol for SES g the produc

or with leading ors of the stator

ponent of th ntermediate c flow. Flux de This flux co ce on an act f decoupled c rent contribu eglecting the similar to tha can be contro onents. To c

to the magn e and Park (

OC control

SG some sim ced torque. T

g stator current, current [1]

he armature circuit volta enoted as Ψ orresponds to

ive compone control the lo utes to the m e stator resis at of the stato

olled indepen control the po

etizing direc (α-β and d-q

properties

mplification h The load torq

flux Ψssdλ, age will be Ψ_ssqλ 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

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214

controlled strategy, t vector cur equal to 9 ity. The to is the one

where: p i After s after a few

Fig. 6

From very simp torque and

2. INVE

Contro drive app mediate v In a m active and provides c zero beca lating blo

d by controll the d axis cu rrent is align 90°. This is o

orque equati derived in:

is a number o substituting w simplificat

. Vector diagram

above equat ple to imple d machine cu

ESTIGATE

ol technique plications. M voltage contro manner of fie

d reactive cu constant DC ause of rotor ock (Machine

ling the torq urrent is kep ned with the one of the mo on for a SES



Te p 2

3  of pole pairs the d-q curre tions the torq

m for constant

Te

tion it can b ement, just b

urrent.

ED SYSTEM TO T

is based on Main advantag

ol with mean eld oriented

urrents. Acti C link voltag

current crea e equations

Maciej Kozak

que angle. In pt zero for a e q axis in o ost used cont SG, taking in

d sq

mi (L L



and ψm is fie ents in equat que value is t

torque control o

sq m

e i

T p

2

3

be easily obs by represent

M WITH G THE INVER

n FOC meth ge of propos ns of control control ther ive current i e Udc value, ating magnet

– see Fig. 7 k

n the constan ll of the ope order to mai

trol strategy nto account b



sd sq q i i L )

eld winding tion (3) as p the one prese

of SESG in stea

served that t ting the line

GENERAT RTER

hod known f

sed method active curren e can be ind i_sq control lo while reacti tic flux. In ad 7) which can

nt torque ang eration time,

ntain the tor because of it both isd and i

flux.

presented in ented in equa

ady state operat

the control p earization be

OR CONN

from inducti is possibility nt loop.

dependently oop in d-q c ive current i ddition, ther n 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

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sensorless was utiliz Clark tran

Fig. 7.

The co space vec phase qua es. For ex expressed plex plane

The α- nitudes ac

Anothe the station the rotor i

s control of zed to obtain nsformations

Vector control

ore of FOC ctor propertie

antities as eas xample, spac d by two-pha e. Mathemati

-β componen ccording to:

x

er very usefu nary abc refe is called Park

generator. In n rotational sp

.

structure of the

is use of tra es there’s pos

sy to control ce vector x_s ase magnitud

ically this re xs  x_  jx_ nts of the spa

 

Re _s x_  x

 

Im _s x_  x ful set of equ

erence frame k transform.

n the investi peed and sta

e self-excited sy entation

ansformation ssibility of p l constant va representing des called x_α lationship ca

2



3 x^a a

  

ace vector ca

2 1

3 x^a 2

  





^²₃^^ ₂³^x^a

 uations trans e to the d-q r

igated system ator field ang

ynchronous gen

s calculated rojection sin alues of curre g aforementio

and x_β in th an be written



2

b c

ax a x an be calcula

1

b 2 c

x  x 



3 2 x^b

 

 forms stator reference fra

m incrementa gle needed in

nerator with stat

in real-time nusoidal bala ents, voltages

oned quantit he real-imagi n as:

ated from the

phase quant ame which ro

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

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216

Equati time just chine inv need for r know valu nous gene functions indirect m or using m

2.1.

While ciple of m pacitors. B is minima tion.

Fig. 8. Cha





  

  

   

0 

2 3

d q

x x x

ions given in to obtain va erter and DC rotating fram ue of rotation erator in the

and α-β pla methods of c machine state

Stand-alon

in so called maintaining c

Because of n al value of ge

ange of generat



 





cos cos

sin sin 1

2

n 4-8 are ha alues of curre

C link volta me angle θ c

nal speed of steady state ane voltages calculating ro

e simulators

ne work of t

volta

island opera constant DC nonlinear flux

enerator RPM

tor phase curren under re

Maciej Kozak

 



 

 

 

 

 

 

 

s 2 c

3 n 2

3 1 2 ard coded in

ents and vol age. To prop calculation. T f the shaft. In

angle is calc and currents otor position

and observer

the generat age control

ation synchro C voltage on

x current dep M’s which e

nt due to speed esistive load of k

 



 

 

  

  

 

 

 

 cos 2

3 sin 2

3 1 2 nto VDSP++

ltages needed per operation

To obtain ro n this particu culated with s. In the liter n angle by in

rs [3].

tor in dc in l loop

onous genera inverter’s in pendency on ensure proper

decrease (1500 f 2,1 kW.



 

 

  

  

 



a b c

x x x

+ and execut d for easy co n of algorithm

otor angle it ular system o utilizing trig rature there ntegration of

ntermediate

ator works in ntermediate n rotational s r excitation a

0 min^-1 down to

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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)

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In pres like in the depends o machine i rotor angl load, the stand-alon ing for sh cal load a term shou

With s values of voltage er ting more When DC the voltag ble.

To pre SESG inv diodes mu portant w used in po

As it c be fair en ing diode Although some adv

sented system e generator w on temporary inverters con le calculation voltage fre ne operation hort time nee applied to DC uld also chan

Fig. 9. Depend

significant e proportional rror increase e significant C voltage sign

ge overshoot

3. US

event circula verter and a ust be imple while parallel

ower electron can be seen i nough but in s are placed the negativ antages espe

m minimal s working with y rotational ntrol algorith

n and instan quency chan intermediat eded by PI re C bus can va nge.

dence between

rror value in l gain isq cha es above min

so the set v nal error dec t is relatively

SE OF AU

ating current another conv emented into ing generato nics circuits in Fig. 10 pl presented sy in series wi ve leg diode ecially while

speed of ope h constant sp

speed and t hm. The con nt inverter fr

nges to the e circuit vol egulators to s ary in wide r

proportional ga

n actual DC anges in the m

nimal allowa voltage value creases propo y small and t

CTIONEE

ts flow thro verter conne o the power or in DC pow

with control acing only o ystem was u th both the p

adds losses ground fault

eration was s peed the resu this relations ntrol method requency cha stable poin ltage Udc cha set error valu range so the

ain term and DC

voltage wh manner show able level pro e can be obt ortional gain the oscillatio

ERING DIO

ough conduc

cted in para circuit [5].

wer grid. Su led rectifiers one diode on used another

positive and s to the syst t operation.

set to 800 RP ulting electri ship is inclu d is based on

anges. With nt of operati anges with lo ue to zero. T

values of pr

C bus voltage e

hile transient wn in Fig. 9.

oportional te tained in sho

term also de ons are gettin

ODES

ting transist allel the auc This is espe uch systems a s and power d positive out approach. A negative out tem this app

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

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218

Since sion, com tion of th eering dio downstrea buses and lead to hi needs to b

3.1. P Auctio rect volta controlled asynchron sen as it equipped nying dev Another g batteries a synchroni of voltage sented sys in Fig. 10 trols level maintaine

Fig. 10. Sch

symmetry is mmon mode a he dual groun

odes in the p am loads du d the asymm gh common be managed b

arallel work oneering diod age sources d rectifiers in nous squirrel

is shown in with dedica vices reduces great benefit and work w izers because es to the lev stem is robus 0 while paral l of DC bus ed voltage ch

heme of propose

desirable in and different nd fault scen

ositive feed uring a dual metry of aucti

mode circul by converter k of the self des allow pa

like DC-DC n DC grid. I l cage genera

Fig. 10. In ated inverters

s weight and of using DC ith varying r e connecting el of commo st power dist llel grid ope

voltage. Be hanges sligh

Maciej Kozak

ed system with

n systems tha ial mode beh nario leads t

only will mi ground faul ioneering dio lating curren r controls and

excited sync arallel operat C converters In the invest ator (SCG) w

such a grid s but lack of d size of ship C system wo

rotational sp g other DC s on bus voltag

tribution alg eration works ecause of its htly accordin k

use of auctione

at involve sw havior is imp o following itigate voltag t of opposite odes in the p nts between D

d protections chronous ge tion of SESG

or other typ tigated system with its mach

all of the A f transformer ps electrical orking in par peed. Such s sources is ba

ge. Another orithm. Gen s as a so cal power (this ng to electric

eering diodes

witching pow portant. The conclusions ge doubling s

e polarity on positive feed DC/DC conv s [6].

enerator in d G along with pes of gene m as other D hine inverter AC consumer rs and other system of ab rallel is possi

ystem doe n ased on sligh great thing erator denote lled “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

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generator active cur generator ly unlimit algorithm

In para SESG an change an

Fig.

in parallel m rrent i_sqand

and inverter ted number o m.

Fig. 1

allel operatio d another so nd such type

12 Process of p

mode works in case of e r decreases to of converters

11 Process of pa

on control alg ource in ord

of control is

parallel operatio

in power sh exceeding th oo. This met s and genera

arallel connecte

gorithm can der to synch s shown in Fi

on SESG in pow change

haring mode his value vol thod allows c ators equippe

ed SESG load s

be set to dis hronous gene

ig. 12.

wer sharing mod

e. There is li ltage drops connection th ed with curre

haring

stribute powe erator rotatio

de while rotatio

imit set on so load of heoretical- ent control

er between onal speed

onal speed

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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 important all characteristics can be customized for flawless work with digital energy 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 operation. 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)