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CHAPTER 1 ; INTRODUCTION
1.1
General Introdxjction
1.2
MHD Generator-Power
System In te g ra tio n
1.3
Scope and O u tlin e o f
the th es is
1.*
CHA P T E R
1
I N T R O D U C T I O N
!• !
Om eral Introduction
Nagnetohydrodynamlc (NHD) d ireot energy conversion was
f i r s t observed by Michael Faraday In 1932*
The phenomenon was
e ^ erlm e n tally v e r ifie d by him, by sensing, through galvonometer, the emf Induced In the sa lin e and hence conducting water
o f riv e r Thames due to Its flow In the magnetic f i e ld o f the
earth [ 1 , 2 ].
The extraction o f e le c t ric a l energy from such a
natural source could not gain momentum as the conductivity o f
water Is quite low and the earth’ s magnetic f i e l d very weak to
y ie ld appreciable amount o f e le c t r ic a l power.
Besides the
Thames r iv e r In vestigations, most o f the Faraday’ s work was
with conductive liq u id metals, lik e murcury.
I t was some
times la t e In the 19th century that the e le c t r ic a l properties
o f thermally Ionised gases and th e ir Interaction with magnetic
f i e ld were studied and the f e a s i b i l i t y o f MHD generation was
re a lise d .
A fte r the World War I I , Magnetohydrodynamlcs (MHD)
emerged as one component o f developing Interests In the under­
standing and u t iliz a tio n o f ionized gases or plasmas under
high temperatvire conditions.
Kinetic theory o f gases, s t a t is t
t lc a l mechanics and quantum theories were developed in large
part as explanations o f what happened in gas discharge tiJbes
[ 3 ].
2
In 1 9 ^ K arlovitz patented the design o f an MHD
device and around 1950*s, Dr. Arth\ir R, Kantrowitz and yomg
researchers did the o rig in a l work fo r the MHD power genera­
tion [ 3 ] .
In the year 1959 AVCO.s f i r s t generator, the Mark-
I was built by Dr. Richard J* Rosa [4 ,5 ].
From I960 to mid
seventies number o f te s t and esqperimental f a c i l i t i e s were
setup with international exchange o f informations [6 ].
The ch aracteristics o f an MHD generator lik e it s
compactness, sim plicity and high power density makes i t
s p e c ia lly su ita b le fo r a v ariety o f applications in the
m ilitary area.
That is why i t was developed duriiig the world
war I I as mentioned above*
An MHD system has no moving p arts,
operates a t high temperature and has low weight to power ra tio .
As s\jch i t is found to be very appropriate fo r space ap p li­
cations lik e space veh icles, space power systoas, space p la te forms in addition to power systems in a ir c r a ft s and ship
propulsion.
A unique use o f an MHD generator is as a portable
source o f power fo r emergency, standby or mobile reqtiirwents
including temporary power fo r s c ie n t ific e^qperiments in iso­
late d areas [2 ].
The area in MHD development that has received the
maximum international attention and has a potential fo r power
generation is the MHD conversion in f o s s i l fu e l based systems.
Inspite o f advent o f other energy soiirces coal s t i l l remains
the most important fu el fo r power generation a l l over the world
and about 79 percent o f e le c t r ic a l power is sxqpplied by
thennal power p lan ts[7 ].
With the Increasing energy demands
and diffllnlshlng reserves o f natural f o s s il fu e ls , l o t o f
time and energy has been spent in evaluating the altern ative
concepts in the energy conversion techniques.
In mid seven­
tie s Energy Conservation Alternatives Study (EGAS) was con­
ducted and the resu lts proved that open cy c le » c o a l-fir e d ,
d ire c t preheat MHD systams have p o ten tially one o f the
highest c o a l-p ile to bxjs-bar e ffic ie n c y and also one o f the
lowest cost o f e le c t r ic it y (COE) among the varioue advanced
power plant concepts xjsing coal or coal derived fu els [8 ] •
For b e tte r e ffic ie n c y and cheap-er e le c t ric a l power generation
an MHD generator and associated components could be coupled
to an existing conventional steam plant as a topping cycle
and could be optimized fo r f u l l load operation as a base load
plant [ 9 ] .
However occasional demands fo r p art load opera­
tion may also occur during emergencies.
A f i r s t stage coal fir e d MHD plant is projected to
achieve a cycle efficiesaey o f ^
to 32 percent compared to
conveoitional plants o f about 29 to 35 percent and second
stage MHD generation is expected with an o v e ra ll efficien cy
o f 60 percent. The consumer cost o f e le c t r ic it y per KVh with
MHD system w i l l be approximately 18 to 20 percent lower than
the conventional thermal power plan ts, and the fu el saving
betweoi 20 to 30 percent with le s s heat ' wastage par unit
k
o f the useful power generated [1 0 ]«
In addition to cost and effic ie n c y consideration, MHO
technology has also been viewed fo r commercial applications
because o f Its b e tte r thermal and a i r pollution control
features.
The improved e ffic ie n c y in MHD plants implies a
reduction o f pollutants*
A fu rth er reduction is achieved
with the anisslon control schemes.
NO
once formed in the
combustor can be e ffic ie n tly reduced along the flow path in
the MHD channel with the optimised d istrib u tio n o f temperature,
pressure and velo city .
is automatically ronoved as seed
m aterial combines with sulphur as the gas steam cools [ l l ] .
I t is reported that 99 percent oxides o f sulphtjr out o f the
gases emitted fxx>m the MHD generator are eliminated and thus
the p ollu tion kept within the perm issible lim its*
Also two
stage cyclonic combxjustor with ammonia in fection reduces the
NO
emission w e ll below the perm issible lim its .
Waste d ls -
posal Involves several m illion tonnes o f ash per year from
any normal power station a t present.
A reduction in coal
consunptlon Implies considerable ash reduction [1 2 ]. Further
studies are being c arried out and new technologies are being
developed fo r b e tte r environmental and p ollu tion control
[13 ,1 4].
Because o f the above promising p oten tials Magnetohydrodynamic power generation from the f o s s i l fu els is
onerging as the most e ffe c tiv e energy conversion method
[1 5 ].
The construction o f a commercial f u e l-fir e d MHD
generator unit with a capacity o f 500 MW has already begun
a t the s it e o f the Ryazan Coatral Power Station (USSR).
The commissioning is planned in near future.
At the outset
o f 1990*8 the f i r s t f u e l-fir e d commercial power generating
MHO \jnit having a capacity o f 500-1000 MW is expected to be
in sta lle d [1 6 ].
MHD involves many d iscip lin es*
The knowledge o f
d iscip lin es sixih as, gasdynamicsi thermodynamics» plasma
physics* Electromagnetics* flu id mechanics etc* is necessary
fo r the analysis and design o f MHD power generators.
In ter­
national cooperation is very much helpfuQ. in the system
design* material development* plasma effects* componait
design and development, diagnostics and power system integ­
ration.
Tr mendous progress has already been made in these
areas and commercial generators fo r la rg e scale integration
o f MHD power into existing power g rid are expected very soon,
MHD a c t iv it ie s in various countries depend upon the type o f
systems being considered and the le v e l o f e ffo r ts Involved.
The international liaso n group on MHD e le c t r ic a l power gene­
ration is providing exchange o f informations between the
s p e c ia lis t 's o f various countries.
Research and develop­
ment work has been stepped \jq[) a t an advance le v e l to cOTimerc ia liz e th is technology by the « i d o f th is century in many
coiaitries o f the world p a rtic u la rly , USA, USSR, Japan,
Netherland, China, Atastralia and India [17 -2 6].
1.2
MHD Generator Power System Integration
MHD power is delivered in d ire ct current form which
could be used d ire c tly in the normal chemical processes
without the need fo r d ,c ./ a .o . inversion but most econcoiio
and useful application o f MHD generation is fo r the inters
connection o f MHD generator with e le c t ric a l supply u t i l i t y
fo r bulk power transmission.
Interfacing MHD power plants
with the stablished e le c tric u t i l i t y g rid depends \jpon the
a b i li t y to e f f ic ie n t ly and r e lia b ly in vert d.c* pov;er to
conv«itional a ,c . power*
Therefore the in verter systems,
which accomplish this process are an important subsystem o f
an MHD power plan t.
They should be e ffic ie n t , r e lia b le
economical and c a re fu lly con trolled systons.
So fa r two
types o f in verter systons have bem developed, namely, a
rotary in v erter and the other is a s ta tic inverter,
A novel rotary in verter system makes use o f a p a ra lle l
connected rotary motor (PCRM) driving a conventional a lt e r ­
nator J27].
The design, fa b ric a tio n , testing and s t a b i li t y
studies o f Such a system has already been performed (28 ,2 9].
A conceptual design o f an . MHD motor is also proposed [3 0 ].
The advantages o f rotary in verter systems are that 7
(i)
a ll
the electrode-p airs o f an MHD generator can be
connected to a sin g le machine while maintaining
e le c t r ic a l iso la tio n between electrode p a ir s »
(ii)
the
systttB provides a means o f control to obtain the
desired d istribu tio n o f load along the MHD channel*
(iii)
the
sjrstan does not require elaborate f ilt e r in g
equipment and
(iv )
owing to i t s mechanical in e rt ia , the Inverter system
decouples the MHD generator from sudden load d istu r­
bances.
In this system MHD power is f i r s t converted
into mechanical power which in tu rm ^ converted to
e le c t r ic a l power.
Therefore th is s y s t ^ is somewhat
c o stly and in e ffic ie n t .
Static inveirters provide a d irect e le c t ric a l connec­
tion between the a .c . and d.c» systems by means o f s o lid state
th y risto r switches.
The use o f so lid state inverters in con­
ventional applications has achieved mattarity but more p e rfo r­
mance data from inverters operating with MHD generators under
fxjiLl load, p a r t ia l load and tra n sim t conditions miost be
acquix*ed fo r optimal MHD in verter design fo r maximum power
tran sfer.
MHD in verter requirements include high power
fa c to r, low harmonic power, d.c* and a.c* transient protec­
tion and a.c* power control.
8
s ta tic lin e commutated inverters have an stablished
technology and are being operated a t power le v e ls o f hundreds
o f magawatts in high voltage d.o« transmission*
Therefore
e a r lie r studies are mainly devoted to lin e commutated inver­
ters fo r in terfacin g HHD generator to power g rid and they are
referenced elsewhere [3 1 ].
Recent studies with lin e commu­
tated inverters include series capacitive compensation fo r
improved power facto r, increased commutation s t a b ilit y and
H all voltage is o la tio n [3 2 ].
A lin e commutated in verter
system has also b e « i used as load regulator o f the NHD gene­
rato r [3 3 ].
Stable operation o f the MHD-inverter lin k has
been studied and s ig n ific a n t harmonic reduction has been
achieved with higher pulse numbers [3A-,35].
The in verter
system was connected with s ta tic VAR generator and o s c illa ­
tion behaviour and it s elemination by current rate feedback
is described [3 6 ].
T i^ s ie n t interaction and s t a b ilit y
studies o f a mill t ip i e^electrode MHD generator have appeared
[3 7 ].
Despite a l l the e ffo r ts fo r b e tte r performance, power
facto r compensation is necessary because o f low power facto r
and inorder to operate near unity power fa c to r a t maximum
conversion effic ie n c y approximately 1,5 MVA o f power fac to r
correction must be provided fo r each MW o f power generated
[4 4 ],
J*Lne commutated inverters cannot work with iso lated
loads.
Static forced commutated in verter systems are an
9
onerglJig technology with many advantages namely
( a ) the a.c* power fa c to r can be ste p le ssly contro­
l le d over any required range,
(b ) b e tte r voltage and harmonic controli
(c ) commutation fa ilu r e induced fa u lts are minimised,
(d ) fa s t system response and
(e ) b e tte r s t a b ilit y .
With the above advantages, extensive th eoretical and
e^cperimoital studies must be made*
However, so fa r , world
wide research in this area has been rather meagre and publica­
tions on th is subject have been lim ited to a very small nuBber
o f papers such as [38] to [ ^ ] .
Reference [38] is the comparative st\idy o f the p erfor­
mance o f lin e and forced commutated in verters, References[39]
and lUo] describe an in verter systan having two controls
(a ) notch width control to determine the ra tio between the
MHD generator d .c . output and the a .c « lin e voltage and
(b ) a phasing control to regulate the angle, leading or
laggin g, between the in verter output and the a .c , lin e vol­
tage.
The process o f voltage regulation prodixjes varying
harmonics which are cancelled in the s p e c ia lly designed magne­
tic s*
Reference [4 l ] discusses about the trad eoffs o f voltage
and current source in verter technologies,
• 10
References [U 2 ], [^3] and [W f] are the studies
concerning MHD channel - in verter interactions including
fa u lt protection, open and closed loop control and s t a b ilit y .
Reference
[45] describes the requirements o f MHD, in verter
and u t i l i t y g rid fo r successful operation using PWM techni­
ques,
Reference [ ^ ]
describes some vsaique consideration o f
four electrode consolidation in verter systems with computer
control.
Above mentioned references are mostly related with
design testing and fabricatio n o f forced commutated inverters
upto few mega-watts.
MHD generation a t commercial le v e ls is
in the range o f 300 to 2000 MW a t the f i r s t stage and there­
fore above informations may not be s u ffic ie n t to provide
answers to many questions that may come tgp about the integra­
tion o f MHD generators into a .c . network using forced comm\>tated in verters.
Therefore d etailed investigations must be
made fo r the switching lo s s e s , flu id dynamic coupling, harmonic
control under v a ria b le operation, s t a b ilit y and maximum power
tran sfer from MHD dtjct with desired power fac to r.
1,3
Soepe and outline o f the thesis
The work z*eported in th is thesis pertains to the
problems associated with the force commutated in verter fo r
the integration o f an MHD generator into an a .c . power network.
Special attention has been paid to evolve a simple control
11
systan and improved power fac to r fo r the conversion and
control o f MHD power.
The th eoretical and eoqperimental
studies presented, ill\ js tra te the f e a s i b ilit y o f using a
simple square wave In verter to convert and control the out­
put power o f an MHD duet feeding Into a power system under
maximum power tran sfer and unity power fa c to r conditions.
Bower flow eqvatlons have been derived treating MHD
in verter lin k as a sin g le machine connected to In fin it e b\]s.
The operating point is located fo r the maximum power tran sfer
from an MHD duet with p a rtic u la r values o f the series reactor
and the phase s h if t angle.
Modified power flow equations are
derived with f i l t e r s added to eliminate harmonics.
A computer
programme has been prepared to simulate the connection o f
eith er a square wave or a PWM in verter to the MHD dtct.
static changes in flu id
Quasi
dynamic conditions have been allowed
fo r, by considering the variatio n in the MHD output voltage
and the in tern al resistance.
Detailed th eo retical and experi­
mental resu lts are presented in graphical and tab\;Q.ated forms.
Inverter and associated components have been designed and
special fuse protection provided.
An automatic feedback c o n tro lle r fo r power control
from MHD duct has been designed, fabricated and te sted ..
Excellent response m der steady state as w e ll as dynamic
condition is demonstrated with a stingle control loop and a
12
simple lo g ic c iro u ita ry .
The V -I ch aracteristics o f an
MHD generator have been computed using a time dependent
computer model o f the generator*
The resu lts are thai
u tiliz e d fo r power control o f the generator undar various
operating conditions.
A voltage source in verter (VSI)
in terface is considered fo r transient stvidies o f the genera­
tor in verter lin k .
The s t a b i li t y studies are also carried
out with sudden load changes from operating point.
The organization o f the thesis Is as follow s i
Chapter 2 t This chapter is devoted fo r the d ig it a l
simxilation o f MIO generator in verter lin k .
The analysis o f
an MHD generator^inverter lin k has been made to reduce swit­
ching losses by using a square wave in verter working at lin e
frequency.
A sin g le c o n tro lle r has bean used to tran sfer
maxiouB power from HMHD gen erator-inverter lin k to the
in fin it e bus a t the desired power fiactor and without any
power fa c to r correction needed.
Generalized power flow
equati(»i8 have been derived which can connect za square wave
or PWN in verter to an MHD dust with any electrode configuration.
Modified power flow equations with f i l t e r s have been also
derived.
The quasi s ta tic changes in the flu id dynamic condi­
tions have been allowed fo r by considering the variations in
the MHD output voltage and the in ternal resistance from the
operating point.
The d etaile d d ig i t a l computer study is
f
*T3
carried o\rt fo r the variatio n o f re a l and reactive powers
with quasi s ta tic changes in the f lu id dynaaic conditions.
The control o f power to an a«c« system with input and out­
put parameters using phase s h if t is also studied.
Results
o f the d ig it a l simulation are plotted and tabulated.
Chapter 3 t This chapter describes a scheme to
simulate the MHO-inverter lin k in the laboratory*
The e3q>eri-
mental resuls are obtained and compared with those o f the d ig i­
t a l simulation in the la s t chapter.
An in verter has been
designed to interconnect 2MW, 20 electrode p a ir MHD-test
f a c i l i t y to a ,c « g rid .
Special design considerations are
made to choose the inductor fo r the commutation c irc iiit fo r
high frequency operation,
A d ig it a l computer st\jdy is made
to Bbov the ctjrrent variatio n under d iffe re n t operating condi­
tions fo r safe in verter operation.
F in ally a special te st is
conducted to p red ict the approximate length o f a rew irable
fuse wire in water to be used fo r protection o f power elec­
tronic devices.
The changes in the input parameters o f the
MHD duct are made to control the power manually \jsing phase
s h iftin g transformer.
Chapter 4 r This chapter is devoted fo r the design,
fabricatio n and testing o f an automatic feedback c o n tro lle r
to doDonstrate the steady state and the dynamic response o f
the simulated system.
In order to predict the behaviour o f
lif
the feedback co n tro U er» the diglteil simulation o f the
co n tro lle r is done losing a 1st order c o n tro lle r in a closed
loop system to achieve greate r acctiracy, improved
response and reduced e ffe c ts o f distuxi>ances.
dynamic
The d ig it a l
computer study neglects the resistances in the syston and
time constants involved in the fir in g c ir c u it and power
transducer.
The mathematical model o f the c o n tro lle r is
obtained and then simulated on the d ig it a l computer following
step by step calculations to determine the approximate para­
m eter o f the c o n tro lle r.
An analogxxe simulation is done to
evaluate exact parameters.
designed and fabricated.
The feedback c o n tro lle r is then
The transient response o f the
c o n tro lle r is calcxilated and experimentally v e r ifie d fo r a
step change in the MHD duet voltage and the resistance. The
steady state response o f the simulated system is also deter­
mined eaqperimentally.
are compared.
Theoretical and e:q>erimental resu lts
The control lo g ic has been designed and fa b r i­
cated with CMOS integrated c irc u its fo r a low power consump­
tion and a high noise immuaity.
Chapter 9 t This chapter is devoted to the th eo retical
calculations o f the V -I curves using the time dependent model
o f the MHD generator fo r d iffe re n t modes o f loading.
These
curves are \jsed fo r d ig it a l simulation o f power control as
described in Chapter 2.
A segmented Faraday generator with
fin e segmentation to minimize H all e ffe c t is considered in
15
th is chapter.
Calcxilations are made fo r 2MW (therm al), 20
electrode p a ir generator.
The re su lts o f the steady state
analysis is compared with the resu lts in Chapter 3«
transient interaction o f a
The
a u lt ip le electrode NHD generator
with a force commutated in verter s y s t ^ is described.
The
transient behaviour with a step change in current is also
described.
A voltage source in verter (V SI) in terface is
considered with the above compxrter model with a d«c» input
f i l t e r in oz*der to lim it the in verter rip p le in;)eotion to the
MHD channel and the influence o f the channels voltage rip p le
on the converter.
A set o f current and voltage equations are
derived fo r the MHD generator in verter lin k .
D ig ita l simula­
tion is carried out with these equations fo r the transient
analysis o f the above system.
The duct is sii)Jected to sudden load changes from
operating point to near open c ir c u it and short c ir c u it condi­
tions and re su lts are obtained fo r space and time variation
o f temperature, pressure, v e lo c ity , mach number, current and
voltage.
Also the resu lts are obtained with diffez*ent input
capacitor and an optimtm s iz e o f the same is suggested fo r the
c h o s ^ system.
Chapter 6 i This is the concluding chapter.
I t summa­
rise s the re su lts obtained in the previous chapters and proposes
furth er z*esearch work.
16
In carrying out th is study the follow ing objectives
are parsued.
(
( l ) The analysis is generalised to include any type
o f force coBUBUtated inverter for the conversion
and control of MHD duet power.
(11) The resu lts obtained are such that they can be
e ^ e r ia e n t a lly v e r ifie d with a laboratox*y gene­
ra to r o f 2 MW (therm al) with 20 electrode p a irs.
17