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THYRISTOR CONTROLLER FOR INDUCTION SMELTING FURNACE
THYRISTOR CONTROLLER FOR INDUCTION SMELTING FURNACE A. Dmowski, K. Stankowski, R. Zajac and P. Fabijanski Technical University of Warsaw, Inst. of Control and Electronics, Koszykowa 75, Gm. E, pok. 316, PI 00-662 Warszawa, Poland
ABSTRACT In the paper the principle and the way design of thyristor power controller (75 kVA - 50 Hz) for 50 Hz electric heating are presented· Described controller is a model of industral controller for 650 kW smelting furnace· This controller enables compatible work with minicomputer which controls whole technological process·
INTRODUCTION The introduction of computers for controlling industrial processes made necessary for conventional control equipment to obtain a greater compatibility with the new computer technique retirements. An example here can be the automatic control system for the induction smelting furnace when a specialized minicomputer is used· Such a solution makes possible to attain the optimal utilization, both energetical and technologicalf of the furnace· In the present paper the thyristor controller model of 650 kVA, 50 Hzf device designed as 75 kVAf 50 Hz# is described. The model connected with the electronic interface system Camac enables a computer control of metal smelting processes· An industrial controller (650 kVAj is being built at present. Before experimental work the following assumptions have been made: 1 · The controller should make possible to supply the furnace with the controlled sinusoidal voltage 50 Hz, with a high watt>hour efficiency· It enables to control the supplied power in every moment of the industrial process· 2. The power coefficient of supply system for the whole technological cycle should be close to one· 3· The system should enable the supply of single-phase surface from threephase mains by symmetrical load of all phases· 4. The control networks of the controller enables interfacing with the computer. In Fig. 1 the block diagram of the controller is presented. 96 l
962
A. Dmowski et
dl
Uni 15%
Fig. 1. Schematic diagram of
(20%r100%) Unt Wo
the Power Regulator
The controller consists of the following blocks. 1· 2. 3. 4.
Thyristor controllers of symmetrical load of three-phase mains R-S Thyristor controller of alternating voltage R-Ü Thyristor controllers of reactive power R-R Central network for interfacing with the Camac module E-S.
The principle of operation of the power controller is as follows. At first the furnace is supplied with the voltage of 20 % U which permits for the action of the reactive power controller R-R and the three-phase mains symmetrical load controller R-S. When the two controllers reach the assumed compensation coefficients (cosip = 1 ß 0.5 % and phase load unbalance is less than 10 %)f the voltage controller R-U is automatically switched on. The R-U controller increases the voltage to the required value. Any rise in the load voltage U is possible only in the case when both the R-R and the R-S reached the assumed coefficients of compensation and symmetrisation. The negative feedback loop makes the whole controller insensitive to the changes of mains supply voltage. The additional current feedback loop prevents from overloading the controller and supply mains.
CURRENT SYMMETRIZATION NETWORK R-S The supply of one-phase induction furnace requires the load balance of three-phase transformers. This symmetrization can be reached by means of a static network which consists of the inductance coil and the condenser battery. One of the most popular networks is shown in Fig. 2. The condenser battery sections are switched with the aid of thyristor inverse parallel switches. The whole network is composed of the following blocks: I. The main circuit
963
Thyristor controller for induction smelting furnace
R
i
S
T
Q
?
PP1
rt
EtL
17
Pt/
PP2
-nAjc
I
LPN2
us
PN3 DL
^5
rSh
A*
i*
RI
or"L11 1i /.tfZ\yr\
Ì^ô
LN1
SA
\KM1\ ÊÌËÉ
TÏÏT 1TÏÏ
LN2
Fi g. 2 Schematic diagram of R-S regulator
1. thyristor switch L11 (T1; T2), L12 (T3| T4) f LN1 (T5| T6), LN2 (T7j T8) 2· permanent battery section C5 3· switched condensers sections C1f CN 4· coil for symmetrization DL 5. resistor for preliminary battery load R II. The control circuit 1. error measurement circuit PU 2. A/C converter 3. the circuit which memorize digits of measurement error - AK 4· the block of timeliness measurement US 5· clock pulse generator G 6. the distribution circuit of thyristor gate pulses RI 7. the circuit for setting in which prepares switched sections for steady state conditions SA 8. gate control circuits of thyristor switches KM1f...fKMN. The operation principle of the device is as follows. The permanent battery section symmetrises the supply mains· The other sections are loaded via resistor R and thyristors T1, T5 by positive half waves of the line-voltage U^g. The transit is about 0.2 sec. and after that time the resistor R is short circuited by the thyristor S1 and actual action is started then. The output voltage error appears as a signal which is a difference between the two phase currents I R and I . The signal is changed by the A/C converter into a digital signal in the 8421 code. The error signal sign determines whether a certain number equal to that of clock pulses synchronized at the instant by
964
A. Dmowski et
al
the US circuit i s added or subtracted in the reverse counter of the A/C converter· After the measurement action the AK circuit memorizes digits of measurement error· The output signal of AK circuits controls inputs of pulse amplifiers of thyristors KM1 . . · KMN· On switching on the thyristor switch one of the sections C1 · . . CN i s immediately connected in parallel to the inductance coil or to the permanent battery section CL· A proper moment for the on-and -off action of the thyristor switch prevents from overloading in the circuit· The appropriate current symmetrization in transformers coils i s possible when the following conditions are satisfied: Ip-YTIu (4)
Ip=VTIc where
(2)
û ^ = UgT = UTR
and the capacity reactance is equal to the inductive reactance of the symmetrization coil· Ó*0ç=×1
(3)
If (3) is fulfilled, the conditions (1) and (2) are fulfilled when I D = ô or I« a Ig· (This relation is present in the circuit)· Rrom (3) appears that the total reactive power of the condenser battery must satisfy:
EQcn-Ql.
«>
where Q, is the reactive power of inductance coll· The reactive power of the smallest section of condenser battery which ensures more than 5 per cent accuracy is given byt
(5) where P is a load active power· The smallest power of permanent section is given by:
QC5 =^ø
(6)
where P 0 ^ is the minimum value of load active power. By diminishing the value of Q ^ (5) one can obtain a smaller asymmetrization of phase currents but a number of condenser sections will then increase· The tests of the model 75 kVA, 30 Hz have confirmed the validity of the theoretical assumptions which were the basis for designing the current symmetrization circuit· In Fig· 3 an oscillograph record of the following signals in the A/C converter and synchronization circuit are presented: 1· the voltage signal for comparison with the control error P2; 2· the signal of measurement time of the control error P; 3· the output signal of the comparator C; 4· the control signal of reverse counter C3; In Pig. 4 f respectively
Thyristor controller for induction smelting furnace
P2
J3m$
(U^s)
(UST)
(URSVT*)
965
51/·
51/-
51/
P1
—
C3
Rg.3.
Rg.4.
1. the signal synchronizing the circuit with the U ™ - A voltagei 2. the signal synchronizing the circuit with the \J^ - B voltagei 3· the signal proportional to the measurement time of the control error P1| In the presented model an asymmetry of the phase circuit is less than 5 per cent·
THEft-UVOLTAGE CONTROLLER The main task of R-U controller is to control voltage supply U of the load· This voltage should be controlled within 20 % - 100 % of U · The block diagram of the controller is presented in Pig· 5. This controller consists of 1 · Autotransformer AT with five taps A, B # Cf D f E (in which U
A = Ü B "Ü C - Ü D -Ü Eâ 20 * V
2. Booster transformer T. 3. Eleven double thyristor switches T1 f T2f.< T21, T22 4. Electronical circuit of regulator control. The control block consists of: a) A circuit which controls the difference between the set point voltage (U t ) and the controller output voltage (U ) b) two electronical reverse counters which control the switch of appropriate thyristor pairs (Lt - thyristor pairs T1, T2, ..· T9f T10 and L 2 - thyristor pairs T11f T12f ... T21, T22j c) L,. and L^ counters switching generator d) a circuit for synchronization and distribution of thyristor control pulses. The principle of controller: The supply voltage U switches on the pairs of thyristors T1, T2 and T21, T22. This stage refers to the zero state of both counters hA$ L . In the case when the voltage control value Ü . increases id
SX
and coefficients of synmetrization and compensation have appropriate values
966
A. Dmowski et
Fi g,5
Voltage
Regulator
ai
R-U
the generator G is unblocked· At each end of positive half - wave of supply voltage U on the generator G output a pulse for counter switch appears· Each state of these two counters causes the conduction of an appriopriate thyristor pair. These thyristors conduct unless the output voltage U reaches the value of the control voltage· In the case when U = Ü + the pulse train is subtracted up to the moment when U Q = u s t"U T i l z s eaoh change of counter state causes the change of U 0 110 T h e voltage of ^ z val · principle of the regulator is illustrated in Fig. 6 in which ^ e oscillograph of U Q £ E 2 voltage versus time when step unit signal of magnitude (20 % U s t - 120 % Uflti was applied. (The load current = 50 A and cosip = 0.9 ind. ). Each moment of thyristor switch causes a high harmonic dystribution in U voltage· The magnitude of these harmonics can be evaluated on the base of the equivalent circuit given in Pig· 7· In this diagram 1T and 2T are on the state of conduction and 3Tf 4T will be switched in the next instant. On the basis of Fig. 7 the following set of equations can be written
967
Thyristor controller for induction smelting furnace
S
^]5
~
Rg.6.
Fig.7 Equivalent
circuit
U=EMa+^EL2
dt M
(7;
(8)
t h i s s e t of equations can be rewritten i n a matrix form
The solution of ( 9 ) for non-sin i n p u t s takes form (when
X n = C[A-jok^]
B Fmke
ø ^ 0 el.)
(9)
(40)
968
A. Dmowski et
al
when:
f K ^ " · ^ 1 ), a i =0 ) ^é = ð"[ - 2 +ÉÃ "^ ^2"C0H
" firsi
harmomc
oiher wise F m = < |-( a fe-jt> ) K
m
QK=0
b,=
+
2(i-k)
TT
+
2(l-k)
Fig. 8 and Fig. 9 give the values of harmonic contains i n output voltage caused by the switches of t h y r i s t o r s . These r e s u l t s permit for evaluation of the number of autotransformer AT taps (A,B,C,D#Ej and thyristor pairs for given harmonic contain i n controller output voltage (for assumed commutation angle).
1
E,(K=11
1
^J<=1
E,(K=1)
0,8 L=0
Oß 0,i
RzO
Ofi
\K=I
ot
K=3
ß&º
02
0.2 0
ºú5
0° 20° 40° 60° SO 100° 120° W° 160° 1SCP 90°
ö Fig. â
110°
Fig.9
' 130°
150° 170°Ë
ö
REACTIVE POWER CONTROLLER ft-R The main task of reactive power controller is to keep the value of power factor (cos ö ) dose to 1. Uhcompenated furnace has this coefficient within the range 0.15-0.22 depending on the technological process. The reactive power is compensated by an additional condenser battery the capacity of which should be adjustable in the case when the value of cos ø close to 1 is desired. A diagram of reactive power controller is presented in Fig. 10. This controller consists of
a) the condensor battery divided into n sections with capacities C^t··· C weighted binary i . e . c f c 2 a 2 C . M # I C = 2 n " ' 1 c 1 b) semiconductor swithes (dioda - thyristor) for bettery switching c) voltage meassurent block with adjustable on 50 Hz f i l t e r and phose-shift
Thyristor controller for induction smelting furnace
969
Uli
\FU
\PU\ PK ΤΓΊ E
k,
P
w L
X' I
^
lus
G
N
I
ðM
KM C*2
Figt10 dj ej f) gj h) i) 3)
Schematic
n-1
AQC1
diagram of R-R regulator
c i r c u i t FU and FU current measurement block with 50 Hz f i l t e r and phase and PI the phase - s h i f t angLe FK measurement block (the angle voltage and current supplying the furnace) the reverse counter block L the clock pulse generator G the block of synchronization i g n i t i o n pulses with mains the block of synchronization block L output pulses with the amplifiers of thyristors KM i g n i t i o n pulses
s h i f t c i r c u i t FI i s measured between
voltage UST UST-U5
The principle of the controller i s as follows: Signals proportional to the f i r s t harmonics of the load voltage and current are lead from the output of FU and PI blocks to the input of FK block. The block FK output rectangular pulses have a width proportional to the phase s h i f t between supply voltage and load current. In the case when the load i s inductive the output signal appears on the |p L output, otherwise in the øÃ output. The block FK output signals If L and [fc cause addition or sub—traction of counter d i g i t s , respectively. The block L consists of two counters f i r s t of which controls the switch the battery with capacity from CL to (1 , the second from C.K+1Ë to Cn. These counters for large values cos ö are connected in series otherwise the paraller connection is used (in the case of cos y small valuesj. This
A. Dmowski et
970
al
counter control enlarges the dynamics of whole controller R-R. The set-point voltage ö controls the tolerancy of cos ö (cos ifca = 1 - ö %)f the second set - point voltage ö limits the boundary value of phase - shift which causes the switch from series to paraller counter connection· The task of US, USTf KM blocks is appropriate control of thyristors ignition· The compensation accuracy depends on the capacity of the smalest battery section (C .·· C )· For given reactive power of the smallest battery section Qoi the minimal accuracy of compensation can be evaluated via the following expressions fcamin
Qa
=arctg
Pmin
w 03)
Qcmax = Pmin ^ g ^ m a x Qcmin = Pmin -tgiftmin
- constant (unregulated) term
Qc reg - Q ca\ax ~ Qcmin
- controlled term
w c«)
the number of battery section is given by (16)
19 n =
Lg 2
0,f6 0,20 0,21. 0fr0?2 0ί6 Qfί O^fS]
Fig. 11 In Fig. 11 the computed transients of the regulation excited by unit step are presented (time t^) for the following set of parameters:
P=4KW,
ip L =84°el, Qa=225Var , f=2, 4,40 KHz , Qcmin =0
In Fig. 12 to 15 the oscillographs of the mains current for the parameters l i s t e d above are presented. (The controller action start at the moment
t » 0).
Thyristor controller for induction smelting furnace
Fig. 12
Fig. 13
95A
» r«
ft
3801/
(f=4KHz)
971
Fig. 14.
m
J -{
Fig. 15. CONTUSIONS The r e s u l t s of modelling and experimental works prove the p o s s i b i l i t y of thyristor c i r c u i t application for 50 Hz e l e c t r i c heating devices· The d i g i t a l inputs of thyristor controller are compatible with the minicomputer outputs which make p o s s i b l e to control the whole group of thyristor controller by one minicomputer, simultaneously i n technological process· Different parts of power controller (R-U, R-S) can be u t i l i z e i n other application, f o r example, mains compensation, t h e i r voltage s t a b i l i z a t i o n etc· REFERENCES
(1) H· Frank and B, Landstrom, Electronics Division ASEA, Power-factor correction with thyristor - controlled capacitors ASEA Journal 6 (1971) (2) A# K. Sziblowski, Simmetrirowanie riezima mnogofaznej sistemy pri pitaniu adnofazowych nagruzok, Ehergia (1974 (3) S« Lloyd, P. Marshall, Dinamical performance of a single-phase thyristor a.c· regulator, Proc· Instn· Electr· Bngrs· 122, 12 (1975)
ABSTRACT In the paper the principle and the way design of thyristor power controller (75 kVA - 50 Hz) for 50 Hz electric heating are presented· Described controller is a model of industral controller for 650 kW smelting furnace· This controller enables compatible work with minicomputer which controls whole technological process·
INTRODUCTION The introduction of computers for controlling industrial processes made necessary for conventional control equipment to obtain a greater compatibility with the new computer technique retirements. An example here can be the automatic control system for the induction smelting furnace when a specialized minicomputer is used· Such a solution makes possible to attain the optimal utilization, both energetical and technologicalf of the furnace· In the present paper the thyristor controller model of 650 kVA, 50 Hzf device designed as 75 kVAf 50 Hz# is described. The model connected with the electronic interface system Camac enables a computer control of metal smelting processes· An industrial controller (650 kVAj is being built at present. Before experimental work the following assumptions have been made: 1 · The controller should make possible to supply the furnace with the controlled sinusoidal voltage 50 Hz, with a high watt>hour efficiency· It enables to control the supplied power in every moment of the industrial process· 2. The power coefficient of supply system for the whole technological cycle should be close to one· 3· The system should enable the supply of single-phase surface from threephase mains by symmetrical load of all phases· 4. The control networks of the controller enables interfacing with the computer. In Fig. 1 the block diagram of the controller is presented. 96 l
962
A. Dmowski et
dl
Uni 15%
Fig. 1. Schematic diagram of
(20%r100%) Unt Wo
the Power Regulator
The controller consists of the following blocks. 1· 2. 3. 4.
Thyristor controllers of symmetrical load of three-phase mains R-S Thyristor controller of alternating voltage R-Ü Thyristor controllers of reactive power R-R Central network for interfacing with the Camac module E-S.
The principle of operation of the power controller is as follows. At first the furnace is supplied with the voltage of 20 % U which permits for the action of the reactive power controller R-R and the three-phase mains symmetrical load controller R-S. When the two controllers reach the assumed compensation coefficients (cosip = 1 ß 0.5 % and phase load unbalance is less than 10 %)f the voltage controller R-U is automatically switched on. The R-U controller increases the voltage to the required value. Any rise in the load voltage U is possible only in the case when both the R-R and the R-S reached the assumed coefficients of compensation and symmetrisation. The negative feedback loop makes the whole controller insensitive to the changes of mains supply voltage. The additional current feedback loop prevents from overloading the controller and supply mains.
CURRENT SYMMETRIZATION NETWORK R-S The supply of one-phase induction furnace requires the load balance of three-phase transformers. This symmetrization can be reached by means of a static network which consists of the inductance coil and the condenser battery. One of the most popular networks is shown in Fig. 2. The condenser battery sections are switched with the aid of thyristor inverse parallel switches. The whole network is composed of the following blocks: I. The main circuit
963
Thyristor controller for induction smelting furnace
R
i
S
T
Q
?
PP1
rt
EtL
17
Pt/
PP2
-nAjc
I
LPN2
us
PN3 DL
^5
rSh
A*
i*
RI
or"L11 1i /.tfZ\yr\
Ì^ô
LN1
SA
\KM1\ ÊÌËÉ
TÏÏT 1TÏÏ
LN2
Fi g. 2 Schematic diagram of R-S regulator
1. thyristor switch L11 (T1; T2), L12 (T3| T4) f LN1 (T5| T6), LN2 (T7j T8) 2· permanent battery section C5 3· switched condensers sections C1f CN 4· coil for symmetrization DL 5. resistor for preliminary battery load R II. The control circuit 1. error measurement circuit PU 2. A/C converter 3. the circuit which memorize digits of measurement error - AK 4· the block of timeliness measurement US 5· clock pulse generator G 6. the distribution circuit of thyristor gate pulses RI 7. the circuit for setting in which prepares switched sections for steady state conditions SA 8. gate control circuits of thyristor switches KM1f...fKMN. The operation principle of the device is as follows. The permanent battery section symmetrises the supply mains· The other sections are loaded via resistor R and thyristors T1, T5 by positive half waves of the line-voltage U^g. The transit is about 0.2 sec. and after that time the resistor R is short circuited by the thyristor S1 and actual action is started then. The output voltage error appears as a signal which is a difference between the two phase currents I R and I . The signal is changed by the A/C converter into a digital signal in the 8421 code. The error signal sign determines whether a certain number equal to that of clock pulses synchronized at the instant by
964
A. Dmowski et
al
the US circuit i s added or subtracted in the reverse counter of the A/C converter· After the measurement action the AK circuit memorizes digits of measurement error· The output signal of AK circuits controls inputs of pulse amplifiers of thyristors KM1 . . · KMN· On switching on the thyristor switch one of the sections C1 · . . CN i s immediately connected in parallel to the inductance coil or to the permanent battery section CL· A proper moment for the on-and -off action of the thyristor switch prevents from overloading in the circuit· The appropriate current symmetrization in transformers coils i s possible when the following conditions are satisfied: Ip-YTIu (4)
Ip=VTIc where
(2)
û ^ = UgT = UTR
and the capacity reactance is equal to the inductive reactance of the symmetrization coil· Ó*0ç=×1
(3)
If (3) is fulfilled, the conditions (1) and (2) are fulfilled when I D = ô or I« a Ig· (This relation is present in the circuit)· Rrom (3) appears that the total reactive power of the condenser battery must satisfy:
EQcn-Ql.
«>
where Q, is the reactive power of inductance coll· The reactive power of the smallest section of condenser battery which ensures more than 5 per cent accuracy is given byt
(5) where P is a load active power· The smallest power of permanent section is given by:
QC5 =^ø
(6)
where P 0 ^ is the minimum value of load active power. By diminishing the value of Q ^ (5) one can obtain a smaller asymmetrization of phase currents but a number of condenser sections will then increase· The tests of the model 75 kVA, 30 Hz have confirmed the validity of the theoretical assumptions which were the basis for designing the current symmetrization circuit· In Fig· 3 an oscillograph record of the following signals in the A/C converter and synchronization circuit are presented: 1· the voltage signal for comparison with the control error P2; 2· the signal of measurement time of the control error P; 3· the output signal of the comparator C; 4· the control signal of reverse counter C3; In Pig. 4 f respectively
Thyristor controller for induction smelting furnace
P2
J3m$
(U^s)
(UST)
(URSVT*)
965
51/·
51/-
51/
P1
—
C3
Rg.3.
Rg.4.
1. the signal synchronizing the circuit with the U ™ - A voltagei 2. the signal synchronizing the circuit with the \J^ - B voltagei 3· the signal proportional to the measurement time of the control error P1| In the presented model an asymmetry of the phase circuit is less than 5 per cent·
THEft-UVOLTAGE CONTROLLER The main task of R-U controller is to control voltage supply U of the load· This voltage should be controlled within 20 % - 100 % of U · The block diagram of the controller is presented in Pig· 5. This controller consists of 1 · Autotransformer AT with five taps A, B # Cf D f E (in which U
A = Ü B "Ü C - Ü D -Ü Eâ 20 * V
2. Booster transformer T. 3. Eleven double thyristor switches T1 f T2f.< T21, T22 4. Electronical circuit of regulator control. The control block consists of: a) A circuit which controls the difference between the set point voltage (U t ) and the controller output voltage (U ) b) two electronical reverse counters which control the switch of appropriate thyristor pairs (Lt - thyristor pairs T1, T2, ..· T9f T10 and L 2 - thyristor pairs T11f T12f ... T21, T22j c) L,. and L^ counters switching generator d) a circuit for synchronization and distribution of thyristor control pulses. The principle of controller: The supply voltage U switches on the pairs of thyristors T1, T2 and T21, T22. This stage refers to the zero state of both counters hA$ L . In the case when the voltage control value Ü . increases id
SX
and coefficients of synmetrization and compensation have appropriate values
966
A. Dmowski et
Fi g,5
Voltage
Regulator
ai
R-U
the generator G is unblocked· At each end of positive half - wave of supply voltage U on the generator G output a pulse for counter switch appears· Each state of these two counters causes the conduction of an appriopriate thyristor pair. These thyristors conduct unless the output voltage U reaches the value of the control voltage· In the case when U = Ü + the pulse train is subtracted up to the moment when U Q = u s t"U T i l z s eaoh change of counter state causes the change of U 0 110 T h e voltage of ^ z val · principle of the regulator is illustrated in Fig. 6 in which ^ e oscillograph of U Q £ E 2 voltage versus time when step unit signal of magnitude (20 % U s t - 120 % Uflti was applied. (The load current = 50 A and cosip = 0.9 ind. ). Each moment of thyristor switch causes a high harmonic dystribution in U voltage· The magnitude of these harmonics can be evaluated on the base of the equivalent circuit given in Pig· 7· In this diagram 1T and 2T are on the state of conduction and 3Tf 4T will be switched in the next instant. On the basis of Fig. 7 the following set of equations can be written
967
Thyristor controller for induction smelting furnace
S
^]5
~
Rg.6.
Fig.7 Equivalent
circuit
U=EMa+^EL2
dt M
(7;
(8)
t h i s s e t of equations can be rewritten i n a matrix form
The solution of ( 9 ) for non-sin i n p u t s takes form (when
X n = C[A-jok^]
B Fmke
ø ^ 0 el.)
(9)
(40)
968
A. Dmowski et
al
when:
f K ^ " · ^ 1 ), a i =0 ) ^é = ð"[ - 2 +ÉÃ "^ ^2"C0H
" firsi
harmomc
oiher wise F m = < |-( a fe-jt> ) K
m
QK=0
b,=
+
2(i-k)
TT
+
2(l-k)
Fig. 8 and Fig. 9 give the values of harmonic contains i n output voltage caused by the switches of t h y r i s t o r s . These r e s u l t s permit for evaluation of the number of autotransformer AT taps (A,B,C,D#Ej and thyristor pairs for given harmonic contain i n controller output voltage (for assumed commutation angle).
1
E,(K=11
1
^J<=1
E,(K=1)
0,8 L=0
Oß 0,i
RzO
Ofi
\K=I
ot
K=3
ß&º
02
0.2 0
ºú5
0° 20° 40° 60° SO 100° 120° W° 160° 1SCP 90°
ö Fig. â
110°
Fig.9
' 130°
150° 170°Ë
ö
REACTIVE POWER CONTROLLER ft-R The main task of reactive power controller is to keep the value of power factor (cos ö ) dose to 1. Uhcompenated furnace has this coefficient within the range 0.15-0.22 depending on the technological process. The reactive power is compensated by an additional condenser battery the capacity of which should be adjustable in the case when the value of cos ø close to 1 is desired. A diagram of reactive power controller is presented in Fig. 10. This controller consists of
a) the condensor battery divided into n sections with capacities C^t··· C weighted binary i . e . c f c 2 a 2 C . M # I C = 2 n " ' 1 c 1 b) semiconductor swithes (dioda - thyristor) for bettery switching c) voltage meassurent block with adjustable on 50 Hz f i l t e r and phose-shift
Thyristor controller for induction smelting furnace
969
Uli
\FU
\PU\ PK ΤΓΊ E
k,
P
w L
X' I
^
lus
G
N
I
ðM
KM C*2
Figt10 dj ej f) gj h) i) 3)
Schematic
n-1
AQC1
diagram of R-R regulator
c i r c u i t FU and FU current measurement block with 50 Hz f i l t e r and phase and PI the phase - s h i f t angLe FK measurement block (the angle voltage and current supplying the furnace) the reverse counter block L the clock pulse generator G the block of synchronization i g n i t i o n pulses with mains the block of synchronization block L output pulses with the amplifiers of thyristors KM i g n i t i o n pulses
s h i f t c i r c u i t FI i s measured between
voltage UST UST-U5
The principle of the controller i s as follows: Signals proportional to the f i r s t harmonics of the load voltage and current are lead from the output of FU and PI blocks to the input of FK block. The block FK output rectangular pulses have a width proportional to the phase s h i f t between supply voltage and load current. In the case when the load i s inductive the output signal appears on the |p L output, otherwise in the øÃ output. The block FK output signals If L and [fc cause addition or sub—traction of counter d i g i t s , respectively. The block L consists of two counters f i r s t of which controls the switch the battery with capacity from CL to (1 , the second from C.K+1Ë to Cn. These counters for large values cos ö are connected in series otherwise the paraller connection is used (in the case of cos y small valuesj. This
A. Dmowski et
970
al
counter control enlarges the dynamics of whole controller R-R. The set-point voltage ö controls the tolerancy of cos ö (cos ifca = 1 - ö %)f the second set - point voltage ö limits the boundary value of phase - shift which causes the switch from series to paraller counter connection· The task of US, USTf KM blocks is appropriate control of thyristors ignition· The compensation accuracy depends on the capacity of the smalest battery section (C .·· C )· For given reactive power of the smallest battery section Qoi the minimal accuracy of compensation can be evaluated via the following expressions fcamin
Qa
=arctg
Pmin
w 03)
Qcmax = Pmin ^ g ^ m a x Qcmin = Pmin -tgiftmin
- constant (unregulated) term
Qc reg - Q ca\ax ~ Qcmin
- controlled term
w c«)
the number of battery section is given by (16)
19 n =
Lg 2
0,f6 0,20 0,21. 0fr0?2 0ί6 Qfί O^fS]
Fig. 11 In Fig. 11 the computed transients of the regulation excited by unit step are presented (time t^) for the following set of parameters:
P=4KW,
ip L =84°el, Qa=225Var , f=2, 4,40 KHz , Qcmin =0
In Fig. 12 to 15 the oscillographs of the mains current for the parameters l i s t e d above are presented. (The controller action start at the moment
t » 0).
Thyristor controller for induction smelting furnace
Fig. 12
Fig. 13
95A
» r«
ft
3801/
(f=4KHz)
971
Fig. 14.
m
J -{
Fig. 15. CONTUSIONS The r e s u l t s of modelling and experimental works prove the p o s s i b i l i t y of thyristor c i r c u i t application for 50 Hz e l e c t r i c heating devices· The d i g i t a l inputs of thyristor controller are compatible with the minicomputer outputs which make p o s s i b l e to control the whole group of thyristor controller by one minicomputer, simultaneously i n technological process· Different parts of power controller (R-U, R-S) can be u t i l i z e i n other application, f o r example, mains compensation, t h e i r voltage s t a b i l i z a t i o n etc· REFERENCES
(1) H· Frank and B, Landstrom, Electronics Division ASEA, Power-factor correction with thyristor - controlled capacitors ASEA Journal 6 (1971) (2) A# K. Sziblowski, Simmetrirowanie riezima mnogofaznej sistemy pri pitaniu adnofazowych nagruzok, Ehergia (1974 (3) S« Lloyd, P. Marshall, Dinamical performance of a single-phase thyristor a.c· regulator, Proc· Instn· Electr· Bngrs· 122, 12 (1975)