SPRS
SPRS
SPRS
INTRODUCTION BASIC CONFIGURATION OF SPRS TECHNICAL DETAILS SELECTION OF SPRS SCHEME SPRS DIMENSIONING LATEST DEVELOPMENTS
1.INTRODUCTION :
What is SPRS :
It is a system used for Energy conservation. Large fans & pumps are required at different places in a process plant. The volume of the fluid flow needs to be regulated as per the process requirement. The common method to achieve this is to run the fan/pump at a fixed speed. The flow is then adjusted by artificially increasing the resistance in the system by using a valve or damper. This method though simple,gives rise to high power losses as throttling losses. Further it results in higher vibrations also. Another method for speed regulation is by rotor resistors LRS or GRC.Again with this system losses and vibrations are at higher side,whereas speed regulation is poor.
If a variable speed motor with an AC/DC Drive is used in the fan/pump,there will be considerable saving in power results. It is because of higher efficiency of the Drives Drive is a static device and its efficiency is between 75-85%. In case of AC drive the efficiency is 85% but the accuracy level is 2-3%. Efficiency of a DC drive is around 75% and the accuracy level is less than 1%. SPRS is basically a DC drive. SPRS recovers the slip power which is normally lost in the rotor resistors.It also reduces the vibrations in the motor and regulates the speed accurately.
HT BREAKER
STATOR
MOTOR
ROTOR
SLIP POWER
SPRS
LOSSES
WHY SPRS ?
IT AVOIDS LOSSES IN DAMPER. IT RECOVERS THE SLIP POWER NORMALLY LOST IN THE ROTOR RESISTOR. REDUCE VIBRATIONS IN THE FAN MOTOR & OTHER MECHANICAL COMPONENTS. REGULATE THE SPEED FINELY AS PER PROCESS REQUIREMENT.
SPRS EVOLUTION:
6 Pulse scheme (Analog Controller) with Relay logic & Harmonic filters: 12 Pulse scheme (Analog) with Relay logic: 6/12 Pulse scheme (Analog) with PLC : 1984 1986 1991 & 1993
SPRS INSTALLATION: ( B C L )
CV FAN 800
12 PULSE
6 PULSE
12 PULSE
6 PULSE
(SERIES)
(SERIES)
80-95% 3200
78-95 2298
66-95% 2878
55-95% 1200
% s = (Ns-Nr)x100/Ns
P1
Pressure N/m2
P3
Flow m3/sec
Q2
Q1
Case 1:At full speed & Damper fully open: Power delivery by fan = P1 * Q1 Power input to motor = P1*Q1/nm *nf Case 2 : At full speed & damper control Power input to Motor = P2 * Q2/nm * nf Case 3 : At reduced speed ,without damper Power input to motor = P3 * Q2/nf * nm So Energy saved = (P2-P3)*Q2/nf*nm Here nm = Motor efficiency, nf = fan efficiency
Flow Q
Speed % : N
From the Curve it is clear that FLOW is directly proportional to the SPEED,whereas POWER is proportional to the cube root of the speed.
Y B N
CAPACITOR BANK
R Y B
Q1
A2TH M
K1 :GRC CONTACTOR
K2:SPRS CONTACTOR
DIODE MODULE
TH2
R Y B (G.R.C.)
2. DC REACTOR (LINE CHOKE) 3. FEED BACK TRANSFORMER 4. SWITCH GEAR 5.H.T. BREAKER 6. MOTOR 7. GRC
POWER ELECTRONICS:
It consists following components: a) Diode Bridge: It rectifies the Rotor voltage.It converts Rotor AC voltage in to fixed DC voltage and feeds it to the Thyristor module. The Rotor voltage is high at lower speed and low at higher speed.A Diode bridge has 6 Diodes.
DC Bus Caps
+ -
DC Reactor
POWER ELECTRONICS:
b) Thyristor inverter (SCR module) The Rotor output is balanced by the Thyristor. bridge. It operates in regenerative mode at fixed frequency of 50 Hz. A Thyristor module has 6 thyristors. +
R B
4 6 2 1 3 5
V output
30 60 90
Firing angle
For 90-150 degree Firing angle,Output is Negative. Since the current is unidirectional the power flows in the bridge and it is called Regenerative mode.
Motor
A1ER
AC M TO R D RI VE O 0. 75 KW 200 V v 1. 3
Speed reference
HE ALT H S E EQ
LO CA RE F L PR O G L R
Electronic Controller
TG Feedback
JO G RU N
F W RE V D ST O P RE SET RE SET
CONTROL LOOP
ACC DEC
SPEED CONTROLLER
:
TRIGGER CIRCUIT THYRISTOR
415 VAC
INV CONV
0:+10V
SA
0:-10V
CA
0:+10V
CURRENT AMPLIFIER
PG
CURRENT LIMIT
SPEED LOOP
TG
REF. +ve
RAMPO/P +ve
SA O/P -ve
CA O/P +ve
Nfb -ve
Ifb +ve
CONTROLLER:
TYRISTOR FIRING ANGLE CONTROL. IT ALLOWS A SMOOTH VARIATION OF THE SPEED THROUGOUT THE CONTROL RANGE. THE CONTROL SECTION CONVERTS THE SPEED REFERENCE SIGNAL TO A LEVEL WHICH IS GIVEN TO DETERMINE THE INSTANT OF FIRING. THIS REFERENCE LEVEL SETTINGS CAN BE ACHIEVED BY RAMP GENERATOR , PLC, SPEED AMPLIFIER AND CURRENT AMPLIFIER CIRCUIT. THIS IS FOR
SPEED AMPLIFIER
THIS FORMS THE ERROR AMPLIFIER OF THE SPEED LOOP. SPEED REFERENCE IS GIVEN THROUGH THE PLC. THIS REFERENECE IS COMPARED WITH THE ACTUAL SPEED FEED BACK COMING FROM THE TACHO GENERATOR. INCASE OF TACHO FAILURE THE ROTOR VOLTAGE FEEDBACK ACTS AC A SPEED FEEDBACK. THE OUTPUT OF THE SPEED AMPLIFIER IS 0 TO -10 VOLT. IT HAS P & I GAINS TO TUNE THE SPEED LOOP, THE EFFECT OF WHICH IS ON THE OUTPUT OF AMPLIFIER. THE OUTPUT OF SA GOES TO CA. IT IS ALWAYS NEGATIVE BECAUSE CURRENT FEEDBACK IS UNIDIRECTIONAL.SO POSITIVE LIMIT IS CLAMPED.
P1
P2 t t1 t2
CURRENT AMPLIFIER
THIS FORMS THE ERROR AMPLIFIER FOR CURRENT LOOP. INPUT CURRENT REFERENCE REQUIRED IS 10 VOLT FOR RATED FULL LOAD CURRENT. THE CURRENT FEEDBACK IS TAKEN FROM CT FROM THE INPUT SIDE . TIME CONSTANT OF CA IS VERY SMALL (30ms) AS COMPARED TO SA (400 ms) HENCE ITS P & I GAINS ARE VERY SMALL . DUE TO THIS REASON THE CURRENT LOOP IS ALWAYS KEPT INSIDE. OUTPUT OF THE CA DECIDES THE FIRING ANGLE. CA OUTPUT GOES TO THE PULSE GENERATOR, WHICH ACCEPTS ONLY POSITIVE SIGNAL SO HERE THE NEGATIVE LIMIT IS CLAMPED.
PULSE GENERATOR
INPUT SIGNALS TO THE PULSE GENERATOR ARE SYNCHRONIZING SIGNALS (WITH 3-PHASE) AND OUTPUT FROM CURRENT AMPLIFIER. ACCORDING TO THE VARIATION IN THE CA OUTPUT,FIRING ANGLE VARIES BETWEEN 0-180 DEGREE. PRACTICALLY IT IS ADJUSTED SUCH THAT IT REMAINS BETWEEN 30 - 150 DEGREE. THE CONVERSION & INVERSION LIMIT WILL DECIDE THE CLAMP ON THE FIRING ANGLE, ALPHA. IN SPRS MODE FIRING IS DONE IN INVERSION MODE ONLY,SO THE FIRING ANGLE IS BETWEEN 90-150 DEGREE ALWAYS.
2) DC Reactor (Link Coke) : The reactor (Inductor) acts as a de-coupling element,It has the property that the voltage across it is proportional to the rate of change of current through it hence it limits, sudden current change Di/Dt . When the SCR is triggered by applying a pulse at the gate, it does not start conducting immediately.At the beginning the flow of current is limited to a small area near the gate and from there it spreads to other areas. Due to this local hot spots are created near the gate and if the rate of rise of current (Di/Dt) is too high,the SCR may be damaged.To limit the rate of rise of current a Choke is used in series with the SCR.
b) Circulating current: At firing angle between 90 - 120 degrees the Inverter voltage output is positive.During this time,the diodes become forward biased. Current thus flows from the Transformer to the DC link and back through Diodes without passing through the Rotor.This is called circulating current. At rated speed,the current can build up to 20% of the rated current.The diodes & thyristors needs to be overrated due to this. The Choke reduces the current to some extent. So the DC Reactor reduces stress on motor windings and also motor losses are less
3. FEED BACK TRANSFORMER: Primary winding is connected to HT BUS through HT Isolator. Secondary connected to Thyristor Inverter(s) and designed for blocked rotor voltage of motor and GRC to SPRS changeover speed. Tapping of Tertiary winding is used for synchronization of firing pulses and sensing supply dips.
4. SWITCH GEAR : Isolators : These are used to isolate the feedback transformer from the supply in case of a fault or if maintenance is required.Isolators also provide an earthing switch interlock.The opening of the isolators automatically closes the earthing switch.This is very important from the safety point of view. Contactors :For GRC K1 is used whereas for SPRS K2 contactor is used.Both are power contactors having higher current capacity.Auxillary contactors are low current contactors used for interlocking purpose. CTs & PTs (Instrument Transformer): These are used for stepping down high value quantities like current & voltage for protection and measurement. These should be very accurate.
4. SELECTION OF SPRS
SELECTION OF DRIVE MAINLY DEPEND UPON THE
POWER RATING OF THE MOTOR. IF THE POWER REQUIRED IS BELOW 1 MW, A VARIABLE FREQUENCY DRIVE CAN BE USED. FOR HIGH POWER OUTPUT AND SPEED REDUCTION UPTO 50% OF THE BASE SPEED,SPRS MAY BE SELECTED. A COMPARISION OF THE COSTS AND BENEFITS OF THE SPRS AGAINST SIMPLE DAMPER CONTROL AND GRC/LRS SHOULD ALWAYS BE MADE. SINCE GRC/LRS IS COMPULSORY,SPRS IS AN EXTRA COST BURDEN.SO PAY BACK CALCULATION MUST.
SCHEME OF SPRS
ONCE DECIDED FOR SPRS,THE NEXT STEP IS,
WHICH TYPE OF SPRS TO BE USED. BASED ON THE OPEN CIRCUIT ROTOR VOLTAGE Rv & ROTOR CURRENT RA SPRS SCHEME IS SELECTED. GENERALLY FOR POWER RATING BELOW 1000 kW, 6 PULSE SPRS SCHEME IS USED. FOR POWER RATING ABOVE 1000 Kw 12 PULSE SPRS SCHEME IS USED. 12 PULSE SCHEMES ARE FURTHER CLASSIFIED INTO 3 MORE CATAGORIES.
DIODES & ONE THYRISTOR MODULE HAVING 6 THYRISTORS, IS USED. FOR EXAMPLE, KILN STRING FAN SPRS IS 6 PULSE SPRS SYSTEM.
HT Bus HT Breaker HT Isolator
USED. EACH THYRISTOR MODU HAS 6 THYRISTORS IN SERIES. THE HIGH VOLTAGE IS BEING SHARED BY BOTH THE THYRISTOR MODULE WHEREAS THE ROTOR CURRENT REMAINS THE SAME. EXAMPLE: BAG HOUSE & CALCINER FAN SPRS
HT Bus HT Breaker HT Isolator
USED. THE HIGH ROTOR CURRENT IS BEING SHARED BY BOTH THE THYRISTOR MODULE WHEREAS THE ROTOR VOLTAGE REMAINS THE SAME.
HT Bus HT Breaker HT Isolator
LOW,SPRS RUNS IN SERIES MODE AND HIGH ROTOR VOLTAGE IS BEING SHARED BY BOTH THE THYRISTOR MODULES.
HT Bus
HT Breaker HT Isolator
K2
Motor K1 K3
Diode 1
Thy 1
L Diode 2 Thy 2
THIS IS ACHIEVED BY MAKING K2 ON AND K3 OFF. ABOVE CERTAIN SPEED, SAY 80% AND ABOVE WHERE ROTOR
CURRENT RA STARTS INCREASING AND ROTOR VOLTAGE Rv STARTS DECREASING,K3 CONTACTOR BECOMES ON. NOW SPRS STARTS RUNNING IN PARALLEL MODE AND THE HIGH ROTOR CURRENT RA IS BEING SHARED BY BOTH THE MODULES. IN THIS MANNER WE CAN CONTROL BOTH Rv & RA
HT Bus
HT Breaker HT Isolator
K2
Thy 1
5.SPRS DIMENSIONING
THE OBJECTIVE OF THIS EXERCISE IS TO RATE THE POWER CIRCUIT COMPONENTS AND THE VOLTAGE & CURRENT LEVEL IN THE CIRCUIT. THIS IS MAINLY REQUIRED AT THE TIME OF DESIGNING AN SPRS SYSTEM / UPGRADATION OF EXISTING SPRS. FOLOWING ARE THE MAIN COMPONENTS OF AN SPRS SYSTEM: FEED BACK TRANSFORMER DIODE & THYRISTORS DC REACTOR GRC / SPRS CONTACTOR
EXAMPLE: FOR A MOTOR OF GIVEN DATA,WE CAN CALCULATE THE RATING OF DIFFERENT COMPONENTS. MOTOR DATA:
STATOR VOLTAGE STATOR CURRENT ROTOR VOLTAGE ROTOR CURRENT OPERATING SPEED RATED RPM SLIP
Sv = 6.6 KV +/- 110% SL = 87 A Rv = 1175 V RA = 380 A = 60-100 % Ns = 1000 RPM Sm = 100-60/100 = 0.4
KVA = SQ RT3 * Vrms * IAC KVA = SQ RT3 * Vrms * IAC = 453KVA F/B TRANS. = 6600 /597V,453 KVA
# DIODE RATING:
SAFETY FACTOR
# THYRISTOR RATING:
SUPPLY VOLTAGE VARIATION
SAFETY FACTOR
L = 2.7 mH
By using these equations we can upgrade the existing SPRS systems.
6.LATEST DEVELOPMENTS
TILL 2001, ANALOG CONTROLLERS WERE USED FOR SPRS SYSTEMS. DIGITAL SPRS CONTROLLER HAVE BEEN INTRODUCED BY L&T IN THE YEAR 2001 WITH THE NAME ACEREG SPRS.IN THE YEAR 2003 SOME MORE MODIFICATION DONE IN THIS ACEREG SPRS CONTROLLER. ACEREG SPRS IS A MICRO PROCESSOR BASED DIGITAL CONTROLLER FOR SPRS APPLICATION. IT HAS FREELY PROGRAMMABLE INPUTS & OUTPUTUS. IT HAS 16 INPUTS & 8 OUTPUTS. ALL THE PARAMETERS CAN BE VIEWED ON 7 SEGAMENT DISPLAY. PARAMETERS CAN BE ALTERED THROUGH KEY- PAD EASILY. DETECTION OF FAULT IS EASY. DIGITAL TACHO CAN BE CONNECTED DIRECTLY
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