CONTROL OF FIRING OF THYRISTORS OF A TCI WITH CONSTANT CAPACITOR
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31-12-2010, 04:43 PM
1CONTROL OF FIRING OF THYRISTORS OF A TCI.ppt.pptx (Size: 636.3 KB / Downloads: 58)
Supervisor: Dr. R. Mahanty
Ram Prasad Sinha
Instantaneous power, p(t) = v(t)i(t)
Power, p(t) value
positive – power transmit/dissipate by load
negative – power return from the load
Since p(t) is power transmits by load, then it is the average power, P at load
P is also known as active power, real power or true power measured in unit of Watts.
p(t) = v(t)i(t) = VI sin 2t
Average power is zero
The product of VI is called reactive power (QL or PQ) with unit Volt-Amp Reactive (VAR)
Reactive power (inductive)
QL = VI = I2XL = V2/XL (VAR)
Z = – jXC (capacitive)
Reactive power (capacitive)
QC = VI = I2XC = V2/XC (VAR)
To distinguish between inductive reactive power (QL) and capacitive reactive power (QC), we use two different signs (+ or –) depending on our reference (i or v), for example jQL and – jQC or otherwise.
TCI With Constant Capacitor
Consists of an inductor in series with two anti parallel connected thyristors and in parallel with a constant capacitor .
The effective value of inductance varies with firing angle α as:
Firing angle can be varied in range 00-90o
Lagging current through inductor,
Leading current through capacitor= VsωC
Net Leading current drawn by TCI
I = VsωC -
The lagging current of the system load is given by = Is Sinφ
For unity power factor condition
Is Sinφ= VsωC -
Max reactive power demand at the load end =2KVAr
At maximum load demand only C will compensate for the load reactive power demand. Thus,
L must compensate whole of the C at no load.
with Vs = 230 V
Control of firing angle
I Sinφ = VsωC –
Thus a relation between (firing angle) and lagging current is obtained. By knowing I sin φ (value of current at voltage zero crossing) corresponding value of α can be find and unity power factor can be achieved.
This can be done by using a micro-processor and other components (ZCD , PD , Sample and Hold ckt, 8253 etc.) where α can be find from a look-up table having various values of leading load currents and corresponding compensating firing angles. This α is then fed to the gate driving circuit of thyristors.
Conversion of firing angle into delay
Clock of 8253 Programmable Interval Timer is given from the clock of micro-processor (8085) and it is taken as 2MHz.
Thus the counter is operating at a clock frequency 40,000 times of source frequency (50 Hz). The counter is loaded with a value which is calculated as follows:
One cycle i.e. ‘2π’ radian of source voltage contains 40,000 clock cycles.
Hence ‘α’ radian will correspond to N = (40,000/2π)*α clock cycles.
Thus, if the counter is loaded with count equal to N, then after N clock cycles thyristor will be triggered i.e. desired delay will be produced.
Scheme For ControllingFiring Angle
Zero crossing detector:
The zero crossing detector is used to obtain the value of current at the instant when voltage is crossing zero and going from positive to negative
Since the firing angle α is measured from the positive peak of voltage, a peak detector is used to give a pulse to the gate of the 8253.
Sample and Hold Circuit:
The sample and hold circuit samples the instantaneous value of AC current signal and maintains it at a constant level. It makes this constant voltage available to A/D converter which requires a constant input during the conversion period.
8253 Programmable Interval Timer:
The programmable interval timer is used for generation of hardware triggered strobe signal. This signal is then inverted and given to the driving circuit of the thyristors
8255A Programmable Peripheral Interface:
Its main function is to interface peripheral devices(ADC0808) to the microprocessor
ADC-0808 Analog-to-Digital Converter:
The A/D converter is used to convert the analog value of the current at the desired instant into digital value.
8085 8-bit Microprocessor
The microprocessor is used to calculate the firing delay of thyristors used in TCI. It performs many other functions such as giving logic pulse to S/H circuit, sensing the zero crossing of voltage, loading the counters of 8253 Programmable Interval Timer with suitable values and storing the output of ADC0808.
Gate Drive Circuit:
An isolation circuit is required between an individual thyristor and its gate-pulse generating circuit. The isolation can be achieved by pulse transformers.
1. Check the output of zero crossing detector.
2. If the output is zero , goto step 1.
3. Send signal to Sample Hold circuit.
4. Provide some delay for acquisition time.
5. End the logic pulse of S/H.
6. Switch on the multiplexer connected with the analog input.
7. Give the Start of Conversion pulse to ADC-0808
8. Check the End of Conversion pin of ADC-0808.
9. If EOC=0, goto step 8.
10. Store the value at data bus of ADC0808 in accumulator (A) of 8085.
11. Compare the value stored in A with the values saved in the look-up table and store the value of corresponding firing angle.
12. Load the counters 0 and 1 of 8253 and provide gate pulse via Pulse Detector
Power Electronics Circuits, Devices and Applications by Muhammad H. Rashid
Linear Integrated Circuits by D. Roy Choudhury and Shail Jain
Power Electronics: Converters, Applications, and Design by Ned Mohan, Tore M. Undeland and William P. Robbins
Microprocessor Architecture, Programming and Applications with the 8085 by Ramesh Gaonkar
Fundamentals of Microprocessor and Microcomputers by B. Ram
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