Power Electronics Laboratory Manual for PSIM Experiments 1. Experiment 8: Title: Simulation of single phase controlled converter with and without source inductance. Objective: To study the supply voltage, output voltage and output current waveforms for a single phase controlled converter with and without source inductance. Theory:
1) Single phase controlled converter without source inductance: Figure below shows a fully controlled bridge rectifier, which uses four thyristors to control the average load voltage and the half-controlled bridge rectifier "semi converter rectifier"; which uses two thyristors and two diodes.
Thyristors T1 and T2 must be fired simultaneously during the positive half wave of the source voltage V s to allow conduction of current. Alternatively, thyristors T3 and T4 must
be fired simultaneously during the negative half wave of the source voltage. To ensure simultaneous firing, thyristors T1 and T2 use the same firi ng signal.
controlled converter with source inductance: 2) Single phase controlled
Fig. 1 shows a single phase fully controlled converter with source inductance. For simplicity it has been assumed that the t he converter operates in the continuous conduction mode. Further, it has been assumed that the load current ripple is negligible and the load can be replaced by a dc current source the magnitude of which equals the average load current. Fig. 2 shows the corresponding waveforms. It is assumed that the thyristors T and T were 3
4
conducting at t = 0. T and T are fired at ωt = α. If there were no source inductance T and T 1
2
3
4
would have commutated as soon as T and T are turned ON. The input current polarity would 1
2
have changed instantaneously. However, if a source inductance is present the commutation and change of input current polarity cannot be instantaneous. Therefore, when T and T are 1
2
turned ON T T does not commutate immediately. Instead, for some interval all four 3
4
thyristors continue to conduct as shown in Fig. 2. This interval is called “overlap” interval.
Fig 1: Single phase controlled converter with source inductance circuit diagram
During this period the load current freewheels through the thyristors and the output voltage is clamped to zero. On the other hand, the input current starts changing polarity as the current through T and T increases and T T current decreases. At the end of the overlap 1
2
3
4
interval the current through T and T becomes zero and they commutate, T and T starts 3
4
1
2
conducting the full load current. The same process repeats during commutation from T T to 1
T T at ωt = π + α. 3 4
2
Fig 2: Single phase controlled converter with source inductance waveforms
From Fig. 2 it is clear that, commutation overlap not only reduces average output dc voltage but also reduces the extinction angle γ which may cause commutation failure in the inverting mode of operation if α is very close to 180º. In the following analysis an expres sion of the overlap angle “μ” will be determined. The required formulas are:
Circuit Design: Single phase controlled converter with and without source inductance Circuit parameters.
Block Name
Library
Value
Sinusoidal Voltage Source
Elements/Sources/Voltage/Sine
Peak Amplitude=230V Frequency=50Hz
Thyristor
Elements/Power/Switches/Thyristor
Source Inductance(Ls)
Elements/Power/RLC Branches/Inductor
Ls = 0.01H
R,L
Elements/Power/RLC Branches/RL
R=23 Ω; L=0.1 H
Elements/Power/Switches/Gating Block
Frequency=50Hz No. of Points=2 Switching Points=30 180
Gating Block(G3,G4,G6,G8)
Elements/Power/Switches/Gating Block
Frequency=50Hz No. of Points=2 Switching Points=210 360
Voltage Probe
Elements/Other/Probes/Voltage Probe
Current Probe
Elements/Other/Probes/Current Probe
Voltage Probe (between two nodes)
Elements/Other/Probes/Voltage Probe(node-to-node)
Ground
Elements/Sources/Ground
Simulation Control
Simulate/Simulation Control
Gating Block(G1,G2,G5,G7)
Total Time=0.06
Figure 1: Single phase controlled converter without and with source inductance
Apparatus Required:
a. PC b. Windows 95/98/NT/XP/7 c. PSIM
Conclusion/Comments: The single phase controlled converter with and without source inductance has been simulated and the waveforms has been studied.
2. Experiment 9: Title: Simulation of step up and step down chopper using MOSFET, GTO & IGBT. Objective: To study the Gating Pulse, Inductor Current, Output Voltage and Output Current for step up and step down chopper using MOSFET, GTO & IGBT. Theory:
Dc chopper primarily finds its application in dc drives. The output voltage of dc choppers is controlled by adjusting the on time of a switch, which in turn adjusts the width of a voltage pulse at the output. This is the so-called pulse width modulation (PWM) control. The dc choppers with additional filtering components form PWM dc-dc converters. Four basic dc-dc converter topologies are known in general buck, boost, buck-boost, and Cuk converters. a)
Buck converter: The step-down dc-dc converter is commonly known as a buck converter. It consists of dc input voltage source VS, a controlled switch S, diode D, filter inductor L, filter capacitor C, and load resistance R. 1) Duty Cycle,
k
V O V S
2) Critical Values of Inductance and capacitance, (1 k ) * R
LC
C C
2 * f
1 k
16 * L * f 2
b) Boost converter: The step-up dc-dc converter is commonly known as a boost converter. It consists of dc input voltage source VS, a controlled switch S, diode D, filter inductor L, filter capacitor C, and load r esistance R. V S 1) Duty Cycle, k 1 V O
2)
Critical Values of Inductance and capacitance, LC C C
(1 k ) * k * R
2 * f k 2 * f * R
Through the simulation of the buck and boost converters with the calculated values from the above formula we have studied the operation of the step up and s tep down converters.
Circuit Design: a) Step-down Converter Parameters
Block Name
Library
Value
DC Voltage Source
Elements/Sources/Voltage/DC
12V
MOSFET/GTO/IGBT
Elements/Power/Switches/MOSFET or GTO or IGBT
Diode
Elements/Power/Switches/Diode
R,L,C
Elements/Power/RLC Branches/Resistor,Inductor,capacitor
Gating Block
Elements/Power/Switches/Gating Block
Voltage Probe
Elements/Other/Probes/Voltage Probe
Current Probe
Elements/Other/Probes/Current Probe
Ground
Elements/Sources/Ground
Simulation Control
Simulate/Simulation Control
R=500 Ω; L=0.00583 H ; C=0.0000004 F
Frequency=25000 Hz No. of Points=2 Switching Points=0 150
Total Time=0.0016
Figure 1: Step-down converter (buck converter) with MOSFET and GTO.
b) Step Up Converter Parameters
Block Name
Library
Value
DC Voltage Source
Elements/Sources/Voltage/DC
5V
MOSFET/GTO/IGBT
Elements/Power/Switches/MOSFET or GTO or IGBT
Diode
Elements/Power/Switches/Diode
R,L,C
Elements/Power/RLC Branches/Resistor,Inductor,capacitor
Gating Block
Elements/Power/Switches/Gating Block
Voltage Probe
Elements/Other/Probes/Voltage Probe
Current Probe
Elements/Other/Probes/Current Probe
Ground
Elements/Sources/Ground
Simulation Control
Simulate/Simulation Control
R=30 Ω; L=0.000133 H ; C=0.00000044 F Frequency=25000 Hz No. of Points=2 Switching Points=0 240
Total Time=0.0016
Figure 2: Step-up converter (boost converter) with MOSFET and GTO.
Apparatus Required:
d. PC e. Windows 95/98/NT/XP/7 f. PSIM
Conclusion/Comments: The step up and step down chopper using MOSFET & GTO has been simulated and the Gating Pulse, Inductor Current, Output Voltage and Output Current have been studied.
3. Experiment 10: Title: Simulation of single phase half controlled symmetrical and asymmetrical bridge converters. Objective: To study the supply voltage, output voltage and output current waveforms for single phase half controlled symmetrical and asymmetrical bridge converters. Theory:
During the positive half-cycle, from α to π, a triggering pulse is applied to the forward biased thyristor T1. It turns ON and thyristor T 2 turns OFF by line commutation. Therefore load current flows from T 1 and D1. During the negative half-cycle, at the instant (π+α) to 2π, a triggering pulse is applied to the forward biased thyristor T 2. It turns ON and thyristor T 1 turns OFF by line commutation. Therefore load current flows from T 2 and D2. Here we see that the conduction period of the thyristors and the diodes are equal, theref ore the circuit is called symmetrical configuration.
During the postive half cycle of the ac supply, T1 and D1 are forward biased. T1 is turned ON at a firing angle α. Current flows through the path L — T1 — A — L — R — B — D1 — N . Hence T1 and D1 conduct from α to π. Similarly, T2 and D2 conduct from (π+α) to 2π in negative half cycle of the ac supply. The freewheeling action of the diodes D1 and D2 from 0 to α and from π to (π+α) in each cycle. In this converter configuration the conduction period of thyristors and diodes are unequal. Hence, this circuit confi guration is known as the asymmetrical configuration.
Fig i) Symmetrical Configuration waveforms
Fig ii) Asymmetrical Configurations waveforms
Circuit Design: Simulation of single phase half controlled symmetrical and asymmetrical bridge converters.
Block Name
Library
Value
Sinusoidal Voltage Source
Elements/Sources/Voltage/Sine
Peak Amplitude=230V Frequency=50Hz
Thyristor
Elements/Power/Switches/Thyristor
Diode
Elements/Power/Switches/Diode
R
Elements/Power/RLC Branches/Resistor
R=10 Ω;
Elements/Power/Switches/Gating Block
Frequency=50Hz No. of Points=2 Switching Points=30 180
Gating Block(G2,G4)
Elements/Power/Switches/Gating Block
Frequency=50Hz No. of Points=2 Switching Points=210 360
Voltage Probe
Elements/Other/Probes/Voltage Probe
Current Probe
Elements/Other/Probes/Current Probe
Voltage Probe (between two nodes)
Elements/Other/Probes/Voltage Probe(node-to-node)
Ground
Elements/Sources/Ground
Simulation Control
Simulate/Simulation Control
Gating Block(G1,G3)
Total Time=0.06
Figure 1: Single phase half controlled symmetrical and asymmetrical bridge converters.
Apparatus Required:
a. PC b. Windows 95/98/NT/XP/7 c. PSIM
Conclusion/Comments: The single phase half controlled symmetrical and asymmetrical bridge converters have been simulated and the supply voltage, output voltage and output current waveforms are plotted.
4. Experiment 11: Title: Simulation of PWM bridge inverter using MOSFET with R-L load. Objective: To study the PWM generation, supply voltage and output voltage waveform for a 3-phase PWM bridge inverter using MOSFET with R-L load. Theory: Pulse width modulated (PWM) inverters are among the most used power-electronic circuits in practical applications. These inverters are capable of producing ac voltages of variable magnitude as well as variable frequency. The quality of output voltage can also be greatly enhanced, when compared with those of square wave inverters discussed in. The PWM inverters are very commonly used in adjustable speed ac motor drive loads where one needs to feed the motor with variable voltage, variable frequency supply. For wide variation in drive speed, the frequency of the applied ac voltage needs to be varied over a wide range. The applied voltage also needs to vary almost linearly with the frequency. PWM inverters can be of single phase as well as three phase types. Their principle of operation remains similar and hence in this lesson the emphasis has been put on the more general, 3-phase type PWM inverter. There are several different PWM techniques, differing in their methods of implementation. However in all these techniques the aim is to generate an output voltage, which after some filtering, would result in a good quality sinusoidal voltage waveform of desired fundamental frequency and magnitude.
Unlike in square wave inverters the switches of PWM inverters are turned on and off at significantly higher frequencies than the fundamental frequency of the output voltage waveform. The typical pole voltage waveform of a PWM inverter is shown in figure below over one cycle of output voltage. In a three-phase inverter the other two pole voltages have identical shapes but they are displaced in time by one third of an output cycle. Compared to the square pole voltage waveform, the pole voltage waveform of the PWM inverter changes polarity several times during each half cycle. The time instances at which the voltage polarities reverse have been referred here as notch angles. It may be noted that the instantaneous magnitude of pole voltage waveform remains fixed at half the input dc voltage (E ). When upper switch (S ), connected to the positive dc bus is on, the pole voltage is + dc
0.5 E
U
dc
and when the lower switch (S ), connected to the negative dc bus, is on the L
instantaneous pole voltage is -0.5 E
dc
.
Fig 1: Typical Pole voltage waveform of a PWM inverter.
The switching transition time has been neglected in accordance with the assumption of ideal switches. It is to be remembered that in voltage source inverters, meant to feed an inductive type load, the upper and lower switches of the inverter pole conduct in a complementary manner. That is, when upper switch is on the lower is off and vice-versa. Both upper and lower switches should not remain on simultaneously as this will cause short circuit across the dc bus. On the other hand one of these two switches in each pole (leg) must always conduct to provide continuity of current through inductive loads. A sudden disruption in inductive load current will cause a large voltage spike that may damage the inverter circuit and the load.
1. 2. 3. 4.
Various PWM control methods are: Single-pulse width modulation (SPWM) Multiple-pulse width modulation (MPWM) Sinusoidal-pulse width modulation (sin PWM) Space Vector-pulse width modulation (SVPWM)
Circuit Design: Simulation of PWM bridge inverter using MOSFET with R-L load.
Block Name
Library
Value
DC Voltage Source
Elements/Sources/Voltage/DC
Vdc,Amplitude=100V VDC1,Amplitude=0 M,Amplitude=0.92
Three phase VSI
Elements/Power/Switches/VSI3(MOSFET)
PWM Controller
Elements/Others/Switch Controllers/PWM pattern Controller
Comparator
Elements/Control/Comparator
Sinusoidal voltage source
Elements/Sources/Voltage/Sine
Peak Amplitude=100V Frequency=50 Hz
Step Voltage Source (Enable)
Elements/Sources/Voltage/Step
Vstep=1
Three phase RL load
Elements/Power/RLC Branches/RL3
R=1 Ω;L=0.1H
Voltage Probe
Elements/Other/Probes/Voltage Probe
Voltage Probe (between two nodes)
Elements/Other/Probes/Voltage Probe(node-to-node)
Ground
Elements/Sources/Ground
Simulation Control
Simulate/Simulation Control
Total Time=0.05
Figure 1: Three phase of PWM bridge inverter using MOSFET with R-L load.
Apparatus Required:
a. PC b. Windows 95/98/NT/XP/7 c. PSIM
Conclusion/Comments: A 3-phase PWM bridge inverter using MOSFET with R-L load has been simulated and the PWM generation, supply voltage and output voltage waveform has been plotted.
5. Experiment 12: Title: Simulation of three phase AC regulator (α = 1200) Objective: To study the supply voltage, output voltage waveforms for three phase AC regulator. Theory:
The circuit of a three-phase, delta-connected ac regulator (termed as ac to ac voltage converter) with balanced resistive load is shown in Fig i. It may be noted that the resistance connected in all three phases are equal. Two thyristors connected back to back are used per phase, thus needing a total of six thyristors. As stated earlier, the numbering scheme may be noted. Since the phase current in a balanced three-phase system is only
1 3
of the line
current, the current rating of the thyristors would be lower than that if the thyristors are placed in the line.
Fig i) Three Phase Delta connected AC Voltage regulator
Assuming the line voltage EAB as the reference, the instantaneous input line voltages are, eAB =
2 Es sin ωt ; eBC =
2 Es sin (ωt-120ᵒ) ; eCA =
2 Es sin (ωt-240ᵒ) .
It may be noted that is the rms value of the line voltage in this case. The waveforms of the input line voltages, phase and line currents, and the thyristor gating signals, for shown below.
α
= 120 ᵒ are
Fig ii) Waveforms for three Phase Delta connected AC Voltage regulator
Circuit Design: 0 Simulation of three phase AC regulator (α = 120 )
Block Name
Library
Value
Sinusoidal Voltage Source
Elements/Sources/Voltage/Sine
Peak Amplitude=110V Frequency=50Hz
Thyristor
Elements/Power/Switches/Thyristor
R
Elements/Power/RLC Branches/R
Gating Block(G1,G4)
Gating Block(G3,G6)
Elements/Power/Switches/Gating Block
Elements/Power/Switches/Gating Block
Gating Block(G2,G5)
Elements/Power/Switches/Gating Block
Voltage Probe
Elements/Other/Probes/Voltage Probe
Current Probe
Elements/Other/Probes/Current Probe
Voltage Probe (between two nodes)
Elements/Other/Probes/Voltage Probe(node-to-node)
Ground
Elements/Sources/Ground
Simulation Control
Simulate/Simulation Control
R=11 Ω
Frequency=50Hz No. of Points=2 Switching Points(G1)=120 210 Switching Points(G4)= 0 30 300 360
Frequency=50Hz No. of Points=2 Switching Points(G3)=240 330 Switching Points(G6)=60 150
Frequency=50Hz No. of Points=2 Switching Points(G2)=180 270 Switching Points(G5)=0 90
Total Time=0.06
Figure 1: Three phase AC Regulator
Apparatus Required:
g. PC h. Windows 95/98/NT/XP/7 i. PSIM
Conclusion/Comments: The three phase AC regulator has been simulated and the supply voltage, output current and voltage waveforms has been studied.