TSTE19 Power Electronics Lecture 11 Tomas Jonsson ISY/EKS
TSTE19/Tomas Jonsson 2015-12-08 2 Outline Converter control Snubber circuits Lab 3 introduction
TSTE19/Tomas Jonsson 3 Basic control principle. Voltage Source Converter (VSC) U v U v adjustable { amplitude phase angle frequency
TSTE19/Tomas Jonsson 4 Control of Active and Reactive Power I d U d DU S b Power transformer X tr U c I R U F I f I t U t n U ac I ac X c r S b = P + jq = r r * 3 U F I R P = U F U C wl sind Q U = - F ( U -U cosd ) F wl C
TSTE19/Tomas Jonsson 5 Control of Active Power Rectifier Inverter DU DUU U F U C U F d U C - d I R I R If the Uc is in phase-lag, the active power flows from AC to DC side (rectifier) If the Uc is in phase-lead, the active power flows from DC to AC side (inverter)
TSTE19/Tomas Jonsson 6 Control of Reactive Power Reactive power consumption Reactive power generation DU DU U F U F U C U C I R I R If U F > Uc, there is reactive power consumption. If U C > U F, there is reactive power generation.
TSTE19/Tomas Jonsson 7 Control of P and Q in 4 quadrants
TSTE19/Tomas Jonsson 8 Calculation of Control Signal P, Q X Converter Bridge U 1 U 2 U 2 calculator PWM U 2 ref Amplitude & phase X P Q
TSTE19/Tomas Jonsson 9 Voltage Source Converter with Pulse Width Modulation I d U d DU S b Power transformer X tr U c I R U F I f I t U t n U ac I ac Converter voltage X c Fundamental component +U d -U d
TSTE19/Tomas Jonsson 10 PWM- generation of pulses 1.50 Sawtooth LIGHTA240.CIR Voltage reference 0.00-1.00 20.00m 24.00m 32.00m 40.00m v(saw) v(ref1) T 200.00K 160.00K 80.00K 0.00K -80.00K -160.00K -200.00K 20.00m 24.00m 28.00m 32.00m 36.00m 40.00m v(conv1) v(conv2) T Switched voltage Fundamental component
TSTE19/Tomas Jonsson 2015-12-08 11 Feed-forward control = + = + = + + = cos + I d U d DU S b Power transformer X tr U c I r U f I f I t U t n U ac I ac X c
TSTE19/Tomas Jonsson 2015-12-08 12 Vector control = + = = = cos = sin = + + PLL required to define wt I d U d DU S b Power transformer X tr U c I R U f I f I t U t n U ac I ac X c
TSTE19/Tomas Jonsson 2015-12-08 13 System with feed-forward and vector control
TSTE19/Tomas Jonsson 14 Control of AC - DC AC converter Bäcks Näs u DC1 u DC2 - u AC-ref1 + q ref1 u AC1 AC voltage control i PWM internal current control + DC voltage control - u - u DC-ref2 DC-ref1 + p ref1 p ref2 DC voltage control PWM internal current control i u AC2 AC voltage control u AC-ref2 q ref2 Principle control of HVDC-Light
TSTE19/Tomas Jonsson 15 DC Power versus DC voltage Control of: U1, f1 Control of: P1, Q1 Control of: P2, Q2 Control of: U2, f2 u DC1 u DC2 P1, Q1 P2, Q2 Without DC voltage control U DC P1 P2
TSTE19/Tomas Jonsson 16 DC voltage control Control of: U1, f1 Control of: Ud, Q1 Control of: P2, Q2 Control of: U2, f2 u DC1 u DC2 P1=f(U dc1 ) Q1 P2, Q2 With DC voltage control U DC P2 P1
Lecture 11 Snubber circuits
TSTE19/Tomas Jonsson 2015-12-08 18 Problem with switching converters Switches turns on/off while conducting large currents Recovery time create large power dissipation di/dt generates EMI (Electromagnetic interference) Small size requires higher switching frequencies
TSTE19/Tomas Jonsson 2015-12-08 19 Example, full bridge leg Finite di/dt and dv/dt Parasitics: L, C, R I O can be both positive and negative
TSTE19/Tomas Jonsson 2015-12-08 20 Hard switching waveform Stray inductance gives voltage overshoot Stray capacitance gives current overshoot Parasitics limits di/dt and dv/dt P T- = v T- i T-
TSTE19/Tomas Jonsson 2015-12-08 21 Switch voltage and current Short moments of high power dissipation Device must cope with the power dissipation Overshoot increases required SOA
TSTE19/Tomas Jonsson 2015-12-08 22 Component characteristic goal High di/dt and dv/dt to reduce power loss Short recovery time diodes Components must cope with short time full power dissipation Large stress on components due to power dissipation changes (material stress etc)
TSTE19/Tomas Jonsson 2015-12-08 23 Snubber circuits Connected in parallel/serial with the switches Turn-on snubber Inductor limits di/dt Capacitance limits dv/dt Power now lost in snubber instead of switch
TSTE19/Tomas Jonsson 2015-12-08 24 Snubber circuits Protect semiconductors Limit voltages applied during turn-off transients Limit device currents during turn-on transients Limit the rate of rise (di/dt) of currents Limit the rate of rise (dv/dt) of voltages Shape the switching trajectory at turn on and turn off Three major classes Unpolarized series RC snubbers Polarized RC snubbers Polarized LR snubbers
TSTE19/Tomas Jonsson 2015-12-08 25 Diode snubbers L σ stray inductance R C snubber circuit Problem when switch turns on Current starts to flow in wrong direction When diode turns off then Lσ tries to force continued current Diode breakdown if + >
TSTE19/Tomas Jonsson 2015-12-08 26 Snubber circuits for controlled switches Step-down example Lσ = L1 + L2 + L3 stray inductances Voltage and current overshoot due to inductances Three snubber types Turn-off, turn-on, over-voltage
TSTE19/Tomas Jonsson 2015-12-08 27 Turn-off snubber circuit for controlled switches Diode to only include R s at switch turn-on Simplified circuit for switch turn-off = 1 Switch current at switch turn-off not affected by snubber circuit
TSTE19/Tomas Jonsson 2015-12-08 28 Overvoltage snubber function At transistor switch-off completed t = 0 V COV = V D I 0 goes through D f Fast switch give i Lσ (t=0) = I 0
TSTE19/Tomas Jonsson 2015-12-08 29 Turn-on snubber To limit di/dt through switch at turn-on Turn-off Thyristors (GTO, IGCT) has limited di/dt capability At switch turn-off, stored energy in L s must be dissipated. D LS and R Ls forms discharge circuit for the inductor L s
www.liu.se