ELEC-E8421 Components of Power Electronics. Protection of Power Semiconductor Devices

Transcription

1 ELEC-E8421 Components of Power Electronics Protection of Power Semiconductor Devices

2 Protection of power semiconductor devices Overvoltage protection du/dt protection di/dt protection Overcurrent protection Protection against overheating

3 Overvoltage protection Overvoltages occur due to Internal causes Reverse current cutoff of diodes or thyristors Switching-off of (inductive) current, transistors and GTOs External causes Lightning strokes Transients due to switching operation Overvoltage protection is needed due to Breakthrough of semiconductor component Switching-off capacity of current drops without protection

4 du/dt protection du/dt of semiconductor circuits occurs due to Reverse current Current turn-off du/dt protection is needed because Thyristors can turn on with high du/dt even without gate control Turn-off losses can be reduced by reducing simultaneous voltage and current EMC can be reduced Voltage reflection of long cables

5 di/dt protection High di/dt of semiconductor circuits occurs due to commutation of low inductance circuits di/dt-protection can be needed to reduce turn-on losses, limits simultaneous voltage and current over the component

6 Overcurrent protection Overcurrents of semiconductor circuits occur due to failures in Load Power semiconductor devices Control Overcurrent protection is needed due to Overcurrent capability of power semiconductor devices is low Risk of fire Risk of explosion

7 Thermal protection (Overheating) Overheating of semiconductor circuits occurs due to Impaired cooling (flow of cooling air or liquid prevented, dust, poor location of device) Overload

8 Snubber Circuits (RC-circuits), turning-off of a thyristor or diode Current of RC-circuit Circuit diagram Voltage of thyristor

9 Losses of RC-protection a) Turn-off W 1 = E 1 t 1 t 2idt C U 2udu L U 1 i 1 i 2idt Energy Capacitor Inductor from the supply t 1 = 0, t 2 = U 1 = 0, U 2 = E 1 i 1 =I 1, i 2 = 0 idt = Cdu W 1 = E 1 = E 1 C 0 U 2Cdu C U 1 E 1du C = CE 1 2 C 1 2 E L 1 2 I 0 2 = 1 2 LI CE U 2udu L U 1 E 1udu L I 0 0 idt i 1 i 2idt 1 Here 2 CE 1 2 represents the extra losses because of the snubber

10 Losses of RC-protection b) Turn on, snubber capacitor discharges and energy is transferred to heat W 2 = 1 2 CE 2 2 c) Total losses of the snubber resistor in a switching cycle is W tot = W 1 + W 2 = 1 2 LI CE CE 2 2

11 Losses during sinusoidal voltages In line-commutated converters line voltage causes corresponding current to flow through the snubber when the thyristor is not conducting, increases resistor power rating Voltage over a thyristor in a three-phase rectifier when control angle is 110 and 160 dgrees

12 Losses Current in the snubber can be approximated with and power is P H~ 1 l 2 360o R I Total losses in the RC-snubber are where n is the total number of transients P HRC = P H~ + f I = n i=1 U R + 1 jωc W 1i + W 2i

13 Polarized RC-protection snubber Inrush current at turn-on of T1 can be controlled with R1 R2 can even be zero Can only be used with converters with no reverse blocking requirement (D2 in parallel with T1, reverse current o D2)

14 Surge current free RC-protection e.g. for TRIACs T is turned on simultaneously with components to be protected snubbber

15 RC-protection of GTO-thyristor di/dt reduction GTO sensitive to du/dt at turn-off Normal snubber introduces large losses There are circuits in which energy of Cs is fed back to the dc-bus but, more expensive IGCT doesn t need du/dt reduction Voltage clamp

16 Overvoltage protection of transistor Clamp-circuit

17 Saturating inductor Saturated at normal current => no effect High inductance at low currents Reduces the effect of reverse recovery current without

18 Overvoltage protection. Metal Oxide Varistor (MOV) Varistor current-voltage characteristics for zinc oxide (ZnO) and silicon carbide (SiC) devices

19 Metal Oxide Varistor (MOV) Voltage vs Current characteristics of varistors Circuit symbol of varistor

20 MOV Lifetime (number of operating cycles/pulses) is reduced with higher currents Current rating of varistor as a function of impulse duration

21 Avalanche diodes Used for overvoltage protection too Works with both polarities, pnp-structure Used also in balancing voltages in series connections, Chapter 7

22 Breakover diodes, overvoltage protection of thyristors Dk turns on at overvoltage and protects T D needed because of lacking reverse blocking capability of Dk Diode in gate control circuit of thyristor Current vs. voltage characteristics of breakover diode, similar to thyristor

23 Fast fuses for semiconductors Fuse element W = R 0 R = constant t si(t) 2 dt = constant 0 t si(t) 2 dt = I 2 t s = constant I 2 t s is fuse specific value Seating plate Sand (inside the body) Body

24 Operating principle of a fuse Prospective short-circuit current Fuse current Supply voltage t s = Pre-arcing time t v = Arcing time

25 Fuse pre-arcing time curve Pre-arcing time as function of prospective current normally given in data sheets Scaling K is used for other voltage values

26 Fuse dimensioning (Overload) If load changes a lot, some cheking needs to be done, can be classified e.g. according to LK-NES Overloads (1 60 s) Once a month I max < 0,8 * I ts 1 or 2 times per week, I max < 0,7 * I ts Hourly, I max < 0,6 * I ts Fast duration overload (< 1 s) Seldom, I max < 0,7 * I ts Often, I max < 0,6 * I ts

27 Fuse dimensioning (Cyclic load) RMS value for the current I eff = k n=1 I 2 n t n k Load cycle t n n=1 Current is higher than RMS I effhot = 2 n I n hot t n hot n t n hot G = Ieffhot + I effcold 2 2 I effhot Current is lower than RMS I effcold = 2 n I n cold t n cold n t n cold

28 G-factor of fuse If load cycle is more than an hour, G can be used as such If load cycle is les than an hour, scaling as shown below Continuous current rating of the fuse needs to fulfiil I b I effhot G Duration of load cycle (min)

29 Fuse in DC-circuit DC has no zero crossings Arc-voltage has to bel large enough to drive current to zero Time-constant of the circuit has an effect on dimensioning, voltage rating changes Overcurrents are problematic, fuse can explode! Resonan LC-circuits problematic too, fuse dosen t necessarily work in the first zero crossing Voltage rating as a function of time constant of shortcircuit

30 Fuses in 6-pulse bridge rectifier circuit Three or six fuses Protection against load in rigth figure

31 Fast circuit breakers Operation time < 10 ms Expensive Maintenance costs are cheaper compared to fast fuses, no need to change the fuse, lost operation time minimal

32 PTC Resistor PTC = Positive Temperature Coefficient Resistance 2500 x nominal 120 C => Fault current limited Needs time to cool and recover