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ELEC0445 - High Voltage Direct Current grids
Part I. Line Commutated Converters
Chapter 2. Thyristors
Thierry Van Cutsem(in replacement of Patricia Rousseaux)
[email protected] www.montefiore.ulg.ac.be/~vct
February 2020
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Chapter 2. Thyristors Ideal power device
Ideal power device
In HVDC applications, power-electronic devices are used as switches.
Those switches are grouped into valves.
The ideal power device should :
have zero resistance in on-state
support an infinite voltage in off-state
switch between on and off states in zero time
have a zero power dissipation
be small, light and cheap.
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Chapter 2. Thyristors Semiconductors
Semiconductors
Class of materials whose electrical conductivity at ambient temperature isbetween that of a conductor and that of an insulator
10−5 Ω.m < ρ < 107 Ω.m
in metallic materials, the conduction is linked to free electrons :movement of negative charges, positive charges are fixed
in semiconductors, two conduction modes :I movement of free electrons : negative chargesI displacement of holes : positive charges
ne : concentration of free electrons np : concentration of holes
doping : adding impurities in the material to control conductivityI add donors of electrons : type n semiconductors with ne >> npI add acceptors of electrons : type p semiconductor with np >> ne
high sensitivity to temperature
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Chapter 2. Thyristors The diode
The diode
A layer of type n semiconductor joined to a layer of type p semiconductor
n layer : ne np, positive ions (donors of electrons)
p layer : np ne , negative ions (acceptors of electrons)
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Chapter 2. Thyristors The diode
Actual i − v characteristic
forward bias mode : high current, smallresistance above threshold voltage Vf
reverse bias mode : small reverse current if|v | < VZK
breakdown mode : for v < −VZK , avalanchecurrent
Ideal i − v characteristic
zero on-state resistance
zero on-state voltage
very negative breakdownvoltage VBR
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Chapter 2. Thyristors The thyristor
The thyristorEssential component of HVDC valves in the LCC technologyoperates as a controllable diodecan have high power ratings : up to 8.5 kV, 4500 A capabilityis robust and efficient.
Four-layer, three-terminal device.Three pn junctions J1, J2, J3
equivalent to two bipolar transistors
assume VAK > 0 and inject IG
both transistors remain insaturation even if IG is suppressed
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Chapter 2. Thyristors Modes of operation of the thyristor
Modes of operation of the thyristor
Three modes of operation depending on :
the sign of the anode − cathode voltage vAKwhether a current IG is injected at the gate terminal
1 Reverse blocking mode
A reverse voltage vAK < 0 is applied.I Junction J2 is in forward bias modeI junctions J1 and J3 are in reverse bias modeI the thyristor acts as a diode in reverse bias mode; it is in off-stateI breakdown occurs when vAK is more negative than the reverse
breakdown voltage VBR . Most often this is associated with junction J1I in HVDC applications, the breakdown mode must be avoided since it
can lead to material destruction.I hence, thyristors with high |VBR | values must be used, and measures
taken to limit the avalanche current.7 / 23
Chapter 2. Thyristors Modes of operation of the thyristor
2 Forward blocking mode
A forward voltage vAK > 0 is applied but no current IG is injected.I Junctions J1 and J3 are in forward bias modeI junction J2 is in reverse bias modeI the thyristor behaves as a diode in reverse bias mode; it is in off-stateI breakdown occurs when vAK is larger than the forward breakdown
voltage of junction J2
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Chapter 2. Thyristors Modes of operation of the thyristor
3 Forward conduction mode
A forward voltage vAK > 0 is applied and a current IG is injected
I the current injection results in an avalanche processI “as if” layer p2 would become of n-type. Hence, the thyristor behaves
as a pn diode in forward bias mode : it switches to on-stateI the thyristor resistance is dramatically reduced (from 106 Ω to 10−1 Ω)I the larger IG , the smaller the value of vAK needed to initiate the
avalanche.
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Chapter 2. Thyristors Modes of operation of the thyristor
Operation of the thyristor in on-state
once the anode current i reaches IL, the latching current, the thyristorswitches to on-state
once the thyristor is in on-state, the gate current can be removed
the gate current is usually a short pulse lasting 10-50 µs
if i falls below IH , the holding current, the thyristor switches tooff-state.
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Chapter 2. Thyristors Modes of operation of the thyristor
The ideal characteristic
closed order given by gate current
in open state, VBR and VBF are assumed infinite
when the thyristor conducts, a zero internal resistance is assumed
when the thyristor conducts, a zero terminal voltage is assumed.
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Chapter 2. Thyristors Switching characteristics
Switching characteristics
From off-state to on-state :
initially, the thyristor isforward biased (vAK > 0)
one pulse of IG initiatesconduction
switching takes place duringthe on-time ton (a few µs):
I the current increasesI the voltage decreases to
about 0.5 - 2.5 V
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Chapter 2. Thyristors Switching characteristics
From on-state to off-state :
the thyristor turns off wheni falls below IH and reverses
recombinations of chargecarriers take place injunctions J1 and J3, whichswitch to reverse bias mode
a maximum reverse currentis observed and the voltageis reversed
an over-voltage can beobserved if currentextinction is too fast
once the charge carrierrecombination is completed,i goes to zero.
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Chapter 2. Thyristors Switching characteristics
Commutation
Process of turning off a conducting thyristor.
commutation is not instantaneous : thethyristor is in reverse blocking mode afterthe reverse recovery time trr
(undesired) natural commutation : the current falls naturally to zeroI in HVDC applications, this can arise when operating with very low DC
currentI this can be avoided by applying repeated gate pulses
forced commutation : an external circuit forces the current to zeroI for thyristors used in AC/DC converters : the commutation is forced by
firing another thyristorI the DC current commutes to this other thyristor
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Chapter 2. Thyristors Switching characteristics
Extinction time and commutation failure
On turning off :
the thyristor can immediately withstand a full reverse blocking voltage
but it cannot withstand immediately a forward voltage and remain inblocking condition
it must remain reverse biased for a minimal time to allow fullrecombination of charge carriers
if not, it may switch back to on-state without a gate pulse. This isreferred to as commutation failure
can be destructive for the thyristor !
the minimum reverse bias time is called theextinction time tq
for high-power thyristors used in HVDC :tq ∈ [300− 1000]µs
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Chapter 2. Thyristors Constraints on current and voltage variations
Constraints on current and voltage variations
di/dt protection
at turn-on, the current in junction J2 spreads gradually across theentire junctionif the current rise is too fast, this can lead to local meltingthe rate of increase of the anode current must be limited :
di
dt|max ' 100A/µs
dv/dt protection
in blocking mode, the pn junction behaves as a capacitor (chargespace layer)a large voltage variation dv/dt can produce too large an anodecurrent i = C dv/dtthis current can trigger undesired thyristor conductionthe rate of increase of the forward blocking voltage must be limited :
dv
dt|max ' 1000V /µs
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Chapter 2. Thyristors Constraints on current and voltage variations
Snubber circuit
The thyristor is provided with a snubbercircuit 1 implementing the required protections
The series inductance Ls allows limiting therate of variation di/dt
The parallel Rs -Cs circuit allows limiting therate of variation dv/dt
The same circuit also allows limiting thepossible reverse over-voltage during turn-off
1 en francais : circuit d’amortissement
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Chapter 2. Thyristors Structure of a thyristor valve
Structure of a thyristor valve
A thyristor module includes :I a thyristor driver including the gating
control circuitI the protection circuits Rs − Cs and LsI the grading resistor Rdc aimed at
balancing voltages among the variousthyristor modules (ideally, all modulesshould work at identical voltages in alloperating conditions)
thyristor modules are associated inseries to form a thyristor valve.Objective : reach the HVDC linkvoltage rating :
thyristor : up to 5 to 9 kVHVDC link : 500 to 800 kV
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Chapter 2. Thyristors Structure of a thyristor valve
Valve rack assembly grouping severalthyristor modules
Thyristor valve hall at SylmarHVDC station, Los Angeles
valves are suspended from theceiling of the valve hall
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Chapter 2. Thyristors Structure of a thyristor valve
Sinusoidal AC voltage and ignition angle α
the thyristor is used as a switch in AC/DC converters
the input is a sinusoidal voltage with frequency f =ω
2πthe thyristor is “fired” once per period
with an ignition delay t0 : time delay from beginning of forwardvoltage (v > 0)
generally expressed in terms of the ignition angle α
α = ωt0
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Chapter 2. Thyristors Summary
Summary
A thyristor can be used as a controllable bistable switch
the control is performed by injecting a current at the gate inputI the thyristor is ON and conducts when it is forward biased and the gate
receives a current pulseI the thyristor keeps on conducting as long as it is forward biasedI the thyristor is turned OFF when the anode current falls below the
holding current threshold IH or when it is reverse biasedI the thyristor remains in blocking mode until it is triggered by a new
gate pulse current
the process of turning off is called commutation
when commutating, the thyristor cannot immediately withstand aforward voltage; it should remain reverse biased for a minimum time,namely the extinction time tq
otherwise, commutation failure can take place.
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Chapter 2. Thyristors Exercise 2-1
Exercise 2-1
This exercise illustrates the use of a thyristor in a rectifier and theimportance of the snubber reactor Ls .
The circuit below shows a single-phase half-wave rectifier. AssumeVS = 230 V (RMS), fS = 50 Hz and a resistive load R = 50 Ω. The firingangle of the thyristor is α = 80.
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Chapter 2. Thyristors Exercise 2-1
1 Without snubber reactor :
I draw the time evolution of the load voltage vLI draw the time evolution of the load current iLI show that the current derivative at the switching instant is very high.
2 With the snubber reactor :Assuming that the source voltage is constant during the thyristorswitching :
I compute the minimum inductance Ls required to limit the rise ofcurrent diL
dt to 1 A/µsI draw the time evolution of the load voltage vLI draw the time evolution of the load current iL
Use the ideal thyristor model.
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