The thyristor, first introduced in the mid-1950s and originally known as the silicon con- trolled rectifier (SCR), is a solid-state device that allows current flow in one direction and blocks in the other, but can also be triggered “on” by an external pulse. The internal structure of the conventional thyristor is shown in Figure 2.4. It can be seen that the thyristor consists of two transistor structures that are connected together, with the col-
lector of a pnp transistor forming the base of an npn transistor. It is the regenerative
action of these two component transistor structures that determines the function of the thyristor. A brief summary of thyristor operation is given herein; a detailed discussion can be found in [11].
Figure 2.4: Thyristor structure.
With the thyristor forward biased, i.e. the anode (A) is positive with respect to the cathode (K), and with the gate current held to zero, both of the outer p-n junctions are forward biased whilst at the central p-n junction a depletion region builds up, restricting current flow across this junction. This is the forward-blocking, or off-state of the thyristor. In order to turn the device on, a positive current is injected into the gate; this causes the total current flowing out of the cathode to increase. After removal of the gate current, the cathode current must be sufficiently large (greater than the holding current) such that the p- and n-emitters (P1 and N2) inject enough carriers to maintain the device in
conduction.
The regeneration action of the thyristor can be described by considering the two
transistors formed by P1-N1-P2 and N1-P2-N2. If the npntransistor is turned on by the
gate, it supplies current to the pnp base, which in turn supplies current to the npnbase,
allowing the gate signal to be removed. At the section of the pnp collector and the npn
collector the anode current can be described as
IA=αpnpIA+αnpnIK+IL (2.1)
where α represents the transistor current gains, IK is the cathode current and IL is the
leakage current in the blocking state. Because the cathode current is equal to the sum of the anode and gate currents, the anode current can be redefined as
IA=
αnpnIA+IL
1−αpnp+αnpn (2.2)
As such, it can be seen that the thyristor switches to the on-state when the sum of thepnp
and npntransistors is unity. This condition can also be attained without the provision of
any gate current, by increasing the applied forward voltage so that carrier multiplication (M>>1) at the N1-P2 junction increases the internal leakage current, which increases the current gains of both transistors. Turn-off of the thyristor requires that, along with zero gate current, the cathode current is reduced to below the holding current level for a certain minimum time; this is achieved by the external current crossing zero (natural
commutation) or by forced commutation using additional external components. The
current-voltage characteristics of the thyristor are illustrated in Figure 2.5.
IA VAK Forward conduction Forward blocking Reverse blocking
Figure 2.5: Thyristor voltage-current characteristics.
and current ratings of single units, there are several limitations of this device. First, as mentioned previously, is the restriction in turn-off capability of the device. Secondly is a
limitation ofdi/dt, which arises due to the fact that the gate-cathode junction first turns
on in the vicinity of the gate contact, taking time for adjacent regions to turn on and
thus limiting the safe operating area of the device. If thedi/dt is too high, then only part
of the thyristor is in conduction, causing excessive heat dissipation in the regions that
have turned on and risking device failure. Finally, a third limitation relates to the dv/dt
applied to the device, which, if high enough, can create a current which can exceed the holding current of the thyristor and trigger it on. Though the inability to turn-off the thyristor is an inherent feature of the device, design improvements now mean that the
di/dtanddv/dtlimitations are less of a constraint; as an example, an 8.5 kV 2180 A Phase
Control SCR from Dynex Semiconductor [12] is rated for di/dt of 100 A/µs and dv/dt
of 2000 V/µs. However, as outlined in [11], it has been shown that by using excessive
gate current under certain operating conditions, it is possible for an inverter SCR to be
operated reliably at 10,000-20,000 A/µs.
The lack of turn-off capability of the SCR restricts its practical use to LCC schemes, whereby the source is the AC system voltage. With reference to Figure 2.6, LCC is reliant on the natural current zeroes that are created by the external circuit in order to transfer current from switch to switch. This operation is explained with the aid of Figure 2.7. Here, an approximated DC voltage is obtained from the three-phase AC voltage as the SCRs are cycled on in pairs, as described at the top of the Figure. The firing delay angle
of the SCRs is denoted here by α; it is noted that α = 0 gives identical operation to a
three-phase diode rectifier. This converter acts as a rectifier for firing delay angles from 0
toπ/2, giving a positive output, whilst for firing delay angles from π/2 to πthe converter
acts as an inverter, giving a negative output.
1 3 5
2 4 6
A B C
Figure 2.7: Operation of a SCR-based three-phase converter.
Although this is the least flexible solution for DC transmission, it was, until very recently, still the most common solution due to its relative maturity when compared to SCC schemes. Unfortunately, though the LCC configuration is simple, it has significant shortcomings in the form of large reactive power requirements, injection of low-order harmonic currents, risk of inverter commutation failures and the dependence on strong AC systems for the provision of commutation voltages. As mentioned previously, the use of SCC schemes which utilise more advanced power electronics devices can eliminate these problems. This has meant that, nowadays, LCC schemes are typically only employed for very long distance point-to-point bulk power transmission at voltages in excess of 320 kV, where the advantages of SCC transmission are not required. At the current time, IGBT- based VSC is the favoured SCC technique for HVDC technology; as such, this is discussed in the next Section.