Silicon Thyristor with Low ON-State Voltage Drop for the Next Generation HVDC Transmission
The third generation of Phase Control Thyristor (PCT) technology is the flagship of the latest generation HVDC Classic product family from ABB Semiconductors. It has been specially designed to further increase the efficiency of converter valves with nominal rating power of 1.5 GW, which are based on the Line Commutated Converters with PCTs to provide the most energy efficient systems for bulk power transmission. Having the size of about 100 mm (4”) wafer, the lowest possible ON-State voltage drop of VTmean = 1.65 V at IT = 1.5 kA and T = 90 °C is achieved for symmetrical forward and reverse blocking capability of 8.5 kV. The device is supplied in ceramic (hermetic) press-pack housing with 100 mm pole pieces.
The third generation of Phase Control Thyristor (PCT) technology is the flagship of the latest generation HVDC Classic product family from ABB Semiconductors. It has been specially designed to further increase the efficiency of converter valves with nominal rating power of 1.5 GW, which are based on the Line Commutated Converters with PCTs to provide the most energy efficient systems for bulk power transmission. Having the size of about 100 mm (4”) wafer, the lowest possible ON-State voltage drop of VTmean = 1.65 V at IT = 1.5 kA and T = 90 °C is achieved for symmetrical forward and reverse blocking capability of 8.5 kV. The device is supplied in ceramic (hermetic) press-pack housing with 100 mm pole pieces.
The Phase Control Thyristor (PCT) maintains a significant market share thanks to its attractive price, relatively simple handling and the lowest ON-state losses from all existing device concepts. PCTs can be found in motor control, induction heating, power quality systems, power supplies and other industrial applications. In contemporary Line Commutated Converters (LCC) for High-Voltage Direct Current Transmission (HVDC), where the lowest ON-State voltage drop is a must, PCT is the main semiconductor switch.
The HVDC Classic concept is intended for a transmission of bulk electric power over very long distances (> 300 km, typically up to ≈ 2,000 km), which is more economic than the classical Alternating Current (AC) systems. Existing HVDC systems with the LCCs are employed to transmit the power of 1 8 GW using the DC voltage rating up to 800 kV. The future systems, which are planned in China, will break the 10 GW level soon using the DC voltage above 1 MV. The device presented in this paper is being applied in the converter poles with nominal rating power of 1.5 GW, which is usually applied in North and South America or India.
Fig.1: Transmission of bulk energy from renewable energy sources at the level of units of GWs happens via DC transmission. AC voltage is transformed, filtered and rectified into the few thousand km long DC cable to be converted back to AC voltage at the end of the transmission line.
The achievement of so high rated voltage of the LCC is possible only by serial connection of up to one hundred PCTs (see Fig.2b). The serial connection requires a precise adjustment of the relation VT – Qrr (voltage drop – reverse recovery charge) at technology curve (see Fig.2a). All parts must have the Qrr adjusted at a narrow band in order to minimize excessive voltage overshoots during turn-off, which would otherwise require redundant PCTs to compensate for this extra voltage and also damping losses would grow. The adjustment of Qrr is performed by electron irradiation, which reduces the recombination lifetime of free carriers (electrons and holes) and accordingly the Qrr. The advantage of this method is that it can be performed after the PCT is fully processed.
Fig.2: Electron irradiation is used to adjust the Qrr of all PCTs into a narrow band at the technology curve VT – Qrr (a). This minimizes voltage overshoots during turn-off process of serially connected PCTs in converter valve (b).
Another special feature of the PCTs for HVDC is that it has to survive several sorts of electrical disturbances during operation. One of them appears when a lightning strike hits the transmission line. In this situation, the PCT can be abruptly reverse biased into the knee of reverse I-V curve and subjected to the reverse current in the range of tens of amperes. To achieve such robust blocking capability, high demands are laid on junction termination including its surface passivation. Another demand is about the surge current capability, where devices have to survive several current pulses in the order of tens of kA. Last but not least, there are also special demands on in-rush current capability, when some of neighboring parts would lose its blocking capability.
TECHNOLOGY FEATURES:
A general approach to reduce ON-state losses consists in maximizing the cathode area and reducing the device thickness. The former is determined by the wafer size, so that there are 4, 5 and 6 inch PCTs available for the HVDC systems. The latter is more difficult, because the PCT is a non-punch through device, which means that one cannot reduce the thickness of the N base and P-base without losing the blocking capability. The only possible thickness reduction is that of the anode and partially of the P-base. The extent of this reduction depends on the type of the used junction termination (negative, positive or combined positive-negative bevel).
Fig.3 shows the new hybrid concept (b), which is used to reduce the device thickness without losing the blocking capability contrary to the classical design (a). The thicker P-type layers at the periphery are necessary to achieve a required blocking capability of the negative bevel. In the bulk, the P-type layers can be thinned, because there is no need to stretch the electric field along the surface like at the wafer edge. As a result, the leakage current caused by the punch-through effect is minimized in the bulk. At the periphery, the punch-through effect remains in its original magnitude. However, as the bulk represents about 90 % of the total area, the leakage current is greatly reduced and the loss of breakdown voltage due to thinning is compensated. The benefit of the new concept (b) is further shown by comparing the devices (a) and (b) with equal forward and reverse blocking voltages of 8.5 kV.
Fig.3: New hybrid concept of thinner p-type layers (anode and P-base) at the bulk than at junction termination (b) allows one to thin PCT without sacrificing the blocking capability to ON-state losses contrary to classical concept (a).
STATIC AND DYNAMIC PERFORMANCE:
Thyristor current and voltage during a typical operation cycle is shown in Fig.4a). The energy losses in the ON-state are characterized by the ON-state voltage VT. PCT is commutated from the ON-state by the AC voltage crossing the zero level. The charge stored in the N-base and P-base during the ON-state is removed by carrier recombination and after the zero cross-over it is also swept by the reverse voltage. The current integrated after zero cross-over is known as the reverse recovery charge QRR, which represents the switching losses of the PCT. At the end of the circuit commutated recovery time tqRp, the PCT must block the forward voltage (may not re-trigger) to be ready for control of power in the next cycle.
Fig.4a): Typical operation of PCT: ON-State, Reverse Recovery, Forward Blocking. Parameters VT and QRR are read out from this test.
Fig.4b): Technology curve VT – QRR of Classical and New devices having equal blocking capability of 8.5 kV.
At a typical rectifier mode of operation, the ON-state losses may amount to 25 – 65 %, while the turn-off losses 15 – 20 %. The lowering of VT has therefore the biggest impact on the power efficiency of the HVDC valve. The reduction of VT at the latest PCT generation from Fig.4b) brings about 6 – 8 % lower valve losses. This translates to a significant saving of energy losses during system operation, to a higher rating current of the HVDC system and surge current of the PCT as well.
CONCLUSIONS
Thanks to the improvements in the PCT ratings presented above, Line Commutated Converters for HVDC Transmission will continue to satisfy the rising demands on power transportation with very high throughput from renewable energy sources to outlying centers of consumption. The new four inch PCT generation, which complies with the growing demands on voltage- and current-carrying capability of future HVDC systems, will expand to five and six inch PCT platforms. This will make future HVDC systems operating within the 1 – 12 GW range at any place in the world even more efficient.
