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Posted on 06 July 2019

Six Inch Thyristors for UHVDC Transmission

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The making of the largest thyristor around.

The most powerful converter available has recently been erected in China. To realise the required rating of 6.4 GW for an UHVDC (Ultra High Voltage Direct Current) transmission, a new thyristor, that can operate the high DC-current of 4 kA at the highest possible blocking voltage was required. To accomplish this task a six inch thyristor with blocking voltage 8500 V was developed. In this article we look at some challenges faced when developing this device, as well as the testing done to prove that it could fulfil the high reliability and life-time requirements that are put on an HVDC-converter.

By Virgiliu Botan and Jürg Waldmeyer , ABB Switzerland Ltd, Semiconductors

 

Introduction

In spite of the higher initial investment, the DC transmission system proves to be more economical than the AC one at long distances. For example, for a 400 kV 1200 MW transmission line, the break over distance after which the DC system becomes more economical is about 250 km. The increased distances between the available energy sources and the users, as in China and Brazil where the new large hydro power plants are more then 1000 kilometers away from the large cities, is therefore expected to lead to a continuing demand for new HVDC systems.

The HVDC transmission systems also enable the connection of asynchronous AC grids and the linking of grids with different frequencies, as for instance has been done through the so-called back-to-back stations between Brazil with 60 Hz and Argentina with 50 Hz.

For the ever increasing power levels of these systems, the thyristor has a long history of higher voltages and larger diameters resulting in the very powerful components available today. The latest step in this evolution is the thyristor presented here, with its 6 inch wafer diameter and blocking voltage of 8500 V.

HVDC

Transport of high amounts of electric energy over long distances has been known as a valuable technology for several decades. Whereas most of the electricity transported through the power grid is AC, for very long distances the use of DC voltages proves to be more economical. For transmissions through sea-cable the distances do not even have to be very long to make DC the more economic solution. Starting with the 1954 Gotland link in Sweden using mercury arc valves, DC power transmission became a viable solution for transporting significant amounts of energy over long distances.

Mercury valves used in the Gotland HVDC transmission inaugurated in 1954

Following this first project, the distances and rated power of the HVDC connections have increased steadily. HVDC was also an early adopter of the thyristor technology and during the last 40 years the thyristor-based HVDC-converters have been refined to have very high reliability and efficiency. Nowadays, there are numerous HVDC links with rated voltages of ± 500 kV DC and rated power of 1 to 3 GW in operation with some even exceeding these values. It is also possible to design HVDC-systems with VSCs (Voltage Source Converters) based on turnoff devices such as IGBTs. These types of converters, as HVDC Light from ABB, offer enhanced flexibility in power transmissions.

Nevertheless, when it comes to really higher power, the thyristor based converters are the solution of choice because they can handle up to one order of magnitude more rated power than their IGBTs counterparts.

Modern HVDC converter hall

These GW-transmissions are needed, for instance, in the case of the today’s booming economy of China. The industrial centers in the east and south are consuming electricity from remote locations rich in hydropower. An example of such a line is the Ultra-High Voltage DC (UHVDC) transmission between Xiangjiaba and Shanghai, presently under construction, with a rated power of 6400 MW and a rated voltage of ±800 kV. This power rating is more than twice the rating of the former HVDC systems transmitting power from the Three Gorges area to the industrial centers on the Chinese coast.

The 2071 km long Xiangjiaba - Shanghai UHVDC transmission

Thyristors for HVDC

For operating an HVDC line, phase control thyristors (PCTs) are being used. For cost and power loss optimization reasons, the most economical solution for a thyristor performance increase is normally a simultaneous increase in blocking capability and current handling capability. ABB, or at that time its founding companies ASEA and BBC, started to produce thyristors for HVDC systems as early as 1969. One of the first projects was the refitting of the Gotland Link, where a two inch thyristor, with a 3.2kV/200 A, was used. Since then, a lot of progress has been made, culminating with the development from a five inch 7.2 kV/3000 A thyristor to a six inch 8.5 kV/4000 A device.

Six inch UHVDC thyristor and its wafer

Manufacturing

Considering that 12” is a normal size for low voltage chip manufacturing it may sound strange that silicon was a challenge for the 6” thyristor development. Since the starting material needed for high power devices is quite different to the chip manufacturing, it was though a key for the development that there had been progress in the Si manufacturability. With the neutron doped (NTD) float zone (FZ) technique, it is feasible today to grow six inch Si crystals essentially defect free, with a very homogeneous resistivity profile, which is a prerequisite for the manufacturing of large area high voltage thyristors.

Additional improvements were necessary in the clean room processing line. The process parameters like temperature, time, gas flows had to be recalibrated in order to ensure a homogenous atmosphere needed for different diffusion processes. Particularly challenging was controlling the gas distribution with a Si wafer that is only marginally smaller than the diffusion tube itself, without getting any gradients along the tube or the wafer itself.

Electrical Characteristics

The 8.5 kV rating is needed in forward and reverse direction for single voltage pulses but as well for 50 Hz operation. In real world applications, the voltages applied to the thyristors are composed of sine waves with short peaks superimposed. These short peaks can be repetitive (i.e. caused by commutation overvoltages) or single peaks (i.e. caused by lightning or network transients). For single voltage pulses the thyristor is tested with half-sine wave pulses of 10 ms duration and 8.5 kV peak voltage. In order to simulate the repetitive peaks that appear in applications a voltage waveform composed of a 50 Hz sine wave and a short pulse on top is used, as seen in Fig. 5.

Waveforms for repetitive blocking

The timing of the two pulses was chosen such that it emulates the worst case scenario. Despite the severe testing, the thyristor withstands the test successfully.

Fig. 6 shows typical blocking characteristics. As can be seen, low leakage currents can well be realized on six inch silicon up to 8.5 kV. Also worth mentioning is the fact that the I=f(V) curve is very symmetrical, although in the forward bias direction the amplification factor contributes to the leakage current.

Breakdown characteristics

The cathode short pattern was further optimized for the new six inch PCT. As a result, the thyristor is robust against self-firing at high dv/dt stress. In Fig. 7 we present a test with a dv/dt of more than 4 kV/µs at the full 8.5 kV blocking voltage.

dv/dt capability

Despite the high dv/dt capability, the thyristor exhibits safe latching properties. Fig. 8 shows transients of gate current, anode voltage and anode current in a lab experiment on latching sensitivity. As we can see, the thyristor safely latches with as little as VAK = 50 ... 60 V and a gate pulse of IGp = 12 ... 14 A and tG 4 µs.

Latching at 5 ... 90 degree celcius

A very stringent requirement in the application is also that the PCT should take large amounts of current in a very short time. This translates into high di/dt capability, like the one presented in the fig. 9. The current ramp in this measurement exceeds 3000 A/µs, which can be safely handled by the thyristor, at room temperature as well as at 90°C. This performance is achieved with a well designed, one-stage amplifying gate.

di/dt capability

Reliability

For an UHVDC thyristor it is mandatory that it exhibits very high reliability. ABB's six inch thyristor has withstood all necessary tests to ensure this. One important test is the IOL (Intermittent Operation Load) test, also called load-cycle test. The thyristor underwent a test with 5000 cycles at a temperature swing of 80 K and as can be seen in Fig. 10, VT remained constant during the test, proving that the internal interfaces could handle the thermal stress.

On-state voltage of two groups of wafers during IOL

The second very important reliability parameter is AC leakage current stability at high temperature. This was proven in a test with continuous AC voltage for a duration of 1000 hours, as can be seen in fig. 11.

AC blocking at 90 degrees celcius

Dr. Jürg Waldmeyer was a well-known expert in HVDC-thyristors. During the past 15 years he was involved in the development of HVDC-thyristors in sizes 4, 5 and 6” with voltage up to 8500 V for many large projects, such as the Garabi back-to-back stations in Brazil/Argentina and the transmissions from the 3-Gorges dam in China. His last project was the 6” thyristor presented in this article for the Xiangjiaba-Shanghai project. Due to a tragic accident he passed away on the 29th of May 2010.

 

 

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