Two different designs of thyristorbased line-commutated valves have been developed by ABB: a conventional base-mounted design with support insulators standing on the floor, and another (more frequently used) design with the valve suspended from the ceiling of the valve hall ➔ 1. The latter design is particularly suitable for withstanding seismic stresses, but also offers cost advantages.
1 Valve suspended from the ceiling of the valve hall
The design uses a modular concept to ensure high reliability while retaining the flexibility to design valves according to customer requirements and preferences. A valve is built up with one or several layers depending on the required voltage withstand capability ➔ 2. Each layer consists of series-connected thyristor modules with intermediate current-limiting reactors, corona shields and piping for the cooling liquid. The individual parts are supported by a mechanical structure with a central shaft running vertically through the structure with ladders and working platforms for easy access.
2 Single layer of a valve
3 Thyristor module
Thyristor modules are of a mechanically standardized compact design. For the sake of reliability they have a minimum of electrical components and connections. The main components are thyristors and their voltage dividers, control units and heat sinks. The modules are of compact design for easy access during maintenance. The design allows components to be exchanged without opening the water cooling circuit ➔ 3.
Each module contains a number of thyristors connected in series. Continuous thyristor development enables them to handle ever-increasing voltages and currents, while also reducing conducting and switching losses. The first HVDC valves designed by ABB were for the upgrading of the Gotland transmission – they used thyristors with an area of 5cm2. The next valve design (used for the Skagerrak transmission) featured double-sided cooling of the thyristors, which had an area of 8cm2. Today thyristors have an area of up to 130cm2 capable of withstanding continuous currents up to 4,500 A and short-circuit current up to 50 kA, eg, for the Xiangjiaba- Shanghai ± 800 kV Ultra-HVDC project .
4 Voltage across thyristor level
A switching position is built up from a thyristor with two parallel circuits, each consisting of a damping circuit and a DC grading circuit, as well a TCU (thyristor control unit). The TCU converts the optical firing pulses from the control system to electrical signals to trigger the gate that fires the thyristor. The TCU includes state-of-the-art built-in functions that protect it against overvoltages during the reverse recovery period (after a thyristor turns off) as well as from high voltages in the forward blocking state ➔ 4.
By using this hybrid technique, ABB is able to provide a very compact TCU. ABB’s service record is also unbreakable: Of the more than 19,000 thyristors installed since 2000, only four thyristor failures have been reported. This demonstrates the superior design of electrically triggered thyristors.
IGBT-based voltage source converter valves
The use of IGBT-based voltage source converters (VSCs) in HVDC power transmission was a breakthrough. Its first application was in a 3 MW HVDC Light test installation in Hällsjön in 1997. Since then, 20 VSC HVDC power transmissions have been installed or are under construction by ABB alone .
Two types of VSCs have been developed for HVDC transmission: the "switch" type and the cascade twolevel (CTL) or "controllable voltage source" type. The choice of converter type mainly depends on the application.
A switch-type valve has a close apparent resemblance to conventional thyristor valves: A large number of series-connected IGBT devices are switched simultaneously. Pulse-width modulation (PWM) is used to achieve a good approximation of a sinusoidal output voltage (AC voltage) ➔ 5.
A CTL converter integrates the DC capacitors into the valve. The valve consists of seriesconnected voltage cells that can produce a sinusoidal voltage ➔ 6. The greater the number of cells connected in series, the more sinusoidal the waveform. A CTL converter valve for the DolWin1 HVDC transmission project is shown in figure 3 page 25 (see full article via link below.)
6 Voltage output of a CTL-type converter with seven voltage cells in series connection
In this project, one valve consists of thirty-six voltage cells.
7 StakPak IGBT used in ABB's VSC valves
The modular design concept for valves is found in both switch and CTL types. ABB’s VSC valves use StakPak IGBTs ➔ 7. These switching modules create an internal short circuit should they fail, making them similar to thyristors. This function allows a current to continue to conduct through a faulty IGBT device without calling for an external bypass circuit (as is the case for other IGBT types). The decreased deployment of components at high potential can therefore augment the valve's availability, reliability and compactness. StakPak follows the conventional press-pack design advantages such as better cooling and robust mechanical module structure .
8 Overview of a typical valve cooling system
9 Typical skid-based modularized redundant cooling system
The purpose of the cooling system is to dissipate the power losses generated in the valves. Coolant fluid is circulating through the heat sink in close contact with the semiconductors. This efficiently transports heat away from the device to be cooled through heat exchangers using either air or a secondary circuit. The liquid in the closed-valve cooling system is continuously passed through a de-ionizing system to keep its conductivity low.
A typical valve cooling system is shown in ➔ 8 and ➔ 9.
ABB Power Systems, Grid Systems
Raleigh, NC, United States
ABB Power Systems, Grid Systems
 J. Waldmeyer and W. Zhengming, "Six Inch Thyristors for UHVDC," in Proc. 2006 International Conference on UHV Transmission Technologies, Nov. 27–29, 2006, Beijing, China.
 G. Asplund, "Application of HVDC Light to Power System Enhancement," IEEE/PES Winter Meeting, Singapore, January 2000.
 B. Jacobson, et al., 'VSC-HVDC Transmission with Cascaded Two-Level Converters,' B4-110, CIGRE 2010, Paris, France.