The art of triggering an HVDC valve

HVDC > HVDC Classic > HVDC technology > Thyristor valves > Valve triggering

Deflating some myths about light triggered thyristors in HVDC.

ABB’s HVDC converters employ the extremely successful electrical triggering (ETT) of HVDC valves that we have been using for over 35 years. The ABB ETT valve is virtually maintenance-free.

ABB has been successfully testing light triggered thyristors in a commercially operating HVDC project (Konti-Skan 1) since 1988 but we concluded that there were no technical or commercial benefits to the customer that would justify commercializing the LTT concept. However, the introduction of light triggered thyristors (LTT) has been claimed by some to be a major achievement in the HVDC field in recent years.

ABB´s HVDC valve control system

ABB’s firing control employs a thyristor control unit (TCU) at every thyristor position, which provides electrical gate signal. The TCU gets its power from the snubber circuit across the thyristor, and it is ready for firing immediately after a network short-circuit. (Not several cycles as is the case in some competitors' designs!) It communicates with the valve control and monitoring on ground potential via two fiber optic channels, one for the firing pulses and one for the indication pulses.


In ABB’s ETT system the optical signals do not need high power; light emitting diodes (LEDs) are sufficient for communication purpose. LEDs working at a fraction of their maximum power output give excellent operating life. On the other hand, if you have to directly trigger a thyristor by light impulses (LTT), then you have to use an intense light source, most likely a laser diode or a hard driven LED. In certain designs a star coupler is also necessary to split the light signal to a number of thyristors.

Moreover, not all the electronics on each thyristor position can be eliminated in an LTT valve unless the monitoring and protection are sacrificed. The arrangement for the critical recovery protection is complicated if the redundancy of the thyristor positions is to be secured. Recovery protection operating on groups of thyristors, or even on a complete valve, has proven less reliable. Thus, when using LTTs, the electronics and the associated arrangements for power supply on each thyristor position cannot be eliminated. Use of LTTs instead of ETTs will in practice end up in a compromise without the full advantage of either concept.


ABB’s customers report their HVDC operating experience to the ABB HVDC Customer Support Department. Thyristor failures are also reported, even if it is extremely rare that they lead to any outage.

These are the failure statistics from ABB’s HVDC plants with water-cooled ETT valves commissioned during the 1980’s and 90’s:

Time period	Thyristor-years	Failed thyristors	Percentage
1984 - 1999	640 512	499	0.078 %
1997 - 1999	171 360	43	0.025 %

It should be noted that only some fraction of the above failures are related to the valve firing system.

Data from the year 2000 and beyond has not been systematically collected since the failed thyrisors have been so few that most owners don't find it meaningful to report them. For example, the two 3 000 MW transmissions from Three Gorges to Changzhou and Guangdong that were commissioned in 2003 and 2004 have had no failed thyristors in 21 800 Thyristor-years (Oct. 2006)!

With the overwhelming experience record with the ABB triggering system, we cannot see any good technical or commercial reasons to change over to a system that can do no more than our present system and which still requires electronics at each thyristor level for protection and monitoring.

Based on the extremely low thyristor failure rate, ABB does not recommend any periodic check of thyristor positions - the ABB valve is virtually maintenance-free.

The electrically triggered HVDC valve is simply a better solution!