RELIABILITY

RELIABILITY :

The reliability of  DC  transmission systems is quite good and comparable to that of  AC systems. An exhaustive record of exhaustive record of existing HVDC links in the world is available from which the reliability statistics can be computed. It must be remembered that the performance of thyristor valves is much more reliable than mercury arc valves and further developments in devices, the development of direct light triggered thyristors (LTT) is expected to improve reliabiliyt because of elimination of high voltage pulse transform - ers and auxiliary7 supplies for turning on the devices . 

            There are two measures of overall systems reliability -energy availability and transient reliability .

Energy availability 

     This is defined as 

                 Energy availability = 100(1- 

where equivalent outage time is the product of the actual outage  time and the fraction of system capacity lost due to outage.

Transient reliability :

                              This is a factor specifying the performance of HVDC systems during recordable faults on the associated AC systems.

Recordable AC systems faults are those faults which cause one or more AC bus phase voltage to drop below 90% of the voltage  prior to the fault, It is assumed that the short circuit level after the fault is not below the minimum specified for satisfactory converter operation.

              Both energy availability and transient of existing DC systems with thyristor valves is 95% or more.

         The average failure rate of thyristors in a valve is less than 0.6% per operating year. It is common practice to provide redundant thyristors in the series string composing a HVDC valve, so that failed thyristors  can be replaced during scheduled maintenance (once or twice a year). The maintenance of thyristor valves also much simpler that the earlier mercury arc valves.

Equipment	MTTF(years )	MTTR (hours )
Thyristor group	13.7	6.1
Converter transformer	16.1	1700.0
Smoothing reactor	76.8	1700.0
DC filter	19.7	7.9
AC filter	12.6	9.3
Master control	25.0	6.9
Pole control	9.0	8.6
Pole of transmission line	1.25/100 km	1.5
DC line switch	147.2	7.8

           

MTTF = Mean time to failure 

MTTR = Mean time to repair

      Some of the HVDC outage statistic is given in table1.1 . In comparing the reliability of various alternatives , it must be kept in mind that bipolar DC line can be as reliable as a double circuit AC line with the same power capability. This is because of the fact that failure of one pole does not affect the operation of the other pole (with ground return). If the DC line conductor has adequate overload rating and if the healthy pole, the prefault power level can be maintained even with permanent outage of one pole.