Back-to-back HVDC converter stations are integrated within ac transmission links and play a role similar to that of transmission tie stations (or substations) on an ac transmission system. Often, the back-to-back HVDC converters are permitting power interchange between two weak and/or unsynchronized ac systems.
In some applications, the back-to-back tie is used for only a few hours per day, and power may flow either way.
In these cases, the “availability” is a logical RAM parameter to specify as one of the design goals, and the specification should treat the entire back-to-back tie as a single system. Other RAM specification terms should be similar to the terms used in IEEE Std 859-1987.
In other applications, it might be desirable to transmit the maximum amount of energy possible at all times and in only one direction. In such cases, one may treat the HVDC converter station as a generator, and “energy availability” may be a better parameter to specify than “availability.”
Deferred maintenance and planned outages may be combined and called “scheduled outages.”
However, the RAM specification should not count “scheduled outages” that are called “operations related outages” in IEEE Std 859-1987. An “operations related outage” is when the unit is removed from service to improve system operating conditions.
The RAM specification also needs to define whether “scheduled outages” for equipment modifications to the HVDC converter station are to be counted. IEEE Std 859-1987 defines four types of “forced outage,” as follows:
a) Transient forced outage: A forced outage where the unit is undamaged and is restored to service automatically.
b) Temporary forced outage: A forced outage where the unit is undamaged and is restored to service by manual switching without repair, but possibly with on-site inspection.
c) Permanent forced outage: A forced outage where the unit is damaged and is not restorable to service until repair or replacement is completed.
d) System-related outage: A forced outage that results from system effects or conditions and is not caused by an event directly associated with the unit.