HVDC Light reliability and availability

The reliability and availability requirements on a particular HVDC transmission can be quite different. They are particularly high for links supplying major parts of a load (e.g. a city, an oil or gas production platform or an island) or evacuating a major power plant. Less stringent requirements can be found for network interconnections that serve mainly to provide access to lower power costs but not essential for maintaining service to load.

The basic configuration of HVDC Light is monopolar. Where it is essential to have at least 50% power if an outage occurs and for large size transmissions, a second circuit has to be added.

One of the main objectives during the design of a HVDC Light converter station is to minimize the outage frequency and outage time due to both forced and scheduled outages. This can be achieved by:

• a simple and well proven station design
• use of reliable and well-known components
• automatic monitoring of all critical subsystems
• use of redundancy

Most of the HVDC Light converter station equipment is located inside a building - a fact that provides the station with a good protection against flashovers.
Redundancies are used in e.g.:

• control and protection systems (MACH2)
• station service power (AC and DC)
• cooling systems (for valves, transformers, etc.)
• IGBT positions in the valves

Spares are normally provided based on the ABB´s extensive experience. For some items which are essential for the operation and which may cause extensive downtime if failure happens, a complete unit is normally provided for each project. This is normally the case for: reactors, wall bushings, instrument transformers, filter reactors and resistors, etc.

HVDC Light converter stations require little maintenance. Most HVDC Light station owners schedule an annual maintenance period at a time when the utilization of the transmission is low. Maintenance can be performed on redundant equipment, such as ABB´s MACH 2^TM , when the link is in full operation.

The majority of equipment in a HVDC Light station is normal high-voltage and low-voltage equipment ( breakers, disconnectors, transformers, capacitors, reactors, low-voltage power distribution and motor control systems, etc) that require normal service. ABB´s IGBT valves require a minimum of maintenance.

To further reduce the scheduled, and forced, outage time a facility for Remote Fault Tracing and Maintenance can be included where the station can be monitored from virtually any remote location.

In the tendering phase of a HVDC project predicted values for reliability and availability for the HVDC substations (i.e. excluding the DC cable) are calculated.

The reliability measure is the forced outage rate in No./year. The unavailability measure is made up by the forced and unforced (scheduled) unavailability, i.e. the times (in % of one year) when the transmission is out of service. (Availability% = 100% - unavailability%.) Since both the sending and the receiving station needs to function, the calculation normally covers both.

For a normal HVDC Light pole ( excluding the DC cable ) the following values are typical:

HVDC Light Pole
Forced outage rate	1 - 2 outages/year
Forced unavailability	0.3 - 0.5%
Unforced (scheduled) unavailability	< 0.4%
Availability	>99.0%

These values are statistical averages but correspond well with ABB delivered HVDC Light projects.

Study Committee B4 (HVDC and Power Electronics) of Cigré has since 1968 collected reliability and availability data on HVDC projects in operation. These reports cover also HVDC Light transmissions. The data is split up on:

• AC and Auxiliary Equipment (AC-E)
• Valves (V)
• Control and Protection (C&P)
• DC Equipment (DC-E)
• Other (O)
• Transmission Line or Cable (TL)

These reports give very useful information.