Transmission configuration
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 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.
Where it is essential to have at least 50% power if an outage occurs and for large size transmissions, a bipolar HVDC transmission is the natural choice. For network interconnections of moderate size often a monopolar configuration is chosen.
Bipolar HVDC converter stations are designed such that there shall be no risk of having a forced outage of both poles at the same time.
The most probable type of line fault: a ground fault due to lightning, affects only one pole. Bipolar HVDC line faults only happens in case of a fallen line tower.
Since bipolar faults are very rare, one can regard a HVDC bipole as being equivalent to a double circuit AC line from the reliability point of view.
Converter station design principles
One of the main objectives during the design of a HVDC 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
Redundancies and spares
Redundancies are used in e.g.:
- control and protection systems (MACH2)
- station service power (AC and DC)
- cooling systems (for valves, transformers, etc.)
- thyristor positions in thyristor valves
- AC-filters (and in some projects DC-filters)
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 station. This is normally the case for: converter transformers, smoothing reactors, wall bushings, instrument transformers, filter reactors and resistors, etc.
Maintenance
Modern HVDC converter stations require little maintenance. Most HVDC station owners schedule an annual maintenance period at a time when the utilization of the transmission is low. For a bipolar link, one pole can be serviced while the other pole is live. Maintenance can also 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 converter 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 thyristor valves require a minimum of maintenance.
To further reduce the scheduled, and forced, outage time a facility for Remote Fault Tracing and Maintenance is included where the station can be monitored from virtually any remote location.
Predicted and observed reliability and availability
In the tendering phase of a HVDC project predicted values for reliability and availability for the HVDC substations (i.e. excluding the DC line or 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 modern HVDC pole ( excluding the DC line or cable ) the following values are
typical:´
| Pole |  |
|---|
| Forced outage rate | 3 - 4 outages/year |
| Forced unavailability | 0.3 - 0.5% |
| Unforced (scheduled) unavailability | < 1.0% |
| Availability | >98.5% |
When the above are applied on a bipole, one can calculate the probability of having bipolar outages and having at least one pole in operation.
| Bipole | | |
|---|
| Forced outage rate | for 100 % power
for 50 % power | 6 - 8 outages/year
0.05 outages/year |
| Forced unavailability | for 100 % power
for 50 % power | 0.6 - 1.0%
0.003% |
| Unforced unavailability | for 100 % power
for 50 % power | < 2.0%
<0.1% |
| Availability | for 100 % power
for 50 % power | >97.0 %
>99.9 % |
These values are statistical averages but correspond well with recent ABB delivered HVDC Classic projects.
Cigré Study Committee B4 HVDC statistics
Study Committee B4 (HVDC and Power Electronics) of
Cigré has since 1968 collected reliability and availability data on HVDC projects in operation. 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.