IEEE Power & Energy Magazine - March/April 2016 - 70

the thyristors meant that up to 280 pairs had to be connected
in series for the 133 kV voltage rating per valve group. It
proved challenging to ensure that all of the pairs were always
conducting, especially after transients, as new valves could
reduce the number of thyristors per valve function from 268
to just 36. The inefficient oil-based cooling system could
be replaced with a modern water-based cooling system and
losses in the converter station could be reduced by 18 MW.
The new ac filters also contributed to this loss reduction.
One of the most important goals with respect to the design
of the new thyristor valves was to integrate them into the
present station layout and to reuse as much of the surrounding equipment as possible. The decision was taken to use
thyristor valves and combine three transportable enclosures
to make up one valve housing for a complete six-pulse group
positioned on two of three old valve tables. Removing one
old table for each valve group created space for the new cooling towers.

figure 4. The new outdoor thyristor valves at the Apollo
station of the Cahora Bassa HVdc link. (Image used with
permission from ABB.)

Control and Protection

For maximum flexibility and best reliability and maintainability (RAM), the new Apollo control and protection systems are designed for any conceivable configuration of the
main system topology. (Figure 5 shows the installation of
the control system.) This includes normal bipolar and monopolar operation, crossed-over monopolar operation utilizing
the internal cross-over link in either station, and utilizing
any one of the two lines for any mode of monopolar operation. It was also possible to reintroduce the original parallel operation of two Apollo poles on one line, which had
been unavailable for many years. This feature may prove
to be a significant benefit for extended outages of either of
the two monopolar lines or the desire to upgrade the facility to higher power transfers in the future. This is consistent
with the original design to operate both poles up to 3,300 A,
which is the design rating of the lines, but would require that
the earth return current and the earth electrodes in both stations be capable of sustaining this current.
By retaining the original station layout and outdoor platforms based on eight six-pulse converter bridges, the reuse
of existing civil works could be maximized. The new dualredundant control and protection systems are now based
on pseudo 12-pulse converter pairs formed by combining
adjacent star-star and star-delta converter transformer converters into common systems, while yet maintaining the
independence of the original six-pulse topology. This control
topology has been extended to the thyristor cooling where
each "12-pulse group" now has one cooling system. This
simplification reduced the total number of cooling systems
(fans and pumps) from eight to four, while enhancing redundancy and RAM, providing 100% standby variable-speed
water pumps and 25% fan capacity based on 40 ÂșC ambient. Having two converters on one cooling system creates
additional redundancy when one converter is off. The topology means that controls and protection are halved by sharing
march/april 2016

figure 5. An ABB employee working with a control
and protection cabinet in the factory system test area in
Ludvika, Sweden. (Image used with permission from ABB.)

systems for two bridges yet at the same time are doubled for
the dual redundancy.
Installation and Commissioning

The new thyristor bridge enclosures were the most challenging part of the electrical equipment installation at Apollo
because it was performed in a live ac/dc yard. The concept
was to replace the existing thyristor bridges by reusing the
existing foundations and support insulators for the new thyristor bridges. The installation work required replacement of
two existing thyristor bridges simultaneously in the following sequence:
1) dismantling work of two existing bridges
2) installing new thyristor bridge enclosures for two
bridges
3) installing one common thyristor bridge cooling system for two bridges
4) installing thyristor valve control and other control and
protection equipment.
ieee power & energy magazine

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2016

IEEE Power & Energy Magazine - March/April 2016 - Cover1
IEEE Power & Energy Magazine - March/April 2016 - Cover2
IEEE Power & Energy Magazine - March/April 2016 - 1
IEEE Power & Energy Magazine - March/April 2016 - 2
IEEE Power & Energy Magazine - March/April 2016 - 3
IEEE Power & Energy Magazine - March/April 2016 - 4
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IEEE Power & Energy Magazine - March/April 2016 - 97
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IEEE Power & Energy Magazine - March/April 2016 - 114
IEEE Power & Energy Magazine - March/April 2016 - Cover3
IEEE Power & Energy Magazine - March/April 2016 - Cover4
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