In September 2012, reserve transformer 6TR was commissioned, completing a plan to provide 6kV backup power to Rivne 1&2 (also known as Rovno in Russian).

The need for a second backup transformer for the twin VVER-440s at Rivne first became apparent in 1981, after startup of Rivne 2. While one 32,000 kVA standby transformer (type 1TR TRDN-32000/110/6, 3/6, 3) was sufficient for one unit, it was not for two.

In the spring of 1983, the station experienced a blackout when the existing backup transformer GRP-330 kV broke down because of bad weather. According to a 1984 analysis by Pivdentehenerho (Lviv), the one backup transformer had insufficient power reserves; but with a second 32,000 kVA backup transformer could have restored station power.

In 1990 the backup transformer was replaced by a 40,000 kVA model. However, a second analysis (by LvivORGRES) found that it could not supply all the needs of the station on its own in every possible mode. It again proposed another backup transformer. For this purpose, a redundant 32,000 kVA-capacity transformer stored at the station was chosen, because of its size and its availability.

The second problem was the routing of cables to the second backup transformer. There were various options:

  • Construction of a new 110 kV transmission line across the station site (rejected due to interference with a busy industrial area)
  • Construction of a new 110 kV transmission line outside the station (rejected due to lack of required corridor width)
  • Re-use of existing 110 kV transmission line (for 1TR) by doubling up (rejected because of required length of outage required for the work, which would be unacceptable for the safe operation of Rivne 1&2).

Only in 2008 was the decision taken to connect the transformer to a dry single-phase 110 kV cable with cross-linked polyethylene (XLPE) insulation; such cables are unusual in the former Soviet Union and are not made in Ukrainian cable factories.

A contract for the development of design estimates was signed by JSC Dex of Kiev in 2009; this work was received in April 2010. That same year, the backup transformer was named 6TR. Tenders were concluded with ART-Energo and CJSC Techenergo for construction and commissioning.

The most time-consuming and costly step was the construction and electrical work of installing cable runs and the 110 kV line from the GRP transformer (cell 5) to 6TR, situated within a complex of several transformers located near the main building of the unit 2 switchyard.

The buried three-phase cable line had to pass through 860m of a site congested with underground utilities. There were eight penetrations underneath railways and roads. Underneath the highway, concrete 20cm thick had to be broken up and removed to lay the cable trench.

At the bottom of the 2m-deep trench was a concrete cable tray, on which sit the three 110kV XLPE cables in a trefoil arrangement (two on the bottom, one on top). After being filled with a sand-cement mixture, the cable tray was closed and covered with earth. This stage took seven months.

Ten different departments cooperated on the upgrade project, including capital construction, electrical engineering, design-engineering, transportation, heating and underground utilities, and general service. Four departments within the power equipment maintenance enterprise were also involved: electrical equipment maintenance, special work, site systems repairs and maintenance equipment operation, and construction and maintenance.

The 6TR transformer protection and control relays (RZTA) were then installed. As part of this phase, the RZTA panel was mounted on the open switchyard 110/330 kV relay board, the 6TR transformer control and alarm panel was installed in the central control room, and 16km of cables were laid. To reach Rivne 1&2, 245m of underground cable trays were dug up.

After construction finished, a 110 kV disconnector was mounted, with overvoltage and end couplings for 11 kV cable lines and a 110 kV busbar arrangement. The backup transformer was not mounted like others, but at a 90° angle because of lack of space. It was installed with two KSHT-50.01 cranes.

The final step was installation of 6kV type KRU-6C switchgear, 6kV mains power backup sectionalizing switches and a 6kV circuit of auxiliary power for Rivne 1&2. Also during this phase, 6 kV switchboard power inputs for units 1&2 were installed. Busbar backup power lines for units 1 and 2 were separated and replaced with overhead-mounted switches. All of the work was done to route backup power for unit 1 from transformer 1TR, and backup power for unit 2 from transformer 6TR.

The project working group consisted of experts from RNPP and contractors, under direction of RNPP deputy chief engineer Vladimir Komarytsi.

Sergey Semenov, head of electrical engineering, Rivne Nuclear Power Plant, Kuznetsovsk 34400, Rovno Region