Upgrading Main Low Voltage Systems

By Stefan Ritzmann and Lars Krantz, ABB Power Generation, Switzerl and Mikael Strömbäck, ABB Power Generation, Finland

This article presents an example of an ongoing upgrade project at a nuclear power plant site in Finland. As a result of this upgrade project, a major part of the low voltage systems within two nuclear power plant units will be replaced during a period of six years. An upgrade project of this type requires special solutions and methods in order to fulfill boundary requirements as:

  • Design and manufacturing of new low voltage switchgear considering the footprint of existing switchgear and existing cables
  • Design and manufacturing of new transformers considering the present switchgear room layouts and dimensions as well as logistical issues within the existing buildings
  • Ensure and improve system safety
  • Authority approval of systems and delivery process
  • Dismantling of existing equipment and installation of new systems without disturbing or extending the annual service outages during which the upgrades are done

This manuscript exemplifies a number of successful approaches and technical solutions done by ABB in close co-operation with the nuclear power plant owner.

Annual outages

The nuclear power plant site consists of two reactor units that undergo regular annual outages. A two weeks service outage is carried out every other year during which, in addition to refueling, scheduled inspections, preventive maintenance and repairs, significant modifications and modernization are undertaken along with the inspection of the condition of the unit. Every other year there is a shorter refueling outage where only minimal changes are made.

Cable connections in existing switchgears

In addition to these regular outages, there are more extensive maintenance outages for implementing major changes in the power plant units. The aim of the nuclear power plant owner is to maintain the power plants in such shape that the expected remaining lifetime always is 40 years. Through this methodology the owner can secure power plant safety and a high reliability of the production.

The capacity factors

One illustration of the nuclear power expertise of the owner can be found in the high capacity factors of the nuclear power plant units, which for many years have been at the top of the chart by international comparison. From the beginning of the 1990s, the capacity factors of unit 1 and unit 2 have varied between 93 and 97 percent. High capacity factors testify to the reliable running of the power plant units, achieved by keeping the units in top condition through annual outages, expert operation and constant improvement of everyday operations and safety.

Scope of supply

In this rehabilitation project, ABB's scope of supply is to deliver completely engineered electrical systems including system design, detailed design, manufacturing, factory acceptance testing, installation, commissioning, documentation and quality control. ABB also provides comprehensive support to the customer during the process of applying for the necessary approvals for the renewal of the electrical systems.

New equipment supplied by ABB for the renewal project:

  • Low voltage switchgear
  • Auxiliary transformers
  • Busducts
  • Relay protection

Main challenges and solutions of each boundary requirement?

Design and manufacture of low voltage switchgear considering the footprint of existing switchgear and existing cables

Transformer in the ABB assembly hall

In order to replace existing electrical systems with new equipment without need for additional outage times, the planning of new low voltage switchgear had to be designed to utilize the existing footprint. In order to address these requirements, ABB made special solutions and adaptions in the construction of the switchgear for this project. For example, the construction of cable sections have been designed so that installation can be done on present locations with minimum work and time needed. As a result of the developed solution, there is no need to change cable installations nor open or change the cable throughings, including fire protection sections, in the floors of the switchgear rooms. As the existing cables remained, the positioning of cable sections as well as positioning of individual feeders have been considered carefully and checked at site during design stage, where the length and possible reuse of individual existing cables were considered.

Design and manufacturing of new transformers considering the present switchgear room arrangements and requirement for increased safety

The auxiliary transformers are based on standard products of ABB. As the transformers are installed in existing switchgear rooms special attention have been given to dimensions, existing cable routes, transformer phase connection points and installation methods. Increased safety and operation reliability have also been important factors in the transformer design. In order to minimize and remove any potential risk for disturbance from either internal or external sources, the construction of the transformers is designed and manufactured based on a principal of safety-on-safety.

All transformers are anchored to the floor in order to withstand the required forces during a potential earthquake. The space available for installation of transformers in the existing switchgear rooms is limited and normal assembly solutions cannot be used. Therefore, to solve this issue ABB has developed a special assembly plate, which enables the needed anchoring within the space available.

Ensure system safety and authority approval of equipment, systems and delivery process

The new switchgear, busducts and auxiliary transformers are installed in safety-related subsystems. Consequently, the delivered equipment and solution need to be qualified and approved by the Nuclear Authority in the country. Overall system qualification is done by the nuclear power plant owner, but ABB performs all special tests, analyses and documentation needed to support the system approval of the supplied equipment.

In addition to tests done in a standard system delivery, several project and power plant specific tests and qualifications are being performed:

  • Coordination test of motor starters
  • Earthquake test of switchgear
  • Arc test of switchgear
  • Qualification of switchgear construction
  • Qualification of switchgear components
  • Transformers tests and analysis
  • Fault and consequence analysis for motor starters
  • Qualification of protection relay
  • Qualification of project management and overall implementation process

Transportation of new equipment to installation locations within the power plant

Switchgear and transformers are generally large equipment that put special requirements on transportation routes. In rehabilitation projects, where the equipment is transported inside existing power plant, transportation has to be given special attention. The electrical systems that are replaced during this ongoing project are located in multiple electrical rooms within the power plants. Dimensions of available and planned transportation routes have been considered in the design of both switchgear and transformers. In order to secure that no unexpected issues endanger the transportation inside the power plant, wooden models representing the new equipment were built and transported along the planned routes.

Dismantling of existing equipment and installation of new systems without disturbing or extending the annual service outages during which the upgrades are done

The power plant owner has detailed outage plans for several years ahead. In order to maintain a high efficiency ratio, the switchgear and transformer replacements are done within the framework of the fixed outage planning of the owner. Based on the main outage time schedules, ABB made more detailed implementation plans. Considering the usual maintenance work to be carried out during the annual outages, the time available is limited. The complete replacement work at site needs to be completed within two to three days. The fact that there are a huge amount of people on the site at the same time also generates special requirements related to logistics and mobilization. During an annual outage about 1000-1500 temporary workers are on site working with multiple service and upgrade tasks. The overall amount of people at site has been considered when the mobilization of the ABB crew, consisting of over 100 workers handling the dismantling and installation work, is planned. Much attention has been given to the detailed planning of the work to be done during the outage including: a detailed step-by-step description of all work phases to be done for every single switchgear, resource planning (staff, back-up, defining a leader of each assembly-team, accommodation, etc.) and training of team members. The installation crew is subdivided into smaller, dedicated installation teams consisting of preselected and experienced installation workers. The installation teams, lead by an installation supervisor, are assigned tasks related to predefined switchgear. In order to mitigate risks as well as to train the installation team leaders, complete units consisting of switchgear, busduct and transformer are pre-assembled at the factory. The replacement work is done without interruptions, i.e. the work is performed in two shifts, a day shift and night shift. Each shift is lead by experienced shift supervisors. Every single work step is defined in detail so that each person knows exactly what their task is and in which order it shall be performed. With a comprehensive and well organized installation plan each and every task is carried out with highest precision and the replacement work is completed without need for additional outage.

What are the main customer needs?

For nuclear power plant owners, there is always a basic need for safety. The main requirement and driver for the replacement project was to increase the safety and reliability of the two nuclear power units.

Test assembly at factory

Another customer goal was to extend the life time of the unit without a significant production loss, driving the need for the upgrade work to be completed during the normal and scheduled annual outage time.

In order to ensure the system upgrades are completed without need for additional outage time, it was necessary to consider the existing footprint and existing cabling in the operating power plants. Consequently, the power plant owner was looking for a reliable supplier who can work in close cooperation and deal with a large quantity of work regarding planning, pre-assembling and logistics.

As the systems are part of the nuclear safety system and must fulfill all the authority's requirements, the power plant owner also needed a reliable contractor with experience in the Finnish nuclear power plant sector.

Chronological workflow Years of commissioning: 2010-2015

2010: Pilot phase which includes replacement of original switchgear, transformers and busducts in the non-safety area. The goal of the pilot installation was to test the execution plan for upcoming years and gain lessons learned that could be addressed and implemented in order to further secure a successful system replacement within the planned short outage times during the remaining years of delivery

2011: Replacement of one of the four subsystems within the safety-related area

2012-14: Two subsystems per year will be delivered and replaced

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