Minimizing reactive power in mixed overhead line and cable networks

2014-02-11 - ABB and the Finnish utility Savon Voima Verkko Oy have launched a research and development project to develop solutions that improve electricity supply, safety and efficiency in Savon Voima Verkko Oy’s distribution network in Eastern Finland.

Savon Voima Verkko Oy is Finland's fourth largest distribution system operator in terms of network length. The company has a network of about 25,000 km and has already taken a number of steps towards an intelligent network. The pilot project aims to create new network solutions for securing a high-quality voltage and reliable electricity supply in mixed networks consisting of both underground cable and overhead line networks.

Photo courtesy of Savon Voima Verkko/Kari Vesterinen, photographer.
Field tests on ABB's decentralized compensation technology combined with smart grid functionalities are planned in Lapinlahti and Ahmo. Seven decentralized compensation devices, integrating both reactive power and earth fault compensation inductors, will be installed in Savon Voima Verkko Oy’s network.

Over the next two years, a 20 kV transmission link between the substations in Lapinlahti and Ahmo will be installed underground, and 48 transformer substations will be replaced. A fiber-optic connection will also be built between the substations. At the same time, 90 km of low-voltage network will be refurbished and overhead lines replaced with underground cables where necessary, as part of refurbishing the entire distribution network.

For this project, ABB has supplied compact secondary substations (CSS) made up of SafePlus, a compact medium voltage ring main unit designed for smart grid applications, and REC615, a grid automation intelligent electronic device designed for remote control and monitoring, protection, fault indication and power quality analysis in in medium-voltage secondary distribution systems.

Decentralized compensation
The project will contribute to developing ABB’s products and technologies further through the piloting of new energy-efficient solutions to compensate reactive power and earth faults. The combined device is particularly suitable for 20 kV cable networks in rural and urban areas. The device can also be placed in existing transformer substations, which provides a technically and economically optimized solution.

Photo courtesy of Savon Voima Verkko/Kari Vesterinen, photographer.
The basic idea behind the decentralization is that the reactive power is compensated as close to where it occurs as possible. This way, transmission losses are reduced and the voltage remains balanced. At the same time, decentralization can prevent an increase in earth-fault currents and ensure successful compensation when clearing the earth fault. The decentralized compensation device will also improve personnel safety as contact voltages are reduced.

Searching for standardized solutions in mixed networks
This research and development project is expected to help create expertise in a new type of mixed networks, where the core section will consist of underground cables, although the network will have overhead lines too. Both cable and overhead line networks are much farther in the development of standardized solutions compared to mixed networks.

Towards the intelligent network of the future
ABB will also develop network automation solutions in accordance with the zone concept for the network between the substations in Lapinlahti and Ahmo. This zone concept concentrates on improving the reliability of the power supply in a fast and cost-efficient manner through increasing automation and adding light substations, zone breakers and zone disconnectors to the existing network. The main principle of the zone concept is to confine the impact of a network fault to as limited area as possible.

Help is sought from solutions applied to overhead line networks. Remote assistance can be used to limit the faults in cable networks to as restricted area as possible, as the so-called chained transformer substation topology will be built in the project area.

"Overhead line networks have recloser stations, whereas new transformer substations with intelligent functionalities are being developed for cable networks. The new transformer substations can provide information based on measurements of power quality and load data. Measurement data can be used to determine where the fault is located, and in the best case, the fault can be detected even before it becomes permanent. This also requires the construction of an optical-fiber data network between transformer substations," says Jussi Antikainen, planning manager at Savon Voima Verkko Oy. "We have achieved a lot together with ABB. This project has attracted considerable interest among other utilities too. It can be considered an important step towards developing the future urban and rural networks."

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