What are power cuts due to?
Most power cuts experienced by customers are due to faults in the medium-voltage network. A power cut experienced by a single low-voltage customer is not visible in the operating centre of the electricity network.
In Helsinki, the operation of the electricity network is monitored 24 hours a day from the operating centre of the electricity network. In addition, a stand-by duty system has been created for fault situations. These measures and the solid professional competence of the operating staff guarantee that faults can rapidly and effectively be diagnosed.
High-voltage network
The total length of the 110 kV high-voltage power lines that connect the substations and power plants in Helsinki is approx. 200 km. The high-voltage network consists of both overhead power lines and underground cables, the latter of which are used in the inner city in particular.
The structure and operating mode of the high-voltage network are looped. Thanks to looping, the supply of electricity is not interrupted, if one line gets disconnected from the network as a result of, e.g., a stroke of lightning.
The most common reason for disturbances on high-voltage networks is thunder. This is due to the large number of overhead power lines. However, thunder does not usually cause permanent damage, and a power line that has become disconnected from the network can usually be rapidly taken into operation again.
Medium-voltage network
In Helsinki, there are approx. 1,600 km of medium-voltage networks (10 kV and 20 kV), 99,5% of which are cabled. Most of the power cuts experienced by customers are caused by faults in the medium-voltage network.
The most common reasons for disturbances on the medium-voltage network are excavation, subsidence of ground and component failures in the electricity network. Among other reasons for power cuts are, e.g., thunder, trees, branches and animals. Because the medium-voltage network is cabled to a high degree, the role of such disturbances in creating disturbances is minor, however.
Low-voltage network
The length of the low-voltage network in Helsinki is approx. 4,500 km, more than 96,5% of which consists of underground cables. Via the low-voltage network, electricity is transmitted from distribution substations either directly or through electricity distribution cabinets to low-voltage customers.
Faults in the low-voltage network are very similar to those in the medium-voltage network. The difference is that a single fault in the low-voltage network causes a power cut in a much smaller area. For example, if a crane breaks the connection line of a building, there will be a power failure only in the building concerned. On the other hand, when a connection line between a distribution substation and a distribution cabinet breaks, it may cause power failure in an area covering a few blocks.
Other factors causing power cuts on a low-voltage network are, e.g., mischief and planned maintenance measures. Customers are informed in advance about maintenance work that will cause power cuts so that they can better prepare themselves for the inconveniences that will result from the power cut.
