Monitoring, controlling and automation of secondary substations

Implementation of secondary substations

The power system was designed for a unidirectional power flow from the power plants to the end-consumer over different voltage levels. The secondary substation is the interface between the medium voltage (MV) and low voltage (LV) power system.

This is shown in Figure 1 below. In the MV the ring network is used as the common grid structure, the mode of operation could be closed or opened controlled by the Nodal Opening Point (NOP).

In the LV radial networks are often used or in some case meshed networks.

Regarding Figure 1 below, the secondary substation consists normally of the disconnectors in the MV-network, the high-voltage high-power fuse (HH-fuse) for protection of the transformer, the MV/LV-transformer and the low-voltage high-power fuse (LH-fuse) for protection of the outgoings in the LV.

Different requirements and deductive different implementations arise to the secondary substation depend on the operating places.

These could be:

• Urban, suburban or rural areas
• Inside of buildings, outside in compact stations or free standing transformers

Based on the stochastic operation of consumers, this means that not all devices works at the same time, the dimensioning of the equipment ensues with the coincidence factor (CF):

g = S_dimensioning / S_installing

Concerning the network planning it should be mentioned, that a CF of one has to adopt, because it is possible that all works at the same time and an active power control is not provided.

A feed-in of a reactive current by the DG in the LV is not planned or foreseeable to fulfil the requirements of voltage support during a fault now.

In Germany, the feed-in in MV networks have been required by law for wind power plants since 2010.

General view

In case of a fault in the MV, the protection relays in the Primary Substation, normally overcurrent-time protection is used, clear the fault. After this, the ring network and all consumers are disconnected from the remaining power system.

For searching of fault location, the service team checks all short-circuit indicators (in case of earth fault the earth passage indicators) along the path to find the fault location and to separate it by open the disconnectors.

They are normally hand-operated and no remote-controlling or remote-monitoring is provided.

These are only some facts, in which automation, controlling and monitoring can help to reach the targets, decrease the cost or increase the reliability.