An employee of Amprion GmbH in the transformer station / substation.

Substations are the nodal points of our transmission grid. They perform a range of tasks that are crucial to smooth operation of our network. Their chief functions are to switch the power lines “on” and “off” and to transform the electrical energy to a different voltage level. Engineers make a distinction between “switchgear” and “substations”, but for the sake of simplicity, we’ll refer here to both as “substations”.

High-availability digital units belonging to the protection and control equipment are used to control and monitor the substations. The data is sent to the control centres and the substations via our own extremely secure communication network that we operate completely independently of our publically accessible information network.

Voltage up, voltage down – transformation

Our substations connect the transmission grid with the distribution networks, the plants of large industrial enterprises and the power generating facilities. If this power transfer is to function smoothly, the voltage must be matched to the requirements of the customers. Voltage transformation – from 380 to 110 kilovolts, for example – is performed by powerful transformers.

Line on, line off – switching

Our substations – better said, switchgear – see overhead lines and underground cables converge, which can be switched on or off as required. Switching is performed by circuit breakers, which, during normal operation, facilitate safe and reliable de-energising of the current flow – automatically if a fault occurs. Our substations are controlled and monitored by our engineers in the network control centres in Rommerskirchen and Hoheneck. Measured values from all of our substations are sent to these control centres, indicating exactly how much electricity is being transmitted through the individual conductors at any one time. This data allows us to ensure that the electricity reaches its designated destination and does not overload our grid in the process. Furthermore, we monitor the voltage level and regulate it so that it is always within the permissible range.

Playing it safe – redundancy

Substations have been an integral part of the extra-high voltage network for more than 80 years. Over the years, its design and components have undergone fundamental development and seen dramatic improvements. Today, we work with innovative technologies and circuitry concepts that offer us a whole range of benefits. These include equipping our substations with multiple busbars – giving the system, ourselves and consumers the real benefits of redundancy.

It means we have multiple options to connect the power lines that come into and leave the substation to one another. This flexibility also ensures that our grid enjoys greater reliability; in the event of a fault, we can fall back on a back-up busbar and redirect the electricity via an alternative route. Another advantage of this flexibility in interconnection of the conductors is that it allows us to control the flow of the electrical energy in the grid within certain limits. This helps us prevent individual route sections from becoming overloaded. What’s more, as our switchgear is equipped with an extra transfer bus, we are able to keep the line in operation even when carrying out maintenance or repair work on the substation. This additional complexity and redundancy built into our substations ensures that we can operate the vital overhead lines continuously and avoid bottlenecks or congestion in the transmission grid. Such congestions can be relieved by issuing a request to power plants or other producers to adjust their power output; these measures are known in the industry as redispatch, which results in high costs.

Keeping everything stable – reactive power

Our substations are increasingly responsible for another important function: they stabilise the voltage level in the grid. During the transmission of AC current, magnetic and electric fields are constantly being generated and reversed – it’s a physical characteristic of AC power lines. This necessitates what’s known as “reactive power”. The transfer of reactive power, however, puts a load on the transmission lines and reduces the usable transmission capacity, known as the “active power” (or real power).

Until now, reactive power has been provided primarily by the generators of conventional, large power stations. As many of these conventional power stations are to be taken off line as the energy transition progresses, Amprion is installing more and more reactive-power compensation devices, more commonly referred to as power factor correction (PFC) equipment. These include reactors, which are already installed in many of Amprion’s substations. Reactors are similar to large transformers. We only switch them into the grid if the voltage on a line is too high. The reactors correct the power factor and lower the voltage on the line again. If the opposite is the case, that is, the voltage on the line is too low, capacitor banks are cut in in order to increase the voltage. In future, we will also be using power-electronic PFC equipment and synchronous condensers (also known as synchronous compensators), which can supply reactive power flexibly and raise or lower the voltage level.

The design of a substation

Although there are different designs of substations, all of Amprion’s substations are similar and built in line with a modular principle. This ensures that the construction and operation of our substation installations run efficiently. The switchgear and measuring instruments required for each power line circuit and transformer within the substation are arranged in close proximity to one another. We talk here about “switchgear bays”. These switchgear bays are interconnected by means of busbars and bus couplers. Specifically, a substation comprises the following key components:

The path taken by electricity through a substation

On its path from the incoming power line to the outgoing line, the electricity flows through a series of switchgear, busbars and transformers, which change the voltage level. Each power line circuit has three conductor cables. Consequently, when the electricity reaches the substation, it is fed through three separate switchgears located next to each another.