Netze Solingen: Smart City starts with smart rectifier stations

Horstmann supplies short-circuit and earth fault indicators for diesel-free public transport

Solingen has the largest trolleybus network in Germany. The buses are supplied via 23 rectifier stations. Many of the stations are clearly getting on in years and do not yet support networked diagnostic and warning functions. As part of an EU-funded project, the network operator Netze Solingen, together with Horstmann, has begun to equip and digitise the rectifier stations with the proven ComPass B 2.0 short-circuit and earth fault direction indicators.

Solingen relies on battery trolleybuses (BOB)

Trolleybuses in Solingen have only been capable of running under the overhead line to date. In order to make local public transport emission-free, battery-powered trolleybuses are now being procured to replace diesel buses. This conversion is accompanied by a lighthouse project that is unique in Germany and is scheduled to run for six years. An important part of the project is to make parts of the medium-voltage grid intelligent and in this way to make the energy management of the overhead line network more transparent. Numerous stakeholders are working closely together in the project. These include the city of Solingen, “Stadtwerke Solingen” (Solingen public utility company), the public transport company “Verkehrsbetrieb der Stadtwerke” as operator of the overhead line network, and “Netze Solingen”. The research work is carried out by the Bergische Universität Wuppertal at three chairs.

Batteries replace diesel generators

The new battery-powered trolleybuses have four batteries installed in the rear with a total capacity of 60 kWh. They power the buses on 2 axles with up to 160 kW each and a range of up to 20 km in a purely electrical manner. The use of the batteries ensures the trolleybuses flexibility for journeys beyond the range of the overhead network, for which a diesel bus was previously used. The electricity from the overhead line is not only used for the ongoing operation of the buses, but also for "in-motion charging", i.e. charging the battery while driving. The battery is also supplied with recuperated braking energy.

Retrofit makes trolleybus network intelligent

Netzen Solingen supplies the overhead line network with medium voltage. In the rectifier stations of Stadtwerke Solingen, the voltage is converted into the DC voltage required for trolleybuses. Since the overhead line network has not been equipped with any intelligence and networking to date, there is very little information about its current condition and approaches for efficient energy management. Faults in the overhead line network or in the stations constitute a particular problem here. As central trolleybus route monitoring from the control centre is not possible, a fault clearance team has to go out to find the cause. That costs time and money. Sub-project manager of Netze Solingen, Christian von Kalben, explains as follows: "It was important for us to create more transparency and understand in real time what happens where and how in the overhead line network. That's why we chose a partner who can provide us competent support with their medium-voltage experience."

New sensor technology for old substations

As Netze Solingen has been working with medium-voltage technology specialist Horstmann for many years, they decided to commission him with the project. An initial analysis and inventory in the stations showed that the tested ComPass B 2.0 short-circuit and earth fault direction indicators met all the necessary requirements At the same time, however, it also became apparent that the rectifier stations differed considerably in terms of their type of construction, technical condition and respective connections. Many of the stations lacked any sort of capacitive interface to tap the voltage signal. Some of them are open installations. Some of the cables were designed as earth cables. It was thus clear that no standard solution could be used to connect all the stations, but that a suitable connection strategy had to be developed for each station.

Creating right technical conditions

The selected fault direction indicator ComPass B 2.0 requires two signals to monitor the line network and assess its condition: the current and the voltage. The current is measured via current sensors placed around each phase. The voltage is measured via a capacitive tap that is a standard feature in new installations and control panels. The basis for this often fist has to be created in old installations. Depending on the conditions in the respective station, a capacitive (insulator) C1I1 sensor or an easy-to-mount C1A2 sensor was required. The latter has proved particularly useful when space was at a premium during installation. Both sensors provide one voltage signal per phase, which is transmitted to the ComPass B 2.0. The capacitive voltage sensor is connected to the Wega 1.2 C voltage testing system and shows at a glance whether or not voltage is present on the three phases. It thus contributes to occupational safety without the need for further testing, and forwards the signals to the ComPass B 2.0.

Short-circuit and earth fault direction indicator ComPass B 2.0

The ComPass B 2.0 is equipped with monitoring and optional control functions and specially designed for the medium-voltage grid. It provides highly accurate measurement data for current, voltage and power, which can then be transmitted to the control room. All measured values can be provided with limit values that trigger an alarm in the control room when exceeded or fallen short of. It is also optionally possible to monitor the transformer and station temperature using the ComPass B 2.0. Thanks to the integrated, high-contrast OLED display, all data including a fault’s direction can also be read directly on the unit. It has a robust design and can be used in temperatures ranging anywhere from -30 to +70 °C.

Detecting faults more quickly

Once connected and commissioned, the ComPass B 2.0 monitors the overhead line network and registers any short circuits and earth faults along the medium-voltage line. If the latter are caused by excavation work, they can usually be located quite easily. Given that some of the cables are up to 50 years old, short circuits and earth faults can also be the result of material fatigue due to age or mechanical stress. It is precisely in such cases that the information provided by the short-circuit and earth-fault indicator is of enormous help. This is because the ComPass B 2.0 can transmit the collected information to the control room and indicate in which section of the line and in which direction the respective fault was detected. This helps the fault clearance teams locate faults more efficiently and repair any damage more quickly. The new source of intelligence also improves overhead line network monitoring and the evaluation of all information in real time.

Better overview of the trolleybus network

Several pilot stations have already been successfully converted. Instead of a black box, they are already providing much more concrete information about the network’s current utilisation status. This facilitates planning and significantly increases grid efficiency. The information about load peaks, fluctuating energy supplies and the characteristics of current and voltage curves help increase network stability and efficiency. Subproject leader von Kalben is already looking ahead to the next steps: “We not only want to make the overhead line network more intelligent, but also monitor large parts of the medium-voltage grid. This will allow us to considerably increase efficiency and, for example, better coordinate and support the integration of photovoltaic power or the connection of charging stations for electric vehicles."

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