Electric power grids have been facing enormous challenges for several years. Increased input from renewable energy sources and decentralized power generation are only two major trends that grids were not designed to deal with in the past. In addition to grid expansion, for example in the high-voltage grid, smarter grids at distribution level are also necessary. In any case, the basis is the increased use of measurement technology. A particularly clever solution for local grid distributors is used by the municipal utilities of Schwäbisch Gmünd.
The grids for electrical energy were originally planned and built as one-way streets – decentralized feeding was not planned. „Today, we often don’t even know when and how much electricity flows as well as in which direction,“ explains Michael Gold, who is responsible for construction site management in the electrical engineering department at the municipal utilities of Schwäbisch-Gmünd. The construction department handles projects in new development areas to supply households and commercial customers with electrical energy.
Schwäbisch Gmünd is a small town with 60,000 inhabitants; the municipal utilities connect the approx. 30,000 households in Schwäbisch Gmünd to the supply lines for electricity, water, gas and district heating as well as to a fibre optic network. A lot is currently changing in the grids for electrical energy, as Gold emphasises: „In addition to the infeed of photovoltaic systems, the charging points for electric vehicles are also creating challenges“. The distribution grids in the low-voltage range must often be adapted to new requirements. In order to be able to carry out these adjustments in a targeted manner, a basic requirement is indispensable: The exact knowledge of the load flows in the distribution grids.
Measuring the exact grid parameters directly in the local distribution grids is a task that the specialists at the municipal utilities are also tackling. In this case they decided to use a Smart Grid Interface Module (SGIM) from EFEN. „We wanted to test this kind of measurement technology in a first step“, Gold reported. In a suburb of Schwäbisch Gmünd a commercial customer had applied for a power increase, which was approved by the municipal utilities. Higher power supplies for individual connections can always lead to difficulties in the grid. „It must therefore be ensured that the customer does not use more power than agreed,“ Gold points out. Above a certain performance limit, the customer has to pay a construction cost subsidy for the provision. For very large consumer requiring more than 160 kVA, the customer should set up a medium-voltage connection with its own transformer station. However, there is another reason for measuring the parameters directly at the outgoing feeder of the local distribution grid: „This allows us to document the grid quality for the customer.“
The local grid distributors are constructed with the usual distribution cabinets in which a 185 mm busbar system is installed. The individual outgoing feeders are protected by size 2 NH fused switches. In order to be able to react quickly to any necessary expansions, there is practically always a reserve space available. The SGIM by EFEN is designed to be installed in exactly this spare space with a width of 100 mm. „This space-saving installation convinced us“, says Gold: „With other solutions the space requirement would have been greater.“ With these solutions, an installation on the spare space in the distribution cabinet would not have been possible.
The SGIM has a modular design and consists of the installation panel and various plug-in units. Power is supplied directly via the busbar system. In addition to a power supply unit and the controller of the SGIM, two Schuko-sockets are also included in the basic configuration. These are very useful if, for example, an additional device is to be connected during maintenance work. The SGIM can be flexibly equipped with different measuring plug-in units. In the model purchased by the municipal utilities of Schwäbisch-Gmünd, five modules for Rogowski coils are installed. To keep the installation effort to a minimum, there are pre-assembled Rogowski sensors for 3-phase measurements. The SGIM-Rogowski cable set is connected to the Rogowski module with a plug and has three sensor rings at the other end. „The installation of the coils is very simple and can be completed in a few minutes with just a few simple steps,“ says Gold, who is enthusiastic about the well thought-out solution. Since two cable sets can be connected to each module, a total of ten outgoing lines can be monitored simultaneously.
An additional module slot is equipped with a communication module. It is used to transfer data to a cloud-based server via mobile radio. One slot is currently still free. „If required, an additional IO module can be integrated here, with which we could, for example, implement fuse monitoring of the NH fuses,“ explains Gold.
Uninterrupted supply is one of the top priorities in electrical distribution grids. The municipal utilities always try to avoid even short-term interruptions. „That’s why it was particularly important for us to be able to install our SGIM under live conditions,“ says Gold, citing another advantage of the metering system. For example, when the SGIM was installed in the local distribution grid in spring 2019, the relevant connections did not have to be shut down. „Immediately after the installation, the currents and voltages at the respective outgoing lines could be measured,“ Gold explains. In addition, the SGIM calculates the important grid parameters such as reactive, apparent and active power, energy consumption, power factor, phase angle and grid frequency.
All measurements are transmitted via mobile radio at 15-minute intervals directly to a cloud-based server, where the data is available for evaluation. „The SIM card was already integrated when the system was installed,“ remembers Gold: „So we didn’t have to worry about anything.“ The SGIM uses the proven com.tom-Kolibri transmission protocol for data transmission, which reduces the amount of data to a minimum. If limit values are undercut or exceeded, the current measured value is transmitted immediately. An alarm message can be generated in the cloud solution, which is sent by e-mail or SMS.
The municipal utilities of Schwäbisch Gmünd did not have to worry about the provision and installation of the cloud solution itself. „We booked a complete package from EFEN, which already includes the mobile phone connection and the cloud solution“, says Gold. This is not only very convenient, but also has advantages in terms of security. „Our IT security department is always very critical of a connection to our own server, as the technical security management is very strict due to our ISMS certification.“ A direct connection via mobile radio to the municipal utilities control room would therefore be very difficult to implement. The transmission and evaluation of data in the cloud solution is therefore the much better solution in terms of the security of the distribution grid. „We now receive regular reports by e-mail that contain all the information we need,“ says Gold. With monthly fees of around ten euros, this solution is also very cost-effective compared to a proprietary server. „And it also gives us a high degree of flexibility,“ explains Gold, highlighting another advantage of this solution: „When we no longer need the service, we simply terminate it.“