Section: D.4.1.0 Overview
Content: An energy-automation domain that now has standards based support by mobile-network technology is the backhaul electricity grid, i.e. the part of the distribution grid between primary substations (high voltage medium voltage) and secondary substations (medium voltage low voltage), and other smart grid services. In figure D.4.1.0-1 we depict a medium-voltage ring together with energy-automation use cases that either are already deployed or are anticipated within the near future. Figure D.4.1.0-1: Functional, topological sketch of a medium-voltage ring. AMI: advanced metering infrastructure; CB: circuit breaker; DMS: distribution management system; FISR: fault isolation and system restoration; HEM: home energy manager; PQ: power quality; RMU: ring main unit. The primary substation and the secondary substations are supervised and controlled by a distribution-management system (DMS). If energy-automation devices in the medium-voltage power line ring need to communicate with each other and /or the DMS, a wireless backhaul network needs to be present (orange "cloud" in figure D.4.1.0-1). A majority of applications in electricity distribution adhere to the communication standard IEC 60870-5-104. However, its modern "cousin" IEC 61850 experiences rapidly increasing popularity. The communication requirements for IEC 61850 applications can be found in EC 61850-90-4. Communication in wide-area networks is described in IEC 61850-90-12. Usually, power line ring structures have to be open in order to avoid a power-imbalance in the ring (green dot in figure D.4.1.0 1). Examples for energy-automation that already is implemented in medium-voltage grids (albeit in low numbers) are power-quality measurements and the measurement of secondary-substation parameters (temperature, power load, etc.) [13]. Other use cases are demand response and the control of distributed, renewable energy resources (e.g. photovoltaics). A use case that could also be realised in the future is fault isolation and system restoration (FISR). FISR automates the management of faults in the distribution grid. It supports the localization of the fault, the isolation of the fault, and the restoration of the power delivery. For this kind of automation, the pertinent sensors and actuators broadcast telegrams about their states (e.g., "emergency closer idle") and about actions (e.g., "activating closer") into the backhaul network. This information is used by the ring main units (RMUs) as input for their decision algorithms. We illustrate this use of automation telegrams for an automated FISR event in figure D.4.1.0-1. Let us assume the distribution lines are cut at the location indicated by the bolt of lightning in the figure. In that case, the RMUs between the bolt and the green load switch (open) will be without power. The RMUs next to the "bolt" automatically open their load switches after having sensed the loss of electric connectivity between them. They both broadcast these actions into the backhaul network. Typically, these telegrams are repeated many times while the time between adjacent telegrams increases exponentially. This communication patterns leads to sudden, distributed surges in the consumed communication bandwidth. After the RMUs next to the "bolt" have opened their switch, the RMU that so far has kept the power line ring open (green dot in figure D.4.1.0-1) closes the load switch. This event too is broadcasted into the backhaul network. The typical maximum end-to-end latency for this kind of broadcast is 25 ms with a peak experienced data rate of 10 Mbit/s. Note that the distribution system typically subscribes to telegrams from all RMUs in order to keep abreast with the happenings in the distribution grid. Automatic fault handling in the distribution grid shortens outage time and offloads the operators in the distribution control centre for more complicated situations. Therefore, automated FISR can help to improve performance indexes like System Average Interruption Duration Index and System Average Interruption Frequency Index. Automation telegrams are typically distributed via domain multicast. As explained above, the related communication pattern can be "bursty", i.e. only few automation telegrams are sent when the distribution network operates nominally (~ 1 kbit/s), but, for instance, a disruption in the power line triggers a short-lived avalanche of telegrams from related applications in the ring (≥ 1 Mbit/s).