The preprocessed and combined data from different sources enables for assessment of the smart meter infrastructure for the given use case by the following three characteristics “overall reliability”, “data volume” and “latencies”.
Overall reliability
We have conducted 2039 power limitations on one SMGW with varying parametrization. Thereof, 36 commands failed, leading to a success rate of 98.2%. As reasons for the failures we found, among other things, a concurrent closing of an existing TLS session with an attempt to start a new session at the GWA. This leads to a stop of transmitting the current power limitation to the SMGW. Here, the GWA could change the system configuration to conduct various attempts to connect with the SMGW after it had failed. When regarding the communication from the SMGW to the emulated EVSE, we currently see a success rate of 51.0%. The relatively low success rate between SMGW and EVSE can be attributed to a software issue on the SMGW side, the fix for which was available at the time of publication but could not yet be tested. It is worth mentioning that this communication channel is still under development and not yet applied in real operation.
Data volume
In our tests, we varied the parametrization of power limitations and the temporal distance between them. When a power limitation setting is sent within 3 min since the last communication, the still open TLS session is used, and thus establishing a new session is unnecessary. This decreases the amount of data volume per power limitation. On the other side, when we waited for the TLS session to close until sending another power limitation, we assigned one session to one limitation setting, including establishing and closing a communication channel. Figure 5a depicts the average data volume for the transmission of one power limitation depending on the number of sent settings per session. We clearly see a decrease in the necessary data volume per transmission of one power limitation when an open session is used in contrast to the single usage of a session. Consequently, we can estimate the average data volume for establishing and closing a session to approximately 5.5 kB, and assign approximately 9.5 kB of the data volume to transmit one power limitation.
Figure 5b shows a box plot representationFootnote 1 with the data volume for transmitting a power limitation within one session. One can recognize two major groups of needed data volume. We suppose this might be due to a frequent retry of sending packets.
Latencies for preventive load management
For preventive power limitations, the command is sent to the GWA 5 min before \(t_{start}\). Figure 6 depicts the statistical occurrences of events along the process of a preventive power limitation setting, respective to its planned starting time \(t_{start}\) each. It shows, that on average the GWA starts connecting to the SMGW 50 seconds before the planned starting time. This is due to the parametrization at the GWA and could be set to an earlier point in time. After establishing the connection and receiving the power limitation via a PUT request, the profile in the SMGW has changed on average 14 s before \(t_{start}\). The exact moment, when the EVSE accepts the power limit, cannot be stated here because of too few successful communication occurrences between SMGW and EVSE. The GWA is informed about the changed profile in the SMGW on average by \(t_{start}\), and the session is closed approximately 180 s after that. Note that the GWA is informed about a successful profile change in the SMGW, independent from the successful acceptance by the EVSE.
The box plot in Fig. 7 shows the duration of procedures between the above-shown events. This representation emphasizes the single long-lasting and strongly differing procedures. Our data shows that processing the power limitation request within the SMGW takes the longest amount of time: approximately 27 s, with a standard deviation of roughly 4 s.
Power limitations that are transmitted to the SMGW within an opened session and that are being sent to the GWA 5 min before start, depict vaguely the same pictures as above; they solely do not contain the events of opening and closing their session. Therefore, it would be unnecessary to show those figures here.
Latencies for curative load management
For curative power limitations, the command is sent to the GWA at the start time \(t_{start}\). Figure 8 depicts the occurrences of events along the process of a curative power limitation, respective to the starting time \(t_{start}\) each. On top of that, Fig. 9 shows the duration of two additional procedures between the beforehand shown events. The other duration box plots are not depicted because they are similar to the values in Fig. 7.
After the DSO has sent the power limitation request to the GWA, we can observe a delay of an average of 11 s at the GWA before starting to connect to the SMGW with a noticeably differing duration. There is definitely potential for improving rapidity. The following steps creating a connection to the SMGW, receiving the power limitation in the SMGW and changing the profile, have similar latencies as in the case of preventive load management. After an average time of 48 s, the SMGW has updated its profile. And 3 s later, after roughly 51 s since sending the command, the EVSE has accepted the power limitation. Power limitations transmitted to the SMGW during an already started session depict approximately the same results as before. Thus, it would be unnecessary to show those graphics again.