4 PLC concept performance evaluation 63
4.5 Latency and data transmission tests
Latency and data transmission tests were performed between the HomePlug 1.0 compliant modems modified for the application. Besides the HomePlug 1.0 circuitry, the PLC modems include a small-signal diode over-voltage protection circuit as presented in Publication IV. The data transmission tests were carried out for the PLC channel in the LVDC laboratory setup and the LVDC field installation grid. The data transmission test setup comprises PLC modems connected with inductive couplers differentially between the two DC conductors of the AXMK and AMCMK cable in the laboratory setup, the AMCMK cable and the Ethernet switch in the grid branch in the
4.5 Latency and data transmission tests 77 field system, and two PCs connected between the modems. The data transmission tests were performed with Iperf software (Gates et al., 2003), which gives the data rates in the TCP/IP layer from the server to the client over the channel (PLC modems work as bridges in the channel). The latency tests between the modems were carried out with the ping functions. The data transmission results in the laboratory and the field pilot installation grid are presented in Table 4.4 (Publication VI). The C-R and R-C in the table refer to the direction from the CEI to the rectifier and vice versa. LN1 in the LVDC laboratory setup is the LN coupling case in the AMCMK cable.
Table 4.4: Data transmission rates between two PCs via HomePlug 1.0 compliant PLC modems with the inductive coupling interfaces in different load conditions, between CEI and rectifier (C-R), and vice versa, respectively in the LVDC laboratory setup, and in different intervals in the LVDC field grid.
LVDC Laboratory grid LVDC field grid
CEI load Data rate (Mbps)
Based on the data transmission results in Publication VI, the data transmission in the LVDC field installation worked every time, when the communication was performed between the PLC modems connected next to the CEIs and/or the cable connection cabinet, where the PLC signal was relayed and repeated. The ping between the modems was < 10ms every time when communication was carried out directly between modems.
When the communication between the modems was relayed through the Ethernet switch in the CCC, the latency was slightly increased, but the ping was below 15 ms. However, the load supplied by the CEIs has a major effect on the PLC performance, shown as dramatically decreased data rates when loads are connected. The communication between the rectifier and the CEI1 was not successful. This is because the signal is attenuated strongly in the 815 m grid segment. Based on these results, the concept is applicable to the LVDC system, if the communication signal is repeated at 500 m as proposed. The latency requirement is met for the present smart grid applications. Some applications may require even lower latencies (<10 ms) as in (ABB, 2010; Bose, 2010).
To respond to these, lighter protocols than TCP/IP should be used. For example UDP packets do not use an acknowledgment (ACK) function, that is, confirmation from the receiver to the transmitter that the packet has been correctly received. This would decrease the latency to half. In addition, a novel 6LowPAN protocol is a lightweight version of TCP/IP; it applies a shorter packet structure than the one used in Ethernet.
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5 Conclusion
In this doctoral thesis, an HF band PLC-based data transmission concept based on a commonly used IP protocol and commercially available PLC modems for an LVDC electricity distribution system was introduced and studied. The LVDC system is considered an example implementation of novel smart grids. The modern distribution grids provide smart applications, such as remote metering, customer load control signals as part of demand side management operated by power grid vendors, and grid protection carried out by communications between the IEDs in the grid. Thus, besides remote network connection between the low-voltage network and the grid operators, also a local network for the system is required. The LVDC distribution network differs significantly from the traditional low-voltage AC grids, because the parts of the medium-voltage AC grid, the public distribution MV/LV transformer, and the low-voltage AC grid are replaced with a rectifier, a DC grid, and customer-end inverters in the other ends of the network. Switched-mode power-electronic devices in the ends of the network generate voltage and current harmonics and impulses with a certain frequency and duration to the DC line, and thus, the channel contains impulsive noise up to HF frequencies. Therefore, to avoid the most adverse effects of the noise generated by the rectifier and the CEIs, the HF band is considered more suitable over low frequencies, and is thereby applied to the PLC. Furthermore, the LVDC grid architecture and structure between the rectifier and the CEIs provide an opportunity to apply the HF band in the PLC; signal repeaters are implemented in the grid branches for the purpose. By this, the communication network is divided into network segments, which makes it simpler to control and manage.
First, one of the most popular topics globally today, smart grids, was outlined. The most common applications that make the grids smart were covered, starting from the remote meter reading, which was first introduced in the late 19th century and carried out with PLC. The smart grid concept platform for an LVDC electricity distribution network was presented with its main features. After the introductory chapter, the PLC concept for the LVDC system was addressed, and the frequency band for the PLC was chosen based on the assumptions of the channel characteristics and noise in the channel. This was partly based on studies carried out in a similar environment as presented in Publications I and II. In the next chapter, the LVDC PLC channel was studied in detail by an analysis of channel characteristics. The underground voltage cables typically used in the low-voltage distribution networks as a communication channel in the frequency band of 100 kHz–30 MHz were studied. The signal attenuation in the cables is one of the most essential factors when the HF band PLC is applied. It defines the maximum signaling range when the network topology, signaling power, and noise characteristics in the channel are known. Thus, the frequency band of 100 kHz–30 MHz covering the band typically used for PLC was analyzed. Furthermore, a two-port channel modeling approach was selected for modeling the cables, and a lumped model with its parameters for the cables was formed. The two-port modeling method is justified, because the PLC modems are coupled differentially between two cable conductors, which are connected
between the neutral and the other DC pole at the rectifier and the CEIs. Consequently, each component in the LVDC PLC channel was represented by a two-port input impedance model. The channel component models in the PLC channel were formed, parameterized, and verified by measurements. The channel model was constructed with a circuit simulator by connecting all channel components together according to the LVDC PLC channel topology. The channel model was verified by channel gain measurements.
Noise samples in the PLC channel ends were measured to study the effects of noise, which is mainly impulsive and generated by the rectifier and the CEIs to the DC grid where the PLC modems are installed. Based on the noise measurements, signal attenuation in the cable, channel gain measurements, and the channel modeling, the signaling range was analyzed with the SNR estimation. In addition, a theoretical channel information capacity analysis was carried out. To support the analysis, practical data transmission tests in the LVDC laboratory setup and in the LVDC field grid were performed. The studied aspects were throughput, reliability, and latency, which are defined as the most critical factors when smart grid applications and functionalities are considered. The analysis shows that the HF band PLC and HomePlug 1.0 modems are applicable to the LVDC system if the PLC signal is repeated in the grid branches.
Suggestions for future work
In the course of study, the author has identified the following subjects, which should be investigated further. In this doctoral thesis, the communication method was based on the HomePlug 1.0 specifications, and commercial HomePlug 1.0 compliant modems were applied to the LVDC PLC concept. To optimize the performance of the communication system, also the novel HomePlug Green PHY compliant modems designed especially for smart grids should be studied. The coupling interface used in the system can also be adopted to other HF band techniques without any modifications. The application of the circuit simulator should be considered further. It also provides simulation profiles in the time domain, and thus, impulsive noise generation in the rectifier and the CEIs could be modeled and its effects on different types of PLC techniques and modulation schemes simulated. Furthermore, the low-voltage cables studied in this doctoral thesis are four-conductor cables. Consequently, to improve the cable model, and thereby the channel model, four conductor cable models could be designed. Moreover, the LVDC PLC channel model should be completed by designing and constructing models for branched channel with noise sources. However, the circuit simulator for channel modeling is rather heavy and very sensitive to parameter changes in the model. Therefore, the channel model is planned to be implemented in a MatLab Simulink environment as single filters. By doing this, channels with different types of topologies could be simpler and faster to model.
In this study, only the HF PLC was considered. Therefore, new high-speed NB-PLC modems, such as G3-PLC should be tested to consider their performance in such a channel.
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