Power Line Communication – The Weakest Link?

There is no doubt that power lines are part of critical infrastructure. Power engineering is now at its most dramatic development phase. Unfortunately, progress in technologies and political decisions tend to complicate development.

After the EU strategic plan 20-20-20 is realised, we are likely to face the paradox of a reliable supply of power still not being ensured. Fortunately, however, even empty slogans such as ‘smart grid’ bring rational considerations and decisions to the area of stability and safety of power supply.

The way to control end devices, including residential e-meters, is very clear… despite reservations PLC technology will prevail. It is the only technology with reliability that equals that of the power supply.

Shannon schematics of a communication system
Shannon schematics of communication systems

The scheme above reveals the structure of both modem and surroundings that influence total efficiency of transferred information. A real communication system is obviously significantly more complicated, however, this simplified model is sufficient to identify the key effects.

Challenges of the Transmission Channel

Based on this, it is clear that transmission capacity does not depend on how we wish it to look, but, depends on the characteristics of the transmission channel. In the case of PLC technology, that channel means power lines.

The distribution can be generalised on serial-parallel structures with relatively high attenuation (because of significant parallel capacity and serial inductance). Parallel ordering of single consumption places with highly dynamic connection and disconnection brings strong and very dynamic change of impedance.

Simplified structure of PLC Network
Idealised distribution network. Elements L1,2,3 and S1,2,3 have character of lines, and they could be described by spare schematics (following picture).

Elements ZG (source impedance), and ZL, and Z1,2,3 (load impedance) describe both the source and appliance part of the power network (impedance characteristics defines that elements have frequency dependency).

Unfortunately, there is always the customer’s consumption at the end. Customers can through their behaviour, influence the distribution network and therefore the communication channel.

Equivalent connection circuit
The spare schematic characteristics highlight the real value of power lines which are frequency dependent. This capacity in transverse admittance leads to high noise attenuation at higher frequency. (This is one of the reasons why BPL communication has a limited reach).

How to address the Weakest Link in Communication?

The communication channel is always the weakest link in the transmission chain. It would seem that based on the frailties of this channel (disturbance, dynamic topology change), it would be more convenient to leave this channel. The exact opposite is, however, true.

The reason is the unique characteristics of PLC, such as enabling power supply together with immediate availability of the communication channel. It is enough to accept the characteristics of this channel, and not to add expectations around unrealistic communication rates.

What are the Limitations of Current Standards?

New interoperability standards (i.e. PRIME, or G3-PLC) show ‘how not to move ahead’. Using these trendy OFDM systems, (PRIME, G3-PLC), which reach high transmission capacity on other transmission channels across the power line environment, means that we sow the seeds of a serious problem.

Furthermore, if we add ineffective transmission protocols (DLMS), and reduce processing capacity to enable a low-cost solution, we will reach satisfactory results on paper only, not in field. We cannot increase emitted power due to legislation. Neither can frequency range increase be considered.

There are two reasons for this:

  1. we are limited by assigned frequency, and
  2. channel characteristics at higher frequencies do not physically allow higher reach (not more than a few hundreds of meters).

It is also necessary take into account requests at the output stage. The output stage and method of modulation have to be designed in such a way that significant changes to impedance will not influence the stability of this stage.

Toward Reliable and Efficient PLC Solutions

That is why, this stage has to have low inner impedance, which is a request hardly applied at OFDM, which needs a linear output amplifier in order to eliminate distortion, and at the same time, disruption of sub-carriers orthogonality. The input circuit of the receiving modem must be able to change its amplification in a very dynamic way, in order not maintain the level of the processed signal.

Nowadays, there are two possible choices. The first one is already being used in realised projects. It assumes a significant reduction in transferred data volume. These proprietary solutions do not need to transmit ballast information. They thus reach high reliability during data transmission, and moreover, they are also competitively priced.

The second scenario takes into account prevailing requests for transmission of higher volumes of real information. In this case, it is necessary to increase the complexity of both transmitting and receiving modem parts significantly.

The Path Forward for PLC Development

The growing element integration of silicon chipsets could help. It enables turbo code implementation (for transmitted information renewal) and efficient usage of the soft decision approach (decision by code words).

If these methods are expanded on, together with a suitable form of modulation, we find the only way for PLC communication development to progress. It ensures transmission of higher data volume, and, in such a case, even the DLMS protocol transmission could not be unfulfilled utopia.

Conclusion: Shaping the Future of PLC

We are convinced that the progress in the nearest future will continue by a parallel process. On one hand, we believe that there will be a public rational interoperable protocol. On the other hand, technological progress in the field of a single communication channel performance – all geared towards increased robustness and defined latency conforming – which is the key request for proper smart grid functioning.

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