Interoperability? OFDM? Forget about it (for now)


By Bedrich Benes

In comparison with the situation in the recent past, today’s power engineering faces new situations. In the mature world, power is promoted as one of the basic human needs. Normal societal behaviour cannot be ensured without electricity. The politics of CO2 emissions reductions and significant financial support for renewable resources is leading to their massive deployment, with all the associated impacts.

Previously, the massive deployment of verified systems was realized, i.e. the development preceded practice. Today the situation is the opposite. Power engineering waits for technical development, in order to safely face all the operational states. The situation is serious. A breakdown of the power system in Central Europe lasting more than 10 days, for example, could very seriously jeopardize the functioning of all the European Union. Thus we can say that to some extent, power engineering in Europe is permanently five days ahead of the declaration of an emergency, with all its impacts.

So we have a tense balance situation, and, moreover, new requirements are appearing for the rollouts of automated metering and load control. In spite of this positive step with beneficial impacts on the environment, it is not sufficient to improve the energy balance situation. Firstly, these systems are not able to react to situations sufficiently quickly. Secondly, the large scale rollouts will impose a financial burden on the system and we are likely to face a lack of resources for the deployment of more secure (and perhaps more financially demanding) technology.

The first conclusions can be drawn based on the already realized pilots/rollouts. Usage of the public operator (i.e. data transmission via GPRS) is problematic. Operators are not able to guarantee time security of transmission. This was demonstrated with the transmission collapse over the Christmas holidays (SMSs were delivered with long delays, phone calls could not be put through due to network overload). If the main criterion is operational economy, whole area servicing can be talked about on the theoretical level only.

The existing powerlines are thus the only reasonable solution for data transfer. The positive aspects of PLC definitely prevail, but two other aspects have to be taken into account. Firstly, there are legal restrictions (modulation voltage level and assigned frequency band), and secondly, there is the possibility of data transfer via the communication channel, which is powerline, with all its physical impacts.

The truth is that governments have become conscious of the tense situation in the power engineering industry, which can be clearly seen from the issuing of many subsidies and grants to realize the desired solution. However, these projects have the disadvantage of not operating from the physical point of view, or are very expensive. The root cause of this is one thing: interoperability.

Interoperability is definitely a desirable aim. Today’s systems for data exchange between meters which do not reflect the characteristics of the communication channel, and which are technically obsolete, are inappropriate, and their demands for communication channel characteristics are almost lethal. This was the case for systems using broadband modulation OFDM for data transfer. It is disputable to use this way of modulation in the dedicated frequency range 9-132 kHz, where single subcarriers are squashed one by one, and do not disturb each other due to system orthogonality only. DLMS standard implementation means all subcarriers transfer single parts of the data of transferred message. There is almost 100% probability that some disturbance will hit the subcarrier frequency, which leads to loss of the whole message.

Such a system is very sensitive to disturbance, according to testing and comparative measurements, and the first installations of systems based on OFDM technology, like PRIME, or G3. From the presentations plenty of important information is missing, e.g. description of the transfer condition (communication channel type – cable, overhead, mixed, age, etc.), number of communications with single electronic meters (15 m profiles transfer, time delay of command realization), and the rollout results.

Measurements have proven that systems with such OFDM communication are (especially in mixed lines with old installations) significantly worse in comparison with narrowband communication. If common methods used within other OFDM systems are used (greater distance of subcarriers, and redundancy of data messages or their parts), that is the right way. However, a significant decrease in the transmission rate is an inevitable result.

It is necessary to answer a key question: Do we need reliable and robust, or fast communication? Both states in such a limited communication channel are mutually excluded. If we respect the physical characteristics of a physical channel, we have to consider which quantities we will transfer. Transmission of all information to the control centre and its subsequent decision making is a utopic idea. If we take into account the physical rules, it is necessary to choose the opposite approach: distribution of control to the lowest level possible. Electronic meters and their controlling data concentrators should not send single data items, but rather reports to the control centre. According to prepared scenarios, they should help the system maintain safe operational state.

Research and development is the way to advance and evaluate mistakes. It is necessary to descend from a single solution, and to precisely define the essential requirements for fulfilling all the needs of energy system participants, while respecting the requirements of transmission reliability and system stability. If we look at system creation in other commodities, we can see the request for interoperability arises at the very end, after all the technical requests are solved. Power engineering will face a similar process: first of all, it is necessary to create a data transmission system that reflects the transmission characteristics of the channel used and the time requests of the power network operational states. This system may become standard. But it is clear, unfortunately, that it is not possible to move in the opposite direction.

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