By Zhang Baojun, Shenzhen RMS Technology Co., Ltd.
The power line is a pre-existing communication channel with the widest coverage. Power line carrier technology can realise data signal transmission in an intelligent network with the least extra wiring cost. So, most of engineers are paying attention to the development of PLC technology which has gradually become the main research hot spot in recent years. But the high impedance variation, high noise and the strong attenuation characteristic of the power grid in different regions, as well as in different periods of time, makes the actual data communication result a completely diversified one. The unsatisfied power grid environment becomes the biggest challenge to the power line carrier technology.
As you know, it has become the national policy for most countries to protect the environment by trying to reduce the carbon dioxide emissions. And the improvement to the management of power departments by the timely collection of data from every electricity user to an effective data analysis platform will become one of the realistic methods of reducing the energy consumption. PLC technology can surely be assumed to be the front-runner of all the other available AMR/AMM solutions.
In China, there are currently about four million electronic power meters in communicating via PLC technology. The State Power Grid plans to install around 150 million electronic power meters with AMR functionality, of which 80 million meters will adopt PLC technology. Owing to such a huge market demand, all kinds of PLC technology solutions are under development and in pilot run now.
In China, due to the historical reasons, the actual power line laying situation is quite diversified. The interference of harmonics and noise to the power network is not strictly regulated. The power network pollution in some areas is very high, which greatly increases the challenge of deploying PLC technology. In recent years, PLC technology solutions from different vendors have all failed in the field, returning unacceptable data collection results.
A reliable carrier system electric meter data needs to be able to communicate across long distances and have strong networking abilities. Normally, the length of a power line under one transformer is between 1 km to 2 km, while the existing communication distance of low-voltage carrier power line from point to point is 100m to 500m. The relay function is critical for all the meters to be able to send data to the concentrator. Each solution adopts various technical methods to improve the communication distance, including spread spectrum, the zero transmission, filter, increasing power, and improving the acceptance sensitivity etc. But these methods do have the limitations. Due to the bad power grid environment, point-to-point communication distance sometimes declines to several meters from 300 meters. The resistance changes in the grid, the noise impact — especially the various electric start-ups and shutoffs — capacitance and inductance distribution function of the power,; multifarious interference waves of the reflection; standing waves; resonance; the line to spread across multi-levels; the intervals; each link: all these will affect etc, point-to-point communication. Therefore, the carrier electric meters under a transformer must have the networking relay ability.
In the area of unpredictable network topology, the specific circumstances and the mathematical models of successfully networking are very intricate, and the composed network should be able to adapt to various external conditions such as the change of the power grid environment and deployment and replacement of meters.
At present, the greater part of the networking solution is the static network. A static network is a search and static network, similar to the objectively geometric topological structure in the low-power network. During the setting process, it uses the approximate algorithm which goes through multiple signal transmissions and surveillance, after the first several hours to tens of hours of commissioning process (or “learning process”) to establish the reading network relay point and the reading path.
In subsequent reading process, according to the power grid environment changes, it can be adjusted and optimised. This is a good method for the power grid environment. But in bad power grid environments, the data reading time will be greatly extended. Sometimes it will take more than a week to read the data. The reasons for this are as follows:
- After all, “learning” is a process based on historical time, place, power grid environment, and the external conditions. Any differences may cause path error and a reading failure
- A bad power grid environment makes the optimal path of reading meters in a constantly changing system. It is extremely difficult for the concentrator’s algorithm to cover various complex combinations accurately. Now only several parameters, such as the successful frequency of meter reading, listening and phase identification, can be sent to the concentrator to analyse. But it is difficult to calculate the accurate network topology through such parameters to the concentrator
- When the relay consists of more than three layers, the concentrators have to perform an increasing amount of calculations, with subsequent delays. Outside the three layers, the successful reading rate and the stability of the carrier electric meters will be very difficult.
- We started the development of electric meter carrier using the traditional method of static network design. In testing a bad power grid environment, we have 40 meters with air wire connected. And each interval is 30-50 meters. The result is not satisfactory. So we performed an experiment. We gave the topology structure of the 40 meters with artificially fixed relay points and reading paths. After 24-hours of uninterrupted reading, the results showed that the fastest meter reading time is 30 seconds and the slowest time is eight hours.
Due to the uncertainty of local meter reading time, we have to seek a new way of meter reading. After several designs, tests, validations, improvements and using carrier chip based on the Lonworks agreement, the technology of the dynamic network carrier meter reading was launched in the domestic market. The so-called dynamic network is not the network topology under one transformer: it uses dynamic network technology, until every concentrator issues a command that it starts without any networking historical memory. Throughout all the equipment across the entire network involved in gathering and transmitting process information, the technology is geared to establish more transmission links rapidly and efficiently in order to seek out the correct relay route to target. It requires all the equipment in the system to be interactive and synchronised in order for the automatic relay to find target meter instantly on the basis of random relay and context of the geological hazards.
The technology has the following features:
- Concentrators issue the meter reading orders; multi-meters receive and forward the orders until the order to the target meter, then the data of the target meter returns to the concentrator through multi-meters receiving and forwarding
- Impervious to historical data, and non-pursuant of the network topology. Seeks out the best path by using real-time dynamics and real-time networking technology
- Each meter is self-adapting and self-controlling instead of the original concentrator calculation control
- Precise timing control and prevention of collision detection makes each meter work consistently, and avoids data loss and prevents network collapse due to losing control
- Because we do not use the complex routing relay algorithm, the relay series relief is not restricted, and the 10 layer relay can easily be realised, albeit somewhat timeconsuming.
In the development process, from the pilot runs to mass production with 8000 meters, the whole system performed well under the rigorous - 30OC climate conditions.
- The successful rate of 24 hour meter reading is 100%, and the average reading time is five to eight seconds for each
- The successful rate of the master station reading meter data point to point at one time is 97%
- A after the installation the static network concentrator is efficient. One transformer only needs one concentrator.
THE REASON FOR GREAT RESULTS
The static network enables the concentrators to calculate out one or several optimal paths according to the historical data. The dynamic network means to choose several paths, according to the real-time grid environment. The concentrator issues the order to read the meter and multi-meters receive and forward the command for “free competition”. This real-time option is completely according to the grid’s instantaneous changes to choose the relay and the relay series. The relay series and paths are different from the data to the concentrator and return to the target. And the path is different for the continuous reading meter (on the other hand, which proves the fluidity of the grid).
Parallel circuits failure mode is used to replace the series circuits failure mode
A static network issues the orders to read the meters according to the fixed design of the path in advance, the target receives the orders and return in the round. In the process, any abnormality can cause a reading failure. This is the series circuits failure mode. The dynamic network designed by us receives and forwards orders by multiple meters, frequently over 10 sets in participation. As long as one meter can transmit the data to the next one, the communication will be successful. The probability of success for parallel circuit’s failure mode is very much better than that of static network mode.
Avoiding the short board effect
A static network in a charge process (the effect of multiple relays is more apparent) must ensure that each relay that is engaged is able to accomplish the task to enable successful reading. The dynamic network consists of multiple meters receiving and forwarding data and is frequently the least interfered with, transmitted power, and provides the highest sensitivity for the receiving meter to operate in. In other words, in the static network, only when the worst meter is operating well, it can be successful, while in the dynamic network, all meters need to be performing well in order to be successful. This way of working with the means of a network improves the system noise reduction and anti-jamming ability significantly to avoid the short board effect.
Higher baud rate can reduce the data interference probability
Currently the baud rate of the static network in the Chinese market is from 100 to 600 while using spread spectrum technology. We adopt the 5400 baud rate, so for the same data transferred, the total time is only 1/9 of the original one, and the probability of destruction by grid disturbance is only 1/9.
To conclude, the advantages of dynamic network technology is obvious and it also achieves satisfactory results in the actual operation. As the carrier meters with dynamic network technology are used with big quantities, the advantage of this technology is increasingly convincing.