The transition from the traditional power grid to the new smart grid, and one of the main challenges it will face is the need for a good communication network to receive all user information and control its load in real time. To solve this problem, the most recognized and reliable solution is PLC (Power Line Carrier) technology with power grid as the communication medium. This paper introduces the PLC technology and its development process, and compares the traditional narrow-band single-carrier FSK modulation scheme with two new schemes based on OFDM, PRIME and G3.
I Introduction
The traditional grid is changing. In the past century, the power grid was a system used to transmit the electrical energy generated by a certain number of power stations to a large number of users of different levels. The standard for designing and operating the power grid is to transfer electrical energy from hundreds of power stations to millions of users in an efficient manner. The function of this system to store electrical energy is very limited, so how to predict the user's electricity consumption becomes crucial. The control of the power grid is based on daily predictions, and electrical energy is transmitted from the power station to the distribution network through the transmission network. Most power generation needs to be controlled by regulators.
Now, in some countries, and in more countries in the future, green energy will contribute more and more to the power grid. Its proportion in the power grid has risen from 5% of hydropower to 40% of solar and wind power. In most green electrical energy, the regulator has little control. In addition, electric vehicles have joined the transformation team. The large-scale promotion of electric vehicles will double the power consumption of the power grid and bring huge storage capacity on a large scale. The rise in electricity consumption, the promotion of green power and uncontrolled power generation, and the storage capacity of electric vehicles are considered perfect storms for the power grid. This solution is called a smart grid. It combines embedded intelligent technology and real-time communication and control functions, and can communicate with any user in real time and control its load at any time. To realize such a communication function, it is necessary to adopt PLC technology with the power grid as the main communication medium.
PLC technology was used in the medium voltage field to control the power grid as early as 20 years ago. However, the large-scale use of PLC on the low-voltage side has only recently begun. A typical successful case of PLC technology is that the Italian ENEL power supply company uses a narrow-band PLC system based on FSK and BPSK modulation to build an AMM (Automatic Meter Management) system for 35 million users. This system can automatically read 35 million meters every 2 months. But its average baud rate is not enough to support more real-time communication and control, as well as future applications based on IPv6 and other communication protocols.
For more real-time communication and control, and future applications based on IPv6 and other communication protocols, a new generation of PLC technology based on OFDM modulation is needed. The two main OFDM schemes are the current G3 and PRIME technologies. G3 is a program initiated by French EDF power company and developed by MAXIM and SAGEMCOM. This plan was announced in 2009, and EDF plans to try 2,000 meters using G3 technology in 2013.
PRIME is an open multi-vendor solution launched by the PRIME Alliance. The alliance includes more than 30 members from power supply companies, meter manufacturers, and chip suppliers such as ADD Semiconductor, FUJITSU, STM and TI. The watch manufacturers include SAGEMCOM, ITRON, LANDIS + GYR, ISKRA-MECO, ZIV and SOGECAM. IBERDROLA was the first power supply company to promote this solution, but now EDP, CEZ MERENI and ITRI have joined this camp.
IBERDROLA began to install 100,000 meters with PRIME technology in 2010. The power supply company also plans to release a new standard with a demand of 1 million meters at the end of 2010, and complete the installation of 10 million meters in Spain in the next 3-5 years. Some other power supply companies have also adopted PRIME technology. Both G3 and PRIME are OFDM schemes, but the development history is different. G3 originally used a chip designed by MAXIM, which can provide IEEE 802.15.4 2006 communication for the PHY layer and some existing software layers, 6LowPAN for the MAC layer, and IPv6 communication for the network layer.
PRIME is an alliance of a power supply company, industry manufacturers, and university research institutes to cooperate in the development of a new open standard for OFDM power line technology. The alliance uses a systematic design process for the PHY layer, starting with meeting the most basic requirements. The next step is to define the physical medium in terms of noise level, noise rhythm, signal attenuation and impedance mode. Industry manufacturers develop new automation products for these purposes and have collaborated many times with power supply companies. This resulted in a large database containing elements such as noise level, noise rhythm, signal attenuation, and impedance patterns, as well as accurate data statistics models for the power grid.
In the second step, they used this model to estimate the different combinations of multiple parameters including the first implementation of OFDM technology, bandwidth allocation, number of subcarriers, subcarrier modulation, and error correction through simulation, and used new equipment on the ground. During the test, the best plan was estimated. After many repetitions and a lot of field tests, they chose the best combination of parameters according to the conditions of the European power grid and the specifications of the power supply company. In addition, the MAC and upper-end communication layer were also developed by an alliance that includes chip suppliers, watch manufacturers, and power supply companies.
After hard work, they developed PHY, MAC and centralized communication layer. The PHY layer sends and receives MPDUs between neighboring nodes. It uses a frequency bandwidth of 47.363 kHz located at the high frequency of the CENELEC A band, with an average transmission rate of 70 kbps and a maximum rate of 120 kbps. Under this condition, the probability of direct communication between nodes in the network is 92%. At other times, routing can ensure 100% connection success.
The MAC layer provides core MAC functions such as system access, bandwidth allocation, connection creation / maintenance, and topology resolution.
The service-specific concentration layer (CL) can classify information transmission and associate it with a suitable MAC connection. It can measure any data transmission that may be included in the MAC SDU, and it can also have payload header compression. At the same time, multiple sub-layers are used to implement various data transmissions in MAC SUD.
In the basic FSK or BPSK scheme, information is transmitted on a single carrier. The baud rate of transmission depends on the size of the bandwidth, and noise and selective attenuation will limit communication. In the OFDM scheme, information is transmitted through multiple subcarriers. The baud rate of transmission depends on the bandwidth and the complexity of DBPSK, DQPSK or D8PSK subcarrier modulation. By using multiple sub-carriers, coding and error correction, the noise and selective attenuation in communication are better eliminated.
The size of the symbol is determined by the sampling frequency and the number of subcarriers. The larger the sign, the more reliably the impulse noise can be suppressed. Coding improves stability, but also increases complexity and power consumption. The more subcarriers, the higher the communication stability, but it does not mean that the baud rate is higher.
The G3 technology uses 36 subcarriers, a classification symbol of 0.735 ms, a sequence of 6.79 ms, and a beginning of 9.5 ms. It requires repetition and RS error correction to improve communication stability.
PRIME uses 97 sub-carriers, a long symbol of 2.24ms, a sequence of 2ms and a beginning of 4.48. In order to avoid the complexity of the repetition method and RS error correction, it uses a symbol with 3 times higher energy efficiency to improve communication stability. This is a solution that provides stability but at a lower cost.
In short, the traditional power grid is developing towards a smart grid that requires more advanced communication capabilities. PLC technology is a more convenient technology for realizing necessary functions and stability. PLC technology is also changing towards the OFDM scheme, while G3 and PRIME are the two main schemes.
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