IT Focus – Communication innovation

The market for Power Line Carrier (PLC) systems is growing worldwide, with utilities requiring systems with greater functionality and digital capabilities. A new PLC system from Siemens fits the bill.

Siàƒ¢n Green, Managing Editor

Power Line Carrier (PLC) systems have become an essential and integral part of electricity transmission networks in many countries. Whether digital or analogue, they provide the utility with an effective means of supervising and protecting the network.

The worldwide market for PLC systems is estimated to be worth g150 million, with approximately 7000 PLC systems installed per year. Almost all countries use PLC systems to carry out three types of service: speech transmission, data transmission, and protection signalling. Today, emerging nations are the main investors in PLC technology, with countries in South America and the Middle East topping the list. In industrialized countries, PLC systems are being renewed, although the replacement cycle is long in comparison to other communication technologies at about 15 years.

A key trend in the current market is the replacement of analog PLC systems with digital PLC systems. This has been driven by the need to connect PLC systems to digital networks, a lack of capacity in the frequency range and the availability of new technologies such as digital signal processing. In addition, while only the highest voltage levels have traditionally been automated, the trend now is to build communication technologies for lower voltage levels to allow utilities to automate bigger parts of their energy networks.

With these trends in mind, Siemens Power Transmission and Distribution is developing a new generation of PLC technology. The new system ” named PowerLink ” is currently undergoing testing and will be made available in June 2004. PowerLink is a digital product but is able to accommodate both digital and analog interfaces. Siemens believes that it is therefore suited to the utility communication environments of today, and is also able to meet the needs of the future. In addition, PowerLink will be the first commercially available PLC system with video surveillance capabilities.

The PowerLink system can be used to transmit protection signals, voice, faxes, data and video surveillance in a wide range of network configurations, and with both analog and digital interfaces
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Latest technology

PowerLink has been developed on the basis of the latest communication standards for power networks and operates with a transfer rate of 76.8 kbits/s at a bandwidth of 8 kHz. It is able to transmit up to three uncompressed analog voice channels and up to three independent protection signalling systems (each with up to four independent commands). PowerLink can be integrated into other communication systems that operate with fibre optic or satellite transmission technologies.

The availability of both analog and digital interfaces in PowerLink is advantageous for utilities, especially those whose communications networks are only part-digital. No new hardware is needed to upgrade PowerLink from analog to digital mode.

One of the key advantages that PowerLink has over other PLC systems is its “multi-service” capability, i.e. the ability to transmit both analog and digital services in parallel. This allows users to maximize the number of services in the available frequency band, and is a key advantage to utilities that have little frequency space available. According to Siemens, PowerLink uses the frequency space so efficiently that typically four traditional PLC systems can be replaced with a single PowerLink unit.

The PowerLink system uses three voice channels without voice compression instead of just two as found on conventional PLC systems. An intelligent subchannel allocation scheme ” known as Optimized Subchannel Allocation (OSA) and for which Siemens has filed a patent ” makes this added capability possible. Using multiplexed voice compression, the amount of channels increases (up to 12). For example, the use of voice compression enables the transmission of up to ten voice channels and additional data channels.

PowerLink contains a number of other new components and features for which Siemens has filed patents. For example, data-driven frequency control (DDFC) is used to optimize the quality of the digital receiver and provide the best possible performance when it comes to digital PLC transmission. With the system, the datapump can be adapted to individual needs by selecting from one of three different modes. The optimized mode adapts precisely to transmission path characteristics and permits higher bit rates than in the fast mode. The fast mode is used when absolute minimum synchronization time is essential, e.g. for Scada applications. The dynamic mode incorporates all the advantages of the optimized mode and it also automatically adjusts the bit rate to changing power line characteristics over time. This mode allows high bit rates under fair weather conditions while it enables the transmission of the most important channels even during adverse weather conditions.

In the past, contiguous band mode required high frequency hybrid tuning to minimize the transmitter’s influence at the receiver’s input. The automatic crosstalk canceller (AXC) replaces the tuning unit and improves transmission quality. Invented by Siemens, AXC automatically subtracts the interfering transmit signal from the receive signal. As a result, time-consuming hybrid tuning becomes unnecessary and transmission quality increases.

The PowerLink system makes it possible to plan beyond the limitations imposed by single- or double-channel terminals, according to Siemens. The system gives the flexibility to configure various services within the available bandwidth, taking frequency planning to another level. Naturally, the operator can use the system in the 2.5 and 4 kHz frequency grid. But even more importantly, when the operating company’s available frequency spectrum reaches its limits, the user can choose between band-to-band and band spacing for all bandwidths (2, 2.5, 3.75, 4, 5, 7.5 and 8 kHz). This makes the frequency planning much easier, and more services than ever before can be transmitted.

Securing systems

The ability to transmit video data will help utility companies to monitor and protect unmanned sites such as transformer substations. Two parallel developments made this new application possible: the ability to transmit increased data volumes in comparison with current PLC systems, and improvements in image compression algorithms.

Video signals can be transmitted to a local control centre or to an external security company. The camera system can be configured so that the video signal is only transmitted when movement is detected, reducing the bit rate needed. In addition, the PLC system can initiate direct action when a security breach is detected ” for example, transmitting speech to a loudspeaker at the site in question.

A key function of PLC systems is protection signalling. PowerLink’s protection signalling system is used to transmit protection signals over high-voltage lines and via digital fibre-optic networks. Up to two independent protection signalling systems can be integrated into one PowerLink system, and represents a more cost-effective solution compared with the installation of a stand-alone protection signalling system.

The optimized subchannel allocation (OSA) enables 50 per cent more transmission capacity. Both examples shown are not possible with traditional double-channel PLC systems
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The Siemens product portfolio is unique. For example, the same, advanced stand-alone version of the protection signalling system SWT 3000 can be integrated into the PowerLink system. Thus, familiarity with one system covers an expansion into others and spare parts are reduced to a minimum, thereby saving both time and money.

Siemens developed impulse noise compression technology (INC) to ensure that the system does not misinterpret impulse noise, the most severe source of error for teleprotection, as a command and accidentally actuate protection relays. In case that the protection signalling system is used as a separate and external accessory, one can establish a fibre-optic connection to the PowerLink system. This arrangement enables a high immunity against internal substation interference.

Protecting networks

The protection signalling system features a unique variety of operating modes. The single-purpose mode provides great transmission ranges and high security, while transmitting protection signals only. The single-purpose 2 kHz bandwidth is designed especially for dense frequency schemes. This means that transmit and receive channels are allocated within a 4 kHz band. The simultaneous multi-purpose mode transmits voice and data simultaneously to the protection signal. By using the alternate multipurpose mode the transmission capacity is completely used for voice and data, provided that no protection is required. The pilot of the PowerLink system is used as the guard tone. If it is necessary to transmit a protection command, voice and possibly also data transmission (depending on parameterization) are interrupted during the brief period required for the command to be sent.

Siemens has also designed the PowerLink system to be based strongly on software rather than hardware. A reduced number of hardware units means that utilities will be required to stock fewer components, and also gives them flexibility in terms of upgrades.

Siemens has developed an innovative upgrade concept called “ease-up” (easy and secure upgrade) for PowerLink. The concept allows upgrades to PowerLink to be performed quickly and without the need for additional hardware.

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