Metering communications and services using the low voltage distribution network

The last twenty years has seen an increase in power communications technology, as utilities view the process as a means to provide better services to customers, both industrial and residential. Improving access to customer information, and providing customers with information, can be accomplished if economic and reliable communications are developed.

Considerable work is being carried out in different parts of the world to achieve these aims, particularly in the field of communications using the power distribution network as the medium. Applications such as distribution automation, load control and remote metering have been proposed and tested. Slow and medium speed data communications systems are being developed for medium voltage, mixed underground and overhead networks for the purpose of remote switch control.

These developments will allow the deferment of primary substation reinforcement and expenditure on networks. Improvements in supply availability and reduced operating costs will also result. More functions can be performed using the network as a higher speed data communication link, such as remote meter reading, credit control, protection signalling. This will require at least an order of magnitude increase in data rate for reliable and efficient transmission.

Hybrid systems

The development of hybrid systems that combine radio communication with power line carrier between the "pole top" radio transceivers and the customer meter, has meant that this area has been of particular interest recently. However, only proprietary solutions rather than open system standards for PLC are being produced by developers for these systems.

Communications for use with metering systems allow data to be sent or retrieved from meters so as to control loads, change tariff rates, read meters and provide information for customers. Current metering communication systems use telephone wires, cable TV facilities, radio communications or PLC techniques. These developments offer advantages to utilities and customers in the form of ever decreasing costs, in line with costs of existing metering systems.

Efforts are going on in the rest of the world in particular the USA, Europe and Japan to develop standards and systems for PLC communications into buildings. In the USA this has been through sponsorship by the Electric Industry Association (EIA) as part of the CEBUS Standards Association for home automation and entertainment.

The IEEE Automatic Meter Reading Association (AMRA) committee SCC31 is investigating a range of communication methods, including the CEBUS power line carrier, as an option in the distribution system. EIA has asked AMRA to continue to maintain the utility context definitions for CEBUS use in distribution automation and automatic metering. Vendors are committing themselves to CEBUS and hybrid radio/PLC systems.

In Europe, CENELEC, through committees TC105 and S/C105A (respectively Home and Building Electronic Systems (HBES) and Mains Communication Systems) has established a number of working groups to investigate and develop standards for protocols, data integrity and interfaces for PLC systems, together with investigations for filters and equipment impedance for devices connected to the mains.

PLC systems have thus attracted interest from large generators of electricity power down to the individual home owner. This has caused concern, since there has been no unified approach to the design of the common functionality of electrical impedance and interfaces with the associated protocols to ensure compatibility and non-interference into and inside the building.

Successful application of such techniques depends on the extent to which reliable operation can be achieved and maintained. Matters of safety cannot be neglected not only the possible shock hazard but also the inadvertent switching of appliances by transient or other disturbances. Reliable operation in user terms means guaranteed performance despite the inevitable variations which must occur during any operation period.

Studies have shown that propagation of signals on mains wiring at frequencies up to 1 - 10 MHz is reasonably efficient. Attenuation of only a few dB per kilometre occurs on a cable without any loads connected. While the attenuation increases significantly in actual systems (with no loads in circuit) signals can still be transmitted with reasonable efficiency over relatively short distances. The designer of a mains communication system must ensure that adequate dynamic range is incorporated into the receiver to take account of impedance and attenuation variations.

The European and world scene

Global standardisation of the world's communication networks is developing rapidly. The spearhead for this is through the constant evolution of telecommunications and information technologies. To provide a high degree of integration, the principal standards bodies of the International Telecommunications Union (ITU), the International Electrotechnical Commission (IEC) and the European Economic Commission (EEC) through the standards electrotechnical body CENELEC, are investigating the key parameters for successful communication over different media connecting companies and utilities to their customers. These developments will provide guidance for communications into different buildings.

Deregulation of the utility

Deregulation of the world's utility markets has allowed companies to develop new freedom within the communications services market. In particular, power distribution companies are investigating the low voltage and medium voltage power lines as a carrier for mains signalling and data communications. In this new utility networks environment the traditional electric meter is rapidly becoming a poor asset. Providing two way communications to enhance the asset will help extend services to customers.

The customer interface involves the remote operation and co-ordination of functions that communicate with devices at the individual customer site.

Commercial customers normally consume most power by operating environmental systems such as air conditioning. Other loads such as lighting and security are also prominent. Residential customers are a significant proportion of a utility's customer base, varying between 30% and 90%. The load normally controlled is limited to space heating and water heating. Providing a bridge into commercial and residential buildings would offer an excellent opportunity to provide information, communication and secondary functions within the buildings.

Three services in particular are demand side management (DSM); pricing strategies and customer services. These can all be provided by low voltage distribution networks.

Low voltage distribution networks

Despite being a difficult channel for communication because of signal attenuation due to variation of impedance, low voltage distribution lines have always been considered by electric utilities as the most attractive resource for supporting bi-directional communications for home automation, with consequent reduction in operating and capital expenditure. Compared with other communication media such as coaxial cable, twisted pair and telephone, distribution networks are solely owned by the distribution company. This allows a natural communications platform into buildings, providing a range of services.

Although early development of PLC signalling was limited to low data rates of 1000 bits/sec, recent progress in hard and software systems are driving data rates to 20 000 bits/sec. This will be ex-tended further as more signal processing techniques are applied to conditioning the low voltage power line.

Power line signalling standards

The different components of the line must be taken into account in developing a potential communication channel using the low voltage network. This will involve the low voltage distribution substation

(11kV/230V) with its transformers, switch gear and cables. The different consumer loads interacting with street lighting will limit the degree of transmission. Further low voltage distribution cables will vary considerably according to the type of cable, age and current and voltage specification. Mismatches will exist due to the variation of cable impedance, leading to poor signal transmission.

The physical characteristics of the network are therefore a cardinal parameter for efficient transmission over the low voltage power line.

To address these conditions and provide a sound foundation for potentially efficient LV communications, CENELEC is developing a standard for Europe - EN50065. The standard has identified a number of key parameters to be specified, which will provide the potential for efficient communications between utilities and customers. These include frequency range; modulation systems; signal power; protocols; impedance characteristics and filters; immunity against disturbing signals.

Current progress has shown these are formidable parameters to evaluate and specify for efficient, seamless communication. Progress is, however, most encouraging.

EN50065 specifies the various parameters both qualitatively and quantitatively to provide efficient and seamless interoperable communication into a building and for electrical equipment within the building. Although limited by frequency range, the potential exists for integrating this technology with other media such as coaxial cable, telephone and twisted pair, to provide a fully integrated communications system for a wider range of services.