Approximately 900 German distribution network operators (DNOs) use about 44 million meters for the measurement of electrical energy in residential customer installations. 80% are designed as three-phase meters 3x230/400V and maximum current of 40/60 amps. In addition there are three-phase meters with 100 amps and single-phase meters with 40/60 amps maximum current. Customer installations with an annual consumption greater than 100,000 kWh are equipped with load profile meters (1/4-h-load profile active and reactive energy) and daily remote metering takes place via PSTN or GSM communication.
Meters for electricity, gas and water must be type approved by the German certified body PTB (Physikalisch Technische Bundesanstalt). Before meters are brought into service, they must be calibrated in one of the 140 meter calibration centres. (DNOs and meter producers are allowed to run meter calibration centres under special legal conditions). After being calibrated, induction-type meters may stay in service for 16 years, and electronic meters for eight years. If it can be proved by a legal sample test that the quality of a meter batch (same meter type, same year of calibration) matches the requirements, the whole meter batch may stay in service for another five years. The length of time that meters are in service is not, therefore, decided by the number of years of operation; it depends on the quality of the meters, which is measured at regular intervals.
Fig. 1 - Three-phase residential meter (new concept)
INDUCTION METERS ARE FAVOURED
At first it was assumed that the suppliers in a liberalised market would distinguish their products with new meter implementations like multi-tariff or communication interface to the customer, but this did not happen. The market requirements are to keep costs to a minimum, measure accurately, read the meter once a year, and forward the meter reading information promptly.
More and more meters will be fitted by meter installers and not by the staff of the DNO, in order to reduce costs. The meters are designed as indoor meters and meet the requirements of IEC 60521 (induction-type meters) or IEC 61036 (electronic meters). National standards also exist for meter cabinets (DIN 43870).
A NEW CONCEPT FOR ELECTRONIC METERS
In addition, we will only be able to establish a technical base for an economic communication platform if we use electronic meters. We therefore intend to build a base product for future development, which links the meter design with the installation design. We expect cost advantages in the product itself, and in the areas of meter test and calibration, logistics chain, installation and meter reading.
The main reasons for starting the project were:
- The realisation that there is no further development of induction-type meters.
- The fact that there is no production of induction-type meters in Germany.
- The undertainty about the requirements of a liberalised electricity market, with additional requirements for meters to satisfy.
- The need to be prepared for automatic meter reading for residential customers.
The essential requirements of this development were:
- The requirements of the liberalised market must be fulfilled.
- The requirements of the European measuring instruments directive (MID) and the IEC standards must be fulfilled.
- Meter installation should be possible without the need for special tools.
- There must be no break in supply during installation.
- The space taken up by the meter in the meter cabinet should be reduced.
- Calibration must be done by using the interface (infra red).
The working group liaised closely with meter manufacturers, and the concept was developed step by step. A specification sheet has been compiled, and a pdf file of the German version can be downloaded at www.emsycon.de. Several manufacturers have indicated an interest in producing these meters, including Iskraemeco (Slovenia); EMH Elektrizitätzähler (Germany); Landis+Gyr (Switzerland); Enermet (Finland) and Circutor (Spain). Iskraemeco and Hager and Geyer (both of Germany) are ready to manufacture the mounting and contacting unit.
The DNOs responsible for electricity metering expect that these new meters will have a minimum lifetime of 18 years. This is in accordance with the law in Germany today, which stipulates a calibration period of eight years and the possibility of prolonging meter life for a further ten years by way of legal sample tests.
The meter has an LCD with six digits registering kWh. The physical dimensions are 135 x 90 x 80 mm (see Figure 1). This makes it possible to reduce the space required in the meter cabinet. In future we will be able to install two electronic residential meters in the same space as is used for one induction-type meter today.
The new meter will not have a terminal block, but there are seven blade contacts at the back (see Figure 2). The meter clings to the holding fixture (mounting and contacting unit) with four claws, and stability is achieved by plugging the meter into the holding fixture and moving it downwards. The blade contacts are designed in such a way that the downward movement is possible without exertion. The meter has a locking device which can be sealed, so that it is impossible to remove the meter without damaging the seal.
The design of the meter and the holding fixture has focused on safety, and specialists will not be needed to install them.
The meter has an interface (IEC 62056-21, Mode D). At regular intervals the meter transmits information about the register and additional data (meter number, status and so on) which allows a meter reading by mobile data units.
Special holding fixtures have been developed in today’s installations (see Figure 3) with the base dimensions of an induction-type three-phase meter. The variable concept of the holding fixture makes it possible to use the electronic residential meters in different contact alignments.
In autumn 2004 we will have available electronic residential meters (with a type approval by the PTB) and the dedicated holding fixtures. About 20 DNOs will start a field test with 25,000 of the new meters, to evaluate the impact on all the areas of metering mentioned above.
In the meantime the further development of the specification has been launched. Additional requirements (double tariff, communication, etc.) will be included in the next version (Version 2.0) of the paper. We intend to finish the work and publish version 2.0 of the ‘VDN-specification for electronic residential meters’ at the end of March 2005.