Electronic watt-hour meters have been replacing the electro-mechanical Ferraris
meters increasingly in industry for some years. Due to their many advantages such as extended functionality with system capability, insusceptibility to mechanical wear, small size and higher achievable accuracy, they are also being used increasingly in private households world-wide. Their areas of application cover a wide range from the local single device through prepayment meters up to (supra-)regionally networked remote control and remote readout systems (example see figure 1).
Figure 1: Block diagram of an electronic watt-hour meter (single-phase)
One of the key components in an electronic watt-hour meter is the current transducer. It supplies an accurate measuring variable (signal voltage) for the primary current to the metering chip and some types ensure electrical insulation
from line potential if needed. It must meet the requirements defined in the various technical standards for the respective device accuracy class. In Europe, these are usually the standards IEC 62053-21 and ...-23 (former IEC 61036 and IEC 61268) for directly connected and IEC 62053-22 (former IEC 60687) for indirectly connected meters, on the Anglo-American market the standards of the ANSI C12.xx series for both connection types.
There are a number of functional principles for implementation of the current transducer. Table 1 shows an comparative overview with respect to the application in meters according to IEC 62053-21 and ...-23 which have to be able to measure not only bipolar sinewave but also unipolar rectified currents with specified accuracy. The shunt resistor is one of the favourite choices because of its very low cost and good linearity but designers have to beware of its disadvantages. Because of the regulations about maximum power consumption (max. 2 W per phase acc. to IEC 62053-21 and ...-23) its resistance is limited to some hundreds of microohms. This low value results in very low voltages (typ. some ten microvolts) at low primary currents. These have to be very carefully filtered and amplified to keep the meter’s specified accuracy in the low current region. Heat dissipation within the meter is another critical point to be considered. In cases of multi-phase meters or single-phase meters with external interface additional galvanic separation has to be provided to prevent hazardous operation or short circuit – conditions between the phases. Mostly optocouplers and separation transformers will be additionally needed increasing the meter’s overall cost. Another favourite principle is the Rogowski coil which does not exhibit saturation effects due to its coreless operation. The disadvantage of this is common to all open magnetic circuits and results in a very interference sensitive operation. Costly shielding has to be provided to keep measurement errors small at low primary currents. The designs using semiconductor hall-effect devices have to be clearly separated: the low cost types can suffer from ageing effects which can deteriorate accuracy in the course of years; stabilized designs will control these effects but at the cost of a complicated compensation circuitry.
The toroidal core current transformer with low burden resistor has several clear application advantages. The closed magnetic circuit makes it less sensitive to interference fields so that normally no additional shields are necessary if the meter has the appropriate design. The purely magnetic functional principle requires no semiconductor components and therefore achieves a high long-term stability with only little additional circuitry expense. The simple assembly with few parts (core with winding, connecting wires and casing) keeps assembly work to a minimum and enables compact designs. Internal power consumption is in the region of milliwatts even at high primary currents and relatively high burden voltages. These features lead on the whole to attractive prices for the meter at high quality and reliability level.
The properties of the toroidal core current transformer such as amplitude and phase error and their linearity as well as the maximum transmissible primary current are basically due to:
the application variables such as operating frequency f and the necessary burden voltage (this gives the burden resistor RB)
* the material properties of the magnetic core used (saturation induction Bsat, permeability µ and loss angle δ as functions of the excitation level)
* the design variables such as number of secondary turns Nsec, resistance of
the secondary winding RCu, core cross section AFe and iron path length lFe.
The equations for the electrical properties of the current transformer can be derived from theoretical electrical engineering whereby some approximations simplify the calculation without noticeably impairing the accuracy of the results:
Optimised solutions adapted quickly and simply to the respective customer requirements can be determined with a newly developed computing program using the relationships shown. The respective materials for the three fields of application mentioned are subject to different requirements:
For meters in accordance with IEC 62053-22 and ANSI C12.xx materials with high permeability and linear characteristics in connection with the comparatively high flux density ranges of the metallic materials as well as only slight changes in property as a function of the temperature are of advantage.Current transformers for these applications usually work with crystalline NiFe alloys. Meanwhile rapid solidification technology on industrial scale of VACUUMSCHMELZE GmbH & Co. KG, Hanau makes new alloys available with particularly noticeable advantages for the customer. The high and almost constant permeabilities over a wide flux density range lead to a very small, easily compensatable phase error. The low strip thickness of the core material of typical 22 µm and the very low eddy current losses in the magnetic core achieve a very small amplitude error. The specific properties of this novel alloys structure lead in turn to very low coercive field strengths and low temperature dependence of the magnetic properties. This causes a temperature dependence of the transformer errors which is basically determined by the linear temperature behaviour of the copper winding. Figure 2 shows the diagram of the transformer errors as functions of the primary current (phase error in degrees and amplitude error in %) for a current transformer various temperatures from –40°C to +85°C as an example. This transformer is designed for indirectly connected industrial meters according to IEC 62053-22 with a maximum primary current of 6 A at 50 Hz.
Figure 2: Diagram of the transformer errors as functions of the primary current for a 6 A current transformer
Remarkable here are the amplitude error which is well below – 0.1% and the linearity of the phase errors, the variation width of which is only 0.03° at room temperature. Due to this material and design-related precision, compensation can be done by very simple means even in applications in high-precision meters. In the NiFe or Ferrite core based transformers or transducers with Hall sensors usually used, a complex compensation assembly is unavoidable and usually a cost factor that is not to be underestimated.
Meters according to IEC 62053-21 and ...-23 must be insensitive to a certain amount of direct current components e.g. from power supply units with primary side diodes (“direct current tolerance”). Conventional current transformers with crystalline NiFe alloys saturate when unipolar alternating currents occur. For this reason current transformers were excluded from engineer’s choice for this application in the starting phase. Nowadays magnetic cores made of the particularly linear and nevertheless highly excitable alloy of VACUUMSCHMELZE GmbH & Co. KG, Hanau have overcome this limitation. So meter designers have to include them in their current transducer selection guide.
These novel alloys lend the current transformer excellent properties. The standard compliant dc tolerance is achieved without an air gap which minimises the sensitivity to interference fields. The excellent soft magnetic properties of the core material lead to a negligible small amplitude error as well as to an extremely low and linear temperature dependence. Due to the low permeability, a phase error of typically 4° to 5° occurs which is easy to compensate on account of its high constancy of typically ï‚± 0.05°. The compensation can be made digitally by appropriate correction in the microprocessor and analogously by an RC low pass in front of the input of the A/D converter. A number of major metering chip providers supply tailored solutions for optimum performance and accuracy in combination with these CT types. The behaviour of a CT for 60 A with direct connection according to IEC 62053-21 and ...-23 is shown in figure 3 as an example of the typical transformer error.
Figure 3: Diagram of the transformer errors as functions of the primary current for a 60 A DC tolerant current transformer
It has shown that the structural properties of rapid solidified soft magnetic alloys lend magnetic cores for current transformers excellent properties even in the case of IEC 62053-21 and ...-23 recommendations. Because of these advantages, the use of this innovative further development of the proven functional principle of the current transformer is highly advantageous particularly for the present technological change from mechanical Ferraris meters to electronic meters.