New materials increase performance of transformers for watt-hour meters
The trend towards system-capable electronic watt-hour metering demands new, high precision but low cost transformers. Due to the excellent properties of rapid solidified soft magnetic materials paired with optimum designs, current transformers offer noticeable advantages. Thus the use of this innovative development is recommended, particularly for the present technological change from mechanical Ferraris meters to electronic watt-hour meters.
One of the key components in multiphase meters and sometimes also in single phase meters is the current transducer. It ensures electrical insulation from line potential if needed and supplies an accurate measuring variable (signal voltage) for the primary current. 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 1036 for directly connected and IEC 687 for indirectly connected meters, and in the Anglo-American market in 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, of which 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. These features lead on the whole to attractive prices.
The properties of the toroidal core 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 d 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 theory, 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 using these relationships. The respective materials for the three fields of application mentioned are subject to different requirements.
Meters in accordance with IEC 687 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. Today rapid solidification technology on an industrial scale makes amorphous and nanocrystalline alloys available with noticeable advantages.
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 – typically 22 µm – and the very low eddy current losses in the magnetic core achieve a very small amplitude error. The specific properties of the amorphous and nanocrystalline 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 1). In this example the transformer is designed for indirectly connected industrial meters according to IEC 687 with a primary current of 6 A at 50 Hz.
Notable 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 that are usually used, a complex compensation assembly is unavoidable and the cost factor should not be underestimated.
Meters in accordance with IEC 1036
These must be insensitive to certain amounts of direct current components, for example 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 the engineer’s choice for this application in the starting phase. Today magnetic cores made of the linear but nevertheless highly excitable amorphous alloys have overcome this limitation. Engineers therefore have to include them in their design tool kits (see Table 1 for an overview). 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 in the part-per-million range as well as to an extremely low and linear temperature dependence. Due to the low permeability, a phase error of approximately 4° to 5° occurs which is easy to compensate on account of its high constancy of typically ± 0.05°. The compensation can be made digitally by appropriate correction in the microprocessor and in analogue by an RC low pass in front of the input of the A/D converter. As example, the behaviour of a transformer for 60 A for direct connection according to IEC 1036 is shown in Figure 2.
The structural properties of rapid solidified soft magnetic alloys have been shown to lend magnetic cores for current transformers excellent properties, even in the case of IEC 61036 recommendations. Because of these advantages, the use of this innovative development of the proven functional principle of the current transformer is highly advantageous, particularly with the present technological changes from mechanical Ferraris meters to electronic energy meters.