Measuring current transformers. Production.
► application, operation of measuring current transformers
К Instrument current transformers have high accuracy requirements. Often, a current transformer is performed with two or more groups of secondary windings: one is used to connect protection devices, the other, more accurate, to connect metering and measuring devices (for example, electric meters).
The secondary windings of the current transformer are necessarily loaded. If the secondary windings are not loaded, a high voltage arises on them, sufficient for the breakdown of the transformer insulation, which leads to the failure of the transformer, and also poses a threat to the life of the operating personnel. In addition, due to increasing losses in the core, the magnetic circuit of the transformer begins to overheat, which can also lead to damage (or at least wear) of the insulation and its further breakdown. For these reasons, during the operation of the current transformer, its secondary winding cannot be kept open.
The normal operating mode of the measuring current transformer is the short-circuit mode of its secondary circuit (for example, for a current transformer with a rated secondary load power S2н = 5 VA and a rated secondary current I2n = 5A, the maximum external load in the secondary circuit should not exceed the rated one: Z2max & lt; Z2n = S2n / I2n2 = 5/52 = 0.2 Ohm). The maximum load of the secondary circuit Z2max is equal to the sum of the resistances of the wires Z2pr (in the short circuit mode, the resistance of the wires cannot be neglected) and the resistance Z2IP of the serial circuits of the measuring instruments connected to the current transformer: Z2max = Z2pr + Z2IP. In this mode, an induced current I2 passes through the secondary circuit of the current transformer, which, with its magnetomotive force, creates in the magnetic circuit a secondary flux of magnetic induction Ф2, directed according to the law of electromagnetic induction opposite to the flux of magnetic induction Ф1 generated by the magnetomotive current of the primary circuit I1 (Figure 6.1). As a result, a relatively weak total nominal flux of magnetic induction F0 = F1-F2 is established in the core in a stationary mode (it is 2-3% of F1), which induces a small EMF in the secondary winding (no more than 1 V), which maintains the current in the secondary circuit in the range (0-100)% of the rated current I2n proportional to the value of the primary circuit current I1 = (1-100)% I1n. The primary circuit current does not depend on the load of the secondary circuit and can vary from zero to nominal, and in the event of a short circuit in the primary circuit (Z1 = 0), exceed the nominal by tens of times. In this case, the safety of the secondary circuits and their loads is ensured by entering the core of the current transformer into saturation - while the permissible overload is determined by the safety factor of the current transformer, which is usually equal to 3..5.
If the secondary circuit of the current transformer is opened (emergency mode), then the disappearance of the secondary current I2 and the magnetic flux Ф2 created by it will lead to a significant increase in the magnetic flux Ф0 = Ф1 from the magnetomotive current of the primary circuit and, accordingly, an increase in the EMF in the secondary winding (up to several kilovolts ), which can cause insulation breakdown and danger of electric shock for the service personnel. In addition, with a large magnetic flux, significantly different from the nominal, the core losses increase sharply, the transformer begins to vibrate (hum) and heat up, which is, in particular, one of the reasons for the early aging of its magnetic circuit. Therefore, during operation, the secondary circuit of the current transformer must not be ruptured if the subscriber has a load (Z1 <> 0), and if it is necessary to replace the meter connected to the transformer, the secondary winding of the transformer must be must be short-circuited (modern current transformers contain paired terminals for this in the secondary circuit).
From the theory of operation of a current transformer, it follows that its errors (current error, or the error of the actual transformation ratio, and the angular error - the phase difference between the currents of the primary and secondary circuits) are determined by two factors: limited magnetic permeability? magnetic circuit and the final, non-zero value of the secondary load. If magnetic permeability? core would be infinite (which would mean that its reluctance is zero), or the secondary load is zero (full short circuit mode), then the errors would be zero. In practice, both conditions are not met.
At the same time, the lower the magnetic resistance of the magnetic circuit, the lower the errors of the current transformer, i.e. the greater the magnetic permeability of the material, the larger the cross-section of the core and the shorter its length, and also the less its secondary load. It is important to take into account that the magnetic permeability of a ferromagnetic material depends on the strength of the magnetic field (depending on its value, we can talk about weak, medium and strong fields), and the graph of this dependence has a bell-shaped form: with a small value of n in low fields, the maximum value max in medium fields and again the minimum value in strong fields. Since current transformers operate in a steady state in low fields, it is essential for them to use a material not only with a high maximum magnetic permeability, but also with a high initial magnetic permeability.
These qualities are fully ensured by nanocrystalline alloys. It is the high initial magnetic permeability, the linearity of the magnetization characteristics and the narrow hysteresis loop that explains the stability of the metrological characteristics of the measuring current transformer with magnetic circuits made of nanocrystalline alloys to direct current magnetization: complete magnetization reversal of the core when the alternating current is applied occurs in them already at low magnetic field strength and primary current values 1-2% I1n. For cores made of electrical steel, this is difficult to achieve even by increasing the cross-section of the magnetic conductor. In general, nanocrystalline cores are characterized by lower material consumption, dimensions and weight compared to cores made of electrical steel for current transformers of a similar nomenclature.
Measuring current transformers.