SU957130A1 - Device for revealing three-phase current system small asymmetry - Google Patents

Description

(54) DEVICE FOR DETECTING SMALL NONSYMMETRY OF THREE-PHASE CURRENT SYSTEM

one

The invention relates to electrical engineering and can be used to measure the unbalance of a three-phase system of currents and for relay protection of electrical installations in violation of the symmetry of phase currents.

A device for monitoring the symmetry of a three-phase system of currents and voltages is known, comprising intermediate transformers, a three-phase rectifier and a reacting element 1.

However, the device does not allow the control, Q to block small current imbalances.

The closest in technical essence to the invention is a device for controlling the symmetry of a three-phase system of currents, containing a reacting organ and three single-phase saturated transformers of a torus, the primary windings of which are connected in a star, and the secondary windings in an open triangle to which a single band filter is connected of the harmonic components of the output voltage of an open triangle 2.

A disadvantage of the known device is the relatively small change in the output signal with a small asymmetry of the measured three-phase system. This disadvantage does not allow accurate measurements of small asymmetries due to the limited sensitivity of the reacting organ.

The aim of the invention is to improve the measurement accuracy.

The goal is achieved by the fact that the device for detecting small asymmetry of a three-phase current system contains three single-phase transformers, a first harmonic filter and a reacting organ, the primary windings of the transformers are connected in a star and are the input terminals of the device, the secondary windings are connected in an open triangle and connected to the filter of the first harmonic, the filter of the fifth harmonic is introduced, two formers of the module and the adder, the input of the filter of the fifth harmonic is connected to the input of the filter of the first harmonics and, the outputs of the filters of the first and fifth harmonics through the shapers of the modules are connected to the inputs of the adder, the output of which is connected to the reacting body.

Claims (2)

The drawing shows a block diagram of the device. The device contains three single-phase transformers 1, filters of the first 2 and fifth of the 3 harmonics, shapers 4 and 5 of the module, adder 6 and reacting organ 7. In the normal mode of operation of the electrical installation with the symmetry of the three-phase system of currents in the secondary voltage group of saturated transformers contain only voltage the third harmonic component (most) and its multiples. This voltage is not passed by the filters, and there is no signal at the input of the reacting organ. In the asymmetry mode of the three-phase current system, the output voltage of the transformer group contains the entire spectrum of odd harmonic components, including the first one. The filters select the first and fifth harmonic components of the output voltage, respectively, and after the module formers, the selected components are summed up. The reacting organ measures this amount and records the asymmetry of the currents. With a small asymmetry (deviation from the normal mode of the phases of currents by 2-3 °), the voltage of high harmonics at the output of a group of transformers exceeds the voltage of the first harmonic. It is impractical to use the third harmonic component of the voltage, since it exists at the output in a symmetric mode. Of the higher harmonic components, it is most appropriate to use the fifth. Its magnitude and rate of increase are greatest with small asymmetry. The use of harmonic components of a higher order (seventh, eleventh, etc.) gives a smaller effect, since the amplitude of the harmonic component decreases with an increase in its number. Allocation of one higher harmonic component is also not effective. With certain asymmetries, the voltage of the higher harmonic component drops sharply, as the higher harmonic components contained in each phase voltage of the saturable transformers begin to be subtracted. Thus, the accuracy of the measurement of small incompletion is increased by measuring the sum of the first and fifth higher harmonic components. The sum of the first and fifth harmonic components is steeper than each: from the individual components, and therefore has a large signal increment on the reacting organ. The proposed highly sensitive devices to the small asymmetry of the three-phase system of currents are necessary, for example, in relay s; inspect motors for detecting coil short circuits in the stator winding. With the closure of a small number of turns (2-3 turns), the three-phase system of currents changes slightly, and it is almost impossible to detect this asymmetry by known methods. The invention is a device for detecting small asymmetry of a three-phase current system containing three single-phase transformers, a first harmonic filter and a reacting organ, the primary windings of the transformers are connected in a star and are the input terminals of the device, the secondary windings are connected to an open triangle and connected to a first harmonic filter, characterized in that, in order to increase the measurement accuracy, a fifth harmonic filter, two modulator and an adder are introduced into it, the input of the fifth harmonic filter CQI: a first input coupled to garmsniki filter filters the outputs of the first and fifth harmonics conditioners through modules are connected to inputs of an adder whose output is connected with the reactant body. Sources of information taken into account in the examination 1.Avtsfskoe certificate of the USSR No. 565261, cl. G 01 R 29/16, 07.15.77. 2.Lisotsky N.V., Khuduguev V.I. and Kulaga Yu.K. Relay of equal sensitivity to various types of damage. - Lime Tomsk Polytechnic Institute. Tomsk, 19 € .9, t. 179, p. 85-89.