Evaluation of the Oscillating Power in Asymmetrical Non-Sinusoidal Modes of Electric Networks

Abstract

The possibility of detection and evaluation of the influence of distortion sources (asymmetry and higher harmonics) on the quality of electricity of three-phase networks using instantaneous conditional capacities of the reverse and zero sequences is analyzed. Such sources are nonlinear consumers, whose share in total load increases year by year. Nonlinear consumers consume active power at the main frequency and at the same time generate power to the higher harmonics in the network. Significantly in low voltage networks, there is also a share of consumers with asymmetric load of phases. On the basis of the theory of instantaneous power, instantaneous values are allocated on the basis of which informative values can be formed that allow characterizing the influence of nonlinear and asymmetric consumers on deterioration of the quality of electricity, as well as assessing the inefficiency of the distribution network of the electricity supplying organization.

To evaluate the impact of nonlinear and asymmetric consumers, it is expedient to use instantaneous conditional capacities  and , which are essentially the product of instantaneous direct current voltage on the current of the reverse and zero sequence, and to assess the fault of the electricity supply organization, which consists in the ineffective construction of the distribution network — instant conditional capacities  and , which is essentially a product of instantaneous current of direct sequence on the back and zero sequence voltage. The formation of these values is carried out using the system of α, β, 0-coordinates, which greatly simplifies the construction of measuring devices.

In the presence of harmonic distortions, the RMS values of conditional capacities  and  grow and make up from one to ten percent relatively to the main power generated by the voltage and current of the main frequency of the direct sequence. In the case of load asymmetry, the average values of these quantities increase. At the same time an increase  and testifies to the lack of power of power transformers and the inappropriate use of transformers with the scheme of winding connection "star / star with zero sequence" for powering nonlinear consumers.