METHODS OF REDUCTION VOLTAGE UNSYMMETRIES IN ELECTRICAL NETWORKS 0.4–10 KV

ABSTRACT

The article describes the processes associated with the appearance of asymmetry of the three-phase voltage system in electrical networks, and explains the reasons why this phenomenon is undesirable. The reasons for the occurrence of voltage asymmetry and their impact on the operation of electrical equipment and on the energy system as a whole are considered. The issues of reducing the negative sequence voltage asymmetry coefficient are considered and basic recommendations are given for improving the quality of electricity to comply with the requirements of GOST 32144-2013. In conclusion, it was noted that at the moment the problem of voltage asymmetry requires an immediate solution, since electrical networks of 0.4–10 kV are characterized by significant voltage asymmetry.

Keywords: quality of electrical energy, asymmetry coefficient voltage, asymmetry, electricity losses, balancing device, low voltage electrical network.

One of the reasons for the deterioration in the quality of electrical energy is voltage asymmetry in the electrical network. If the three-phase voltage has the same magnitude and is in a phase displacement equal to 120°, then the three-phase voltage is called symmetrical. When the symmetry of a multiphase system is broken, the latter can be decomposed into three symmetrical components - a positive sequence system and the negative and zero sequence systems superimposed on it. Typically, three-phase voltage balance is the ideal situation for a power system [1]. However, single-phase loads, unbalanced three-phase equipment and devices, poor connections to electrical connectors, and many other factors cause voltage imbalances in the power system and reduce power quality. Thus, voltage asymmetry is one of the pressing problems of electrical power systems. In addition to the above, the cause of voltage asymmetry can be emergency situations in networks, such as asymmetrical short circuits or phase breaks. Let's consider the main consequences of voltage asymmetry.

1. Additional power losses.

2. Abnormal engine operation.

3. Reduced equipment life cycle.

4. Impact on relay operation.

5. Inaccurate measurements.

6. Impact on the operation of the transformer.

7. Increased voltage asymmetry when the neutral wire is damaged.

Overall, the consequences of voltage imbalances in power systems are wide-ranging and serious. Voltage imbalances can significantly shorten the life cycle of equipment, speed up replacement, and increase system operation and maintenance costs.

To eliminate non-random voltage asymmetry in the power system, it is necessary to develop a scheme for uniform connection of loads at the early stages of design. In this case, it is necessary to take into account their capacities and work schedules. Uniform phase-by-phase distribution of loads in low-voltage networks guarantees a reduction in direct and negative sequence voltage asymmetry coefficients in electrical networks by 20%. In accordance with GOST 32144-13, the asymmetry of a three-phase voltage system is assessed by such indicators of power quality (QE) as the negative sequence asymmetry coefficient K2U, % and zero sequence K0U, %. These coefficients are normalized quality indicators. EC for asymmetry coefficients is considered to comply with the requirements of this standard if the coefficient values averaged over a 10-minute interval do not exceed 2% for 95% of the time in one week and do not go beyond 4% for 100% of this interval.

There are several measures to solve these problems.

1. Increasing the power of the transformer in the system. The work examines the dependence of the negative sequence voltage asymmetry coefficients for high and low voltage on the power of the power transformer. During the experiment, as the transformer power decreases at low voltage, the negative sequence voltage asymmetry coefficient increases, and at high voltage it decreases. Moreover, these dependencies are observed at different powers of the voltage source; the only difference is that at a higher power of the source, the dependencies for both high and low voltages begin with lower values of the negative sequence voltage asymmetry coefficient.

An increase in the negative sequence voltage asymmetry coefficient in a low-voltage network with a decrease in transformer power can be explained by the fact that the load power increases relative to the transformer power and the transformer power reserve decreases. In this case, with an increase in the power of the power source, the degree of increase in the values of the negative sequence voltage asymmetry coefficient increases with a decrease in the power of the power transformer.

A decrease in the negative sequence voltage asymmetry coefficient in a 10 kV voltage network with a decrease in transformer power can be explained by the fact that the power of the voltage source relative to the power of the transformer increases and, accordingly, the power reserve of the power source increases.

At the same time, with an increase in the power of the power source, the degree of decrease in the values of the negative sequence voltage asymmetry coefficient decreases when the power of the power transformer decreases.

2. Use of interphase variable resistances. One way to reduce voltage asymmetry is to equalize the loads across phases. The technical solution of this method is the introduction of additional interphase variable resistance. The identified dependencies in the work make it possible to determine the direction and order of change in resistance values. In this case, it becomes known how the values of linear voltages will change, which will make it possible to find the right direction to reduce the voltage asymmetry coefficient in the negative sequence. By varying these dependencies, it is possible to select the final values of interphase resistances at which the value of the negative sequence voltage asymmetry coefficient will be within the limits standardized by GOST 32144-3013.

3. Use of a balun. Another technical solution for balancing loads across phases is the introduction of a transformer-type balun. It is realized by changing the capacitance value of the capacitor or the inductance of the coil.

Based on the above, it should be noted that voltage asymmetry negatively affects the operation of motors, transformers, capacitor banks, rectifiers and power lines, causing additional energy losses and creating safety problems for the power system [4].

The issue of solving the scientific and technical problem of determining the voltage asymmetry coefficient by negative sequence has not been resolved due to the lack of an appropriate standard. Therefore, there is a need to develop an algorithm for its determination, which will allow timely detection of this interference and the likelihood of its occurrence in electrical networks of any voltage level and thereby assess the level of danger from this interference. This algorithm will make it possible to take a step forward towards suppressing voltage asymmetry in the negative sequence, which is relevant today.

References:

1. GOST 32144-13. Electric Energy. Electromagnetic compatibility of technical equipment. Standards for the quality of electrical energy in general-purpose power supply systems. Enter. 2014–07. M.: Standartinform, 2014. 16 p.
2. I.I. Kartashev, V.N. Tula. Power quality management / R.G. Shamonov. Publishing house MPEI, 2008.
3. Rudi D. Yu., Tkachuk N. A. Negative influence of asymmetry and methods for their elimination in the power supply system // Theory and practice of modern science: collection. scientific tr. based on materials of the XX International. scientific-practical conf. M.: Olimp, 2017. pp. 87–91.
4. Sabirzyanova, A. S. Digital substation as a tool for increasing reliability of power supply / A. S. Sabirzyanova, G. Z. Gilyazieva // Энергетика и энергосбережение: теория и практика : СБОРНИК МАТЕРИАЛОВ VII МЕЖДУНАРОДНОЙ НАУЧНО-ПРАКТИЧЕСКОЙ КОНФЕРЕНЦИИ, Кемерово, 07–09 декабря 2022 года. – Кемерово: Кузбасский государственный технический университет имени Т.Ф. Горбачева, 2023. – P. 339-1-339-4. – EDN DZRZQH.