EVALUATION OF THE APPLICATION OF VARIOUS METHODS AND EQUIPMENT FOR PROTECTION FROM EMERGENCY VOLTAGE IN 6-10 kV ELECTRIC NETWORKS OF OIL PRODUCTION FACILITIES

EVALUATION OF THE APPLICATION OF VARIOUS METHODS AND EQUIPMENT FOR PROTECTION FROM EMERGENCY VOLTAGE IN 6-10 kV ELECTRIC NETWORKS OF OIL PRODUCTION FACILITIES

Piriyeva Najiba Malik

Associate Professor of the Department "Electromechanics" Azerbaijan State University of Oil and Industry,

Azerbaijan, Baku

Rzayeva Sona Vaqif

Head of laboratory Department of Electromechanics, Azerbaijan State University of Oil and Industry,

Azerbaijan, Baku

Mustafazadeh Elnur Matlab,

Master of the Department "Electromechanics" Azerbaijan State University of Oil and Industry,

Azerbaijan, Baku

ABSTRACT

One of the main reasons for the high accident rate and outages in distributed power supply systems of 6–10 kV oilfields is overvoltage due to lightning. The vast majority of lightning outages in distribution networks of 6–10 kV associated with overvoltages are due to the insufficient level of impulse strength of linear insulation, which leads to the overlapping of insulators and, subsequently, to a blackout of oilfield consumers. To ensure the required lightning resistance of a distributed power supply system for oil fields, protective equipment is installed in various ways: surge arresters, arresters of various types. At the same time, protective equipment changes the electrical characteristics of the distribution network, which affects the characteristics of transients during lightning. Therefore, it is necessary to evaluate the effect of the most commonly used protective devices on overvoltages in a distributed power supply system for oil fields under lightning conditions.

Keywords: surge arresters, oil fields, surges, overhead power lines.

Introduction

Currently, in the distributed power supply system of oil and gas producing enterprises, there is a high accident rate, reaching 40% of the total number of outages, due to the impact on electrical equipment of impulse, in particular lightning, overvoltages [1, 2].

In oilfield electrical networks with a voltage of 6(10) kV, to increase lightning resistance, non-linear surge arresters (SA) and arresters are installed in places recommended by regulatory documents. The vast majority of lightning impacts in distribution networks with a voltage of 6 (10) kV falls on the overhead power transmission line (PTL), while the most severe case is a direct lightning strike on the wire [3].

To reduce the levels of lightning surges, in most cases, arresters are installed on power transmission towers, and arresters are installed on the approaches of transformer substations (TS) 35/6(10) and 6(10)/0.4 kV and in their switchgear [4–6]. Failures of electrical equipment that occur during lightning surges lead to overlapping and damage to the insulation of electrical installations and downtime of technological equipment [1, 2, 6], so the task of assessing the impact of various protection methods and devices on the magnitude of surge voltages in the electrical network is relevant. Of scientific and practical interest is the choice of the number and installation location of surge arresters and arresters in electric networks with a voltage of 6(10) kV at oil and gas producing enterprises.

Object and methods of research

As an object of study, a section of a distribution network with an isolated neutral voltage of 6 kV of an oil field was taken.

Figure 1 shows the power supply diagram for two sections of 6 kV busbars of two clusters of oil wells, powered by block packaged switchgears BPS1, two transformer substations 6/0.4 kV connected by overhead power lines to the block packaged transformer substation BPTS 35/6 kV. Non-linear surge arresters are installed on 6 kV busbars. Long-spark arresters are installed on supports of the PS10P 27M type of an overhead power line.

According to the power supply circuit diagram, in order to determine overvoltages along the overhead power line due to the impact of a lightning impulse, a simulation model of a typical section of the power distribution network for oil well clusters of enterprises was developed in the MATLAB Simulink software environment, the diagram of which is shown in Figure 2.

Figure 1. Schematic diagram of oil producing wells power distribution network section

Figure 2. Simulation model of oil producing wells power distribution network section

When developing the simulation model, the following assumptions were made:

1. Section switches BPS1 and 2 are disabled, 6 kV bus sections operate independently, and power is supplied through one 6 kV input of KTPB 35/6 kV.

2. The parameters of the three-phase symmetrical load of transformers TSLT 630 6/0.4 kV are reduced to the voltage of the power supply distribution network with a voltage of 6 kV.

  3. The parameters of overhead transmission lines with a voltage of 6 kV - the type of support, the brand of wire, the amount of wire sag - are the same along the entire length of the section.

The simulation model makes it possible to calculate the time and frequency characteristics of surge overvoltages and currents at arbitrary points of impact of a lightning impulse on a 6 kV power distribution network. In Figure 2, the impact of a lightning impulse falls on a distribution network point located at distances X and Y from the beginning of the outgoing line to BPS1, respectively. Each section of the power distribution network is represented by a simulation model of a three-phase line with distributed parameters.

A simulation model of a lightning impulse has been developed, the scheme of which is shown in Figure 3.

Figure 3. Simulation model of lightning impulse

The procedure for calculating lightning surges on a simulation model is as follows:

1. The parameters of overhead lines are calculated and entered into blocks of 6 kV overhead lines: fundamental frequency, linear resistances, inductances, capacitances of direct and zero sequences, the length of the overhead line (or section of the overhead line) depending on the type of support, brand of wire, wire sag, resistivity soil.

2. The arrester parameters are accepted according to the passport data: coefficients of approximation of the current-voltage characteristic I(U)=I_pr(U/U_pr)^a, where U_pr is the protective voltage, I_pr is the reference current.

3. By means of Linear Analysis Tools, the amplitude and phase-frequency characteristics (Bode Diagram) of impulse overvoltages of the investigated section of the overhead power transmission line of the distribution network are constructed, limited by the input Input Perturbation and output Output Measurements by model points.

4. The calculated time functions of stresses are stored by the module for outputting information to a file in the form of a data array.

The simulation results are shown in fig. 4–7.

Figure 4. Oscillograms of: a) lightning impulse; b) phase-to-phase voltage at the input block switchgear № 1 6/0,4 kV without protection devices

Figure 5. Phase-to-phase voltage oscillograms at the input block switchgear № 1 6/0,4 kV and phase-to-ground voltage non-linear surge arrester: a) without arresters on the overhead power line 6 kV; b) with arresters on the overhead power line 6 kV

Figure 6. Phase-to-ground current oscillograms at the non-linear surge arrester without arresters (a), with arresters (b); phase-to-ground current oscillograms at the arrester near the place of lightning effects (с)

Figure 7. Frequency characteristics of impulse overvoltages on the input block switchgear № 1 6/0,4 kV – U_ab^áfñ, U_bc^áfñ, U_ca^áfñ respectively

Conclusion:

The use of surge arresters limits the voltage at the input of BKRU 1 6 / 0.4 kV at a level 1.5 times higher than the operating values. The installation of arresters on the supports of power transmission lines and surge arresters at transformer substations 35/6 and 6/0.4 kV reduces the overvoltage level to 5% of the operating voltage, which confirms the correctness of the recommendations given in the regulatory documents, provided that the electrical installations are in good technical condition. Thus, a 20% increase in the ground resistance of power transmission towers leads to a threefold increase in overvoltage on the linear insulation.

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