Analysis of earth faults in a wind farm integrated into the electric power system

The aim of this paper is to investigate the processes of earth faults occurrence in a wind farm network, integrated into the electric power system. The wind power farm is realized with squirrel-cage induction generators (SCIG). Each generator has a 0.69/21kV step-up transformer. The connection of the wind power farm to the electric power system is implemented through a step-up substation with a single 21/115kV transformer. The star center of the transformer 21kV winding is grounded by means of a 40 ohm resistor. A synthesized in MatLab/Simulink computer model investigates the processes of earth faults occurrence in the medium-voltage (MV) network of the wind farm. Results have been obtained, concerning the influence of the earth fault location on the transient voltages and currents at both rated torque of the generators and no-operating generators. When there is an earth fault in one of the lines, the voltages and currents of the MV network are asymmetrical and higher harmonics of the currents and voltages are generated. A harmonic analysis of the current and voltage results has been made and the total harmonic distortion (THD) has been determined for several characteristic points of the wind farm network. The parameters of voltage contamination of the power system with higher harmonics in case of an earth fault occurrence in the wind power substation have been determined. It has been found that the earth faults in the MV network of the wind farm generate voltages and currents contamination of the power system with higher harmonics.


Introduction
Wind farms are one of the main renewable sources of electricity. The trends in the development of the modern energy systems include an increase in the relative share of electricity produced by wind power farms, as well as increase in their capacity. Wind farms usually work integrated into the region's electricity system. Changes in wind speed and direction over time are largely random, which affects the joint operation of the wind farm and the power system [1]. The wind farms integrated into power systems mainly use squirrel-cage induction generators (SCIG) and doubly-fed induction generators (DFIG). It is known that when taking into account the fluctuation of the parameters of the wind energy these generators have certain advantages if operating in parallel with the electricity system [2]. The reactive power required to excite the magnetic field of the wind farm generators is provided by the synchronous generators in the system or by capacitor banks. The wind farms can consist of a large number of generators (tens, hundreds). There is usually a low-voltage/medium-voltage (/LV/MV) step-up transformer for each generator. Several generators form a group and connect to each other through a main feed cable line. The individual groups are connected by radial lines to a substation that increases the voltage in accordance with the voltage of the electric power system.
The processes in the wind farms are different from those in the traditional power plants with large synchronous generators. The wind farms exert a specific influence on the parameters of the electricity system to which they are connected. This applies both to normal operation and to emergency modes (three-phase and two-phase short circuits, and earth faults). A thorough study of the impact of these processes on the quality of electricity in the electric power systems is of great importance. The wind farms are characterized by a mixed radial-magistral network and presence of elements with nonlinear characteristics (transformers, generators, etc.). Therefore the mathematical models for describing the emergency mode processes in a wind farm integrated into the power system are rather complex [3], [4]. A computer model in MatLab/Simulink, described in [3], examines the normal mode operation processes, as well as the processes in case of a three-phase short-circuit occurrence and in case of a two-phase short-circuit occurrence. In [4], based on a simplified model, presenting the wind farm through an equivalent generator, studies of the effect of both the transformer parameters and the type of generators on the currents at short-circuits and earth faults have been carried out. It is known that the particular method of neutral grounding has a significant influence on the voltages and currents in case of an earth fault in the substation of the MV wind farm. The effect of the different methods of grounding the neutral (on the currents and voltages in case of an earth fault in the MV substation of the wind farm) has been analyzed in [5]. A resistance grounded system is recommended as a compromise option. The studies in [3], [5] show that higher harmonics of voltage and current are generated in the wind farm network in case of earth fault occurrence in it.
The aim of this paper is to study the effect of the earth faults in a wind farm on the voltages and currents of the electric power network. The wind farm under consideration is integrated into the electric power system. The studies have been performed by simulating earth faults by means of an especially designed in MatLab/Simulink computer model.

Description of the studied wind farm
The single-line diagram of the wind power farm under study is given in Figure 1. The farm contains eight identical SCIG generators with a step-up LV/MV transformer (0.69/21kV) to each of them. The

Computer model
The computer model was synthesized in MatLab/Simulink based on the connection scheme in Figure Figure 2 shows the transient voltages for the case of generators, operating at their rated torque and a phase L1 earth fault in point 2. The moment of the earth fault occurrence is when the phase L1 current passes through the zero value.  In case of a single-phase earth fault, higher harmonics of the currents and voltages with odd and even numbers are generated in both points 2 and 3. The maximum contamination of the network with higher harmonics occurs in the beginning of the transient process when the moment of the earth fault coincides with the moment when the current passes through the zero value.

Results
The earth faults in the MV network contaminate the voltage of the power system with higher harmonics in the point of the wind farm connection to the system.
In case of an earth fault after the transient process has completed, the contamination coefficients (THD U ) for all cases are up to 2%. Table 4 gives the results, concerning the maximum coefficients of voltage contamination (THD U ) in points 1 and 2.