Farhan Mahmood

Investigation of Lightning Electromagnetic Fields on Underground Cables in Wind Farms

Due to the current spreading of wind farms all over the world and their vulnerability to lightning strikes, it is important to investigate the lightning electromagnetic transients in wind farms. Underground cables are one of the essential parts in wind farms that link the turbines with the electrical grid. This paper investigates how the ground conductivity and relative permittivity, struck wind tower position, and rise time of lightning current influence the lightning electromagnetic fields impinging the underground cable sheath. The three-dimensional finite-difference time-domain method is implemented for this study. The lightning current through the cable sheath and its associated electric field can be effectively mitigated by connecting the grounding systems of the wind towers by underground bare wires called as counterpoise. Accordingly, the mitigation effect of counterpoise on the lightning current through the sheath and its associated electric field is also investigated. The results show that the lightning current through the cable sheath and its associated electric field is higher with lower ground conductivity. Moreover, it is revealed that the effect of both ground permittivity and struck wind tower position on the associated electric field and the sheath current is dependent on the ground conductivity.

An improved technique to determine the wave propagation velocity of medium voltage cables for PD diagnostics

Investigation of wave propagation characteristics for high frequency pulses is an important aspect for the partial discharge (PD) diagnostics in medium voltage cables. Precision in determining the electromagnetic wave propagation velocity in the cables determines the accuracy of developing cable model and localization of partial discharge initiation site. It has been observed that commonly used time domain reflectometry (TDR) and time difference of arrival (TDA) methods pose certain limitations while finding out the propagation velocity of the cables. This can cause serious inaccuracy during localization of PD faults. This paper presents an alternative quarter wave length (QWL) method in frequency domain to measure the propagation velocity. It is based on transmission line model of the cables and eliminates the practical limitations of TDR and TDA techniques. The comparative assessment of these methods is presented based on experimental measurements. The comparison asserts the improved performance of proposed technique. This improved technique can be used to enhance the diagnostics capability for power networks.

Influence of Highly Resistive Ground Parameters on Lightning-Induced Overvoltages Using 3-D FDTD Method

It is important for the proper insulation design of the distribution system that lightning-induced overvoltages ( LIOVs) are accurately computed. This paper investigates the impact of ground resistivity, considering wide range up to 20 kΩm, on LIOVs impinging an overhead line due to nearby return stroke using the three-dimensional finite difference time domain (3-D FDTD) method. The investigation considers two values of both ground permittivity as well as the rise rate of lightning current. Subsequently, it is inferred that the influence of both ground permittivity and rise rate of lightning current on the peak values of LIOVs depends on the ground resistivity. Furthermore, horizontal conduction and displacement current densities are also calculated on the ground surface under the line to analyze the behavior of ground surface impedance. Consequently, it is deduced that the behavior of the impedance changes from inductive to capacitive at high values of ground resistivity, thus increasing the time to the peak value of LIOVs. The peak values of LIOVs computed using the 3-D FDTD method are compared with those calculated using Darvenizas formula. It has been revealed that this formula considerably underestimates the peak values of LIOVs at high values of ground resistivity. Thereby, an interpretation is presented for the reason of this underestimation from an electromagnetic perspective. Accordingly, Darvenizas formula is appropriately modified to improve the computation accuracy.

Probabilistic Risk Assessment of MV Insulator Flashover Under Combined AC and Lightning-Induced Overvoltages

The effect of lightning-induced overvoltages is more profound in overhead distribution lines due to their limited height and low insulation level. Consequently, there is a high risk of line insulation flashover when exposed to lightning-induced overvoltages. This paper presents a probabilistic method to assess the risk of insulator flashover in medium-voltage overhead lines due to lightning-induced overvoltages. In order to accomplish this, a modified Gaussian cumulative distribution function has been used to predict the probability of single-phase, two-phase, and three-phase flashover of insulators under combined ac- and lightning-induced overvoltages. The validity of the modified probabilistic model is confirmed through experiments carried out in the high-voltage laboratory. Next, Monte Carlo simulations were performed on the simplified Ruscks model to generate the distribution of peak lightning-induced overvoltages. Finally, the risk of insulator flashover is calculated based on the distributions of lightning-induced overvoltages and insulator flashover voltages. The proposed procedure could be considered beneficial to select the optimum insulation level required against lightning-induced overvoltages by distinguishing between single-phase and multiphase flashover faults.

Experimental investigation of lightning initiated flashover faults in MV lines

Direct lightning stroke to an overhead line conductor can produce a flashover across insulator causing a fault on the power network. A medium voltage (MV) overhead line with grounded cross arm will experience a single-phase fault whereas a multiphase (two-phase or three phase) fault is likely to occur when the cross arm is ungrounded in the event of a lightning strike. This paper investigates the characteristics of lightning initiated flashover faults in unshielded medium voltage (MV) lines with different types (metallic/wood) and configurations (grounded/ungrounded) of cross arm. The lightning impulse voltage is applied to one phase of a three-phase overhead line and the experimental flashover characteristics of single-phase and two-phase flashover faults are investigated. The total CFO of the line is also calculated using up and down method. The effect of cross arm braces on the flashover path and the variation in the magnitude of voltage build-up in the nearest phase is also analyzed. The tests have been performed under dry and wet conditions with both positive and negative polarities.