Investigation of the Overvoltages in Electrical Power Lines with a Grounded through a Resistance Neutral

The paper examines the variation of electrical variables during phase-to-ground faults on medium voltage electrical power lines with a grounded through a resistance neutral. The studies were performed with simulation models of a medium voltage electrical power line. The parameters under consideration have been the voltages and the currents in the transient and steady state operation of the line and more precisely their dependence on the variation of the arc resistance in the place of the fault and the distance from the fault to the supply source. Calculated, accordingly, is the overvoltage during the transient process, the maximum magnitude of the overvoltage and phaseto-earth fault current, the duration of the transient phase-to-ground fault current as well as the maximum magnitude of the overvoltage оn the neutral.


Introduction
In medium voltage (MV) power line, single phase to earth faults are the most common faults. The method of grounding of the neutral of the electric power line medium voltage is determined by many factors, such as (Etarski, 1994, Shabad, 2002, Feldman, 2001, Shalin, 2005, Karafeizov, 2003, Mehmed-Hamza, 2007 ─ the magnitude and nature of phase-to-ground currents; ─ the overvoltage during phase-to-ground faults; ─ the reliable and selective operation of the relay protections; ─ values not exceeding the permissible limits for touch and step voltages to the earthing devices for humans and animals; ─ limited damages caused by earth faults, etc. According to the regulation requirements in Bulgaria, the connection of the power source neutral is chosen in relation to the type of network and the magnitude of the earth fault current. Grounding of the neutral is necessary when the capacitive current at phase-to-ground current is greater than 10 A, such as (Ministry of Energy. 2004, Ministry of Energy. 2015): ─ for overhead power lines -by means of a reactor or a combination (a reactor and a resistor ); ─ for cable lines -through a resistor; ─ for mixed lines -through a resistor when the cable share is greater than 40 % of the total length of the network and combined (reactor in parallel to resistor) when the cable share is less than 40 %.
The paper explores the variation of the electrical variables during single phase-to-ground fault in medium voltage electrical power lines with neutral grounded through a resistor, commensurate with the arc fault resistance and the distance between the fault and the power source.
The investigation was performed within a MATLAB/Simulink-based programming environmentfocusing exclusively ontwo MV electric power lines, namely,. overhead and cable power lines with a total length of 130 km and cable share of 40 %. Standard blocks of the software were thereupon used to construct the relevant models. The rating of the power transformer for the tested medium voltage lines is 40 MVA ). Outline in Fig.1 is the model diagram of the lines under study.

Results and analysis
Obtained, for the purposes of the studied power lines, are the simulation results for the phase conductors overvoltage,while for the neutral voltage and the single phase-to-ground fault current derived is : ─ An arc fault resistance 0 ; ─ a varying arc fault resistance 0 Ω to 100 Ω; The single phase-to-ground faults were investigated in overhead power lines with ACSR 70 and ACSR 95 conductors for different lengths.
Reported further were the findings for the magnitude of the voltage of the undamaged phases and in the neutral. An analysis was made for: ─ the instantaneous maximum measured amplitude of the overvoltages on the undamaged phases conductors and on the neutral; ─ the maximum KTOV value.
The results for the tested electrical line with 40 % cable share are presented in Tables 1, 2 and 3 and  Figures 2 to 5.  Table 1 presents the results for a single phase-to-ground faults at zero arc fault resistance on an overhead power line constructed with ACSR 70 conductor. The failure is simulated at 0, 10, 20, 30 and 40 km away from the power source of an overhead line with a length of 40 km.
Conducted aslo were studies into the change in overvoltage dependent on the value of the arc resistance at a single line-to-ground fault.

ISSN 2603-316X (Online)
Published: 2020-06-30 Page | 75  Fig. 2 are the results derived for the overvoltage measured on the undamaged phases for a single phase-to-ground fault on an overhead power line and through a varying arc resistance (from 0.001 Ω to 100 Ω).  The results observed for an overhead line with ACSR 95 conductors are presented in Table 2 and Fig. 4 and 5.   Arranged in Table 3 are the results of the maximum overvoltage measured on the undamaged phases and calculated in accordance with (BDS EN IEC 60071-1:2019) and conditional upon the arc fault resistance and the fault distance from the source. For the investigated electrical grid, the transient overvoltage duration is up to 1 fundamental frequency cycle.

Recorded in
From the findings generated for the tested electrical lines it appears that in the power transformer's neutral grounded with a resistor, the overvoltage consistent with (BDS EN IEC 60071-1:2019) and measured on the undamaged phases tends to decrease, varying from 1.87 p.u. to 1.12 p.u. in response to the arc resistance and the fault distance for the overhead power line with ACSR 70 and ACSR 95 conductors. The overvoltage magnitude decreases by 40% with an increase in the arc resistance. The maximum magnitude of the phase voltages on the undamaged phases during single phase-to-ground fault reaches up to 43 kV. The maximum voltage on the neutral does not exceed 17 kV throughout a single phase-to-ground fault.