Investigation of the Effects of the Arc Fault Resistance and Transient Resistance on the Overcurrent Ground Relay Protection Setting in Medium Voltage Electrical Power Lines

In the medium voltage electrical power lines, the most common fault is the phase to ground fault. The determining factors for selection of the neutral grounding mode of the medium voltage electrical power lines are the selectivity of the relay protection, the transient voltages during phase to ground fault, the magnitude of the fault currents, etc. The paper studies the influence of the transient resistance of phase to ground fault on the selectivity and sensitivity of the earth fault protection when the neutral of the power transformer is grounded through active resistance in medium voltage power grids.


Introduction
The medium voltage power lines (MV) are the most common in Bulgaria with about 70% of their faults being phase to ground faults. xpansion/Expanding the medium voltage grid will also result in increased length and number of the cable lines. The most commonly used methods for grounding of the MV transformer neutral in Bulgaria are through active resistance and combined grounding -reactor in parallel to active resistance.
The operation of the electrical lines under normal conditions does not depend on the grounding of the neutral. For phase to ground faults, the grounding mode greatly defines the requirements for the insulation level of the line and the equipment that is electrically connected to it. The grounding mode also affects the level of the switching overvoltages and the methods for their limitation, the magnitude of the currents in the earth fault point, the operation of the relay protection, the safety conditions, the capital investments for the construction of the line, etc. (Shabad, 2002;Shabad, 2004;Etarski, 1994, Yordanova, 2016. Grounding modes and their pros and cons are well known. The determining factors for selecting the neutral grounding mode are the type of power grid and the magnitude of the phase-to-ground fault current. Regulatory requirements for cable lines require grounding of the neutral via active resistance. For mixed overhead and cable lines -through active resistance when the cable part share is greater than 40% of the total grid length. And combined (reactor with active resistance) when the overhead line share is more than 60% (Ministry of Energy Ordinance № 3, 2015; Ministry of Energy Ordinance № 2015).
The purpose of thе paper is to investigate the influence of the transient resistance of the phase to ground fault on the selectivity and sensitivity of the earth fault protection when the MV transformer neutral is grounded through active resistance.

Model scheme and settings of the earth fault relay protection
Considered hereto is a medium voltage electrical line with a total length of 130 km, composed of overhead and cable lines,. The power transformer is rated at 40 MVA and the neutral is grounded through a 40  resistor. The investigated electrical line has 40 % of underground cables and the remaining 60 % are overhead sections. The studies on the change of the electrical variables and the effect of the arc resistance at ground fault point were conducted in MATLAB Simulink software environment.
The model of the studied lines is shown in Fig. 1.

Fig. 1. Model scheme for the studied line
Earth fault protection can be used to protect against phase to earth faults in MV electrical power lines with a grounded neutral through active resistance. The setting of earth relay protection covers the calculation of the thershold current, the time delay and the sensitivity of the relay protection. In order to increase the selectivity and sensitivity of earth fault relay protection, a three-stage earth protection is preferably used, which takes into account the effect of the transient resistance in earth damage.
. , A is the coefficient allowing for the sudden discharge of the cable line in the case of external faults and equals to: 4÷5 -in the absence of delay in relay protection, 2÷3 -in the presence of delay in the relay protection, 1 -when the relay protection acts on a signal, 0 C I is the zero-sequence overhead line capacity. The first stage is instantaneous (without delay) and the start time of the first stage is 0s t = .
The second stage of the earth relay protection recognizes the impact of transient resistance at the location of the fault. In order to provide sufficient sensitivity of the earth fault protection at ground connection through high transient resistance, the threshold current of the second stage is determined by the earth fault current with an arc resistance 100 Ω (TSO LTD). The second stage of the earth fault protection is without delay 0s t = .
The sensitivity coefficient sensitivity K is used to check the sensitivity of the protection in line with (Ministry of Energy Ordinance № 3, 2015; Avramov, 1986;Andreev, 2005;Vichev & Todorova, 1998;Domeland, 1981;Fedoseev, 1976 where LG I − is the estimated current at zero arc resistance phase to ground fault at the end of the line. , for overhead lines -1,5 for the first stage and 2,5 for the second stage. The current threshold setting of the third stage provides sensitivity of the protection for phase to ground fault through high arc resistance. It is performed with the current threshold set at 10 A. Subsequently, the time delay is 3,6 s t = and acts on the signal (TSO LTD).

Results and analysis
Investigated, for the purposes of the surveyed MV line, was the variation of the electrical variables for earth faults through an arc resistance of 0.001 Ω to 100 Ω . The examined variation of the magnitude of the phase to ground fault current for the overhead power lines constructed with ACSR 70 and ACSR 95 conductors revealed that their length varied from 10 to 40 km.
Presented in Table 1 are the results for the value of the phase to ground fault current at an arc resistance from 0.001 Ω to 100 Ω for an ACSR 70 conductors. With increasing the In short, increased arc resistance of the fault produces decreased value of the phase to ground fault current. Figure 2 illustrates the results obtained for the effect of the arc resistance on the value of the phase to ground fault current for conductor ACSR 70 overhead line with 10 km and 40 km lengths for the tested electrical line.  Table 2 are the results for the value of the phase-to-ground fault current at a transient resistance of 0.001 Ω to 100 Ω for an ACSR 95 conductor.   To determine the sensitivity factor of the first stage of the earth fault protection, it is necessary to know the magnitude of the fault current for a zero arc resistance phase-to-ground fault at the end of the line. The results derived for the investigated lines can be further used to determine the coefficient of sensitivity.
In the setting of the second stage of earth fault protection, applied is the magnitude of the current for a phase-to-ground fault through a 100 Ω arc fault.
For a phase-to-ground fault through a transient resistance, the value of the fault current is reduced, which leads to decreased sensitivity of the protection. For a phase to ground fault through a transient impedance of 100 Ω at the end of a 40 km long terminal, the earth connection current decreases by about 65%.
Taking 1,1 security K = and 5 discharge K = after substitution in (1)  Formula 4 gives the restrictive condition for setting the first stage of the earth fault relay protection. In order to meet the prerequisite for a first-rate sensitivity coefficient for the selected values of the coefficients, it is necessary to fulfill the requirement of formula 4.

Conclusion
1. Results have been obtained for the range of variation of the phase-to-ground fault current depending on the value of the arc fault resistance. The results achieved can be used to adjust the sensitivity of the first stage of earth protection. 2. The outcomes produced for changes in the value of the phase to ground fault current at a transient impedance of 100 Ω, can be further used to set the second stage of earth fault protection. 3. A formula has been devised for the threshold value of the zero-sequence capacitive current at which the condition for the sensitivity of the first stage of earth fault relay protection is fulfilled.