Evaluation of different solutions of faulted phase earthing technique for an earth fault

The study is focused on evaluation of different types of prototypes of automatics for additional faulted phase earthing (FPE), which are used for earth fault (EF) current reduction in resonant earthed distribution network. Three prototypes of these automatics have been installed in Czech distribution network, the first one utilises direct connection of faulty phase–to-earthing system of supply substation, the second one utilises connection through resistor and the last one through reactor. The contribution is mainly focused on detail analysis of operational differences of these types of FPE systems based on case study of compensated distribution network. The main aim is to specify and describe benefits and disadvantages of individual FPE applications. The results could be used for evaluation of best solution of FPE application which could be chosen for an EF current reduction in compensated distribution network.


Introduction
The continual growth of consumption, especially in urban areas with increasing numbers of technological centres, leads to the expansion of cable distribution network and therefore to increasing of earth fault (EF) current levels in such networks. One of the possibilities how to reduce the level of the residual EF current in compensated networks is utilisation of an automatic system for the faulted phase earthing (FPE). The faulted phase earthing method involves creating of conductive path for the residual current of the EF directly to the earthing system of an HV/MV substation (Fig. 1). The detailed description of the method can be found in [1][2][3][4][5][6][7][8].
There is no doubt that the method yields positive results in case of resistive earth faults that represent a vast majority of all earth faults in MV distribution networks. With the view to find best solution of the FPE application, which could be applied to national distribution system operator (DSO's) standard, three prototypes of FPE automatics have been installed under pilot project to different distribution MV networks. These automatics are designed to earth faulted phase to the earthing system directly, through resistor or reactor, based on its type. Individual types of FPE are differed in shunt impedance (Z SH ), which is connected between faulty phase and earthing system of supply substation as shown in Fig. 1. Type 1 (T1) presents design of FPE, where faulty phase is directly (Z SH = 0 Ω) connected to earth by single pole circuit breaker (no. 3. -Fig. 1), Type 2 (T2) is label of resistor earth FPE automatic (R SH = 10 Ω) and labels Type 3 (T3) or Type 4 (T4) is used for FPE automatics utilising shunt reactor with value X SH = 10 Ω and X SH = 4 Ω, respectively (value of reactance is optional from 4 to 10 Ω). The presented case study of FPE applications will respect only difference between shunt impedances of individual types of FPE (T1-4).

Factors influencing FPE method
For the purpose of proposing an optimal test network configuration and case study parameters, the basic factors which can influence characteristics of individual types of FPE are discussed in the following sections. All these factors will be respected and evaluated in the case study for each type of FPE application.

EF current reduction (fundamental component):
The crucial factor affecting the affectivity of an EF current reduction by FPE is ratio of fault circuit impedance and shunt loop impedance (impedance between faulty phase and earth). Just the ratio of these impedances has significant impact to ability of FPE to reduce an EF current as it is described in [3,8].
EF loop impedance: this impedance is given by positive and zero sequence impedance of the line to fault, fault resistance and partly by soil resistivity (earth resistance).
Shunt loop impedance: This is given mainly by shunt impedance of FPE (Z SH ) and earthing system impedance of supply substation.
To evaluate FPE applications, the case study has to contain sensitivity analyses of parameters, which can affect the impedance ratio and have high variability of its value, as line impedance to fault and fault resistance are.

Reduction of higher harmonic component of an EF current:
In case that conventional arc-suppression coils are used, which are tuned into resonance with the network capacity on system frequency (50 Hz), the EF current is predominately composed of harmonic components. These components are injected to the EF from non-linear loads and it is not compensated by the arc-suppression coil. As it is mentioned in [3], the RMS value of EF current is mainly given by third, fifth and seventh harmonic component especially in urban or sub-urban distribution network. For this reason, the possibilities of reducing these harmonics by given types of FPE have to be necessary analysed in case study.

Load transmission through EF point:
The next problem related with FPE principle is transfer of a load current of faulty feeder through earth and fault point. It can cause significant increase of an EF current and thus deterioration of touch or step voltage levels around faulted area. This phenomenon can occur only during low-impedance EF at heavily loaded areas of distribution network, the issue is in detail discussed in the contributions [3,8]. low-impedance earthing of faulty phase, the phase voltage of healthy phases is increased up to L-L operation voltage at least regardless of value of an EF resistance. High level of the overvoltage could then cause insulation breakdown of healthy line leading to ignition of the second EF i.e. ignition of short-circuit and interruption of power supply. Since this overvoltage is significantly influenced by the used shunt impedance of FPE, the evaluation of overvoltage level is also included to the study.

EF current level during cross-country EF:
Regarding to safety against electric shock, a cross-country EF (double-EF) is the most hazardous state, it is state when EF current reaches highest values in compensated network. This fault current increases earth potential rise not only in the area of second EF but also in the area of supply substation earthing system. Therefore, assessment of the level of the EF current during the cross-country EF is also subject of the case study.
1.1.6 Overvoltage during cross-country EF: High overvoltage can also arise during cross-country EF ignition as well as it was described in Section 1.1.4 for single EF. This overvoltage can damage insulation of distribution network component what could affect continuity of power supply in the future. Owing to this, overvoltage strongly depends on R, L, and C conditions during fault ignition, the case study is also focused on evaluation of overvoltage for individual types of FPE, the respecting variation of fault distance, capacitive current, faulty phase, moment of the fault ignition etc.

Case study
The testing network showed in Fig. 2 was designed to be able evaluate all factors mentioned in Sections 1.1.1-1.1.6 for individual applications of FPE separately. The testing network simulates mixed compensated network 22 kV, which is supplied from 110 kV network over three winding supply transformer YnYD with the power 63 MVA and u k = 16.5%. Contribution of symmetrical short-circuit current from HV network is 16 kA. MV distribution network consists of two healthy feedersoverhead line AlFe110/22 and cable line AXEKVCEY120. The cable line varies its length base on its operation variants (V1-V6) listed in Table 1.
The third (faulty) feeder is overhead line 70AlFe6 with length 40 km, this feeder is used for simulation of earth fault with variations of fault distance 0, 10, 20, 30, and 40 km (P1-P5). For the case study, four basic values of earth fault resistance R f are respected 10, 300, 600, and 1200 Ω. To evaluate of all aspects of the study, not only ideally compensated state is respected but also under compensated (compensation current is 20% lower than the capacitive current of the network) and over compensated state (+20%).

Analyse of earth fault current reduction by FPE
The difference in the effectiveness of different types of FPE automatics can be evaluated based on Fig. 3. The figure presents relative value of an earth fault current after application of individual types of FPE during 10 Ω earth fault in ideally compensated distribution network. This relative value I f indicates percentage value of an earth fault current which is flowing through fault point after application of FPE. In terms of earth fault current reduction by each type of FPE, the type 1 is the best solution because of lowest value of shunt impedanceZ SH . The relative value of an earth fault current reaches value 0-30% in this case. On the other hand, the type 3 is the least effective, the relative value of an earth fault current is reaching 90% in this case.   maximum, and minimum relative values of earth fault current during all testing states in ideally compensated network categorised based on earth fault resistance 10, 300, 600, and 1200 Ω.

Impact of load transmission on earth fault current level
As it was expected, the rising of fault resistance leads to increase of FPE effectivity (reduces the level of the load transmitted through the earthing system), similarly increasing distance of the EF from the loaded distribution transformer reduces the level of the load transmitted through the earthing system as it can be seen in Fig. 4.
In terms of comparison of the impact of each solution of FPE on load transmission, the shunt impedance of FPE plays key role. Therefore lowest effectivity of EF reduction is reached by Type 1 (direct FPE), next is Type 4 ( Z SH = 4 V . Then solution Type 2 and Type 3 are comparable when impact of load is respected, absolute value of the shunt impedance is for these types equal Z SH = 10 V . Table 3 is depicted to compare individual types of FPE base on its ability to reduce EF current when impact of load transmission is respected. The table summarises average, maximum, and minimum relative value of earth fault current during all testing states in ideally compensated network categorised base on earth fault resistance 10, 300, 600, and 1200 Ω.

Overvoltage analysis
This analysis shows that states when FPE is applied during high impedance EF (300, 600, and 1200 Ω) in cases of network with high capacitive current I c .300 A (V4, V5, and V6) are most problematic from hazardous overvoltage occurrence point of view. Especially, Types 3 and 4 reach the highest values of overvoltage after FPE of high impedance EF, where overvoltage of healthy phase exceeding 25 kV. The maximum RMS value of overvoltage is 28.8 kV when Type 3 is used and 26 kV for Type 4. This overvoltage is caused by oscillation of neutral voltage due to energisation of shunt inductance of the FPE automatic (X sh ). As this inductance and also current flowing through this inductance (I sh ) will be higher, as high value of overvoltage is possible achieve.
Regarding the solution Type 1, the level of self-overvoltage is mainly caused by discharge current given by faulty phase, the duration of this overvoltage is very short (about a quarter of a period) and it is usually suppressed in real systems. The recorded values of neutral voltage and healthy phase L3 voltage are shown for all modelled cases in Figs. 5 and 6.
Waveforms of instantaneous value of recorded phase voltages for case of FPE through reactor 10 Ω (Type 3) are presented in Fig. 7. There can be seen extreme overvoltage exceeding value 40 kV (>210% of nominal voltage). The overvoltage duration is approximately a period and it is caused by neutral voltage oscillation (U 0 ), as it can be seen in Fig. 7. Value of the neutral  Fig. 4 Relative earth fault current after FPE application respecting impact of loadideally compensated network, R f = 10 Ω

Complex evaluation of respected types of FPE
It follows that any solution of FPE automatic has its advantages and disadvantages with respect to characteristics of an earth fault and distribution network configuration. Comprehensive comparison of respected types of FPE applications in terms of the case study is possible on the basis of Table 4, which shows the percentage values of the suitability of each type of FPE. For this comparison, the value of 100% is used for most appropriate solution of FPE and the remaining are proportionally lowered with respect to their ability (efficiency) to reduce an earth fault current and overvoltage.

Evaluation of FPE automatics during cross-country earth faults
Lowest value of an earth fault current during cross-country fault can be achieved with application of FPE Type 2 eventually Type 3. In case that FPE with shunt impedance 10 Ω (Type 2) is used, the earth fault current of cross-country fault is limited up to 1140 and 1340 A in case of Type 3 (reactor 10 Ω), respectively. The least suitable solution of FPE automatics is a direct earthing of faulted phase (Type 1), where the level of an earth fault current during the cross-country EF with resistance of 10 Ω is approximately two times higher (2190 A). Similarly, with regard to highest value of overvoltage, the best solution is also Type 2 (R sh = 10 Ω) and the worst one FPE with reactors (Types 3 and 4). Maximal recorded values of overvoltage for individual types of FPE application are listed in Table 5.
Presented overvoltage was prepared for cross-country EF with fault resistance 10 Ω, where phase L2 was additionally earthed and phase L3 was affected by second EF. Complex comparison of respected types of FPE applications in terms of the cross-country earth faults is possible on the basis of Table 6, which shows the percentage value of the suitability of each type of FPE. For this comparison, the value of 100% is used for most appropriate solution of FPE and the remaining are proportionally lowered with respect to their ability (efficiency) to reduce an earth fault current and overvoltage during cross-country EF.

Conclusion
The results show that any solution of FPE automatic has its advantages and disadvantages with respect to characteristics of an earth fault and distribution network configuration (capacitive current, harmonics, load, neutral point connection etc.). With respect of the case study conditions, the most suitable type of FPE application for compensated distribution network is earthing of faulted phase through 10 Ω resistor. This type of FPE automatic achieves the best results while respecting all the key influences (impact of loads, harmonics, overvoltage, and fault current levels) not only during the earthing of faulted phase but also during the upcoming cross-country faults (double-earth faults).