Results of earth fault measurements in an earth fault compensated 110-kV-system. P. Märtel H.-J. Radtke P. Schegner O. Seifert

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1 Results of earth fault measurements in an earth fault compensated 11-kV-system P. Märtel H.-J. Radtke P. Schegner O. Seifert ESAG - Energieversorgung Sachsen Ost AG DREWAG - Stadtwerke Dresden GmbH Technology Laboratory of Electrical Power Systems Technology Laboratory of Electrical Power Systems A great part of the German 11-kV-systems and the medium voltage systems are earthed through arc suppression coils. This method of neutral-point connection enables to compensate the capacitive earth-fault current I CE by the inductive current of the suppression coil I L except for a small residual current at the earth-fault location. The big advantage for system operation consists of the fact that an arc-over vanishes automatically in general, i.e. out intervention of the operation personnel, and thus remains out effects on the customers. A substantial prerequisite for a safe ground fault extinction and thus a reliable line operation is the exact knowledge of the damping, the detuning of the arc suppression coil and the unbalance of the network as well as the harmonics in the residual current. These quantities are the basis for the planning of further network expansion. The scale of the mentioned parameters can be evaluated from operational measurements. The exact values, in particular the harmonics of the residual current can however only be determined by earth fault measurements. For that reason two utilities in Saxony (ESAG - Energieversorgung Sachsen Ost AG, DREWAG - Stadtwerke Dresden GmbH) decided to carry out corresponding earth fault measurements in the 11-kV-system. The theoretical background of the evaluation methods is explained in detail. The obtained results are represented, together a short description of the measuring targets, the measurement set-up and the procedure of the measurement. The special emphasis is put on the comparison between the results of the indirect and the direct measurement of the network parameters. With the indirect measuring procedures, the determination of the detuning and the damping of the network takes place for example out of the envelope curve of the evanescent residual voltage after an earth fault. In another procedure the complex voltages and the coil position are analysed, whereby the resonant circuit is calculated using the least square method. This gives additional information about the magnitude and the position of the unbalance. The disadvantage of the two described indirect procedures is the fact that only the fundamental component of the residual current can be determined. For the direct measurements an artificial earth fault was installed. This enables the measurement of the actual residual current including the harmonics. In the context of the earth fault measurements, also the coupling of the residual voltage during an earth fault from the 11-kV-system to the medium voltage systems is analysed. At several 11//1-kV transformers connected arc suppression coils at all three voltage levels completing measurements were carried out. It could be shown that by an optimal selection of the tuning of the arc suppression coil in the medium voltage systems the coupled residual voltage will be minimized.

2 RESULTS OF EARTH FAULT MEASUREMENTS IN AN EARTH FAULT COMPENSATED 11-KV-SYSTEM P. Märtel H.-J. Radtke P. Schegner O. Seifert Energieversorgung Sachsen Ost AG, Germany DREWAG - Stadtwerke Dresden GmbH; Germany Technology; Germany Technology; Germany ABSTRACT The earth fault compensation has well known favourable effects on the operation of distribution networks. The biggest benefit is the fact that an arc-over vanishes automatically in general, i.e. out intervention of the operation personnel, and thus remains out effects to the customers. The exact knowledge of the damping factor, the detuning and the unbalance as well as the harmonics in the residual current are substantial prerequisites for safe ground fault extinction and thus for a reliable line operation. These quantities are the basis for the planning of further network expansion. To get reliable information, earth fault measurements in the described 11-kV-system were carried out. In this paper the evaluations of the above-mentioned parameters via operational indicators are compared the results of direct earth fault measurements INTRODUCTION Great parts of the German distribution networks are earthed via arc suppression coils. This method of neutral point connection enables the compensation of the capacitive earth fault current I CE by the inductive current of the arc suppression coil I L except a small residual current at the earth fault location. The unbalance, the damping and the detuning influence the maximum zero-sequence voltage during normal operation. The magnitude of the residual current depends on the dumping factor, the detuning and the harmonics. According to the German standard DIN VDE 8 part the maximum residual current is limited to = 13 A for 11-kV-systems. In general there is a dumping factor of d = 1 % assumed. That limits the maximum network size to an capacitive earth fault current of I CE = 13 A. The additional installation of cables increases the capacitive earth fault current rapidly. Therefore it is important to get information about the real proportions in the network. The scale of the above mentioned parameters could be determined from operational indicators. The exact values can however only be determined by ground fault measurements. For that reason two utilities in Saxony (ESAG - Energieversorgung Sachsen Ost AG, DREWAG - Stadtwerke Dresden GmbH) decided to carry out corresponding earth fault measurements together the Technology. PARAMETERS OF COMPENSATED SYSTEMS The most important parameters of compensated systems are the detuning v, the damping factor d and the unbalance factor k. These parameters are defined in the following way: IL IL v = = 1 (1.1) capacitiveearth-fault current IL arc suppression coil current d = (1.) watt component of the residual earth-fault current k C + a C + a C U Ea Eb Ec en max = = d CEa + CEb + CEc ULa (1.3) C Ea,C Eb,CEc line a,b,c to earth capacitance Uen max maximum of the open delta connection voltage Another important parameter is the harmonic content in the residual current. The damping factor, the detuning and the unbalance can be evaluated indirectly by analysing operating indicators. But only the measuring of the earth fault current itself gives the real values of these parameters. Up to now the harmonic content of the earth fault current can only determined by the direct measurement of the earth fault current. One goal of the research program was the assessment of the indirect evaluation method. Therefore short descriptions of these different indirect methods are following. Indirect earth fault parameter evaluation methods Indirect evaluation methods use on the one side the tuning process of the arc suppression coil and on the other side earth fault records to calculate the parameters of the network. The advantage of these methods is that only operational indicators are needed for the calculation. To find the optimal tuning of the arc suppression coil, it is necessary to change the inductive current of this coil

3 during normal operation conditions. This follows changes of the line to earth voltages and of the open delta connection voltage (figure 1). These changes can be used to evaluate the parameters of the compensated system. Evaluation via the resonance curve of the open delta connection voltage. During normal operation, the open delta connection voltage can be described accordingly (1) as a function of the arc suppression coil current. K Uen = (1.4) + j ( - IL) K komplex factor If the current of the arc suppression coil I L has the same value like the capacitive earth fault current I CE the open delta connection voltage get its maximum value. K Uen max = (1.5) If the difference between the two currents I L and I CE is exact I w the open delta connection voltage has the following value. K Uen w = (1.6) I wr + j The result of the division oft the last two equations is: U en w I wr 1 = = (1.7) U en max ( + j I wr ) The change of the arc suppression coil current from the maximum to the 1 value of the open delta connection voltage is equal to the watt component of the earth fault current. The following figure explains this so called - method. Evaluation via circle diagram of the zero voltage. The second evaluation method uses the absolute value and the phase angle of the line to earth voltage. During the tuning of the arc suppression coil, the neutral point of these voltages describes a circle diagram. With the help of the least square method, the optimal parameters of this circle can be estimated (, 3). Figure shows an example of this estimation. The measured values are shown as crosses. The estimated circle is drawn as a dotted line. The method has two benefits, at first it works very small changes of the absolute value of the open delta connection voltage and it gives information on the direction of the unbalance in the system. a a.) Im {U en } kv c b U en U en max U en max I w b.) Figure : kv 1 Re {U en } Explanation of the circle diagram method a.) Direction of the unbalance in the system b.) Measured values and estimated circle The parameters of the compensated network, which are calculated by means of the circle diagram method are indicated by an index C (v C, d C and k C ). Figure 1: Explanation of the - method The parameters v R, d R and k R can be calculated the equations (1.1), (1.) and (1.3). The index R indicates that the evaluation is made via the resonance curve of the open delta connection voltage. I L Evaluation via the damped oscillation of the open delta connection voltage. The damped oscillation of the open delta connection voltage can also be analysed. The advantage of this method is, that each earth fault record, including records of transient earth faults, can be used to determine the parameters of the compensated network. The envelope curve of the open delta connection voltage (figure 3) after an earth fault can be described in the following way. $ t u = u e δ (1.8) en env en env

4 Assuming that a simple parallel oscillation circuit can model the zero system, the parameter of this circuit can be calculated. 1 1 δ= = (1.9) R C T The watt component of the residual current and capacitive earth fault current are defined. Uen = (1.1) R =ω C Uen (1.11) By rearranging the equation (1.8.) to (1.11) the damping factor of the network could be calculated. δ do = = (1.1) ω The detuning can be carried out by the two frequency measurements. f f P vo = 1 1+ (1.13) fp f frequency of the damped oscillation (zero sequence system) fp frequency of the positiv sequence system The index O indicates that the evaluation is made via the damped oscillation of the open delta connection voltage. Figure 3: Damped oscillation of the open delta connection voltage Direct earth fault parameter evaluation methods The direct evaluation methods need the installation of an artificial earth fault in the network. The current via this earth connection, the line to earth voltages and the open delta connection voltage had to be measured. During the measurements the suppression coil had to be tuned. The results of these measurements are presented as socalled V-curve. This presentation comprises the information of the damping factor d D and the detuning factor of the arc suppression coil v D. The results of these measurements are not only the fundamental wave but also the r.m.s value of the earth fault current. Therefore it is possible to define the dumping factor in two different ways. The first one uses only the fundamental wave of the residual earth fault current d D and the second one is based on the r.m.s value of this current d Drms. The comparison between the resonance curve and the V-curve gives information on saturation effects of the arc suppression coil. Further important results are the harmonics of the earth fault current. The earth fault measurements are the only way to get these information. Up to now, the influence of these harmonics on the arc quenching capability has not been clearly pointed out. THE MEASUREMENT LAYOUT The 11-kV-System of the ESAG and the DREWAG are directly connected. The network area has an extension of nearly 68 km² and approximately about 1.6 million inhabitants. It consists of 1473 km overhead lines and 48.4 km underground cables, this results in an calculated capacitive earth fault current of I CE = 135 A. In the network 14 earth-fault suppression coils are installed a total inductive current of I L = 1817 A. This 11-kV-System is fed via 4 connections to the 38/-kV-System and one base-load power plant. Normally the network can be operated subdivided in two parts. The measurements were carried out in one of these parts and in the whole network. The results, which are reported, refer to measurements in the whole network. The artificial earth fault was installed in an overhead line bay, that is currently out of operation, where the necessary voltage and current transducers were still available. The fault was switched on a circuit breaker, which gives in the case of a double earth fault the possibility to switch off the fault in very short time. The measurements were carried out not only in the substation where the earth fault was installed, but also on four additional locations. Different kinds of measurement equipment were used, but at each location the data registration was made digital voltmeters, power quality analysers and transient recorders. The recording procedure was realized in the following steps: resonance curve switch on the earth fault V-curve switch off the earth fault. RESULTS OF THE MEASUREMENT The following table presents the measurement results for the dumping factor, detuning factor and the unbalance factor at the substation where the earth fault was installed.

5 Table 1: Parameter of the whole network Damping Detuning Evaluation factor factor method d [%] v [%] Resonance curve Unbalance factor k [%] d R = 1.6 v R = k R =.11 Circle diagram d C = 1.64 v C = k C =.13 Damped d oscillation O = 1.84 v O = * Direct method d (fundamental) D = 1.67 v D = -4.9 * Direct method d (r.m.s) Drms = 3.9 v Drms = -4.9 * * this method the evaluation of the parameter is not possible The calculated damping factors d R, d C and d D gives identical results according to the measurement accuracy. The value of factor d O is 11 % higher. The reasons are some problems the program that should estimate the damping of open delta connection voltage. Nevertheless the indirect and direct methods calculate the dumping factor very well for the fundamental current. The measured damping factors are much lower than the up to now assumed value of 1 %. One reason could be the fine weather at the day of measuring, but this corresponds also further measurements in other distribution networks (4). It seems that the dumping factor for the fundamental current is not higher 8 A 6 operate the networks a very small detuning factor, if the open delta connection voltage is not too high. The calculated unbalance factors are small. This corresponds the low value of the open delta connection voltage at the resonance point. Figure 4 shows the resonance curve as a solid line square markers. The maximum value of the open delta connection voltage is U en max = 5 kv. This is a characteristic of a well-balanced network as mentioned above. There are nearly no harmonic components in the open delta connection voltage. Before switching on the earth fault, the detuning factor was brought to a value of v R = -1,69 %. That value was selected to measure the whole V-curve only by shifting one arc suppression coil. Then the earth fault was switched on. The transients were recorded. The range of the detuned arc suppression coil was not large enough to reach more than the minimum of the V- curve. To get the left side of the V-curve the current of an arc suppression coil in another substation was increased by about I L = 3 A. That is the reason that the V-curve has a discontinuity at I L = 198 A. In Figure 4, the curve the dashed line and the triangle markers shows the r.m.s-value of the earth fault current. There is an visible minimum of rms = 4.8 A at I L = 131 A. This curve can be subdivided into the fundamental part of the current (5 Hz) (solid line the rhomboid markers) and the r.m.s.-value of all other I L = 45 A 6 V IF = 4,8 A d Drms = 3,9 % =,1 A d D = 1,67 % 3 Uen - V-curve total (fundamental) V-curve total (r.m.s. of harmonics) V-curve resistive component (fundamental) V-curve reactive component (fundamental) 1 V-curve total (r.m.s) resonance curve A 14 Figure 4: Resonance curve and V-curve I L than 3 % in distribution systems. The value d Drms shows clearly the influence of the harmonics. The saturation of the arc suppression coils is the reason for the differences in the calculation of the detuning factor between the indirect and direct methods. This effect is well known. This result should be an impulse to reconsider the normal operation method of the arc suppression coil in Germany. It should be advised to harmonics (dotted line cross markers). This r.m.s.- value of all harmonics is nearly independent of the tuning of the arc suppression coil. The fundamental part of the earth fault current shows the characteristic V- shape. The minimum value is f =.1 A. This curve can be divided into a watt component (solid line cross markers), which is nearly constant and a reactive component (dotted line circle markers) that is

6 linear changing the setting of the arc suppression coil. Figure 4 clearly shows the difference between the maximum of the resonance- and the minimum of the V- curve. The difference of 45 A is related to the whole current of the suppression coil only 3.5 %. This is quite small. Table shows the absolute and relative value of the harmonic components of the residual earth fault current at three different settings of the arc suppression coil. The fundamental current changes the setting of the arc suppression coil, all other harmonics are nearly constant. The 3 rd harmonic has the maximum value of all harmonics approximately 34 A. At the minimum of the V-curve the 3 rd harmonic is therefore bigger than the fundamental. The 3 rd harmonic is close to the resonance frequency of the connected positive, negative and zero sequence system of the network. This could be the reason why this harmonic is so strong. The increase of the harmonic components in the earth fault current corresponds measurements in other distribution networks. The influence of these harmonics to the self-extinction of an arc is not clearly pointed out. Table : Absolute and relative value of the harmonic components of the residual earth fault current v R = -1,69 % v R = 3,7 % v R = 7,11 % Harmonic v D = -4,9 % v D = % v D = 4,17 % [A] [%] [A] [%] [A] [%] After all the earth fault was switched off. All voltages came to a normal operation value. By analysing the damped oscillation of the open delta voltage (figure 3) the results of this method were a little bit higher than all other methods for fundamental frequency analysis (table 1). In the context of the earth fault measurements, the coupling of the residual voltage from the 11-kVsystem to medium voltage systems during a earth fault is analysed. Measurements were carried out at several 11//1-kV transformers connected arc suppression coils at there three neutral points. These measurements pointed out that the coupling between the systems depends on the current of the arc suppression coil, which is connected to the neutral point of in the 11-kV-transformer and the detuning of the arc suppression coil of the medium voltage system. By an adjusted detuning of this arc suppression coil the coupled residual voltage could be reduced. CONCLUSION The damping factor of the fundamental is evaluated by the indirect and direct methods high accuracy. The harmonics are not taken into account by the indirect methods, because they are not part of there theoretical background. Nevertheless the calculated dumping factors are very useful for the assessment of earth fault compensated networks. The advantage of these evaluation methods is, that they need only operational indicators. That makes it possible to supervise the network continuously. The estimated dumping factors are very small. These results are confirmed by long-term operational measurements. The up to now used reference - dumping factor of 1 % should be reduced to about 3 %. It makes no sense to compare the calculated detuning factors. The indirect methods have no information on the saturation of the earth fault suppression coils. Therefore the results of the indirect and direct methods can not be identical. The measurements have clearly shown, that the tuning of the earth fault suppression coils to the maximum of the resonance curve results to optimal compensation conditions during an earth fault. Because of the saturation of the earth fault suppression coils the network is still overcompensated. This gives on the one hand optimal conditions for the arc quenching capability; on the other hand the voltage of the faulted phase increases extremely slow. Therefore these results should be an impulse to reconsider the common setting of the earth fault suppression coils. It seems to be advantageous to operate the networks at the resonance point, if the open delta connection voltage is not too high. The discovered reduction of the damping factor and the recommended operation of the network increase the limit of the maximum network size of earth fault compensated networks considerably. REFERENCES 1. Technische Universität Dresden, Fakultät Elektrotechnik, 1995, Sternpunktbehandlung in Elektroenergiesystemen, Dresden, Institut für Elektroenergieversorgung. Hauser G., Rüger R., Schelksi U, 1986, Ermittlung der Resonanzkurve mit Hilfe der Leiter- Erd-Spannungen, Elektrizitätswirtschaft, 11, Funk G., Kizilcay M., 1988, Begrenzung der Sternpunktspannungen von erdschlußkompensierten Netzen bei Unsymmetrie der Erdkapazitäten, etzarchiv, 1/4, Lüke E., 1997, Langzeitbetrachtung der Erdschlußströme in gelöschten Netzen, Elektrizitätswirtschaft, 19,