Presented By:
Moritz Pikisch
OMICRON electronics Corp USA
TechCon 2018
Abstract
Grounding System tests must be performed after construction of electric facilities and on a regular basis every 4-5 years. This is to guarantee personnel safety during a single phase fault and to check the quality of the grounding system’s construction according to the dimensioning during the planning period. Therefore, it must be proven that no hazardous step and touch voltages in and around the substation or at the pole of an overhead transmission line are caused. For the construction check the measured ground impedance can be used for a comparison to the ground impedance value resulting from simulations during the planning period. This paper gives an overview of the motivation of grounding system testing and its relevant theoretical aspects.
Furthermore, it explains the fall-of-potential, the step and touch voltage, and the reduction factor measurement, by referring to relevant standards and their recommendations regarding test methodology and assessment. Grounding System tests must be performed after construction of electric facilities and on a regular basis every 4-5 years. This is to guarantee personnel safety during a single phase fault and to check the quality of the grounding system’s construction according to the dimensioning during the planning period. Therefore, it must be proven that no hazardous step and touch voltages in and around the substation or at the pole of an overhead transmission line are caused. For the construction check, the measured ground impedance can be used for a comparison to the ground impedance value resulting from simulations during the planning period. This paper gives an overview of the motivation of grounding system testing and its relevant theoretical aspects. Furthermore, it explains the fall-of-potential, the step and touch voltage, and the reduction factor measurement, by referring to relevant standards and their recommendations regarding test methodology and assessment.
Introduction
During a ground fault the fault current circulates between the fault location and the transformers which are driving the fault current. In order to establish a low-ohmic return path for the fault current grounding systems allow a conductive, low ohmic connection between the soil and the neutral of the network respective to the fault location (e.g. a pole). In principal a grounding system consists of conductive elements such as wires, rods, etc. These elements have direct contact to soil and, therefore, allow a current between the soil and the neutral. The more conductive elements are brought into soil the better (resp. low-ohmic) the grounding system is.
Figure 1 shows the potential in the event of a ground fault at the pole of an overhead transmission line. The return current through soil causes a potential rise of the grounding system and the pole where the fault occurs in regards to reference ground. The reference ground potential is represented by the green flat plain. In the vicinity of the grounding system and the pole the fault current causes an upward and downward cone shaped potential rise according to electromagnetic field theory.
The resulting ground potential rise VG is considered as the voltage between the grounding system and an infinitely remote location. Practically the potential of this remote location is represented by the flat part around the grounding system’s potential rise. This zone is considered as not being influenced by the grounding system anymore. The ground impedance ZG is introduced as follows:
A high potential rise respectively a high ground impedance reveals a “bad contact” to reference earth. In order to reduce the ground impedance, the grounding system must be extended by additional conductive elements or replacement of eroded conductive elements.
Figure 2 and Figure 3 illustrate the potential rise of a ground grid in detail. In contrast to the simplified illustration in Figure 1 the potential contour inside the grounding system is not flat. Therefore, step and touch voltages have to be considered in and outside the substation for personnel safety.
A touch voltage is defined as the difference in potential between a grounded object and a location 1 m away in the event of a ground fault. This scenario represents the worst case for a person touching this object as a maximum arm span of 1 m is assumed.
A step voltage is defined as the difference in potential between two locations 1 m apart from each other in the event of a ground fault. This scenario represents the worst case for a person being exposed to a step voltage by standing with his feet 1 m apart.
In order to recommend limits for step and touch voltage EN 50522 and IEEE 80 define permissible body currents as shown in Figure 4. IEEE 80 even proposes three different limits (according to Biegelmeier and Dalziel) but doesn’t recommend any explicitly. The permissible body current depends on the maximum fault duration. The longer the fault duration, the lower is the permissible body current. For the assessment of step and touch voltages the body impedance is considered as 1 kΩ in both standards. This means that the permissible step and touch voltage is the same value in Figure 4 as the permissible body current by reading the vertical axis in V. However for the measurement and the assessment of step and touch voltages the two standards define different approaches.
Usually overhead transmission lines are equipped with a ground wire which results in a current split in the event of a ground fault. This means that one portion of the entire fault current returns via the ground wire whereas the other portion returns via the soil. This results in a lower ground potential rise as well as in lower step and touch voltages as they are only caused by the current through soil (the grid current IG). The same applies to cables equipped with a conductive cable shield.
Ground Impedance Measurement
The test current is driven by an AC source which causes a potential rise of the grounding system as it would be the case for a real fault. The only difference is that the current which is injected during the measurement is smaller than the maximum fault current. In general there are three common test methods for grounding system testing which allow effective noise suppression as interference must be carefully considered: The frequency selective measurement used by the CPC 100 and CP CU1, the polarity reversal method used by DNV GL and the beat method. Please refer to 3 for detailed information and comparative measurements on these methods. In order to measure true values it is important to ensure that the two grounding systems’ cone shaped potential rises are not overlapping. If this would be the case the ground impedance would be determined too small which means that a “better” (or smaller) value than the actual one would be measured. EN 50522 recommends to have a minimum distance to the auxiliary electrode of 1 – 5 km. IEEE 81 recommends at least 5 times the biggest dimension of the grounding system under test.
The voltage is initially measured between a grounded reference point in the substation and a location at the edge of the ground grid. The connection at the location is realized by driving a metallic rod at least 20 cm into the soil. Since the ground grid’s edge is hard to estimate the substation’s fence is also a good reference point to start from. The voltage referring to the initial measurement is supposed to be quite small since the rod is close to the grounding system which theoretically has the same potential at each location. For the next measurements the rod’s distance to the grounding system increases as shown in Figure 5. Increasing the rod’s distance results in an increase of the impedance relative to the voltage. The measurement can be stopped as soon as the results for impedance and voltage do not change anymore as it is the case for the last 3 points of both curves in Figure 6. The value of the impedance curve’s flat part is the ground impedance, the value of the voltage curve’s flat part is the ground potential rise.
Further it must be considered that the angle between the measurement trace and the current’s injection path is 90° as recommended in EN 50522 and IEEE 81. This is not always possible due to obstacles and inaccessible private property. These standards, therefore, require a minimum angle of 60°. The main reason for this recommendation is the inductive coupling between the line which is used for injection and the voltage measurement. If the trace for the voltage measurement would be in parallel to the injection path the injected current would couple into the voltage measurement and would therefore interfere with the voltage caused by the potential rise. Regarding the calculation of the ground impedance, the impact of the current split caused by the ground wire relative to the cable shield must be considered in case an Overhead Transmission line or a power cable has been used for the injection of the test current. Therefore the ground impedance is correctly calculated by the following equation. r is the reduction factor of the line used for injection:
For the calculation of the ground potential rise the maximum grid current must be taken into account by multiplying the ground impedance and the maximum grid current. For the assessment of the fall-of-potential measurement EN 50522 states that if the ground potential rise is less than double the permissible touch voltage then the step and touch voltage measurement can be skipped. IEEE 80 doesn’t recommend any limit neither for ground impedance nor ground potential rise. If reference values for the fall-of-potential obtained by ground grid simulation are available they could also be compared to the measured fall-of potential in order to cross check simulation and measurement.
Step and Touch Voltage Measurement
For step and touch voltage measurements the injection of the test current remains the same as for the ground impedance measurement. The only difference is the voltage measurement which is now performed at selected locations in and outside the substation.
EN 50522 suggests the personnel simulation method by measuring the touch voltage across a 1 kΩ resistor and using a metal plate which simulates bare feet 1 m apart from the object. The plate must have dimensions of 20 cm x 20 cm and be loaded with at least 50 kg, ideally a person who steps on it (Figure 7). EN 50522 also recommends to wet the soil under the metal plate in order to simulate the worst case. For the assessment of measured touch voltages, the limits in Figure 4 apply after the measured voltage has been calculated by taking into account the maximum current to earth IG as shown in equation ( 3 ). EN 50522, Table 1 outlines the calculation of IG for every neutral configuration. Measuring and assessing step voltage is not mentioned explicitly in EN 50522.
IEEE 81 recommends to measure touch voltage with a rod which is driven at least 8 inches into soil by measuring with a high-ohmic volt meter. Thereby the prospective touch voltage is measured which is higher than the touch voltage a person would be exposed to. For the step voltage 2 rods are driven into the soil 1 m apart from each other. For the assessment of step and touch voltages IEEE 80 therefore considers additional resistances which lead to higher permissible step and touch voltages than shown in Figure 4. Please refer to IEEE 80 chapter 8.3 in order to get the exact equations for the calculation of permissible step and touch voltages.
Reduction Factor Measurement
The reduction factor measurement determines the portion of the injected test current which is returning via the soil respective to the ground wire. Therefore a test current is injected as for the ground impedance measurement and the return current is measured by using e.g. a Rogowski coil which is wrapped around a grounded conductor. This grounded conductor could be the connection of the ground-wire to ground as shown in Figure 8. Please note that modern Rogowski coils produce a voltage which is proportional to the measured current and also consider the phase angle correctly. That’s the reason why a voltmeter is shown in Figure 8 for the measurement of the return current. If the entire return current can’t be determined at once the measurement is repeated at all conductors which are serving as the return path. The individual currents must then be added by considering their phase angles in order to obtain the true value for the overall return current. The reduction factor r is then calculated according to formula ( 4 )
For the assessment of the reduction factor there are no limits defined in the standard. One way to assess the reduction factor measurement is to check if the measured reduction factor is lower than the reduction obtained by simulation. If this is true the consideration of the grid current resulting from simulation is even more conservative than considering the grid current resulting from the reduction factor measurement. Alternatively, the measured reduction factor can also be used directly for the determination of step and touch voltages according to formula ( 3 ).
Case Study
This case study illustrates a grounding system test at a grounding system with the configuration in Figure 9. Close to the measured 110 kV / 20 kV substation a 20 kV / 0.4 kV substation is located. The shield of the 20 kV cable interconnects the grounding systems of both substations and merges them into one combined grounding system. The test current has been injected via a 110 kV line which has been grounded at a remote substation. For the sake of simplicity the two substations are called 110 kV and 20 kV substation.
First the fall-of-potential measurement has been performed which provides the two curves in Figure 10. Additionally it also includes a third measurement which will be discussed later. It is quite common that 2 measurements are performed in different directions in order to cross check the correctness of the determined ground potential rise respective to the ground impedance. Here the ground potential rise has been determined to be 1792 V. According to EN 50522 the step and touch voltage measurement can be skipped if the ground potential rise does not exceed double the permissible touch voltage which is 176 V in this case due to a maximum fault duration of 600 ms. As this is not the case here, step and touch voltages must be measured.
Figure 11 shows the results of the touch voltage measurement. Locations # 1 – 40 are inside the 110 kV substation or in its close proximity, e.g. its fence. These locations don’t show any critical values as they all are lower than 176 V. Locations # 41 – 46 are close to or right at the 20 kV substation. Here the touch voltage significantly exceeds the permissible touch voltage of 176 V. After these failed touch voltages were measured the shield of the interconnecting 20 kV cable was isolated from the 110 kV substation (orange dot in Figure 9) and locations # 41 – 46 were measured again and showed much lower touch voltages. Due to the connection of the 20 kV substation to the 110 kV substation via the cable shield a significant portion of the grid current is returning via the 20 kV substation’s grounding system which is obviously under-sized for this purpose. If the cable shield is disconnected there is no grid current returning via the 20 kV grounding system anymore and therefore touch voltages are much lower.
In addition to the touch voltage at selected locations # 41 – 46 the fall-of-potential has been also measured by leaving the cable’s shield disconnected from the 110 kV substation as shown in the third curve in Figure 10. The ground potential rise has been determined to be 2328 V here which is clearly higher than for the measurement where the cable shield was connected. The increase of the ground potential rise is caused by the disconnection of the 20 kV substation’s grounding system which results in a higher ground impedance as the individual ground impedances of the 20 kV and the 110 kV substation are considered as parallel impedances.
This case study clearly demonstrates the necessity of grounding system tests by determination of touch voltages inside and outside the substation. It further demonstrates the accuracy of the test methods for the determination of the ground potential rise and touch voltages by comparing the measurements in Figure 10 as well as comparing touch voltage measurements at identical locations for different cable shield connections.
References
- EN 50522:2010: Earthing of power installations exceeding 1 kV a.c.
- IEEE 80-20135: IEEE Guide for Safety in AC Substation Grounding
- IEEE 81-2012: IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System
- G. Valtin, S. Böhme, M. Pikisch: “Methods for Grounding System Testing – A Comparison of the Polarity Reversal, Beat, and frequency-selective Method”
