2 Why measurement/inspection of the grounding system?

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1 Grounding System Testing and Assessment Authors: Moritz Pikisch, OMICRON electronics Deutschland GmbH Sirko Böhme, DNV GL Dresden 1 Abstract Grounding System tests must be performed after construction of electric facilities and on a regular basis every 4-5 years as mentioned e.g. in DGUV Vorschrift 3. 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 & 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. 2 Why measurement/inspection of the grounding system? Requirement from national standards or safety regulations regarding personnel safety Grounding measurements need to be carried out for verification of the grounding system s effectiveness regarding personnel safety and livestock during single-phase-to-ground faults according to the European Standard EN 5522:21 [1] and are often a requirement from national accident prevention regulations (e.g. the German DGUV Vorschrift 3 [4]) for safe operation of the plant. Triggers for measurement/inspection The necessity for ground measurements and/or inspections may be driven by one or more of the following triggers: Increase of fault currents, Commissioning of new substations/power plants, Extension of plants, Bad corrosive state from visual inspection, Change of neutral treatment leads to higher fault currents or different tripping times. 3 Ground Impedance Measurement Remote Substation Current injection to remote substation A ~ V Measurement perpendicular to current injection Figure 1: Fall-of-potential measurement by using an existing line for injection OMICRON electronics GmbH, 215 1

2 Fall-of-Potential / V Impedance / mω For the determination of a ground grid s ground impedance resp. the ground potential rise a test current is injected into the soil via a remote ground electrode. Usually the remote ground electrode is another substation where the current is injected via an existing power line between the substation under test and the remote substation. The line used for injection must be taken out of service for this purpose. If no line is available for testing purpose the test can also be injected via an adequately remote current probe. 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 1 and CP CU1, the polarity reversal method used by DNV GL and the beat method. Please refer to [5] 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 5522 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 2 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 1. Increasing the rod s distance results in an increase of the impedance resp. 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 2. 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. Fall-of-Potential Impedance Distance / m Figure 2: Fall-of-Potential and Impedance Diagram Further it must be considered that the angle between the measurement trace and the current s injection path is 9 as recommended in EN 5522 and IEEE 81. This is not always possible due to obstacles and inaccessible private property. These standards therefore require a minimum angle of 6. 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 the voltage caused by the potential rise. OMICRON electronics GmbH, 215 2

3 2 cm x 2 cm metal plate loaded with min. 5 kg 1 kω 4 Step and Touch Voltage Measurement For step & 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. V Figure 3: Touch voltage measurement setup according to EN 5522 EN 5522 suggests the personnel simulation method by measuring the touch voltage across a 1 kω resistor and using a metal plate which is simulating bare feet 1 m apart from the object. The plate must have dimensions of 2 cm x 2 cm and be loaded with at least 5 kg, ideally a person who steps on it (Figure 3). EN 5522 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 ( 1 ). EN 5522, Table 1 outlines the calculation of IG for every neutral configuration. Measuring and assessing step voltage is not mentioned explicitly in EN I G V Tmeas,max I meas r < V ( 1 ) Tp,EN5522 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. Hereby 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 soil 1 m apart from each other. For the assessment of step and touch voltages IEEE 8 therefore considers additional resistances which lead to higher permissible step and touch voltages than shown in Figure 4. Please refer to IEEE 8 chapter 8.3 in order to get the exact equations for the calculation of permissible step and touch voltages. OMICRON electronics GmbH, 215 3

4 Permissible Body Current in ma Permissible Touch Voltage in V Fault Duration in ms Biegelmeiers curve EN 5522 Dalziel 5 kg Dalziel 7 kg Figure 4: Permissible body currents and step & touch voltages 5 Reduction Factor Measurement I injected A ~ I return V I G Figure 5: Reduction factor measurement setup The reduction factor measurement determines the portion of the injected test current which is returning via the soil respectively 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 5. 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 5 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 return path. The individual currents must then be added by considering their phase angle in order to obtain the true value for the overall return current. The reduction factor r is then calculated according to formula ( 2 ) r = 1 I return I G = ( 2 ) I injected I injected OMICRON electronics GmbH, 215 4

5 Fall-of-Potential / V 6 Case Study I Test 2 kv /,4 kv Substation 11 kv / 2 kv Substation 11 kv line Remote Substation I E,I I E,II Figure 6: Grounding system configuration of case study This case study illustrates a grounding system test at a grounding system with the configuration in Figure 6. Close to the measured 11 kv / 2 kv substation a 2 kv /.4 kv substation is located. The shield of the 2 kv cable interconnects the grounding systems of both substations and merges them to one combined grounding system. The test current has been injected via a 11 kv line which has been grounded at a remote substation. For the sake of simplicity the two substations are called 11 kv and 2 kv substation. Trace 1, Shield connected Trace 1, Shield disconnected Trace 2, shield connected Distance / m Figure 7: Fall-of-potential measurement First the fall-of-potential measurement has been performed which provides the two curves in Figure 7. Additionally it also includes a third measurement which will be commented subsequently. 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 respectively the ground impedance. Here the ground potential rise has been determined to 1792 V. According to EN 5522 the step & touch voltage measurement can be skipped if the ground potential rise does not exceed double of the permissible touch voltage which is 176 V in this case due to a maximum fault duration of 6 ms. As this is not the case here step & touch voltages must be measured. OMICRON electronics GmbH, 215 5

6 Touch Voltage / V connected open Location # Figure 8: Touch voltage measurement Figure 8 shows the results of the touch voltage measurement. Locations # 1 4 are inside the 11 kv substation or in its close vicinity, e.g. its fence. These locations don t show any critical values as they all are lower than 176 V. Locations # are close to or right at the 2 kv substation. Here the touch voltage significantly exceeds the permissible touch voltage of 176 V. After these failed touch voltages have been measured the shield of the interconnecting 2 kv cable has been isolated from the 11 kv substation (orange dot in Figure 6) and locations # have been measured again and showing now much lower touch voltages. Due to the connection of the 2 kv substation to the 11 kv substation via the cable shield a significant portion of the grid current is returning via the 2 kv substation s grounding system which is obviously under-dimensioned for this purpose. If the cable shield is disconnected there is no grid current returning via the 2 kv grounding system anymore and therefore touch voltages are much lower. In addition to the touch voltage at selected locations # the fall-of-potential has been also measured by leaving the cable s shield disconnected from the 11 kv substation as shown in the third curve in Figure 7. The ground potential rise has been determined to 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 2 kv substation s grounding system which results in a higher ground impedance as the individual ground impedances of the 2 kv and the 11 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 7 as well as comparing touch voltage measurements at identical locations for different cable shield connections. OMICRON electronics GmbH, 215 6

7 7 Literature [1] EN 5522:21: Earthing of power installations exceeding 1 kv a.c. [2] IEEE 8-2: IEEE Guide for Safety in AC Substation Grounding [3] IEEE : IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System [4] DGUV Vorschrift 3: Elektrische Anlagen und Betriebsmittel [5] Methods for Grounding System Testing - A Comparison of the Polarity Reversal, Beat, and frequency-selective Method 8 Authors Moritz Pikisch studied Electrical Engineering at the University of Karlsruhe / Germany. After working as an instructor at OMICRON between 21 and 213, he switched to a product management role at the beginning of 214. In this new capacity, he is responsible for the development of testing solutions for line impedance measurement and the testing of grounding systems. Sirko Böhme is a qualified electrical engineer who, from 1995 onwards, has worked as a construction manager for industrial clients and wind power operators at various major construction sites around the world. Since 213, he has been working as a coordinator for various tasks including ground measurements, at DNV GL -Energy in Dresden. He is involved in the development of measurement tools and is responsible for performing high-quality ground measurements on customers' behalf. The preferred approach is the polarity reversal method, which has proven itself over many years. Other measurement tasks are associated with lightning protection, thermography, EMC, and the impact of high voltage levels. OMICRON electronics GmbH, 215 7

8 OMICRON is an international company serving the electrical power industry with innovative testing and diagnostic solutions. The application of OMICRON products allows users to assess the condition of the primary and secondary equipment on their systems with complete confidence. Services offered in the area of consulting, commissioning, testing, diagnosis and training make the product range complete. Customers in more than 14 countries rely on the company s ability to supply leadingedge technology of excellent quality. Service centers on all continents provide a broad base of knowledge and extraordinary customer support. All of this together with our strong network of sales partners is what has made our company a market leader in the electrical power industry. For more information, additional literature, and detailed contact information of our worldwide offices please visit our website. OMICRON