Online Monitoring of Substation Grounding Grid Conditions Using Touch and Step Voltage Sensors

Transcription

1 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 2, JUNE Online Monitoring of Substation Grounding Grid Conditions Using Touch and Step Voltage Sensors Xun Long, Student Member, IEEE, MingDong, Student Member, IEEE, Wilsun Xu, Fellow, IEEE, and Yun Wei Li, Member, IEEE Abstract A grounding grid of a substation is essential for reducing the ground potential rises inside and outside the substation during a short-circuit event. The performance of a grounding grid is affected by a number of factors, such as the soil conductivity and grounding rod corrosion. Industry always has a strong desire for a reliable and cost-effective method to monitor the condition of a grounding grid to ensure personnel safety and prevent equipments damage. In view of the increased adoption of telecom and sensor technologies in power industry through the smart grid initiative, this paper proposes an online condition monitoring scheme for grounding grids. The scheme monitors touch and step voltages in a substation through a sensor network. The voltages are created by a continuously-injected, controllable test current. The results are transmitted to a database through wireless telecommunication. The database evaluates the grid performance continuously by comparing the newly measured results with the historical data. Many of the limitations of the offline measurement techniques are overcome. Computer simulation studies have shown that the proposed scheme is highly feasible and technically attractive. Index Terms Online monitoring, step voltage, substation grounding, thyristor, touch voltage. I. INTRODUCTION P ROPER grounding is the first line of defense against lightning or other system contingency to ensure the safety of operators and power apparatus. A poor grounding system not only results in unnecessary transient damages, but also causes data and equipment loss, plant shutdown, as well as increases fire and personnel risk. As a result, Utility companies are actively seeking techniques that can effectively and reliably evaluate the grounding grid conditions to ensure personnel safety and prevent equipments damage. The performance of grounding grid is affected by various factors such as unqualified jointing while building, electromotive force of grounding current, soil erosion and theft of grounding rods [1]. Thus, monitoring and diagnosing the conditions of grounding grid has been an active research field for many years. However, almost all techniques implemented or proposed for grounding monitoring are offline types where special instruments are installed for grounding condition check on a regular Manuscript received May 11, 2011; revised August 28, 2011; accepted October 14, Date of publication February 13, 2012; date of current version May 21, This work was supported by icore. Paper no. TSG The authors are with the Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada ( xlong@ualberta.ca; mdong@ualbeta.ca; wxu@ualberta.ca; yunwei.li@ece.ualberta.ca). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TSG or as-needed basis. These existing methods can generally be categorized into two types: measurement of grounding impedance and detection of grounding integrity. Fall-of-Potential (FOP) method is the basic scheme for grounding impedance measurement and it has been implemented for many years [2]. Its key point is to correctly locate the potential probe, which is quite time-consuming. A lot of variations have been proposed to improve this scheme, such as by using variable frequency source [3]orimplementingmultiple electrodes [4]. The methods taking account of current split in transmission and distribution grounding system are further developed in [5], [6] for accurately measuring the impedance of in-service substations. However, the potential probes are still indispensable in these FOP-based schemes. Several enhanced grounding grid computer models are developed recently with considering soil layer depth in [7], [8] or based on electromagnetic field methods [9], [10]. But, the accuracy of these models relies on the soil resistivity measurement. Once the soil condition is changed [11], potential electrode needs to be relocated and it obviously increases the labor. Monitoring the integrity of grounding grid is another way to evaluate the performance of grounding grid [12], [13]. However, the computation of this method depends on many uncertain factors such as soil conductivity, humidity and climate [14]. A device based on measuring magnetic induction intensity is designed to diagnose the grounding grid corrosion in [15]. It requires the current injection between all possible grounding leads on the ground surface to increase accuracy, which is not practical in a large scale substation. All of the aforementioned methods are offline-based, which at best give one-shot measurement results. If another set of results is needed, the measurement system must be redeployed. The offline-based methods have significant disadvantages. Firstly, the results are largely dependent on the soil condition at the time of measurement. Secondly, sudden changes of the grounding grid such as those caused by theft cannot be identified timely. To solve these problems, the methods that can monitor the grounding condition on a continuous, i.e., online, basis is highly desired. This paper proposes an online substation touch and step voltage monitoring scheme, which can continuously inject testing current into a grounding grid and then measure the corresponding touch and step voltages. The testing current is created by a thyristor-based signal generator which is connected between single energized phase conductor and ground to stage a temporary and controllable fault. There is no extra cable needed for current flow as power line is utilized as a path /$ IEEE

2 762 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 2, JUNE 2012 Fig. 1. The process of the proposed online monitoring scheme. for current injection. Touch and step voltages, which directly reflect operational safety of substation, are used as indicators of grounding condition. As the measurement of touch/step voltages does not require long cables extending outside of a substation [16], it is very suitable for long-term online monitoring. Supported by the historical data made available through the online database which wirelessly communicate with the voltage sensors, the variation of the measured data can be used to infer the change of grounding grid conditions. Fig. 2. The remote current injection scheme. II. THE PROPOSED ONLINE MONITORING SCHEME As shown in Fig. 1, the process of the proposed online monitoring scheme includes a) testing current generation and injection; b) touch voltage and step voltage measurement; and c) safety assessment based on both data variation and actual values. If the variation between the newly measured data and historical data is below the preset threshold plus the measured data does not exceed the safe value defined in the IEEE standard [1], the grounding grid under test is considered to be in a good condition and the next test will be made after a preset period. Otherwise, a warning event will be created and then mandatory inspections in the suspected spots with high touch voltage or step voltage will be carried out. A. Testing Current Generation and Injection The signal generator for testing current generation consists of a pair of thyristors connected to the supply via a singlephase step-down transformer, which convert high voltage to low voltage for the normal operation of the thyristors. When the thyristors are fired under a preset firing angle, a testing current will be injected into the system from the primary side of the transformer [17]. The two thyristors are operated alternately to create a sinusoidal waveform. To reliably measure the resulting touch and step voltages, the duration of injected current cannot to be too short to establish stable potential profiles [18]. The minimum time for tolerable touch or step voltage calculation is 30 ms according to [1]. On the other hand, the injected current is required not to interrupt the normal operation of grounding fault protection relay, in which the minimum trip time is about 100 ms [19]. In this work, the duration of current injection is therefore setup as 50 ms, which is within the range between 30 ms to 100 ms. Not like grounding impedance measurement, which needs square waveform to obtain various frequency components to avoid the fundamental frequency interference from power system or requires lightning waveforms to measure transient impedance, this paper focuses on safety evaluation at substations and the sinusoidal waveform is used to mimic a shot-circuit fault. Fig. 3. The local current injection scheme. This signal generator can be installed either remotely or locally. In the remote source scheme (see Fig. 2), the signal generator is installed at a downstream site far from the substation to minimize the impact of the current injection to the ground potential profile. As the ground can be utilized for current path from the injected site to the substation grounding grid, the extra current cable is not necessary [20]. In the local source scheme, the signal generator including a step-down transformer is installed in the substation as shown in Fig. 3. The current is directly injected from the substation and it returns from the remote power source. Since the device is located in a substation, maintenance can be conveniently achieved, which is important for long-term monitoring. However, a large transformer is needed as the signal generator has to be installed at the high voltage side in a substation for the local source scheme. This signal generator cannot be installed at the grounded secondary side in a substation, since a current loop is established by the grounded neutral and the test current will not pass through the remote earth [21]. B. Touch/Step Voltage Based Sensor Network The current injected into the grounding grid results in rises of touch voltage and step voltage, which directly indicate the safety situation in and around the substation under test. Touch voltage is defined as the potential difference between an exposed metallic structure within reach of a person and a point where that person is standing on the earth, while step voltage is defined as the difference in potential between two points in the earth spaced 1 meter (or a step) apart [22]. The measurement of touch and step voltages can be easily conducted at many points of interest in a substation, which is very suitable for long-term online monitoring. Moreover, the interference with potential electrode and long cable installation when conducting impedance measurement is also eliminated. We further proposed to use a wireless sensor network for touch and step voltages monitoring (see Fig. 4). Typically, a

3 LONG et al.: ONLINE MONITORING OF SUBSTATION GROUNDING GRID CONDITIONS USING TOUCH AND STEP VOLTAGE SENSORS 763 Fig. 6. The Thevenin equivalent circuits of: (a) touch voltage measurement; (b) step voltage measurement. Fig. 4. The sensors network of touch/step voltages measurement. (2) Fig. 5. (a) The voltage measurement (1-step voltage, 2-touch voltage). (b) The structure of the voltage sensor. grounding grid is buried 0.5 m 1 m under ground, which results in potential difference at the surface of ground. The touch/ step voltage sensors are distributed at corners of a grounding grid and some other frequently visited spots with special concern of human safety. All of these sensors can transmit signals wirelessly to a computer which could be located indoors. The computer is responsible to collect, classify and update the data recorded in the database. According to the IEEE Standard 81.2 [22], the simulatedpersonnel method is recommended for touch and step voltages measurement. This method utilizes a resistor with 1000 resistance represents human body resistance and is connected between two feet. Each foot is made by a metallic plate with 200 cm surface area and 20 kg weight. A voltage meter is installed across the resistor with high internal impedance so as not to influence the measurements. A device is designed to measure either touch voltage or step voltage as seen in Fig. 5. Note that, the distance between two feet is adjustable, which is 0.5 m for measurement and 1 m for measurement, respectively. Moreover, an extra probe is used to contact the exposed conductive surface for touch voltage measurement. It uses a voltage transducer to measure the voltage across, and then the measured value is converted to the digital format by an ADC module. A MCU processes the data and the results are finally transmitted to a central computer through a RF module. In the project, Zigbee 2.4 GHz wireless signal transmission is recommended and its range can be reach up to 300 ft, which is adequate for a small or medium size distribution substation. Moreover, it can be easily configured to handle wireless sensor networking application at a low cost. From the Thevenin equivalent circuit of the touch/step voltage measurement as shown in Fig. 6, it is found that the measured or is not the same as the potential difference on the ground, and the touch or step voltage can be expressed as (1) where is the potential difference between the feet and the touch point, is the touch voltage, is the foot resistance when two feet are in parallel, is the human body resistance (1 k ), is the potential difference between two feet, is the step voltage, is the foot resistance when two feet are in series. However, the metallic plates installed on the surface of the ground are likely to be corroded due to humidity or other factors, which results in the increase of their resistance accordingly. Equation (1) and (2) indicate that the measured voltage ( or ) decreases with the increase of the feet resistance ( or ) under the same potential difference ( or ). In this case, the measured touch/step voltage will be lower than the normal value and it may mislead the assessment. To eliminate the effects of the feet resistance variation, the voltages are measured twice, one in a close circuit during one signaling period and the other in an open circuit during the next signaling period. As the switching is operated after current injection, it would not cause arcing and it also has no requirement on the switching speed. As shown in Fig. 6, an electrical contactor is utilized for the switching purpose. Apparently, the voltage or in an open circuit is equal to the potential difference or. Resolve (1) and (2), the resistance of or can be obtained. If the variation between the estimated (or )andits nominal value is larger than a predetermined value, the measured (or ) cannot be directly used. In this case, the metallic feet need to be replaced by a new pair of plates. Alternatively, these voltages ( and ) can be adjusted according to the following equations: where is the nominal value of is the nominal value of. From the study of the potential profile of a substation, it is found that there are several suspected spots in or around a substation, particularly in a substation s corners or around the fences. Therefore, before installing the measurement tools, it (3) (4) (5) (6)

4 764 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 2, JUNE 2012 is necessary to inspect those suspected points. One solution is pretest. A workman walks across a substation with the test device to record the locations where the measured voltages exceed the preset threshold. Further measurements can then be made by online monitoring in the suspected locations, thus reducing the time and cost of measurements. Another solution is to find the suspected spots in the potential profile from a computer simulation results. In this case, the accuracy of locating the danger points highly depends on the accuracy of the simulation model. According to [1], the limit of touch/step voltage is a function of a) shock duration (i.e., fault clearing time); b) system characteristics; c) body weight; and d) foot contact resistance as shown in (7) and (8). The constant is0.116forapersonwith50kg body weight, while it is for 70 kg. Since the injected current is much smaller than the maximum fault current, the measured touch/step voltage is therefore much lower than the limits defined in (7) and (8). Thus, the original measured data is intentionally increased to maximum value by (9) and (10) when the data transfer to the database. The decision is then made by the comparison of the measured data with historical data or with the maximum tolerable values provided by (7) and (8). (7) (8) (9) (10) For personnel safety evaluation, touch voltage is more severe than step voltage [23]. The current caused by touching an exposed conductor flows through the heart, whereas the one caused by step voltage bypass the heart. Therefore, the tolerate touch voltage is much lower than tolerate step voltage. Generally, satisfying the touch voltage safety criteria in a substation automatically ensures the satisfaction of the step voltages safety criteria. In this project, most areas in the substation are examined for touch voltage, and only the edges of the grid are examined for step voltage. C. Intelligent Evaluation Techniques With Database Another novel feature of the proposed scheme is the implementation of online database. It is known that the resistivity of the surface soil layer would be changed in different seasons, which may results in touch/step voltages moving to the hazard side [14]. For example, if the thickness of the low-resistivity soil layer in raining season is smaller than the buried depth of the grounding grid, the touch voltage increase. In another case, the high resistivity soil layer formed in freezing season would cause the increase of the touch/step voltage with the thickness or resistivity of the freezing soil layer. One major defect of the existing offline monitoring method is the inability of tracking seasonal influences on the safety of substationgroundingsystem. With the support of database, we can continuously monitor and record the change of touch/step voltage. Particularly, during the severe conditions, like continuous raining or freezing seasons, the frequency of online monitoring can be increased in order to find the potential hazards in time. Corrosion, which can damage the effective connections among the conductors, is another factor affecting the safety of the grounding system. The grounding grid corrosion is caused by acid or alkali in soil and the corrosion rate can reach up to 8.0 mm per year according to statistic results [24]. This situation becomes more serious as the steel-grounding or galvanized steel-grounding system is widely used, which is more easily corroded than copper so that it requires more accurate, timely assessment of grounding grid. While the corrosion of grounding grids may be detected by regular off-line measurements as it is a slow process, the theft of grounding rods, another major concern to utility companies, can suddenly change the integrity of the grounding grid. Failing to detect this change in a timely manner will cause serious consequences. In the proposed online monitoring scheme, the change of touch and step voltages at the same point are recorded, so that synthesized and reliable estimation can be made not only depending on the IEEE standard but also on the variation due to seasonal influence, corrosion or theft. Based on the above analysis, an intelligent evaluation (see Fig. 7) can be made as follows: 1) Generate and inject a testing current into a grounding grid periodically. Measure the resulting touch/step voltages with the sensor network and transfer the data to the central database. 2) Scale the measured touch/step voltages to the maximum touch/step voltages. 3) Compare the maximum touch/step voltages with IEEE standard under the same parameters, like fault clearing time and the body weight, etc. If it exceeds the safe value, a warning event is created and the suspected location is reported to substation operators for further analysis. 4) Compare the measured touch/step voltages with the historical data at the same location. If the variation is larger than the preset threshold, a warning event is created even though the actual value does not exceed the standard. A mandatory examination will be taken around the suspected point to check if the conductors are stolen or broken due to corrosion. 5) If no suspected spot is found, the database is updated with the new measured data and meteorological parameter, such as temperature and humility. Then, after a preset period, go back to 1). III. STUDY OF CURRENT DISTRIBUTION A simulation model is built in PSCAD to study current distribution of both remote and local injection schemes as shown in Fig. 8. The distribution substation under test transfers power from 125 kv to 25 kv via a Delta-Yg connection transformer. An overhead ground wire, so called skywire, accompanies with transmission lines and the ground resistance of a transmission line tower is 32. At the secondary side, the neutral line of distribution system is multiple-grounded with 15 at each

5 LONG et al.: ONLINE MONITORING OF SUBSTATION GROUNDING GRID CONDITIONS USING TOUCH AND STEP VOLTAGE SENSORS 765 Fig. 7. The evaluation process with database. Fig. 10. Current distribution of the remote scheme with respect to distance from subtation. Fig. 8. Computer simulation for current distribution study. Fig. 11. Current waveforms of current distribution study. Fig. 9. The structures of transmission line tower and distribution line pole. TABLE I PARAMETERS OF COMPUTER SIMULATION FOR CURRENT DISTRIBUTION STUDY grounded connection. The structure details of the transmission line tower and the distribution line pole are illustrated in Fig. 9. Other parameters are listed in Table I. In the remote injection scheme, a temporary fault is staged at downstream of the under-test substation to create a fault current flowing from the faulted phase to ground and back to the substation. However, with the presence of the multiple grounded points on the neutral, such as pole grounds and transformer grounds, the current is divided before reaching the substation grounding grid. As shown in Fig. 10, the current division of the remote injection scheme depends on the distance between the location of the staged fault and the substation. When the staged fault is located 5 km away from the tested substation, only 37% current flows back through substation grounding grid. The local injection scheme requires a pair of thyristors connected between a single phase of transmission line and the grounding grid by a step-down transformer. Because of the existence of overhead ground wires and neutral lines, not all fault current flow through the grounding grid to the remote earth [21]. The simulation results (see Table II) show that 73.58% current across the grounding grid, 26.70% current in the skywire and 10.59% current in the neutral line. Disconnecting the skywire and the neutral line can largely increase the grounding grid current. However, it is impossible to disconnect these ground wires for long time monitoring in reality. From touch voltage simulation which will be discussed later, 60 A grounding grid current can result in about 3 V 13 V touch voltage, which is large enough for effective detection. The current waveforms for the local injection scheme are shown in Fig. 11. Typically, there are relays implemented in the substation for ground fault protection. These protective relays have an inverse current/time characteristic, which suggests they can tolerate high current with a short duration. As the duration of the injected 60 A current is about 50ms, shorter than 0.1 s, it does not interrupt the normal operation of the protective relays [19]. The proposed local scheme is also applicable to the substations with Yg-Yg or Y-Yg connection. As shown in Fig. 12, both the primary side and secondary side of the transformer are Yg connection and the neutral points are connected in the grounding grid. A staged fault is created at the primary side when the thyristors are turned on for 50 ms. The computer simulation results are listed in Table III. If all the grounded wires

6 766 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 2, JUNE 2012 TABLE II CURRENT DISTRIBUTION OF THE LOCAL SCHEME WITH A DELTA-YG CONNECTION TRANSFORMER Fig. 13. The designed grounding grid in the computer simulation. Fig. 12. Current distribution study of Yg-Yg connection substation. TABLE III CURRENT DISTRIBUTION OF YG-YG AND Y-YG CONNECTION TABLE IV THE SOIL RESISTIVITY MEASUREMENTS DATA WITH THE FOUR-PIN METHOD PROVIDED BY IEEE STANDARD are connected, the current ratio of is 81.9% for Yg-Yg connection and it is 76.8% for Y-Yg connection. TABLE V COMPARISON OF THE SIMULATION RESULTS AND IEEE VALUES IV. COMPUTER SIMULATION OF THE PROPOSED ONLINE MONITORING SCHEME Computer simulations have also been conducted in CYMGRD [25] to measure touch/step voltages, illustrate the process of intelligent evaluation scheme and analyze the influence of seasons, corrosion and theft. The designed grounding grid as shown in Fig. 13 is 150 m long and 100 m wide. All conductors are buried at a depth of 0.5 m. X-axis has 8 conductors and Y-axis has 10 conductors. The diameter of all conductors is 19.1 mm. Plus, 30 grounding rods are vertically connected to the grounding grid. Each rod is 5 m long with diameter cm. Moreover, the station surface is with crushed rock of 2500 ohm-meter resistivity at a thickness of 0.3 m and the exposure duration is 0.36 s with 4000 A fault current. To begin touch/step voltages simulation, we firstly interpret the soil resistivity measurements and obtain a soil model for the subsequent analysis. A two-layer soil model is implemented in this simulation. From the data provided by IEEE standard (see Table IV), both the upper and lower layers resistivity can be calculated and the depth of the upper layer can be estimated as well. The result of soil model calculation is consistent with the IEEE calculated values (see Table V), which proves the validity of the designed two-layer soil model. Furthermore, the maximum permissible touch and step voltages in accordance with the substation surface and the shock time can be calculated by (7) and (8), which is V and V respectively. The potential profile of the grounding grid diagonal line is shown in Fig. 14. Apparently, touch voltage at the corner is much larger than in the middle center. It is due to less conductors buried around corners than around center. The suspected points can be clearly located from this potential profile, which is very useful for installation of the voltage sensors. This profile also confirms that the value of maximum permitted touch potential has a dominant role in determining the design of the grounding grid. If a grid satisfies the requirements for safe touch potentials, it is very unlikely that the maximum permitted step potential will be exceeded. In Fig. 14, the margin between the calculated touch voltage and the permissible touch voltage is about 200 V 800 V, while this margin for step voltage is as large as 3500 V. As the injected current through the grounding grid is actually about A, the concern here is if the A current is able to result in detectable touch/step voltage. The profile in Fig. 15 is obtained with 60 A grounding grid current, which causes the touch voltage between 3 13 V. The voltage in this range can be easily detected by the voltage sensors. For safety evaluation, the actual voltages are scaled up to the maximum values in the database according to (9) and (10). With the support of database, synthesized and reliable estimation can be made depending on IEEE standard constraint and recorded data variation. To better clarify the concept of the

7 LONG et al.: ONLINE MONITORING OF SUBSTATION GROUNDING GRID CONDITIONS USING TOUCH AND STEP VOLTAGE SENSORS 767 Fig. 14. The potential profile of the grounding grid diagonal line. Fig. 17. The potential profile after theft. Fig. 18. The soil resistivity in different seasons. Fig. 15. Fig. 16. The potential profile with 60 A grounding grid current. The conductors on the left edge are stolen. safety evaluation with database, we consider two scenarios, one is theft and the other is the change of soil resistivity due to seasonal influence. Theft is a serious threat for the safety of substation. As shown in Fig. 16, the conductors on the left edge of substation are stolen so that touch voltage around that area is largely increased as illustrated in Fig. 17. When comparing the profiles of Figs. 14 and 17, it is easy to detect the difference in the corner area. An alarm is created immediately and investigation in the corner should be made as soon as possible. On the other hand, touch voltages at some spots also exceed the limit, a mandatory examine is required at these locations. As mentioned above, soil resistivity depends on a number of factors: soil type, chemical composition, moisture, temperature, etc. For an existing grounding grid, it is mainly affected by seasonal variations. Especially in North America, the frozen soil in winter is a hazard for grounding grid safety. Fig. 18 is the field test data of soil resistivity in 12-month study [11]. It is apparent that during the summer, the resistivity becomes lower due to high precipitation, while the resistivity goes higher during the winter because of the frozen soil. The influence of these variations on the touch/step voltages are analyzed in the following simulation. Three locations are picked up from the left-bottom corner to the middle of the grid as shown in Fig. 19 and the touch voltages are investigated for 12 months (Fig. 20). When a fault occurs in June, all touch voltages are under the limit. However, if a fault happens in December, the increase of soil resistivity cause the touch voltages increase. It is clear that some of the touch voltages exceed the limit. In this case, if an offline test is taken in June but not in December, these danger spots cannot be founded; whereas, in the proposed online monitoring, these spots can be reported in time with monthly evaluation. The frequencies of the measurement can be adjusted according to the requirement of a utility company. More frequently the measurements are conducted, more timely the danger spots can be found. V. CONCLUSION In this paper, an online monitoring scheme for substation safety assessment is proposed, which periodically measures touch and step voltages with a preset frequency and effectively evaluates the grounding grid conditions with the help of a database. The test current is generated by firing a pair of thyristors

8 768 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 2, JUNE 2012 For example, the proposed scheme is focused on touch and step voltage indices which are related to personnel safety concerns. Since grounding design has other objectives such as facilitating equipment protection, the proposed scheme needs to be further expanded to include sensors and indices that address equipment protection concerns. It is possible that acceptable touch and step voltages at sufficient locations in a substation may imply an acceptable grounding condition from equipment protection perspectives. But research is needed to verify this postulation. The proposed scheme involves a sensor network and its data collections. There are many challenges to build and maintain such networks. The reliability of the network needs to be confirmed as well. These are exactly the subjects of interest to ICT- (information and communication technology) oriented smart grid researchers. Fig. 19. Three suspect spots are picked up from the diagonal line. REFERENCES Fig. 20. Touch voltage of three suspect spots in different seasons. for 50 ms. The current can be injected remotely or locally. The local injection scheme has a larger portion of the injected current flowing through the grounding grid, but it costs more than the remote scheme due to high rating voltage and high capacity of the step-down transformer. The condition monitoring is achieved with a wireless touch/step voltage sensors network installed at various locations of a substation. These sensors are connected to a central database where an evaluation process is carried out by comparing the newly measured data to the limits from IEEE standard, or checking if the data variation at the same spot exceeds safety thresholds. Furthermore, current distribution has been studied with computer simulations, which verified the effectiveness of the proposed local and remote schemes. From the case studies of conductor theft and seasonal influences, the advantage of online monitoring is very clear since some danger spots cannot be found in time without continuous measurement. With further research, the proposed scheme could be used to locate broken section or missing grounding electrode based on the step/touch voltage profile obtained from the sensors. Compared to offline methods, which at best gives one-shot assessment, the proposed online grounding grid monitoring scheme is more effective and reliable, and it could become an important component of a smart substation. The paper has presented an overall concept of the proposed monitoring scheme. A lot more research works are still needed. [1] IEEE Guide for Safety in AC Substation Grounding, IEEE Std , [2] IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System, IEEE Std , [3] I. Lu and R. 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9 LONG et al.: ONLINE MONITORING OF SUBSTATION GROUNDING GRID CONDITIONS USING TOUCH AND STEP VOLTAGE SENSORS 769 [18] R. Verma and D. Mukhedkar, Fundamental considerations and impulse impedance of grounding grids, IEEE Trans. Power App. Syst., vol. PAS-100, no. 3, pp , Mar [19] P. Rush, Network Protection & Automation Guide, ALSTOMT&D Energy Automation & Information [20] J. Sarmiento, H. G. Fortin, and D. Mukhedkar, Substation ground impedance: Comparative field measurements with high and low current injection methods, IEEE Trans. Power App. Syst., vol. PAS-103, no. 7, pp , Jul [21] S. Patel, A complete field analysis of substation ground grid by applying continuous low voltage fault, IEEE Trans. Power App. Syst., vol. PAS-104, no. 8, pp , Aug [22] IEEE Guide to Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems, IEEEStd. 81.2, [23] R. Kosztaluk, R. Mukhedkar, and Y. Gervais, Field measurements of touch and step voltages, IEEE Trans. Power App. Syst., vol. PAS-103, no. 11, pp , Nov [24] P. Sen and N. Mudarres, Corrosion and steel grounding, in Proc nd Annu. North Amer. Power Symp., pp [25] Cooper, Dec. 2009, CYMGRD Substation Grounding Program [Online]. Available: Xun Long (S 08) received the B.E. and M.Sc degrees in electrical engineering from Tsinghua University, Beijing, China, in 2004 and 2007, respectively, and iscurrentlyworkingtowardtheph.d.degreeintheelectrical and Computer Engineering Department, University of Alberta, Edmonton, Canada. His main research interests include power line signaling, distributed generation and fault detection. Ming Dong (S 08) received the B.Eng. degree in electrical engineering from Xi anjiaotonguniversity,china,in2004.heiscurrentlyworkingtowardthe Ph.D. degree in electrical and computer engineering with the University of Alberta, Edmonton, Canada. His research covers smart grid, grounding systems, and power quaility. Wilsun Xu (F 05) received the Ph.D. degree from the University of British Columbia, Vancouver, Canada, in From 1989 to 1996, he was an Electrical Engineer with BC Hydro, Vancouver and Surrey, respectively. Currently, he is with the Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Canada, where he has been since His research interests are power quality and distributed generation. Yun Wei Li (S 04 M 05) received the B.Sc. degree in engineering from Tianjin University, China, in 2002 and the Ph.D. degree from Nanyang Technological University, Singapore, in In 2005, he was a Visiting Scholar with the Institute of Energy Technology, Aalborg University, Denmark. From 2006 to 2007, he was a Postdoctoral Research Fellow in the Department of Electrical and Computer Engineering, Ryerson University, Canada. After working with Rockwell Automation Canada in 2007, he joined the Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Canada, as an Assistant Professor. His research interests include distributed generation, microgrid, power converters, and electric motor drives.