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IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 2, JUNE 2012
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Online Monitoring of Substation Grounding GridConditions Using
Touch and Step Voltage SensorsXun Long, Student Member, IEEE, Ming
Dong, Student Member, IEEE, Wilsun Xu, Fellow, IEEE, and
Yun Wei Li, Member, IEEE
AbstractA grounding grid of a substation is essential for
re-ducing the ground potential rises inside and outside the
substationduring a short-circuit event. The performance of a
grounding gridis affected by a number of factors, such as the soil
conductivityand grounding rod corrosion. Industry always has a
strong desirefor a reliable and cost-effective method to monitor
the conditionof a grounding grid to ensure personnel safety and
prevent equip-ments damage. In view of the increased adoption of
telecom andsensor technologies in power industry through the smart
grid ini-tiative, this paper proposes an online condition
monitoring schemefor grounding grids. The scheme monitors touch and
step voltagesin a substation through a sensor network. The voltages
are createdby a continuously-injected, controllable test current.
The resultsare transmitted to a database through wireless
telecommunication.The database evaluates the grid performance
continuously by com-paring the newly measured results with the
historical data. Manyof the limitations of the offline measurement
techniques are over-come. Computer simulation studies have shown
that the proposedscheme is highly feasible and technically
attractive.
Index TermsOnline monitoring, step voltage, substationgrounding,
thyristor, touch voltage.
I. INTRODUCTION
P ROPER grounding is the first line of defense against
light-ning or other system contingency to ensure the safety
ofoperators and power apparatus. A poor grounding system notonly
results in unnecessary transient damages, but also causesdata and
equipment loss, plant shutdown, as well as increasesfire and
personnel risk. As a result, Utility companies are ac-tively
seeking techniques that can effectively and reliably eval-uate the
grounding grid conditions to ensure personnel safetyand prevent
equipments damage.The performance of grounding grid is affected by
various fac-
tors such as unqualified jointing while building,
electromotiveforce of grounding current, soil erosion and theft of
groundingrods [1]. Thus, monitoring and diagnosing the conditions
ofgrounding grid has been an active research field for many
years.However, almost all techniques implemented or proposed
forgrounding monitoring are offline types where special
instru-ments are installed for grounding condition check on a
regular
Manuscript received May 11, 2011; revised August 28, 2011;
accepted Oc-tober 14, 2011. Date of publication February 13, 2012;
date of current versionMay 21, 2012. This workwas supported by
iCORE. Paper no. TSG-00175-2011.The authors are with the Department
of Electrical and Computer Engineering,
University of Alberta, Edmonton, AB T6G 2V4, Canada (e-mail:
[email protected]; [email protected]; [email protected];
[email protected]).Color versions of one or more of the
figures in this paper are available online
at http://ieeexplore.ieee.org.Digital Object Identifier
10.1109/TSG.2011.2175456
or as-needed basis. These existing methods can generally be
cat-egorized into two types: measurement of grounding impedanceand
detection of grounding integrity.Fall-of-Potential (FOP) method is
the basic scheme for
grounding impedance measurement and it has been imple-mented for
many years [2]. Its key point is to correctly locatethe potential
probe, which is quite time-consuming. A lot ofvariations have been
proposed to improve this scheme, such asby using variable frequency
source [3] or implementing mul-tiple electrodes [4]. The methods
taking account of current splitin transmission and distribution
grounding system are furtherdeveloped in [5], [6] for accurately
measuring the impedanceof in-service substations. However, the
potential probes are stillindispensable in these FOP-based schemes.
Several enhancedgrounding grid computer models are developed
recently withconsidering soil layer depth in [7], [8] or based on
electro-magnetic field methods [9], [10]. But, the accuracy of
thesemodels relies on the soil resistivity measurement. Once
thesoil condition is changed [11], potential electrode needs to
berelocated and it obviously increases the labor.Monitoring the
integrity of grounding grid is another way to
evaluate the performance of grounding grid [12], [13]. How-ever,
the computation of this method depends on many uncer-tain factors
such as soil conductivity, humidity and climate [14].A device based
on measuring magnetic induction intensity isdesigned to diagnose
the grounding grid corrosion in [15]. It re-quires the current
injection between all possible grounding leadson the ground surface
to increase accuracy, which is not prac-tical in a large scale
substation.All of the aforementioned methods are offline-based,
which
at best give one-shot measurement results. If another set
ofresults is needed, the measurement system must be redeployed.The
offline-based methods have significant disadvantages.Firstly, the
results are largely dependent on the soil conditionat the time of
measurement. Secondly, sudden changes ofthe grounding grid such as
those caused by theft cannot beidentified timely. To solve these
problems, the methods that canmonitor 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 injecttesting
current into a grounding grid and then measure thecorresponding
touch and step voltages. The testing current iscreated by a
thyristor-based signal generator which is con-nected between single
energized phase conductor and groundto stage a temporary and
controllable fault. There is no extracable needed for current flow
as power line is utilized as a path
1949-3053/$31.00 2012 IEEE
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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
directlyreflect operational safety of substation, are used as
indicatorsof grounding condition. As the measurement of
touch/stepvoltages does not require long cables extending outside
of asubstation [16], it is very suitable for long-term online
moni-toring. Supported by the historical data made available
throughthe online database which wirelessly communicate with
thevoltage sensors, the variation of the measured data can be
usedto infer the change of grounding grid conditions.
II. THE PROPOSED ONLINE MONITORING SCHEME
As shown in Fig. 1, the process of the proposed online
mon-itoring scheme includes a) testing current generation and
in-jection; b) touch voltage and step voltage measurement; andc)
safety assessment based on both data variation and actualvalues. If
the variation between the newly measured data andhistorical data is
below the preset threshold plus the measureddata does not exceed
the safe value defined in the IEEE standard[1], the grounding grid
under test is considered to be in a goodcondition and the next test
will be made after a preset period.Otherwise, a warning event will
be created and then mandatoryinspections in the suspected spots
with high touch voltage orstep voltage will be carried out.
A. Testing Current Generation and Injection
The signal generator for testing current generation consistsof a
pair of thyristors connected to the supply via a single-phase
step-down transformer, which convert high voltage to lowvoltage for
the normal operation of the thyristors. When thethyristors are
fired under a preset firing angle, a testing currentwill be
injected into the system from the primary side of thetransformer
[17]. The two thyristors are operated alternately tocreate a
sinusoidal waveform. To reliably measure the resultingtouch and
step voltages, the duration of injected current cannotto be too
short to establish stable potential profiles [18]. Theminimum time
for tolerable touch or step voltage calculation is30 ms according
to [1]. On the other hand, the injected currentis required not to
interrupt the normal operation of groundingfault protection relay,
in which the minimum trip time is about100 ms [19]. In this work,
the duration of current injection istherefore setup as 50 ms, which
is within the range between30 ms to 100 ms. Not like grounding
impedance measurement,which needs square waveform to obtain various
frequency com-ponents to avoid the fundamental frequency
interference frompower system or requires lightning waveforms to
measure tran-sient impedance, this paper focuses on safety
evaluation at sub-stations and the sinusoidal waveform is used
tomimic a shot-cir-cuit fault.
Fig. 2. The remote current injection scheme.
Fig. 3. The local current injection scheme.
This signal generator can be installed either remotely or
lo-cally. In the remote source scheme (see Fig. 2), the signal
gen-erator is installed at a downstream site far from the
substationto minimize the impact of the current injection to the
groundpotential profile. As the ground can be utilized for current
pathfrom the injected site to the substation grounding grid, the
extracurrent cable is not necessary [20].In the local source
scheme, the signal generator including a
step-down transformer is installed in the substation as shownin
Fig. 3. The current is directly injected from the substationand it
returns from the remote power source. Since the deviceis located in
a substation, maintenance can be convenientlyachieved, which is
important for long-term monitoring. How-ever, a large transformer
is needed as the signal generator hasto be installed at the high
voltage side in a substation for thelocal source scheme. This
signal generator cannot be installedat the grounded secondary side
in a substation, since a currentloop is established by the grounded
neutral and the test currentwill not pass through the remote earth
[21].
B. Touch/Step Voltage Based Sensor NetworkThe current injected
into the grounding grid results in rises
of touch voltage and step voltage, which directly indicate
thesafety situation in and around the substation under test.
Touchvoltage is defined as the potential difference between an
exposedmetallic structure within reach of a person and a point
where thatperson is standing on the earth, while step voltage is
definedas the difference in potential between two points in the
earthspaced 1 meter (or a step) apart [22]. The measurement of
touchand step voltages can be easily conducted at many points of
in-terest in a substation, which is very suitable for long-term
onlinemonitoring. Moreover, the interference with potential
electrodeand long cable installation when conducting impedance
mea-surement is also eliminated.We further proposed to use a
wireless sensor network for
touch and step voltages monitoring (see Fig. 4). Typically,
a
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LONG et al.: ONLINE MONITORING OF SUBSTATION GROUNDING GRID
CONDITIONS USING TOUCH AND STEP VOLTAGE SENSORS 763
Fig. 4. The sensors network of touch/step voltages
measurement.
Fig. 5. (a) The voltage measurement (1-step voltage, 2-touch
voltage). (b) Thestructure of the voltage sensor.
grounding grid is buried 0.5 m1 m under ground, which re-sults
in potential difference at the surface of ground. The touch/step
voltage sensors are distributed at corners of a groundinggrid and
some other frequently visited spots with special con-cern of human
safety. All of these sensors can transmit signalswirelessly to a
computer which could be located indoors. Thecomputer is responsible
to collect, classify and update the datarecorded in the
database.According to the IEEE Standard 81.2 [22], the
simulated-
personnel method is recommended for touch and step
voltagesmeasurement. This method utilizes a resistor with 1000
re-sistance represents human body resistance and is connected
be-tween two feet. Each foot is made by a metallic plate with200 cm
surface area and 20 kg weight. A voltage meter is in-stalled across
the resistor with high internal impedance so as notto influence the
measurements. A device is designed to measureeither touch voltage
or step voltage as seen in Fig. 5. Note that,the distance between
two feet is adjustable, which is 0.5 mfor measurement and 1 m for
measurement, respec-tively. Moreover, an extra probe is used to
contact the exposedconductive 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
byan ADC module. A MCU processes the data and the results
arefinally transmitted to a central computer through a RF module.In
the project, Zigbee 2.4 GHz wireless signal transmission
isrecommended and its range can be reach up to 300 ft, whichis
adequate for a small or medium size distribution
substation.Moreover, it can be easily configured to handle wireless
sensornetworking 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
themeasured or is not the same as the potential difference onthe
ground, and the touch or step voltage can be expressed as
(1)
Fig. 6. The Thevenin equivalent circuits of: (a) touch voltage
measurement;(b) step voltage measurement.
(2)
where is the potential difference between the feet and thetouch
point, is the touch voltage, is the foot resistancewhen two feet
are in parallel, is the human body resistance(1 k ), is the
potential difference between two feet, isthe step voltage, is the
foot resistance when two feet are inseries.However, the metallic
plates installed on the surface of the
ground are likely to be corroded due to humidity or other
fac-tors, 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
thenormal 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
onesignaling period and the other in an open circuit during the
nextsignaling period. As the switching is operated after current
in-jection, it would not cause arcing and it also has no
require-ment on the switching speed. As shown in Fig. 6, an
electricalcontactor is utilized for the switching purpose.
Apparently, thevoltage or in an open circuit is equal to the
potentialdifference or . Resolve (1) and (2), the resistance ofor
can be obtained.
(3)
(4)
If the variation between the estimated (or ) and itsnominal
value is larger than a predetermined value, the mea-sured (or )
cannot be directly used. In this case, the metallicfeet need to be
replaced by a new pair of plates. Alternatively,these voltages (
and ) can be adjusted according to the fol-lowing equations:
(5)
(6)
where is the nominal value of is the nominalvalue of .From the
study of the potential profile of a substation, it
is found that there are several suspected spots in or around
asubstation, particularly in a substations corners or around
thefences. Therefore, before installing the measurement tools,
it
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764 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 2, JUNE
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is necessary to inspect those suspected points. One solutionis
pretest. A workman walks across a substation with the testdevice to
record the locations where the measured voltagesexceed the preset
threshold. Further measurements can thenbe made by online
monitoring in the suspected locations, thusreducing the time and
cost of measurements. Another solutionis to find the suspected
spots in the potential profile froma computer simulation results.
In this case, the accuracy oflocating the danger points highly
depends on the accuracy ofthe 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
charac-teristics; c) body weight; and d) foot contact resistance as
shownin (7) and (8). The constant is 0.116 for a person with 50
kgbody weight, while it is 0.157 for 70 kg.
(7)
(8)
Since the injected current is much smaller than the maximumfault
current, the measured touch/step voltage is therefore muchlower
than the limits defined in (7) and (8). Thus, the originalmeasured
data is intentionally increased to maximum value by(9) and (10)
when the data transfer to the database. The decisionis then made by
the comparison of the measured data with his-torical data or with
the maximum tolerable values provided by(7) and (8).
(9)
(10)
For personnel safety evaluation, touch voltage is more se-vere
than step voltage [23]. The current caused by touchingan exposed
conductor flows through the heart, whereas the onecaused by step
voltage bypass the heart. Therefore, the toleratetouch voltage is
much lower than tolerate step voltage. Gener-ally, satisfying the
touch voltage safety criteria in a substationautomatically ensures
the satisfaction of the step voltages safetycriteria. In this
project, most areas in the substation are exam-ined for touch
voltage, and only the edges of the grid are exam-ined for step
voltage.
C. Intelligent Evaluation Techniques With DatabaseAnother novel
feature of the proposed scheme is the imple-
mentation of online database. It is known that the resistivity
ofthe surface soil layer would be changed in different
seasons,which may results in touch/step voltages moving to the
hazardside [14]. For example, if the thickness of the
low-resistivitysoil layer in raining season is smaller than the
buried depth ofthe grounding grid, the touch voltage increase. In
another case,the high resistivity soil layer formed in freezing
season wouldcause the increase of the touch/step voltage with the
thickness orresistivity of the freezing soil layer. One major
defect of the ex-isting offline monitoring method is the inability
of tracking sea-sonal influences on the safety of substation
grounding system.With the support of database, we can continuously
monitor and
record the change of touch/step voltage. Particularly, during
thesevere conditions, like continuous raining or freezing
seasons,the frequency of online monitoring can be increased in
order tofind the potential hazards in time.Corrosion, which can
damage the effective connections
among the conductors, is another factor affecting the safety
ofthe grounding system. The grounding grid corrosion is causedby
acid or alkali in soil and the corrosion rate can reach up to 8.0mm
per year according to statistic results [24]. This situationbecomes
more serious as the steel-grounding or galvanizedsteel-grounding
system is widely used, which is more easilycorroded than copper so
that it requires more accurate, timelyassessment 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
ofgrounding rods, another major concern to utility companies,
cansuddenly change the integrity of the grounding grid. Failing
todetect this change in a timely manner will cause serious
conse-quences. In the proposed online monitoring scheme, the
changeof touch and step voltages at the same point are recorded,
sothat synthesized and reliable estimation can be made not
onlydepending on the IEEE standard but also on the variation due
toseasonal 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 gridperiodically. Measure the resulting
touch/step voltageswith the sensor network and transfer the data to
the centraldatabase.
2) Scale the measured touch/step voltages to the
maximumtouch/step voltages.
3) Compare the maximum touch/step voltages with IEEEstandard
under the same parameters, like fault clearingtime and the body
weight, etc. If it exceeds the safe value,a warning event is
created and the suspected location isreported to substation
operators for further analysis.
4) Compare the measured touch/step voltages with the his-torical
data at the same location. If the variation is largerthan the
preset threshold, a warning event is created eventhough the actual
value does not exceed the standard. Amandatory examination will be
taken around the suspectedpoint to check if the conductors are
stolen or broken due tocorrosion.
5) If no suspected spot is found, the database is updated
withthe newmeasured data and meteorological parameter, suchas
temperature and humility. Then, after a preset period, goback to
1).
III. STUDY OF CURRENT DISTRIBUTIONA simulation model is built in
PSCAD to study current dis-
tribution of both remote and local injection schemes as shownin
Fig. 8. The distribution substation under test transfers powerfrom
125 kV to 25 kV via a Delta-Yg connection transformer.An overhead
ground wire, so called skywire, accompanies withtransmission lines
and the ground resistance of a transmissionline tower is 32 . At
the secondary side, the neutral line ofdistribution system is
multiple-grounded with 15 at each
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CONDITIONS USING TOUCH AND STEP VOLTAGE SENSORS 765
Fig. 7. The evaluation process with database.
Fig. 8. Computer simulation for current distribution study.
Fig. 9. The structures of transmission line tower and
distribution line pole.
TABLE IPARAMETERS OF COMPUTER SIMULATION FOR CURRENT
DISTRIBUTION STUDY
grounded connection. The structure details of the
transmissionline 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
currentflowing from the faulted phase to ground and back to the
sub-station. However, with the presence of the multiple
groundedpoints on the neutral, such as pole grounds and
transformergrounds, the current is divided before reaching the
substationgrounding grid. As shown in Fig. 10, the current division
of theremote injection scheme depends on the distance between
thelocation of the staged fault and the substation. When the
stagedfault 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
Fig. 10. Current distribution of the remote scheme with respect
to distancefrom subtation.
Fig. 11. Current waveforms of current distribution study.
grounding grid by a step-down transformer. Because of
theexistence of overhead ground wires and neutral lines, not
allfault current flow through the grounding grid to the remoteearth
[21]. The simulation results (see Table II) show that73.58% current
across the grounding grid, 26.70% current
in the skywire and 10.59% current in the neutralline.
Disconnecting the skywire and the neutral line can largelyincrease
the grounding grid current. However, it is impossibleto disconnect
these ground wires for long time monitoring inreality. From touch
voltage simulation which will be discussedlater, 60 A grounding
grid current can result in about 3 V13V touch voltage, which is
large enough for effective detection.The current waveforms for the
local injection scheme areshown in Fig. 11. Typically, there are
relays implemented in thesubstation for ground fault protection.
These protective relayshave an inverse current/time characteristic,
which suggests theycan tolerate high current with a short duration.
As the durationof the injected 60 A current is about 50ms, shorter
than 0.1 s, itdoes not interrupt the normal operation of the
protective relays[19].The proposed local scheme is also applicable
to the substa-
tions with Yg-Yg or Y-Yg connection. As shown in Fig. 12,both
the primary side and secondary side of the transformerare Yg
connection and the neutral points are connected in thegrounding
grid. A staged fault is created at the primary sidewhen the
thyristors are turned on for 50 ms. The computer sim-ulation
results are listed in Table III. If all the grounded wires
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766 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 2, JUNE
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TABLE IICURRENT DISTRIBUTION OF THE LOCAL SCHEME WITH A
DELTA-YG
CONNECTION TRANSFORMER
Fig. 12. Current distribution study of Yg-Yg connection
substation.
TABLE IIICURRENT DISTRIBUTION OF YG-YG AND Y-YG CONNECTION
are connected, the current ratio of is 81.9% for Yg-Ygconnection
and it is 76.8% for Y-Yg connection.
IV. COMPUTER SIMULATION OF THE PROPOSED ONLINEMONITORING
SCHEME
Computer simulations have also been conducted inCYMGRD [25] to
measure touch/step voltages, illustratethe process of intelligent
evaluation scheme and analyze the in-fluence of seasons, corrosion
and theft. The designed groundinggrid as shown in Fig. 13 is 150 m
long and 100 m wide. All con-ductors are buried at a depth of 0.5
m. X-axis has 8 conductorsand Y-axis has 10 conductors. The
diameter of all conductorsis 19.1 mm. Plus, 30 grounding rods are
vertically connected tothe grounding grid. Each rod is 5 m long
with diameter 2.858cm. Moreover, the station surface is with
crushed rock of 2500ohm-meter resistivity at a thickness of 0.3 m
and the exposureduration 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
thesubsequent analysis. A two-layer soil model is implemented
inthis simulation. From the data provided by IEEE standard
(seeTable IV), both the upper and lower layers resistivity can be
cal-culated and the depth of the upper layer can be estimated as
well.The result of soil model calculation is consistent with the
IEEEcalculated values (see Table V), which proves the validity
ofthe designed two-layer soil model. Furthermore, the
maximumpermissible touch and step voltages in accordance with the
sub-station surface and the shock time can be calculated by (7)
and(8), which is 1084.2 V and 3551.8 V respectively.The potential
profile of the grounding grid diagonal line is
shown in Fig. 14. Apparently, touch voltage at the corner is
Fig. 13. The designed grounding grid in the computer
simulation.
TABLE IVTHE SOIL RESISTIVITY MEASUREMENTS DATA WITH THE FOUR-PIN
METHOD
PROVIDED BY IEEE STANDARD
TABLE VCOMPARISON OF THE SIMULATION RESULTS AND IEEE VALUES
much larger than in the middle center. It is due to less
conduc-tors buried around corners than around center. The
suspectedpoints can be clearly located from this potential profile,
whichis very useful for installation of the voltage sensors. This
pro-file also confirms that the value of maximum permitted
touchpotential has a dominant role in determining the design of
thegrounding grid. If a grid satisfies the requirements for safe
touchpotentials, it is very unlikely that the maximum permitted
steppotential will be exceeded. In Fig. 14, the margin between
thecalculated touch voltage and the permissible touch voltage
isabout 200 V800 V, while this margin for step voltage is as
largeas 3500 V.As the injected current through the grounding grid
is actually
about 50100 A, the concern here is if the 50100 A currentis able
to result in detectable touch/step voltage. The profile inFig. 15
is obtained with 60 A grounding grid current, whichcauses the touch
voltage between 313 V. The voltage in thisrange can be easily
detected by the voltage sensors. For safetyevaluation, the actual
voltages are scaled up to the maximumvalues in the database
according to (9) and (10).With the support of database, synthesized
and reliable es-
timation can be made depending on IEEE standard constraintand
recorded data variation. To better clarify the concept of the
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CONDITIONS USING TOUCH AND STEP VOLTAGE SENSORS 767
Fig. 14. The potential profile of the grounding grid diagonal
line.
Fig. 15. The potential profile with 60 A grounding grid
current.
Fig. 16. The conductors on the left edge are stolen.
safety evaluation with database, we consider two scenarios,
oneis theft and the other is the change of soil resistivity due to
sea-sonal influence. Theft is a serious threat for the safety of
sub-station. As shown in Fig. 16, the conductors on the left edge
ofsubstation are stolen so that touch voltage around that area
islargely 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
createdimmediately and investigation in the corner should be made
assoon as possible. On the other hand, touch voltages at somespots
also exceed the limit, a mandatory examine is requiredat these
locations.
Fig. 17. The potential profile after theft.
Fig. 18. The soil resistivity in different seasons.
As mentioned above, soil resistivity depends on a number
offactors: soil type, chemical composition, moisture,
temperature,etc. For an existing grounding grid, it is mainly
affected by sea-sonal variations. Especially in North America, the
frozen soil inwinter is a hazard for grounding grid safety. Fig. 18
is the fieldtest data of soil resistivity in 12-month study [11].
It is apparentthat during the summer, the resistivity becomes lower
due tohigh precipitation, while the resistivity goes higher during
thewinter because of the frozen soil. The influence of these
vari-ations on the touch/step voltages are analyzed in the
followingsimulation.Three locations are picked up from the
left-bottom corner to
the middle of the grid as shown in Fig. 19 and the touch
voltagesare investigated for 12 months (Fig. 20). When a fault
occurs inJune, all touch voltages are under the limit. However, if
a faulthappens in December, the increase of soil resistivity cause
thetouch voltages increase. It is clear that some of the touch
volt-ages exceed the limit. In this case, if an offline test is
taken inJune but not in December, these danger spots cannot be
founded;whereas, in the proposed online monitoring, these spots can
bereported in timewith monthly evaluation. The frequencies of
themeasurement can be adjusted according to the requirement ofa
utility company. More frequently the measurements are con-ducted,
more timely the danger spots can be found.
V. CONCLUSIONIn this paper, an online monitoring scheme for
substation
safety assessment is proposed, which periodically measurestouch
and step voltages with a preset frequency and effectivelyevaluates
the grounding grid conditions with the help of a data-base. The
test current is generated by firing a pair of thyristors
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768 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 2, JUNE
2012
Fig. 19. Three suspect spots are picked up from the diagonal
line.
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 injectedcurrent
flowing through the grounding grid, but it costs morethan the
remote scheme due to high rating voltage and highcapacity of the
step-down transformer.The condition monitoring is achieved with a
wireless
touch/step voltage sensors network installed at various
lo-cations of a substation. These sensors are connected to acentral
database where an evaluation process is carried out bycomparing the
newly measured data to the limits from IEEEstandard, or checking if
the data variation at the same spotexceeds safety thresholds.
Furthermore, current distributionhas been studied with computer
simulations, which verified theeffectiveness of the proposed local
and remote schemes. Fromthe case studies of conductor theft and
seasonal influences, theadvantage of online monitoring is very
clear since some dangerspots cannot be found in time without
continuous measurement.With further research, the proposed scheme
could be used tolocate broken section or missing grounding
electrode basedon the step/touch voltage profile obtained from the
sensors.Compared to offline methods, which at best gives
one-shotassessment, the proposed online grounding grid
monitoringscheme is more effective and reliable, and it could
become animportant 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.
For example, the proposed scheme is focused on touch and
stepvoltage indices which are related to personnel safety
concerns.Since grounding design has other objectives such as
facilitatingequipment protection, the proposed scheme needs to be
furtherexpanded to include sensors and indices that address
equipmentprotection concerns. It is possible that acceptable touch
and stepvoltages at sufficient locations in a substation may imply
an ac-ceptable grounding condition from equipment protection
per-spectives. 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 maintainsuch
networks. The reliability of the network needs to beconfirmed as
well. These are exactly the subjects of interestto ICT-
(information and communication technology) orientedsmart grid
researchers.
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Xun Long (S08) received the B.E. and M.Sc degrees in electrical
engineeringfrom Tsinghua University, Beijing, China, in 2004 and
2007, respectively, andis currently working toward the Ph.D. degree
in the Electrical and ComputerEngineering Department, University of
Alberta, Edmonton, Canada.His main research interests include power
line signaling, distributed genera-
tion and fault detection.
Ming Dong (S08) received the B.Eng. degree in electrical
engineering fromXian Jiaotong University, China, in 2004. He is
currently working toward thePh.D. degree in electrical and computer
engineering with the University of Al-berta, Edmonton, Canada.His
research covers smart grid, grounding systems, and power
quaility.
Wilsun Xu (F05) received the Ph.D. degree from the University of
British Co-lumbia, Vancouver, Canada, in 1989.From 1989 to 1996, he
was an Electrical Engineer with BCHydro, Vancouver
and Surrey, respectively. Currently, he is with the Department
of Electrical andComputer Engineering, University of Alberta,
Edmonton, Canada, where he hasbeen since 1996. His research
interests are power quality and distributed gener-ation.
YunWei Li (S04M05) received the B.Sc. degree in engineering from
TianjinUniversity, China, in 2002 and the Ph.D. degree from Nanyang
TechnologicalUniversity, Singapore, in 2006.In 2005, he was a
Visiting Scholar with the Institute of Energy Technology,
Aalborg University, Denmark. From 2006 to 2007, he was a
Postdoctoral Re-search Fellow in the Department of Electrical and
Computer Engineering, Ry-erson University, Canada. After working
with Rockwell Automation Canada in2007, he joined the Department of
Electrical and Computer Engineering, Uni-versity of Alberta,
Edmonton, Canada, as an Assistant Professor. His researchinterests
include distributed generation, microgrid, power converters, and
elec-tric motor drives.