Effective Use of Continuity Testing to Assess Grounding System Integrity Dr Darren Woodhouse, MIEEE, Mr Ian McLagan and Mr Stephen Palmer, MIEEE Safearth Consulting 212 Macquarie Road, Warners Bay, NSW, Australia Email:
[email protected] Abstract— While continuity based testing is a relatively well accepted method for assessing the integrity of a grounding grid, no specific continuity based test has distinguished itself with practitioners at large. Why this continues to be the case is not completely understood but it is likely a combination of ignorance of more effective procedures and the availability of better instrumentation. This paper reviews the various continuity based methods used for integrity testing and the characteristics required of a test procedure to make it effective at assessing integrity. This is considered primarily due to a lack of review of the various methods and instrumentation available to perform integrity testing. Accurate measurement of resistance in the electrically noisy environment of a power substation is a difficult task. In the case of grounding grid measurements, not only are the noise levels quite high but the resistances to be measured are relatively small, and the absolute significance of a specific measurement can be obscure. In practice the context of any given measurement is crucial to determining the significance of the measurement. While continuity testing can be performed at any power installation this paper concentrates specifically on the integrity assessment of a major substation. Most power utility substations are of a reasonable size, typically a few thousand square meters of real estate, which usually defines the extent of the grounding system and hence the extent of the integrity testing. This paper identifies the obstacles to performing an accurate measurement of resistance in the noisy environment of a power substation, establishing measurement targets and specific instances of poor integrity. Keywords—grounding; continuity; integrity.
I.
INTRODUCTION
Grounding systems are integral to the safe operation of a HV installation, particularly power utility substations as they house the neutral return points for ground faults. Verifying the integrity of an installation's grounding system is not just conducted at commissioning, but at regular intervals due to the safety critical role the grounding system plays and the susceptibility to deterioration of the buried components, nominally metallic conductors. An integrity assessment constitutes the condition monitoring program for a grounding system. It should also be performed prior to any injection test of the system to ensure that the extent of the grounding system is well defined, and that any anomalies do not impact any performance assessment based on the injection test. The majority of practitioners use continuity testing to perform an integrity assessment, not realizing that continuity testing can be achieved using a number of methods. The inappropriate application of these methods to the task of
continuity measurement in the electrically noisy environment of a substation can lead to errors and significant labor costs. This paper examines the primary aim of continuity testing as part of an integrity assessment of a grounding system and examines the suitability of the various methods. It also looks at what further inspections and tests should be conducted during an integrity assessment which in appropriate combination effectively achieve the stated aim. II.
INTEGRITY ASSESSMENT
The purpose of an integrity assessment is to verify the physical state of a grounding system, in terms of its physical condition and connectivity against some measure of expectation. In some instances the expectation is that the grounding system aligns with design drawings. In other instances, the expectation is that the condition of the grounding system compares well to a historical condition, so the comparison will be to the documentation produced by previous integrity assessments. In summary, an integrity assessment should report on the extent of deterioration to a grounding system's condition and structure. A. Grid Condition Most grounding grids are built of copper, while some organizations choose steel for security or availability reasons [1]. While copper has a higher corrosion resistance in soils than steel [2], both metals are subject to corrosion when buried, with the rate of corrosion highly dependent on the chemical nature of the soil. The rate of corrosion of steel can be very high, particularly at the air/soil boundary and is particularly susceptible to most acids. Copper corrosion is exacerbated significantly in sulfate soils [3]. The non-homogeneous nature of soil leads to localized corrosion, primarily driven by the level of oxygen in the soil and the soil resistivity. To alleviate this most grids are buried at 500-600 mm [4], a depth at which soil still retains some minor level of aeration, and so are designed with a high level of redundancy. Buried connections, particularly those with sharp cross sections will be subject to electric field intensification and higher rates of corrosion, consequently threads are not, or should not, be used for buried connections. The condition of a buried grid, where it is part of a highly redundant installation, is difficult to measure. The most useful method to perform this assessment is primarily based on an assessment of the soil resistivity, chemical soil analysis and visual inspection of the buried conductors as appropriate. The
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most susceptible area of the grid to corrosion is the connection between the bulk of the grid and aboveground plant, particularly as they pass between media, albeit the soil-air or air-concrete interfaces. These connections can be checked very effectively using continuity tests. B. Grid Structure As mentioned most grounding grids are highly redundant, in part to alleviate the long term effects of any deterioration, but more commonly to achieve other aims of a grounding system design, specifically to facilitate the distribution of fault and lightning energies. The most susceptible locations of a ground grid to high current levels are the conductors connecting the buried grid and above ground plant, as they are the connections between the point of fault and the ground grid.
C. Failure Modes The failure modes for grounding conductors and connections are numerous, but include: loose connections (see Fig 1); high resistance joints - which can be visually uncompromised (see Fig 2), compared to a visually clearly compromised joint (see Fig 3); single bond failure; single failure with parallel bond/s; conductor necking; disconnection; installation error (see Fig 4); corrosion/oxidization; and compromised separation between separated grounding grids.
Figure 3: Visually and Electrically Compromised Joint
D. Key Points In summary, the connections between the buried, redundant part of a ground grid and the aboveground plant are the most susceptible. Figure 1: Example of a Loose Connection
The grounding system at any given installation may consist of multiple ground grids. It is common, particularly in industrial and remote installations, for HV and LV networks to have separate grounding systems. The separation between these grids can be crucial for limiting energy transfer between power networks under specific fault scenarios.
Figure 4: Example of Installation Error
III.
Figure 2: Example of Visually Uncompromised High Resistance Joint
Global Earthing System integrity is currently assessed through current injection testing [5]. Effective continuity testing can indicate the interconnection level of all adjacent grounding components and metallic infrastructure. The number of connections may also be determined.
THE CONTINUITY TEST
A. Operating Environment Accurate measurement of low resistances in the electrically noisy environment of a power substation is a difficult metrological challenge. As substation grounding systems may have an overall impedance of only a few hundred mΩ, the ability to perform mΩ measurements is essential and highly significant. Further, connections through which several thousand amps will pass, a few mΩ results in a lot of heat, for example a 1 kA current level through a 10 mΩ joint results in 10 kW of heat dissipation in the joint. A low impedance system also means that the noise levels are very high as
inductive loops are common and significant currents will be present. Even under ideal conditions there are two further concerns with continuity testing: contact resistance and electropotentials. To overcome contact resistance a four wire configuration is preferred. Electro-potentials created across contacts of differing materials are overcome by using sufficient test current. This is only applicable if the test current is DC. However, the use of AC, except at extremely low frequencies, to test continuity is not recommended as it introduces the more complicated issue of mutual induction. 1) Noise Sources a) AC Noise The level of noise in ground conductors within a substation can be significant during normal operation. It is typical for 1020 A of current to flow between transformer star points, which are commoned to ground. Current levels as high as 200 A have been recorded at some installations with high load conditions and unbalanced line impedances. b) DC Noise Using AC current injection as the basis for continuity measurement will mean current flowing parallel to the voltage measurement lead will induce an emf in the voltage lead. A small separation between the current lead and the voltage lead, such as ensured by using twisted pair cable, will result in a reasonable level of coupling between the current and the voltage leads. At small resistances the voltage measured using the voltage lead will by definition be small and the induced emf will be proportionally larger. For larger resistances this method has acceptably low error but as the resistance reduces the errors will eventually become unacceptable and not easily reconciled. In this application which requires tests to be carried out in a noisy environment, such as zone substations, the injection current will need to be significantly different to power frequency to enable the use of ‘fast’ filtering methods. 2) Capacitive Soil Effects Current flow through the ground has a significant ionic component, and depending what constitutes that flow can result in what appears to be a capacitive or charging effect. DC based test methods are susceptible to this effect. 3) Temperature Considerations Large currents can result in joint and conductor heating which in turn increases their resistance. This can result in resistance measurements creeping during any test involving high currents. B. Theory of Operation 1) Measurement Circuit There is something magic about the 1 Ω limit, particularly in the electrical power industry. For instance AS3000 [5] has for many years used 1 Ω for the upper limit of a ground grid in distribution substations. In terms of making measurements of large electrical objects, 1 Ω represents a boundary between standard testing practice and the necessary use of increasingly more complex test methods.
• Two Probe Test -The preferred method for measuring higher resistances is a two probe test, as shown in Fig 5.
Figure 5: Two Probe Resistance Test Arrangement
For values above 1 Ω the use of two probe resistance measurements are quite accurate, such as implemented in any reasonable quality multi-meter. Below 1 Ω contact resistance and mutual induction become significant factors. • Four Probe Test - The preferred method for measuring low resistances is a four probe test, as described in Fig 6. This method is predominantly used in laboratory test
Figure 6: Four Probe Resistance Test Arrangement
instruments and more expensive field equipment. 2) Voltmeter Characteristics a) Sensitivity The sensitivity is related to the input impedance of the meter and more indirectly how much current needs to be drawn from the circuit to make the voltage measurement. This characteristic is of greatest significance for measuring high impedances, as charge drawn from the circuit impacts the impedance, or at small voltages where there is little electromotive force to drive the measurement. b) Accuracy Absolute and relative errors are predominantly determined by the quality of the instrument used and its calibration. Regular calibration should quantify the accuracy of a given instrument and the accuracy of subsequent field measurements. c) Selectivity Selectivity is an absolute resistance error [6] as defined by (1).
ΔR =
FN (1) I
where
ΔR
F
I N
= absolute error in resistance measurement = selectivity factor or the ability of the instrument to filter the noise = injected current = the noise voltage.
Selectivity is dependent on the filtering employed and the frequency difference between noise and the signal. Where the noise is predominantly power frequency, an AC injection is subject to a finite frequency difference, whereas a DC injection is theoretically infinite. The accuracy then becomes a function of time spent sampling and filtering, the injection current stability and resistance stability. This explains the motivation for higher currents or higher filtering/selectivity. 3) Current Stability The stability of the injected current determines how stable the resistance measured appears. Passing current through a resistance increases its temperature which in turn increases its resistance. Resistance is also dependent on the ambient temperature so comparison requires the ambient temperature to be recorded. In this regard injecting smaller currents is advantageous.
arrangements of the current source and the measuring components of the instrument. 1) DC Test Methods a) Two Probe Test This method is commonly used by multi-meters, see Fig 7. • Strengths - Lightweight and portable. • Weaknesses - Inaccurate, especially with long leads and/or low resistances; and susceptible to DC offset b) Four Probe Test - Known Current This method is primarily used in laboratory standard calibration equipment, see Fig 8. • Strengths - Improved accuracy with low current; and four wire connection accounts for lead and connection impedance. • Weaknesses - Limited portability; susceptible to DC offset; susceptible to open or short circuit; and slow to test entire installation. c) Four Probe Test - Displaced units The general configuration for this method is shown in Fig 9. Specialist grounding instruments usually employ this method for continuity measurement. • Strengths - Measurement displayed in portable unit; split system enable portability and speed; and improved portability enabling single person operation.
Figure 7: Two Probe Test Configuration
Figure 9: Four Probe Test - Displaced Configuration
• Weaknesses - Susceptible to DC offset; susceptible to open or short circuit; soil separated items can appear connected; and current assumed to be constant.
Fi gure 8: Four Probe Test - Known Current Configuration
In some instruments the injected current is assumed rather than measured. This makes the stability of the current even more critical to the operation of such an instrument as there is a direct relationship between the error in the injected current and error in the measurement. C. Test Method Configurations Final test method configurations are categorized by the nature of the current injected, number of probes and the relative
d) Four Probe Test - Polarity Switching The general configuration for this method with uses a DC test with polarity switching and separate intelligent hand unit, is shown in Fig 10. Some specialist grounding instruments employ this method for continuity measurement. • Strengths - Automatically accounts for DC offset; not susceptible to induction or other AC interference; improved accuracy due to current measurement; improved speed, reducing cost of test; high AC immunity allows in-service testing; accurate with long leads - in service systems have achieved over 200 m; detect soil in circuit; detect open and short circuits in leads; low power, test with or without external
power supply; and fastest and most accurate integrity test available. • Weaknesses - None identified.
Figure 10: Four Probe Test - Polarity Configuration
IV.
INTEGRITY PROGRAMME
An integrity assessment is based on a continuity test combined with visual and physical inspections. The recommended process for evaluating continuity test results is: • Establish allowable resistance limits for different connection types, such as fence, plant and busbar; and • Measure continuity to various locations, as per Section V, and compare test results with established limits. The visual and physical inspections should include: • Physical manipulation of connections to check robustness; • Checks for strand damage; • Check for missing or damaged conductor restraints or supports; and • Visual indications of conductor corrosion, contamination and method of installation. V.
CONTINUITY TESTING EXECUTION
A. The Target The purpose of performing continuity testing of the various items of plant in a substation is to verify that they remain effectively connected to the installation's ground grid. The test's measure is the resistance of that connection to a common point, nominally a major ground bar in the installation, and the check is that the resistance is sufficiently low. What constitutes a sufficiently low resistance depends on the installation, as outlined in Section III-A. As a general rule continuity across an installation should be in the mΩ range. The difference in continuity measurements across joints should be in order of 100 µΩ. As the ground system impedance of an installation reduces so the size of these measurements should reduce or the more sensitive the investigator should be to where these values are occurring. As a basis for this position consider an installation with a 10 mΩ ground system impedance. Any joint or connection of 10 mΩ involved directly in a ground fault will effectively double the
GPR and have half the GPR across that element. The power through that element will likely result in that element being the point of failure. The argument regarding the accuracy of continuity measurement is then a relative argument and is very similar to the argument regarding the use of significant figures. A favorite response, but somewhat annoying to some, is ‘It Depends!’. What is significant to the grounding system of a pole mounted substation with a single electrode for a grounding system is markedly different to what would be considered significant in the grounding system of a power station. B. The Process 1. Establish Reference The actions required of this step are: • Connect to a main grounding bar of the system under test with a current and voltage lead. • Verify this a good starting point post establishing that most of the installation is connected to this point. An isolated reference point is a poor choice! 2. Establish Extent In this step measurements are made around the substation and to nearby infrastructure beyond the expected extent of the installation, as follows: • Measurements should be made to determine the things that are clearly bonded or not. This will define the ‘extent’ of the grounding system. • As well as finding what is directly connected to the substation it is also important to identify the things that are meant to be separated aren’t indirectly bonded. Sometimes this can be harder to determine as some items may appear bonded through measurements with certain instruments. This could just be a connection through the soil. An example of this scenario could be a water pipe or a fence approaching the substation system. 3. Identify Anomalies In this step measurements are made around the substation and to nearby infrastructure within the extent of the installation as identified in the previous step. These measurements will indicate various levels of connectivity and if used properly can identify bad connections for general bonds right down to breaks in the buried conductors in some cases. Interestingly this requires the instrument to be stable and sensitive, not necessarily highly accurate. • Example: Testing a bonded substation fence connected directly to the grid at every 2nd post and is bonded indirectly through the fence through rails and cross members. The first (connected) post could measure 10mΩ. The next post, with no direct bond, would be slightly more, say 20 mΩ. The next (connected) post should be back down in the 10 mΩ range. If this value increases again it would indicate either a ‘bad’ or loose connection which could be identified by then testing the grid riser connecting to
the post. If this is still high then the connection issue is more likely in the buried grid. This same method can be used for all bonded equipment. Items that are easily tested can sometimes identify problems with a grounding system prior to the next level assessment of a full injection test. Knowing a cable screen or OHEW is connected to the substation will assist with determining current paths. Alternatively knowing a screen or OHEW is not connected for a separated system could be a key part of the system design for safety compliance. C. Testing Frequency Continuity testing needs to be conducted on a regular basis. The period of checking is dependent predominantly on the likelihood that mechanical damage will occur. For instance in a utility substation it is accepted practice in many countries, such as Australia, to test annually. However, we have also recommended to a number of industrial customers that continuity testing be conducted every 2-3 months. The operation of large machinery around grounding conductors seems to be significant contributor to instances of damage. Continuity testing should also be performed as one of the precursor tests to current injection testing of a grounding system. It is the only means of identifying the core of a grounding system and a location to which the bulk of the grounding system is connected. This is particularly important where the grounding system is aged or where separated grounding systems are suspected of having been employed. VI.
CONCLUSIONS
This paper has examined the various methods used to undertake integrity assessment of grounding grids with a particular focus on the continuity testing component. The following specific conclusions are made: 1. Integrity assessment of a grounding system should be performed as part of the condition monitoring program of any significant HV installation. 2. Continuity testing forms an integral part of an integrity assessment, and appropriate methods executed effectively can identify defects in joints and connections that would otherwise remain hidden. 3. Whilst the benefits of continuity testing can be significant an integrity assessment should also consist other elements, in particular visual and physical inspections. 4. The most effective method to perform continuity testing is the Four Probe Test with Polarity Switching method.
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