Audit of Lightning Protection Systems in Medium and High Voltage Power Substations

This paper proposes a methodology to audit and certify lightning protection systems in any medium or high voltage substation. The methodology deals with aspects, like data recollection, inspection, diagnosis and verification of the installed protection systems, measurements of electrical parameters, geologic analysis and some solutions are proposed.

Keywords: equipotential bonding, surges protection, grounding systems, lightning protection systems (LPS).

Introduction

Direct lightning strokes can cause damages to the substation transformers, measurement, control and communication equipments. In order to guarantee the equipments protection, a LPS is installed, which must be diagnosed, inspected and certificated once in a while, to control their state. There are power substations that were by many years, even decades, unsupervised. Some of them were designed based on old standards, which do not agree with the present technical criteria. This paper has as a main purpose to propose a methodology to audit and certify lightning protection systems in any medium or high voltage operative substation.

Methodology proposed

The authors propose a methodology based on the following considerations:

1) Data recollection and analysis of the LPS.

2) Validation of the protection method applied on the substation.

3) Data recollection on the substations.

4) Physical review of all the elements intervening in the system.

5) Data recollection of the surge arresters.

 Revision of the surge arresters.

 Surge arresters diagnosis using tests.

6) Measurement of electrical parameters.

 Safety procedures.

 Current measurement by grounding wires.

 Equipotential Bonding measurement.

 Grounding resistance measurement.

 Earth Resistivity measurement.

 Touch and step voltage measurement.

7) Analysis of the obtained measurements.

8) Geologic analysis of the substation location zone.

9) Solutions proposed to improve the system installed.

Data recollection and analysis of the LPS

The much common interception devices in substations are metallic masts or air terminations in the top of metallic structures and the use of shielding wires on metallic supports. In order to determine the nature of the grounding system, the plans and original design review are very important.

Validation of the applied protection method

In order to be able of evaluate the LPS, it is necessary to compile a series of data that depend on the initial design, such as the substation importance, ground lightning density of the place and the degree of exposure to direct lightning strokes.

The methods more widely used are:

 Empirical classic methods. Within this category are the method of the angle and the one of the empirical curves.

The first one uses vertical angles to determine the number, position and height of the shielding wires or masts. The angles are determined by the degree of lightning exposure, the importance of the substation being protected and the physical area occupied by the substation. The curves determine the number, position, and the height of the shielding wires and masts.

 Electrogeometric model. The more used version is the rolling sphere method. To apply this method an imaginary sphere of radio S is rolled over the protective elements like terminals and ends, shielding conductors and support structures. In order to be able to apply this method it is necessary to calculate the radius S, the reason why it is required the transitory impedance Zs and the current of probable impact. The standard [1] describes how to calculate the impulse impedance under crown for each bar with different height and conductor type. If the sphere touches some equipment that is desired to protect, the system must be redesigned.

There are companies that use the rolling sphere method in similar or superior voltages than 138 kV and use the angle method for voltages lower than 138 kV [1].

Data collection

In order to verify the effectiveness of the LPS the following data are required: substation nominal voltage, basic insulation level of the bars, diameter and type of the conductor used in the bars, conductors radio or the equivalent radius in case of a group of conductors, conductors height average, basic insulation level of equipments, bars height and the existing equipment in the substation, height either of the mast or the tower, height of the shielding conductors, distances between the air terminations systems and the equipment and bars to protect, separations between elements of the protection system, height differences between masts or air terminations rods and the equipment and bars to protect.

Elements physical review

In order to perform a suitable physical revision of the elements that belongs to the LPS and the grounding system the next recommendations must be followed:

 Verify the existence of down conductors in the structures that have shielding wires; if they do not exist, the thickness of the metallic structure must be measured in order to compare its dimensions with the standard.

 Verify that the interconnections between the metallic elements that belong to the tower are the right ones, creating low way impedance so that the fault currents would be drained to ground in an effective way.

 Verify that the down conductors are connected directly and vertically, in such a way that they have the smaller length and the most direct way to earth. If there are bends, the radius of any bend shall not be less than 20.3 cm (8 in) in their trajectory and the angle of any bend shall not be less than 90 degrees.

 Review the physical grounding grid integrity in visual form and at points where a circulation of current is detected.

In addition, the quality of the connections, the corrosion, the type of material and its dimensions will have to be registered, also the state of the vertical grounding rods, their material and dimensions.

 The connections will have to be fixed, without false contacts. The connectors must not be corroded and must be the suitable ones. The types of connectors that establish the standards [2,3] to use are: exothermic weld, connectors of pressure and other connectors’ certificates.

 To verify that all the metallic elements are connected suitably to the grounding grid. In case of different earthings in the place, there must be verified the interconnection to the substation grounding grid, guaranteeing the equipotential bonding of all system, in order to avoid the existence of potential differences.

 Verify that the ground grid must be covered by a gravel layer of high resistivity. This gravel layer delays the evaporation of the humidity, and also maintains the humidity for a long time in drought periods.

 Verify that there exists physical connection between the grounding grid of the substation and the fence located around it.

In [2] it is demonstrated that the most suitable site to install the fence is when this is located in the outside area that occupies the substation. The interconnection guarantees that a person standing 0.91 m and touching the fence will be under a touch voltage smaller than the tolerable touch voltage.

Data collection of the surge arresters

Surge arresters are used to obtain protection against surges originated by lightning and switching. The protection level of the surge arrester must be coordinated with the insulation level of the protected element.Surge arresters are universally used, even though some companies use gaps in the switches and final points of the installation where there can appear the duplication voltage effect.

The surge arresters of metallic oxides (ZnO) are the most actually used due to their advantages compared with those of silicon carbide (SiC), such as: the surge arrester of metallic oxide has a greater protection margin, their construction is simpler, the possibility of coordinating them with the equipment to protect is superior, they allow the diagnosis method online and can be washed on duty. As a disadvantage could be indicated that they require a greater control when they are in operation, the reason why it is necessary to count on instruments to diagnose a possible fault in time. However, many silicon carbide surge arresters are still installed in our countries.

Diagnosis and Inspection of surge arresters

The inspection and diagnosis of the surge arresters will follow the next recommendations:

 Review the surge arrester integrity, if surface contamination exists, the state of air tightness, the oxidation degree in the different elements, the porcelain or another type of surrounding that covers them. Make sure that the installation fulfils the manufacturer indications.

 Verify the gauge and the state of the wires that connect the surge arrester, verifying the existence of false contact and the installed quality of the connections and the connectors installed.

 Review the lightning counter of the surge arrester.

 Check that the surge arresters that are located at the incoming power lines are connected in derivation instead of giving support.

 Perform the inspection with the surge arrester out of service. In case of some metal oxide surge arresters, the inspection must be performed with them energized.

 Verify that the technical specifications of the surge arrester installed are the appropriate ones for the type and location in the electrical / electronic system being protected.

The tests to perform to the SiC are [4, 5, 6]:

 Insulation resistance measurement.

 Application of high voltage direct current to measure the leakage current.

 Measuring the tgδ of each element to evaluate the integrity of the surge arrester. This test reveals conditions that could affect the protective function of the surge arrester such as: the presence of moisture, salt deposits, corrosion, cracks or fissures in the porcelain, open shunt resistance, pre-ionizing elements and gaps defective or faulty.

The tests to perform to the ZnO are [6,7,8]:

 Insulation resistance measurement.

 Resistive current measurement.

 Voltage reference of direct current measurement.

 Leakage current measurement in direct current (voltage reference in direct current test).

 Voltage reference measurement at industrial frequency.

 During the operation of the surge arrester, monitoring methodologies are most effective, especially the measurement of leakage current at rated voltage.

In general diagnostic methods available for metal oxide surge arresters are total leakage current measurement, leakage resistive current direct measurement and leakage current harmonics analysis.

Safety procedures to perform the measurements

In order to perform any type of measurement within the substation, the following aspects of security will be considered:

 Demarcate the testing area and warn the presents that they must not touch the wires while the tests are being performed.

 Only certified personal can perform the measurements.

Wear insulating gloves, protective glasses, dull with isolation, carpets with high level of isolation, clothesdestined to perform electrical duties.

 Use a current clamp meter to verify the existence of current in the grounding wires, lightning rods wires, equipotential bonding connections and others, before performing any measurement. In case of high current values, the origin of this current will be detected before continuing.

 During electrical storms it is prohibited to perform any measurement.

 In case of any sudden electrical storm on the area where the measurement is being performed, this one must be stopped, the connections between the measurement equipment and the system under testing must be disconnected, and the connectors must be isolated temporarily and also placed in the superior part of the grounding grid under test.

In [9] are mentioned several safety rules that must be considered while the grounding measurements are being performed.

Grounding wires current measurements

In substations due to diverse causes there may be circulating currents by grounding wires. In case of metallic oxides surge arresters, there is always a tiny leakage current within the range of micros or miliamperes, this leakage is increased with the superficial contamination of the surge arrester. In addition, systems connected in wye, the desbalance currents will circulate toward the grounding grid. The electromagnetic induction also can generate during normal operation conditions, the circulation of currents in grounding wires. It is important to indicate that no matter how many measures have been taken to avoid these parasitic currents, it is very difficult to eliminate them in a 100%.

These currents can be dangerous when measurements to the substation grounding systems are being performed.

Based on its magnitudes they can bring about the appearance of high contact voltages. In addition, the circulation of these currents can generate interferences or damages in high sensitivity equipments. It is important to indicate that the IEC and IEEE international standards for substations design and maintenance, do not establish the permissible maximum value of current that can circulate through grounding wires.

Equipotential bonding measurements

It is very important to verify the connections between all the existing elements in the substation, which guarantees the suitable conduction of current towards the grounding grid in case of failures at the substation.

One of the methods more recommended to perform this verification is the “Stakeless” method, also called Clamp On, which measures the resistance and continuity of the loop, not the grounding resistance. There is another method [10] that consists on the injection of direct current with a regulated DC power source and the measurement of the voltage and the injected current.

Grounding resistance measurement

Through the years, the grounding can be degraded, reason why it is recommended periodically to verify all the connections of grounding wires to the grounding grid and the grounding grid of the substation, as a part of its normal plan of predictive maintenance.

In [12, 13] the methods and procedures to measure the grounding resistance in electrical substations are indicated. The measurement of resistance applying the method of Potential Fall is not recommended in substations of great extension, because conductors of great length are required.

The method of the slope is used for systems of grounding that covers a great area; also it is applied, when the grounding system configuration is not known, or if it is interconnected with other groundings. The method of intersection of curves also is recommended for grounding systems that cover great extensions.

The [1, 2, 12, 13, 14, 15] standards establish that the grounding grid resistance for smaller and industrial substations must be ≤ 5 Ω and 1 Ω for greater substations.

In addition, it is considered that to avoid possible errors in the measurement process the reactive component must be taken into account when the resistance value is less than 0.5 Ω and the area of enmeshes is relatively great. This reactive component will have a small effect in grounding grids with impedance values greater than 0.5 Ω. A low resistance value, contributes to control the elevation of the maximum ground potential (Ground Potential Rise).

Nowadays it is world-wide known that the grounding systems impedance behaviour before transitory is totally different from the cuasi-stable conditions, in such sense, with the purpose of knowing its variation and to perform the necessary corrections in case of obtaining high values; it is recommendable to measure the grounding impedance front of atmospheric discharges, in [12] the equipment, and the procedures to measure it are indicated.

Practical criteria to perform grounding resistance measurements

 When placing the measurement electrodes, it must be guaranteed that they are in straight line.

 The current and potential wires must be separated to avoid signal coupling. However, if there are any noises problems that can affect the measurements, a solution could be to thread the wires of the auxiliary electrodes.

 In recent construction grounding systems, the resistance should be affected due to the compaction of the ground.

 The current and potential laying wire when the measurement is being performed will form an angle of 90 degrees with the distribution and transmission lines, to avoid any coupling.

 The measurement will take place at the driest period of the substation land.

 If a very high impedance or outside rank is obtained, it is recommended to spill a small amount of water around the test rods.

 In order to locate the horizontal part of the curve it is recommended to perform five or more measuring at least.

Earth Resistivity Measurement

Earth resistivity measurement in this methodology fulfils two purposes. One of them is that earth resistivity is a determining value to evaluate the ground aggressiveness.

The other one is that knowing their value and in case of obtaining a high resistance value after measuring the resistance of the grounding system substation, the auditor or designer will be able to decide which improvements will have to perform, in order to change the grounding resistance to the values of resistance established by the standards.

Some practical criteria to perform the resistivity measurements are the following:

 The depth of the electrodes must not exceed 30 cm.

It is advisable to perform the measurements in different directions for the same sounding, for example from North to South and from East to West, due to the anisotropic ground characteristic.

 When choosing the exploration depth it is not recommended separations greater than 8 m for exploration depths of 6 m.

 The presence in the sounding zone of metallic bodies (for example, naked canalizations) or of very resistant seams of the land or conductors, which end up arising to the surface, can disturb the resistivity measures, since to cross such obstacles, modifications of the currents trajectories injected in the ground are originated and consequently the electric field, on the measurement point.

 The way of demonstrating that there is always a possible disturbing causes and, also, to verify that there are no sensible variations in the subsoil homogeneity is analyzed to perform measurements in diverse zones of the location of the grounding installation and with different separations between the measurement electrodes.

 If the existing grounding resistivity is going to be measured, is obligatory to perform the measurement in a zone near this one, with similar characteristics and the same geologic conformation, with a separation equal or greater than three times the separation of the electrodes (3 x a).

 When performing the measurements in the different directions (North-South) (East-West), the obtained values of resistance for each separation between electrodes (a) can be divided equally, but values obtained with different separations cannot be divided equally (a).

 The resistivity readings must include annotations of the environmental conditions during the measures (temperature and humidity conditions of the ground), and if it is possible, compared them which the ones that were taken in different periods through the year.

 It is recommendable to perform the earth resistivity measurement of the land in dry periods, trying to reproduce the most unfavourable conditions and if it were not possible, a certain safety factor will be applied that increases the obtained results.

 After obtaining the results of earth resistivity in the performed measurements, the interpretation of them will be performed, to define the suitable land structure (homogenous, two layers and multilayer) that responds to the results obtained in the field measurements.

The method more commonly used by the electricians to perform the resistivity measurement is the Wenner method [2, 9, 10, 11, 12, 14, 15, 16, 17].

Touch and step voltage measurements

All substations must have a grounding system designed in such a form, that for any normally accessible point to people, these are put under the maximum touch and step voltage in case of fault [2].

The maximum touch voltage applied to the human being, who is accepted in any point of an installation, is given based on the grounding fault clearing time, the ground resistivity and the fault current. For effects of the present methodology the maximum touch voltage must not exceed the values given in the Figure 44A of the IEC 60364-4-44.

When the audit of the LPS is being performed, there must be a check out of the voltage that can appear in case of fault, due to damage conductors that belongs to the grounding grid, bringing with itself high voltage, putting in danger the life of the people who can circulate or remain close or within the substation.

The touch and step voltage measurements will be performed in all the points where some danger can be anticipated for the personnel who is within the substation or next to the same and where a greater probability of fault exists, as for example next to the power transformers, entry points of the lines, sites near the grid peripheral, access doors, grids of ventilation, grid vertices, tracks for the power transformers displacement and generally any other type of element, that at certain moment can be energized happening some fault. In [10, 14, 16, 17] are indicated the equipment and procedures to measure the touch and step voltage.

It is recommendable to perform specific simulations using EMTP/ATP and/or other applications to determine the step and contact voltage when the substation is hit by lightning, in order to determine possible zones of high risk under these circumstances.

In [12] the methods and procedures world-wide accepted to measure the potentials in the grounding systems in electrical substations are indicated.

Analyses of the obtained measurements

Based on the obtained registries, it will be known the real state the substation and all the elements interconnected with it. After performing the touch and step voltage measurements, it will be known if the voltage that can appear in the substation in case of fault is below the allowed limits (table 1) and then solutions to the problems will be provided.

Geological analysis of the zone where the substation is located

The methodology proposes to perform the geologic study of the zone where the substation is located.

It is important to consider the distribution of the potential in the land when the grounding system is being crossed by a fault current, essentially has fundamentally as a determining factor the land resistivity and the geologic characteristics in which that one is buried. It is therefore that the conception of a grounding network requires, initially, the analysis of the nature of the land.

With the geologic analysis it is possible to know the soluble salts content and its concentration which is determining in the conductivity of the ground. It is possible to know the water content of the soil, which influences in an appreciable form on the soil resistivity. It allows knowing the sorting level of the ground, which also influences the porosity and the retaining humidity and the quality of the contact with the grounding system. The estratigraphy of the land allows knowing the stratification lands level.

This analysis in combination with the results obtained in the earth resistivity measurement, the equipotential bonding measurement of the grounding system and the resistance measurement, will allow defining whether the elements that conform the grounding grid could be corroded or damaged.

In addition to being required to perform an improvement of the existing grounding system, the designer will define the more effective improvements measures considering the existing terrain features. Some of the most important soil properties to consider are: PH, ground class, chemical composition, ground model, validation of the resistivity of the land, ground humidity level and others [10].

2.8. Solutions proposed to improve the installed protection system Based on the results obtained, if it is necessary the existing LPS will be redesigned, guaranteeing that the entire substation is being protected.

From the evaluation and diagnosis of the surge arresters, it will be decided if those ones are cracking, well selected and if they are coordinated with the insulation of the equipment that they are protecting. If it is necessary new surge arresters will be located considering the parameters established by the standards. Based on the conditions of the down conductors, equipotential bondings, connectors, clamps, etc. the change and the normalization of the same will be decided. In case of high values of grounding resistance, measures will be dictated to diminish their value, such as change of conductors in grounding grids that could be corroded, increase of the area of grounding grid, construction of deep electrodes and the use of ground enhancement compounds with low resistivity.

If elevated voltages appear, there are different solutions that can be studied and applied where it would be appropriate: increase the area of the grounding grid, cover the substation with a gravel layer, increase the number of rods in the grid perimeter, and limit the access to certain areas of danger, among others.

Grounding systems and surge protective devices in relays room The relays room require a special attention, most of all those substations whose control systems and protections are migrating to digital systems. The authors, due to the extent of the topic and its relation with electromagnetic compatibility propose that for future investigations.

Conclusions

This paper proposes a methodology to follow in order to perform the audit of a Lightning Protection Systems in medium and high voltage substations that are in operation and can be used as a guide to audit and to certify the protection systems against lightning of any substation of medium and high voltage.

The methodology proposed deals with the most important aspects to audit and certify medium and high voltage substations.

The methodology proposed in this paper has been applied and practically validated in the audits performed in some power substations of 110 kV located in Bulgaria

REFERENCES

[1] IEEE Std 998-1996, IEEE Guide for Direct Lightning Stroke Shielding of Substations.

[2] IEEE Std 80-2000, IEEE Guide for Safety in AC Substation Grounding.

[3] IEC 60364-5-54, Part 5-54: Selection and erection of electrical equipment-Earthing arrangement, protective conductors and protective bonding conductors.

[4] ANSI/IEEE C62.2-1987, IEEE Guide for the application of gapped Silicon-Carbide Surge Arresters for Alternating Current Systems.

[5] ANSI/IEEE C62.1-1989, IEEE Standard for Gapped Silicon-Carbide Surge Arresters for AC Power Circuits.

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[7] IEC 60099-4 Ed.2, –Surge Arresters –Part 4. Metal-oxide surge arresters without gaps for a.c. systems.

[8] J. L. Silva Menéndez, “Pararrayos de Óxido Metálico”, ECIE Matanzas, CUBA.

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[11] Fluke, “Principios, métodos de comprobación y aplicaciones”.

[12] IEEE Std 81-1983, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System.

[13] IEEE Std 81.2-1991, IEEE Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems.

[14] A. Granero, “Medidas y vigilancia de las instalaciones de puestas a tierra, Mayo 2001.

[15] I. Usunariz, “Medida de la resistencia de la toma de tierra en edificios comerciales, residenciales y en plantas industriales”.

[16] E. N. Stefanov, “Metodología para el diseño de sistemas de puesta a tierra en líneas y subestaciones para tensiones intermedias”. Tesis presentada en opción al grado científico de Master en Ingeniería Eléctrica. Cuba, 2004.

[17] R. G. Márquez, “La puesta a tierra de instalaciones eléctricas y el R.A.T”, España, 1991.

Authors: M.Sc. Ernesto Noriega Stefanov, Eng. Favio Casas Ospina, M.Sc. prof. Manuel Briceño Manuel Briceno, M.Sc. prof. Stefano Mangione, Eng. prof. Anyi Gavidia, Eng. Luis Ignacio Colina Jiménez.