A New Approach to Measure and Control Grounding ... - IEEE Xplore

16th International Power Electronics and Motion Control Conference and Exposition

Antalya, Turkey 21-24 Sept 2014

A New Approach to Measure and Control Grounding Resistance Ilhan GARIP

Gungor BAL

Gazi Electrical Machines and Energy Control Group TRT Broadcasting Company, Department of Strategy Development, Ankara, Turkey [email protected]

Electrical and Electronics Engineering Gazi University Ankara, Turkey [email protected]

Abstract—Grounding is the only security precaution to transfer leakage electrical energy on an electrical system to the ground via electrodes and panels placed under the ground. In order to do this transfer, the contact resistance between grounding elements and ground must be very small. Nevertheless, the contact resistance of ground changes with variation of climate. In high temperature, humidity of ground decreases causing the ground resistance to increase. However, to prevent these changes in the ground, a new grounding system is established to measure and control grounding resistance by using humidity sensors, grounding electrodes and underground watering system. The voltage applied to electrodes creates a current to flow through ground which leads to calculate ground resistance. If resistance value is appropriate for the reference value, the system continues measuring, otherwise underground watering system is activated to decrease grounding resistance. Experimental results show that proposed method for measurement and control of grounding resistance is precise and can be applied to large systems. Key Words: Grounding, microcontroller, magnetic valve, humidity sensors

measurement,

I. INTRODUCTION In any electrical systems, safety of equipment and personnel against the leakage electric and lightening is the first priority. Therefore grounding has to be considered as a protection method for these systems. There are two main factors should be taken into account about the grounding [1]. These are providing minimum resistance between the grounding systems and ground, and preventing a potential difference (voltage) between the electrical system and other elements during the operation. The main aim in grounding is to provide safety of people and systems. In order to arrange security in a possible emergency situation, touching voltages should be at appropriate value for human beings. In electrical systems, to protect components from damages caused by high voltage and electrical leakage, a good isolation and grounding have to be provided [2]. The distance between the grounding electrodes is one of the most important subjects in designing a grounding system. If there is an increase in this distance, the grounding resistance value increases, too. Another subject is the measurement of grounding resistance between electrodes. If there is an increase in length of electrodes, the resistance value decreases [3]. In grounding plants, different methods used to set up electrodes. Generally network grounding systems are preferred in where electrodes and metal plates are placed under the ground and connected to each other by a PEMC 2014

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network. [4].There are different kinds of groundings with respect to protection types; ¾ Safety grounding ¾ Operational grounding ¾ Function grounding ¾ Grounding for static electric ¾ Grounding for lightening The grounding methods are listed below; ¾ Rod grounding or pipe grounding ¾ String or knitted conduction grounding ¾ Sheet grounding Change in weather conditions cause a change in the ground resistance. In electrical transportation or any electrical systems, equipment, personnel and passenger safety are provided by grounding at certain points [5].The resistance between the ground and electrode should be as small as possible. Otherwise, a current conduction at high values may occur between the system and electrodes. This current would turn into a voltage on resistor so that while the resistance value decreases, the voltage value would also decrease [6]. The diffusion resistance is dependent on the specific resistance of ground, the type of designed grounding system and its dimensions. The ground may have different layers that have different specific resistances both in vertical and horizontal ways. To calculate the transmission resistance of ground, the scientists propose different analytical solutions. Along these solutions, Sverak and Schwars methods appeal at the same time with Thpaer-Gerez method that is a noteworthy one. These formulas and methods give proper solutions for regions that have two vertical layers [7]. There are countless articles and some important books, which give a very detailed description of the principle of lightning [8]. Precaution systems for lightning protection are carried out with CLED test device. The results of this test show that for prohibitive factor 0.6-0.8 coefficients are obtained [9]. Iron and steel systems placed in the concrete in buildings are attached to the ground with concrete substructures using electrodes and therefore the grounding of a building is established [10]. Increments in ground resistance put the systems, businesses and human lives in risk. To prevent this situation, different materials to decrease the resistance between grounding electrodes and ground touching surface should be used. In grounding areas, ideal ground conditions cannot always be satisfied so that studies for decreasing ground resistance are carried out as; ¾ Increasing mineral ingredients in ground ¾ Adding metallic salts to ground to increase conductivity

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Providing necessary moisture in ground and making studies for this ¾ Checking the temperature of ground ¾ Replacing the ground bar and electrode burying areas with appropriate ground for grounding [11]. These are; a) Pouring conductive concrete b) Bentonite c) Adding carbon based materials d) Using clay based cushion materials e) Increasing mineral ingredients in ground f) Adding metallic salts to ground for increasing conductivity Safety borders of grounding resistance must be maximum 10 ohms and must not be under 5 ohms according to the Ministry of Public Works [12]. In this study, a new approach to design and implementation of a grounding system have been carried out. The basic aims of the study can be summarized as below: ¾ To keep the grounding resistance constant in its minimal value in every seasonal changes ¾ To visualize the changes in ground resistance of high frequency TV, radio, GSM, radar systems ¾ To introduce a new approach to grounding system designers ¾ To provide security for the personnel and electrical components in case of any lightning and leakage currents ¾ To produce a report on design details This new approach controls the grounding resistance by measuring the ground humidity using sensors and electrodes. Experimental results show the accuracy of the new system developed to measure and control grounding resistance.

A. Equations

(1)

In where; U Ground resistance (ohm-meter)

r

Measured resistance value (ohm) Boring space (m) Same calculation is carried out for all measurements, and then an average ground resistance value is obtained in Equation 2 by using seasonal coefficient. U ( k / 4 ).( U 1 U 2 U 3 U 4 ) (2)

a

In here; k Seasonal coefficient Calculation of the ground resistance value in ohm is given in Equation 3 [14]. R1 ( U / 1 . 915 L )[ln( 96 L / d ) 1 ] (3) In where;

L

Rn

R1 . K / N

(4)

In equation 4;

Rn

Total resistance of the straps ( : )

R1 N K

Resistance of one strap ( : ) Necessary strap number

Conjoining factor The calculation of grounding strip diameter is given in Equation 5 [15]. 2

D

A 3 ,14

(m)

(5)

D = Strip diameter (m) A = Laboratory base area (m2) A horizontal grounding resistance calculation is given in Equation 6.

R

U

y

Ohm

2 D

(6)

In where 6;

Ry

Horizontal grounding resistance ( : )

D = Strip diameter (m)

U

Ground resistance ( : -m) Vertical grounding resistance calculation is given in Equation

Rd Rd

Establishment of proposed new approach to measure and control grounding resistance is done experimentally for a specific electrical machine laboratory which has dimension of 40x20 m. To have a successful grounding system, an accurate design is mandatory by determining the ground characteristics of the building. For that reason, different ground examples from different areas should be taken and their humid and temperature analysis should be carried out. Grounding resistance value can be calculated in Equation 1 [13].

2 S ra

d Diameter of electrode (m) Then in equation 4, number of straps is calculated.

U

4 Lc

(: )

(7)

In Equation 7;

II. SYSTEM ESTABLISHMENT

U


Vertical grounding resistance ( : )

L c = Strap length (m)

U

Ground resistance ( : -m) Equivalent grounding resistance is given in Equation 8.

Re

R y Rd

(: )

(8)

Ry R d In where; Re= Equivalent grounding resistance Ry= Horizontal grounding resistance Rd= Vertical grounding resistance After these calculations, necessary electrode number is figured out and to perceive every electrode’s touching surface, ground moisture and sensors are placed. The information coming from these sensors is amplified to appropriate voltage values for the input of interface control circuit as depicted in Fig. 1, which is composed of the sensor interface cards, humidity sensors, current sensors, a microcontroller, magnetic valves and an LCD.

Fig .1. Control system

Length of electrode (m)

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The most important factor in providing equipotential between terminals is having equal or very close resistance values between ground electrodes [14]. Seasonal changes affect grounding systems in a negative manner. Generally, in dry regions during summer time, ground moisture decreases and this situation causes increment in grounding resistance value. Therefore, equipotential in system grounding is disturbed and becomes vulnerable to electrical leakages and lightning [15]. In the system designed, grounding electrodes are planned to be connected to each other with placing drainage channels between them. These channels will be connected to water utility by using a magnetic valve. The magnetic valve which will be controlled by the microcontroller would be taken into and out of the system according to moisture information coming from sensors and hence, it provides water to underground channels. In Figure 2, flowchart of the system is shown. This follow chart initiates by a starting command, and then continues reading numerical data obtained from sensors. If inputs are “on control” they are written on LCD and the system passes to next step. If one or all of the entrances are “off control”, magnetic valve is taken into system by pulling relay with number 8 exit. Data is written on LCD. This process continues until all entrances become “1”. When all inputs become equal to “1”, it means that grounding resistance is below 10. System is prepared to keep grounding resistance at this value constantly.


underground and the region where grounding electrodes exist are established in conjunction with each other. These conjunctions are connected to water plant with a magnetic valve. On-off control signals coming from sensors are applied to microcontroller inputs. According to data coming from sensors, microcontroller actuates valve and increases moisture ratio with watering the region where grounding electrodes exists with using drainage pipes.

Fig.3. Experiment scheme Hence, grounding resistance decreases and the functions of the grounding system are fully completed. In Figure 3, a block diagram of the experimental set is shown. In where, water network, valves, liquid storage, drainage channel, sensors, grounding networks, grounding pole, grounding line, experimental set and sensor information transmission are given. IV. EXPERIMENTAL STUDIES In Figure 4, the experimental set is illustrated. Every component of the set is shown on the figure separately. These are a microcontroller, a magnetic valve drive circuit, sensors and sensor circuits, grounding electrodes and a magnetic valve (water pomp). The data sent to LCD screen during the application of the experimental set is shown in Figure 5.

Fig.2. Flowchart of the system III.SYSTEM OPERATION Grounding electrodes and underground drainage pipes along grounding components are placed and moisture sensors are installed on grounding electrodes to minimize ground resistance. Drainage pipes that are placed

Fig.4. Photograph of test rig The results on the screen show that resistance values in grounding system are normal or increased. In Figure 5, as seen in LCD, grounding resistance of fifth electrode is high.

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resistance and therefore this new proposed system is applicable to all areas in where grounding is required. REFERENCES

Fig. 5. LCD screen command information Procedures of experiment a) Calculated grounding bars and sensor of the grounding system are buried to ground. b) Drainage pipes are placed between grounding electrodes and underground. Pipes are connected to each other and plugged into magnetic valve by paving pipes until magnetic valve. c) Sensor types are entered to sensor circuit. Exit of sensor circuit is applied to input of microcontroller. d) Microcontroller exit is connected to magnetic valve drive circuit. The magnetic valve is set to operate with the system. e) When energy is given to the system to actuate, the microcontroller starts controlling the numerical data coming from sensors. If all inputs are “1” then grounding resistance is low. System continues to control grounding resistance in normal operating form. If one of the entrances or some of them are “0”, microcontroller takes magnetic valve into circuit with a program prepared in C programming language. The magnetic valve sends water to ground and electrodes from water utility by drainage pipes and continues its application until the resistance becomes equal or less than 10 ohms. When the resistance decreases to a normal value, valve application is stopped, and watering is finished, but control of the system is continued. During this process, information of the system is sent to LCD screen and at the same time the system is followed visually with sensor circuits led (whether the led are on or off.) V. CONCLUSION In this study, a new approach to design and implementation of a grounding system was introduced and then effects of ground moisture ratio on grounding resistance was investigated. Increments in ground resistance were reported mostly due to seasonal changes of temperature. Studies on a system design that would keep underground conditions constant and observation of these conditions were carried out. An experimental setup was established to keep the grounding resistance constant against variable ground temperature. Designed system provides security in case of lightning and electric leakages. Safety for human life was provided and any possible damage occurring in radio, TV, GSM, radar stations was prevented and monitored. Experimental results proved that the accuracy of the new developed system to measure and control grounding

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