The performance of the two systems will be seen, which is the most effective and efficient

86

Zuraidah Tharo^*, Amani Darma Tarigan, Siti Anisah, David Banjarnahor Faculty of Sains & Technology, University of Pembangunan Panca Budi Medan, Indonesia

ABSTRACT

This research will look at the protection system on a 60 MVA power transformer and compare two grounding systems, namely grounding with Neutral Grounding Resistance (NGR) and Grounding Solid (GS). The performance of the two systems will be seen, which is the most effective and efficient. Soil resistance is directly related to water content and temperature;

thus, it can be assumed that the resistance of a grounding system will change according to climate change. To obtain a stable grounding resistance, the grounding electrode is installed at the optimal depth to achieve a constant level of water content. The study began by collecting data and then comparing grounding using NGR with GS and measuring the resistance values of NGR and GS on a 60 MVA Power Transformer. The results of this comparison will obtain data on the NGR and GS resistance values on the 60 MVA power transformer, then analyze and compare these values which will provide a conclusion which of the two systems has the best value for grounding.

Keywords:

Neutral Grounding Resistance Grounding Solid

Comparison

Power Transformer 60MVA

Corresponding Author:

Zuraidah Tharo,

Faculty of Sains & Technology,

University of Pembangunan Panca Budi Medan,

Gatot Subroto Road, Medan, North Sumatera, Indonesia.

Email: zuraidahtharo@dosen.pancabudi.ac.id

1. INTRODUCTION

In general, transformers do not have their own security system, therefore, to maintain security, the maximum current must be limited by using NGR (Neutral Grounding Resistance). NGR is a resistance that is installed in series between the neutral point of the secondary side of the power transformer with ground, aims to control the magnitude of the fault current flowing from the neutral point to the ground. NGR functions to minimize single-phase to ground fault currents that appear in medium voltage networks to prevent damage to power transformers, minimize momentary overvoltages caused by instantaneous disconnects in order to extend the service life of the switchgear, and reduce the step voltage to a value that is not harmful to the operator. The direct grounding system (Solid Grounding) is a system that is connected directly from the neutral point of the power transformer to the ground through a conductor ignoring resistance and reactance. This aims to limit the voltage of the undisturbed phase in the event of a fault to ground.

Figure 1. Solid Grounding System

The power transformer is a type of transformer that is used to increase the value of the electric voltage from the generator. Placement in the substation. The increased electric voltage is then transmitted to the electric power transmission line. The power transformer is composed of an iron core. The other part is a coil which is divided into a primary coil and a secondary coil.

Figure 2. Transformer Parts

The grounding system is a security system for electrical devices from disturbances that occur, especially from power surges caused by lightning. The grounding system is described as the connection between an equipment or electrical circuit and the earth. The grounding system that is used both for neutral grounding of electric power systems, grounding for lightning protection systems and grounding for equipment, especially in the field of telecommunications and electronics, needs serious attention, because in principle the grounding is the basis used for a protection system. Grounding is connecting a point of an electric circuit or a conductor that is not part of an electric circuit with the earth in a certain way. The behavior of the resistance of the grounding system is highly dependent on the frequency (base and harmonics) of the current flowing into the grounding system.

From the two protection systems used in the 60 MVA power transformer, the performance of the most efficient and effective protection system will be seen. The dominant quantity to be considered from a grounding system is the grounding resistance. In general, the resistance is measured using an earth tester, which in principle flows direct current into the grounding system, while in reality, a grounding system never carries direct current. Because usually the current that flows is sinusoidal/alternating (AC) or even in the form of high- frequency impulses (lightning) or in the form of time-changing currents that are very erratic in shape. The magnitude of the grounding impedance is greatly influenced by many factors, both internal and external factors.

As for what is meant by internal factors include the dimensions of the grounding conductor (diameter or length), the relative resistivity of the soil, and the configuration of the grounding system. While what is meant by external factors includes the shape of the current (pulse, sinusoidal, direction), and the frequency that flows into the grounding system. To find out the accurate soil type resistance value, measurements must be made directly at the location used for the grounding system because the actual soil structure is not as simple as it is thought, for each different location the soil type resistance is not the same.

2. RESEARCH METHOD

In this study, the literature study method and observation method or direct observation of a 60 MVA power transformer were used to obtain the desired results. The research steps can be seen in the image below:

Figure 3. Research Stages

The stages carried out in the research, starting with collecting data on the transformer used, then comparing the grounding system used, namely using Neutral Grounding Resistance (NGR) with Grounding Solid (GS) then measuring the value of the NGR resistance and GS resistance on the 60 MVA power transformer. Then, perform calculations and analyze the measurement results of the two systems, then compare the calculation results of the two systems obtained, then determine the best grounding system and meet the requirements for a good and safe transformer neutral grounding.

The data obtained are as follows:

Table 1. Transformer Data Power Impedance Primary

Voltage

Secondary Voltage

Nominal Current 150 KV

Nominal Current 20 KV

Short Circuit Current

Primary Current Lowest Tap

Primary Current Highest Tap 60

MVA

12 % 150 KV 20 KV 230,95

Amper

1732,1 Amper

9,72156 KA

148 Amper

190 Amper Furthermore, the research flowchart can be seen in the following figure:

N Y

Figure 4. Flowchart

Closing

•conclusions

•suggestions

3. RESULTS AND ANALYSIS

Grounding in power transformers can be done in various ways, such as by embedding a conductor rod perpendicular (vertical) to the ground surface or by embedding a conductor rod parallel (horizontal) to the ground surface, with a depth of several tens of centimeters below the ground surface. This is done to secure the equipment and people around the grounded equipment. Difficulties are also often encountered in planting rods from these grounding electrodes such as in rocky locations, even so this grounding is very important which is prioritized in efforts to secure or protect equipment and people.

3.1. Grounding Calculation Using NGR

The results of the analysis of one-phase short-circuit current to ground before the NGR wire breaks and after the NGR wire breaks are as follows:

a. Ihs 1∅ ground on the 20 KV side before the NGR wire breaks using Eq

INGR:I1∅20 = ^{𝑉3 .1000}_{√3.𝑁𝐺𝑅} (1)

= ^{20 .1000}

√3.60

= ²⁰⁰⁰⁰_√180

= ²⁰⁰⁰⁰

√180

= _13.416²⁰⁰⁰⁰

= 1.490,757 A

b. Ihs 1∅ ground on 20 kV side at break of NGR wire. Calculation of the value of the short circuit current one phase to ground after the break of the NGR wire, it is necessary to calculate the value of short circuit power, transformer reactance, transformer source impedance, positive sequence impedance and negative sequence as follows:

Transformer Short Circuit Power MVA ha = √3 xKVLL X IhsMVA.

= 1,732 x 20 x 9,72156.

= 336 MVA Impedansi Dasar Trafo Zd = KV²dasar

MVA dasar

= 20²/60

= 6,67 Ohm

Impedansi Trafo dengan rumus persamaan Zt = 12% x Zd

= 12% x 6,67 Ohm

= 0,8004 pu.

Reaktansi trafo dengan menggunakan persamaan.

Xt = Zt x Zd

= 0,8004 x 6,67

= 5,34 Ohm.

Impedansi sumber trafo dengan menggunakan persamaan.

Zs = (KVLL)

MVA hs

= 20²

336

= 1,190 Ohm

Impedansi urutan positif dan negatif menggunakan persamaan:

Z1eki dan Zzeki = Zd + Zs

= 6,67 + 1,190

= 7, 86 Ohm.

X0 = 3 x Xt

= 3 x Vph

2 (Z1eki + Zeki) + Z0eki

= 3 x 2000/√3

2 (7,86) + 23,46

= 34641,016

39,18

= 884,15 A

3.2. Calculation of Grounding Using Grounding Solid (GS)

The Grounding Solid (GS) value at one phase to the ground can be calculated by the following equation:

GS = tset x ((1F1∅ ) 0,02-1) (3)

1 set primer_____

0,14

GS = 0 x ((884,15 x 10³ ) 0,02-1) 0,233

0,14 GS = 0

Based on the results of these calculations at the time of a single-phase short-circuit fault to the ground, the solid grounding can be calculated based on the applicable TMS value, namely:

t = TMS x 0,14

((1F1∅ )0,02-1 (4)

t = ____0 x 0,14________

((8884,15 x10³ ) 0,02-1) 0,233 t = 0 detik

GS fault simulation calculation using the equation under certain circumstances on the winding of phase "a" if there is a short circuit fault of one phase to ground with a distance of 30% from the neutral point of the transformer, so the analysis results obtained are:

I∅a = ^𝑋

100 X 1F I∅a = ³⁰

100 X 884,15 I∅a = 265,245 A

This calculation means that there is a disturbance to the ground at phase "a" of 265.245 Amperes, resulting in a current flowing towards the neutral point of the transformer of: 265.245 A, then the IR is:

I0 = I∅a = IP = 265,245 A IR = (I∅a X N)- 0,4 IR = (265,245 X ⁵

2000) - 0,4 IR = 0,263 A

From the results of the analysis of pass-to-ground faults, an IR of 0.263 A was obtained which indicates a safety indication. Attention to security issues both for equipment and work, it is necessary to make an effort to create a security system that can protect the equipment and work from threats or electrical

disturbances that occur. One of the most important security systems for equipment and work is the transformer at the substation. The main function of grounding equipment is to channel electric current into the ground through a ground electrode that is planted in the ground if a disturbance occurs, besides that it also prevents the occurrence of electric shock voltages that are harmful to humans.

Equipment grounding resistance which is still so large is very detrimental because it will endanger the personnel who are working and the equipment that is being used at the substation. To reduce the effect of changes in resistance, routine maintenance is needed on the grounding system so that the price of grounding resistance remains as small as possible on the power transformer. In the analysis of these calculations on the grounding system using solid grounding, the neutral point of the transformer on the secondary side through resistance needs to be done because if the neutral point is grounded without resistance (solid) it will affect impedance and affect the amount of short circuit fault current in the feeder, thus around fault point is very dangerous for the equipment in its path and is also expected to cause damage to the conductor cable around the fault point.

3.2. Comparative Analysis

The calculation analysis determines the REF relay settings sourced from the data used, namely the current transformer ratio (N) is 2000/5 A, the current transformer secondary coil resistance (RCT 2) is 3.0 Ohm, the current transformer magnetization current is 0.1, the number of current transformers in parallel (n) is 4, the loop resistance (2 RL) is 2.5 Ohm, and the power burden of the REF relay is 1.0 VA, the external resistance of the relay is 1.5 Ohm. The calculation of the I mean value can be calculated by the equation:

I Mean = √IMax x I Min I Mean = √190 x 148 I Mean = 167,6

Furthermore, the sensitivity of the transformer safety is analyzed by the following equation:

g =10% + (I Max- I Mean x100%) I Mean

g =10% + (190- 167,6 x100%) 167,6

g =23,3%

Based on these calculations, IR = 23.3% from 1 Ampere = 0.233 Amperes.

Calculation of the primary current sensitivity (IP) on solid grounding can be calculated by the following equation:

IP = N x ((IR +(n x IE)) IP = ²⁰⁰⁰

5 x ((0,233 +(4 x 0,1)) IP = 252 A.

Then the minimum voltage can be calculated as follows:

VS ≥I hsmax trafo X (RLT + 2Rloop ) N

VS = ⁹⁷²⁰

400 X (3 +2,5) VS = 133,65 V

The value of the knee point voltage (VK) can be calculated by the following equation:

VK = 2 x VS

VK = 2 x 133,65 VK = 267,3

NGR and GS retention values can be calculated using the following equation:

RST = ^𝑉𝑆

𝐼𝑅 - ^𝑉𝐴

𝐼𝑅² RST = ^133,65_0,233 - _0,233^1,0₂ RST = 573,6 -18,45 RST = 555 Ohm

From the results of the analysis and calculation of grounding according to the calculation is 555 Ohm, where the basic setting facilities installed are Is: 0.1; 0.15; 0.2; 0.25; 0.3; 0.35; 0.4 A, Frequency = 50 Hz, Ip =

NGR 844,15 A

GS 265,245 A

The results of the calculation of the comparison of Neutral Ground Resistance (NGR) with Solid Grounding against single-phase-to-ground short circuit fault currents show that the NGR remains relatively large and is expected to damage equipment and conducting cables around the fault point and is also very dangerous for the people around it. In order to stop the short circuit current is to use a protective relay setting so that it can activate the power breaker (PMT) when the disturbance occurs.

From this analysis, the security obtained in using grounding as a grounding system is using NGR, because the incoming current is greater than using GS, so the risks posed are smaller using NGR than using GS, or single-phase short-circuit fault currents. to the ground becomes larger using NGR than GS.

The grounding system using NGR (Neutral Ground Resistance) on the transformer is inseparable from short circuits, therefore safety equipment is needed. Safety equipment for special transformers is generally not available, therefore, to maintain safety, the maximum current must be limited by using NGR (Neutral Ground Resistance). If the NGR does not work, then the fault current will flow directly to the neutral point of the transformer so that it cannot protect the transformer. Direct earthing system is a system that is connected directly with or without including an impedance. This aims to limit the voltage of the undisturbed phase in the event of a fault to the ground, or also known as the grounding system. The grounding system using NGR is used in the secondary system of power transformers to reduce fault currents, because it is feared that there are many disturbances that can harm the transformer.

4. CONCLUSION

From the results of the analysis carried out on the comparison of grounding using Neutral Grounding Resistance with Solid Grounding on a 60 MVA transformer, it can be concluded that the grounding system using Neutral Grounding Resistance (NGR) on a 60 MVA power transformer is the main key that determines the resistance of the electrode and at what depth the electrode should be planted in order to obtain low resistance. Soil resistance varies from place to place and tends to change with the weather. Soil resistance is also determined by the electrolyte content in it, the content of water, minerals, and salts. Dry soil usually has a high resistance however, wet soil can also have a high resistance if it does not contain soluble salts. Soil resistance is directly related to water content and temperature thus it can be assumed that the resistance of a grounding system will change according to climate change every year. In order to obtain a stable grounding resistance, the grounding electrode is installed at the optimal depth to achieve a constant level of water content.

The results of the analysis of the calculation of the NGR and GS phase to ground grounding currents from the calculation results on the 60 MPA transformer show the results of the calculation of a single phase to ground short circuit fault on a transformer with a capacity of 60 MVA, that the result of the calculation of the single phase grounding current to the ground is 884, 15 A for NGR, and the GS grounding phase is 265.245 A From this analysis, the security obtained in using grounding as a grounding system is using NGR, because the incoming current is greater than using GS, so the risks posed are smaller using NGR than using GS, or single-phase short-circuit fault currents. to the ground becomes larger using NGR than GS. Analysis of the disturbance calculation on the transformer winding is based on the simulation calculation of the disturbance on the 60 MVA transformer winding, that the amount of current flowing is 0.233 A at NGR, while at GS it is 0.263 A at the time of a single-phase short-circuit fault to ground with a working time of 0 seconds or without delay.

Suggestion

a. The transformer safety system must be equipped with a Restricted Earth Fault (REF) relay to protect against a singlephase short circuit to ground, because the REF relay

b. Analysis of the calculation of the REF relay setting as protection against single-phase short-circuit faults to ground, therefore a good and safe location analysis is needed, so as not to cause losses.

c. In order to reduce the value of a grounding resistance, it can be done by adding parallel electrodes or adding the depth of embedding the electrodes.

ACKNOWLEDGEMENTS

At the end of this article I thank my colleagues who have helped with research and writing, as well as the Panca Budi Development University for providing funding for research, and also the Medan Polytechnic for organizing this conference. May Allah SWT, the Almighty God, repay the kindness of all of us.

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Jurnal Teknik Elektro, Vol. 1, No. 1, diakses Agustus 2019 ISSN 2622 – 7002 (online), https://doi.org/10.30596/rele.v1i1.2257

How to Cite

Tharo, Z., Tarigan, A. D., Anisah, S., & Banjarnahor, D. (2023). Performance of Neutral Grounding Resistance and Solid Grounding on 60 MVA Power Transformer. International Journal of Research in Vocational Studies (IJRVOCAS), 2(4), 86–93. https://doi.org/10.53893/ijrvocas.v2i4.172