Matters that need attention and are about the operation and off-stream of power transformer

  Transformer is outage for one month or put aside and outage for more than 6 months. 

Transformer

What test should be done before it is put into use?

1. If the transformer is outage for one month, we should use the insulation resistance tester 

DC Winding Resistance Meter

to measure insulation before resumption of power.

2. If the transformer is outage for more than 6 months, we should do insulation resistance test and insulation oil pressure test.

3. If the special transformer of drainage and irrigation which is in a dry and cold area, the outage period may be appropriately extended, but not more than eight months.

What are the requirements for transformer operation and outage?

(1) The new transformer must be tested at rated voltage for 5 times, and after the overhaul, the impulse voltage test should be carried out at least 3 times.

500kV25KJ Impulse Voltage Generator 

(2) when the transformer is put into operation, the cooler should be put into use first, and the cooler runs for a period of time (about 15min).

(3) In the 110kV and above neutral grounding system, when the transformer is put into operation and shutdown, the neutral point must be grounded first.Transformer neutral point of the arc suppression coil should be returned and then investted. The neutral point of the two transformers shall not be connected to the neutral bus of an arc suppression coil at the same time.

(4) The power operation, stop the load side of the switch, and then stop switch power supply side (side power from low to high stop); the first pull transformer side switches, pull the bus side gate. Power supply operation is opposite.

(5) Putting into standby transformer should be based on the actual location of the device and meter instructions to determine whether it is carried out the load.  the load has been carried out in order to enable the operation of the transformer power outage. The transformer on the diagonal and the 3/2 connection, although the transformer has been cut off, but the heavy gas and differential protection of the transformer can still cause the closing of the switch. It shauled be based on the actual situation and site regulations to change the location of the gas protection signal or gas out.

(6) The station transformer is not allowed long-term coordination. It can be used to cut the low pressure side of the low voltage side of the circulation with high pressure knife gate cut off the transformer station.

How to dry transformer?

Transformer is the principle of electromagnetic induction to change the AC voltage of the device. The main components are the primary coil, secondary coil and iron core (magnetic core).

The purpose of drying transformer is to remove water from the transformer insulation material and increase the insulation resistance to improve its flashover voltage. Transformers above 3kV must be dried. The transformer body is mainly composed of an iron core , a coil and an insulating material. After assembled, transformer must through the drying process to remove the water and gas of insulating materials before joining the transformer oil. It is to ensure the transformer insulation strength and service life enough. For high voltage transformer, the insulation material should be less than 0.5%. 

1. Induction heating method

The instrument itself is placed in the tank, the use of fuel tank wall eddy current loss of heat to dry. At this time the wall temperature should not exceed 115-120 degrees, the body temperature should not exceed 90-95. In order to winding the coil of convenience, as far as possible the number of turns of the coil less or less current, the wire can be  35-50mm2 wire, the general current is 150A. The oil tank wall can be filled with a plurality of asbestos bars, and the wire is wound on the asbestos strip.

2. Hot-air drying

The transformer itself is placed in a dry room through the hot air drying. The inlet hot air temperature should be increased gradually, and the filter should be installed at the inlet of hot air to prevent the entry of Mars and dust. The maximum temperature should not exceed 95. Hot air blowing device not directly, as uniform as possible from the body of the wind in all directions, so that the moisture released by the box cover hole.

Problems should be paid attention to in transformer drying:

(1) If the drying chamber is not vacuum, the vent should be opened on the lid of the box or the throttle hole can be used to make the moisture release.

(2) When the oil is heated, the insulation layer should be installed outside the tank. The insulating layer can be made of insulating material such as asbestos cloth, glass cloth and so on.

(3) In order to improve the drying quality of windings, there are two major factors to be considered: the first is to control the drying temperature; the other is to improve the vacuum of the equipment. On the first point, the general drying equipment can meet the technical requirements, to second points, subject to a number of factors, must be taken into account, reasonable arrangements in order to achieve good drying effect.

(4) The process of vacuum drying, drying in low temperature stage, not in the vacuum or low vacuum drying conditions, otherwise it is not conducive to the elimination of core temperature and moisture. When the temperature rose to 70 to 80 to improve the vacuum degree. When drying is carried out from 1 to 2h, the water vapor in the oil tank is more, the heat radiation ability is improved, the internal temperature tends to be uniform, the water is gradually reduced, and the heat radiation ability is reduced.

      (5)A method of identification of insulation after drying and transformer technical specifications for the test.

 After the transformer is dry, it is necessary to use insulation resistance tester to make a comprehensive identification of its insulation performance to check its drying effect. Identification of the project, in addition to casing, the rest are the same as the hoisting of the heart of the test items.

Shielding Design of High Voltage Test Hall

Introduction

Electric design of high voltage test hall is mainly involved in three key points: grounding, shielding and safety. The well-designed shielding system of high voltage test gall is vital to smooth operation of high voltage test hall. As for high voltage test, roles of good shielding are as follows:

(1) Ensure the reliability and precision of measuring equipment (including display and record instrument).

(2) Prevent shock wave from disturbing low-voltage control circuit or other test equipments in the impulse test.

(3) Stop shock wave from affecting outside the high voltage test room and disturbing the power supply.

(4) Restrain external electromagnetic field from disturbing PD measurement test, which leads to measurement error.

(5) Protect the health of human body within the range of electromagnetic radiation and interference.

Combined with the construction example of a high voltage test hall, its shielding design and measures are presented in this paper.

1. Project Overview

The high voltage test hall covers 700m². The single-floor high voltage test hall is divided into east area and west area. With 1620m² building area, the east test hall is 27m × 60m × 20m while west test hall is 37m × 24m × 32m covering 888m². North of east test hall is used as capacitive room and equipment shed; south of east and west test halls is three-layer control room, debugging room and office. The high voltage test hall mainly conducts the AC and DC withstand voltage, PD test, impulse voltage, insulation test and over-current test for UHVAC power transmission and distribution equipment . Most tests fall into the category of high voltage and low current (600 ~ 880kV) and a minority of tests belong to the category of high current and low voltage (2000 ~ 3000A). The frequency range that high voltage test hall needs to shield is 500kHz~100MHz. The  shielding effectiveness SE ≥ 55 dB. 

2. Issues Needed to Be Pay Attention to When Designing High Voltage Test Hall   

(1) Correctly analyze properties of electromagnetic shielding field of high voltage test to determine shielding material, thickness as well as the method to deal with pores.

(2) Pay attention to the integrity of shielding body. The overall shielding effectiveness depends on the weakest link on the shielding body. In order to make shielding effectiveness reach some value, all parts and components on the shielding body need to reach this value. Therefore, shielding effectiveness of each component should be greater than the one the design required. The match of shielding effectiveness level of each component in the shielding system is very important.    

(3) The shielding design is involved in many majors, such as architecture, water heating and electrical engineering etc. One major among these majors had better be responsible for organizing and coordinating. Generally speaking, the electrical engineering major bears the responsibility.

(4) Owning to big volume of high voltage test hall, adopting shielding material with excellent quality and reasonable price should be taken into account when pursuing good shielding effect. Besides design factor, factors which can determine shielding effectiveness also include construction factor and its constructability.  

(5) The pores on shielding body have a significant effect on the shielding effectiveness, which needs to be carefully dealt with.   

3. Analysis and Measures of Shielding Field of High Voltage Test Hall

3.1 Shielding Situation of Part of High Voltage Test Hall at Home and Abroad

High voltage test halls in overseas advanced countries mostly adopt multi-layer steel plates to shield while part of halls use aluminum plate, copper plate or expanded metal to shield. The shielding effectiveness is above 78 dB. One-layer expanded metal is mostly utilized in China and the shielding effectiveness is 40~50 dB.

3.2 Shielding Material and Structure of High Voltage Test Room

JBJ7-1996 Code for Design of Machinery Factory Building stipulates shielding material and structure of electromagnetic shielding room, which is shown in Table 1.

      

  In the project, high voltage and low current interference source is focused on electric field while high current and low voltage interference source is focused on electromagnetic field, belonging to variable electromagnetic field shielding. Hence, electric and electromagnetic field need to be taken into account at the same time. The electromagnetic wave has electric field component and magnetic field component. The high magnetic permeability is as important as high conductivity.     

The effectiveness of shielding body is measured by shielding effectiveness (SE). The definition of SE is as follows:

SE = 20 lg(E₁/E₂)dB        (1)

In the formula:

E₁ — field strength without shielding

E₂ — field strength with shielding 

Generally, the shielding effect can be classified into following categories:

0~10 dB  almost no shielding effect

10~30 dB small shielding effect

30~60 dB middle shielding effect, which can be applied in the general industry or commercial electronic equipment

60~90 dB high shielding effect, which can be used for shielding the aerospace and military equipments

＞90 dB best shielding effect, which is suitable for products with high precision and sensitivity. This project belongs to the middle shielding requirement.

The bigger the attenuation value is, the better the shielding effect is. According to the principle of electromagnetic shielding, shielding effect of the material can be expressed as:

SE = SE_R + SE_A + SE_B      (2)

In the formula:  

SE_R — single return loss of electromagnetic shielding body material (dB)

SE_R — absorption loss of electromagnetic shielding body material (dB)

SE_B — multiple return loss correction terms of electromagnetic shielding body material (dB)  

A＞10 dB, SE_B can be neglected. The formula (2) can be expressed as:

SE = SE_R + SE_A           (3)

SE_R = 168 + 10 lg (ơ_r/μ_rf)  (4)

 SE_A = 1.31 t (f ơ_rμ_r )^1/2      (5)

The plate thickness adopts the following formula:

t = SE - 168 - 10 lg (ơ_r/μ_rf)/1.31 (f μ_rơ_r )^1/2

In the formula:

t — material thickness (cm)

SE — shielding effectiveness (dB)

ơ_r — material to copper conductivity steel-0.17

μ_r — material to copper magnetic permeability  steel-200   

f — frequency  

The steel is a good conductor and the permeability magnitude is satisfactory as well. It is relatively cheap and can provide the material with strong mechanical strength. Hence, the satisfactory shielding effectiveness can be obtained via using cheap steel. The main interference source of high voltage test hall is low-frequency electromagnetic wave, which has higher magnetic-filed component than high-frequency electromagnetic wave. Thereby, as for low interference frequency, magnetic permeability of shielding material is far more important than high frequency. In the strong electromagnetic environment, the material is required to shield electric field and electromagnetic field so the ferromagnetic material with perfect structure is necessary. The shielding effectiveness is directly affected by the thickness of material and the method to lap and ground.  

The frequency that high voltage test hall needs to shield ranges from 500kHz to 100MHz. Based on the relation between wave length and frequency, corresponding wave length can be obtained. 500kHz corresponds to 600m wave length and 100MHz corresponds to 3m wave length.  

In order to prevent shock wave in the impulse test from disturbing the low voltage control circuit or other test equipments, high voltage test hall can be viewed as interference field source. Meanwhile, control room can be viewed as receiving point. The project belongs to low-frequency alternating electromagnetic field. Because electric field and magnetic field exist at the same time, shielding of electric field and magnetic field need to be taken into consideration. When the frequency is low, the electromagnetic interference mainly displays in the near-field area. In the near field, because properties of interference source are different, the size of electric field is greatly different from that of magnetic field.

When the interference generated by interference source appears in the manner of high voltage and low current and is focused on the electric field, electric field shielding method can be taken into account. The electric shielding can view electric field induction as coupling of distributed capacitance. Key points of the design are as follows: the shielding shell had better be totally-enclosed metal box and be grounded well. Bad grounding of metal shielding body will reduce shielding effectiveness. The material of shielding plate might as well use good conductor but there is no requirement for thickness. When the interference generated by interference source emerges in the manner of low voltage and high current and is focused on magnetic field, magnetic field shielding method can be taken into account. If the interference source belongs to low frequency field, shielding plate should select high magnetic conductive material and increase the thickness of shielding body. Pay attention to the structure design of shielding body and deal with pores which may increase the magnetic resistance of shielding body, thus reducing the shielding effect.

The baseboard adopts double-layer expanded metal and mesh size is 9mm × 25mm. The lower layer adopts 1.5mm expanded metal while the upper adopts 3mm expanded metal. The thickness of liner shielding plate is 0.65m and lapping＞120m. The interior adopts metal ceiling and all metal ceilings and part of metal walls adopt the micro plate.           

In order to prevent the shock wave in the impulse test from imposing an impact on outside the high voltage test room and disturbing the power source, high voltage test hall can be viewed as internal electric field shielding of interference source. However, the equipment of high voltage test hall cannot be completely independent, such as water and electricity. It is inevitable that the equipment is related to surrounding while the shock wave affects outside the high voltage test room and power source via this connection. When designing, the shielding shell of high voltage test hall had better adopt the totally-enclosed metal box and the grounding should be good. At the same time, 10kV cable and water pipe enter the lab 20m and should be grounded. Due to single pointing grounding of high voltage test hall, this connection grounding should be separated from shielding grounding.        

To prevent external electromagnetic field from disturbing PD measurement test and resulting in measurement error, interference field source can be viewed as electromagnetic wave of far-field high frequency. The key point of design is higher requirement for dealing with pores on control room shielding body.  

Take GB 7195-1988 Hygienic Standard for Environmental Electromagnetic Waves and GB 8702-1988 Regulations for Electromagnetic Radiation Protection for the design standard in order to protect the health of human body within the range of electromagnetic radiation and interference. The design point is to strengthen shielding measures at the office area, which is adjacent to high voltage test hall, and adopt metal plate to shield and ground.

4. Methods to Deal With Pores

Methods such as welding, spring leaf contact, manganese metal mesh and cut-off waveguide etc can be used to handle pores on the shielding body.

4.1 Cut-off Waveguide

The heating, ventilating, water pipe, power cable and control measurement cable on the shielding body are connected externally and the connection cannot cause electromagnetic leakage, pores have to be handled. Methods such as welding, spring leaf contact, manganese metal mesh etc can be applied to handle pores on the shielding body. Or the cut off waveguide can also be used. When applying cut-off waveguide to shielding body, here are some tips:  

(1) The waveguide must be cut off. The waveguide pipe does not play any role in attenuating the electromagnetic wave, which is above cut off frequency. The requirement for applying the above formula is to make waveguide cut-off frequency five times of highest shielding frequency. This project pick the diameter d of waveguide pipe based on f = 500MHz and λ = 60mm. As for round waveguide pipe, d (cm) ≤ λ/1.71 while d (cm) ≤ λ/2 for rectangular waveguide pipe. The attenuation S is proportional to the length of waveguide pipe L. L ≥ S_d/32 for round waveguide pipe while L ≥ S_d/27.3 for rectangular waveguide pipe. A section of honeycomb plate can be added on the waveguide pipe if the diameter of metal pipe exceeds the maximum diameter corresponding to cut off frequency.        

(2) Prohibit metal material from passing through the cut-off waveguide pipe. Otherwise, it will cause serious electromagnetic leakage.

(3) Continuous welding should be adopted between the waveguide pipe and shielding body; adopt Flange to fix the waveguide pipe on the shielding body or take gap shielding measures.  

4.2 Shielding Measures of the Gap

(1) Shielding of Construction Joints. In order to ensure the safety, lapping length between plates and internal corner as well as external corner sheet should be lengthened. The interval adopts 40 copper meshes as filling zone. Welding effect of shielding hall is the best but only riveting or screw is used to fix owing to restrictions of construction condition. When using riveting or screw to lap, starts with middle gap, and then extend to both ends in order to avoid bending of mental surface. The riveting distance should be less than 1% of highest working frequency and at least no more than 1/20λ. The result in this project is 150mm.      

(2) Shielding of Pipelines. Connect a section of non-mental insulating pipeline before ventilation and drainage pipelines lead to the shielding area. Its length is 1.5-2 times of pipeline diameter. Set copper mesh or waveguide filter in the non-mental insulation pipeline which goes through the shielding layer.   

(3) Shielding of Doors and Windows. The shielding layer of observation window must be tightly connected with shielding body. The shielding door adopts steel plate door, which should be equipped with compress equipment. The upward and both sides of the door is set with shielding bucket. The seal adopts comb grid.    

(4) Shielding of Cable and Cable Channel. Measuring cable and primary cable are applied respectively in the metal cable tank, which multi-connects to test hall shielding body. The cable channel gap in the junction between shielding area and non-shielding area uses the aluminum foil to shield.   

(5) Shielding of Luminaries. All the luminaries adopt 2mm × 2mm copper mesh to shield.  

5. Conclusions

After the project is completed, the minimum of shielding effectiveness measured on site is 58dB, which meets the design requirement. That proves that shielding design and construction of this high voltage test hall are successful.

By reviewing the whole design process, conclusions are made about this kind of design. First, correctly analyze properties of electromagnetic shielding field of high voltage test hall and then select reasonable shielding material; pay attention to the integrity of shielding body and its good grounding; shielding effectiveness of each shielding component selected should be greater than required effectiveness; pay great attention to ways to deal with pores, and strengthen the coordination of each major; take implementation feasibility into consideration (cost and construction cycle). 

Design of Grounding for High Voltage Test Hall

Introduction

The electric design of high voltage test hall is mainly involved in three aspects: grounding, shielding and safety. In terms of high voltage test, good grounding plays a role in sharing the voltage and fixing the potential.

The well-designed shielding system of high voltage test gall is vital to smooth operation of high voltage test hall. This paper discusses the design of the main grounding system.

1. Grounding System of High Voltage Test Hall

The 1.4.1 term in the standard for national electric power industry DL 560-1995 Safety Code of Electric Power Industry (High Voltage test part) stipulates that the high voltage test hall should have a very good grounding system to ensure the accuracy measurement of the high voltage test and the personal safety. The grounding resistance should not be higher than 0.5Ω. Table 1 presents part of high voltage test grounding resistance at home and abroad.

Tab. 1 Part of high voltage test grounding resistance at home and abroad

The grounding system must meet the requirements as follows:

(1) Ensure the reliability and precision of measurement equipment (including display and record instrument)

(2) Prevent the shock wave in the impulse test from generating overvoltage on the low voltage control circuit and other test equipment.

(3) Deter the shock wave in the impulse test from affecting outside the high voltage test room and disturbing the power source.

(4) Reduce the external interference into the minimum.  

(5) Provide a shortest and most advantageous discharge channel for the partial discharge.

(6) Provide one-point grounding channel for the impulse test (transient process).

(7) Provide stable and reliable grounding for the power frequency test (steady process).

(8) Provide the reliable equivalent potential for test personnel safety.  

The grounding system of high voltage test hall comprises following three parts:

(1) Grounding circuit formed by the six-face Faraday Cage of test hall (east and west test hall, and six-face shielding body of control room).

(2) Main grounding system composed of horizontal grounding belt under the six-face shielding ground of test hall and vertical grounding body.

(3) Auxiliary voltage-sharing round grounding body around the building.   

Due to providing shortest and most advantageous discharge channel for the partial discharge, main grounding system should be set within the range of east and west test hall six-face shielding body. How to design the main grounding system? We start by analyzing the grounding resistance formula.

The calculation formula of grounding resistance of vertical grounding electrode is as follows:

R = pln (4l/d)/2πl           (1)

The calculation formula of grounding resistance of horizontal grounding electrode is in the following:

R = p[ ln (l²/hd ) + A ]/2πl    (2)   

The calculation formula of grounding resistance of compound grounding electrode is as follows:

R_n ≈ 0.5p/S½             (3) 

Or  

R_n ≈ p/4r + p/l              (4)

In the formula:

p— soil resistivity (Ω · m)  

d—equivalent diameter of grounding electrode (m)

h—buried depth (m)

L— length of grounding electrode (m)

A—shape factor

S—horizontal projected area of grounding grids (m²)

R—circle radius equal to S (m)

The effect of steel section size of grounding electrode on the calculation of grounding resistance is so small that we can neglect it. Assume that soil resistivity and buried depth of grounding electrode are known, the formula (1) indicates that vertical grounding electrode must be lengthened to reduce the grounding resistance value of vertical grounding electrode; the formula (2) shows that horizontal grounding electrode must be lengthened or expand the horizontal projected area of grounding grids to reduce the grounding resistance of horizontal grounding electrode; it is clear from formula (3) and (4) that only expanding the horizontal projected area of grounding grids and lengthening vertical grounding electrode can reduce the grounding resistance value of compound grounding electrode.

1.1 Horizontal Grounding Body

1.1 Impact of Horizontal Grounding Ring Area on Grounding Resistance

When the grounding grid adopts grounding ring to ground, p = 40 Ω · m; h = 1.2m; A = 1.69; grid width and the equivalent diameter of grounding electrode are different; we can calculate the grounding resistance based on the formula (2). Please refer to Table 2.

Tab. 2 Effect of horizontal grounding ring area on grounding resistance

Based on Table 2, the grounding ring works if the grounding requirement is not high and geological condition is excellent. However, as for the high voltage test hall, it is infeasible. The high voltage test hall in this project is ranked as No. 1 in scale and area after its completion. The results indicate that only adopting the grounding ring cannot meet the requirement of grounding resistance.

It is also clear that the section of grounding body has little impact on the grounding resistance. Hence, the grounding body only needs to meet the requirement of thermal stability, mechanical strength and corrosion resistance.

1.1.2 Effect of Number of Grounding Grids on Grounding Resistance

Area of grounding grids is 48×48m, p = 40Ω· m, h=1.2, d=0.02m; Table 3 shows that different number of grounding grids correspond to different grounding resistance, which is shown in Table 3.    

Tab. 3 Effect of number of grounding grids on grounding resistance

If relevant grounding resistance is taken as cardinal number 1, according to the above calculation method, grounding resistance rate under different grid number is obtained. Use the dot to represent the data on the table and we can draw Figure 1-the effect of number of grounding grids on grounding resistance. 

Fig.1: Effect curve of number of grounding grids on grounding resistance  

American AFSC-DH1-4 Handbook of EMC Design stipulates that based on the economic benefit of the design, the grounding grids should cover the area as much as possible to reduce the grounding resistance while the number of mesh should be less than 16. This is consistent with test conclusions made by Chinese relative department. When the grounding mesh is more than 16, the grounding resistance reduces slowly. Comparing 16-mesh square grounding with 2-mesh one, the grounding resistance reduces by 23%; for 4-mesh square grounding, the resistance only reduces by 10%. Obviously, compound grounding should be adopted to reduce grounding resistance of high voltage test hall.    

1.2 Compound Grounding Coordinated with Vertical Grounding Body  

The calculation formula of grounding resistance of compound grounding electrode is as follows: In

 (5)

S = l₁l₂

In the formula:

p—soil resistivity (Ω · m)  

d—steel equivalent diameter (m)

h—buried depth (m)

l—total length of grounding body (m) , including vertical grounding body

l₁—horizontal length of grounding grids (m)

l₂—horizontal width of grounding grids (m)

1.2.1 Effect of Vertical Grounding Body and Horizontal Grounding Grids Area on Grounding Resistance

When the compound grounding coordinated with short vertical grounding body (focuses on horizontal grounding body and the margin is enclosed) is adopted, the grounding grids is 6m × 6m round steel vertical grounding electrode, p= 40Ω·m, h=1.2m, d=0.02m; for different horizontal grounding area, the grounding resistance is shown in Table 4 based on the formula (5).

Tab.4 Effect of vertical grounding body and horizontal grounding body area on grounding resistance

 According to Table 4, when the area of horizontal grounding grids is small, vertical grounding body do play a certain role in reducing the grounding resistance; when the area of horizontal grounding girds is large and the length of vertical grounding body is one-grade smaller than grounding grid equivalent radius,  vertical grounding body plays subtle role in reducing the grounding resistance. Therefore, the area of vertical grounding grids should be expanded as large as possible. Only when grounding resistance of vertical grounding body cannot reach the requirement or the grounding resistance needs to be further reduced, can the vertical grounding body be used.

1.2.2 Effect of Distance between Vertical Grounding Bodies in the Large Horizontal Grounding Grids on the Grounding Resistance   

The horizontal grounding grids is set in a certain area; the grounding grids area is 48m × 48m, and the round steel vertical grounding electrode  is used with l=6m, d=0.02m, p=40 Ω·m, for the distance between different vertical grounding bodies, the grounding resistance is calculated according to formula (5), the result of which is shown in Table 5.  

Tab. 5 Effect of distance between vertical Grounding Bodies on grounding resistance

Based on Table 5, when the distance between vertical grounding bodies is above 2 times length of grounding body, the role in reducing the resistance is great; while the distance between vertical grounding bodies is within 2 times length of grounding body, large quantities of vertical grounding electrodes play little role in reducing the grounding resistance. Because the current will be affected by adjacent grounding electrode flow when the current flows to the ground via grounding electrode. In other words, the current shielding between grounding electrodes will not have a significant impact on resistance reduction no matter how many electrodes are connected to the grounding grids. Therefore, in order to reduce the shielding function of grounding body, the interval of economic and reasonable vertical grounding bodies should not be less than 2 times of its own length.

1.2.3 Effect of Vertical Grounding Body Length on Grounding Resistance

Assume p= 40Ω·m, h=1.2m, d=1.2m; grounding grids area is 48m×48m; grounding grids is 12m×12m; when the vertical body adopts d=0.02m round steel vertical grounding electrode, as for different lengths, the grounding resistance can be calculated according to formula (5) and its result is shown in Table 6.

Tab. 6 Effect of distance between vertical grounding bodies on grounding resistance

From Table 6, when the distance between vertical grounding bodies is 2 times length of grounding body,  the grounding resistance of 6m grid and 3m vertical grounding electrode proposal only reduces by 5.4 % in contrast with 12 grid and 6m vertical grounding electrode; however, the ditch quantity and steel quantity respectively increase 80% and 71.3%; compared with 24 grid and 6m vertical grounding electrode, although the ditch quantity and used steel quantity reduce by 40% and 51.6%, grounding resistance increases by 10.5%. That is consistent with previous analysis. So it is clear that when the area of horizontal grounding grids is large, the length of vertical grounding electrode has little effect on the grounding resistance. Generally, the length of vertical grounding electrode is 2.5-3m, which is most economic.    

Taking skin effect of grounding body into account, deep and long grounding body should be used around the test equipment which needs the current to be diffused quickly. Based on the project condition, adopting non-equal grounding and expanding  equipotential area can break the limit of grounding area and make long grounding body play a good role indiffusing the current, providing shortest and most advantageous channel for partial discharge.  

2. Measures of Some High Voltage Test Hall Grounding  

The high voltage test building covers 4857m². The high voltage test hall is one-story building and is divided into east area and west area. The wast test hall is 27m × 54m × 21m while east test hall is 44m × 54m × 34.8m. The total building area is 3897m² . North of east and west test halls is used as capacitive room and equipment shed; south of east and west test halls is three-layer control room, debugging room and office. According to the geological drilling report, the soil resistivity is 22Ω·m. Owning to many factors such as seasonal coefficient in the design, the soil resistivity p=40Ω·m to conduct the design calculation. The owner requires the grounding resistance R ＜ 0.25Ω.

The interior wall of high voltage test hall, roof steel plate and double plate grid in the floor constitute grounding circuit formed by six-face shielding Faraday Cage; the independent six faces shielding body grounding circuit is composed of interior wall of control room, roof and ground steel. The six- face shielding body and main grounding system are connected with main grounding grid through one fixed grounding well and 11 active grounding wells.

The main grounding system adopts 12m ×12m grid; the number of girds is 16; 2-3 balancing rings with short distance are set around according to actual condition; the outer margin of grounding grids is closed; each corner of the outer margin is arc-shaped; the radius of the arc should be greater than 1/2 of balancing voltage belt distance; the place where people often go and leave is laid with asphalt or two “hat brim shaped” balancing belts connected with grounding grids are set under the ground. A certain amount of 3m length Ø 20mm vertical grounding electrode are set around; 6m length Ø 20mm vertical grounding electrode is set between grounding well of equipment discharge and fixed grounding well of one-point grounding; the distance of vertical grounding electrode is 12m; the section of horizontal grounding body is 50mm × 5mm; the compound of long and short vertical grounding electrode forms pyramid-shaped three-dimension space. The main grounding system adopts copper-clad steel.        

The grounding resistance is 0.22Ω through calculation. That conforms to the requirement of grounding resistance owners.

3. Issues Needed to Pay Attention to in the Design of High Voltage Test Hall Grounding

The grounding of high voltage test hall is greatly different from that of general industrial buildings and power plants. So its distinctiveness needs attention during the design.

(1) The grounding circuit is formed by six-face shielding Faraday Cage, which is composed of test hall interior wall, top steel sheet and double-layer steel grid; the inner wall of the control room, top roof and ground steel comprise independent six-face shielding grounding circuit. The impulse test (transient process) requires one-point grounding.

(2) Main grounding grid of high voltage test hall should adopt compound grounding mode coordinated with vertical grounding body. Expand the area of horizontal grounding grid as large as possible. The number of main grounding grids should be less than 16.

(3) The distance between vertical grounding bodies should be greater than two times of its length. The length of vertical grounding is between 2.5m and 3m. The long and deep vertical grounding body is only applicable to discharge point of the equipment and enlarged potential area in order to reduce the resistance.

(4) The main grounding grid is separated from Faraday Cage by means of insulated materials. Except one-point grounding place, any equipment and fitting have to be connected to one grounding.   

(5) The impulse test (transient process) requires the method of one-point grounding; there is certain distance between discharge point and one-point earthing point. Thereby, the discharge point is led to one-point grounding point via shortest path and enlarge the grounding wire section to reduce the resistance of the grounding wire.

(6) The high voltage test hall has two kinds of discharge tests: low current and high voltage or low voltage and high current. According to technical requirement, the thermal stability should be calibrated for grounding system.

(7) Two grounding terminals at each grounding well have reliable electric connection with main grounding grids and steel plate of Faraday Cage. The two grounding terminals of active grounding well adopt feasible connection knife.  

(8) The buried grounding body should adopt the same material to prevent ionization in the soil from eroding the grounding body because of different materials. To reduce the contact resistance between the surface of grounding body and the soil, copper-clad steel grounding body is recommended.

(9) If the Faraday Cage of high voltage test hall serves as down-lead, the Faraday Cage should be insulated with building foundation. In the rainy weather, test operation is prohibited and all connection knives of active grounding wells are in the “closed” position at the same time, making Faraday Cage in multiple points grounding.     

(10) Measures of reducing step voltage should be taken around the Faraday Cage and entrance; the margin of horizontal grounding ring should close and the corner should have a certain arc.

(11) Except high resistance area, the resistance reducing agent should be used with caution as a main method to reduce the resistance. The effect reduces quickly in the low resistance area. The agent does not play a role in the large grounding grids. The penetration degree of agent is far lower than equivalent radius of grounding grids. The application of agent cannot increase the area of grounding grids.

4. Conclusions

After calculating grounding resistance of high voltage test hall grounding system in the paper, compound grounding mode coordinated with vertical and horizontal grounding is recommended for the grounding of high voltage test hall.   

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