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Power grounding and distribution

2016/03/04

First, grounding

  1. Foreword The grounding grid acts as the grounding and lightning protection grounding of the DC equipment of the substation, which plays an important role in the safe operation of the system. Since the grounding grid is easily overlooked as a hidden project, it is often only necessary to pay attention to the measurement results of the final grounding resistance. As the voltage level of the power system increases and the capacity increases, accidents caused by poor grounding often occur. Therefore, the grounding problem is getting more and more attention. Due to its important position in safety, the substation's ground network has received attention in engineering construction due to its one-time construction and maintenance difficulties. In addition, it is not easy to control during design and construction, which is one of the difficulties in engineering construction. Therefore, in order to ensure the safe operation of the power system, how to reduce the cost of grounding engineering, this paper discusses the related issues in the grounding design of the substation from the design point of view.

2. About grounding resistance

2.1 Grounding resistance "Technical Regulations for Grounding Design of Power Equipment" (SDJ8-79) have specific provisions for grounding resistance values, generally not more than 0.5Ω. In areas with high soil resistivity, when the grounding device is required to achieve the specified grounding resistance, which is extremely unreasonable in terms of technology and economy, the grounding resistance of the large grounding short-circuit current system is allowed to reach 5 Ω, but measures should be taken, such as preventing high-potential external lead. Potential isolation measures, check contact potential, step voltage, etc. According to the regulations, the grounding potential rises no more than 2000V when the ground fault occurs, and the grounding resistance is not more than 0.5Ω and 5Ω. Therefore, it is generally believed that in the 110kV and above substation, the grounding resistance value is less than 0.5Ω, which is considered qualified, and more than 0.5Ω is unqualified, no matter how much the short-circuit current is, no need to take measures. This is unreasonable.

2.1.1 The essence of grounding is to control the earth potential of the fault point when the grounding short circuit occurs in the substation. Because the grounding is mainly for the safety of the equipment and the human body, the potential is not the resistance, and the grounding resistance is the grounding grid. An important parameter of qualification, but not the only one.

2.1.2 As the capacity of the power system continues to increase, the single-phase short-circuit current value is generally large. The short-circuit current in single-phase grounding in an effective grounding system generally exceeds 4kA, while the grounding resistance of most substations in Qinghai is difficult to achieve 0.5Ω. Therefore, from the perspective of safe operation, the contact potential and stride voltage of the ground net should be checked regardless of the circumstances, and isolation measures to prevent high-potential external lead should be taken when necessary.

2.2 Ground Short-Circuit Current Analysis When a ground fault occurs in the system, the generated short-circuit current flows into the system grounded neutral point in three ways.

(1) via overhead ground line-rod system;

(2) After the equipment is grounded to the lower line, the ground network flows into the neutral point of the transformer in the station;

(3) After passing through the earth network, it will flow back to the neutral point of the system through the earth. What is decisive for the grounding resistance of the grounding grid is only the short-circuit current to the ground. Therefore, correctly considering and calculating the short-circuit current values of each part has a great influence on the rational design of the ground network.

2.2.1 Impact of overhead grounding system For the effective grounding system above 110kV substation, the line overhead grounding line is directly connected to the substation internal outgoing line architecture. When a grounding short circuit occurs, a large part of the short-circuit current is shunted through the overhead grounding system. Therefore, in the calculation, the partial shunting action should be considered. When a ground fault occurs, the total short-circuit current is constant, as long as the overhead ground wire is increased. The shunt current can reduce the short-circuit current to the ground. Therefore, reducing the impedance of the overhead ground wire is also an important branch of the safety ground design. The use of good conductors for overhead ground lines and the proper use of overhead ground system shunting will make the design conditions of the ground grid more favorable.

2.2.2 Short-circuit current into the ground From the above analysis, it is known that the short-circuit current to the ground is the total grounding short-circuit current minus the shunting of the overhead ground line, and then the current flowing through the neutral point of the transformer (that is, the zero flowing through the transformer) Sequence current). In this way, the value of the short-circuit current to the ground is relatively small. Since the grounding resistance allows the value R≤2000I, the corresponding allowable value of the grounding resistance is relatively large, and the design is also easy to satisfy. In addition, for a given ground network, the grounding resistance is also basically determined: from R ≈ 0.5 ρ / S, it has a great impact on the actual grounding grid area reduction.

3. About the design problem of the grounding device

3.1 Measurement of soil resistivity The measurement of soil resistivity is an important first-hand data for engineering grounding design. Due to the limitations of measuring equipment and methods, the measurement of soil resistivity is often not accurate enough. Our province is located in the eastern part of the Qinghai-Tibet Plateau. The geological structure is complex. Although the substation occupies a small area, it is mostly uneven geological structure. Now the actual measurement, often only take 3 to 4 measuring points, too simple. It is recommended to improve the measurement accuracy. The design uses the method of calculating the average resistivity provided in the Design Manual to reduce the design error value.

3.2 Grounding grid layout According to the estimation formula of grounding grid grounding resistance: R≈0.5ρ/S where ρ——soil resistivity (Ω•m), S—grounding area (m2)R—grounding grounding resistance (Ω) Once the network area is determined, the grounding resistance is basically constant. Therefore, when designing the ground network layout, the full available area of the substation should be fully utilized. If the ground area does not increase, the grounding resistance is difficult to reduce. .

3.3 Vertical grounding pole function In the 110kV substation, a horizontal grounding wire is mainly used, and a composite grounding grid with a vertical grounding pole is generally used. According to R=0.5ρ/S, the grounding resistance of the grounding grid has little relationship with the vertical grounding pole. Theoretical analysis and experiments have proved that the horizontal grounding grid with an area of 30×30m2—100×100m2 has a vertical grounding pole with a length of 2.5m and 40mm, and its grounding resistance is only reduced by 2.8-8%. However, the vertical grounding electrode has a better effect on the impact current.

Therefore, a vertical grounding pole should be placed at the down conductor of the independent lightning rod, lightning protection line and lightning arrester to strengthen the concentrated grounding and the lightning current. For example, in the grounding design of the 330kV Alan Substation, the design of the grounding grid is composed of a horizontal grounding body. Only a small number of vertical ground poles are placed near the lightning rod and the arrester. The actual operation proves to be better.

3.4 Design of the grounding network equalizing network According to the design regulations, when the total number of equalizing strips including the four grounding wires outside the grounding grid is less than 18, a long-hole grounding net should be used, as shown in Figure 1(a). Shown: (a) n = 8 (b) n = 8 Figure 1 Since the 110kV substation area does not generally exceed 100 × 100m2, considering the shielding effect between the equalizing lines, the total number of equalizing lines is generally 8 ~ About 12, so according to the regulations, generally arranged in a long hole way, but there are several problems.

3.4.1 The vertical and horizontal pressure equalizing belts of the square hole ground are interlaced, so the shunting effect of the ground net is better than that of the long hole ground net. The equalizing effect is better than the long hole ground net and the reliability is high, as shown in Fig. 1(b). Show.

3.4.2 The connection between the equalizing line of the long hole ground network and the main network is weak, and the distance of the equalizing line is long. When a ground fault occurs, the voltage drop along the equalizing line is large, which may cause damage to the secondary control cable and equipment. When a certain pressure equalization line is disconnected, the shunting action of the pressure equalizing belt is obviously reduced, and the pressure equalizing belt of the square hole grounding grid is crisscrossed. When a certain pressure equalizing line is disconnected, the shunting effect on the grounding grid has little effect. . Therefore, it is recommended to use a square hole equalizing network design to improve the grounding safety when designing the grounding grid of the substation.

3.5 Corrosion of the grounding grid

3.5.1 Corrosion of the grounding grid In the design of the substation in the 1980s and before the province, little or no corrosion of the grounding grid was considered. Safety accidents caused by grounding grid corrosion occur frequently. For example, if the grounding down conductor is disconnected and the high-voltage operating equipment is in a non-grounded state, the underground main network corrosion fracture causes the ground net to be divided into several pieces, and the secondary equipment is burned out when grounding occurs. Wait. In addition, since the ground net is a concealed project, it is not easy to inspect and repair after being buried in the ground. Therefore, from the design point of view, the investigation and research on the corrosion of the ground net should be intensified to facilitate the safe operation of the system. From the transformation of 330kV garden, 110kV Xichuan, Gonghe, Hongwan, etc., the corrosion problem of the ground network in Qinghai is serious. The garden has only been running for about 11 years, and the corrosion rate of the grounding wire. Up to 40% or more; Xichuan transformer grounding wire uses 6mm round steel, almost corroded. The design life of a general substation is considered in 25 to 30 years, but the actual safety life of the ground network is only about 10 to 15 years, which is extremely unsuitable for the design period of the substation. In addition, due to the increase in system capacity, the level of short circuit is increased, and the ground network after corrosion cannot meet the requirements for safe operation.

3.5.2 Anticorrosion design of the grounding grid The material of the grounding grid is generally flat steel and round steel. The corrosion state should be calculated according to the local corrosion parameters of the substation. However, in general, its corrosion parameters are difficult to measure. Therefore, when there is no actual data in the engineering design (see Table 1). Table 1 Annual average maximum corrosion rate of grounding wire and grounding body (total thickness) Soil resistivity (Ω.m) Corrosion speed (mm/a) Flat steel round hot dip galvanized flat steel 50~300 0.2~0.1 0.3—0.2 0.065 >300 0.1~0.07 0.2~0.07 0.065 In the calculation, the influence of different corrosion conditions of different laying parts should also be considered. Refer to Table 2 for relevant data. Table 2: Annual corrosion rate of flat steel grounding grid. Horizontal grounding equipment. Horizontal grounding equipment. Lower grounding belt in the cable trench. Annual corrosion rate mm/a (total thickness) 0.1~0.12 0.2~0.3 0.47

For the general substation, the design period of the network should not be less than 30 years, and the network life of the important hub substation should be considered for 50 years. Neither of these cases is greater than the design life specified by the regulations, but is closer to reality. Regarding the selection of grounding materials, flat steel and round steel are generally used. The contact surfaces of flat steel and round steel with the same cross section are inconsistent with the surrounding soil medium. The flat steel is about 50%, but the corrosion mechanism is not complete. Consistently, the corrosion results are basically the same. This has been confirmed from the ground grid corrosion investigation of Shaanxi Power Grid and Qinghai Power Grid, and different corrosion data are also provided in the regulations. Therefore, there is no big difference in whether the grounding material is made of flat steel or round steel. Regarding the design of anti-corrosion, it should generally be considered to use hot-dip galvanized materials within the design period.

3.6 Contact potential and step voltage Contact potential and step voltage are two important parameters for ground network safety design. The new specification states that these two parameters should not exceed the following values:

Ut=174+0.17ρ+tu0=174+0.7ρ+t

Where: Ut - maximum allowable contact potential (V)

Uo──Maximum allowable step voltage (V)

ρ+──Soil surface resistivity (Ω•m)

T──ground short circuit duration (s)

It can be seen from the above that in the new specification, ut and u0 are more demanding and more secure than the requirements of the design procedure, but the design of the ground network is more difficult. For a given substation, the maximum contact potential and the maximum stride voltage generated by the short circuit are also determined. It can be seen from the above equation that increasing the allowable value of ut and u0 by increasing the ρ+ value is also an aspect of rationally designing the ground network. Therefore, ρ+ is a more important data. When the contact potential and step voltage of the substation do not meet the requirements, the equipment area can be used as an insulation operation platform to be a local equalizing network. The road is treated with a high soil resistivity pavement structure such as gravel, gravel or asphalt concrete. It is not advisable to use bricks, bricks and other materials, so the ground construction should be carried out in strict accordance with the design requirements.

3.7 The grounding depth specification and new specifications clearly indicate that the buried depth of the grounding grid should be 0.6m. It is added in the Design Manual that it should be laid below the frozen soil layer in the frozen soil area. The ground net is completely buried below the frozen soil layer.

3.7.1 Influence of grounding depth on maximum contact coefficient The maximum contact potential is an important parameter in the design of the ground network. One of the problems in the design of the ground network is how to reduce the maximum contact potential of the grounding grid. The maximum contact coefficient Kjm of the contact potential of the ground network and the buried depth of the ground network have a relationship as shown in FIG. 2. It can be seen from Fig. 2 that when the buried depth of the grounding grid increases from zero, the contact coefficient is reduced, but after the buried depth exceeds a certain range, Kjm starts to increase again. This is because the relationship between the maximum contact coefficient Kjm and the buried depth h of the grounding grid 2 (grounding area A=40×40 m2, grounding body diameter d=0.01 m, number of meshes n=400) is different. The change in the electric field strength generated on the ground at the center of the mesh determines that the potential difference generated between the ground center of the mesh and the ground net is different. When the buried depth increases to a certain depth, the current tends to flow deep in the formation, and the current density on the ground becomes smaller and smaller, so the potential difference between the ground center of the mesh and the ground net begins to increase again. Therefore, the regulations stipulate The laying depth is reasonable.

3.7.2 Influence of laying depth on grounding resistance The substations currently encountered are generally in seasonally frozen areas. If the ground net is laid at a depth of 0.6 m according to the regulations, the ground net will be in the frozen soil layer in winter. Since the resistivity of the soil will increase by more than three times after the soil freezes, it will have a certain influence on the grounding resistance of the grounding grid. At present, the grounding grid used is a composite grounding grid with a horizontal grounding wire as the main edge and a vertical grounding pole. The vertical vertical grounding in the winter extends to the lower non-frozen soil. At this time, the soil structure can be equivalent to two soil structures with different resistivities. Studies have shown that for vertical electrodes in a two-layer soil medium, the diffuse density of each part is inversely proportional to the resistivity of the surrounding medium, except at the tip of the electrode, which has ρiJi = constant (where Ji is at the resistivity ρi The diffuse density of the electrode portion in the soil). At this time, when a part of the electrode enters the lower layer soil, the stray resistance of the entire electrode will mainly depend on the underlying soil. At this time, the grounding resistance of the ground net will also depend mainly on the non-frozen soil of the ground net. Therefore, in seasonal frozen soil areas, the use of such a composite grounding grid with vertical grounding poles has great advantages. If the soil is not frozen in winter, it has no significant influence on the grounding resistance. It is not necessary to bury the ground net below the frozen soil. It is certainly advantageous to bury the ground net below the frozen soil layer for the grounding resistance of the ground network. If the depth of the frozen soil is 2m, if the depth of the frozen soil is 2m, such as Dawu Substation, the maximum frozen soil depth is 2.4m, simply laying on the ground net will greatly increase the amount of earthwork excavated by the project. Increased, construction is difficult. The project cost has also increased. The regulations also stipulate that the grounding resistance should meet the requirements of changing seasons throughout the year, which is difficult to achieve in actual engineering. The freezing of soil in winter definitely has an impact on the grounding resistance, but it can be comprehensively compared through various factors of safety requirements. , reasonable control. Therefore, the buried depth of the ground net should be reasonably determined in the engineering design.

3.8 About the use of resistance-reducing agents In recent years, the resistance-reducing agent has been widely used in power system grounding engineering. The main function of the grounding device is to spread the grounding fault current, and the main function is the ground-scattering property. It is not the local soil resistivity of the ground contact. And reduce the main body of the resist


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