Optimal construction method and demonstration application of multi-in-one station grounding system

0 Introduction

Multi-in-one substations with multiple functions will constitute a new mode of substation development in the future and an important direction for new power-system development.Mining substation space,power and optical fiber,and other resources can be leveraged to transform a substation from single power transmission to multi-service integration[1].Multi-in-one substations focus on the construction and optimization of business[2-4],and AC and DC power integration[5].

At present,the research on grounding systems,both domestic and abroad,mainly aim at substation grounding grids with different voltage levels[6-9].Relevant achievements have been made in safety assessment[10-13],design optimization[14-15],lightning protection[16-17],and fault diagnosis[18-21].Grounding systems such as substations,energy storage stations,and data centers have their own models and operating mechanisms,and there is no relevant grounding standard for multi-in-one substations at present.The influence of strong current on weak current caused by multi-in-one substations shared by grounding networks has not been analyzed[22].The influence of ground potential difference(GPD)between adjacent grounding networks on many secondary devices in multiin-one substations has not been studied either[23-24].Moreover,the design and operation experience concerning multi-in-one substations is insufficient.The existing research on power-frequency short-circuits[25]and powerfrequency magnetic-fields[26]of the integrated grounding grid does not consider the impact of lightning strikes.The research on the locations of data centers[27]only considers the function and architecture,omitting safety issues caused by substation short-circuits.In reference[28],the authors used CDEGS software to model the charging station,and simulated the ground potential rise(GPR)when different lightning strike positions were struck by lightning.In reference[29],the authors built a model of fused grounding grid based on CDEGS software,and evaluated the safety of fused grounding system.Therefore,based on CDEGS software,this paper studys the construction scheme of grounding system integration in multi-in-one substations to ensure their safe and stable operation.

In this study,Current Distribution,Electromagnetic Fields,Grounding and Soil Structure Analysis(CDEGS)software was used to evaluate the safety of grounding-grid plane layouts from the point of view of power-frequency short-circuit,and the safety of data centers and substation grounding-grid longitudinal layouts is analyzed in terms of power-frequency short-circuits and lightning strikes.Reasonable optimization methods are described,and the rationality of the two construction schemes was verified.

1 Description of multi-in-one substations

1.1 Introduction of grounding-grid simulation

CDEGS is a powerful integrated engineering simulation software[28].In this study,the MALT(grounding timedomain calculation and analysis)module was used to simulate and calculate the safety parameters of a grounding grid,and the FFTSES(Fourier transform)and HIFREQ(high-frequency analysis)modules were used to analyze the lightning-current impact characteristics.

Given that the shape of grounding grids has no substantial influence on the problems addressed in this study,we assumed that all grounding grids are rectangular,their scope overlaps that of the building for each station,and the grounding resistance of each grounding grid meets their own protection requirements.Therefore,we only studied the changes of ground potential rise,touch voltage,step voltage,lightning protection,etc.caused by the integration and construction of each grounding grid.

1.2 Safety index of grounding-grid construction

(1)GPR

According to article 4.2.1 of GB/T 50065-2011 Grounding design specification for AC electrical devices,in effective and low-resistance grounding systems,the GPR is 2 kV when R ≤ 2000/I.In accordance with the provisions of regulation 4.3.3,the GPR can be improved to 5 kV.When necessary,the potential rise of grounding grids can be further improved through special calculations and measures taken to ensure the safety and reliability of people and equipment[29].

(2)Touch voltage and step voltage

According to the calculation formula provided in GB/T 50065-2011 Grounding design specification for AC electrical devices,the touch voltage safety threshold of a grounding grid is:

The safety threshold of a step voltage is:

where ρsis the resistivity of the surface layer;Csis the surface attenuation coefficient;and tsis the grounding fault current duration.Usand Utunits are V.

1.3 Tolerance and protection measures of secondary equipment in multi-in-one substations

Secondary equipment in multi-in-one substations mainly includes a power conversion system(PCS)energy storage converter;a battery system as energy storage station;an energy management system;communication equipment in the energy storage station;a server host and an uninterruptible power supply(UPS)in the data center;a relay;and a microcomputer line protection device.The withstand voltage and corresponding protective measures of secondary equipment are shown in Table 1.

Table 1 Withstand voltage and protective measures of secondary equipment

Secondary equipmentWithstand voltage Protective measure Energy storage station Energy storage converterDC 500-850 V No special protection measures.Battery management system DC 1000 V When the withstand voltage is exceeded,it will automatically disconnect from the load and stop the output.Communication equipment DC 220 V allowable variation:-20%-+15%No special protection measures.Data center Server hostAC 460 V The host will automatically shut down when the time exceeds the limit.UPSAC 110-300 V The UPS will stop outputting when it exceeds the time limit.Transformer substation Protector Strong current loop:5 kV Weak current loop:1 kVWithout special protection measures,if the allowable value and time are exceeded,the protection device or relay will be damaged.AC current loop 2 IN,continuous work 10 IN,allow 10 s 40 IN,allow 1 s AC voltage loop 1.2 UN,continuous work 1.5 UN,allow 30 s DC power supply circuit 80%-115% UN,continuous work

According to Table 1,the secondary equipment is weak in withstanding voltage and sensitive to the GPR.Thus,the communication equipment and the data-center UPS should be given priority.The communication equipment has no special protective measures and can withstand DC 220 V voltage,but to preclude affection on the normal operation of the data-center UPS,the GPR should be limited below AC 110 V voltage.Therefore,to ensure the normal operation of the secondary equipment,it is necessary to consider the GPR of the grounding grid.

2 Plane-layout construction scheme of multiin-one substations

2.1 Introduction of construction scheme of grounding-grid plane layout

When it is necessary to build a substation,data center,and energy storage station independently,or when the scale of each station is too large to be built in one building,it is necessary to arrange the stations in different buildings,that is,each grounding grid adopts a plane layout.The flow of the grounding-grid plane-layout construction scheme proposed in this study is shown in Fig.1.

Fig.1 Construction process of plane layout scheme

2.2 Simulation verification

We analyzed,according to the construction process shown in Fig.1,the actual construction case of transforming a 35 kV substation in Shanghai into a multi-in-one substations.

(1)Parameter acquisition

The parameters of this multi-in-one substations are shown in Fig.2.The site is a uniform single-layer soil with a resistivity of 50 Ω·m.The ground surface is covered with 15-cm-thick gravel with a resistivity of 3000 Ω·m.The grounding-grid conductors are 40 mm × 6 mm hot dip galvanized flat steel,and the buried depth is 0.8 m.

Fig.2 Plane distribution of multi-in-one substations

(2)Short-circuit current calculation

According to the data provided by the system,the short-circuit current of the 35-kV bus in this station is approximately 9.2 kA according to Appendix B in GB/T50065-2011 Grounding design specification for AC electrical devices.The effective value of the maximum asymmetric current of grounding fault entering the ground through the grounding grid can be calculated according to the following expression:

where Sfis the fault current shunt coefficient;Dfis the attenuation coefficient;and Ifis the effective value of ground fault current.According to the relevant parameters,the shunt coefficient is calculated to be 0.4,the attenuation coefficient is approximately 1.08,and the short-circuit grounding current of the substation is approximately 4 kA at most.

(3)Determine the appropriate grounding-grid connection mode

To make the fault current effectively leak into the earth,three forms of main grounding grids are designed for multiin-one substations:independent grounding grids for each part,independent grounding grids interconnected through conductors,and integrated grounding grid.

In the plane layout,a lightning rod or roof lightning strip needs to be newly built in multi-in-one substations to provide sufficient protection capability for the data center,energy storage station,etc.It can be assumed that the lightning protection of multi-in-one substations can meet the safety requirements.Therefore,we only study the safety in multi-in-one substations when power-frequency shortcircuits occur in substations.

The grounding-grid model is provided in the SESCAD tool of the CDEGS software.A plurality of observation lines distributed parallel to the X axis at equal intervals are generated to cover the whole area.The grounding grid and observation line models are shown in Fig.3.

Fig.3 Grounding grid and observation line model

Input conductor and soil parameters are introduced into the grounding-grid model established in SESCAD,and a 4-kA power-frequency current is set to be injected into the substation grounding grid.The power-frequency current will leak into the ground through the grounding-grid conductors,inducing potential on the ground surface.The model established in SESCAD is submitted to the MALT module for running,and the output results can be viewed in the GRServer toolbox after execution.

Fig.4 shows the distribution of GPR for three construction modes in the simulated multi-in-one substations.

Fig.4 Ground potential distribution in multi-in-one substations.

The observation lines are straight lines set on the ground plane,and the GPR above can be calculated by the same aforementioned method.The distribution of GPR on the observation lines is shown in Fig.5.

Fig.5 GPR distributed along observation lines

Note from Fig.4 that the voltage sharing effect of the internal conductor of each grounding grid is notable,and the distribution of GPR is uniform.Only in the area between the grounding grids,the GPR is large and decreasing.

Actually,it is not the absolute GPR that affects the safety of the whole field,but the GPD between the grounding grids.The change in ground potential in the area(40 m ≤ x ≤ 50 m)between the grounding grids can be seen more intuitively in Fig.5(a),where the maximum GPD when the grounding grids are independent is approximately 1000 V;Fig.5(b)shows that when the grounding grid is connected by a single row of conductors,the maximum GPD is approximately 400 V;the maximum GPD of the whole grounding grid in Fig.5(c)fluctuates within a range of 100 V.Therefore,when the grounding grid is arranged in plane layout,the whole grounding grid can achieve better voltage sharing effect and meet the safety requirements of many secondary devices in data centers and energy storage stations.

(4)Analysis of touch voltage and step voltage

After determining the connection mode of the planelayout grounding grid,it is necessary to calculate whether the touch voltage and step voltage are out of their corresponding limits.After transferring the model established in SESCAD to the MALT module for operation,we analyzed the 2D color block diagram of the touch voltage and step voltage in the output interface.The simulation results are shown in Fig.6.

Fig.6 Distribution of touch voltage and step voltage

Note from Fig.6 that the maximum touch voltage and step voltage of the whole grounding grid are 820.15 V and 206.23 V,respectively.According to Eqs.(1)and(2),the touch-voltage and step-voltage safety limits of the whole grounding grid are calculated to be 896.91 V and 3010.74 V,respectively.Therefore,the touch voltage and step voltage of this arrangement can meet the safety requirements for personnel.

To make the construction scheme of the groundingsystem plane layout of multi-in-one substations more universal and provide reference for future construction,it is necessary to consider that when the GPR,touch voltage,and step voltage do not meet the safety requirements,a reasonable optimization method should be adopted.To reduce the excessive GPD caused by the GPR between grounding grids,the method of increasing the number of connecting conductors between grounding grids can be adopted.When a whole large ground grid is finally formed with the increase in the number of connecting conductors,the GPR in the whole area should tend to be consistent.According to Fig.6,the maximum values of touch voltage and step voltage are reached at the corners and edges of the grounding grid because the potential drops at the highest rate there.Adding vertical grounding rods around the grounding grid can reduce the influence of touch voltage and step voltage.The specific adding method and improvement effect will be introduced in following studies.

To sum up,if this multi-in-one substations adopts the plane-layout scheme of the grounding grid,it is necessary to build an overall grounding grid to meet the safety requirements of secondary equipment in the station.

3 Longitudinal-layout construction scheme of multi-in-one substations

When the scale of the data center and energy storage station is smaller than that of the substation,we suggest a longitudinal layout for the grounding grid,that is,the data center and energy storage station are arranged on the upper floor of the substation,their equipotential equalizing network is laid also on the upper floor,and the grounding grid is shared with the substation through the down-lead line.Given that secondary equipment in the data center and the energy storage station require higher GPD between grounding grids,the voltage of their equipotential equalizing networks is consistent after adopting the proposed longitudinal arrangement.Thus,the arrangement does not need to consider the GPD between adjacent grounding networks.However,in this scheme,it is necessary to analyze not only whether the touch voltage and step voltage are within the safety limits,but also the protection capability of the lightning protection belt on the roof of the original substation.The longitudinal layout diagram of a multiin-one substations in the construction scheme is shown in Fig.7.The parameters needed for the simulation were introduced in Section 2.2.

Fig.7 Longitudinal layout diagram of the multi-in-one substations

3.1 Analysis of touch voltage and step voltage

The corresponding parameters of the grounding-grid model established in SESCAD were introduced.Then,the model was transferred to the MALT module for operation,resulting in the 2D color block diagram of the touch voltage.Fig.8 shows the distribution of touch voltage and step voltage when the substation is short-circuited by simulation calculation when the grounding grid is longitudinally arranged.

Fig.8 Distribution of touch voltage and step voltage

Note from Fig.8 that the maxima of the touch voltage and step voltage caused by a substation short-circuit when the grounding grid is arranged longitudinally are 1028.61 V and 257.10 V,respectively.Compared with the plane layout,the touch voltage and step voltage of this layout are increased;in particular,the touch voltage exceeds the safety limit of 896.91 V.Therefore,the construction scheme based on sharing the grounding grid when this is longitudinally arranged cannot meet the safety requirements of the touch voltage.This means that measures are required to optimize the structure of the integrated grounding grid and improve the safety of the construction scheme.Common measures to improve the safety of grounding grids are increasing the vertical grounding rods and setting the optimal compression ratio.

· Conventional horizontal grounding grid and vertical grounding rods are usually used to form a composite grounding grid,that is,a certain number of vertical grounding rods are connected to the edge and corner of the horizontal grounding grid.Vertical grounding bars with different numbers and sizes were set in the longitudinal-layout grounding-grid model established in SESCAD.The touch voltage was calculated several times,and the output results for each calculation were counted in the MALT module.Fig.9 shows the relationship between the number of grounding rods uniformly distributed at the edge of the grounding grid and the maximum touch voltage when the diameter and length of the vertical grounding rods are changed.

Fig.9 Variation of maximum touch voltage

According to the variation curve in Fig.9,the longer the vertical grounding rods with the same diameter is,the better the reduction voltage effect is.The longer the diameter of vertical grounding rods with the same length is,the better the reduction voltage effect is.However,the effect of reducing the maximum touch voltage by increasing the diameter is not as evident as by increasing the length.Therefore,the touch voltage and step voltage in substation short-circuits can be reduced to a value below the safety limit by selecting a suitable type and a certain number of vertical grounding rods.

· Setting the optimal compression ratio implies improving the voltage sharing effect by optimizing the conductor distribution of the horizontal grounding grid.In the design of grounding grids,horizontal grounding grids are usually arranged at equal intervals.Because of the mutual inductance between conductors,the mutual impedance causes uneven leakage for each part of the grounding grid,and the leakage density of conductors near the center of the grounding grid is much smaller than that at the edge[30].Therefore,it is necessary to arrange more conductors at the edge of the grounding grid.Fig.10 is the conductor layout of the horizontal grounding grid.

Fig.10 Distribution of conductors in grounding grids

Assuming that the conductor spacing of the central mesh is dmax,the conductor spacing of the mesh with n-level distance from the central mesh is:

where Cis a compression ratio - a constant not greater than 1 indicating the uniformity of conductor spacing in the grounding grid.The larger Cis,the more uniform the conductor spacing is.When C= 1,equal spacing is obtained.The touch voltage under different compression ratios was calculated using the CDEGS software.Table 2 shows the calculation results.

Table 2 Maximum touch voltage at different compression ratios

Compression ratioMaximum touch voltage/V 0.4998.69 0.5986.30 0.6985.04 0.7991.93 0.81002.53 0.91014.91 1 1028.61

Note from Table 2 that the maximum touch voltage reaches a minimum value of 985.04 V when the compression ratio is 0.6,which is the optimal compression ratio of horizontal grounding grids.

To sum up,before the construction of the grounding grid in a multi-in-one substations,the conductor spacing should be arranged according to the optimal compression ratio,and if the safety requirements are still not met,the optimization should be carried out by adding vertical grounding rods.

3.2 Field model simulation verification

Concerning the real case of transforming a 35-kV substation in Shanghai into a multi-in-one substations,owing to the limited indoor building space of the original substation,and considering the need to protect some secondary equipment,the actual construction scheme of the station results from arranging the energy storage PCS and communication room and data center in the upper layer of the substation(2.6 m above the ground).These components are connected with the main grounding grid at a single point through the down-lead line.Other equipment,such as the box transformer in the energy storage station,is arranged outdoors and connected with the main grounding grid through two conductors.This construction scheme is consistent with the longitudinal layout scheme proposed in this paper.Equipment in the energy storage station is arranged outdoors.Thus,it is necessary to verify through the plane-layout construction scheme that the GPD cannot exceed the safety value of other secondary equipment.Considering the feasibility and economy of construction,and leaving a certain safety margin for subsequent construction,24 copper-clad-steel grounding rods of 14.2 mm × 2.5 m type were evenly arranged around the grounding grids of the substation and energy storage station.The simulation model based on CDEGS software according to the real on-site situation is shown in Fig.11.

Fig.11 Longitudinal layout simulation model

First,the relevant parameters in the SESCAD module of CDEGS software are entered.Then,4-kA powerfrequency current is injected into the main grounding grid.The observation surface covering the whole grounding grid is generated on the ground surface and transferred to the MALT module for operation.According to the GPR value displayed in the operation output report from simulation,we found that the GPD between the outdoor grounding grid of the energy storage station and the main grounding grid of the substation when the power frequency of the substation is short-circuited does not exceed 300 V.Given that the energy storage communication room was arranged on the upper layer of the substation,the GPD can meet the safe operation of other equipment.The maximum touch voltage and step voltage were calculated to be 803.56 V and 201.28 V,respectively.These values can also meet the safety requirements.

3.3 Safety analysis of lightning strike

The original substation was built indoors,and the roof lightning protection belt was used for lightning protection.Subsequently,the new data center,PCS of energy storage station,and communication room were added indoors.It was necessary to verify the impact of lightning strikes on the newly added equipment.To verify the influence of substation lightning protection on the data center,indoor energy storage PCS,and communication room,the lightning protection model shown in Fig.11 was built in the SESCAD module.When lightning strikes the lightning strip on the roof,lightning current is injected into the ground along the down-lead line,which will inevitably cause the ground potential of the grounding conductor to rise.

The typical double-exponential lightning current is used to excite the roof lightning belt.It is expressed as follows:

where Im=30 kA,α=1.4 × 104 s-1,β=6 × 106 s-1.

As shown in Fig.11,five typical lightning strike points were selected in the roof lightning belt.After the lightning strike,the lightning current is injected into the grounding grid through the down-lead lines.A total of 40 conductor segments in the data-center equipotential voltage sharing grid are shown in Fig.12,and the maximum GPD of each conductor segment is calculated,respectively,as shown in Fig.13.

Fig.12 Number of conductor segments in the data center

Fig.13 Maximum GPR of conductor segments

According to the calculation results of GPR in the conductor section in Fig.13,the instantaneous overvoltage of the voltage sharing grid in the data center can reach more than 160 kV under lightning impulse.This will have a massive impact on the equipment.Therefore,protective measures must be taken to reduce this impact.The main lightning protection measures of the equipment room,UPS room,and communication room in the data center are equipotential connection and overvoltage protection equipment.Equal potential connection can play a significant role in voltage sharing,but it is difficult to reduce the amplitude of the surge current.Therefore,to protect the safety of the equipment,a surge protective device(SPD)is usually employed to limit the overvoltage within the range that the equipment can withstand,injecting the surge current into the ground.

It can be seen from Fig.13 that the location of the lightning strike point has a significant impact on the GPR of the equipotential voltage sharing grid conductors.The farther the lightning strike point is from the down-lead lines,the higher the ground potential of conductor segments caused by lightning current is.When the lightning strike point is on the down-lead line,the closer the down-lead line is to the voltage sharing grid of the data center,the higher the ground potential of the conductor segments caused by the lightning current.Therefore,when lightning protection is built in a multi-in-one substations,the down-lead line is set to connect to the main grounding grid in the lightningintensive area of the roof lightning belt,and the voltagesharing grid is arranged in a position far from the down-lead line to reduce the impact of lightning current.

4 Field test verification

The actual site layout of the multi-in-one substations transformed from the 35-kV substation adopts the plane and longitudinal layouts of the grounding grid proposed in this study.After simulation verification and application of the corresponding lightning protection methods,co-location operation of the substation,data center,and energy storage station can be achieved.According to GB/T 50065-2011 Grounding design specification for AC electrical devices,a grounding resistance R ≤ 2000/I,combined with a fault current of the station of 4 kA,implies that the grounding resistance of the multi-in-one substations should not exceed 0.5 Ω.The GPR,touch voltage,and step voltage were verified to meet the safety requirements through simulation of the actual parameters of the multi-in-one substations.Then,the actual grounding resistance was measured by an automatic grounding resistance tester.The results are shown in Table 3.

Table 3 Ground-resistance test results

Test equipment:LEM automatic grounding resistance tester Test positionResistance/Ω Box-type transformer 1#0.313 Box-type transformer 2#0.309 UPS battery warehouse 1#0.298 UPS battery warehouse 2#0.314 UPS battery warehouse 3#0.325 Energy storage PCS room0.318 Data-center equipment room0.296 Relay protection communication room0.303 Climate:sunnyTemperature:25 °CHumidity:65%

It can be seen from Table 3 that the grounding resistance tested at different positions in the multi-in-one substations meets the requirements,confirming the effectiveness of the proposed grounding-grid construction scheme.

5 Conclusions

In this study,the construction scheme of grounding systems in multi-in-one substations was studied.The conclusions can be summarized as follows:

· When it is necessary to build a substation,a data center,and an energy storage station independently,or when the scale of each station is too large to be built in a single building,we propose to implement a plane layout of the grounding network.The rationality of the proposed construction process was confirmed on a 35-kV multi-in-one substations in Shanghai.To meet the safety requirements of secondary equipment in the station,the multi-in-one substations requires an overall grounding network.

· When the scale of the data center and energy storage station is smaller than that of the substation,we propose the implementation of a longitudinal layout scheme of the grounding grid,thereby greatly reducing the influence of power-frequency short-circuits on upper equipment.The effectiveness of this construction scheme was verified by software simulation and field tests.

· In the future construction of multi-in-one substations,if the longitudinal layout scheme is feasible,it is preferred,and the GPR,touch voltage,and step voltage caused by power-frequency short-circuits can be improved by adding vertical grounding rods.Setting the optimal compression ratio for the grounding grid can reduce the number of vertical grounding rods and the construction cost.

Acknowledgements

This work was supported by National Natural Science Foundation of China(No.92067105)and the science and technology project of State Grid Shanghai Municipal Electric Power Company(No.5209211900VD).

Declaration of Competing Interest

We declare that we have no conflict of interest.