GROUNDING GRID DESIGN CRITICAL PARAMETERS BASIC INFORMATION

The following site-dependent parameters have been found to have substantial impact on the grid design: maximum grid current IG, fault duration tf, shock duration ts, soil resistivity ρ, surface material resistivity (ρs), and grid geometry.

Several parameters define the geometry of the grid, but the area of the grounding system, the conductor spacing, and the depth of the ground grid have the most impact on the mesh voltage, while parameters such as the conductor diameter and the thickness of the surfacing material have less impact.

Fault duration (tf) and shock duration (ts)

The fault duration and shock duration are normally assumed equal, unless the fault duration is the sum of successive shocks, such as from reclosures. The selection of tf should reflect fast clearing time for transmission substations and slow clearing times for distribution and industrial substations.

The choices tf and ts should result in the most pessimistic combination of fault current decrement factor and allowable body current. Typical values for tf and ts range from 0.25 s to 1.0 s.

Soil resistivity (ρ)

The grid resistance and the voltage gradients within a substation are directly dependent on the soil resistivity. Because in reality soil resistivity will vary horizontally as well as vertically, sufficient data must be gathered for a substation yard.

Because the equations for Em and Es given assume uniform soil resistivity, the equations can employ only a single value for the resistivity.

Resistivity of surface layer (ρs)

A layer of surface material helps in limiting the body current by adding resistance to the equivalent body resistance.

Grid geometry

In general, the limitation on the physical parameters of a ground grid are based on economics and the physical limitations of the installation of the grid. The economic limitation is obvious. It is impractical to install a copper plate grounding system.

Clause 18 describes some of the limitations encountered in the installation of a grid. For example, the digging of the trenches into which the conductor material is laid limits the conductor spacing to approximately 2 m or more.

Typical conductor spacings range from 3 m to 15 m, while typical grid depths range from 0.5 m to 1.5 m. For the typical conductors ranging from 2/0 AWG (67 mm2) to 500 kcmil (253 mm2), the conductor diameter has negligible effect on the mesh voltage.

The area of the grounding system is the single most important geometrical factor in determining the resistance of the grid. The larger the area grounded, the lower the grid resistance and, thus, the lower the GPR.

WORST FAULT LOCATION BASIC INFORMATION (GROUND POTENTIAL RISE CONSIDERATION

A complex power station may have a large number of rights-of-way (ROW) with multi circuit power lines on each ROW. These circuits may be operated at different voltage levels. A fault current study for an L-G fault at each transformer voltage level should be produced. Each fault current study should be examined as follows:

a) If the vectorial sum of all zero-sequence fault current contributions to the transformer bus fault from all transmission and distribution lines entering the station under study is greater than the sum of all current contributions from all grounded sources at that station (including generators, grounded transformers, shunt capacitors, etc.), then at the voltage level for which the fault current study is presently being examined, the bus fault will usually produce a worse GPR than the line fault.

b) If the reverse is true, that is, the vectorial sum of the line contributions is smaller than the local ground source current sum, the line fault will produce a greater GPR.

This is because the local ground current will return partially, in the case of the line fault, through the station ground impedance, adding to the GPR caused previously by the line current contribution.

In the bus fault case, the current merely circulates through the faulted transformer winding, the station ground bus, and the fault impedance.

Having determined the worst-fault location (bus versus out on the-line), to select that fault current study with the highest fault current is not appropriate.

Variances between grounding networks of lines with the various voltage levels may, for instance, cause the study showing lower zero-sequence fault currents to result in a GPR greater than that caused by the higher currents. Instead, all faults should be investigated for fault locations as determined above.

GROUNDING GRID DESIGN IN DIFFICULT CONDITIONS BASIC INFORMATION AND TUTORIALS

In areas where the soil resistivity is rather high or the substation space is at a premium, it may not be possible to obtain a low impedance grounding system by spreading the grid electrodes over a large area, as is done in more favorable conditions.

Such a situation is typical of many GIS installations and industrial substations, occupying only a fraction of the land area normally used for conventional equipment. This often makes the control of surface gradients difficult.

Some of the solutions include

a) Connection(s) of remote ground grid(s) and adjacent grounding facilities, a combined system utilizing separate installations in buildings, underground vaults, etc. A predominant use of remote ground electrodes requires careful consideration of transferred potentials, surge arrester locations, and other critical points.

A significant voltage drop may develop between the local and remote grounding facilities, especially for high-frequency surges (lightning).

b) Use of deep-driven ground rods and drilled ground wells.

c) Various additives and soil treatments used in conjunction with ground rods and interconnecting conductors.

d) Use of wire mats. It is feasible to combine both a surface material and fabricated mats made of wire mesh to equalize the gradient field near the surface.

A typical wire mat might consist of copper-clad steel wires of No. 6 AWG, arranged in a 0.6 m × 0.6 m (24 in × 24 in) grid pattern, installed on the earth’s surface and below the surface material, and bonded to the main grounding grid at multiple locations.

e) Where feasible, controlled use of other available means to lower the overall resistance of a ground system, such as connecting static wires and neutrals to the ground. Typical is the use of metallic objects on the site that qualify for and can serve as auxiliary ground electrodes, or as ground ties to other systems. Consequences of such applications, of course, have to be carefully evaluated.

f) Wherever practical, a nearby deposit of low resistivity material of sufficient volume can be used to install an extra (satellite) grid. This satellite grid, when sufficiently connected to the main grid, will lower the overall resistance and, thus, the ground potential rise of the grounding grid.

The nearby low resistivity material may be a clay deposit or it may be a part of some large structure, such as the concrete mass of a hydroelectric dam.

BASIC ASPECT OF SUBSTATION GRID DESIGN BASIC INFORMATION AND TUTORIALS

Conceptual analysis of a grid system usually starts with inspection of the substation layout plan, showing all major equipment and structures. To establish the basic ideas and concepts, the following points may serve as guidelines for starting a typical grounding grid design:

a) A continuous conductor loop should surround the perimeter to enclose as much area as practical. This measure helps to avoid high current concentration and, hence, high gradients both in the grid area and near the projecting cable ends. Enclosing more area also reduces the resistance of the grounding grid.

b) Within the loop, conductors are typically laid in parallel lines and, where practical, along the structures or rows of equipment to provide for short ground connections.

c) A typical grid system for a substation may include 4/0 bare copper conductors buried 0.3–0.5 m (12–18 in) below grade, spaced 3–7 m (10–20 ft) apart, in a grid pattern. At cross-connections, the conductors would be securely bonded together.

Ground rods may be at the grid corners and at junction points along the perimeter. Ground rods may also be installed at major equipment, especially near surge arresters. In multilayer or high resistivity soils, it might be useful to use longer rods or rods installed at additional junction points.

d) This grid system would be extended over the entire substation switchyard and often beyond the fence line. Multiple ground leads or larger sized conductors would be used where high concentrations of current may occur, such as at a neutral-to-ground connection of generators, capacitor banks, or transformers.

e) The ratio of the sides of the grid meshes usually is from 1:1 to 1:3, unless a precise (computer-aided) analysis warrants more extreme values. Frequent cross-connections have a relatively small effect on lowering the resistance of a grid.

Their primary role is to assure adequate control of the surface potentials. The cross-connections are also useful in securing multiple paths for the fault current, minimizing the voltage drop in the grid itself, and providing a certain measure of redundancy in the case of a conductor failure.

RESISTANCE OF HUMAN BODY BASIC INFORMATION AND TUTORIALS

For dc and 50 Hz or 60 Hz ac currents, the human body can be approximated by a resistance. The current path typically considered is from one hand to both feet, or from one foot to the other one.

The internal resistance of the body is approximately 300 Ω, whereas values of body resistance including skin range from 500 Ω to 3000 Ω, as suggested in Daziel, Geddes and Baker , Gieiges, Kiselev [B94], and Osypka [B118].

The human body resistance is decreased by damage or puncture of the skin at the point of contact. Conducted extensive tests using saltwater to wet hands and feet to determine safe let-go currents, with hands and feet wet.

Values obtained using 60 Hz for men were as follows: the current was 9.0 mA; corresponding voltages were 21.0 V for hand-to-hand and 10.2 V for hand-to-feet.

Hence, the ac resistance for a hand-to-hand contact is equal to 21.0/0.009 or 2330 Ω, and the hand-to feet resistance equals 10.2/0.009 or 1130 Ω, based on this experiment.

Thus, for the purposes of this guide, the following resistances, in series with the body resistance, are assumed as follows:

a) Hand and foot contact resistances are equal to zero.

b) Glove and shoe resistances are equal to zero.

A value of 1000 Ω in Equation (10), which represents the resistance of a human body from hand-to-feet and also from hand-to-hand, or from one foot to the other foot, will be used throughout this guide. RB = 1000 Ω

BONDING, SHIELDING, AND GROUNDING RELATIONSHIP BASIC INFORMATION AND TUTORIALS

a. The simple grounding of elements of a communications facility is only one of several measures necessary to achieve a desired level of protection and electrical noise suppression. To provide a low impedance path for

(1) the flow of ac electrical current to/from the equipment and

(2) the achievement of an effective grounding system, various conductors, electrodes, equipment, and other metallic objects must be joined or bonded together.

Each of these bonds should be made so that the mechanical and electrical properties of the path are determined by the connected members and not by the interconnection junction. Further, the joint must maintain its properties over an extended period of time, to prevent progressive degradation of the degree of performance initially established by the interconnection.

Bonding is concerned with those techniques and procedures necessary to achieve a mechanically strong, low-impedance interconnection between metal objects and to prevent the path thus established from subsequent deterioration through corrosion or mechanical looseness.

b. The ability of an electrical shield to drain off induced electrical charges and to carry sufficient out of-phase current to cancel the effects of an interfering field is dependent upon the shielding material and the manner in which it is installed.

Shielding of sensitive electrical circuits is an essential protective measure to obtain reliable operation in a cluttered electromagnetic environment. Solid, mesh, foil, or stranded coverings of lead, aluminum, copper, iron, and other metals are used in communications facilities, equipment, and conductors to obtain shielding.

These shields are not fully effective unless proper bonding and grounding techniques are employed during installation. Shielding effectiveness of an equipment or subassembly enclosure depends upon such considerations as the frequency of the interfering signal, the characteristics of the shielding material, and the number and shapes of irregularities (openings) in the shield.

Interference-causing signals are associated with time-varying, repetitive electromagnetic fields and are directly related to rates of change of currents with time. A current-changing source generates either periodic signals, impulse signals, or a signal that varies randomly with time.

To cause interference, a potentially interfering signal must be transferred from the point of generation to the location of the susceptible device. The transfer of noise may occur over one or several paths. There are several modes of signal transfer (i.e., radiation, conduction, and inductive and capacitive.

MOBILE SUBSTATION ITEMS THAT NEEDS TO BE GROUNDED BASIC INFORMATION AND TUTORIALS

What are the substation items that needs grounding?

Fence

The fences may be either connected to the mobile substation grid or isolated. A dangerous voltage may exist if an isolated conducting (metallic) fence and the trailer can be bridged by a person standing between them. 

Connecting the fence and trailer together will lower the voltage between them, even if they may not be touched at the same time. If an ungrounded conducting temporary fence is installed adjacent to a grounded substation fence, an isolation section may be necessary to prevent a transfer voltage on the temporary fence.

Gate

A conducting gate should be bonded to a conducting fence. If the gate swings outward, a loop conductor should be

installed to control the touch voltages when opening the gate. If the gate opens inward, no special grounding may be

needed unless the fence is isolated from the mobile substation’s ground grid.

Trailer frame

The trailer frame and/or trailer ground bus should be connected to the grid with a conductor of a size adequate to carry the available fault current. If the trailer has no ground bus for the equipment connections, all equipment and the trailer should be connected to the mobile substation ground grid using separate conductors of proper size for each piece of equipment.

Multiple trailers (e.g., a switch and fuse trailer used with a mobile transformer trailer) should be connected via the grid or a direct cable tie to prevent voltages between them. Caution should be taken while any maintenance is performed on the trailer (e.g., changing or removing tires and adjusting jacks) while the unit is in service. Movable (e.g., slide-out switch bases, etc.) or removable conducting parts (e.g., steps, dollies, etc.) should be connected to the trailer ground bus or the ground grid, or both, to eliminate voltages between them.

Cable shield grounding

Cable shields should be grounded. If the cable shield is to be grounded at both ends, the shield must be sized to conduct fault current or have a separate parallel conductor to prevent excessive current in the shield. Refer to IEEE Std 525-1992 for further guidance.

Operator platforms or plates

In the absence of a properly designed ground grid, operator platforms or plates should be installed and connected to the grid at all switch handles that are accessible from the ground. In the case of a properly designed ground grid, operator platforms and plates may be installed as a supplement to the ground grid and connected to the grid. If the trailer jacks are to be adjusted while the mobile substation is in service, a platform or plates connected to the grid should be installed at the jack handle.

Neutral grounding

Neutral conductors of adequate fault-current capacity should be installed from the mobile transformer to the grid. Feeder neutrals should be connected to the provided attachment point (neutral bushing) or directly to the grid. If possible, transmission line shield wires should be connected to the grid.

Due to the higher resistance of a separate mobile grid not connected to the main substation grid, a high percentage of the fault current will flow on the neutral and transmission line shield wires. In some cases, the current limits of these wires may be exceeded by return currents.

When the mobile substation is located outside the substation fence, the grids should be connected together, if practical, to lower the ground potential rise. Installation of two or more ground cables is desirable to reduce the inductive reactance between the two ground mats and to lower the transient overvoltages.

Temporary equipment

Any temporary equipment connected to the trailer or equipment on the trailer should be grounded to the grid or trailer ground bus.

NEC GROUNDING TERMS AND DEFINITIONS BASIC INFORMATION

Bonding Jumper, Main: The connector between the grounded circuit conductor (neutral) and the equipment-grounding conductor at the service entrance.

Conduit=Enclosure Bond: (bonding definition) The permanent joining of metallic parts to form an electrically conductive path which will assure electrical continuity and the capacity to conduct safely any current likely to be imposed.

Grounded: Connected to earth or to some conducting body that serves in place of the earth.

Grounded Conductor: A system or circuit conductor that is intentionally grounded (the grounded conductor is normally referred to as the neutral conductor).

Grounding Conductor: A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

Grounding Conductor, Equipment:The conductor used to connect the noncurrent-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor and=or the grounding electrode conductor at the service equipment or at the source of a separately derived system.

Grounding Electrode Conductor: The conductor used to connect the grounding electrode to the equipment-grounding conductor and=or to the grounded conductor of the circuit at the service equipment or at the source of a separately derived system.

Grounding Electrode: The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding conductor connection to the system. The grounding electrode shall be:

(1) the nearest available effectively grounded structural metal member of the structure; or

(2) the nearest available effectively grounded metal water pipe; or

(3) other electrodes (Section 250-81 & 250-83) where electrodes specified in (1) and (2) are not available.

Grounding Electrode System: Defined in NEC Section 250-81 as including: (a) metal underground water pipe; (b) metal frame of the building; (c) concrete-encased electrode; and (d) ground ring. When these elements are available, they are required to be bonded together to form the grounding electrode system.

Where a metal underground water pipe is the only grounding electrode available, it must be supplemented by one of the grounding electrodes specified in Section 250–81 or 250–83.

Separately Derived Systems: A premises wiring system whose power is derived from generator, transformer, or converter windings and has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

REASONS FOR HAVING GROUNDING IN ELECTRICAL INSTALLATION

There are three basic reasons for grounding a power system: personal safety, protective device operation, and noise control. All three of these reasons will be addressed.

Personal Safety

The most important reason for grounding a device on a power system is personal safety. The safety ground, as it is sometimes called, is provided to reduce or eliminate the chance of a high touch potential if a fault occurs in a piece of electrical equipment.

Touch potential is defined as the voltage potential between any two conducting materials that can be touched simultaneously by an individual or animal. Figure 29.2 illustrates a dangerous touch potential situation.  

The ‘‘hot’’ conductor in the piece of equipment has come in contact with the case of the equipment. Under normal conditions, with the safety ground intact, the protective device would operate when this condition occurred.

However, in Fig. 29.2, the safety ground is missing. This allows the case of the equipment to float above ground since the case of the equipment is not grounded through its base. In other words, the voltage potential between the equipment case and ground is the same as the voltage potential between the hot leg and ground. If the operator would come in contact with the case and ground (the floor), serious injury could result.

In recent years, manufacturers of hand held equipment, drills, saws, hair dryers, etc. have developed double insulated equipment. This equipment generally does not have a safety ground. However, there is never any conducting material for the operator to contact and therefore there is no touch potential hazard. If the equipment becomes faulted, the case or housing of the equipment is not energized.

Protective Device Operation

As mentioned in the previous section, there must be a path for fault current to return to the source if protective devices are to operate during fault conditions. The National Electric Code (NEC) requires that an effective grounding path must be mechanically and electrically continuous (NEC 250–51), have the capacity to carry any fault currents imposed on it without damage (NEC 250–75).

The NEC also states that the ground path must have sufficiently low impedance to limit the voltage and facilitate protective device operation. Finally, the earth cannot serve as the equipment-grounding path (NEC-250–91(c)).

The formula to determine the maximum circuit impedance for the grounding path is:

Ground Path Impedance = Maximum Voltage to Ground (Overcurrent Protection Rating x 5)

Noise Control

Noise control is the third main reason for grounding. Noise is defined as unwanted voltages and currents on a grounding system. This includes signals from all sources whether it is radiated or conducted.

As stated, the primary reason for grounding is safety and is regulated by the NEC and local codes. Any changes to the grounding system to improve performance or eliminate noise control must be in addition to the minimum NEC requirements.

When potential differences occur between different grounding systems, insulation can be stressed and circulating currents can be created in low voltage cables (e.g., communications cables). In today’s electrical environment, buildings that are separated by large physical distances are typically tied together via a communication circuit.

An example of this would be a college campus that may cover several square miles. Each building has its own grounding system. If these grounding systems are not tied together, a potential difference on the grounding circuit for the communication cable can occur.

The idea behind grounding for noise control is to create an equipotential grounding system, which in turn limits or even eliminates the potential differences between the grounding systems. If the there is an equipotential grounding system and currents are injected into the ground system, the potential of the whole grounding system will rise and fall and potential differences will not occur.

Supplemental conductors, ground reference grids, and ground plates can all be used to improve the performance of the system as it relates to power quality. Optically isolated communications can also improve the performance of the system.

By using the opto-isolators, connecting the communications to different ground planes is avoided. All improvements to the grounding system must be done in addition to the requirements for safety.

GROUNDING FOR NOISE CONTROL BASIC AND TUTORIALS

Noise control is the third main reason for grounding. Noise is defined as unwanted voltages and currents on a grounding system. This includes signals from all sources whether it is radiated or conducted.

As stated, the primary reason for grounding is safety and is regulated by the NEC and local codes. Any changes to the grounding system to improve performance or eliminate noise control must be in addition to the minimum NEC requirements.

When potential differences occur between different grounding systems, insulation can be stressed and circulating currents can be created in low voltage cables (e.g., communications cables). In today’s electrical environment, buildings that are separated by large physical distances are typically tied together via a communication circuit.

An example of this would be a college campus that may cover several square miles. Each building has its own grounding system. If these grounding systems are not tied together, a potential difference on the grounding circuit for the communication cable can occur.

The idea behind grounding for noise control is to create an equipotential grounding system, which in turn limits or even eliminates the potential differences between the grounding systems.

If the there is an equipotential grounding system and currents are injected into the ground system, the potential of the whole grounding system will rise and fall and potential differences will not occur.

Supplemental conductors, ground reference grids, and ground plates can all be used to improve the performance of the system as it relates to power quality. Optically isolated communications can also improve the performance of the system.

By using the opto-isolators, connecting the communications to different ground planes is avoided. All improvements to the grounding system must be done in addition to the requirements for safety.

Separation of loads is another method used to control noise. Figure illustrates this point. Figure shows four different connection schemes. Each system from left to right improves noise control.

As seen in Figure, the best case would be the complete separation (system on the far right) of the ADP units from the motor loads and other equipment. Conversely, the worst condition is on the left of Fig. where the ADP units are served from the same circuit as the motor loads.