substation grounding

36

Upload: alvin-lagasca

Post on 19-Sep-2015

68 views

Category:

Documents


10 download

DESCRIPTION

grounding complete guide

TRANSCRIPT

  • The PRIMARY goal of the grounding system throughout any facilities is SAFETY.Why ground at all?PERSONNEL SAFETY FIRSTEQUIPMENT PROTECTION SECOND

  • Types of Grounding SystemsGround rodEarth / Ground BasicsMany different types availableChoice depends on local conditions and required function

  • Types of Grounding SystemsGround rod groupEarth / Ground Basicsground rod group

    typically for lightning protection on larger structures or protection around potential hotspots such as substations.

  • Types of Grounding SystemsGround plateEarth / Ground BasicsFor areas where there is rock (or other poor conducting material) fairly close to the surface ground plates are preferred as they are more effective

  • Types of Grounding SystemsGround meshEarth / Ground BasicsA ground mesh consists of network of bars connected together, this system is often used at larger sites such as electrical substations.

  • An electrical substation is a critical resource in a power system. Safe operation of asubstation calls for a properly designed and installedgrounding system. A well-designedgrounding system will ensure reliable performance of the substation over its entire servicelifeSubstation Grounding

  • The following steps, when put into practice, will ensure areliable,safeandtrouble-freesubstation grounding system:Size conductors for anticipated faultsUse the right connectionsGround rod selectionSoil preparationAttention to step and touch potentialsGrounding using building foundations**Grounding the substation fence**Special attention to operating points**Surge arrestors must be grounded properly**Grounding of cable trays**Temporary grounding of normally energized parts**

  • 1. Size Conductors For Anticipated Faults

    Conductors must be large enough to handle any anticipated faults without fusing (melting).

  • 2. Use the Right ConnectionsIt is very evident that the connections between conductors and the main grid and betweenthe grid and ground rods are as important as the conductors themselves in maintaining apermanent low-resistance path to ground

  • 3. Ground Rod Selection

    In MV and HV substations, where the source and load are connected through longoverhead lines, it often happens that the ground fault current has no metallic path and hasto flow through the groundmass (earth). This means that the ground rods of both sourceand load side substations have to carry this current to or from the groundmass.

  • 4. Soil PreparationSoil resistivity is an important consideration in substation grounding system design.The lowerthe resistivity, the easier it is to get a good ground resistance.

  • Resistivity MeasurementThe purpose of resistivity measurements is to quantify the effectiveness of the earth where a grounding system will be installed.

    Differing earth materials will affect the effectiveness of the grounding system.

    The capability of different earth materials to conduct current can be quantified by the value E (resistivity in W.m).

    Resistivity measurements should be made prior to installing a grounding system, the values measured will have an effect on the design of the grounding system. Ground Testing Methods (1)

  • Resistivity Measurement ( Wenner method)Resistivity measurements are performed by using a four wire method.

    Used to determine which KIND of earthing should be used, so BEFORE placing earth stakesGround Testing Methods (1)

  • Recommended values of earth resistance

  • 1-Chemical Rods

    Chemical rods are electrodes with holes along their length, filled with mineral salts. The specially formulated mineral salts are evenly distributed along the entire length of the electrode.The rod absorbs moisture from both air and soil. Continuous conditioning of a large area insures an ultra-low-resistance ground which is more effective than a conventional electrode.

  • 5. Attention to Step and Touch Potentials

    Limiting step and touch potential to safe values in a substation is vital to personnel safety.

  • Step potential Step potential is the voltage between the feet of a person standing near an energized grounded object. It is equal to the difference in voltage, given by the voltage distribution curve, between two points at different distances from the electrode. A person could be at risk of injury during a fault simply by standing near the grounding point.

  • Touch potential Touch potential is the voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the energized object and a point some distance away. The touch potential could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it.

  • Step and touch voltages

  • 6.Grounding using building foundationsConcrete foundations below ground level provide an excellent means of obtaining a low-resistance ground electrode system. Since concrete has a resistivity ofabout 30 -m at20 C,a rod embedded within a concrete encasement gives a very low electrode resistance compared to most rods buried in the ground directly.

  • The use of Ufer grounds (named after the person who was instrumental in thedevelopment of this type ofgrounding practice) has significantly increased in recentyears. Ufer grounds utilize the concrete foundation of a structure plus building steel as agrounding electrode

  • 7. Grounding the Substation FenceMetallic fences of substations should be consideredjust as other substation structures.

  • 8. Special Attention to Operating PointsTo protect the operator in case of a fault, it should be ensured that he is not subjected tohigh touchorstep potentialswhen a fault happens in theequipment he is operating.

  • There are four types of safety mats.1.A steel grate or plate on supporting insulators.2.A steel grate on the surface, permanently attached to the grounded structure.3.Bare conductor buried (in a coil or zig-zag pattern) under the handle area andbonded to the grounded structure.4.Prefabricated equipotential wire mesh safety mat buried under the handle areaand bonded to the grounded structure

  • 9.Surge ArrestorsMust be Grounded Properly!

    When there is a surge in the electrical system (by indirect lightning strikesor due toswitching) surge arrestors placed near all critical equipment divert surge energy to groundand protect the equipment from being subjected to thesurges

  • 10. Grounding ofCable Trays

    All metallic tray sections must bebonded together with proper conducting interconnections.

  • 11. Temporary Grounding of Normally Energized Parts

    When personnel work on high-voltage electric structures or equipment,any conductivebodies should be groundedas a measure of safety

  • Installing an EARTHLINK 101 earthling strip is simple: Dig a trench and lay in the wire.

  • Pour EARTHLINK 101 conductive cement, using the handyapplicator bag, and shovel in a thin protective layer of soil.

  • Backfill the remaining soil using a front-end loader and restore the surface to grade.

  • THANK YOU!! ^_^Philippians 4:13I can do all thingsthrough Christ who strengthens me

    Reporter: Alvin R. Lagasca

    *The next slides are to bring the point out that theres a huge variety of ways of creating a ground system.For simplicity in the presentation we refer to ground electrode or grounding system to describe these different methods of grounding.It should be noted that there are many different types of grounding systems available.The type installed will depend on the local conditions and the required function of the grounding systemThe simplest form of grounding element is the ground stake, this can take many forms with a variety of lengths from a few feet to many feet long made of materials such as brass, galvanised or stainless steel, the size and material as required locallyThe simple ground rod can be used for lightning protection on stand-alone structures such as pole mounted transformers or radio towers, it can also be used as a back up to a utility ground.

    *A group of connected rods will have a more complicated interaction, typically configurations like this are present around substation sites and sensitive buildings.A slightly more complicated version of the rod system is the ground rod group, this is typically for lightning protection on larger structures or protection around potential hotspots such as substations.*Ground plates are used widely in telecoms applciations. They are particularly good where the deeper ground has high resistivity.For areas where there is rock (or other poor conducting material) fairly close to the surface ground plates are preferred as they are more effective*Ground meshes can be part of the foundations of structure. At substations and generating site the metal parts of the foundations will all be bonded together and form part of the overall grounding systems. At substation site an area of ground could be reserved at the start of the life of the substation with a ground mesh under the whole of the site. As the site grows over a period of years new equipment can easily be installed and grounded by the mesh. This ensures that the whole of the site remains at the same potential should a fault occur.

    *Resistivity should be measured prior to ground system design as it checks the effectiveness of the local geology.*Four stakes are used during resistivity testing. The distance between the stakes should be measured. This distance depends on the total area and the local resistivity. Typical distances are 6 and 12 feet, but if large areas have to be surveyed the distances may need to be larger, in that case it would be important to have oversized stakes, maybe even 3 to 6 feet long. Those types of surveys are more specialised. The outer two spikes inject test current and the inner two spikes measure voltage, a value of resistance is then derived