System grounding – What Is Piping

Earthing or Grounding can be defined as a conducting connection whether intentional or accidental by which an electrical circuit or equipment is connected to the earth Types of GroundingSystem GroundingEquipment Grounding System Grounding The System grounding is the intentional connection of neutral conductor to earth. Purpose of System Grounding Controlling the voltage to earth within predictable limitsProvides flow of current that will allow detection of an unwanted connection between system conductors and Ground and which may instigate operation of automatic devices to remove the source of voltage from conductors with such undesired connection to ground Methods of Grounding Ungrounded Systems (No intentional Grounding) Provides continuity of supply in case of 1-ph to ground faultsNo expenditure required for Grounding systemExcessive overvoltages during arcing, resonance ground faults Grounded Systems Greater SafetyFreedom from excessive over-voltagesEasier detection and location of ground faults Resistance Grounded Limits earth fault current and subsequent effects on the connected equipmentReduces momentary line-voltage dipControl of transient overvoltage Reactance Grounded Ground fault current should be preferably 60% of 3-ph fault current to prevent serious transient overvoltage. This is considerably higher fault current than resistance grounded system Grounding Fault Neutralizer Reactance is tuned to System charging current so that the resulting ground-fault current is resistive and of low magnitudeCurrent and voltage are in phase, So if a ground fault is in the air (insulator failure) it is self-extinguishingSolidly Grounded Systems: Direct connection for systems with Ro<=X1 and Xo<=3X1 Equipment Grounding The Equipment grounding refers to interconnection and grounding of all non-electrical metallic elements of a system Purpose of Equipment Grounding To reduce electric shock hazard to personnelTo provide adequate current-carrying capability both in magnitude and duration to accept ground-fault current permitted by the overcurrent protection systemTo provide a low impedance return path for ground-fault current necessary for the timely operation of the overcurrent protection system Safe Grounding Design Objectives for safe grounding design To provide means to carry electric current into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity in serviceTo assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock Safe grounding strives at controlling the interaction of the two grounding systems as follows: The intentional ground consisting of ground electrodes buried at some depth below the earth surfaceThe accidental ground temporarily established by a person exposed to a potential gradient in the vicinity of the grounded facility Basic shock situations (Fig. 1): Fig. 1: Figure showing the basic shock situation Ground potential rise: The maximum electric potential that substation grounding grid may attain relative to a distant grounding point assumed to be at a potential of remote earthMesh Voltage: The maximum touch voltage within the mesh of the ground gridMetal to metal touch voltage: The difference in potential between metallic objects within substation site that may be bridged by direct hand to hand or hand to feet contactStep Voltage: The difference in surface potential experienced by a person bridging a distance of 1m with the feet without contacting any other grounded object.Touch Voltage: The potential difference between ground potential rise (GPR) and the surface potential at the point where a person is standing while at the same time having a hand in contact with the grounded structureTransferred Voltage: A special case of touch voltage where a voltage is transferred into or out of substation from or to a remote point external to the substation site. Metal to metal touch situation (Fig. 2): Fig. 2: Figure showing metal to metal touch situation Grounding Design Designed based on IEEE 80-2000Critical parameters for Design:Maximum grid currentFault Duration and Shock durationSoil resistivityThe resistivity of the Surface layerGrid Geometry General Concept of Grounding Design Combination of Vertical rods and horizontal conductorsHorizontal conductors (grid) installed in a shallow depth (0.3 – 0.5m) are most effective in reducing the danger of high step and touch voltages on earth’s surfaceIf the magnitude of current dissipated to earth is high it is seldom possible to install a grid with resistance so low so as to assure that rise of ground potential will not generate surface gradients unsafe for human contact. The hazard can be eliminated only by control of local potentials through the entire area by using ground rods. Soil Treatment for Grounding It is often impossible to achieve the desired reduction in ground resistance by adding more grid conductors or ground rods An alternate solution is to increase the diameter of the electrode by modifying the soil surrounding the electrode by use of the following materials Use of Sodium chloride, magnesium and copper sulfate, etc.Use of Bentonite,Ground enhancement materials viz. Marconite, Terec+ Field Measurements –Fall of the potential method (Fig. 3): Ground resistance measurement consists of measuring the resistance of the grounding system with respect to the remote ground electrodeIt has difficulties and errors when used for large grounding systems Fig. 3: Field measurements Survey of Potential contours and Touch and Step voltages: Use the existing power lines and remote substation as current electrodePass test current through substation ground grid via remote current electrode as in substation ground resistance measurementsMeasure Touch and Step voltages.The test values are multiplied with a ratio of actual fault current to test current to obtain potential under fault conditions.Disturbance due to noise and electrical interference influences the results. A new approach of measurement: OMICRON CPC-100 can be used to generate current at a Lower and higher frequency than power frequency. Using digital filter algorithms the test set will measure only the signal with a frequency that is currently generated and filters out signals at other frequencies. Disturbance due to noise and electrical interference thus no longer influence the result.