IEEE 3003.1:2019 pdf free download – IEEE Recommended Practice forSystem Grounding of Industrialand Commercial Power Systems
In normal operating conditions, with balanced three-phase voltages applied to the lines, the capacitive chargingcurrent, Ic, in each phase will be equal and displaced 120° from one another. The phase voltages to groundwill also be equal and displaced 120° from one another. The phasor relationships can be determined. Since theneutral of the distributed capacitances is at earth potential, it follows that the neutral of the transformer is alsoat earth potential, being held there by the capacitance to ground.
lf one of the system conductors, phase C for example, faults to ground, current flow through that capacitanceto ground will cease, since no potential difference across it now exists. The voltage across the remainingtwo distributed capacitors to ground will increase from line-to-neutral to line-to-line.The capacitive chargingcurrent, Io, in the two phases without a fault will therefore increase by the square root of 3.As shown inFigure 9, the line-to-ground voltages are no longer 120° , but 60° apart. This causes the excessive terminalvoltages during faults, along with the absence of a neutral conductor, precludes support of line-to-neutral loadson an ungrounded system.
In an ungrounded system, it is possible for destructive transient overvoltages to occur throughout the systemduring restriking ground faults. These overvoltages, which can be several times normal in magnitude, resultfrom a resonant condition being established between the inductive reactance of the system and the distributedcapacitance to ground.Experience has proven that these overvoltages may cause failure of insulation atmultiple locations in the system, particularly at motors.Transient overvoltages from restriking ground faultsare the main reason why ungrounded systems are no longer recommended and grounded systems of someform are the predominant choice. To reduce transient overvoltages during restriking ground faults, one shouldground the system using either solid or impedance grounding as indicated in Figure 10.
Various detection schemes are used to detect the presence of a single line-to-ground fault. The simplestscheme employs three light bulbs rated for line-to-line voltage, each connected between line voltage andground.Under normal operation, the three bulbs are illuminated with low equal intensity. When a single line-to-ground fault occurs, that bulb connected to the faulted phase is extinguished. The remaining two bulbsincrease in intensity since the voltage on the phases without a fault increascs from line-to-neutral to line-to-line. It should be noted that the light bulbs are a high resistance and to some extent provide a ground referencefor the ungrounded system.
Another scheme frequently used takes the form of three voltage transformers with their primary windingsconnected in wye and the neutral point grounded.The secondary windings of the transformers are connectedin broken delta, with a voltage relay connected in the open corner and used to operate an indication or alarmcircuit. Using this scheme, loading resistors may be required either in the primary neutral or secondary circuitto avoid ferroresonance.
Locating a single linc-to-ground fault on an ungrounded system can be time consuming. Usually, the first stcpis to open the secondary feeders, one at a time, to determine which feeder on which the fault is located. Thisis verified by observing the three lights to determine when the ground fault has been cleared. This process isrepeated downstrcam until the faulted device is detccted.
lf a ground cannot be located before a second line-to-ground fault occurs on a different phase, a line-to-line fault will result. The current must be carried either by the protective conductor,metallic raceways, orby the earth. This will be contrasted later to a grounded system that develops enough fault current to clear,automatically and sclectively, cach faulted circuit.