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  • with industry expertsQ&AResistance Grounding

    ASK THE EXPERTS

    806CSTMfcig.indd 1 5/30/2008 12:40:51 PM

  • ask the experts

    What Is Grounding? The term grounding is commonly used in the electrical industry to mean both equipment grounding and system grounding. Equipment grounding means the connection of earth ground to non-current-carrying conductive materials such as conduit, cable trays, junction boxes, enclo-sures, and motor frames. System grounding means the connection of earth ground to the neutral points of current-carrying conductors such as the neutral point of a circuit, a trans-former, rotating machinery, or a system, either solidly or with a current-limiting device. Figure 1 illustrates the two types of grounding.

    What Is a Grounded System? It is a system in which at least one conductor or point (usu-

    FIGURE 1

    System Grounding Definition and Different Types

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    ding ally the middle wire or neutral point of trans-

    former or generator windings) is intentionally grounded, either solidly or through an imped-ance (IEEE Standard 142-1991 1.2).

    The types of system grounding normally used in industrial and commercial power systems are solid grounding, low-resistance grounding, high-resistance grounding, and ungrounded.

    What Is the Purpose of System Ground-ing? System grounding, or the intentional connection of a phase or neutral conductor to earth, is for the purpose of controlling the voltage to earth, or ground, within predictable limits. It also provides for a flow of current that will allow detection of an unwanted connec-tion between system conductors and ground [a ground fault].

    What Is a Ground Fault? A ground fault is an unwanted connection between the system conductors and ground. Ground faults often go unnoticed and cause havoc on plant pro-duction processes. Shutting down power and damaging equipment, ground faults disrupt the flow of products, leading to hours or even days of lost productivity. Undetected ground faults pose potential health and safety risks to

    806CSTM2ig.indd 1 5/30/2008 12:41:15 PM

  • ask the experts

    personnel. Ground faults can lead to safety hazards such as equipment malfunctions, fire, and electric shock.

    Ground faults cause serious damage to equipment and to your processes. During a fault condition, equipment can be damaged and processes shut down, seriously affecting your bottom line.

    I Have Over-Current Protection; Do I Need Additional Ground Fault Protection? The over-current protection will act to interrupt a circuit for currents for which it was designed and set to operate. However, some ground faults, particularly low-level arcing faults, will produce significant damage and create a fire-ignition source without ever reaching the level necessary to activate the over-current protective device.

    Ungrounded 480-Volt SystemIs there any danger in running a 480-volt ungrounded system in an old manufacturing plant? Should we ground the system?The main danger in running a 480V ungrounded system is that, when a ground fault occurs, the only indication you will have is the three lights. The voltage on the ungrounded phases will in-crease to 480V with respect to ground, the volt-age on the grounded conductor will be 0V with respect to ground. With this system, the only way to indicate the presence of a ground fault will be when two lights are of greater brilliance than the faulted phase light. In order to locate the ground fault, you must cycle every feeder breaker until all three lights appear at equal brilliance again. Once this is done, you continue down that feeder until you find the fault. This sounds very easy to do but proves to be very difficult in the real world.

    The plant is normally ungrounded because it is a continuous operational plant, and isolation due to a ground fault should be avoided. This un-fortunately translates to locating the ground fault. The only way to locate the ground fault is through cycling of the feeder breakers. This is what you are trying to avoid. So at the end of the day, the

    ground fault remains on the system, because there is no easy way to locate it. This is danger-ous because any maintenance being performed on the system in a grounded state is subject to full line-to-line potential with respect to ground. The good news is that there is a solution. Ungrounded facilities can be easily converted to high-resistance grounded facilities, and the detection and loca-tion of a ground fault can accomplished without power interruption.

    Floating GroundWhat is the impact, if any, on moving equipment de-signed for a plant with a floating ground or ungrounded secondary to a plant that has a true grounded system? My thoughts are, it shouldnt really matter, but I could be mistaken.In your case (from an ungrounded system to a sol-idly grounded system), no, it does not matter.

    However, if you were going the other way (from an SG to a UNG system), then, yes, it would matter. During normal operation, it more than likely will not matter; however, during a ground fault it will. In an ungrounded system, the faulted phase voltage collapses to ground potential (or ~0V), and the unfaulted phases rise to phase-to-phase voltage with respect to ground. For ex-ample, a 480V system will have ~277V phase-to-ground voltage during normal operation, so it should work adequately. However, a ground fault on one phase makes its voltage go to 0V, and the other two phases will rise from 277V to 480V, phase-to-ground.

    Since this doesnt happen on a solidly ground-ed system, anything rated only 300V phase-to-ground will explode, such as TVSSs, VFDs, meters, etc.

    Ungrounded SystemHow can we measure earth fault and unbalanced current in an ungrounded system? And how do open delta PT and CB-CT function against faults? What is the effect on open delta PT?In an ungrounded system, the phase-to-ground voltages of the three phases change when there is an earth fault. Under normal conditions,

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    grounding system grounding system

    grounding system grounding

    806CSTM3ig.indd 1 5/30/2008 12:41:37 PM

  • ask the experts

    phase-to-ground voltage equals phase-to-phase voltage/1.73, and it is the same for the three phases. On the occurrence of an earth fault, the phase-to-ground voltage on the faulted phase is 0, and for the other two phases, it becomes equal to phase-to-phase voltage. The unbalance in phase-to-ground voltages is measured by YY-connected PTs and is used to indicate an earth fault. Open delta connected PTs are connected line-to-line, and do not see this change; they are unaffected, hence cannot be used for earth-fault detection.

    Any energized conductor will have capaci-tance-to-ground, so in a three-phase system, there are three balanced distributed capacitances to ground due to natural conductor insulation. In an ungrounded system, these three capacitive-charging currents, Ico, are normally balanced, and there is no net ground current flow. In an earth fault, as the voltage-to-ground on the faulted phase goes to 0, the charging current in that phase also goes to 0; however, the charging current in-creases in the other two phases, which are driven at line-to-line voltage magnitude. And they add up to become 3 Ico. This charging current can be detected by a zero-sequence sensor (also called CB-CT) to indicate an earth fault. These currents are normally 0.1A to 2A in LV systems and up to 20A in MV systems.

    Grounding SystemWhy are HV and LV systems always solidly earthed, and an MV system resistance-earthed?Solid earthing lowers the power conductor insula-tion requirements. The insulation level only needs to be suitable for the phase-to-ground voltage, not the line-to-line voltage. This has major cost ben-efit for insulators on transmission lines, cables, and lightning arrestor ratings. Solid earthing reduces the impedance in the return path in the event of a phase-to-ground fault. A fault current can then operate protective relays. With solid earthing, no transient over-voltages due to earth faults occur. So solid earthing is preferred for the HV systems.

    The consequence of high ground-fault cur-rent with solid grounding is the very high energy dissipation at the fault point, which can be devas-

    tating. For MV systems, insulation systems suit-able for phase-to-phase voltage usually can be ap-plied. The penalty for a higher insulation level at MV is not significant, and the benefit of low fault damage far outweighs the cost. For systems up to 5KV, in many jurisdictions, continuous operation of the faulted circuit is permitted if the current in the ground path can be 10A or less. This pro-vides a major benefit to the power systems where trip-out due to a phase-to-ground fault cannot be tolerated and causes consequential loss due to power interruption.

    LV systems have been solidly earthed to maxi-mize the phase-to-ground-fault current so that circuit protective devices such as fuses and break-ers will open. With modern electronic relays, de-tecting and locating ground faults can be done even when ground currents are small. Hence, in the industrial LV distribution systems, where a neutral conductor is often not required, the prac-tice of limiting the earth-fault current to less than 10A allows power continuity. The fault can be de-tected, alarmed, and located using electronic re-lays. This allows the faulted feeder to be serviced when convenient. So LV three-phase, three-wire systems can be high resistance grounded; such sy

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