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Performance Evaluation for Ground Relays

  •  Performance Evaluation for Ground Relays

    When ground relays are connected in the neutral-to-ground circuits of the power system (3I0), or if I0 is obtained from a transformer delta tertiary,  is not applicable because the positive sequence load current is not involved. If power circuits are connected to the transformer delta, CTs in each leg of the delta connected in parallel are necessary to provide 3I0.

    For the common connection of the phase-and-ground relay in fig Section 5.6 applies with the ground criteria outlined in fig

     

    Performance Evaluation for Ground Relays

     

    Phase-and-ground relays for the protection of a circuit and the current distribution for a phase-and-ground fault.

     

    Normally, ground relays can be set much more sensitively than the phase relays, especially on higher-voltage circuits for which the zero-sequence unbalance is very small. On lower-voltage circuits, such as distribution lines, the effect of single-phase loading may result in ground relay settings that approximate those of the phase relays.

     

     

    Flux Summation Current Transformer

     

     

    Also known as doughnut or ring CT, flux summation CT consists of a magnetic core with a distributed winding. Power conductors are passed through the center opening. Typical openings are approximately 4–10 in. These CTs are useful in protection at the lower voltage.

    When the three-phase conductors are passed through the opening, the secondary measures Ia + Ib + Ic = 3I0, the ground current. When the same phase conductor on the two ends of a device is passed through the opening, the net current for a load or fault current passing through the device is zero. For an internal fault, with one or both supplying current of different magnitude or phase angle, the net or sum equals the fault current.

    This is flux summation, rather than individual summing of separate transformer secondary currents. The advantages are that the CT ratio is independent of the load current or kVA of the circuit and that it avoids the possible difficulties of unequal individual CT saturation or performance with paralleled CTs. The disadvantage is the limitation of the size of conductors that can be passed through the opening. A typical ratio for this CT is 50:5, and the diameter of the maximum opening is about 8 in.

    The CT is commonly used with a 0.25 A instantaneous overcurrent unit. The combination provides a primary pickup of 5 A, rather than 2.5 A, if the exciting current were negligible. Specific applications are discussed in later chapters.

    Metallic sheath or shielded cables passed through the toroidal CT can result in cancellation of the fault current. This is illustrated in Fig. This applies either to three-phase cables, as shown, or to single-phase cables. The cancellation may be partial or complete, depending on the sheath grounding. This sheath component of fault current can be removed from passing through the CT by connecting a conductor, 

     

     

    Reference: Protective relay 4th edition.

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