Your task is to study the operation of the directional elements of a 132 kV tran

Question

Your task is to study the operation of the directional elements of a 132 kV transmission line protection scheme with the 90-degree connection. The phase elements have a MTA of 45 degrees (current leading) and the ground element has a MTA of 60 degrees (current lagging). Two faults were simulated with the following results for the phase voltages and currents “seen” by the relays:

For each case, determine which of the four directional relay elements will operate. Use graphical and analytical methods. Use:

Determine the operation time in each case for the directional overcurrent phase and ground relays if the overcurrent element settings are:

Phase relays:

Ground relay:

Case a Angle (degrees) Magnitude (Primary kV and amps) 16 76 76 4389 40 40 Quantity 120 120 -80 -84 -84 Case b Angle (degrees) Magnitude (Primary kV and amps) 21.4 76 76 1238 24 31 Quantity 120 120 98 109

Explanation / Answer

STANDARD I.D.M.T. OVERCURRENT RELAYS:

The current/time tripping characteristics of IDMT relays may need to be varied according to the tripping time required and the characteristics of other protection devices used in the network. For these purposes, IEC 60255 defines a number of standard characteristics as follows:

Standard Inverse (SI)

Very Inverse (VI)

Extremely Inverse (EI)

Definite Time (DT)

Relay Characteristic Equation (IEC 60255)   

Standard Inverse (SI) t TMS I r = × 0 14 1

Very Inverse (VI) t TMS I r = × 13 5 1

Extremely Inverse (EI) t TMS I r = × 80 1

Long time standard earth fault t TMS I r = × 120 1

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