9-16 D30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUALGROUND DIRECTIONAL OVERCURRENT CHAPTER 9: THEORY OF OPERATION9Consequently, the following signals are applied to the phase AB distance element:Eq. 9-1Eq. 9-2This results in the following apparent impedance:Eq. 9-3This value is a correct measure of the distance from the VT location to the fault. For relay location 2, this certainly includesthe positive-sequence impedance of the transformer:Eq. 9-4Thus, 0.127 Ω ∠90° + 0.05779 Ω ∠85° = 0.1847 Ω ∠88.4° primary side or 2.569 Ω ∠88.4° on the secondary side.This example illustrates how the relay maintains correct reach for fault behind power transformers. When installed at X,the relay needs to be set to 0.687 Ω ∠85° secondary in order to reach to the fault shown in the figure. When installed at H,the relay needs to be set to 2.569 Ω ∠88.4° to ensure exactly same coverage.See the Application of Settings chapter for information on setting calculations.9.3 Ground directional overcurrent9.3.1 DescriptionConsider the negative-sequence directional overcurrent element. As shown, the negative-sequence voltage can be lowduring internal fault conditions.Figure 9-6: Offset impedance augmentationV 13------V AB V BC– 10.861 kV 59.9° primary or 94.45 V 59.9° secondary∠∠= =I 3I B– 58.860 kA 27.6° primary or 36.75 A 27.6° secondary–∠–∠= =Z appVI--- 94.45 kV 59.9°∠36.75 kA 27.6– °∠---------------------------------------- 2.570 Ω 87.5°∠ secondary= = =Z T at 13.8 kV( ) 10100-------- 13.8 kV( )2150 MVA------------------------× 0.127Ω 90°∠= =Z L at 13.8 kV( ) 30.11 13.8315--------- 2× 0.05779Ω 85°∠= =837728A2.CDRURV_2(a)URV_2 >I_2 * Z_2line(c)I_2ECAI_2 x ZV_2 -V_2S_polS_op(b)I_2ECAI_2 x ZV_2-V_2S_pol S_op(d)-V_2
