CHAPTER 9: THEORY OF OPERATION PHASE DISTANCE APPLIED TO POWER TRANSFORMERSD30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL 9-119VA_1M = 64.71 V ∠0.0°j × I_0 × ej3 = 1.37 A ∠24.8°IA _2 × ZD = 1.37 V ∠19.8°I_0 × Z D = 1.37 V ∠19.8°IA × ZR + I_0 × K0 × ZR + IG × K0M × ZR – VA = 87.6 V ∠–109.2°IA × ZR + I_0 × K0 × ZR = 91.5 V ∠–93.0°IA × ZL + I_0 × K0 × Z L + IG × K0M × Z L – VA = 57.0 V ∠108.7°IA × ZL + I_0 × K0 × Z L = 45.8 V ∠82.9°• Overcurrent supervision: | 3 × I_0 | = 4.09 A > 3 A• Reactance difference angle = | –3.9° – 24.8° | = 28.7° < 75°• Zero-sequence difference angle = | –19.8° – 0.0° | = 19.8° < 75°• Negative-sequence directional difference angle = | –19.8° – 0.0° | = 19.8° < 75°• Right blinder difference angle = | –93.0° – (–109.2°) | = 16.2° < 90°• Left blinder difference angle = | 82.9° – 108.7° | = 25.8° < 90°• Fault-type comparator difference angle = | 19.8° – 19.8° | = 0.0° < 50°All six comparators and the overcurrent supervision are satisfied, so the Quad Phase A ground element operates for thisfault.9.2 Phase distance applied to power transformers9.2.1 DescriptionAs a Wye-Delta transformer introduces discontinuity for the zero-sequence circuit, the ground distance protection cannotbe applied, except special circumstances, to respond to faults behind the transformer.The phase distance elements, however, can be developed so that both accurate reach and correct fault phaseidentification is retained for faults behind the power transformer as seen from the relaying point. Without appropriatecompensation, the reach of the relay depends on a type of fault, creating considerable difficulties in applying the relay.The D30 provides for any location of the VTs and CTs with respect to the involved power transformer and the direction ofany given zone.In the following equations, the VT and CT locations are referenced as “None” if the transformer is not present between theCT/VT and the intended reach point. Otherwise, the location is to be selected as a type of a transformer as seen from theVT/CT position towards the intended reach point. The following figure explains the adopted rules.
