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S R Q ZQR=10W ZRT=10W Z T Q Z ZQR=10W R S X T ZRT=10W S R Q ZQR=10W IQR=10A VQ=100V Z X VR=0V ISR=10A ZRT=10W ITR=10A T S R Q ZQR=10W IQR=10A VQ=100V Z X VR=0V ISR=10A ZRT=10W T ITR=10A At bus Q, relay Z correctly measures the impedance to the fault as: Z=VQ / IQR (Ohm’s Law) Z = 100 V / 10 A = 10 W S R Q ZQR=10W ZRT=10W Z T S R Q ZQR=10W ZRF=5W Z ZTF=5W X T S R Q IQR=5A VQ=100V Z ISR=5A ZQR=10W ZRF=5W ZTF=5W T X IRF=10A ITF=13A VR=50V VF=0V At bus Q, relay Z now measures the impedance to the fault as Z=VQ / IQR (Ohm’s Law), where VQ = ZQRIQR + VR VR = ZRFIRF IRF=IQR+ISR VQ = ZQRIQR + ZRF (IQR+ISR) S R Q ZQR=10W IQR=5A VQ=100V Z ISR=5A ZRF=5W ZTF=5W T X IRF=10A ITF=13A VR=50V VF=0V Z= [ZQRIQR + ZRF (IQR+ISR)] / IQR Z= ZQR + [ZRF (IQR+ISR) / IQR] Z= ZQR + ZRF (IRF / IQR) For the above example, the actual impedance to the fault from bus Q is 10 + 5 = 15 ohms. However, due to infeed from bus 2 into R, the relay at Q measures 10 + 5(10/5)=20 ohms. Q R S Z T ZRU=10W U In the case of bus breakups, the relay is simply at the bus measuring all sources into a bus. Q R IQR=5A Z S ISR=5A T ITR=15A ZRF=5W ZUF=5W U IRF=25A X ITF=13A The calculation is the same. The relay at R measures: Z= VR / (IQR+ISR), where VR = ZRFIRF Z= ZRF [IRF / (IQR+ISR)] Z=5 * [25/(5+5)] = 12.5 ohms Q R IQR=5A Z S ISR=5A T ITR=15A ZRF=5W ZUF=5W U IRF=25A X ITF=13A The calculation is the same. The relay at R measures: Z= VR / (IQR+ISR), where VR = ZRFIRF Z= ZRF [IRF / (IQR+ISR)] Z=5 * [25/(5+5)] = 12.5 ohms Q R S Z T ITR=5A V ITR=20A ZRF=5W ZUF=5W U IRF=25A X ITF=13A After the breakup relay operates, reverse zone 3 elements on the lines on the bus section must operate (one additional line added to illustrate infeed). R Z T ITR=5A V ITR=20A ZRF=5W ZUF=5W U IRF=25A X ITF=13A Again, the calculation is the same. The relay at R on line R-T measures: Z= VR / ITR, where VR = ZRFIRF Z= ZRF(IRF / ITR) Z=5 * (25/5) = 25 ohms