Xi’an Jiaotong-Liverpool University Page 1 of 5 EEE340 Protective Relaying Assignment 2 Notice: – The number in the column on the right indicates the approximate marks for the question. – Only electronic copy (Word or PDF) submission is required. Q1 a) A set of unsymmetrical three phasors can be decomposed as sum of a set of symmetrical sequence components. What are these symmetrical sequence components? Please explain in detail by their magnitude, phase displacement and sequence. (6) b) For a set of unsymmetrical phasors ̇, ̇ and ̇, if the symmetrical sequence components of phase A are given as ̇(ଵ), ̇(ଶ), ̇(), and = ଵଶ , calculate ̇, ̇ and ̇ by ̇(ଵ), ̇(ଶ), ̇(). (6) c) If ̇, ̇ and ̇ are given, calculate the zero sequence component ̇() of phase A. (4) d) In Figure 1.1, there is a single line-to-ground fault on phase a, the symmetrical sequence components at fault location have following relations: (ଵ) + (ଶ) + () = 0 (ଵ) = (ଶ) = () Draw the interconnected sequence network for the single line-to- ground fault. (4) Total 20 Xi’an Jiaotong-Liverpool University Page 2 of 5 Figure 1.1 Q2 a) In Figure 2.1, if the input voltage and current to the directional overcurrent protection 1 are phase voltage UA and current IA, what is a dead zone for this protection? (4) b) For zero sequence current directional protection, is there any dead zone for the directional element? Why? (5) c) In Figure 2.2, the fault current through line A-B is indicated as IAB, the fault current supplied by A’ is indicated as ᇱ , the fault current through line B-C is indicated as IBC. The instantaneous protection (zone I) of 1 is set as point d. How to calculate the setting value for Zone II of protection 2? Include figures in your explanation. (6) d) What is an inverse time overcurrent protection? What is different between its starting current and instantaneous operating current? Include figures in your explanation. (5) Total 20 Figure 2.1 Xi’an Jiaotong-Liverpool University Page 3 of 5 Figure 2.2 Q3 a) The input voltage and current for an impedance relay are set as: = , = , can this relay be used to detect a line-to-line fault on phase B and phase C? Why? (6) b) With a potential transformer PT, a current transformer CT, a reactance transformer UR and a voltage transformer T given, please design a circuit of impedance relay by amplitude comparison to get a directional circular characteristic. (6) c) With the same conditions of (b), what is the operating characteristic equation of this relay? (4) d) In Figure 3.1, the amplitudes of power sources EM and EN are equal, their phase angle difference is δ, equivalent impedance between two power sources is , the impedance phase angles of the whole system are identical. Draw a phasor diagram to indicate relations among voltage EM and EN of power sources and voltage of any point F on line lM-N. (4) Figure 3.1 Total 20 NLM ZZZZ Xi’an Jiaotong-Liverpool University Page 4 of 5 Q4 a) What are the four communication technologies mainly used in pilot protections? (4) b) What are the three working modes for Power Line Carrier Pilot Protection? (3) c) If a distance pilot protection is the main protection for a transmission line and a distance protection works as the backup protection, when the backup protection is under maintenance, the main protection has to be stopped, why? (3) d) If the differential relay for a pilot protection uses static setting value to avoid the possible maximum unbalance current, the sensitivity may not be enough. How can we utilize dynamic setting value to solve this problem? (10) Total 20 Q5 a) Directional Blocking Pilot Protections are installed from protection 1 to protection 6 as shown in Figure 5.1, the construction of each protection is given in Figure 5.2. A three-phase short circuit occurs at point k on line B-C. Please explain the complete working process of all protections in details. (10) b) The operating time for single pole autoreclosure is normally longer than three pole autoreclosure. Please explain the reason in details. (10) Total 20 Figure 5.1 Xi’an Jiaotong-Liverpool University Page 5 of 5 Figure 5.2
