APT4801 Assignment 3 MINOR TEST 1 (DETAILED ANSWERS) 2025 - DISTINCTION GUARANTEED

APT4801 Assignment 3 MINOR TEST 1 (DETAILED ANSWERS) 2025 - DISTINCTION GUARANTEED - DISTINCTION GUARANTEED - DISTINCTION GUARANTEED Answers, guidelines, workings and references , ...Differential protection for a 3-phase, 15 MVA, 132/33 kV, /Y-connected transformer has to be designed. Multi-ratio current transformers with ratios of 600/5, 500/5, 400/5, 300/5, 200/5 and 30/5 are available. The differential relays have taps of 5.0, 5.5, 6.6, 7.3, 8, 9 and 3. Select the CT ratios and taps for the two sides of the transformer so that the percentage mismatch is less than 3%. (20) You are a consulting engineer working in Power Systems Protection. You are required to analyse and assess the protection system for two Eskom power Stations located next to each other in Kriel, Mpumalnga Province, as well as the protection system on a potential distribution network the company plans to build. 1. each stage. Figure 2 gives details (characteristic curves and available pick-up or tap settings) of the electromechanical O/C relays deployed in the protection scheme. Figure 1: Radial feeder with three relaying points (a) Calculate the load currents at each relaying point. (4) (b) Select the relay pick-up (tap) settings at the three relaying points. (Allow for 10% short-time overload.) (6) (c) Propose suitable time settings for each O/C relay that will ensure proper discrimination during fault conditions. Use a margin of 400 milli-seconds between stages to cater for security and CB clearing times. (10) Figure 2: Relay characteristic curves Fig Q2a shows a transformer-feeder system. The power transformer nameplate reads: 20 MVA, 33/11 kV, Dyn1. The feeder is a short length of 11 kV cable of negligible capacitance. Biased differential protection is to be applied to this transformer-feeder system. A numerical relay is available for the implementation. Provide a solution, including the relay settings, that will provide satisfactory protection without risk of mal-operation. Assume the following information: • Transformer has load tap changing of 10%; • CT error of 5%; • The relay has current setting range from 0.2 to 1.2A in steps of 0.01A. • Fig. Q2b (attached) gives the 3-phase transformer vector groups Figure Q2a Fig. Q2b Name and give a brief overview of these two Power Stations. Also highlight the similarities and differences including ratings, technology, applicable protection at power stations such as these, etc. (20) 2. The fault level at a 66 kV substation is 200 MVA. Assuming human body weight and resistance of 50 kg and 1 000 , respectively, a footing resistance of 75 k, and a fault current split factor of 65%, calculate the maximum value of earthing (grid) resistance that will ensure that the touch potential within the substation does not exceed the tolerable limit. The protection clears the fault in 0.75 seconds. The empirical constant k = 0.116 for body weight of 50 kg. This Evaluate available power system simulation software programs and list them (Maximum of 5) that can calculate fault levels and choose a software program to use to evaluate analyse the network in 2. Briefly describe the features of three software programs, including history, owner, how it works, positives and negatives. Describe the rationale in choosing the software for use in this assessment. (15) 3. Three-zone mho relays are used for transmission line protection of the power system shown in figure 1 The line impedances are Line 1 – 2: j60  Line 2 – 3: j40  Line 2 – 4: j70  The rated voltage for the HV buses is 400 kV. The CT and VT ratios at B12 are 2 000/1 and 3 600/1, respectively. Figure 1 power network (a) Determine the zone 1, 2 and 3 settings for the mho relay at B12. (Zone 1 should be set to cover 80% of the protected line, zone 2 to cover 130% of the protected line and zone 3 to cover the longest adjacent feeder.) (12) (b) What will the impedance seen by the relay at B12 be for a fault at F (which is 15% into line 2- 4), given that the infeed (measured CT secondary side) from G1 is 500 A and that from G2 is 300 A? (4) (c) Given that the zone 1, 2 and 3 settings at B32 are 18, 27 and 61 Ω, respectively, determine which of B12 or B32 will operate first for the fault at F, and indicate the zones in which the two relays operate. (6) A 125 MVA, 15.5 kV generator is protected by an unbiased differential relay (R) and neutral earth fault (NEF) relay as shown in figure Q1. The differential relay is set to operate at 0.2 A and the E/F relay at 0.1 A. Determine if the relays will operate for a single phase-to-earth fault at F, with the following earthing resistance (RN) values: : (a) RN = 0 Ω, (b) RN = 1.0 Ω, and (c) RN = 10 Ω. (Assume the generator is not synchronised to the grid.) Use the following data: Generator reactances: X1 = X2 = 0.2 pu, X0 = 0.03 pu Differential CT ratios: 500/5 A (on both phase and neutral ends of generator windings) Neutral earthing CT ratio: 100/5 A Design a network layout and give the network diagram for a potential distribution network Eskom wants to build to be connected to both Power Stations. This network should be drawn in the software chosen in The network should include and show all transformers and protection equipment (You do not need to get the exact equipment that is at these power stations for protection equipment). The network should have 16 substations that are at 22 kV and 11kV being supplied from the power station. These should respectively be in a 50% split. The network should include a ring network with two 22 kV substations. Two of the 11kv substations should be supplied, respectively, from two 22 kV substations (One 22 kV in the ring supplied from the first power station and another not in a ring supplied from the second power station). The substations should be n-1 firm with transformers and supply feeders. The power station should have in feeds into the common 22kV network. (40) APT4801 Assessment 4. Three-zone mho relays are used for transmission line protection of the power system shown in figure 1 The line impedances are Line 1 – 2: j60  Line 2 – 3: j40  Line 2 – 4: j70  The rated voltage for the HV buses is 400 kV. The CT and VT ratios at B12 are 2 000/1 and 3 600/1, respectively. Figure 1 power network (a) Determine the zone 1, 2 and 3 settings for the mho relay at B12. (Zone 1 should be set to cover 80% of the protected line, zone 2 to cover 130% of the protected line and zone 3 to cover the longest adjacent feeder.) (12) (b) What will the impedance seen by the relay at B12 be for a fault at F (which is 15% into line 2- 4), given that the infeed (measured CT secondary side) from G1 is 500 A and that from G2 is 300 A? (4) (c) Given that the zone 1, 2 and 3 settings at B32 are 18, 27 and 61 Ω, respectively, determine which of B12 or B32 will operate first for the fault at F, and indicate the zones in which the two relays operate. (6) Calculate the fault levels at each substation’s secondary bus-bar using the software chosen in 3 and give the results as well as the fault level at the generator transformers’ secondary and primary side. Record the simulation and Discuss the observations/Results. (25) Differential protection for a 3-phase, 15 MVA, 132/33 kV, /Y-connected transformer has to be designed. Multi-ratio current transformers with ratios of 600/5, 500/5, 400/5, 300/5, 200/5 and 30/5 are available. The differential relays have taps of 5.0, 5.5, 6.6, 7.3, 8, 9 and 3. Select the CT ratios and taps for the two sides of the transformer so that the percentage mismatch is less than 3%. (20)