Question

Part B (20 marks) Figure 2 shows a three phase 22kV, 50Hz radial system, which is to be protected by two inverse- time over-current relays B1 and B2. The maximum loads for this system are L 6 MVA, and L2- 5 MVA. The maximum fault currents at bus 1 and bus 2 are 1500A and 1000A respectively. The minimum fault currents at bus 1 and bus 2 are 900A and 650A respectively The line currents are measured using current transformers with a 200:5 turns ratio, and the relays are CO-7 inverse time over-current relays with characteristics as given in Figure 3. The breaker operating time is 4 cycles. Select relay Current Tap Settings (CTS) and Time Dial Settings (TDS) so as to protect the system for faults at bus 2 only, using a coordination interval of 0.5 s. Check coordination for both minimum and maximum fault currents, making sure that coordination time is at least 0.5 s in all cases us Us Figure 2 Radial Power System. Typical time curves type CO-7 over current relay Current tap settings: 4,5,6,7,8,10,12 amperes Time dial 10 1/2 4 5 6 7 8 9 10 12 14 16 18 20 Current as a multiple of tap setting Figure 3 Characteristic ofa CO-7 Inverse Time Overcurrent Relay.

Answer 1

Buo us 2 Lea d GMVA 5MVA MAx fautt turut 1 SODA 10DOA MiyW ult curseut 700A G SOA Leal curnt at buo 2,, V 131-2159 A N3X 22 Minwfault ument et bus 2, , 6s0 A. pick up for Jut shauld not Relayy ateo R frult cuveul nin mwat pek up fr 6 SD A of B2 be 200:5. CT Tatto Actual pick-up unent= 5 A IG 25A Aeem by B2 =(£ x650 Fult eurend Minm 16 25 3.25 55 Currunt tap settig r relay R R should he Fiven least available TDs fer ferter puatian Cuwe Let TDs Fiven Co-7 Over cument decisian tme, te 0.2 sec chanactenistic, 4eycles 4x)008sec. Breakor operabim tinue, t Total bradeer Opening tmo, t20-2+0.08) e 0.28 sec. ba ck up tay Kelny Ri Mut at bus L in wind to Cordinob'm tine fer fault get - kesping amd must be of o.5sec intewal belacen elay t tradce t enure the tine- Coordiuatia If waru tne imerse tine chanactenistic, thime cordrdi vactian fer we can fault urnemt at us 2, thau, because t Ceu earrure the 2 the minw fault euneut at ous 2 an well. G+5)X103 Load cunent S at B C7 ratio = 200:5 288.675A

MaxW auult unent at bus 2-100 A Cat MaRh fruit crment 25A Be peer by umart tap sctbing o c -5 fr TDS Land Qecisim tine = Telerg cunent trp setting Cfrvm tue Tvar raph o.Isec brealer aperetian tine ootrho.08 sec (4csel) Operabian Hue Co-ardinatian ine = 0.5 sec r Teley bscaleer B,t-0.18see Secisian tine Rto. 18+0.5) O.68 sec. urneut tp settirg Talmy nith 4ime of aperobia -068se 5 and TDS= 2. the awailable trfo TDS Cuwe is Now for Minn, fault cuvent Pault curent = G sO A tue cunent tp setting aferr B at bu 2ie ab would be 3.25 200 oeativg Lecisin &TDS= 2, tha CTS 3.25 aud for tinne af Operatin fae tne me for more tuan t whicth is 0.8 sec RI a co-ordinatia and tnne is 0.2 Sec Thus e.8-0.1) 0.6600 (>0.5 sec) "nterwal Carardintia is retainet the Over- went mainteined Relay settings TDS. CTS CT Tatio 2 200:5 3.25 2 200: 5 AN N