standards / manuals / guidelines for small hydro development - AHEC

2.3.4 Existing Insulation Practice of Substation EquipmentsThe substation equipments namely the power transformers circuit breakers anddisconnecting switches are considered for detailing the existing practice.Existing PracticeThe commonly adopted insulation levels at present for the above mentioned equipmentsare given in table 2.4 with reference to Karnataka and Tamil Nadu (66 kV, 110 kV and220 kV) and Punjab (132 kV Bhakra System).A commonly adopted practice is to locate lightning arrester as near to the transformer aspossible. In large substations additional arresters could be required at suitable locations toprotect circuit breakers, isolators and other equipments. Since each of these equipmentscannot be provided with arrester individually, it is the normal practice to adopt higherinsulation to provide the equipment with as good protection as is economically justified.Insulation level for circuit breaker and other equipments connected to the busbar togetherwith the bubars themselves are designed for insulation class about 10% higher than theinsulation level for the transformers (one class higher). Insulation level across the openpoles of the isolating switches is kept about 10 to 15 % higher than that provided betweenthe poles and the earth, so that in the event of a surge at an open isolating switch, theflashover should pass to earth and not across open poles.Table 2.4: Insulation Levels of the terminal Equipments RecommendedS. DescriptionNo.1. Highest systemvoltage kV (rms)2. Power transformerinsulation levels kV(Crest)3. Circuit breaker kV(Crest)4. Disconnectingswitches between poleand earth kV (Crest)Nominal voltage in kV220 132 110 66 33 11245 145 123 72.5 35 12900 550 450 325 170 751050 650 550 350 170 751050 650 550 380 170 752.3.5 Protection with Spark GapsThe spark gap is among the cheapest protective devices used for diverting the surgesfrom line to earth. After the break down of the spark gap the circuit breaker alwaysoperates to interrupt the fault of power frequency current in the circuit. Thus theoperation of the gap generally results in the circuit outage and interruption of supply ofthe power system. It is therefore used as a back up to surge arrestor (lightning arrestor).1-6