standards / manuals / guidelines for small hydro development - AHEC

7.0 Lightning ArrestorsLightning arrestors are the basis of insulation co-ordination (Para 2.3 of section 1) in thesystem and are installed at outdoor transformer terminals for direct protection againstlightning impulse overvoltage spark over (1.2/50 micro second wave) and are capable ofwithstanding dissipation of energy associated with lightning impulse only. This impliesthat temporary overvoltages (at or near power frequency) which are of the order of milisecondmust be withstood to avoid damage. Taking into consideration high temporaryover voltages expected on load throw off 90- 95 % lightning arrestors should beprovided.Metal oxide (gapless) lightning arrestor confirming to following standards are now beingspecified.IEC: 99-4 - Specification part – 4 for surge arrestor without gap for AC systemIS: 3070 - Specification for lightning arrestorsTypical parameters for a 66 kV system are given (table 2.7.0) below.Table 2.7.01. Nominal system voltage (kV rms) 66 kV 33 kV 11 kV2. Highest system voltage (kV rms) 72.5 kV 36 kV 12 kV3. 1.2/50 microsecond impulse voltagewithstand levela) Transformer (kVp)325 170 75b) Other equipment and lines (kVp)4. Minimum prospective symmetrical faultcurrent for 1 second at Arrestor location(kA rms) (based on system studies)325 17031.5 kA To be stated5. Anticipated levels of temporary overvoltage and its duration(based on systemstudies)a > Voltage (p. u.)b > Duration (seconds)1.5/1.2 1.5/1.2 1.5/1.26. System frequency(Hz) 50 2.5 c/s 50 2.5 c/s 50 2.5 c/s7. Neutral Grounding EffectivelyearthedEffectivelyearthedEffectivelyearthed8. Number of Phases Three Three ThreeGeneral Technical Requirements1 The Surge Arrestors should conform to the technical requirements given in table2.7.1.2 The energy handling capability of the Arrestor offered, supported by calculationsshould be obtained with offer.3 The Lightning Arrestor should be fitted with pressure relief devices and arc2-14