Section 6(18) in Tamil Nadu Electricity Grid Code, 2005
(18)Protection System and Coordination: (a) Protection in general(i)Every element of the power system shall be protected by a standard protection system having the required reliability, selectivity, speed, discrimination and sensitivity. Where failure of a protective relay in the system of one entity has substantial impact on the system from which power is drawn by it, it shall connect an additional protection as standby. The entity supplying power to another entity shall provide back-up protection to the system which receives power.(ii)Protection coordination shall be done at State level by STU in consultation with RPC. Each entity shall develop protection manuals conforming to various standards for the reference and use of its personnel in consultation with STU. The System Fault Dependability Index shall be as stipulated in CEA (Grid Standards) for the systems of all entities.(iii)Over current, short circuit, earth fault over voltage and other types of protection systems are generally required to be provided by all Licensees and other agencies connected to the Grid. These protective systems are essentially required for the quick isolation of faulty equipments, lines and UG Cables and protection of the network and its healthy equipments and lines and UG cables from the severe impact of the fault, with the appropriate reliability, sensitivity and selectivity.(b)Fault clearance Time(i)The fault clearance time, for a three phase fault (close to the bus-bars) on agencies equipment directly connected to STS and for a three phase fault (close to the bus-bars) on STS connected to agencies equipment, shall not be more than:(a)100 milli seconds (ms) for 400 kV and above(b)160 milli seconds (ms) for 220 kV & 110 kV(ii)Back-up protection shall be provided for required isolation / protection in the event of failure of the primary protection systems provided to meet the above fault clearance time requirements. If a Generating Unit is connected to the STS directly, it shall withstand, until clearing of the fault by back-up protection on the STS side.(iii)All agencies connected to the STS shall provide protection systems as specified in the connection agreement.(c)Generating Unit Requirements The guidelines mentioned in the "Manual on protection of Generators, Generator Transformers, and 220 kV and 400 kV networks" vide publication No: 274 of CBIP shall be kept in view. All Generating Units and associated electrical equipments connected to the Transmission System shall be protected by adequate protection so that the Transmission System does not suffer due to any disturbance originating from the Generating Unit. The generator protection schemes shall cover at least differential protection, back up protection, stator earth fault protection, field ground/field failure protection (not applicable to brush-less excitation system), negative sequence protection, under frequency, over flux protection, back-up impedance protection and pole slipping protection (applicable to units above 200MW), loss of field protection, reverse power protection etc.(d)Transmission System Requirements: All HV lines taking off from a power station or a sub-station shall have appropriate over voltage protection and distance protection and back up protection schemes. The STU shall notify the users of any change in its policy on protection from time to time.(i)400 kV Lines / UG Cables: Two independent protection systems (Main I & II Protection) with separate DC Battery supply shall be arranged. Main-I Protection shall be preferably with three-zone static non-switched numerical distance protection with appropriate carrier protection and back up protection.Main-II Protection shall be a fast protection scheme using direction comparison or phase comparison carrier relaying scheme. In addition, single pole tripping and single shot, single pole auto-reclosing after an adjustable dead time shall be provided.(ii)230 kV Lines/UG Cables: The main protection shall be through three-zone static, non-switched numerical distance protection with suitable carrier protection. The back up will be micro processor based switched distance protection scheme. One pole tripping and single shot single pole auto-reclosing with adjustable dead time shall be provided for both schemes.(iii)110 kV ami 66 kV lines/UG Cables: Static/micro processor based distance protection with suitable backup protection shall be provided as the main protection scheme. The back up will be directional three phase over current and earth fault protection scheme.(iv)General: For short transmission lines and sub transmission system at or below 33 kV, appropriate protection schemes may be adopted. Relay Panels for the protection of lines of STU taking off from a Generating Station shall be owned and maintained by the STU. Generating Companies shall provide space, connection facility and access to the STU for such purpose.(v)Generator, Auto and Power Transformers: All windings of Auto Transformers and Power transformers of 400, 230 and 110 kV class shall be protected by differential relays and REF (Restricted Earth Fault) relays. In addition, there shall be back up time lag over current and earth fault protection. For parallel operation such back up protection shall have directional feature. For protection against heavy short circuits, the over current relays should incorporate a high set instantaneous element. In addition to electrical protection, gas operated relays, winding temperature protection and oil temperature protection shall be provided. For smaller transformers of FIV class, differential protection shall be provided for 10 MVA and above along with back up time lag over current and earth fault protection (with directional feature for parallel operations). Transformers of 1.6 MVA and above but less than 10 MVA shall be protected by time lag over current, earth fault and instantaneous REF relays. In addition, all transformers of 1.6 MV A and above shall be provided with gas-operated relays, winding temperature protection and oil temperature protection.(vi)Bus bar protection: Suitable (high impedance) bus bar protection shall be provided for the speedy clearance of bus faults at all the generating stations and 400 kV and 230 kV Transmission substations.(vii)Over voltage protection: There shall be proper "Insulation Coordination" studies before implementing the over voltage protection scheme. All the operating equipments, lines and UG cables shall be protected against direct lightning strokes by the use of masts / ground wires. In respect of protection against system generated surges (switching surges) and indirect lightning strokes/ back flash-overs, co-coordinated protective gaps and Metal Oxide Arresters (Gapless Arresters) shall be provided. These protective gapless arresters shall be placed at the entry point of OH lines/Under Ground Cable, at the power/ substation (Line Entrance Arrestors) and at a location very close to Auto/Power Transformers and other protected equipments including shunt capacitor banks. Gapless surge diverters shall be provided at the interface of OH line - UG cable in a composite circuit and also its entry at the power station / substation. To the extent possible, the mixing up of gapped and gapless arresters shall be avoided. Gapless arresters should not be placed on the LV side of a Auto/Power Transformer, when a gapped surge diverter already exists on its HV side. When IIV lines (110 kV or 66 kV) are used for feeding lower voltage substations (33,22 and 11 kV) after its degradation, its insulation level should be brought down to the level required by the substations fed i.e derated HV lines should not be employed directly for feeding lower voltage substations.All power system equipments should withstand twice its rated power frequency voltage for one minute so as to withstand the rigors of temporary over voltages generated in the systems, as a consequence of faults.
