Important considerations when design protection system.1. Types of fault and abnormal Conditions to be protected against2. Quantities available for measurement3. Types of protection available4. Speed5. Fault position discrimination6. Dependability / reliability7. Security / stability8. Overlap of protections9. Phase discrimination / selectivity10. CTs and VTs ratio required11. Auxiliary supplies12. Back-up protection13. Cost14. Duplication of protection

Types of protectionA - FusesFor LV Systems, Distribution Feeders and Transformers, VTs, Auxiliary SuppliesB - Over current and earth fault Widely used in All Power Systems1. Non-Directional2. Directional.

C - DIFFERENTIALFor feeders, Bus-bars, Transformers, Generators etc1. High Impedance2. Low Impedance3. Restricted E/F4. Biased5. Pilot Wire

D - DistanceFor transmission and sub-transmission lines and distribution feeders, also used as back-up protection for transformers and generators without signaling with signaling to provide unit protection e.g.:1. Time-stepped distance protection2. Permissive underreach protection (PUP)3. Permissive overreach protection (POP)4. Unblocking overreach protection (UOP)5. Blocking overreach protection (BOP)6. Power swing blocking7. Phase comparison for transmission lines8. Directional comparison for transmission linesE - Miscellaneous:1. Under and over voltage2. Under and over frequency3. A special relay for generators, transformers, motors etc.4. Control relays: auto-reclose, tap change control, etc.5. tripping and auxiliary relaysSpeedFast operation:minimizes damage and dangerVery fast operation:minimizes system instability discrimination and security can be costly to achieve.Examples:1. differential protection2. differential protection with digital signaling3. distance protection with signaling4. directional comparison with signalingFault position discriminationPower system divided into protected zones must isolate only the faulty equipment or sectionDependability / reliabilityProtection must operate when required to Failure to operate can be extremely damaging and disruptive Faults are rare. Protection must operate even after years of inactivity Improved by use of:1. Back-up Protection and2. duplicate ProtectionSecurity / StabilityProtection must not operate when not required to e.g. due to:1. Load Switching2. Faults on other parts of the system3. Recoverable Power SwingsOverlap of protections1. No blind spots2. Where possible use overlapping CTsPhase discrimination / selectivityCorrect indication of phases involved in the fault Important for Single Phase Tripping and auto-Reclosing applicationsCurrent and voltage transformersThese are an essential part of the Protection Scheme. They must be suitably specified to meet the requirements of the protective relays.1Aand 5A secondary current ratings, Saturation of current transformers during heavy fault conditions should not exceed the limits laid down by the relay manufacturer.Current transformers for fast operating protections must allow for any offset in the current waveform. Output rating under fault conditions must allow for maximum transient offset. This is a function of the system X/Rratio.Current Transformer Standards/Classes:British Standards: 10P, 5P, XIEC: 10P, SP, TPX, TPY, TPZAmerican: C, T.Location of CTs should, if possible, provide for overlap of protections. Correct connection of CTs to the protection is important. In particular for directional, distance, phase comparison and differential protections. VTs may be Electromagnetic or Capacitor types. Busbar VTs: Special consideration needed when used for Line Protection.Auxiliary suppliesRequired for:1. Tripping circuit breakers2. Closing circuit breakers3. Protection and trip relays AC. auxiliary supplies are only used on LV and MV systems. DC. auxiliary supplies are more secure than ac supplies. Separately fused supplies used for each protection. Duplicate batteries are occasionally provided for extra security. Modern protection relays need a continuous auxiliary supply. During operation, they draw a large current which increases due to operation of output elements.Relays are given a rated auxiliary voltage and an operative auxiliary voltage range.the rated value is marked on the relay. Refer to relay documentation for details of operative range. it is important to make sure that the range of voltages which can appear at the relay auxiliary supply terminals is within the operative range.IEC recommended values (IEC 255-6):Rated battery voltages:12, 24, 48, 60, 11 0, 125, 220, 250, 440Preferred operative range of relays:80 to 10% of voltage ratedAC. component ripple in the dc supply: