Important considerations when design protection system
considerations when design protection system.
D - Distance
For 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.:
E - Miscellaneous:
Fast operation: minimizes damage and danger
Very fast operation: minimizes system instability discrimination and security can be costly to achieve.
Fault position discrimination
Power system divided into protected zones must isolate only the faulty equipment or section
Dependability / reliability
Protection 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:
Security / Stability
Protection must not operate when not required to e.g. due to:
Overlap of protections
Phase discrimination / selectivity
Correct indication of phases involved in the fault Important for Single Phase Tripping and auto-Reclosing applications
Current and voltage transformers
These are an essential part of the Protection Scheme. They must be suitably specified to meet the requirements of the protective relays.
1A and 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/R ratio.
Current Transformer Standards/Classes:
British Standards: 10P, 5P, X
IEC: 10P, SP, TPX, TPY, TPZ
American: 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. VT’s may be Electromagnetic or Capacitor types. Busbar VT’s: Special consideration needed when used for Line Protection.
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, 440
Preferred operative range of relays:
80 to 10% of voltage rated
AC. component ripple in the dc supply:
<10% of voltage rated
The cost of protection is equivalent to insurance policy against damage to plant, and loss of supply and customer goodwill.
Acceptable cost is based on a balance of economics and technical factors. Cost of protection should be balanced against the cost of potential hazards there is an economic limit on what can be spent.
Must ensure that all faulty equipment is isolated by protection
Total cost should take account of: