01-08-2012, 09:59 AM
PROTECTIVE RELAYING
PROTECTIVE RELAYING.ppt (Size: 252.5 KB / Downloads: 101)
Purpose of protective relaying
Protective relaying senses the abnormal condition in a part of the power system and gives an alarm or isolates that part of the system from the healthy system.
Main Elements of protection
Current Transformers
Potential Transformers
Relays
Auxiliary relays
Circuit breaker Etc.
General operation of a protective relay
Relay will operate when operating force or torque is more than the Restraining torque
Operating torque is provided by the actuating quantity .
Restraining torque is by control spring and/or restraining quantity.
Current Transformers
Current Transformers(CTs) are used to minimise the amount of current handling at instruments.
General Secondary rating of CTs are 1A(/5A.)
CTs used for measuring purpose are called measuring CTs., and used for protective purposes are called protective CTs.
Measuring CTs.
Measuring CTs will saturate for less currents. These are generally made of nickel-iron alloys having low exciting ampere-turns.
These CTs have high accuracy over 10-120% of rated current. These CTs saturate over this range and hence protect the instruments.
Protective CTs
These CTs have very high saturation levels and are generally have high area of cross section of core and are made up of Grain oriented steel.
These are so designed in order to give correct picture of fault current to the relay.
INTERPOSING CTs
Interposing CTs are those,which are used for balancing the secondary currents in case of a differential protection of transformers. (In case existing CT ratings are not matched).
This will reduce the cost of installation.
Facilitates standardisation of CT ratios.
POTENTIAL TRANSFORMER
Potential transformers are used to reduce the voltage levels at the instruments.
Potential transformers used for measuring are different from that of protection
Measuring PTs are designated with class1.0, 0.5, etc., where as protective PTs are designated with class 3P,10P etc.,