25-08-2017, 09:32 PM
STATIC OVERCURRENT RELAY
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AIM:
To study and test static overcurrent relay at different current settings and plot the characteristics.
APPARATUS REQUIRED:
Microcontroller based overcurrent relay, VPL – 01
THEORY:
Static relays are those relays which consist of electronic circuitry to develop all those characteristics achieved by moving parts in an electro-magnetic relay. As the name implies, these relays do not contain any moving parts.
Static relays are superior to electromagnetic relays in the following aspects:
1. The moving parts and the contacts are largely eliminated. The only moving element in a static relay is the final tripping contact.
2. C.Ts and P.Ts employed are of lesser VA rating as static relays require a very little volt-ampere for their operation.
3. More precise and high speed operation.
Static relay consists of the following units for operation:
1. Auxiliary voltage supply: 24V DC is required for energizing the circuitry of the static relay. This is obtained by converting 230V AC to 24V DC.
2. Comparator: it compares actual quantity with the pre-set quantity. In case of overcurrent relay, it will compare the actual current supplied by CT with the pre-determined set current over which tripping is required.
3. Time delay elements: the required operation time is achieved here by adjusting the values of R and C.
4. Logic circuit: by using gate circuits, conditions of operation of relays are set and relay can only be operated when these conditions are satisfied.
5. Output device: the actual tripping of relay can be achieved by firing the SCR or operating the attracted armature type relay by feeding the signal coming from logic circuit output.
PROCEDURE:
1. Switch on the relay and current source.
2. Set the required value for the current source which is the actual quantity. Then switch of the source.
3. Set the operation of relay as IDMT.
4. Set the value of TMS.
5. Set the reference value for current.
6. Then switch on the current source which is connected to relay.
7. Note down the time of operation.
8. Adjust the actual quantity to different values and take the readings.
9. Also take readings a different TMS value.