Ppt on protection of power transformers

Basic protections of power transformers.

1. Protection of Power Transformer Mr.Siddharam Kantoli

2. DIFFERENTIAL PROTECTION SCHEME This scheme is employed for the protection of transformers against internal short circuits. It provides the best overall protection for internal faults. It compares the current entering the transformer with the current leaving the element. If they are equal there is no fault inside the zone of protection. If they are not equal it means that a fault occurs between the two ends.

3. DIFFERENTIAL PROTECTION SCHEME

4. I p - primary rated current of transformer I s - secondary current of transformer CT of ratio I p /1A at primary side and CT of ratio I s /1A at secondary side of the transformer. Biased Differential Protection Scheme

5. Construction: The secondaries of these both CTs are connected together in such a manner that secondary currents of both CTs will oppose each other. In other words, the secondaries of both CTs should be connected to same current coil of differential relay in such a opposite manner that there will be no resultant current in that coil in normal working condition of the transformer.

6. Operation: But if any major fault occurs inside the transformer due to which the normal ratio of the transformer disturbed then the secondary current of both transformer will not remain the same and one resultant current will flow through the current coil of the differential relay, which will actuate the relay and trip both the primary and secondary circuit breakers. To correct phase shift of current because of star – delta connection of transformer winding in case of three phase transformer, the current transformer secondaries should be connected in delta and star.

7. Limitations of differential protection of transformer: 1) Due to the magnetization characteristics of the CTs used, the ratio errors change with respect to the circulating currents. 2) The pilot wires used may vary in length due to which the unbalance in the secondary circuit parameter (resistance) is created that results in improper scheme. 3) During heavy short circuit conditions the high currents create saturation of the flux in core of CTs that lead to abnormal relaying or unexpected behavior of the relaying circuit.

8. 4) Tap changing may lead to change in settings & improper operation. 5) Inrush of magnetizing current may lead to inadvertent operation & hence the settings are done for higher values of fault current (higher imbalance) due to which accuracy of sensing & operation is decreased.

9. PROTECTION AGAINST INCIPIENT FAULTS INCIPIENT FAULTS : Faults which are not serious at the beginning but which slowly develops into serious faults are known as incipient faults.

10. BUCHHOLZ RELAY It is a gas actuated relay. When a fault develops slowly, it produces heat, thereby decomposing solid or liquid insulating material in the transformer. The decomposition of the insulating material produces inflammable gases. The Buchholz relay gives an alarm when a specified amount of gas is formed. The analysis of the gas collected in the relay chamber indicates the type of the incipient fault.

11. There is a chamber to accommodate Buchholz relay, in between the transformer tank and the conservator. The Buchholz relay is a slow acting device, the minimum operating time is 0.1 s and the average time is 0.2 s.

12. The Buchholz relay also has another set of contacts operated by a float. These contacts stay open when the transformer tank is filled with oil. However, in case of leakage of oil or decomposition of oil, the float sinks causing the contacts to close. Loss of oil will no doubt cause the transformer temperature to rise but does not warrant immediate tripping. Hence, normally these contacts are wired to an alarm which alerts the operator. Working Animation

13. RESTRICTED EARTH FAULT PROTECTION A percentage differential relay has a certain minimum value of pick up for internal faults. Faults with current below this value are not detected by the relay. Winding-to-core faults, which are single phase to ground type, involving high resistance, fall in this category . Therefore for such type of faults RESTRICTED EARTH FAULT PROTECTION is used.

14. An external fault in the star side will result in current flowing in the line CT of the affected phase and at the same time a balancing current flows in the NCT, hence the resultant current in the relay is therefore zero. So this REF relay will not be actuated for external earth fault. But during internal fault the NCT only carries the unbalance fault current and operation of Restricted Earth Fault  Relay takes place. This scheme of REF protection  is very sensitive for internal earth fault of electrical power transformer. 

15. The protection scheme is comparatively cheaper than differential protection scheme. Restricted earth fault protection  is provided in  power transformer for sensing internal earth fault of the transformer. In this scheme the CT secondary of each phase of power transformer are connected together as shown. Then common terminals are connected to the secondary of a Neutral Current Transformer or NCT. The CT connected to the neutral of transformer  is called Neutral Current Transformer .

16. OVER CURRENT PROTECTION In an over current relay, there would be essentially a  current  coil. When normal  current flows through this coil, the magnetic effect generated by the coil is not sufficient to move the moving element of the relay, as in this condition the restraining force is greater than deflecting force.

17. But when the  current  through the coil increased, the magnetic effect increases, and after certain level of current, the deflecting force generated by the magnetic effect of the coil, crosses the restraining force, as a result, the moving element starts moving to change the contact position in the relay.

18. Over current protection for a transformer

19. Fig. shows two numbers of phase fault over-current relays and one ground-fault over-current relay for providing over-current protection to the star-delta transformer. The neutral current under load conditions is quite small. The neutral current is essentially because of load unbalance.

20. EARTH FAULT

21. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over current of the system, it bends and makes normally open contacts to close and close the Trip Circuit of C.B.

22. Thank You!!!