Differential Relay.pptx

The differential relay is one that operates when there is a difference between two or more similar electrical quantities exceeds a predetermined value. In the differential relay scheme circuit, there are two currents come from two parts of an electrical power circuit. These two currents meet at a junction point where a relay coil is connected. According to Kirchhoff Current Law, the resultant current flowing through the relay coil is nothing but the summation of two currents, coming from two different parts of the electrical power circuit. If the polarity and amplitude of both the currents are so adjusted that the phasor sum of these two currents, is zero at normal operating condition. Thereby there will be no current flowing through the relay coil at normal operating conditions. But due to any abnormality in the power circuit, if this balance is broken, that means the phasor sum of these two currents no longer remains zero and there will be non-zero current flowing through the relay coil thereby relay being operated.

1. Differential Relay

2. Percentage (Biased) Differential Relay The percentage differential system consists of a restraining coil connected in the pilot wire as shown in the figure below and the current induced in both the CTs flows through it. The operating coil places between the midpoint of the restraining coil. The restraining coil controls the sensitive characteristic of the relay. It restricts the unwanted tripping of the transformer due to the imbalance current. The restraining coil also restrains the harmonics in the inrush current.

3. Percentage (Biased) Differential Relay Working of Percentage Differential Relay The torque due to the restraining coil prevents the closing of the trip circuit while the torque due to the operating coil tends to close the trip circuit contacts. Under normal operating conditions and through load condition the torque developed by the restraining coil is greater than the operating coil torque. Thus the relay remains inoperative. When an internal fault occurs, the operating torque exceeds the restraining torque then the trip circuit contacts are closed to open the circuit breaker . The restraining torque may adjust by varying the number of turns of the restraining coil. For external faults both i 1  and i 2  increase and thereby the restraining torque increase , which prevents the maloperation.

4. Percentage (Biased) Differential Relay Operating Characteristic of the Percentage Differential Relay For operation: Where:

5. Percentage (Biased) Differential Relay characteristics

6. Differential Protection of a Transformer schematic diagram

7. Differential Protection of a Transformer The restraining coil is connected between the secondary winding of the current transformers. Restraining coils controls the sensitive activity occurs on the system.  The operating coil is placed between the tapping point of the restraining coil and the star point of the current transformer secondary windings. Working of Differential Protection System Normally, the operating coil carries no current as the current are balanced on both the side of the power transformers. When the internal fault occurs in the power transformer windings the balanced is disturbed and the operating coils of the differential relay carry current corresponding to the difference of the current among the two sides of the transformers. Thus, the relay trip the main circuit breakers on both sides of the power transformers.

8. Differential Protection of a Transformer Problem Associated with Differential Protection System When the transformer is energizing the transient inrush of magnetizing current flows in the transformer. This current is as large as 10 times full load current and it decays respectively. This magnetizing current is flows in the primary winding of the power transformers due to which it causes a difference in current transformer output and it makes the differential protection of the transformer to operate falsely. To overcome this problem the kick fuse is placed across the relay coil. These fuses are of the time-limit type with an inverse characteristic and do not operate in short duration of the switch in the surge. When the fault occurs, the fuses blow out and the fault current flows through the relay coils and operate the protection system. This problem can also be overcome by using a relay with an inverse and definite minimum type characteristic instead of an instantaneous type.

9. Differential Protection of a Transformer Kick Fuse

10. CT Connection for Transformer Differential Protection Vector Group of Transformer The connection shown in figure 1 indicates that the LV winding leads the HV winding by 30°. Figure 2 shows a delta-wye transformer connection of vector group dyn1. This connection indicates that the LV winding lags the HV winding by 30°. This phase shift needs to be compensated to avoid the maloperation of the differential protection. Figure 1. YNd11 Transformer Connection Figure 2. Dyn1 Transformer Connection

11. CT Connection for Transformer Differential Protection Phase Angle Compensation by CT Connection Figure 3. Phase Compensation Using DAB Connected CTs

12. CT Connection for Transformer Differential Protection Phase compensation is done by means of wiring current transformers either delta or wye. Cts are wired such that the 30° phase shift is reversed. This is shown in figure 3. It can be seen from the ct connection for transformer differential protection that wye-connected cts are used in the delta side of the transformer while cts on the wye side of the transformer is dab-connected. Using dab-connected cts , secondary currents seen from the relay will lead the actual currents by 30°, thus offsetting the 30° phase displacement (lv lagging hv) introduced by the transformer vector group dyn1.

13. Differential Protection of Generator Differential protection for a generator is mainly employed for the protection of stator windings of generator against earth faults and phase-to-phase faults. The stator winding faults are very dangerous , and it causes considerable damage to the generator. For the protection of stator winding of the generator, the differential protection system is used for clearing the fault in the shortest possible time for minimizing the extent of a damage. Merz-prize circulating current system  Currents at the ends of the protected sections compare.  When the system is in normal operating condition, the magnitude of currents is equal on the secondary windings of the current transformers. On the occurrence of the faults, the short-circuit current flows through the system and the magnitude of current become differ. This difference of current under fault conditions is made to flow through the relay operating coil.

14. Differential Protection of Generator The relay then closes its contacts and makes the circuit breaker to trip and thus isolated the protection from the system. Such a system is called a merz -prize circulating current system. It is very effective for earth faults and faults between phases. Connection for Differential Protection System