**Generator Protection Part 12**

**1. Negative Sequence Relays** The negative relays are also called phase unbalance relays because these relays provide protection against negative sequence component of unbalanced currents existing due to unbalanced loads or phase-phase faults. The unbalanced currents are dangerous from generators and motors point of view as these currents can cause overheating. Negative sequence relays are generally used to give protection to generators and motors against unbalanced currents.

A negative sequence relay has a filter circuit which is operative only for negative sequence components. Low order of over current also can cause dangerous situations hence a negative sequence relay has low current settings. The earth relay provides protection for phase to earth fault but not for phase to phase fault. A negative sequence relay provides protection against phase to phase faults which are responsible to produce negative sequence components.

The Fig. 1 shows the schematic arrangement of negative phase sequence relay.

Fig. 1 Negative phase sequence relay |

Basically it consists of a resistance bridge network. The magnitudes of the impedances of all the branches of the network are equal. The impedances Z

_{1 }and Z_{3 }are purely resistive while the impedances Z_{2 }and Z_{4 }are the combinations of resistance and reactance. The currents in the branches Z_{2 }and Z_{4 }lag by 60^{o}from the currents in the branches Z_{1 }and Z_{3}. The vertical branch B-D consists of inverse time characteristics relay. The relay has negligible impedance.Fig. 2 |

The current I

_{R }gets divided into two equal parts I_{1 }and I_{2}. And I_{2 }lags I_{1 }by 60^{o}. The phasor diagram is shown in the Fig. 2. Ī

_{1}+ Ī_{2}= Ī_{rs}Let I

_{1 }= I_{2}= I The perpendicular is drawn from point A on the diagonal meeting it at point B, as shown in the Fig. 2. This bisects the diagonal.

**.**OB = I^{.}._{R }/2 Now in triangle OAB,

cos 30 = OB/OA

**.**√3/2 = (I

^{.}._{R}/2)/I

**.**I = I

^{.}._{R}/√3 = I

_{1 }= I

_{2 }............(1)

Now I

_{1 }leads I_{R }by 30^{o }while I_{2}lags I_{R}by 30^{o}. Similarly the current I

_{B }gets divided into two equal parts I_{3 }and I_{4}. The current I_{3 }lags I_{4 }by 60^{o}. From equation (1) we can write, I

_{B }/√3 = I_{3 }= I_{4 }...............(2) The current I

_{4 }leads by I_{B }while current I_{3 }lags I_{B }by 30^{o}. The current entering the relay at the junction point B in the Fig. 1 is the vector sum of , and .

I

_{relay }= Ī_{1}+ Ī_{3}+ Ī_{Y} = I

_{Y }+ (I_{R}/√3) (leads I_{R }by 30^{o}) + I_{B}/√3(lags I_{B }by 30^{o}) The vector sum is shown in the Fig. 3 when the load is balanced and no negative sequence currents exist.

Fig. 3 |

It can be seen from the Fig. 3 that,

Ī

_{1}+ Ī_{3}= -Ī_{Y}**.**Ī

^{.}._{1}+ Ī

_{3}+ Ī

_{Y }= 0

Thus the current entering the relay at point B is zero. Similarly the resultant current at junction D is also zero. Thus the relay is inoperative for a balanced system.

Now consider that there is unbalanced load on generator or motor due to which negative sequence currents exist. The phase sequence of C.T. secondary currents is as shown in the Fig. 4(a). The vector diagram of I

_{1}, I_{3 }and I_{Y }is shown in the Fig. 4(b) under this condition. The component I

_{1}and I_{3 }are equal and opposite to each other at the junction point B. Hence I_{1 }and I_{3 }cancel each other. Now the relay coil carries the current I_{Y }and when this current is more than a predetermined value, the relay trips closing the contacts of trip circuit which opens the circuit breaker.Fig. 4 Negative sequence current |

**Zero Sequence Currents**: The zero sequence components of secondary currents are shown in the Fig. 5(a). We know that,

Fig. 5 Zero sequence currents |

_{R}= Ī

_{1}+ Ī

_{2}

Ī

_{B}= Ī

_{3}+ Ī

_{4}

These sums are shown in the Fig. 5(b) and (c). It can be seen from the Fig. 5(d) that,

Ī

_{1}+ Ī

_{3}= Ī

_{Y }in phase with I

_{Y }

The total current through relay is Ī

_{1}+ Ī_{3}+Ī_{Y}. Thus under zero sequence currents the total current of twice the zero sequence current flows through the relay. Hence the relay operates to open the circuit breaker. To make the relay sensitive to only negative sequence currents by making it inoperative under the influence of zero sequence currents is possible by connecting the current transformers in delta as shown in the Fig. 6. Under delta connection of current transformers, no zero sequence current can flow in the network.

Fig. 6 Delta connection of C.T.s |

__1.1 Induction Type Negative Sequence Relay__

Another commonly used negative sequence relay is induction type. Its construction is similar to that of induction type over current relay. The schematic diagram of this type of relay is shown in the Fig. 7.

Fig. 7 Induction type negative sequence relay |

The central limb of upper magnet carries the primary which has a centre tap. Due to this, the primary winding has three terminal 1, 2 and 3. The section 1-2 is energized from the secondary of an auxiliary transformer to R-phase. The section 2-3 is directly energized from the Y-phase current.

The auxiliary transformer is a special device having an air gap in its magnetic circuit. With the help of this, the phase angle between its primary and secondary can be easily adjusted. In practice it is adjusted such that output current lags by 120

So, I^{o}rather than usual 180^{o}from the input._{x }= Input current of auxiliary transformer

I

_{R1 }= Output current of auxiliary transformer

and I

_{R1 }lags I

_{R }by 120

^{o}

Hence the relay primary carries the current which is phase difference of I

_{R1 }and I

_{R }.

Positive Sequence Current : The C.T. secondary currents are shown in the Fig. 8(a). The Fig. 8(b) shows the position of vector I

_{R1 }lagging I_{R }by120^{o}. The Fig. 8(c) shows the vector sum of I_{R1 }and - I_{Y}. The phase difference of I

_{R1 }and I_{Y }is the vector sum of I_{R1 }and - I_{Y}. It can seen from the Fig. 8(c) that the resultant is zero. Thus the relay primary current is zero and relay is inoperative for positive sequence currents.Fig. 8 Positive sequence currents |

**Negative Sequence Currents**: The C.T. secondary currents are shown in the Fig.. 9(a). The Fig. 9(b) shows the position of I

_{R1 }lagging I

_{R }by 120

^{o}. The Fig. 9(c) shows the vector difference of I

_{R1 }and I

_{Y}which is the relay current.

Under negative sequence currents, the vector difference of I

_{R1 }and I_{Y }results into a current I as shown in the Fig. 9(c). This current flows through the primary coil of the relay.Fig. 9 Negative sequence currents |

Under the influence of current I, the relay operates. The disc rotates to close the trip contacts and opens the circuit breaker.

This relay is inoperative for zero sequence currents. But the relay can be made operative for the flow of zero sequence currents also by providing an additional winding on the central limb of the upper magnet of the relay. This winding is connected in the residual circuit of three line C.T. This relay is called induction type negative and zero sequence relay.

The schematic arrangement of induction type negative and zero sequence relay is shown in the Fig.10.Fig. 10 Induction type negative and zero sequence relay |

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ReplyDeleteThis is very impressive information to me starting job for relay. Thank you!!

ReplyDeletePractical circuit is not working :( (Fig. 1)

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