Negative Sequence Relays

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 Z₁ and Z₃ are purely resistive while the impedances Z₂ and Z₄ are the combinations of resistance and reactance. The currents in the branches Z₂ and Z₄ lag by 60^o from the currents in the branches Z₁ and Z₃. 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 I₁ and I₂. And I₂ lags I₁ by 60^o. The phasor diagram is shown in the Fig. 2.

                            Ī₁ + Ī₂=  Ī_rs

Let                       I₁ = I₂ = 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 = I₁ = I₂                                          ............(1)

        Now I₁ leads I_R by 30^o while I₂ lags I_R by 30^o.

       Similarly the current I_B gets divided into two equal parts I₃ and I₄. The current I₃ lags I₄ by 60^o. From equation (1) we can write,

                            I_B /√3 = I₃ = I₄                                                    ...............(2)

       The current I₄ leads by I_B while current I₃ 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  = Ī₁ + Ī₃ + 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,

                                 Ī₁ + Ī₃ = -Ī_Y

.^..                               Ī₁ + Ī₃ + 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 I₁, I₃ and I_Y is shown in the Fig. 4(b) under this condition.

       The component I₁ and I₃ are equal and opposite to each other at the junction point B. Hence I₁ and I₃ 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

                              Ī_R = Ī₁ + Ī₂
                              Ī_B = Ī₃ + Ī₄
       These sums are shown in the Fig. 5(b) and (c). It can be seen from the Fig. 5(d) that,
                             Ī₁ + Ī₃ = Ī_Y in phase with I_Y

       The total current through relay is Ī₁ + Ī₃ +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^o rather than usual 180^ofrom the input.

So,                   I_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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