Question

Discuss the principle of operation of directional overcurrent protection when applied to detect phase and ground faults.

Answer 1

The principle of operation of directional over current protection when applied to detect phase and ground faults is explained below.

Fundamental Principle Thus, if we measure the bus voltage phasor Vp and compute the phase angle of relay current with respect to bus voltage, then we can use the following logic to provide selectivity. If the relay 'detects fault' and current lags VR ( Vp), then permit relay tripping. If the relay 'detects fault and current leads VR (-Vp), then inhibit the relay tripping. The 'discrimination principle' based on phase angle comparison between a set of phasors, one of which is used as reference is called 'directional discrimination principle'. Relays with this principle are called directional relays Zero Torque Line Do not Operate Operate For example, overcurrent relays can be made directional by adding above discrimination logic to well known overcurrent logic. Such relays are called as directional overcurrent relays. They are used in distribution system or subtransmission system where 'ring main' configuration is used to provide more ectional Iippingreliability of service. Cost of this relaying scheme is higher than 'non-directional Logic overcurrent due to additional cost of VT We now discuss the choice of reference phasor for various type of phase and ground faults. Recall that phase relays are used to protect against phase fault (3 phase and L-L) Now, with traditional overcurrent relays, a directional overcurrent relay can be visualized as a cascade connection of 'one directional unit and one overcurrent unit. If the polarity of the current is appropriate, then directional unit picks up. If the current magnitude is above pickup, then the overcurrent unit also picks up and when both units pickup, the trip coil is energized and CB tripping is ensured. In a numerical relay, this can be programmed by a simple 'AND logic Any fault involving ground is called a ground fault. Traditionally, three phase relays and one ground relay have been used to protect a feeder or a transmission line. However, in a numerical relay, all these functions can be integrated into a single relay which acquires 3-phase voltages and 3-phase currents Design of Directional Units for Phase Fault Let us first consider, a three phase fault. In this case, choice of the reference phasor can be the phase voltage itself. For a purely reactive circuit, the fault current in the correct direction lags the reference phasor by 90°. With respect to reference phase Va, we can draw operating line (also called as zero torque line due to legacy of electromechanical realys) which separates the plane into two regions marked as 'operate' and 'Do not operate'. If the fault is in the operating region, then la lags Va and we issue trip decision. In case, fault is behind the relay, the fault current leads Va and hence lies in the "do not operate" region

3 Phase Fault Protection Fig 18.7 shows vector diagram and relationship between different phasors. The threshold or maximum torque line is a line perpendicular to the zero torque line. Again this terminology is because of the legacy of electromechanical relay. The threshold or maximum torque line can be placed at an angle with respect to Va also. This does add complexity to electromechanical relay design. But same placement is a simple programming job in a numerical relay. For example, the common practice is to place the maximum torque line at an angle of 60 degrees lag or 45 degrees with respect to Va (fig 18.8) ac 821 30° Maximum Torque Line 120° 30° cb be Operate Zone cn Do not Zero Torque Line erate Zone Up ca Fig 18.8 Placement of Maximum Torque Line for 30° Unit Fig 18.7 Balanced Three Phase Voltage and Curr ent Phasors As shown in the fig 18.8, since Vbc is in phase quadrature with Va, it is possible to use Vbc as the reference phasor and locate the maximum torque line at 30 degrees leading it. This is what traditionally practiced in legacy directional overcurrent relays (see fig 18.9). With this placement we now show that directional unit will pickup for both 3-phase and L-L faults Maximum Torque 60° I 30° Now consider a line fault involving phase 'a' and 'b'. Then, using 3-phase line model we get, ac Similarly erate be Do not Operate Since, (Zs - Zm) Z1 Z2 of a feeder4 6 2Z Fig 18.9"a"-Phase directional unit response to a-b, a-c and b-c faults If for simplicity we assume Z to be purely reactive, then from fig 18.9 we get that Iab will be at an angle of 60 degrees lagging to Van Thus, 30° unit with Vbc as reference phasor will pickup on both 3-phase fault and L-L fault. For a L-L fault involving phases 'a' and 'c', Vac lags Van by 30°. Assuming purely reactive circuit, the phase current la will lag Vbc by 30°. As seen in the figure, lac will be again in the operate region and the directional unit will pickup. Thus, this unit (30° lead with Vbc as reference phasor) will pickup for all phase faults involving phase 'a'. In contrast, for L-L fault involving phases 'b' andで, lbc will lag Vbc by 90°. Hence, it will lie outside the tripping region of the directional unit. Therefore directional unit will not pick up

Earth Fault Protection Typically, earthfaults are SLG and LLG faults. Earthfaults are distinguished by presence of zero sequence currents T . Since, except for unbalance, normal system operation is devoid of lo component, much more sensitive pickup is possible for earthfault by using component lo = (la + Ib + Ic) / 3 and declaring a fault if "Io" exceeds a threshold. However, in a system with multiple sources or parallel paths, we will require earthfault relays to be directional. The reference phasor is sometimes called as "polarizing quantity". Also both voltage and current polarizing signals are used with ground fault relaying. -3to ag 0 ag 0 bg cg bg Fig 18.11 Computati on of Zero Sequence Voltage Phasor ce Fig 18.10 Voltage Phasor with a SLG Fault on Phase A