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 |  موضوع: Over-current and Earth Fault Protection *****part 2 الأربعاء أكتوبر 27, 2010 9:50 am | |
| Over-current and Earth Fault Protection v Application for CoreBalance CT's with Cable Termination JointsThe termination of a three core cableinto three separate lines or bus-bars is through cable terminal box. Ref. (Fig. 7), theCore Balance Protection is used along with the cable box and should beinstalled before making the cable joint. The induced current flowingthrough cable sheath of normal healthy cable needs particular attention withrespect to the core balance protection.The sheath currents (Ish) flow through the sheath to the cover of cable-box and then to earth through the earthing connection between cable-box. For eliminating the error due to sheath current (Ish) the earthing lead between the cable-box and the earth should be takenthrough the core of the core balanceprotection. Thereby the error due to sheath currentsis eliminated. The cable box should be insulated from earth. 1. Cable terminal box2. Sheath of 3 core cable connection to (1)3. Insulator support for 14. Earthing connection passing through 55. Core balance CT
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]v Frame-leakage ProtectionThe metal-clad switchgear can beprovided with frame leakage protection. The switchgear is lightly y insulated from theearth. The metal-frame-work or enclosure of the switchgear is earthed with a primaryof a CTin between (Fig.  .The concrete foundation of theswitchgear and the cable-boxes and other conduits are slightly insulated from earth, theresistance to earth being about 12 ohms. In the event of an earth fault withinthe switchgear, the earth-fault current finds the' path through the neutralconnection. While doing so, it is sensed by the earth fault relay.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]Circulating current differential protection also responds toearth-faults within its protected zone. v Earth-fault protection can be achieved by following methods:1. Residuallyconnected relay.2. Relayconnected in neutral-to-ground circuit.3. Core-balance-scheme.4. Frameleakage method.5. Distancerelays arranged for detecting earth faults on lines. 6. Circulatingcurrent differential protection. DirectionalOver-current ProtectionThe over-current protection can be given directionalfeature by adding directional element in the protection system. Directionalover-current protection responds to over-currents for a particular directionflow. If power flow is in the opposite direction, the directional over-currentprotection remains un-operative.Directional over-current protection comprises over-currentrelay and power directional relay- in asingle relay casing. The power directional relay does not measure thepower but is arranged to respond to the direction of power flow. Directional operation of relay is used where the selectivitycan be achieved by directional relaying. The directional relay recognizes thedirection in which fault occurs, relative to the location of the relay. It isset such that it actuates for faultsoccurring in one direction only. It does not act for faults occurring inthe other direction. Consider a feeder AC (Fig. 9) passing through sub-section B. The circuit breaker CB3 is providedwith a directional[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]Relay `R' which will trip the breaker CB3 if fault power flow in direction Calone. Therefore for faults in feeder AB, thecircuit breaker CB3 does not trip unnecessarily. However forfaults in feeder BCthe circuit-breaker CB3 tripsBecause it's protective relaying is set with a directionalfeature to act in
direction ACAnother interesting example of directional protection isthat of reverse power protection of generator (Fig. 10). If the prime moverfails, the generator continues to run as a motor and takes power from bus-bars.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]Directional power protection operates in accordance with thedirection of power flow. Reverse power protection operates whenthe power direction is reversed in relation to the normal working direction.Reverse power relay is different in construction than directionalover-current relay.In directional over-current relay, the directional elementdoes not measure the magnitude of power. It senses only direction of powerflow. However, in Reverse Power Relays, the directional element measures magnitudeand direction of power flow.v Relay connections of Single Phase DirectionalOver-current Relay :The current coils in the directionalover-current relay are normally connected to asecondary of line CT. The voltage coil of directional element is connected to aline VT, having phase to phase output (of110 V). There are four common methods of connecting the relay dependingupon phase angle between current in the current coil and voltage applied to thevoltage coil.
3-Phase Directional over current relaysWhen faultcurrent can flow in both directions through the relay location, it is necessary to make the response of the relay directional bythe introduction of directional control elements. These are basically power measuring devices in which thesystem voltage is used as areference for establishing the relative direction or phase of the faultcurrent.Although powermeasuring devices in principle, they are not arranged to respond to the actualsystem power for a number of reasons:1. The power system, apart from loads, isreactive so that the fault power factor is usually low. A relay[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]location on faulted phasesVb2 and Vc2 Voltages remote from fault location Fig.12 Phase voltages for a B-C faultResponding purely to the active component would not develop a high torqueand might be much slower and less decisive than it could be.1. The system voltage must collapse at the point of shortcircuit. When the fault is single-phase, it is the particular voltage acrossthe short-circuited points which are reduced. So a B—C phase fault will cause the B and C phasevoltage vectors to move together, the locus of their ends being the originalline be for a homogeneous system, as shown in (Fig.12)At the point of fault the vectors will coincide, leaving zero voltage across the fault, but the fault voltage to earth will be half the initial phase toneutral voltage. At other points in the system the vector displacement will beless, but relays located at such points will receive voltages which areunbalanced in their value and phase position. The effect of the large unbalance in currents and voltages is to make thetorques developed by the different phase elements vary widely and even differin sign if the quantities applied to the relay are not chosen carefully. Tothis end, each phase of the relay is polarized with a voltage which will not bereduced excessively except by close three-phase faults, and which will remainin a satisfactory relationship to the current under all conditions. RelayconnectionsThis is the arrangement whereby suitable current and voltage quantities are applied to therelay. The various connections aredependent on the phase angle, at unity system power factor, by which thecurrent and voltage applied to the relay are displaced. v Relay maximum torqueThe maximum torque angle (MTA) is defined as the angle by which thecurrent applied to the relay must be displaced from the voltage applied to therelay to produce maximumtorque. Although the relay element may be inherently wattmetric, its characteristic can bevaried by the addition of phase shifting components to give maximum torque atthe required phase angle.A number of different connections have been used and these are discussedbelow. Examination of the suitability of each arrangement involves determiningthe limiting conditions of the voltage and current applied to each phaseelement of the relay, for all fault conditions, taking into account thepossible range of source and line impedances. v 30° relay connection (0° MTA)The A phase relay is supplied with current la and voltage Vac.In this case, the flux due to thevoltage coil lags the applied Vac voltage by 90°, so the maximumtorque occurs when the current lags the system phase to neutral voltage by 30°.For unity power factor and 0.5 lagging power factor the maximumtorque available is 0.866 of maximum. Also, the potential coilvoltage lags the current in the current coil by 30° and gives a tripping zonefrom 60° leading to 120° lagging currents, as shown in (Fig. 13a). The most satisfactory maximum torque angle for this connection, thatensures correct operation when used for the protection of plain feeders, is 0°,and it can be shown that a directional element having this connection and 0°MTA will provide correct discrimination for all types of faults, when appliedto plain feeders If applied to transformer feeders, however, there is a danger that atleast one of the three phase relays will operate for faults in the reversedirection; for this reason a directional element having this connection shouldnever be used to protect transformer feeders.This connection has been used widely in the past, and it is satisfactoryunder all conditions for plain feeders provided that three phase elements areemployed. When only two phase elements and an earth fault element are usedthere is a probability of failure to operate for one condition. An inter-phaseshort circuit causes two elements to be energized but for low power factors onewill receive inputs which, although correct, will produce only a poor torque.In particular a B—C fault will strongly energize the B element with lb current and Vba voltage, but the Celement will receive Ic and the collapsed Vcb voltage, which quantities have a large relativephase displacement, as shown in (Fig. 13b). This is satisfactory provided thatthree phase elements are used, but in the case of a two phase and one earthfault element relay, with the B phase element omitted, operation will depend uponthe C element, which may fail to operate if the fault is close to the relayingpoint.
v 60° No. 1 connection (0° MTA)The A phase relay is supplied with lab currentand Vac voltage. In thiscase, the flux due to the voltage coil lags the applied voltage to the relay by90°, so maximum torque is produced when the current lags the system phase toneutral voltage by 60°. This connection, which uses Vac voltage withdelta current produced by adding phase Aand phase B currentsat unity power factor, gives a current leading the voltage Vac by60°, and provides a correct directional tripping zone over a current range of30° leading to 150° lagging. The torque at unity power factor is 0.5of maximum torque and at zero power factor lagging 0.866; see(Fig.14).It has been proved that the most suitable maximum torque angle for thisrelay connection, that is, one which ensures correct directional discriminationwith the minimum risk of mal-operation when applied to either plain ortransformer feeders, is 0°.When used for the protection of plain feeders there is a slightpossibility of the element associated with the A phase mal-operating for a reversed B—C fault.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]However, although the directional element may mal-operation, it isunlikely that the over current element which the directional element controlswill receive sufficient current to cause it to operate. For this reason theconnection may be safely recommended for the protection of plain feeders.When applied to transformer feeders there is a possibility of one of thedirectional elements mal-operation for an earth fault on the star side of adelta/star transformer, remote from the relay end. For mal-operation to occur,the source impedance would have to be relatively small and have a very low angleat the same time that the arc resistance of the fault was high. The possibilityof mal-operation with this connection is very remote, for two reasons: first,in most systems the source impedance may be safely assumed to be largelyreactive, and secondly, if the arc resistance is high enough to causemal-operation of the directional element it is unlikely that the over currentelement associated with the mal-operation directional element will seesufficient current to operate.The connection, however, does suffer from the disadvantage that it isnecessary to connect the current transformers in delta, which usually precludes their being used for any otherprotective function. For this reason, and also because it offers no advantageover the 90° connection, it is rarely used. v 60°No. 2 connection (0° MTA)The A phase relay is supplied with current la and voltage In this case, the flux of the voltagecoil lags the applied voltage by 90° so the maximum torque is produced when thecurrent lags the system phase toneutral voltage by 60°. This connection gives[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]a correct directional tripping zone over the current range of 30° leading to 150° lagging.The relay torque at unity powerfactor is 0.5 of the relay maximum torque and at zero power factorlagging 0.866; see (Fig.15).The most suitable maximum torque angle for a directional element usingthis connection is 0°. However, even if this maximum torque angle is used,there is a risk of incorrect operation for all types of faults with theexception of three-phase faults. For this reason, the 60° No. 2 connection isnow never recommended.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
v 90° relay quadratureconnectionThis is the standard connection for the type CDD relay; depending on theangle by which the applied voltage is shifted to produce the relay maximumtorque angle, two types are available.
v 90°- 30° characteristic (30° MTA)The A phaserelay is supplied with lacurrent and Vbc voltage displaced by 30° in an anti-clockwisedirection. In this case, the flux due to the voltage coil lags the appliedvoltage Vbc by 60°, and the relay maximum torque is producedwhen the current lags the system phase to neutral voltage by 60°. Thisconnection gives a correct directional tripping zone over the current range of30° leading to 150° lagging; see (Fig.16).The relay torque at unity power factoris 0.5 of the relay maximum torque and at zero power factor lagging0.866. A relay designed .for quadrature connection and having amaximum torque angle of 30° is recommended when the relay is used for theprotection of plain feeders with the zero sequence source behind the relayingpoint. v 90°- 45° characteristic (45° MTA)The A phase relay is supplied with current la and voltage Vbc displaced by 45° in ananti-clockwise direction. In this case, the flux due to the voltage coil lagsthe applied voltage Vbc by 45°, and the relay maximum torque isproduced when the current lags the system phase to neutral voltage by 45°. Thisconnection gives a correct directional tripping zone over the current range of45° leading to 135° lagging. The relay torque at unity power factor is 0.707 of the maximumtorque and the same at zero power factor lagging; see (Fig.17).[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]This connectionis recommended for the protection of transformer feeders or feeders which havea zero sequence source in front of the relay. The 90°- 45° connection isessential in the case of parallel trans-formers or transformer feeders, inorder to ensure correct relay operation for faults beyond the star/ deltatransformer. This connection should also be used whenever single-phasedirectional relays are applied to a circuit Theoretically,three fault conditions can cause mal-operation of the directional element: aphase-phase ground fault on a plain feeder, a phase-ground fault on atransformer feeder with the zero sequence source in front of the relay and aphase-phase fault on a power transformer with the relay looking into the deltawinding of the transformer. It should beremembered, however, that the conditions assumed above to establish the maximumangular displacement between the current and voltage quantities at the relay,are such that, in practice, the magnitude of the current input to the relaywould be insufficient to cause the over current element to operate. It can beshown analytically that the possibility of mal-operation with the 90°-45° connection is, for all practical purposes, non-existent.
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