Over-current and Earth Fault Protection *****part 1

Over-current and Earth Fault Protection
v Introduction
As the fault impedance is less thanload impedance, the fault current is more than load current. If a short circuitoccurs the circuit impedance is reduced to a low value and therefore a fault isaccompanied by large current.
Over-current protection is that protection inwhich the relay picks up when the magnitude of current exceeds the pickup level.

The basic element in Over-currentprotection is an Over-current relay.
The Over-current relays are connected to the system,normally by means of CT's.

Over-currentrelaying has following types:
1. High speed Over-current protection.
2. Definite time Over-current protection.
3. Inverse minimum time Over-current protection.
4. Directional Over-current protection (of above types).

Over-current protection includes the protection fromoverloads. This is most widely used protection. Overloading of a machine orequipment generally) means the machine is taking more current than its ratedcurrent. Hence with overloading, there is an associated temperature rise. Thepermissible temperature rise has a limit based on insulation class and materialproblems.
Over-current protection of overloads is generally provided by thermal relays.

Over-current protection includes short-circuit protection.Short circuits a be phase faults, earth faults or winding faults. Short-circuitcurrents are generally several times (5 to 20) full load current. Hence fastfault clearance is always desirable on short-circuits.

When a machine is protected by differential protection, theover-current is provided in addition as a back-up and in some cases to protectthe machine from sustained through fault.
Several protective devices are used for over-current protectionthese include:
1. Fuses
2. Circuit-breakers fitted with overloaded coils or tripped byover-current relays.
3. Series connected trip coils operating switching devices.
4. Over-current relays in conjunction with current transformers.

The primary requirements of over-currentprotection are:
· Theprotection should not operate for starting currents, permissible over-current,and current surges. To achieve this, the time delay is provided (in case ofinverse relays). If time delay cannot be permitted, high-set instantaneousrelaying is used.
· Theprotection should be coordinated with neighboring over-current protections soas to discriminate.

v Applications ofOver-current Protection
Over-current protection has a wide range of applications. Itcan be applied where there is an abrupt difference between fault current withinthe protected section and that outside the protected section and thesemagnitudes are almost constant.

The over-current protection is provided for thefollowing:
v Motor Protection
Over-current protection is the basic type of protection used against overloadsand short-circuits in stator windings of motors. Inverse time and instantaneousphase and ground over-current relays can be employed for motors above 1200 H.P.For small/medium size motors where cost of CT's and protective relays is noteconomically justified, thermal relays and HRC fuses are employed, thermalrelays used for overload protection and HRC fuses for short-circuit protection.
v Transformer Protection
Transformers are provided with over-current protection against faults, only,when the cost of differential relaying cannot be justified. However,over-current relays are provided in addition to differential relays to takecare of through faults. Temperature indicators and alarms are always providedfor large transformers.
Small transformers below 500 kVA installed in distributionsystem are generally protected by drop-out fuses, as the cost of relays pluscircuit-breakers is not generally justified Line Protection.
The lines (feeders) can be protected by
(1) Instantaneous over-current relays.
(2) Inverse time over-current relays.
(3) Directional over-current relay.
Lines can be protected by impedance or carrier currentprotection also.

Protection of Utility Equipment
The furnaces, industrial installations commercial,industrial and domestic equipment are all provided with over-currentprotection.

v Relays used inOver-current Protection
The choice of relay for over-current protection depends uponthe Time / current characteristic and other features desired. The followingrelays are used.
1. For instantaneous over-currentprotection. Attracted armature type, moving iron type,
permanent magnet moving coil type andstatic.
2. For inverse time characteristic. Electromagnetic induction type, permanent magnet
moving coil type and static.
3. Directional over-current protection.Double actuating quantity induction relay
with directional feature.
4. Static over-current relays.
5. HRC fuses, drop out fuses, etc. are used in low voltagemedium voltage
and high voltage distribution systems,generally up to 11 kV.
6. Thermal relays are used widely forover-current protection.

Not: Now Digital Numerical Relay you can used for all types

v Characteristics of relay units for over currentprotection
There is a wide variety of relay-units. These are classifiedaccording to their type and characteristics. The major characteristic includes:
1. Definite characteristic
2. Inverse characteristic
3. Extremely Inverse
4. Very Inverse

In definite characteristic, the time of operation is almostdefinite i.e.

I⁰ * T = K
Where:
I = Current in relay coil
T = Relay lime
K= Constant.
In inverse characteristic, time is inversely proportional tocurrent i.e.

I¹ * T = K
In more inverse characteristic

Iⁿ * T = K

Where n can be between 2 to 8 the choice depends ondiscrimination desired.
Instantaneous relays are those which have no intentionaltime lag sod which operate in less than 0.1 second, usually less than 0.08second. As suck they are not instantaneous in real sense.

The relays which are not instantaneous are called Time DelayRelay'. Such relays are provided with delaying means such as drag magnet, dashposs. bellows, escape mechanisms, back-stop arrangement, etc.

The operating time of a relay for a particular setting andmagnitude actuating quantity can be known from the characteristics supplied bythemanufacturer. The typical characteristics are shown in (Fig. 1)

An inverse curve is one in which the operating time; becomesless as themagnitude of the actuating quantity is increased. Howeverfor higher magnitudes of actuating quantity the time is constant. Definite timecurve is one in which operating time is little affected by magnitude ofactuating current. However even definite time relay has a characteristic whichis slightly inverse
The characteristic with definite minimum time and of inversetype is also called Inverse Definite Minimum Time (IDMT) characteristics(Fig.1).[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

(Fig.1) Inverse Definite Minimum Time (IDMT) characteristics

Principle of trip circuit
Referring to (Fig. 2) the three current transformers andrelay coils connected in star and the star point is earthed. When short circuitoccurs in the protected zone the secondary current of CT'sincreases.
These current flows through relay coils and the relaypicks-up, the relay contacts close, thereby the trip circuit is closed and thecircuit breaker-operates The over-current protection scheme with threeover-current relays (Fig. 2) responds tophase faults and earth faults including single-phase to earth fault.
Therefore such schemes are used with solidly earthed systems where phase tophase and phase to earth faults are likely to occur.
For proper functioning ofover-current and earth fault protection, the choice of CT's and polarityconnections should be correct.[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Methods of CT Connections in Over-currentProtection of 3-Phase Circuits
v Connection Scheme withThree Over-current Relays
Over-current protection can be achieved by means of threeover-current relays or by two over-current relays (See Table 1).
Table 1

	Fig	Description	Note
1	[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]	One OC with one CT for over load protection.	For balanced load only.
2	[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]	Two OC relays with two CT's for phase to phase fault protection.
3	[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]	Three OC relays with three CT's for phase to phase fault protection.	EF current > two time pick-up phase current
4	[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]	Three OC relays with three CT's for phase to phase fault protection and phase to earth fault.	EF setting less than phase fault setting
5	[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]	Two OC and one EF relays for phase to phase and phase to earth fault protection

v Earth-Fault Protection
When the fault current flows throughearth return path, the fault is calledEarth Fault. Other faults which do not involve earth are called phase faults.Since earth faults are relativelyfrequent, earth fault protection is necessaryin most cases. When separate earth fault protection is not economical,the phase relays sense the earth fault currents. However such protection lackssensitivity. Hence separate earth fault protection is generally provided. Earthfault protection senses earth fault current. Following are the method of earthfault protection.
v Connections of CT's for Earth-fault Protection
1. Residually connected Earth-fault Relay
Referring to Fig. 3 In absence of earth-fault the vector sumof three line currents is zero. Hence thevector sum of three secondary currents is also zero.
I_R+I_Y+I_B=0
The sum (I_R+I_Y+I_B) is called residual current
The earth-fault relay is connected such that the residualcurrent flows through it (Figs.3 and Fig. 4), inthe absence of earth-fault,
Therefore, the residually connected earth-fault relay does not operate.However, in presence of earth fault the conditions is disturbed and (I_R+I_Y+I_B) is no more zero. Hence flows through the earth-fault relay. If theresidual current is above the pick-up value, the earth-fault relay operates.
In the schemediscussed here the earth-fault at any location near or away from the location of CT's can causethe residual current flow. Hence the protected
zone is not definite. Such protectionis called unrestricted earth-fault protection
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Earth-fault Relay connected in Neutral to Earth Circuit(Fig. 5).
Another method of connecting anearth-fault relay is illustrated in Fig 5. The relay is connected to secondary of a CT whoseprimary is connected in neutral to earth connection. Such protection can be provided atvarious voltage levels by connecting earth-fault relay in the neutral-to-earthconnection of that voltage level. The fault current finds the return paththrough the earth and then flows through the neutral-to-earth connected. The magnitude of earth fault current isdependent on type of earthing (resistance, reactance or solid) and location of fault. In this typeof protection,
The zone of protection cannot be accurately defined. Theprotected area is not restricted to the transformer/generator winding alone.The relay senses the earth faults
beyond the transformer/generator winding hence such protection is calledunrestricted earth-fault protection. The earth-fault protection by relay inneutral to earth circuit depends upon the type of neutral Earthing. In case oflarge generators, voltage transformer is connected between neutral and earth
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

v Combined Earth-fault and Phase-fault Protection
It is convenient to incorporate phase-fault relays and earth-fault relay ina combined phase-fault and earth-fault protection. (Fig. 4) The increase incurrent of phase causes corresponding increase in respective secondarycurrents. The secondary current flows through respective relay-units Very often only two-phase relays are provided insteadof three, because in case ofphase faults current in any at least two phases must increase. Hence tworelay-units are enough.

v Earth-fault Protectionwith Core Balance Current Transformers. (Zero Sequence CT)
In this type ofprotection (Fig. 6) a single ring shaped core of magnetic material, encirclesthe conductors of all the three phases. A secondary coil is connected to arelay unit. The cross-section of ring-core is
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Ample, so that saturation is not a problem. Duringno-earth-fault condition, the components of fluxes due to the fields of threeconductors are balanced and the secondary current is negligible. During earthfaults, such a balance is disturbed andcurrent is induced in the secondary. Core-balance protection can beconveniently used for protection of low-voltage and medium voltage systems. The burden of relays and exciting currentare deciding factors. Very large cross-section of core is necessary forsensitivity less than 10 A. This form of protection is likely to be morepopular with static relays due to the fewer burdens of the latter.Instantaneous relay unit is generally used with core balance schemes.
v Theoryof Core Balance CT
. Let I_a, I_b andI_c, be the threeline currents and Φ_a, Φ_b and Φ_c be corresponding components of magneticflux in the core. Assuming linearity, we getresultant flux Φ as,
Φ=k (I_a + I_b + I_c)
where k is a constant Φ= K * I_a_. Referring to theory of symmetrical components
(I_a+ I_b + I_c)=3I_c=I_n
Where, I_ois zero sequence current and In, iscurrent in neutral to ground circuit.During normal condition, when earth fault is absent,
(I_a+ I_b + Ic) =0
Hence Φr = 0 and relay does not operate
During earth fault the earth fault current flows throughreturn neutral path.
For example for single line ground fault,
I_f = 3I_ao= I_n
Hence the zero-sequence component of I_o produces the resultant flux Φr in the core.Hence core balance current transformer is also called as zero sequence currenttransformers (ZSCT).

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» Over-current and Earth Fault Protection *****part 4
» Basic of protection system *****part 2
» Basic of protection system *****part 3