Differential protection
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Differential protection : (protective relaying by Blackburn).

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Differential protection : (protective relaying by Blackburn)

Differential principle is applicable to all part of the power system: generation, motor, buses, transformer, line capacitor, reactor and some times combination of these. During normal condition or external fault the sum of the current flowing into the relays circuit is almost zero .However in internal fault condition the net current into the relay circuit is not zero. For example IP is the primary current is shown in fig (6.1)

To reduce this voltage a non linear resistor should be imposed, this resistor increase the absorbance of current 32 times, if the voltage is doubled, eg.

Voltage across relay current in non-linear resistor

120 0.01

240 0.32

480 10.24

600 30

  • Relay Types ( GEC textbook )

  • Plain Impedance Relay ( non-unit )

  • The relay impedance characteristic is a circle when it plotted on R-X diagram with its center at the origin, became the relay does not take into account the phase angle between voltage and current applied to it Fig(11.5) it is a non directional so that it can operate for all faults a long the vector

  • GL & GM . it has three main disadvantages

  • It is non directional and it requires a directional element

  • It is affected by arc resistance

  • It is highly affected by power swings because of the large area covered by the impedance circle

Mho Relay:

The characteristic of this relay is a circle, whose circumference passes through the origin, when it is plotted on R/X diagram fig (11.10.b) showing the relay is inherently directional and will operate only for a fault in the forward direction. The relay is adjusted be setting Zreach (Zn) and Ø, the angle of displacement of the diameter from Raxis. Angle Ø is known as relay characteristic angle (RCA). The relay operate for Zf within the circle. The mho relay characteristic can be obtained by using a phase comparator circuit which compares input signals S2 and S1 and operates whenever S2 lags S1 by between 90° and 270° [ ( θs1 – θs2 ) between 90° and 270° ]

The two input signals are:

S2 = V – I * Zn

S1 = V

Where: V = fault voltage from VT secondary winding.

I = fault current from CT secondary winding.

Zn = impedance setting of distance relay.


Reactance relay measures only the line reactance and does not vary with the presence of arc resistance. Fig (11-12) shows that any increase in arc resistance will not change the value of the reactance. reach as the relay continue to measure the value of reactance X.

Distance Protection ( Electrical Power System ) :

The basic idea of using distance protection , is to eliminate the pilot . Since the pilot can be used up to 30 km in route length . for a feeder shown in Fig (101) , the line (A-B) is to be protected by Distance relay , which located at bus (A) .

Distance relay compares system voltage to system current presented to it to find out the voltage as the VT primary side :

example : A distance relay , which located at A ( fig below) , protects line AB and BC . the minimum source MVA (3-ø fault level ) at A is 1500 MVA reglect R . The relay characteristic angle 60 . calculated the setting of the relay to give a reach of 80% at stage 1 of AB .

Solution :

Maximum Xs = ((132)/(3)^(1/3)) / (1500/((3)^(1/3)*132))=132^2/1500

= j 11.62 ohm / ph (primary)

Reach of stage 1

= 0.8 * 0.435 * 96.5 = 33.6L70 ohm / ph (primary) = 11.48+j31.6 ohm / ph

Total fault impedance = 11.48+j31.6+j11.26 = 44.6L71.8 ohm ohm/ph

Minimum fault current = (132/(3)^(1/3))/44.6 = 1710 A (primary) = 1710*0/600 = 2.8 A(sec)

Line voltage at relaying point = ((3)^(1/3)) * 1710 * 33.6 = 99516 V = 99516 * (110/132000)=82.9 v (sec)

Reach of compensated impedance= ((32.9) / (((3)^(1/3))*2.85) = 16.8 ohm/ph (scenery)


Reach of compensated impedance 33.6 * CT ratio/VT ratio = 16.8ohm/ph

Relay Setting = relay reach / cost(70-60)

= 17ohm/phase (Typical relay setting range from 320 ohm)

Electrical power system by guile and pates
Electrical power system by guile and pates

Quadrilateral Relay:

Quadrilateral relay combines the advantage of reactance with directional relay and resistive reach control. Characteristic It is applied for earth fault protection of short and medium lines where high fault resistance tolerance is required. Fig(11.13)

Transformer Protection Scheme:

Transformer are costly units. Thus, protection is essential to prevent damages might be caused due to internal or external faults. The types of protective relays are summarized below.

Differential Protection:

This type of protection is only applied to large transformers.

Buchholz Protection:

The Buchholz relay is the best device available for detecting incipient (primary) faults and is specially sensitive to interterm faults. This relay will operate for one of the reason:

1 – The introduction of air during filling or because of mechanical failure of the oil system.

2 – Gas produced by the breakdown of the oil.

3 – Gas produced by the breakdown of the solid insulation.

The gases produced are mainly hydrogen and carbon monoxide. If the quantity is greater than 1% then sort of action must be taken. The analysis of the gases may be as follow;

Directional over-current relay

This type of over-current is usually mounted at the secondary side of the transformer

Highest instantaneous relay

It comes with IDMT over-current relay and they mounted at the primary side of the transformer. This relay detects and operates for the primary faults only since the relay setting is above the maximum of secondary fault level.

Restricted earth fault relay

Usually in transformers, earth fault current is limited by the inclusion of an earthing resistor. The effective operation level of a restricted earth fault relay is normally less than 25% of the resistor rating.

Standby earth fault relay:

This relay has a long time delay and it regarded as a last line of defense and intend to trip only in the event of a sustained earth fault condition.

Overload protection:

This type of protection is used for large transformers fitted with oil and winding temperature indicators

1- If the gas is mainly hydrogen with less than 2 % carbon monoxide then the fault is likely involve only the insulation oil.

2- If the gas is hydrogen with about 20 % carbon monoxide then the fault is concerned with both solid insulation and insulating oil

Balanced Earth Fault Relay:-For secondary earth fault on secondary side the current on delta side is equal and opposite in two phases and therefore the output to the relay will be zero. Thus, the relay will operate only for single earth fault on the delta side.

Generator Protection SchemeGenerator protection requires immediate disconnection due to insulation failures. Other faults may be allowed for some time due to unsatis factory operating conditions.

Insulation Failure:

Stator Protection:

High impedance differential relay is usual for stator protection and is applied on phase by phase basis

Earth – Fault Protection :

Earth fault protection by an over current relay is essential to compliment the differential protection scheme and to provide a back-up protection for the differential relay. When the generator is directly connected to the power system i.e- without generator – transformer, it provides a back-up protection for the bus bars and the whole system. In this case it should have a very long time delay and should be thought of as the last line of defense.

Rotor Earth Fault Protection :

Earth fault an the rotor will not cause any current to flow to earth and does not , therefore , constitute a dangerous condition . It a second fault happened , a portion of the field winding will be short circuited and resulting in an unbalanced magnetic pull on the rotor. Thus , earth fault protection is essential.

Unsatisfactory Operation Condition :

The conditions in general does not require immediate disconnection .

Unbalanced loading :

Unbalanced loading of the generator phases results in the production of negative phase sequence ( NPS ) currents.

These currents will have a phase rotation in the opposite direction to the normal phase rotation, produces a magnetic field which induces currents in the rotor at twice the system frequency. Time will result in considerable heating in the rotor and would cause damage if allowed to persist.


Overload protection can be achieved by embedded a thermometer in the stator winding. Overload relay operates over hundreds to thousands range where an over current relay operates in the one-ten second range.

Failure of prime mover:

In the failure of prime mover, the generator continues to run, but as a synchronous motor and this can cause dangerous condition in the prime mover. To prevent this, a reverse power relay should be applied.

Loss of Field:

Failure of the field system results in acceleration of the rotor to above synchronous speed where it continue to generate power as an induction generator. This condition can be protected by undercurrent relay.

Over speed:

The rotor speed is controlled by the governor and steam valve. A sensitive under power relay is used to detect when rotor is over speed.


Voltage is generally controlled by a high-speed voltage regulator. An instantaneous relay set to 150% is used to cater for defective operation of voltage regulator.

Protection of Generator/Transformer units:

This can be achieved biased differential protection.

مع تحيات فريقالعمل كتاب المحاضرات:

بدر الغامدي خالد المحمادي

أحمد القرني عبدالحميدفقية

رامي النهاري عبدالعزيزالغامدي

اشراف الدكتور: عبدالرحمن المسعود