DISTRIBUTION FEEDER PROTECTION

FUSE PROTECTIONMaterial used:

TinLeadZincSilverAntimonyCopperAluminum

Fuse Law

Heat generated = I2.R = I2.( ℓ. )

Again heat lost ∞ Surface area of the fuse wire

= K1.∏.d.lHence,

K1.∏.d.l = I2.( ℓ. ) or, I ∞ d 3/2

Where, I = current through the fuse wireℓ= resistivity of the fuse materiald = diameter of the fuse wireL = length of the fuse wireK1= parameter constant

Operating characteristics of fuse

CB short circuit current carrying capacity

>Operating current of fuse

>Max over-load current

Tim

e in s

ec

Current in Amp

Current chopping by FuseCurrent

Time

Ichopping

T1 = Melting Time

T2 = Arcing Time

T = Total operating time

Hence, T = T1 + T2

T1

T2

T = T1 + T2

Fault current without fuse

Operating Principle of fuse

Load

Fuse

Source

Series trip When,

Over-load current or fault current > Fuse rating

Fuse will be burnt and load will be OFF

Operating Principle of fuse

Load

Fuse

Shunt tripWhen,

CT secondary current> Fuse rating

Fuse will be burnt and CT secondary current will flow through trip coil and CB will trip.

TC

CLASSIFICATION OF FUSES

Fuses are classified on the basis of

Continuous current carrying capacityOverload capacityResponse characteristicsVoltage rating

Fuses in power system

Use of fuses

Use of fuses

MotorAir-conditionerLaptopCell-phonePrinters

Overcurrent Relay

Load

TC

Relay

Battery

CB

CT Secondary current exceeds relay setting current

Relay NO contact will make according to relay characteristics.

Trip coil will get energized Circuit breaker will trip.

O/C relay Vs. Fuse Protection O/C relay is more accurate and versatile than Fuse

protection

O/C relay is more costly and more complicated than fuse.

Time gradation is difficult in case of fuse protection

Once fuse melts protection cannot be re-commissioned until it is replaced.

Unlike relay, fuse characteristics is temperature dependent.

Types of Overcurrent Relay

B

RN

1. Two-overcurrent & one earth fault relay

Overcurrent Relay2. Three-overcurrent & one-earth fault relay

BYR

N

Overcurrent Relay setting Overcurrent relay current setting = 125% of

nominal load current

Under 3-ph balanced load condition, Earth Fault relay will sense zero current. Thus this relay has to be very sensitive.

Earth fault current setting = 20% of rated current

Phase-Earth short circuit fault

B

RN

1. Two-overcurrent & one earth fault relay

2. Three-overcurrent & one-earth fault relay

BYR

N

E/F relay will operate in both the cases

Phase-Phase short circuit fault

BYR

N

1. Two-overcurrent & one earth fault relay

2. Three-overcurrent & one-earth fault relay

B

RN

Only R-ph O/C relay will operate

Both R & Y-ph O/C relay will operate

Operating principle of O/C relay

Plug setting multiplier (PSM) =

Relay only operates when PSM > 1Let t = operating time from relay characteristicsHence, Relay operating time = t x TMS

Where TMS = time multiplier setting

Operating characteristics of O/C relay

Operating time for different O/C relay characteristics

Let, t = Relay operating time. Then for PSM > 1,

t = 0 for Instantaneous overcurrent relay

= constant for Definite time overcurrent relay

= TMS . for IEC standard inverse curve

= TMS . for IEC very inverse curve

= TMS . for IEC extreme inverse curve

Numerical O/C relay

Now-a-days numerical relay has inbuilt three overcurrent and one earth fault element.

BYR

N

Numerical relay

Typical algorithm of Numerical O/C relay

Protection

healthy

Iinput

Calculate

Irms

Compute PSM

=Iinput/Irms

If PSM>

1

Start timer

T1

If Type

= DT

DT delay top

If Type= SI

Compute top

If Type= VI

Compute top

If Type= EI

Compute top

If Top>T1

Alarm

Trip

yes

yes yes yes

yes

yes

yes

no

no

no

no

no

no

Less wear and tear

Self supervision facility is available

Setting range is comparatively wide

Choice of operating characteristics is possible

Relay resetting time is adjustable

Advantages of Numerical O/C relay over electromechanical relay

Additional facilities of Numerical relay

Waveform record and post-fault analysis

Remote communication and time synchronization

Over-voltage and under-voltage protection

Over-frequency and under-frequency protection

Trip circuit supervision

CT supervision

VT supervision

Typical waveform recorded by Numerical relay

Case Study

Incomer

Out-going Feeder-1

Out-going Feeder-2

Out-going Feeder-3

Assume out-going feeders are radial.For electromechanical relay let T1 = Operating time of the incomer relayT2 = Resetting time of the incomer relay

t1 = Operating time of the feeder relayt2 = Resetting time of the feeder relay

And T1 > t1

For numerical relay T2, t2 = 0

51

51

51

51

Case Study

Incomer

Out-going Feeder-1

Out-going Feeder-2

Out-going Feeder-3

At t = 0

51

51

51

51

Incomer

Out-going Feeder-1

Out-going Feeder-2

Out-going Feeder-3

51

51

51

51

At t = t1

For Electro-mechanical relay

Case Study

Incomer

Out-going Feeder-1

Out-going Feeder-2

Out-going Feeder-3

At t = t1 + Δt

51

51

51

51

Incomer

Out-going Feeder-1

Out-going Feeder-2

Out-going Feeder-3

51

51

51

51

If T1 < (2.t1-Δt)

For Electro-mechanical relay

Case Study

Incomer

Out-going Feeder-1

Out-going Feeder-2

Out-going Feeder-3

At t = 0

51

51

51

51

Incomer

Out-going Feeder-1

Out-going Feeder-2

Out-going Feeder-3

51

51

51

51

At t = t1

For Numerical relay

Case Study

Incomer

Out-going Feeder-1

Out-going Feeder-2

Out-going Feeder-3

At t = t1 + Δt

51

51

51

51

Incomer

Out-going Feeder-1

Out-going Feeder-2

Out-going Feeder-3

51

51

51

51

At t = 2.t1 + Δt

For Numerical relay

Case StudyFor Numerical relayFor Electro-mechanical relay

time

time

time

An

gu

lar

dis

pla

cem

en

t of

dis

c

T1T2

t1t2

Incomer

Feeder-1

Feeder-2

Δt

(T1+ 2Δt)

(2t1+ Δt)

time

time

time

An

gu

lar

dis

pla

cem

en

t of

dis

c

T1

t1

Δt

t1

T1

IDMT grading for a typical system

132kV Incomer-1

132kV Incomer-2

132kV Bus bar

33kV Bus bar

6kV Bus bar

75MVA Tr.-2 Zp.u.=0.2

75MVA Tr.-1 Zp.u.=0.2

20MVA Tr.-1 Zp.u.=0.6

20MVA Tr.-1 Zp.u.=0.6

Out-going feeder

Out-going feeder

B/S

B/S

B/S

I11,T11I11,T11

I12,T12

I13,T13I13,T13

I21,T21

I22,T22

I23,T23

I21,T21

I23,T23

I31,T31 I31,T31

I32,T32

I33,T33 I33,T33

Current setting= I**, TMS=T**

Let base MVA=100

IDMT grading for a typical systemConsidering CB short circuit current rating,

• T11 > T12 > T13

• T21 > T22 > T23

• T31 > T32 > T33Let MVA base = 100MVA Considering 132kV as infinite bus and neglecting cable impedance

Fault MVA contribution by 75MVA Tr. = = 500MVAFault MVA at 33kV Bus bar = 2x500 = 1000MVA

Fault MVA contribution at 6kV bus bar = 100/ ( ) = 250MVA

IDMT grading for different faults

132kV Incomer-1

132kV Incomer-2

132kV Bus bar

33kV Bus bar

6kV Bus bar

75MVA Tr.-2

75MVA Tr.-1

20MVA Tr.-1

20MVA Tr.-1

Out-going feeder

Out-going feeder

B/S

B/S

B/S

Case-1: Fault at 132kV bushing of 75MVA Tr.(T-op) 132kv incomer > (T-op) 132kV B/S > (T-op) 132 kV of

75MVA Tr,

IDMT grading for different faults

132kV Incomer-1

132kV Incomer-2

132kV Bus bar

33kV Bus bar

6kV Bus bar

75MVA Tr.-2

75MVA Tr.-1

20MVA Tr.-1

20MVA Tr.-1

Out-going feeder

Out-going feeder

B/S

B/S

B/S

Case-2: Fault at 33kV Bus bar(T-op) 132kv incomer > (T-op) 132kV B/S > (T-op) 132 kV of

75MVA Tr,

And (T-op) 33kv of 75MVA Tr. > (T-op) 33kV B/S

IDMT grading for different faults

132kV Incomer-1

132kV Incomer-2

132kV Bus bar

33kV Bus bar

6kV Bus bar

75MVA Tr.-2

75MVA Tr.-1

20MVA Tr.-1

20MVA Tr.-1

Out-going feeder

Out-going feeder

B/S

B/S

B/S

Case-3: Fault at 33kV bushing of 20MVA Tr.(T-op)33kV of 75MVA Tr. > (T-op)33kV B/S > (T-op) 33kV of

20MVA Tr.

IDMT grading for different faults

132kV Incomer-1

132kV Incomer-2

132kV Bus bar

33kV Bus bar

6kV Bus bar

75MVA Tr.-2

75MVA Tr.-1

20MVA Tr.-1

20MVA Tr.-1

Out-going feeder

Out-going feeder

B/S

B/S

B/S

Case-4: Fault at 6kV Bus bar(T-op)33kV of 75MVA Tr. > (T-op)33kV B/S > (T-op) 33kV of

20MVA Tr And (T-op) 6kv of 20MVA Tr. > (T-op) 6kV B/S

IDMT grading for different faults

132kV Incomer-1

132kV Incomer-2

132kV Bus bar

33kV Bus bar

6kV Bus bar

75MVA Tr.-2

75MVA Tr.-1

20MVA Tr.-1

20MVA Tr.-1

Out-going feeder

Out-going feeder

B/S

B/S

B/S

Case-5: Fault at 6kV outgoing feeder(T-op)6kV of 20MVA Tr. > (T-op)6kV B/S > (T-op)6kV outgoing

feeder

Directional O/C & E/F relay

Normal direction(forward) of power flow

51

67

51= Non-directional O/C Relay67= Directional O/C Relay

Trip Trip

This relay operates only when fault occurs in a particular direction.For example,1. Parallel interconnector: Sink end CB of the faulty interconnector will trip through Directional relay operation since it is sensing reverse current.

Directional O/C & E/F relay

Normal direction(forward) of power flow

51

67

51= Non-directional O/C Relay67= Directional O/C Relay

Trip Trip

2. Parallel Transformer feeder : LV side CB of the faulty transformer feeder will trip through directional overcurrent relay operation.

Directional O/C & E/F relay

Normal direction(forward) of 30MW power flow

Lock-out

relay

3. Islanding scheme: By the directional relay operation synchronization get lost Lock-out relay operates Two systems get isolated with part of the load is shed.

LoadLoad

Load

100MW

100MW

30MW50MW

80MW 51

Trip 67

Trip

Trip

System-1 System-2

Operating Principle of Directional O/C & E/F relay

It requires both voltage and current inputs. Under normal operating condition residual voltage and current will be zero.Va + Vb + Vc = 0Ia + Ib + Ic = 0

BYR

NDirectional O/C & E/F Relay

Operating Principle of Directional O/C & E/F relay

During any fault a voltage phasor is taken as reference. The position of the fault current phasor w.r.t. the voltage phasor determines the direction of fault – Forward or reverse.

Vref

α

Max. ‘+’ torque line

Max. ‘-’ torque line

Forward zone

Reverse zone

If

α = Max. Torque angle along which relay is max. sensitive

Operating Principle of Directional O/C & E/F relay

In general Vref is cross polarized. For example,

Type of fault Voltage reference

Neutral current V0

R-Phase fault VYB

Y-Phase to B-Phase fault VRN

3-Phase short circuit fault Memorized voltage (Numerical relay)

Switch ON to 3-Phase Short circuit fault

Non-directional behavior

Operating Principle of Directional O/C & E/F relay

Quadrature Connection: In case of numerical relay Vref is rotated through an angle (δ) w.r.t. which ±90⁰ is the forward direction.

Type of faults Value of the angle δ

Phase – Phase fault -90⁰

Phase – Earth fault +90⁰

Operating Principle of Directional O/C & E/F relay

Phase-Phase fault: Let phase ‘Y’ gets shorted with phase ‘B’. Then phase reference will be VRN . For line side CT neutral (say) forward direction of operation is shown.

Directional O/C &

E/F Relay

VR

N

δ = -90⁰

Forward zone

If =IY = -IB

If

VYNVBN

-VBN

VYB

VRef

Reverse zone

Operating Principle of Directional O/C & E/F relay

Phase-Earth fault: Let phase ‘R’ gets shorted with earth. Then phase reference will be VYB . For bus side CT neutral forward direction of operation is shown.

Directional O/C &

E/F Relay

VR

N

δ = +90⁰

Forward zone

If =IR

IfVYNVBN

-VBN

VYB

VRef

Reverse zone

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