REVIEW NOTE ON THE SELECTION

OF PROTECTION SCHEMES OF

POWER TRANSFORMER

1

P.K.Pattanaik

OPTCL- Odisha

OBJECTIVES : Focus on the Best Practice on Protection

Scheme of Transformer.

Out Line ……

1. Notes on Present Protection Scheme in vogue.

2. Review on the Protection Scheme.

3. Bridging the GAP by…

The Choice of SENSORS (CT/ PTs)

The Choice of Primary Relays ( Characteristics, Concepts and

Logics)

The selection of LOCKOUT schemes.

4. Recommendations

5. Conclusion

Notes on Saturation Curve for CT

A

C

B

Different Zone CT

A:- Ankle Zone

B:- Linearity Zone

C: Saturation Zone

Ex

Voltage

Exciting Current ( Ie )

Selection of Sensors (CT/PTs)

Parameters Present Scheme/ Practice Technical Note on

Recommendations

Choice of CTR (Max load current + 25% extra) /

(Rated short time current for 1

sec) / 150.

Lesser between two.

2-2.5 times of the rated

current or higher depending

upon the consideration of

current calculation and

workable linearity zone of

characteristics curve.

Choice of CT

Sec Drive

Voltage

Va= ISF*(Rct + RL+ Rr ),

Vd= Burden x ALF /Is

(Use of Burden with ALF Core)

(Vd > Va is Choice)

The core with Burden and ALF

is not to be used to avoid

limitation of Driving Voltage.

Choice of Knee

point Voltage

(VK) and

Excitation

Current.

V0 = IR ( RCT+RL) and

VK (min) = 2*IF ( RCT+RL)

(Vk> V0 is the choice)

Iex at Vk/2, Vk/4 or at Vk

1. Vk and Iex to be selected

similar for both balance

CTs.

2. Iex <25mA at Vk/4 ( 1Amp)

3. Iex <100 mA at Vk/4 ( 5Amp)

Selection of Sensors (CT/PTs) Contd…

Parameters Present Scheme/ Practice Technical Note on

Recommendations

Choice of

PT/CVT

PT: Inductive path across the

Transformer.

CVT: Capacitive path across the

Transformer.

Use of PT and selection of BUS

PT .

CVT in the circuit to be

avoided ( For Ferro-resonance

effect)

Choice of

Accuracy class

and Burden

No Burden consideration.

Choice with Higher Burden

Burden to be selected lesser

due to use of modern

Numerical relays in the

scheme ( Limited to Max. 30

VA)

Choice of Lead

Resistance

VD = (Burden x RL)/ Voltage,

Considering utilization of full

Burden.

RL = Lead Resistance to be

selected as low as possible

Best use 2x2.5 sqmm cable.

Or Digital conversion with use

of Numerical Relay.

Positioning of CT/PTs …

CT position LA

1. LCT to be used with LA inside the

transformer

2. For the case of TURRET BUSHING CT,

High set element on Protection scheme to

be used.

Selection of CT

Selection of PT

1. Bus PT to be selected as per dis-connector on the bus.

2. Voltage activated Relays to decide the quality of Supply before energisation

of the Transformer.

PROTECTION RELAY SENSITIVITY

Star Connection with Resistive

earthingStar Connection with SOLID

Ground

Fault on DELTA Connected

Winding

Pattern Sensitivity of

Relay

Relay selection

STAR

SOLID

EARTH

REF more

sensitive than

Differential

Both Selection

(REF does not cover

PH-PH fault.)

STAR

RESISTANCE

EARTH

REF is far

sensitive than

Differential

Both Selection

(REF does not cover

PH-PH fault.)

DELTA Differential with

instant OC

relay.

Differential + OC

instant

8

Setting Review of Protection Relay

1. Notes on 2nd Harmonics Effect

2. Notes on 5th Harmonics Effect

3. Notes on CTR, Voltage Tap and others

4. Notes on Selection Practices

Setting Review on Relays

Parameters Present

Scheme/

Practice

Technical Note on Recommendations

2nd Harmonics

Effect.

(Inrush Effect)

Use of 2nd Harm.

Blocking logic in

Relays

Use of CSD with special logic of

individual threshold closure as per adaptive

method with integrated logic

5th Harmonics

(Over

Excitation)

Use of 5th Harm.

Blocking logic

with V/f relay.

1. Use of V/f relay prior feeding from PT

supply.

2. Feature with ( Ph-Ph supply), IDMT

curve

3. Reset /Pickup ≥ 98.5%4. 5th Harm. Blocking for both magnitude and

angle)

CTR /Tap/

Others

Biasing factor ≤ 30

% Biasing factor ≤ 25 %

Selection of Relay Characteristics

Differential Relay

1. High Impedance Relay : Relay with High Resistive coil with Operating Coil

and has no absolute Boundary

2. Low Impedance Relay: Relay has Restraint Coil and Operating Coil and

has selection boundary between Operating and Blocking

Recommendation

Selection of 3rd

drooping slope

Present Practice with DUAL SLOPE.

Our recommendation to have TRIPPLE

SLOPE in co-ordination of HIGHSET

instant trip.

Selection of Relay Characteristics

REF Relay

1. High Impedance Relay : General practice of all utilities due to its stability

and sensitivity.

2. Low Impedance Relay: Rare selection due to its boundary threshold for

tripping and blocking.

Recommendation

1. To be used with HIGH

IMPEDANCE Relay (

with judicious

selection of Stabilizing

Resistance, position

of IPCT and type of

NCT ).

2. NCT to be selected of

Lower ratio for more

sensitivity

Selection of Back up Relay

OC+EF Relay

1. Directional Feature ( 67 ): Polarized relay and actuates for development of

Zero sequence Voltage and current .

2. Non-Directional Relay (51): On the principle of Zero Sequence availability

Recommendation

Should be used with

NON-DIRECTIONAL TYPE

( 51) to meet all situations

like UNBALANCE

SUPPLY

Y phase jumper opening

Review on Mechanical Relays

Parameters Present Scheme/ Practice Technical Note on

Recommendations

Temp. DEVICE 1.Weighted Ambient 32 deg. C

2. Max. Amb. 45 deg C

3. Mercury Float

1.Weighted Ambient 30-35 deg. C

2. Max. Amb. 45-50 deg C

3. Reed type micro switch

4. Selection OTI (Al 80-85°C,

Trip 85-90°C) and for winding

(Al 90-95°C, Trip 95-100°C).

Pressure

Device

1. (0.32kg/cm2/sec as ALARM

and 0.6kg/cm2/sec for

TRIP) for under oil device.

2. Oil flow of 0.85-1.15 m/s for

Inside Oil device ( TRIP)

1. Both UNDER and INSIDE OIL

device to be provided.

2. TRIPPING BLOCKING ON OC

relay actuation.

GAS activated

Device

1. Use of Mercury Float

BUCH Relay of Single

Relay.

2. Setting 100-280 CC on the

basis of Rating.

1. Reed type micro switch

2. Double BUCH relay in

gradation.

3. Setting on Gas

accumulation ( 100 cc AL and

200CC for TRIP)

Review on Mechanical Relays ( Contd..

Parameters Present Scheme/ Practice Technical Note on

Recommendations

AIR CELL

Device

1. Air Cell Ballooning System

Available.

2. But MONITORING DEVICE

is not compulsory.

AIR CELL PROTECTIVE

RELAY must be provided

Environmental

Factors

No such Device presently

used

Provided with TRANSFORMER

MANAGEMENT RELAY ( Monitors

all parameters being provided

with sensors)

OTHER MODERN DEVICES

15

Notes on TRIPPING SYSTEM

Notes on GROUNDING SYSTEM Parameters Present Scheme/ Practice Technical Note on

Recommendations

Body Double RUN to Earth Mat after

separate Earth PIT

Quadruple RUN from each side of

TFR to EARTH MAT after separate

mat.

NEUTRAL 1.Single/ Double Point with

separate Earth Pit. ( Some Utility)

2. Separate Earth Pit then MAT

TRIPOD/ QUADRUPLE Earth

Pit again connection to MAT.

LA Separate Earth PIT no connection

to MATTRIPOD/ QUADRUPLE Earth

Pit again connection to MAT.

16

TAKE AWAY----

1. We recommend for the CHANGE in PROTECTION SCHEME

2. Because We are becoming SMART and our Transformer is

becoming SMART but Abnormality is becoming SMART

Enough……….

“If we don’t strive for CHANGE ,

then CHANGE may drive us away”