06_Transformerprotection

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TransformerProtection

Protection ApplicationHandbook


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Transformer protectionPower transformers in a power system

400 kV AC Transmission

Generation

130 kV Subtransmission

Distribution

M

MV

LV


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Transformer protection

General

A Power Transformer is a very valuable and vitallink in a Power Transmission system

Many sizes and types of power transformer

Transformer faults may cause substantial losses

.The repair time may be long


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General

A fast protection system for transformer isessential to minimize the damage in case of aninternal fault with suitable back-up protection

scheme to take care of uncleared system faults

Faults in a Transformer occur due to insulation

breakdown, ageing of insulation, overheating dueto over excitation, oil contamination and leakageor reduced cooling


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Different vector group connections and ways of neutral connection

Y d + z

YN d11 + zn

Y y

YN yn0 (d1)

Y d

YN d1

D y

D yn11


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Transformer protectionDifferent winding arrangements

Autotransformer+ tertiary

e.g.. YN Auto d1

2-winding transf.

+ Auxiliarytransformer


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Particular power transformer applications

HVDC

Static VARCompensator

M

Motor drive

UV

On- loadtap-changer

Generator-transformer

unit

x x

Double breaker


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Overload Capability

It is possible to overload power transformers

Older transformers may withstand 140% continuously

Overloading and loss of cooling causes overheating


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Withstand capabilty for External Faults

System Shunt Faults

Transformer

reactances

Fault current Permitted fault

duration

% multiple of ratedcurrent seconds

4 25 2

5 20 3

6 16.6 47 14.2 5


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Types of Internal Faults

Earth faults

Short-circuits

Inter turn Faults

Core Faults Tank Faults

Reduced cooling


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Abnormal Conditions

Overload

Over voltage

Reduced system voltage

Overexcitation


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Transformer protectionDifferential Protection

Classical application of differential protection

Interposing current transformers for phase shift andtransformation ratio correction

Protection zone, bushing CTs

Influence from OLTC Harmonic restrained differential protections


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Transformer protectionTransformer differential protection. Current bias characteristics

I 1

I2

Id min

1 5 10

1

5

10

I

High

Trips

Blocks

Differential current |I1+I2 |

Unrestrained

< 25

Irestrain(|I1 |+ |I2 |)/2


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Inrush Current

Inrush currents flow in connection with energisation

Inrush currents may also flow at voltage recovery

Sympathy inrush current


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Inrush Current


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Factors that Affect the Inrush Current

The source impedance

The size of the transformer

The location energized winding

The point of wave when the switch closes The magnetic properties of the core

The remanence of the core

The use of pre-insertion resistors


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Transformer protectionConsequences of Inrush Current

The inrush current has a large DC component that may

saturate the CTs

DC saturation of phase and neutral CTs may cause afalse residual current and a false differential current

There is a risk that sensitive differential protections,residual overcurrent protections and neutral pointovercurrent protections may operate incorrectly


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Magnetizing current at overexcitation


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Differential protection for

Autotransformer


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Transformer protectionGround Fault Currents


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Transformer protectionEarth Fault Protection

Restricted earth fault protection

High-impedance restricted earth fault protections Low-impedance restricted earth fault protections


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Restricted Earth fault protection


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Transformer protectionThermal overload protection

Time

0,63

Rise of temperature


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Over excitation protection

Overexcited transformers become overheatedand damaged

Especially required for Generator Transformers


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Overexcitation Protection

iE= 4.44fNABmax

f = the system frequency [Hz],

N = the number of turns on the core leg [1],

A = the area of the core leg [m2] and

Bmax = the peak value of the flux density [T]


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Over excitation Protection

(B /B) = V fV / f

n

n n

max

Bn = the rated peak value of the flux density [T], V = the RMS value of the terminal voltage [V],

Vn

= the rated RMS value of the terminal voltage [V], and

fn = the rated system frequency [Hz]


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Over excitation protection

Definite

V/Hz inverse

t

V/Hz


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Time overcurrent relays

Used on all feeding circuits of transformer

Provide back up for internal faults

Also provide back up for system faults Instantaneous high element

f


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Residual Overcurrent relays

Used on all feeding circuits of transformer

Provide back up for internal faults

Also provide back up for system faults

Instantaneous high element used

T f t ti


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Impedance Protection

Back-up protection of the local busbars

Back-up protection of the outgoing feeders

Back-up protection of the busbar at the remote end

Plain underimpedance protection/Distance protection

T f t ti


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Monitors

Monitors are very important devices which detect faults and

abnormal service conditions which may develop into fault.

T f t ti


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Oil and Gas Monitoring

Sudden gas pressure protection

Buchholz protection Oil level monitoring

T f t ti


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Temperature Monitoring

The oil thermometer

The winding thermometer

T f t ti


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Protective Relays Used ( Transformers > 5

MVA)

Gas detector relay ( Buchholz) Over load protection

Thermal relays

Temperature monitoring relays

Over current protection

Ground fault protection

Differential protection

Interturn faults

Pressure relay for tap changer

Oil level monitor



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Protective Relays Used ( Transformers IF ( RCT + 2 RL)

VK > 2 VR



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Phase overcurrent relays The pick up current seting must be set above highest possible load

current

It shall also serve as overload protection In many cases it can be considered that overload capacity is 1.4 times

rated power

The lowest setting value can be written as

I pickup = 1.2 . I max/K

It should also be able to detect all faults within the zone

I pickup < 0.7. I scmin

The high set relay should be set so as to be selective wrt other relays

I high > 1.2. K t .I scmax

K t is transient overreach and I scmax is largest fault current on lowvoltage side

Time setting should be set as to get shortest fault time with maintainedselectivity



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Failure to Reset of an Overcurrent Protection

The relay does not reset

t

I

I stI reset



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Inverse Time Characteristics Selectivity

Fault current

Time

t



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Residual over current relays

Earth fault current is fed from the transformer in case ofphase to earth fault in the system. Therefore there is aneed

to trip the transformer in case of such fault. In most cases itshould be considered as back up to line protection. Currentsetting is usually low. Time setting should be long to beselective wrt line protection.



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System Transformer with Impedance Protection

Z

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