HANIFTRANSFOFRMER PROTECTION.pdf

TRANSFORMER PROTECTION

Extent of damage

Fault level Duration of fault

Winding failures Voltage regulating load changers Transformer faults

Transformer bushing failure Transformer core problem Miscellaneous failures

Insulation breakdown Time Improve cooling system of possible

Ageing of insulation Temperature Condition leading to faults

Over heating due to excitation Oil contamination% & oil leakage Reduced cooling

Reduced load

FAULT OF TRANSFORMER

Earth fault on H.V external connection Phases to phase fault on H.V external connection Internal earth fault on H.V windings Internal phase to phase fault on H.V windings. Short circuit between turns L.V windings. Earth fault on L.V external winding Phase to phase fault on L.V external connection. Internal earth fault on L.V windings Internal phase to phase fault on L.V winding Short circuit b/w turn L.V windings Earth fault on tertiary windings Short circuit b/W turns tertiary windings Sustained system phase to phase fault Sustained system earth fault

Differential relay

L.V side three phase over current and earth fault relay 132KV/11KV POWER TRANSFORMER

H.V side three phases over current and earth fault relay

D.C trip circuit supervision relay

Trip and lock out relay

Percentage biased Transformer differential relay

High REF impedance Circulating current differential protection

HV side over current and earth fault relay

LV side over current and earth fault relay

Over excitation relay 220/132/11KV AUTO TRANSFORMER Thermal over load relay

Overall percentage biased differential relay

L.H&H.V Connection circulating protection

Tertiary over current protection

D.C trip circuit supervision relay

Percentage biased Transformer differential relay High REF impedance Circulating current differential protection HV side over current and earth fault relay LV side over current and earth fault relay

Over excitation relay

500/220KV AUTO TRANSFORMER

Thermal over load relay

Overall percentage biased differential relay L.H&H.V Connection circulating protection

Tertiary over current protection

D.C trip circuit supervision relay

Over voltage relay

Transformer buchhloz relay Tap changer buchhloz relay MECHANICAL PROTECTION:

Winding Temperature 0IL temperature Pressure relieve valve

· Factors: · The shape, magnitude and duration of the inrush current depend on the factors: · Size of power transformer · Source Impendence · The magnetic properties of the core i.e. saturation density · The remanence of core · Resistance in power system from source to transformer. · The moment when transformer is switch on.

Effect of magnetising current  Appears on one side of transformer only  Seen as fault by differential relay  Normal steady state magnetising current is less than relay setting  Transient magnetising inrush could cause relay to operate

· The vector group shows the connection of windings of transformer and numerical index (hour numbers) for displacement of vector of two star voltages. · Capital Letter DY11 Small letter ( clock dial reference) · The first capital letter donates the connection of high voltage winding of transformer · The small letter represent the connection of low voltage secondary winding of transformer

· Yy0d5 · · The first capital letter Y is referred to H.V or primary winding, the second letter y is referred as secondary winding and third letter is referred as tertiary winding. · Primary winding is taken as phase referred ‘O’ means that phase angle b/W H.V and M.V winding is zero. Whereas ‘5’ denotes that phase angle b/W H.V and tertiary winding is 150 (5x30)

OVER CURRENT PROTECTION As it names implies, relay will pick up when it exceeds its present value TYPES: The types of over current relay are based on the relay characteristics over can be classified into three groups. · Definite current or instantaneous · Definite time · Inverse time

Over current Relay Applied to a Transformer 51

51

51

HV2

HV1

LV

HV1

HV2

Time LV

IF(LV) IF(HV) 1.2IF(LV)

Current

Use of Instantaneous Over current Protection Source

LV

50 51

Differential Protection · Overall differential protection may be justified for larger transformers (generally > 5MVA). ·

Provides fast operation on any winding

· Measuring principle: · Based on the same circulating current principle as the restricted earth fault protection · However, it employs the biasing technique, to maintain stability for heavy thro’ fault current · Biasing allows mismatch between CT outputs. · It is essential for transformers with tap changing facility. · Another important requirement of transformer differential Protection is immunity to magnetizing inrush current.

PROTECTED ZONE HV

LV

R

·Correct application of differential protection requires CT ratio and winding connections to match those of transformer. ·CT secondary circuit should be a“replica” of primary system. · Consider : · (1) Difference in current magnitude · (2) Phase shift · (3) Zero sequence currents

Biased Differential Scheme Differential Current I1 BIAS OPERATE

BIAS I2

I1 - I2

OPERATE RESTRAIN

I 1 - I2

I1 + I2 2

Bias =

Differential (or Spill)Current Mean Through Current

Mean Thro Current

Restricted E/F Protection Low Voltage Windings (1) ABCN

LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated

Restricted E/F Protection Low Voltage Windings (2) ABCN

LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated

Delta Winding Restricted Earth Fault Relay Source

Protected zone REF

 Delta winding cannot supply zero sequence current to system

 Stability: Consider max LV fault level  Recommended setting: less than 30% minimum earth fault level

`

Combined Differential and Restricted Earthfault Protection

A2

A1

P1 P2

a1

S1 S2

a2

REF

P1 S1

P2 S2

P2 P1 S1 S2

To differential relay

Integral Vectorial and Ratio Compensation Power transformer

Ratio correction

Vectorial correction Virtual interposing CT

Differential element

Virtual interposing CT

In Zone Earthing Transformer P1

P2

a2

a1 A1

A2

S2

S1 T2

T1

P1

P2

P2

P1

S2

S1

Three Winding Transformer 63MVA 132KV

300/5

25MVA 11KV

1600/5

50MVA 33KV

1000/5 4.59

5.51

10.33

2.88

5

2.88

5

All interposing C.T. ratio’s refer to common MVA base (63MVA

Transformer Magnetising Characteristic Twice Normal Flux

Normal Flux

Normal No Load Current

No Load Current at Twice Normal Flux

Parallel Transformers T1

T2

NABC

Inter-Turn Fault E

CT Load Shorted turn Nominal turns ratio Fault turns ratio Current ratio

- 11,000 / 240 - 11,000 / 1 - 1 / 11,000

Requires Buchholz relay

Buchholz Relay Installation 3 x internal pipe diameter (minimum)

Conservator

5 x internal pipe diameter (minimum)

Oil conservator 3 minimum Transformer

Buchholz Relay Petcock Counter balance weight

Alarm bucket Mercury switch

Oil level

To oil conservator Trip bucket

From transformer Aperture adjuster

Drain plug

Deflector plate

Overfluxing Basic Theory V = kf Causes

2m

m

Low frequency High voltage Geomagnetic disturbances

Ie

Effects Tripping of differential element (Transient overfluxing) Damage to transformers (Prolonged overfluxing)

EFFECTS OF OVER FLUXING: · · · ·

Increase in magnetizing current Increase in winding temperature Increase in noise and vibration Overheating of laminations and metal parts (cause by stray flux)

V/Hz Overfluxing Protection V f K Trip and alarm outputs for clearing prolonged overfluxing Alarm : Definite time characteristic to initiate corrective action Trip : IDMT or DT characteristic to clear overfluxing condition Settings Pick-up 1.5 to 3.0

i.e. 110V x 1.05 = 2.31 50Hz

DT setting range 0.1 to 60 seconds

V/H CHARACTERISTIC:

Over-fluxing Relay Ex

G

VT

AVR

RL

THERMAL OVERLOAD: · EFFECT OF OVER LOAD ON TRANSFORMER INSULATION LIFE:

Overheating Protection Trip

I load TD setting Top oil of power transformer

Alarm

On

Fan control

I load Off On

Pump control Off

Temp. indication

Heater Local Thermal replica

Temperature sensing resistor

Remote

Overload Protection · Overcurrent protection designed for fault condition · Thermal replica provides better protection for overload – – – – –

Time

Current based Flexible characteristics Single or dual time constant Reset facility Non-volatile

Current

Thermal Overload Oil Filled Transformers Trip time (s) 10000

Single characteristic:  = 120 mins

1000

Dual characteristic 100

10 1 ZA

2 3 4 5 6 Current (multiple of thermal setting)

Single characteristic:  = 5 mins

DIGITAL RELAYS FOR TRANSFORMER THERMAL WINDING PROTECTION