TRANSFORMER EXPLOSION PREVENTION

TP Successful Activation

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• INTRODUCTION

• TP PERFORMANCE ANALYSIS

• SCADA Recording During the Short Circuit

• Signals Recorded on the TP Control Box After the Fault

• Rupture Disk Activation

• Rupture Disk Opening Section Comparison with TP Live Test No. 25

• Comparison of Activation Parameter with TP Live Test No. 25

• Pressure Relief Valve Behavior During Short Circuit

• BPC INVESTIGATIONS

• Transformer Oil Analysis After the Electrical Fault

• Electrical Arcing in Transformer Tank

• CONCLUSION

• BPC CERTIFICATE OF TP SUCCESSFUL ACTIVATION

• BIBLIOGRAPHY

OUTLINE

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The TRANSFORMER PROTECTOR (TP) installed at Thamaga SS on

a 125 MVA transformer was activated by an internal fault on January 9, 2006.

The Site Engineers report to SERGI that during the short circuit a big explosion

sound was heard in a 3km radius.

It was mentioned that one surge insulator exploded and shorted to the ground.

As a result, a 4.8 KA current fault was generated.

The TP was activated within milliseconds by the first dynamic pressure peak,

before static pressure increased, avoiding transformer tank explosion.

INTRODUCTION

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INTRODUCTION

TPC scientific articles ref. 2, 3, 4 should be read to understand the TP behavior during

the incident:

• The maximum of gas is produced at the exact time of the creation of the electrical arc.

During the first milliseconds, 80 ft³ [2.3 m³]of explosive gas are produced by the first Mega

Joule.

• The gas generated during the first milliseconds creates one dynamic pressure peak,

amplitude from115 to 200 psi [8 to 14 bar].

• As soon as appearing, the dynamic pressure peak travels at the speed of sound inside the

oil, 4,000 feet per second [1,200 meter per second].

• Then, the dynamic pressure peak generates secondary peaks when travelling inside the

tank.

• The integration of the dynamic pressure peak and its secondary peaks are building up

static pressure inside the tank, within 20 to 100 milliseconds depending on tank size.

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TP PERFORMANCE ANALYSISSCADA RECORDING DURING THE SHORT CIRCUIT

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TP PERFORMANCE ANALYSISSCADA RECORDING DURING THE SHORT CIRCUIT

SEQUENCE OF EVENTS

t = 0 ms, Low Impedance Fault is detected by the Reactor Relay Oscillograph.

t = 4 ms, Reactor Relay actuates and sends the signal to trip the Circuit Breaker.

t = 5 ms, TP Rupture Risk opens before the static pressure increases.

t = 14 ms, Electrical Protection signal is received by the TP Control Box.

t = 28 ms, Automatic Nitrogen Injection is initiated by the two signals coming from the Electrical

Protection and the Rupture Disk Burst Indicator.

t = 56 ms, Breaker opens after receiving Reactor Relay signal sent at t = 4 ms.

t = 322 ms, Conservator Shutter closes to avoid draining the Oil Conservator Tank.

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TP PERFORMANCE ANALYSISSIGNALS RECORDED ON THE TP CONTROL BOX AFTER THE

FAULT• TP Control Box

Electrical Protections LED On

• TP Rupture Disk LED On

Indication that the TP Rupture Disk is opened.

• Nitrogen Low Level LED On

Meaning that the TP Nitrogen injection was

performed to evacuate all explosive gases from

tank to render the evacuation safe.

• Oil Filtration Electro-Valve Not Ready LED On

The OLTC oil filtration unit was isolated before

Nitrogen injection.

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TP PERFORMANCE ANALYSIS

TP RUPTURE DISK ACTIVATION

• The Rupture Disk Petals have been

opened.

• The Burst Indicator wire has been cut,

therefore the RD opening signal was

sent to the TP Control Box.

• The RD opening section seems small

because the RD diameter is sized for

the strongest short circuits.

To estimate short circuit energy, the RD opening section is compared with the opening

sections versus energy obtained during the TP Live Tests

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TP PERFORMANCE ANALYSISRUPTURE DISK OPENING SECTION COMPARISON WITH TP LIVE

TEST No. 25

For the same Rupture Disk opening section during the TP Live tests, the characteristics of

the electrical fault and the consequences on the tank pressure are listed on the table

below.

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TP PERFORMANCE ANALYSISCOMPARISON OF ACTIVATION PARAMETERS WITH TP LIVE

TEST No. 25

• The comparison of the RD opening section obtained during the TP live tests have shown

the RD opening section was corresponding to an electrical arc energy of 0.25 MJ, ref. 2

and 4.

• During the TP live tests it was measured that an electrical arc of 0.25 MJ generates 80

ft³ [0.5m³] of explosive gas, ref. 2 and 4.

• Calculations have also demonstrated that 80 ft³ [0.5m³] of explosive gas generated

inside transformer tank resulted to an internal pressure of 44 psi [3 bar] atmospheric

for tank not equipped with TP, ref. 2 and 4.

• Transformer tanks are exploding at 15 psi [1 bar] above atmospheric pressure.

Consequently, if not protected by the TP the transformer tank would have exploded

during this incident. A fire could have occurred, destroying the transformer and all

surrounding equipments.

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TP PERFORMANCE ANALYSISCOMPARISON OF ACTIVATION PARAMETERS WITH TP LIVE

TEST No. 25

• For an energy of 0.25 MJ, the Dynamic Pressure Peak had an amplitude of 75 psi [5

bar], as shown in the graph below, ref. 2 and 4.

• During the fault, the current peak was 4.8kA, closed to the current peak created for the

test No. 25, 7.3kA.

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TP PERFORMANCE ANALYSISPRESSURE RELIEF VALVE (PRV) BEHAVIOR DURING SHORT CIRCUIT

The Pressure Relief Valve yellow pin at the

top center was found in its original position,

showing the PRV was not activated during

the short circuit.

For TPC scientific article ref. 1 should be

read

to understand the PRV behavior during

short

circuit

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TP PERFORMANCE ANALYSISPRESSURE RELIEF VALVE (PRV) BEHAVIOR DURING SHORT CIRCUIT

• During the incident, the PRV was not activated because:

• The dynamic pressure peak of 75 psi [5 bar] amplitude has travelled inside the tank at

the speed of the sound inside oil, which is 4,000 feet per second [1,200 meter per

second], ref. 2 and 4.

• The PRV section is 6 inch, 150 mm, consequently the PRV has seen the dynamic

pressure peak only during 0.1millisecond, ref. 1.

• The PRV springs have an opening inertia of approximately 5 millisecond; therefore, the

PRV had no time to activate because of the dynamic pressure peak speed, ref.1.

Because the PRV had no time to open, the TP has therefore depressurized the

transformer

tank before the static pressure increased.

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BPC INVESTIGATIONSTRANSFORMER OIL ANALYSIS AFTER THE ELECTRICAL FAULT

The results of the transformer oil analysis show that the following gases were present:

• Hydrogen, Oxygen, Nitrogen, Methane, Carbon Monoxide and Carbon Dioxide and

Acetylene.

According to the chemical component present in this sample, the Dissolved Gas Analysis

(DGA) and DUVAL Triangle were used to determine which kind of fault had occurred inside

the tank.

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BPC INVESTIGATIONSTRANSFORMER OIL ANALYSIS AFTER THE ELECTRICAL FAULT

• First the concentration of the three gases in PPM: S = A + B + C

• Second, the calculation of the relative proportions of these three gases:

X = % CH4 = 100(A/S)

Y = % C2H4 = 100(B/S)

Z = % C2H2 = 100(C/S)

• Third, place the coordinates X, Y, Z on the triangle to find the type of fault.

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BPC INVESTIGATIONSTRANSFORMER OIL ANALYSIS AFTER THE ELECTRICAL FAULT

The Yellow Phase Transformer Analysis gives:

• A = 10.41 PPM ; X = CH4 76.65%

• B = 0.72 PPM ; Y = C2H4 5.30%

• C = 2.45 PPM ; Z = C2H2 18.04%

With these percentages, the Duval Triangle

Method shows:

The fault is a D1 Type, “Arc Discharges”.

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BPC INVESTIGATIONSELECTRICAL ARCING IN TRANSFORMER TANK

During the transformer internal inspection, the electrical partial discharges proven by the

oil

chemical analysis has been confirmed.

The 2 pictures below show the arcing points.

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CONCLUSION

The TP has been activated 5 millisecond after the origin of the fault.

The transformer Pressure Relief Valve had no time to open, which demonstrates that the

TP

has been activated by the first dynamic pressure peak of the shockwave before the static

pressure increased.

The activation of the TP has therefore saved the transformer tank from explosion.

The transformer could be repaired on site and was back to service one week after the

incident, thanks to the TRANSFORMER PROTECTOR.

Without the TP, the transformer would have exploded and a fire would have spread to the

all surrounding equipments. According to BPC the total cost of the damages this incident

would have reached USD 120 Millions, ref. 5.

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BPC CERTIFICATION OF TP SUCCESSFUL ACTIVATION

“ On 01.09.2006 an autotransformer of 125 MVA,

located at our Thamaga Substation

developed troubles. A serious internal failure

occurred and the SERGI system activated. The

SERGI system therefore correctly operated and

released the pressure developed internally.

Furthermore and according to transformer

manufacturer investigation, it was reported

that thanks to the Transformer Protector, the

transformer tank was not deformed and

secondary damages were limited. An isolating

Bakelite plate replaced and new tests

performed before the transformer was put

back to service.”

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BIBLIOGRAPHY

1. Pressure Relief Valve efficiency calculations by comparison to the TRANSFORMER PROTECTOR

during transformer short-circuit, ref. AtTPradb01e.

2. Prevention of Transformer Tank Explosion, Part 1: Experimental Tests on Large Transformers,

ref.Arpibp01a.

3. Prevention of Transformer Tank Explosion, Part 2: Development and Application of a Numerical

Simulation Tool, ref.Arpibp02a.

4. An Answer to Prevent Transformer Explosion and Fire: Live Test and Simulations on Large

Transformers, ref. AtTPrajo1e Power-Gen Europe Best Paper Award 2008, Track 5, Electrical and

Information & Control Systems.

5. Transformer Explosion and Fire Incidents, Guideline for Damage Cost Evaluation, Transformer

Protector Financial Benefit, ref. AtTPraeb03e.

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SERGI FRANCE

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78 260, Achères

FRANCE

Phone (0033) 1.39.22.48.40

Fax (0033) 1.39.22.11.11

sales@sergi-france.com

www.sergi-france.com