pd-tr1342-train 2-rasgas-27032013

AYT

AYT INTERNATIONAL LLC PO BOX: 181398, DUBAI-UAE , : +971-43804755 :+971-43804756

PROJECT: ONLINE PD MEASUREMENT ON 33/6.9kV TRANSFORMER

CLIENT: RAS GAS, QATAR

CONTRACTOR: AYT INTERNATIONAL WLL

REPORT No: AYT-PD-RS-002-R00

REPORT DATE: 20TH MARCH 2013

DIAGNOSTIC TEST REPORT OF 33/6.6kV TRANSFORMER (TR-1342)

AYT CONFIDENTIAL ONLINE PD MEASUREMENT ON 33/6.9kV TRANSFORMER

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AYT INTERNATIONAL WLL PO BOX: 10161, DOHA-QATAR , :+974-4660758 :+974 4660739

Table of Contents

S.N Description Page no.

1 Introduction 3

2 Acceptance-Interpretation, Analysis and Test Results 4

3 Conclusion & Recommendation 10

4 References 11

5 Enclosures 12

6 Disclaimer 12

7 Queries 12


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1. Introduction

The scope of the document is to present the overall Analysis of the PD Data and to

document the overall health of the 33/6.6kV, 12.5MVA (TR-1342) Transformer located

in TRAIN 2 of RASGAS (RG) feeding Fuel Gas Compressor (28M001).

The transformer was taken out of the yard and was installed at present location. After

Electrical and Oil Filtration the transformer was taken in operation on 16th

November’2012. Oil Sample for Dissolved Gas Analysis (DGA) and Oil’s properties test

was taken on 17th November’2013. DGA Report and the properties of the oil are well

within the limits and the transformer is in CONDITION 1 according to [1] and is good for

operation. DGA report of 13/02/2013 shows a rapid increase of the H2, with CH4, CO and

Co2. RG decided to carry out the DGA after a month to see the trend. The transformer

was kept under load and another sample was taken after a month on 13/03/2013. The

DGA results show that there is increase in the values of the above gases substantially

and the condition of the transformer has gone to CONDITION 4. As per the latest DGA

report the level of Hydrogen is 14,699ppm with Methane, Carbon Monoxide and Carbon

Dioxide and this shows that there is active fault inside the transformer. The type of fault

is Partial Discharge (PD) or Corona inside the transformer. The transformer as per the

DGA Diagnosis is running under failure mode and there is potential threat of hazard with

fire and stoppage of production, etc.

Transformer Failure History and operation The transformer was built by Pauwels in the year 1997 failed in operation in Train 1. As per the information that was provided to AYT, there was surface tracking on Y phase of the transformer and hence it failed. Please refer to the Photo attached with the report. It was repaired by ISCOSA in 2008. This Transformer was stored in open Yard for more than 4 years. It was reported to us that the transformer was filled with oil (50%) and 50% with N2. Normally the transformer is to be kept under positive pressure to ensure that there is ingress of ambient air and moisture. If the positive pressure is not being maintained while in storage, then there is there is great likelihood of moisture to enter inside the transformer. Under Temperature equilibrium the moisture can settle down with the solid insulation, since the cellulose loves water. This water cannot be easily taken out through Oil filtration. To remove the Moisture, one needs to do filtration for several passes or go through the drying of the transformers. When the transformer is put on load, it generates heat which can boil the water in Solid Insulation and this can create the bubble inside the transformer which can create PD activity inside the solid insulation. The rated load of the transformer is 1049A at 6.6KV. The normal load is about 80%. This transformer is in operation on 24x7basis. The Oil Temperature and the winding temperature were found to be at 73o C and 83O C respectively which is well within the limits.


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Partial Discharge (PD) and Corona Partial Discharge is an Electrical pulse or discharge in a gas filled void or on a dielectric surface of solid or liquid insulation system. This pulse or discharge only partially bridges the gap between phase insulation to ground and the phase to phase insulation. The pulses occur at high frequencies therefore they attenuate quickly as they pass to ground. The discharges are effectively small arcs occurring within the insulation system, therefore, deteriorating the insulation, and can result in eventual complete insulation failure. Corona is an electrical phenomenon caused by a chemical reaction in the air and also known as Partial discharge. It means the PD in air is called as Corona. Corona is the ionization of the nitrogen in the air caused by an intense localized electrical field. Inside transformer, the corona takes place at the location where the lead from winding is connected to the bushing.

The PD activities inside the could be categorized in three groups;

1. Internal PD: Internal PD is the one which starts from the conductor side and movers towards the Earth through the Insulation. This PD activity is of concern since this is internal and it erodes the insulation over period of time and then leads to complete discharge.

2. Surface PD (Internal): The PD activity which takes place above the insulation is considered Surface PD Internal.

3. Surface PD External: The PD activity which takes place on the surface of the insulation due to contamination or dirt is considered Surface PD External.

4. Corona: This kind of PD is predominantly due to sharp edges on the HV conductor or Ground Side. Normally, this type of PD in transformer is in the bottom of the bushing connecting to the windings.

RASGAS engaged AYT to carry out the online PD measurement on this transformer to

confirm that PD activities are taking place within the transformer as a confirmatory test.

AYT carried out the Online PD Measurement using two sensor techniques. The

measurement was conducted in Wide Band to increase the Signal to Noise Ratio. The

noise rejection technique is based on both hardware and software. HFCT was mounted

on the neutral and ground lead of the transformer and the PD was measured using

PDCHECK, Techimp, Italy. Subsequent to the above measurement, UHF sensor

technique was used with AE-150, NDB Technology Canada. The UHF antenna was

installed in the drain valve of the transformer and measurement was taken. Both the

technique validates the presence of PD. Both instruments of two different

manufacturers detected the PD activity within the transformer. Following observations

were made;

1. UHF Sensor through the Drain Valve is more sensitive as compare to HFCT mounted

on the ground lead of the transformer or neutral of the transformer.


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2. The magnitude of ground lead HFCT was 20mV, 35mV on HFCT connected on

Neutral cable of the transformer and 140-150mV on UHF sensor.

Subsequent to the PD Detection by both the instruments, PD localization through

Acoustic Sensor was tried, but the exact localization is difficult due to design of the

transformers and the type of PD. The Transformer is very compact and there are cable

boxes and the radiator on the transformers which restrict the movement and

installation of the Acoustic Sensors on the wall of the transformer. However, when the

PD Pattern was analyzed, it is found that it is Pressboard internal PD; this means that the

PD is taking place within the windings. In such situation the PD is not able to come out

for AE sensors to detect since it attenuates.

Online verses Offline PD Measurement

Offline PD measurement is better than online provided the PD free power source and controlled environment (Faraday Cage) is available at site. PD measurement are carried out in the factory as routine test as per IEC 60270 and IEC60076-1 on all most all MV and HV transformers in a controlled environment (Faraday Cage) whereby the noise level is less than 2pC. The PD free Power Source is also available which is very large and huge. There is an acceptance criterion mentioned in the IEC60076. But the same cannot be applied by for onsite PD measurement. Moreover, the PD Measurement is done on the new insulation at elevated voltage. The Voltage applied is 1.73xtimes the rated voltage between the phase and ground to check the PD inception and PD Extinction Voltage. This method of measurement is mostly called as CONVENTIONAL METHOD OF PD MEASUREMENT and under this method the capacitive tapes or capacitive coupler is used as sensors. In case of online measurement, PD is measured in noisy environment and at rated voltage. This method is called as an UNCONVENTIONAL METHOD OF PD MEASUREMENT, wherein different type of sensors like inductive, capacitive and magnetic are used for the PD Measurement at site. There is no standard for this so far in practice known to the writer as on date. However, there are some studies and recommendations by known agencies such as CIGRE to IEC and IEEE standard committee for standard. For Example, CIGRE has prepared a Guideline [2] and through it they have recommended to IEC committee to form a standard. A prospective standard called as IEC62478 is underway. This standard is coming up with PD measurement with Unconventional Technique. In this particular case, Offline PD though possible is not recommended, since DGA report already suggests that there is PD activity is taking place. This means that the insulation is getting damaged. In case of Offline PD, if the voltage is increased there is great chance that the insulation can fail and hence and the transformer cannot be taken in the service.

2. Acceptance-Interpretation and Analysis


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Partial Discharge (PD) Technique is a proven effective diagnostic tool for the condition assessment of the insulation of the electrical equipment. However, there is no national and international standard which specifies the acceptance limits and pass fail criteria, since there is no standardization of the measuring technique especially for online measurement. Therefore, we need to follow a systematic methodology to analyze and interpret the result of the PD data according to the engineering practices and the PD Instrument manufacturers. The well accepted method to analyze the PD data is through analysis of the PD Pattern which is also called Phase Resolved Partial Discharge (PRPD). In case of PRD, we not only see the magnitude but also we look into the phase relation of the PD activity and the repetition rate of the PD pulses. Moreover, PD occurs at certain angle on the phase and it voltage dependent, unlike noise. It has been established that every type PD mentioned above has certain pattern. We, therefore use the standard pattern to compare with actual PD Pattern gathered during the filed measurement. The Typical PD Patterns of the transformers are as follows; The P.D pattern may appear different while taking the measurement and hence a

through studies and analysis of all PD data are necessary before making any decission.

Moreover, following points also need to be taken into consideration while analyzing the

PD Patterns.

1. Phase 2. Magnitude 3. Polarity of the Pulse: 4. Repetition Rate: The color of the pattern goes from Grey to Red with increase in

repeatation rate. Magnitude and Repeatation rate plays a determnetal role.


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STANDARD PD PATTERN (LIBRARY) FOR OIL FILLED

TRANSFORMER

CORONA PD

Uncontaminated Dry Oil Uncontaminated Dry Oil

Uncontaminated Dry Oil H2O Contamination

Free Metal Particles


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PRESSBOARD INTERNAL PD

SURFACE/INTERNAL (MIXED) PD ON PAPER


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The PD waveform taken with PDCHECK using two HFCT one in neutral of the cable

(140mm dia) and other one installaed in the Ground wire of the Transformer (39mm

dia) measured PD Pulse with magnitude of 35mV and about 20mV respectively. Then

the PD Pattern were capatured and noise were filtered using Map Filter in the software.

The PD Pattern were compared with the STANDARD PD PATTERN FOR TRANSFORMERS

(OIL FILLED) and it is found that there is Pressboard Internal PD in the trasnformer.

It is important to note that ther PD activity coming out from the neutral point of the

transformer will enter through the ground wire of the transformer and vice versa.

Hence, it observed that there is that there would be direct polarity in case of 39mm

HFCT

HFCT (39mm)GROUND LEAD

ENTIRE PD PATTERN

SUB PD PATTERNS

Class-0 (Pressboard Internal PD) Class-1 (Pressboard internal PD)


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HFCT (140mm) NEUTRAL

ENTIRE PD PATTERN

SUB PD PATTERNS

Class-0 (Noise/Pressboard Internal PD) Class-1 (Pressboard Internal PD)


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Class-1 (Pressboard Internal PD)

3. Conclusions and Recommendations Conclusion

Internal PD inside the transformer is active. This is low energy discharge. This is also confirmed

through Dissolved Gas Analysis (DGA) of the Transformers. Please refer to AYT Report AYT-DGA-

DR-RG-001. Low energy discharge at 73OC (oil temperature) causes the generation of the

Hydrogen (H2) and Methane (CH4). Since the internal PD involves the solid insulation of the

Transformer, there is generation of Carbon Monoxide (CO) as well. The detail analysis says that

the PD is taking place in Y phase of the transformer. This means that the transformer is running

under “Failure Mode”.

Recommendations

There is active PD inside the transformer. The transformer is running under failure mode and

time to fail cannot be predicted and hence following recommendations are proposed;

1. Transformer must be taken out of service or if there is no alternative, extreme care should

be taken and required step should be taken to ensure that the load is transferred to some

other source if available.

2. Regular monitoring of this transformer is required.


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3. Carry out some other electrical tests such, Capacitance and Tan Delta, FDR, Leakage

Impedance, SFRA before the transformer is sent for inspection, overhaul and repair.

4. One of the reasons for the failure and the damage of the insulation is 3rd

Harmonics. It is also

recommended to carry out the Power Quality Analysis of the complete network of the

RASGAS.

5. It is also recommended to carry out the root cause analysis of this failure

4. References 1. C57-104-2008: IEEE Guide for the Interpretation of the Gases Generated in Oil-

Immersed Transformer

2. Technical Brochure 444: Guidelines for Unconventional Partial Discharge

Measurement

5. Enclosures: 1. Test Results from the Equipment.

6. Disclaimer:

The information provided in this report is sole property of AYT INTERNATIONAL WLL,

Doha. It is to be used by RAS GAS. No party other than the one who is specified can use

this information. It cannot be produced or copied nor, it can be transmitted to any third

party without written permission from AYT, Doha. Please contact:

[email protected].

AYT shall not be liable for any special, indirect, consequential damages, losses such as,

but not limited to, loss of revenue, loss of use, loss of production, loss of power, cost of

capital, cost of replacement of power, a costs connected with interruption of operation

arising out of this report.

7. Queries:

In case of any clarification or queries, you may reach at following:

AYT INTERNATIONAL WLL

Doha-Qatar

PO Box: 10161, Doha

Qatar

: +974 4660758 :+974 4660739

mailto:[email protected]

pd-tr1342-train 2-rasgas-27032013

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