case I-2

8/11/2019 case I-2

1/9

SESSION I-2

International Conference on Condition Monitoring & Diagnostic Engineering Management of Power Station / Substation Equipment - 2009

CONDITION MONITORING AND RESIDUAL LIFE ANALYSIS OF THE

CIRCUIT BREAKERS, POWER TRANSFORMERS, CURRENT

TRANSFORMERS AND LIGHTNING ARRESTERS

D.R.Dharmadhikari1, Sameeksha Raina

2, B. Karunakaran

3and Ravindra Bhanage

4

North Delhi Power Ltd, Delhi, [email protected];

[email protected];

[email protected];

[email protected]

Abstract: The focus of this paper centers on the Condition Monitoring and Residual LifeAssessment. These are the modern scientific tools which provide accurate measurement on the

condition of the critical equipments like power transformers, Circuit breakers, CTs, LAs etc.The

need of the RLA is based on the idea of operating the equipments till the end of their useful life,rather than its design life. This paves way for the performance based management of the assets.

A systematic approach is adopted for carrying out the study. To assess the condition of the

equipment, it requires the deployment of Non destructive techniques and procedures For RLA, as

a first step, the available data for all the equipments under study is collected. In the second stepthe actual On-line and off-line testing with the required parameters is carried out and data

compared and analysed with available data and the limits specified. Immediate corrective action

if required is taken to correct the minor deviations. If any major deviations are detected, thecorrective action is planned in consultation with the manufacturer. Also, for RLA study the

tripping data is collected along with the magnitude of fault current during the fault. The results of

the tests performed on the equipments are compared against the manufacturer standards andplotted against this tripping data. The results are interpreted to match the fault data and fault

levels of grid stations recorded in the recent past. The knowledge and understanding of the

satisfactory and unsatisfactory condition of the equipments is gained and made explicit from theanalysis of various on-line and off-line test results.

I. Introduction:The increasing demand for quality and reliable power necessitates zero tolerance to any

kind of defects. In order to achieve maximum reliability the critical equipments like Circuit

Breakers, Power Transformers, Current Transformers, and Lightning Arresters are to be

monitored and maintained at regular intervals. In 2002 Distribution in Delhi was privatizedwhich led to formation of three Distribution companies, NDPL (NORTH DELHI POWER

LIMITED - A joint venture of TATA POWER & DELHI Govt.) was one of them. After

formation of NDPL, health check on various equipments across the network had been carriedout, based on which it was decided to run, repair & replacement of the equipments. The

preliminary phase of repairing & replacing of the critical equipments was completed. These

equipments had also completed 2-3 years of service, during which certain equipments had beenstressed by feeding faults of considerable magnitude.

As part of life assessment of the equipments, a study is undertaken on grid station

equipments especially on Circuit breakers, Power Transformers, Current Transformers, and

Lightning Arresters. This paper deals with the analysis made on the Tripping data and faultcurrent seen by the breaker under fault conditions and various tests performed on the equipment.

The tests performed on these equipments are mostly OFF-Line and thermal scanning an effective

ON-Line tool was used to monitor the performance of the equipments.
mailto:[email protected]:[email protected]:[email protected]:[email protected]

8/11/2019 case I-2

2/9

SESSION I-2


II. HOLISTIC APPROACH TO CONDITION MONITORING AND RLA:

Significant amount of money and time are spent in managing critical assets each year and avariety of approach has been employed, yet there are high profile failures that hit the headlines.

Any occurrence in the system and operation costs lots of money, in the forms of:

Operational disruption

Loss of livesInjuries

Assets damaged

Damage to corporate reputation

Environmental damage and pollution

What are the sources of problem?

Lack of maintenance

Ageing equipment

Inadequate design

Poor specification

Time constraintsPoor safety / operational management

O&M Manual not up to date

Poor / inadequate training

Inadequate consideration of environment and human

Performance interaction

Things are no longer running as we planned

Poor construction quality

Performance management:The need to adopt more effective performance and asset management regimes is one of the

important topic for the utility industries. The universal question is: how can we strike the rightbalance between reliability and quality of supply, and capital and operational expenditure?

Commonly used processes adopt a reliability based approach to managing systems. These are

based on understanding the failure rates of equipments and implementing interventions based on

that knowledge. Key elements missing from this approach are the understanding of theconsequences of the failures and risk that such failures present to the operations.

Methodology:! Identify Problems! Analyze the Problems! Evaluate! Treat! Track and Monitor! Review! Communicate and Document

8/11/2019 case I-2

3/9

SESSION I-2


PROCEDURE ADOPTED for RLA:The following procedure is adopted for circuit breakers:

(1) The tripping record of each and every breaker for past 2 years was collected (almost all the

breakers were replaced starting from first half of 2005) and the fault current recorded by theNumerical relays on the breaker in the recent past were noted down, the average value was taken

as reference and multiplied to the tripping and cumulative fault current seen by the breaker wasobtained.

(2) The various Offline tests are performed on the breaker included PI, Contact resistance,Breaker operating time and Dynamic contact resistance.

(3) Thermal scanning on breaker poles with very high contact resistance was used for monitoring

the same.(4) The condition of operating mechanism and auxiliary contacts was also assessed by the timing

and coil signatures.

The following procedure is adopted for Power Transformers:

(1) DGA analysis of oil is carried out once in 6 months.

(2) As part of ON-Line monitoring thermal scanning of the transformers were done.(3) The various test performed on transformer included IR, PI, Magnetic balance,Tan delta,TTR, Winding resistance and Device checks.

The results were analyzed to assess the condition of the Equipment and remedial actions were

taken.

The following procedure is adopted for Current Transformers:

(1) The OFF-Line test performed are , Capacitance and tan delta , Ratio test , knee point voltagetest , IR of Primary and secondary circuits , resistance of secondary windings , Primary current

injection testing.

(2) The As part of ON-Line monitoring thermal scanning of the CTs is done.

The trend of the results is monitored and necessary corrective actions are taken.

The following procedure is adopted for Lightning Arresters:

(1) The OFF-Line test performed is Insulation Resistance testing(2) As a part of ON-Line testing Leakage current Measurement is carried out.

The trend of the results is monitored and necessary corrective actions are taken.

8/11/2019 case I-2

4/9

SESSION I-2


III A. Methodology followed for Condition Based Monitoring and Result Analysis

Circuit Breakers:CBs are expected to protect circuits and plant by interrupting Short-Circuit current within

a time in the range of 80-150ms. The significance of Dynamic contact resistance is that it

determines the condition of the arcing tip which never gets reflected in the conventional tests. In

principle DCRM injects100A DC through the power contacts of Circuit breakers while breaker isundergoing close-open operation. During this short time span it measures the voltage drop across

the contacts dynamically and relays it to the CB operational analyzer. The classical four wire

measurement method is employed to measure contact resistance. Change in Micro Ohm valuesas moving and fixed contacts engage and disengage is plotted against time and a graphical record

is obtained through the analyzer. This dynamic record of micro ohm signature helps to analyze

the condition of the arcing contacts.The significance of trip coil signatures is aimed on the concept that the deterioration in CB

condition also results from its inertia, where the effects of Stictionarising due to non operation

or rusting of mechanical parts can also result in CB failure or slow operation of the breaker whenit is most needed. The deterioration of mechanical parts resulting from frictional forces,

coagulation of lubricants, etc. remains disguised and subsequently neglected. Measurement ofCBs trip signature provides a useful insight into the operation of the CB and its condition. Inaddition to its primary function of controlling of the breaker trip operation, the trip coil performs

a secondary role as a noninvasive condition monitoring sensor. Capturing the trip coil current

signature from the breaker at first trip will serve as record for subsequent breaker condition

assessment. The control or trip unit of the CB transforms the trip signal into the physicaloperation of the breaker mechanism.

Power Transformers:Dissolved gas analysis of Transformer oil is based on the breakdown of the molecules of

oil locally, in certain types of electrical faults such as ionisation, heating, arcing and pyrolysis of

cellulose. It is a powerful diagnostic technique for on line monitoringthe internal conditions ofTransformers due to its capability to detect defects in the early stages before they develop into

major faults. In this method it is possible to check whether a Transformer is subjected to a

normal amount of ageing and heating or whether there are incipient defects. In order to make the

interpretations applicable to those cases of condition monitoring, where rise in concentration ofone or more individual gases is being observed. The simplified approach, which forms the new

method of interpretation, is given in the table.

Sr.

No.

Rise in gas concentration Interpretation

1 Nil Normal Ageing

2 H2 Corona Partial Discharge

3 CH4 and C2H6 Thermal fault of low temperature range up to 300C

4 C2H4 With or without

CH4, C2H6, & H2

Thermal fault of 300C or 700

C or above

5 C2H2 with without H2,

C2H4, C2H6

Arc or flash over or persistent sparking may be along with

overheating.

8/11/2019 case I-2

5/9

8/11/2019 case I-2

6/9

SESSION I-2


IV TREND ANALYSIS

The following are the details of RLA study carried out:

(1)Circuit Breakers

(2) Power Transformers

Pow.tx Equip Equipments Exceptions

Service Tested % DetectedBreakup of excepti

(nos) Completed (nos) Win.tand Win.res Oltc Pi/ir Bus.

66/33KV 5 1 20 3 1 0 0 0 166/11KV 51 38 74.5 42 5 4 3 2 833/11KV 68 44 64.7 42 14 13 0 2 0

Breakup of exceptions

CB

Eq in

Service

Equipments

Tested(nos)

%

Completed

Exceptions

Detected(nos) Timing Crm Dcrm Pi/ir66KV 167 87 52.09 7 4 2 1 033KV 235 69 29.36 37 16 11 9 111KV 946 258 27.27 42 28 3 8 3

8/11/2019 case I-2

7/9

SESSION I-2


Manufacture wise issues

(3) Current Transformers

Ct Eq.ser Equipments % Exceptions Breakup of exceptions

Nos Tested(nos) Completed Detected(nos) Tandelta Ir Sec.res Kne

66KV 241 66 51 14 1

33KV 1054 123 34.53 61 46 14 0

8/11/2019 case I-2

8/9

SESSION I-2


(4) Lightning Arresters

Eq.ser Equipments % ExceptionsBreakup ofexceptions

La Nos Tested(nos) Completed Detected(nos) Ir Leakage current

11KV 33 5 0 533KV 725 66 36.55 3 3 066KV 166 3 2 1

8/11/2019 case I-2

9/9

SESSION I-2


V. Conclusion:By carrying out the RLA/RLE substantial saving is obtained in terms of avoiding the possibledamage to equipment and thereby enhancing the reliability. The saving obtained till date is

approximately seven crore rupees.The maintenance related defects noticed during the study are rectified locally. Critical problems

like of pole discrepancy, timing etc which required high level expertise are referred to themanufacturer and corrective action are taken accordingly. The data obtained from the study is

loaded onto the software supplied by our vendor for further review, trend analysis. The

corrective action is also logged into the software for further reference Those equipments wherethe values were found on threshold of crossing the limits are marked for special attention and

frequency of next monitoring is fixed. These equipments are monitored at regular intervals for

physical and thermal properties. The following benefits are harnessed from the study.1. To understand the condition of the assets and evaluate the risk involved in the operations.2. Tracking of assets.3. Provides focus on the control measures and the causes.4. Helps to measure the effectiveness of controls mechanism in treating the problems.5. Provides an effective tool to compare the performance of different products in system.

References:

1. IEEE transaction on power delivery.

case I-2

Documents

Transcript of case I-2