230KV AND 400KV EQUIPMENTS MAINTENANCE PRACYICES,EXPERIENCES & EXPECTATIONS

230KV AND 400KV EQUIPMENTS MAINTENANCE PRACTICES, EXPERIENCES & EXPECTATIONS

N.KAMALASANAN ACHARY Additional Chief Manager / Electrical / TPSII

R.N.KAMARAJ Deputy Chief Engineer / Electrical / TPSII

NEYVELI LIGNITE CORPORATION LIMITED,

NEYVELI 607 807

NEYVELI LIGNITE CORPORATION LIMITED, NEYVELI is one of the prestigious organization in Indian Power Sector & Lignite Mining Industry. NLC is the biggest lignite based power station in South Asia. There are three power stations viz., Thermal Power Station-I, TPS-II and TPS-I (Exp) of installed capacity 600MW, 1470MW and 420MW respectively. In TPS-II, there are 22 bays of 230kV equipments and 13 bays of 400kV equipments. EHV sub station activities are very vital for smooth evacuation of Power Generated. Only proper routine maintenance practices, retrofitting of switchgear equipments and its up gradation will ensure the complete success. No doubt, our own experiences are our master but the subtle man learns from the others experiences also. This paper discusses our own practices, experiences and expectations.

MAINTENANCE SCHEDULE:

The present system of maintenance schedule is the result of continuous development based on the experiences and events occurred in the system. The details of preventive, periodical and major over haul maintenance are furnished as a part of sharing the information.

MOCBs:

Periodical inspection of oil level, Nitrogen pressure and oil leakage

Half yearly maintenance schedule viz., terminal contacts inspection, tightness checking, cleaning, moving part lubrications, critical dimensions checking (like sinking gap, symmetrical movement, closing armature air gap) etc.

Measurement of breaker closing and opening timing once in two years

Measurement of contact resistance, once in two years

Replacement breaker EHV oil once in two years and as when the breaker operates on severe fault current and also confirming the operation of Nitrogen pressure relief valve

Major Overhaul once in five to seven years

SF6 CIRCUIT BREAKERS:

Periodical inspection of Air / SF6 / Hydraulic oil pressures & their leakages and oil level etc.

For air compressors, cleaning, periodical changing of lubricating oil and removing of water condensation from the air receivers

Measurement of breaker closing and opening timing once in two years

Measurement of contact resistance, once in two years

Half yearly maintenance schedule like termination tightness checking, interrupter insulator& supporting insulator cleaning, etc.

ISOLATORS:

Half yearly maintenance works viz., terminal contacts inspection, tightness checking, insulator cleaning, moving parts lubrication, alignment checking, greasing of gear mechanisms, checking / cleaning of auxiliary contacts etc.

Assessment of damage in Fixed contact assembly & moving contact assembly and replacing them if required

Measuring and maintaining of contact resistance

CURRENT TRANSFORMERS:

Periodical inspection of oil level and oil leakage

Half yearly maintenance works like tightness checking, insulator cleaning, secondary terminal box inspection etc.

Checking of spark gap

Oil tests for BDV, Dielectric dissipation factor and specific resistance, once in two years

Checking of Nitrogen pressure once in two years and charging the same if required

Replacement of EHV oil whenever the oil parameter exceeds specified norms or once in Five to seven years, which ever is earlier

Carrying out all detailed electrical tests viz., Insulation resistance, Ratio, Magnetising characteristic, Dielectric Dissipation Factor etc., once in five to seven years

CAPACITIVE VOLTAGE TRANSFORMER:

Periodical inspection of oil level in EMU and oil leakage

Half yearly maintenance works such as termination tightness checking, capacitor insulator cleaning, secondary termination box inspection etc.

Major Overhaul as when required and all detailed electrical tests like IR value, Capacitance measurement, Dielectric Dissipation Factor, Winding resistance & ratio measurement, etc.

LIGHTNING ARRESTORS:

Periodical monthly inspection and recording of leakage current and counter reading

Half yearly maintenance works like LA stack insulator cleaning, terminal tightness checking

FREQUENT PROBLEMS FACED

1. The half yearly maintenance works are carried out on routine basis. These maintenance works are carried out in systematic manner with checklist. Occasionally, the problem arises after half yearly maintenance also. One of the major reasons identified is that the potential weak points could not be identified before the start of work. This leads to the thinking of introduction of Infra Red Thermal Scanning.

2. 400 kV CVTs were failing very frequently due to the failure of surge arrestor inside EMU unit. Also numbers of failures have occurred due to the failure of damping resistances.

3. Some manufacturers especially WS make has not provided oil sampling provision with valve arrangement in their CTs. The dummy provided could not be opened confidently as there is a chance of jetting out of oil with out control. We ourselves are making steps to provide suitable valve arrangements for taking oil samples easily.

4. Failure of accumulators in Hydraulic mechanism operated 400kV SF6 circuit Breakers.

5. The available bus arrangement is two main bus and one transfer bus system. Improper current sharing especially in 230kV Double break Isolators are noticed due to high contact resistance when the two bus isolators are closed for isolating one of the above main buses / isolators.

6. The oxidization of silver coated contacts of isolators could not be avoided in out door switchyards. The problem is finding a method to hoe with out affecting the silver coating.

7. It is generally felt that the MOM boxes of isolators are not to the tune of withstanding till the life of main equipments. In spite of regular attention, problems like door hinge breaking, key interlock switches failure, improper closing of doors, auxiliary switches failure etc are happening frequently.

SPECIFIC MAJOR INCIDENCES

1. One of the 230kV Current Transformer got blasted and resulted in damaging of insulators of so many near by equipment insulators. The opening of secondary could not be suspected because one day before the blast the feeder had tripped on lie fault and the sensing & operation protections were perfect. The manufacturers may have more point to share in this regard for the sudden failure of that current transformer.

2. The originally available Pneumatic type NGEF make 400kV Generator Transformer breakers were replaced with BHEL make SF6 breakers mainly due to frequent SF6 gas leak and air leak. The cost of power generation loss was so heavy which leads to replacement of four generator transformer breakers. During commissioning of one of that BHEL breakers, the PIR mechanism failed to open which resulted in blasting even before the unit was tried for synchronizing

3. In one of the 400kV BHEL breakers, one pole opened partially during protection tripping which leads to blasting of that particular pole interrupter.

4. One of the 400kV CVT, top capacitor unit has blasted while charging the line which was kept under hand tripped condition due grid over voltage condition in the night.

5. The rare phenomena of Isolator getting struck up in the arcing region had occurred 2 or 3 times.6. 198 kV Lightning Arrestors especially in Transformer yard which are not under the protected area of switchyard lightning master had blasted three or four times. Every time these incidences had occurred on the rainy day in the month of April & May. These occurrences leads to the decision of replacing all the Gap type lightning arrestors with Gapless type lightning arrestors. Up dating of Maintenance Practices:

In any front, continuous up gradation of practices / technologies are must for the continual improvement and to sustain the systems. The problems enumerated above could not be eliminated altogether. The failure of equipments could be at least predicted to the maximum extent possible by adopting suitable maintenance practices. Some aspects are thought over and implemented. The same are discussed below.

High current DC Contact Resistance Measurement

Infra red Thermal Scanning

Circuit breaker operating analyzer

High current DC Contact Resistance

Measurement:

Measuring of DC contact resistance with 100 Amps kit was introduced. This test could ensure the perfect making of power contacts in Isolators and breakers.

Whenever the breaker is tested for timing checks, the contact resistance values are also measured. This will be helpful in identifying any major problem / damage inside the interrupter with out opening the same.

The contact resistances of all isolators of Generator transformers and feeders having high current power flow are being monitored on regular basis. This test will guarantee the proper closing of isolators. This contact resistance measurement is helpful in minimizing the power loss through isolator contacts and also eliminating the unbalance in current sharing as stated above.

INFRA RED THERMAL SCANNING:

The introduction of Infra Red Thermal Scanning is a boon to our maintenance divisions. Before its introduction, routine maintenance activities were carried out with out knowing fully the potential weak points before hand. This instrument can measure the surface temperature of the equipments due to internal failure while in service. Before availing any shut down for routine maintenance, all the equipments of particular bay are scanned and potential weak points are identified before shut down. This is helpful in giving special attention to the identified weak points during routine maintenance. These measurements are very effective especially in CT termination and Isolator & Breaker power contacts.

The main draw back of this instrument is that it is difficult to obtain absolute value of particular surface temperature. This will not affect usage of this equipment in sub-stations because the comparative analysis always possible very easily.

CIRCUIT BREAKER OPERATING ANALYZER

This instrument is exclusively used for circuit breakers. Earlier only tripping time and closing time are tested / checked. The availability of this instrument is helpful in analyzing:

Operation timing of Main and PIR contacts simultaneously

Bounce / Chattering during operation

Coil current characteristic

Auxiliary contact status

Contact travel / mechanism velocity

Wipe of contacts

Dynamic Contact Resistance

The initial signatures of all the breakers are not available to compare with that of present status. It is being attempted first to create the existing status signature, which will be more useful for future comparisons and analysis.

The main draw back of this instrument is that only a comparative study of the parameters of the same equipment can mainly be carried out which necessitates always the old parameters than the independent study. Another major draw back of this instrument is that these measurements require a lot of time, which is normally not available during shut down for routine maintenance works.

UP GRADATION OF EHV SWITCHGEARS:

Majority of our bay equipments have served more than 15 years. It is essential to have more hectic activities like retrofitting, up gradation, major overhauling to keep the equipments in healthy and trouble free operation. The major activities on the anvil are:

1. Now MOCB technology has become absolete.19 numbers of 245kV MOCBs are to be replaced with SF6 Circuit Breakers.

2. Some of the 400kV SF6 Circuit breakers have served 15 to 20 years of service. These breakers are to be taken up for major over haul and requires detailed inspection of gas chambers.

3. Some 70 numbers of 245kV double break isolators have served more than 15 years service. These isolators require retrofitting / replacements to further extend the life of these equipments.

4. The available CVTs are 0.5 class accuracy. The available energy meters are 0.2 class accuracy. It is planned to replace the existing bus CVTs with 0.2-class accuracy CVTs.

5. The major work of replacing more than 50 numbers of 198kV and 360kV Gap type lightning arrestor with the Gapless type lightning arrester is under progress.

EXPECTATIONS:

The expectations listed below are based our experiences. Here some of the expectations, which require further clarifications from the manufacturers, are also listed below.

1. As said already one our 245kV CT had blasted some few years back, which leads to multiple damages to the various neighboring equipments. Really it was a nightmare to replace all the defective equipments within a specified time frame. It is learnt composite bushings suitable to EH Voltages are available in the market which is fracture & explosion proof and easy to handle. These insulators have excellent pollution free layer on account of the hydrophobic insulation. These insulators may be introduced for breaker interrupters and Current transformers, which are liable for explosion.

2. The silver coatings of power contacts of isolators are getting removed while cleaning by usual practices to remove oxide coating. This results in increase of contact resistance and damages further. Is there any other method of removing this oxidation with out damaging silver coating or is there any possibility or method of giving silver coating at site.

3. The failure of Lightning arrestors had occurred. Is there any foolproof method to assess the condition of the lightning arrestors while it is in service?

4. Some pf our OIP condenser bushings and Lightning arrestors are located near Air Condition Plant cooling towers. This result in exposure of one side of insulators to water drifts, which leads formation thick scale in one half of the insulator with in a short period (six months to one year). Whether any body could suggest the method removing scales with any loss in insulator properties or method to avoid formation of such kind of scaling.

5. There is a formati0on of white powder coating in aluminum pad clamps, aluminum bus bar clamps etc., where an air gap is available. No doubt it is creating a high resistance path. Whether it is due to mere oxidization or any other phenomena and how to prevent that coating formation.

6. Corona discharge is a common phenomena in any EHV system. There are some instruments available in the market to measure the corona discharge. Is there any significance in measuring corona discharge? Is there any recommendation from the EHV switchgear manufacturers in this regard?

CONCLUSIONS:

The maintenance schedule discussed here is for our own evaluation purpose. Whatever failures / incidences highlighted are just for the purpose sharing our experiences with the other power utilities and manufacturers for the further improvement system / equipments. Some of our expectations are thrown to this august forum in the form of questions. Any manufacturers or any agency those who some remedy / solutions with them may share for every ones betterment.

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