theory_(1 - 6)_without numbers

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Maintenance and operation of 33/11KV SUBSTATION Page 1


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INTRODUCTION

Maintenance is an important part of the life-cycle of systems, and must be

considered from the design stage through the end-of-life stage of the system. Maintenance

covers two aspects of systems - operation and performance. Maintenance is generally

performed in anticipation of, or in reaction to, a failure. Maintenance is performed to ensure

or restore system performance to specified levels. Improperly performed or timed

maintenance can exacerbate problems because of faulty parts, maintainer error, or decreased

profits. A systematic and structured approach to system maintenance, starting during the

design process, is necessary to ensure proper and cost-effective maintenance.

Past and current maintenance practices in both the private and Government sectors

would imply that maintenance is the actions associated with equipment repair after it is

broken. The dictionary defines maintenance as follows: the work of keeping something in

proper condition; upkeep. This would imply that maintenance should be actions taken to

prevent a device or component from failing or to repair normal equipment degradation

experienced with the operation of the device to keep it in proper working order.

Unfortunately, data obtained in many studies over the past decade indicates that most private

and Government facilities do not expend the necessary resources to maintain equipment in

proper working order. Rather, they wait for equipment failure to occur and then take

whatever actions are necessary to repair or replace the equipment. Nothing lasts forever and

all equipment has associated with it some pre-defined life expectancy or operational life. The

design life of most equipment requires periodic maintenance



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TYPES OF MAINTENANCES

Over the last 30 years, different approaches to how maintenance can be

performed to ensure equipment reaches or exceeds its design life have been developed. In

addition to waiting for a piece of equipment to fail (reactive maintenance), we can utilize

preventive maintenance, predictive maintenance, or reliability centered maintenance.

REACTIVE MAINTENANCE: (Breakdown or Run-to-Failure Maintenance)

Reactive maintenance is basically the run it till it breaks maintenance mode. No

actions or efforts are taken to maintain the equipment as the designer originally intended to

ensure design life is reached.

Basic philosophy

Allow machinery to run to failure.

Repair or replace damaged equipment when obvious problems occur.

This maintenance philosophy allows machinery to run to failure, providing for the repair

or replacement of damaged equipment only when obvious problems occur. The advantages of

this approach are that it works well if equipment shutdowns do not affect production and if

labor and material costs do not matter.

PREVENTIVE MAINTENANCE: (Time-Based Maintenance)

Actions performed on a time- or machine-run-based schedule that detect, preclude, or

mitigate degradation of a component or system with the aim of sustaining or extending its

useful life through controlling degradation to an acceptable level. While we will not prevent

equipment catastrophic failures, we will decrease the number of failures. Minimizing failures

translate into maintenance and capitol cost savings.

Basic philosophy

Schedule maintenance activities at predetermined time intervals.

Repair or replace damaged equipment before obvious problems occur.

This philosophy entails the scheduling of maintenance activities at predetermined time

intervals, where damaged equipment is repaired or replaced before obvious problems occur.

The advantages of this approach are that it works well for equipment that does not run



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continuously, and with personnel who have enough knowledge, skills, and time to perform

the preventive maintenance work.

PREDICTIVE MAINTENANCE: (Condition-Based Maintenance)

Measurements that detect the onset of a degradation mechanism, thereby allowing

causal stressors to be eliminated or controlled prior to any significant deterioration in the

component physical state. Results indicate current and future functional capability. Basically,

predictive maintenance differs from preventive maintenance by basing maintenance need on

the actual condition of the machine rather than on some preset schedule. You will recall that

preventive maintenance is time-based.

Basic philosophy

Schedule maintenance activities when mechanical or operational conditions warrant.

Repair or replace damaged equipment before obvious problems occur.

This philosophy consists of scheduling maintenance activities only if and when

mechanical or operational conditions warrant-by periodically monitoring the machinery for

excessive vibration, temperature and/or lubrication degradation, or by observing any other

unhealthy trends that occur over time. When the condition gets to a predetermined

unacceptable level, the equipment is shut down to repair or replace damaged components so

as to prevent a more costly failure from occurring. In other words, Dont fix what is not

broke.

Advantages of this approach are that it works very well if personnel have adequate

knowledge, skills, and time to perform the predictive maintenance work, and that it allows

equipment repairs to be scheduled in an orderly fashion. It also provides some lead-time to

purchase materials for the necessary repairs, reducing the need for a high parts inventory.

Since maintenance work is only performed when it is needed, there is likely to be an increase

in production capacity.

Depending on a facilitys reliance on reactive maintenance and material condition, it

could easily recognize savings opportunities exceeding 30% to 40%. In fact, independent

surveys indicate the following industrial average savings resultant from initiation of a

functional predictive maintenance program:



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Return on investment: 10 times

Reduction in maintenance costs: 25% to 30%

Elimination of breakdowns: 70% to 75%

Reduction in downtime: 35% to 45%

Increase in production: 20% to 25%.

RELIABILITY CENTERED MAINTENANCE: (Pro-Active or Prevention

Maintenance)

A process used to determine the maintenance requirements of any physical asset in its

operating context. It recognizes that all equipment in a facility is not of equal importance toeither the process or facility safety. It recognizes that equipment design and operation differs

and that different equipment will have a higher probability to undergo failures from different

degradation mechanisms than others. It also approaches the structuring of a maintenance

program recognizing that a facility does not have unlimited financial and personnel resources

and that the use of both need to be prioritized and optimized. In a nutshell, RCM is a

systematic approach to evaluate a facilitys equipment and resources to best mate the two and

result in a high degree of facility reliability and cost-effectiveness. RCM is highly reliant on

predictive maintenance but also recognizes that maintenance activities on equipment that is

inexpensive and unimportant to facility reliability may best be left to a reactive maintenance

approach.

Basic philosophy

Utilizes predictive/preventive maintenance techniques with root cause failure analysis

to detect and pinpoint the precise problems, combined with advanced installation and repairtechniques, including potential equipment redesign or modification to avoid or eliminate

problems from occurring.

This philosophy utilizes all of the previously discussed predictive/preventive

maintenance techniques, in concert with root cause failure analysis. This not only detects and

pinpoints precise problems that occur, but ensures that advanced installation and repair



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techniques are performed, including potential equipment redesign or modification, thus

helping to avoid problems or keep them from occurring.

One advantage to this approach is that it works extremely well if personnel have the

knowledge, skills, and time to perform all of the required activities. As with the predictive-

based program, equipment repairs can be scheduled in an orderly fashion, but additional

improvement efforts also can be undertaken to reduce or eliminate potential problems from

repeatedly occurring. Furthermore, it allows lead-time to purchase materials for necessary

repairs, thus reducing the need for a high parts inventory. Since maintenance work is

performed only when it is needed, and extra efforts are put forth to thoroughly investigate the

cause of the failure and determine ways to improve machinery reliability, there can be a

substantial increase in production capacity.

HOW TO INITIATE RELIABILITY CENTERED MAINTENANCE:

The road from a purely reactive program to a RCM program is not an easy one. The

following is a list of some basic steps that will help to get moving down this path.

1. Develop a Master equipment list identifying the equipment in your facility.

2. Prioritize the listed components based on importance to process.

3. Assign components into logical groupings.

4. Determine the type and number of maintenance activities required and periodicity

using:

a. Manufacturer technical manuals

b. Machinery history

c. Root cause analysis findings - Why did it fail?

d. Good engineering judgment

5. Assess the size of maintenance staff.

6. Identify tasks that may be performed by operations maintenance personnel.

7. Analyze equipment failure modes and effects.

8. Identify effective maintenance tasks or mitigation strategies.



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IMPORTANT SUB-STATION EQUIPMENTS

The expected life of equipment such as circuit breakers, transformers, etc is 25 years as

per O&M manuals of power utilities. However, it is specified to be 35 years in the case of

large sized power transformers. This life expectancy is based on the assumption that the

equipment is operated and maintained as per standards. Abnormal operation like continuous

overloading, non-operation of protective devices, equipment repeated feeding faults, and non-

adherence to preventive schedules will also cause premature ageing and reduce the lifetime.

The following are the main equipment at 33 / 11 KV Substations.

1) CIRCUIT BREAKERS ( 33 KV, 11 KV):

GENERAL:

It is known that low voltage switches and fuses are used at home. The switch is a

mechanical

device which is used to put ON (make) and put OFF (break) the electric circuit. Thefuse

is protective device which blows out during fault condition such as short circuit thereby

protecting wiring etc. In the same way, switching and protection in transmission &

distribution network at high voltage is done / performed by switchgear. Circuit breaker is the

switching and interruption/ breaking device in switchgear.

It serves two purposes

a) Switching during normal operating conditions for operation &maintenance.

b) Switching during abnormal conditions such as short circuits and interrupting the fault

current.There are several types of faults and abnormal conditions in power system. The fault currents

can damage the power system equipment if allowed to flow longer duration. In order to avoid

damage to the equipment protective relaying is provided.

The relays sense the fault and send signal to the circuit breaker to open. The

circuit breaker opens and clears the faults. So circuit breaker is a mechanical switching

device which is capable of making (closing), braking (opening) and carrying current in power

system under normal as well as abnormal conditions. All the equipment, associated with



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switching, interrupting and protection is covered by the switchgear. Circuit breaker is the

heart of switchgear.

Why switchgear, when we have switches and fuses for switching, interruption

and protection. It known that a common experience at home to see a spark in the switch when

putting off. The spark is seen, because the gap between the contacts of switch becomes

conducting and current begins to flow in the gap. The spark is extinguished by the natural

flow of air. At higher system voltage, the natural flow of air is not sufficient to extinguish the

spark (arc) which is dangerous magnitude. Hence switchgear is must & should used for high

voltage system.

VARIOUS TYPES OF CIRCUIT BREAKER:

When currents are interrupted, an arc strikes between the two contacts (fixed &

moving). An arc is column of charged particles moving across the contacts. Various media

used to extinguish arc in the circuit breaker is air, oil, gas (SF6) and vacuum. Now a days

vacuum technology used for circuit breaker at 33/11KV Sub-stations because of they do not

need daily observations and maintenance free and low cost affair.

VACCUM CIRCUIT BREAKER:

Fig. Vacuum circuit breaker



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This breaker employs the principle of contact separation under vacuum (in the vacuum

interrupter or bottle which is provided in top bushing).

VCB is suitable for rapid closing and tripping and its operations i.e. closing

and tripping shall be obtained from compressed /or elongation of spring charging

mechanism.

The closing spring is charged by motor operation /or with manual lever. As

soon as the breaker is closed, the tripping spring are get automatically compressed/or

elongated. Closing spring and associated to spur gears, closing levers, tripping

levers/linkages and trip & close coils are housed in separate operating mechanism box. All

breakers shall be provided with 66 trip free mechanisms. These gear and linkages design

arrangements are varies company to company. But basic theory is same. In the market the

following companies are available and they are used in APCPDCL.

1) S&S (OFVP) -vacuum 2) S&S (OFV) -vacuum

3) System Control -vacuum 4) JYOTHI -vacuum

5) SIEMENS -vacuum 6) BHEL -vacuum

7) ALSTOM/GEC/AREVA-vacuum 8) VICTORY -vacuum

9) G.R.Power -vacuum 10) ABB VCB & SF6

11) A bond strand -vacuum 12) Mega win -vacuum

13) Kirloskar -vacuum 14) VOLTAS -SF6

15) Andrew Yule -vacuum 16) A Lind -vacuum

17) CGL VCB 18) CGL SF6

19) BHEL MOCB 20) OLG -vacuum

21) S&S, SF6 22) HITACHI -Bulk oil

23) BHEL -Bulk oil

The operating mechanism shall be operated by local electrical control. Manual

closing & tripping devices are also provided in the mechanism. Mechanical indicators such

as to show the close or open and spring charge position of the breaker are also provided

in mechanism box.

The VCB / Circuit breaker (except some) shall comprise of three independent

poles filled with a common operation mechanism. Each pole of the circuit breaker shall



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consist of a separate breaking chamber. These breaking chambers shall be mounted on a

common chassis and connected together for common operation i.e. simultaneously tripping /

closing.

Every month oiling / or quarterly black grease shall be applied for mechanism

parts. Do not pour oil / lubrication for close or trip coil plungers. If it is applied, dust & oil

may get jam trip plunger, the breaker will not trip during normal or fault current. Due to,

plunger sluggishness, the breaker opening time will be high and it is possible to burn /

damage relay trip contact. There by protective relaying purpose is defeated. Thus, it is an

operator duty to vigil or daily check up the healthiness of relay contacts. Relay contacts

should be always normally open condition i.e. do not make / touch each other.

It may be happened sometimes that relay trip contacts are got melted touched

each other or broken due to heavy continuous spark which will be generated by trip coil

(Auxiliary switch make / brake operation problem).Healthy trip circuit is checked up daily

once by operator. Even it is found alright, if relay trip contacts are damaged, the breaker did

not trip but relay flag indications will show.

MOCB /OCB:

When transformer oil is used in oil circuit breakers, the main function of the oil is to

extinguish the arc which is formed between the contacts when an electric circuit is opened. In

order to effectively quench the arc formed during operation, the oil must have proper

characteristics so as to offer less resistance to the moving contacts and avoid the risk of fire.

There by, for every 3 No earth faults, the oil shall be replaced with new oil.

The arc consists of conducting ionized particles and is of low electrical

resistance. Electrical stress and temperature are very high. Therefore oil is de ionize and

quench the arc.



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Fig. Bulk oil circuit breaker

Fig. minimum oil circuit breaker



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3) FEEDERS:

A feeder is a conductor which converts the substation to the area where the power is

to be distributed. Generally, no tapings are taken from the feeders so that the current in it

remains the same throughout. The main consideration of a feeder is the current carrying

capacity while the voltage drop consideration is relatively unimportant. It is because the

voltage drop in a feeder can compensate by means of voltage regulating equipment at the

substation. As a good voltage regulation of a distribution system is probably the most

important factor responsible for delivering good service to the consumers, the design of

feeder requires careful consideration.

4) RELAYS:

Function of Protective relays is to cause a prompt removal from service of any

element of a power system when it suffers a short circuit or when it starts to operate in any

abnormal manner that might cause damage or otherwise interface with the effective operation

of the rest of the system. The relaying equipment is added in this task by circuit breakers that

are capable of disconnecting the faulty element when they are called upon to do by the

relaying equipment.5) INSTRUMENT TRANSFORMERS CTs & PTs 33KV, 11KV:

CURRENT TRANSFORMERS:

Current transformers shall be of the steroidal core type preferably encapsulated in epoxy

resin. The current transformers shall contain no hygroscopic materials, which could affect the

moisture contents of the SF6 gas in the CT chamber. The rated short-time thermal current

shall not be less than the through fault capacity of the associated circuit breakers.

The characteristics of current transformers shall be submitted to the Authority for approval

together with details of the protection, instrumentation or measuring equipment with which

each current transformer is to be used. Each current transformer shall be capable of providing

the necessary output to operate the related devices satisfactorily at the lead burdens involved.

Each current transformer shall have a continuous extended current rating of at least l.2 times

the rated current. The characteristics and capacities of current transformers used for

protection circuits shall be calculated by the relay manufacturer who shall prove by



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calculation, the suitability of the CT's being provided in conjunction with the relay

manufacturers requirements for the relays and equipment offered. Where multi-ratio

secondary windings are specified a label shall be provided at the secondary terminals of the

current transformer indicating clearly the connections required for each ratio. These

connections and the ratio in use shall also be shown on the diagram of connections. All

connections from secondary windings shall be brought out and taken by means of separate

insulated leads to a terminal blocks specially designed for the CT circuits, mounted in the

Local Control Cubicle. The secondary windings shall be earthed at one point through a

removable link, which shall be in the relay panels for protection and in the control panel for

instrumentation. CT terminal blocks located in the local control cabinets shall have shorting/

disconnecting links to allow testing with the circuit in service and on load. It shall be possible

to carry out primary injection testing of the CTs including magnetizing curve testing, when

the switchgear is fully assembled, or retesting of the CTs during the service life of the

switchgear without interruption of supply to adjacent circuits. The secondary windings of

each set of current transformers shall be capable of being open circuited for one minute with

the primary winding carrying the rated current. Unless otherwise approved, all current

transformers shall be installed with the P1 terminals adjacent to the bus bars. The polarity of

the primary and secondary windings of each transformer shall be clearly indicated at the

respective terminals and in addition labels shall be fitted in a readily accessible position to

indicate the ratio, class and duty ofeachtransformer.

Fig. 132kv current transformer



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VOLTAGE TRANSFORMERS:

Voltage transformers shall be of electromagnetic type and of metal-enclosed design,

which shall be compatible with the switchgear. They shall contain no hygroscopic insulating

material, which could affect the moisture contents of the SF6 gas in the VT chamber. The bus

voltage transformers shall be provided with motor operated disconnections for disconnecting

the VT for maintenance, testing etc. Line voltage transformers shall be supplied with manual

disconnections.

The voltage transformers shall be capable of discharging the capacitance of line, cables and

switchgear, which may remain connected to them during switching operations. The

Contractor shall declare any limitations of the equipment for this duty. Voltage transformer

secondary and tertiary circuits shall be provided with miniature circuit breakers or fuses asclose to each voltage transformer as possible and shall be labeled with winding and phase

indication. For single-phase voltage transformers separate earth links for each secondary shall

be provided and each neutral lead shall be connected together at a single earth point in the

local control cubicle. Earthing of the VT HV winding shall be through a link separate from

the LV winding. A fixed ladder or other arrangement shall be provided for each voltage

transformer to enable an easy access to the voltage transformer and to the VT MCB/fuse box.

The ratio and phase angle errors of voltage transformers shall not exceed the permissible

limits prescribed in the relevant Standard and shall be capable of meeting the following

additional requirements from 5% rated primary voltage to 90% rated primary voltage: Voltage

error - not exceeding + 3% Phase angle error - not exceeding + l20 minutes. The voltage

transformer shall have a voltage factor withstand rating of 1.2 continuous 1.9 times for 8

hours without saturation. Voltage transformers shall be capable of carrying continuously

without injurious heating 50% burden above their rated burden. Damping resistors shall be

supplied for VT open delta windings.



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Fig. voltage transformer

6) PLCC AND CONTROL PANEL:

This is relevant in power line career communication (PLCC) systems for communicationamong various substations without dependence on the telecom company network. The signals

are primarily tele protection signals and in addition, voice and data communication signals

The line trap offers high impedance to the high frequency communication signals thus

obstructs the flow of these signals into the substation bus bars. If there were not to be there,

then signal loss is more and communication will be ineffective.

The voice signals are converted/compressed into the 300 Hz to 400 Hz range, and this audio

frequency is mixed with the career frequency. The career frequency is again filtered

amplified and transmitted. The transmission of these HF career frequencies will be in the

range of 0 to +32 db.These range is set according to the distance between stations.



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CONTROL PANEL:

The relays and its alarms circuit are placed in control panel, which will be placed in the

maintained building, the secondary of the CTs and pts will be brought to the panel through

the underground cables.

7) EARTHING:

The first step in designing a substation is to design an earthing system. The function of

an earthing system is to provide an earthing system connection to which transformer neutrals

or earthing impedances may be connected in order to pass the maximum fault current. The

earthing system also ensures that no thermal or mechanical damage occurs on the equipment

within the substation, there by resulting in safety to operation and maintenance personnel. Italso improves reliability of power supply.

The earthing is broadly divided as:

SYSTEM EARTHING:

Connection the part of plant in an operating system like LV neutral of the power

transformer winding and earth.

EQUIPMENT EARTHING:

Connecting bodies of equipment like transformer tank, switch gear box, operating

rods of air break switches, LV breaker body, hv breaker body, etc to earth.

The system earthing and safety earthing are interconnected and therefore fault current

flowing through system ground raises the potential of the safety ground and also cause step

potential gradient in and around the substation but separating the two earthing systems have

disadvantages like higher short circuit current, low current flows through relays and long

distance to be converted to separate the two earths.

After weighing the merits and demerits in each case the common practice of common and

solid grounding system designed for effective earthing safe potential gradients is being

adopted the earth resistance shell be as low as possible and shall not exceed the following

limits.



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Power stations - 0.5 ohms

EHT stations - 1.0 ohms

33KV - 2.0 ohms

Tower foot resistance - 10 ohms

D/t structures - 5.0 ohms

8) BUS BAR:

A bus bar in electrical distribution refers to thick strips of copper or aluminum that

conduct electricity within a switch board, distribution board, substation, or other electrical

apparatus.

The size of the bus bars is important in determining the maximum amount of current

that can be safely carried. Bus bar can have a cross-sectional area of as little as 10mm2 but

electrical substations may be use metal tubes of 50mm in diameter.

At extra high voltages(more than 300KV) in outdoor buses, corona around the connections

becomes a source of radio frequency inference and power loss, so connection fittings

designed for these voltages are used.

The most commonly used bus bar arrangements are:

1) Single bus bar arrangement.2) Single bus bar with sectionalisation and

3) Double bus bar arrangement.

9) CAPACITOR BANK:

Capacitors are used to control the level of the voltage supplied to the consumer by

reducing or eliminating the voltage drop in the system caused by inductive reactive load.

They supply fixed amount of reactive power to the system at the point where they are

installed. Its effect is felt in the circuit from the location towards source only. It improves the

power factor of the system and also decreases KVA loading on the source. The location has

to be as near the load point as possible. In practice due to high compensation required it is

found economical to prove group compensation on lines at substation.



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operating volatages, the impedance of lighting arrester, placed in parallel to the equipment to

be protected,is very high and allow the equipment to per form its respective function.

OPERATION AND MAINTAINANCE OF VARIOUS SUB-STATIONEQUIPMENTS:

Regular inspections and preventive maintenance to be done on each of the above

equipment are codified as Preventive Maintenance schedules fixing periodicity for each

item. The main objective of the maintenance is to maintain the insulation in good condition

and to avoid entry of moisture and to remove dirt.

The sustained operating temperature of about 8 to 10 Degrees Centigrade more thanthe operating temperature of 75 Degrees Centigrade will shorten the life of

transformer oil, circuit breaker etc. Hence, over loading should be avoided.

As far as possible the temperature of oil and windings shall be maintained at 400 C

and 450 C above ambient temperature.

If the acidity of oil exceeds 1.0 mg. KOH/Gm of oil, the oil should be replaced with

fresh oil.

Dielectric strength for H.T. equipment (power transformer circuit breaker etc.) shall

be 30 KV for 60 Sec. and 40 KV instantaneous.

Earth resistance should not exceed 2 ohms for 33/11 KV sub-stations. Earth pits are

to be wetted daily.

Fencing should be checked and any missing barbed wire lacings should be replaced to

keep away unauthorized persons or animals entering the sub-station yard.

Grass and weeds growing in the SS yards must be cleared daily.



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----I/P---> ---O/P--->

BLOCK DIAGRAM OF 33/11 KV SUB STATION

The general operation of substation is simple and easy to understand, a sub-station mainly

involves Power transforms, circuit breakers, relays. Its main block diagram consists of circuit

breakers , potential transforms, the power transformers and capacitor banks .The circuit

breakers are given the supply directly from distributor systems then which is, fed to power

transformers with a summation of potential transformers and Capacitance bank.

The power transformers step up or step down based on requirement then they are again

connected with potential transformers and capacitor bank then they are connected to circuit

breakers , the entire arrangement of circuit breakers , potential transformers , capacitor

banks is for the purpose of detecting the faults which occur transmission. Sub-stations play

very important role in distribution hence are must be protected with much care. The circuit

breakers are used to detect the faults onincoming as well as outgoing power, incase of any

faults the circuit breakers trip s itself and thus protects power transformers from engaging

with the fault currents.


CIRCUIT

BREAKERPOWER

TRANSFORMER

CIRCUIT

BREAKER

POTENTIAL

TRANSFORMER

CAPACITANCE

BANK

POTENTIAL

TRANSFORMER

CAPACITANCE

BANK


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Power transformers at generating stations are used to Step up the voltage for transmission

and at receiving stations to step down the voltage for primary and secondary distribution

on. Transformers are generally installed up to lengths of rails fixed on concrete slabs

having foundation of 1 to 1.5 m deep. The potential transformers are employed for voltage above 380V to fed the potential coils

of indicating and metering instruments and relays. These transformers make the ordinary

low voltage, instruments suitable for measurements of high voltage and isolate them from

high voltage.

The current transformers are connected in A.C power circuit to feed the current coil of

indicating and metering instruments and protective relays. The primary is directly inserted

in the power circuit and to the secondary windings, the indicating and metering

instruments and protective relays, is connected. The standard secondary rating adopted

are 5A or 1A. 1A rating is used for major sub-stations which remote controls.

Vacuum circuit breakers are more popular at distribution voltage level up to 33KV

voltage class. They require minimum maintenance except to replace the vacuum

interrupter if a leak has occurred. The reasons being they have no fire risk and have high

reliability with long maintenance free period. For these breakers the main contacts are

housed within a vacuum in an insulating cylinder of glass or ceramic having metal end

plates supporting the contacts.



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RECORD & REPORTS

The following records and reports should be maintained so that the cause of the

fluctuations in voltages and defects in daily operation can be thoroughly examined andrectified immediately.

Some important records and reports are:

1) HOURLY READING BOOK:

All the hourly reading of voltmeters, ammeters, energy-meters, Etc. of all the Feeders

and equipments are noted.

This readings help for calculating M.D. and of the sub-station and it gives us the

information about the extra load which it can take up.

2) DAILY REPORT:

Daily Report of the sub-station should be sent to the Engineer concerned, giving full

information of operations carried out interruptions caused to the feeders during the day.

Load particulars of all the feeders, giving peak load, minimum load, and average loads of

each feeder.



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SOME OF THE OPERATING INSTRUCTIONS:

1) A.B. switches are meant for operating on no load.

2) When workmen are on work spot the line or equipment on which they are working, itshould be first discharges and earthed property

3) Transformers having different KVA ratings may operate in parallel.

The conditions to satisfy for successful parallel operations of the transformer are as follows:

a) They should belong to some vector groundb) Transformers should have the same voltage ratio

c) The percentage impendence of the transformers should be equal when each percentage is

expressed on the KVA base of its respective transformer.

d) It is also necessary that the ratio of resistance in the transformers should be equal



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MAINTENANCE SCHEDULES FOR POWER TRANSFORMERS:

1. Checking the Colour of silica gel in the breather and also oil

level of the oil seal. If silica gel Colour changes from blue to

pink by 50% the silica gel is to be reconditioned or replaced .

Daily

2. Observation of oil levels in (a) main conservator tank (b) OLTC

conservator (c) bushings and examining for oil leaks if any from

the transformer

Daily

3. Visual check for overheating if any at terminal connections (Red

hot) and observation for any unusual internal noises.

Daily in each shift

4. Checking for noise, vibration or any abnormality in cooling fans

& oil pumps of power transformers standby pumps & fans are

also to be run condition to be observed.

Daily

5. Observation of oil & winding temperatures & recording Hourly

6. Visual check of explosion vent diaphragm for any cracks Daily7. Checking for any water leakage into cooler in case of forced

cooling system.

Daily

8. Physical examination of diaphragm of vent pipe for any cracks Monthly

9. Cleaning of bushings, inspect for any cracks or chippings of the

porcelain and checking of tightness of clamps and jumpers

Monthly

10. Measurement of IR values of transformer with 2.5 KV meager

up to 33KV rating and 5.0 KV meager above 33KV rating.

Recording of the values specifying the temperature which

measurements are taken.

Monthly

11. Cleaning of Silica gel breather Monthly

12. Checking of temperature alarms by shorting contacts byoperating the knob.

Monthly

13. Testing of main tank oil for BDV and moisture content Quarterly

14. Testing OLTC oil for BDV & moisture content Quarterly

15. Testing of Buchholz surge relays & low oil level trips for correct

operation

Quarterly

16. Checking auto start of cooling fans and pumps Quarterly

17. Checking of Buchholz relay for any gas collection and testing

the gas collected

Quarterly or during

fault

18. Checking of operation of Buchholz relay by air injection

ensuring actuation alarm & trip

Half yearly or

during shutdown19. Noting the oil level in the inspection glass of Buchholz relay and

arresting of oil leakages if any.

Monthly

20. Checking of all connections on the transformer for tightness

such as bushings, tank earth connection

Quarterly

21. Lubricating / Greasing all moving parts of OLTC mechanism Quarterly or as

given in the

manufacturers

manual

22. Checking of control circuitry, interlocks of oil pumps and

cooling fans for auto start and stop operation at correct



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temperatures and also for manual operation

23. Testing of motors, pumps and calibrating pressure gauge Half yearly

24. Pressure testing of oil coolers Half yearly

25. Testing of oil samples for dissolved gas analysis (for 100MVA

transformers)

Half yearly

26 Testing of oil for dissolved gas analysis of EHV transformers upto 100KVA capacity

Once in a year

27. Overhauling of oil pumps and their motors also cooling fans &

their motors.

Once in a year

28. Testing of oil in main tank for acidity, tan delta, interface tension

specific resistively

Once in a year

29. Bushing testing for tan delta Once in a year

30. Calibration of oil & winding temperature indicators Repeats

31. Measurement of magnetizing current at normal tap and extreme

taps

One in a year

32. Measurement of DC winding resistance Once in a year 33. Turns ratio test at all taps Once in a year

34. Inspection of OLTC mechanism and contacts its diverter switch Once in a year or

number of

operation as

recommended by

manufacturers are

completed

whichever is earlier.

35. Overhaul of tap changer and mechanism One in a year

36. Replacement of oil in OLTC Once in year or

whenever numbersof operations as

recommended by

manufacturer are

completed

whichever is earlier.

37. Calibration of thermometers (temperature indicators) and tap

position indicator.

Yearly

38. Remaining old oil in thermometer pockets, cleaning the pockets

and filing with new oil.

Yearly

39. Checking oil in the air cell (for transformers of 100 MVA &

above capacity)

Yearly

40. Bushings partial discharge test and capacitance (EHV

transformers)

Yearly

41 Filtration of oil / replacement of oil and filtration Whenever the IR

values of

transformer are

below permissible

limits and oil test

results require

filtration /



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replacement of oil

42. General overhaul (consisting 1) Inspection of core & winding (2)

Through washing of windings (3) Core tightening (4) Check-up

of core bolt insulation (5) Replacement of gaskets (6) Overhaul

of OLTC

One in 10 years

MAINTENANCE SCHEDULE FOR BATTERIES:

S. No. ITEM OF MAINTENANCE PERIODICITY

1. Taking specific gravity and voltage of pilot cells. Daily.2. Checking specific gravity and voltage of each cell

a) Lead-acid cell.

b) Nife cells.

(Before and after charging.

Weekly when trickle charge

exists)

Monthly.

3. Cleaning of terminals applying Vaseline and topping up

with distilled water.

Weekly for Lead-acid batteries.

4. Over-haul of Nife Battery recommended by manufacturers. Yearly.

5. Leakage test by lamp or voltmeter method Each shift

6. Checking all connections of charger and battery for

lightness

Quarterly

EARTHING:

Maintenance schedule of station earths includes transformer, and lightening arrestors earths.

S. No. ITEM OF MAINTENANCE PERIODICITY

1. Combined earth resistance Monthly

2. Checking earth connection at joints Monthly

3. Watering of sub-station earth Daily

4. Water distribution transformer earth Twice a week (Earth)

RECOMMENEND MAINTAINENCE SHEDULE FOR CERTAIN IMPORTANT

FACTORS:

S. No. INSPECTION

FREQUENTLY

ITEMS TO BE

INSPECTED

INSPECTION

NOTE

ACTION REQUIRED IF

INSPECTION SHOWS

UNSATISFACTORY

CONDITIONS

1. Hourly Ambient temp.

2. Hourly Winding temp. Check that temp.

rise is reasonable

Shut down the Transformers and

investigate if temp. is persistently

higher than normal.



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3. Hourly Oil temp.

4. Hourly Load (amperes) Check against

rated figure

Note: An improper tap position

can cause Transformers failure.

5. Hourly Voltage

6. Daily Oil level in

Transformer

Check against oil

gauge

If low, examine the Transformers

for leak.7. Daily Relief vent

diaphragm

Replace if cracked or broken

8. Daily Dehydrating

Breather

Check colour of

the active agent

If silica gel is pink, change it with

new one

9. Quarterly Bushing Examine for crack

and dirt deposits

Clean or replace

10. Quarterly Oil in

Transformers

Check the electric

strength and water

content (B.D.V ,

P.P.M)

Take suitable action to restore

quality of oil

11. Quarterly Dehydrating

breather

Check oil level in

oil cap and that airpassages are free.

Make up oil , required

12. Yearly or earlier

if Transformer

can conveniently

be taken out for

checking

Oil in

Transformers

Check for acidity

and sludge

Filter or replace


if Transformer

can conveniently

be taken out for

checking

Insulation

resistance

Compare with

value at time of

commissioning

Filter or replace


if Transformer

can conveniently

be taken out for

checking

Gasket joints Tighten the bolts evenly to avoid

uneven press.


if Transformer

can conveniently

be taken out for

checking

Relays , alarms ,

their circuit etc.

Examine relay and

alarm contacts and

their operations,

fuses etc. check

relay accuracy etc.

Clean the components or replace

contacts and fuses if necessary.

Change the setting if necessary.


if Transformer

can conveniently

be taken out for

checking

Temperature

indicator ; WTI,

OTI

Pockets holding

the thermo meter

should be checked.

Oil to be replenished if required


if Transformer

can conveniently

be taken out for

checking

Dial type Oil

gauge

Check pointer for

freedom

Adjust if required


if Transformer

Earth resistance Take suitable action, if resistance

is high.



29/38

can conveniently

be taken out for

checking

19. Two- yearly Oil conservator Internal inspection Should be thoroughly cleaned

20 Two yearly Buchholz relay Mechanicalinspection Adjust floats, switches etc asrequired.

COMMON DEFECTS NOTICED AND THE CAUSE

S. No PARTS DEFECTS CAUSES

1. Tank a. Leakage of oil

b. Deformation

c. Overheating

Corrosion / mechanical damageGaskets worn out

excessive internal pressureImproper circulation of

cooling oil and / or inadequate ventilation.

2. Radiators a. Leakage of Oil

b. Deformation

c. Overheating




3. Conservator a. Leakage of Oil

b. Deformation

c. Overheating




4. Breather Ineffective Inlet choked Silica gel saturated

5. Explosion Glass broken Mechanical

6. Core a. Loose

b. Increased Losses

c. Excess Noise

Bolts loosening up change in characteristics due to

heating vibration of stampings

7. Winding a. Short Circuited

b. Loosening

c. Insulation Brittle

d. Open circuited

Overloading Air bubbles loss of insulation

shrinkage displacement Overheating decomposition

burn out.

8. Oil a. Discoloration

b. High Acidity

c. Low BDV

d. Sludge

Contamination Increased moisture

Decomposition chemical action with other parts.

9. Terminal

Bushing

a. Breakage

b. Leakage of Oil

Strain Gasket Worn out Loose fit.

10 Tap Switch a. Inoperative

Broken leverb. Burnt Contact

c. Short Circuit

Mal operation Insulation failure Failure of

operation mechanism overheating



30/38

Types of faults against which Buchholz relays gives successful protection:

Visible or Audible Alarm (Upper Float

Actuates)

Trip Circuit Operates (Lower Float Actuates)

1. Core bolt insulation failure 1. Short circuit between phases

2. Short circuited core laminations 2. Winding earn fault3. Bad electrical contacts 3. Winding short circuits

4. Local overheating 4. Puncture of busing

5. Loss of oil due to leakage

6. Ingress of air into the oil system

CONDITIONS FOR THE PARALLEL OPERATION OF TRANSFORMERS:

The conditions that must be observed for the parallel operation to transformers bothprimary and secondary side.

1. Same voltage ratio.

2. Same polarity.

3. Same phase sequence and

4. Zero relative phase displacement.



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A L A R M S

S. No ALARMS ACTION TO FOLLOW

1. Oil temperature Alarm

for a transformer

Cancel the alarm and feel temperature by touching

transformer body and verify winding temperature also and

compare with the other transformer. If no-abnormality is

observed. It is a false alarm. Inform AE & ADE. If

excess temperature is observed cut-off few loads and

inform AE & ADE & ECR.

2 Winding Temperature

Alarm for a transformer

- DO -

3 Buchholz Alarm Cancel the Alarm and isolate if both from HV and L.V.

side. Restrict the loads to the available capacity. Inform

AE, ADE, ECR & MRT.

4 Buchholz TRIP Cancel the Alarm and disconnect the leads connecting to

trip and isolate the transformer. Restrict loads to available

capacity.

5 Neutral displacement

relay of capacitor bank

Cancel alarm, isolate the bank and inform to ADE, MRT.

6 Any feeder breaker Cancel the alarm and follow for operating instruction.



32/38

STATION BATTERIES

PRINCIPLE OF CHARGING AND IDENTIFICATIONS:

TYPE OF BATTERY USED: LEAD ACID BATTERIES

The A.C. 3 phase 11KV supply volts which are stepped down by the STATION

TRANSFORM to 3 phase 440V supply with a neutral. A single phase 230 volts which is

available from 3 phase with a neutral is sufficient for the station batteries to get charged. A

RECTIFIER UNIT is used to convert the A.C. current to unidirectional current. The output of

the rectifier is directly given to the batteries for charging.

When the cells are full charged, the voltage ceases to raise the co lour of plates; on

full charge is deep chocolate brown for positive plate and grey for negative plate. The

approximate value of the E.M.F. is 2.1V. During charge in the density of electrolyte increases

due to absorption of water or the Electrolyte assumes a milky appearance the specific gravity

can be measured with a suitable Hydrometer.

NUMBER OF END CELLS:

When the battery is fully charged with each having an emf of 2.1 V, then the number of

cells required is volts/2.1 =110 cells.

TRICKLE CHARGING:

It is essential that the batteries should be fully charged and ready foe service when an

emergency rises. To keep it fresh, the battery is kept on a trickle charge. The rate of trickle

charge is small and is just sufficient to balance the open circuit losses. It keeps the sells fully

charges with out any passing in good conditions.



33/38

MAINTENANCE OF STATION BATTERIES:

1) The level of the electrolyte should be 10 to 15mm above the top of the plates and mustnot be exposed to air.

2) The battery terminals and metal and supports should be cleaned down to bars metal and

covered with Vaseline or petroleum jelly.

3) The acid and the corrosion on the battery top should be washed off with a cloth moistened

with baking soda or ammonia and water.

4) It must be taken care that the batteries should not be left in discharged condition for long.

Since acid does not vaporize, none should be added



34/38

SAFETY MEASURES

SAFETY MEASURES FOR SUB-STATIONS:

1) Lifting of should be made by lugs or jacks wheels, never by cooling tubes.

2) Transportation of transformer should be on large diameter wheels unless strength and

acidity.

3) Transformer oil should be tapped periodically for checking of dielectric strength and

acidity.

4) Co lour of breather needs daily watch.

5) For proper rating, instrument fuses should be checked.

6) No cotton waste would be used to clean the dirty/dust.

7) At every shift D.C. supply may be checked. It is essential for safety.

8) Maintenance registers of sub-station should be studied thoroughly to gain lot of future

warnings.

9) In order to reduce fire troubles, cable trenches must be fill with sand.

10) Insulators should not be able to work while O.C.B. is closed and so interlocking system

should be checked.

11) No new comers should be allowed to switch gear. Visitors should have authorized person

of sub-station.

12) Do not take chance without safety measures.



35/38

PRECAUTIONS AGAINST FIRE IN TRANSFORMER:

1) Transformers should be installed at a reasonable place from inflammable material.2) For out-door sub-stations.

a) Provided with drainage.

b) Stone chippings are filled around the plinth area.

3) For in-door sub-station

a) Stop the spreading of oil.

b) The floor should be non-flammable.

c) The main space between the transformer and walls should be one meter.

4) Fix fighting equipment should be installed.

5) Emergency relief pipe should point in a safe direction. In case of fault condition, burning

oil may be ejected from it.

PRECAUTIONS AGAINST SHOCK:

1) All parts of the metal tank should be effectively bonded together and properly earthed.

2) The transformer should be fully protected with cable boxes, terminals boxes so that live

metal parts are totally enclosed.

3) When the transformer is energized an enclosure to the transformer with a door or gate

which our be kept locked, should be provided.

4) Precautions should be taken against exposed metal ends of bushings.



36/38

CASE STUDY

At a 33/11kv substation we find many number of equipments involved in it starting from the

input to the output feeders. A 33kv from jubilee hills provided at the input feeder and with the

help of AB switches the direction of the flow is changed depending up on the requirements.

At first the 33kv input is given to the LV side of the first transformer of 12MVA and only up

to 800MW it is used. We are provided with the group circuit breakers (GCB) where the

charging takes places after that only the transformer will be oned. Another transformer with

33kv is provided on the HV side then it gets step down to 11kv on LV side. From the LV side

we have four individual feeders they are city City centre, Rainbow, Tata rao & Erramanzil.

From these four feeders the supply to the required area can be given through transmission

system. The GCBS provided will have energy meters as well as relays which operate at the

time of faults.

Sample readings of energy meter:

Daily reading:

Feeder1(MWH) Feeder2(KWH) Feeder3(KWH)

9955.49 1810157 2199307

9961.88 1811468 2200286

9968.12 1812709 2201234

Monthly reading:

Feeder1(MWH) Feeder2(KWH) Feeder3(KWH)

435620 668040 936140



37/38

CONCLUSION

With the high demand of power, it has become necessary to have high voltage

distribution thus casing construction of high voltage sub-station like 66 KV, 132 KV, and 200

KV and so on. For operating the high voltages, the need for reliable protective devices and

switch gear has become paramount important. When short circuit occurs, an enormous power

can be fed into the fault with considerable damage and interruption of service.

As it is equipped with many protection circuit breakers for the protection of various

buses or lines from faults, even if the faults remains for a moment then, due to high current

all the equipments connected to the line would damage. At the same time identifying and

isolating the faults is important which is done by various relays and circuit breakers in the

substation. If the fault is not cleared then the faulty line is isolated by using the bus couplers

in substation.

Thus maintenance and operation of substation is very important and it will represent the

healthy network.



38/38

REFERENCE

1. Technical reference book of substation (aptransco).

2. www.whitepapers.com3. www.technologyreview.com

4. www.onlinetaken.com.

5. Wikipedia (Google search).
http://www.whitepapers.com/http://www.technologyreview.com/http://www.onlinetaken.com/http://www.whitepapers.com/http://www.technologyreview.com/http://www.onlinetaken.com/

theory_(1 - 6)_without numbers

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