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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 sub- station 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.
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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
13. Yearly or earlier
if Transformer
can conveniently
be taken out for
checking
Insulation
resistance
Compare with
value at time of
commissioning
Filter or replace
14. Yearly or earlier
if Transformer
can conveniently
be taken out for
checking
Gasket joints Tighten the bolts evenly to avoid
uneven press.
15. Yearly or earlier
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.
16. Yearly or earlier
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
17. Yearly or earlier
if Transformer
can conveniently
be taken out for
checking
Dial type Oil
gauge
Check pointer for
freedom
Adjust if required
18. Yearly or earlier
if Transformer
Earth resistance Take suitable action, if resistance
is high.
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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
Corrosion / mechanical damageGaskets worn out
excessive internal pressureImproper circulation of
cooling oil and / or inadequate ventilation.
3. Conservator a. Leakage of Oil
b. Deformation
c. Overheating
Corrosion / mechanical damageGaskets worn out
excessive internal pressureImproper circulation of
cooling oil and / or inadequate ventilation.
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
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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.
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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.
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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
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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.
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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.
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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
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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.
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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/