Components
OF
SUBSTATION
and
TROUBLE
SHOOTING
1
INDEX
PAGE NO.
1. INTRODUCTION
3
2. SINGLE LINE DIAGRAM
4-5
3. MAIN EQUIPMENTS
6
4. COMMON TERMINOLOGY
7
5. LIGHTNING ARRESTER
8-9
6. INSTRUMENT TRANSFORMER
10 - 17
7. ISOLATORS
18 - 19
8. CIRCUIT BREAKER
20 - 23
9. CAPACITORS
24 - 25
10. TRANSFORMER AND ITS TESTING
26 - 46
11. CONTROL AND RELAY PANEL
47 - 50
12. STATION TRANSFORMER & BATTERY
51 - 52
13. MEASURING INSTRUMENTS
53 – 57
14. NUMBER NOTATION
58
15. TROUBLE SHOOTING
59 – 62
2
INTRODUCTION:The Substation may be defined as assembly of apparatus which transforms
the characteristics of electrical energy from one form to another, say for example,
from A.C. to D.C. and from one voltage to another. A.C. electrical energy is
generated at low voltage but for transmission the voltage is stepped up. Higher
the voltage, lesser is the current and lesser is the power loss (I²R) and lesser is
the voltage drop (IR). Similarly the consumers do not use high voltage and so the
same must be stepped down to low voltage. The stepping up and stepping down
of voltage is done in the substations. There are various bays for Incoming Lines,
Outgoing Lines, Transformer, Bus-coupler and Bus transfer etc. Each bay has
equipments such as CT, PT, Circuit Breaker, Isolators, Post Insulators, etc.
Substation up to 11 KV is generally Indoor substation with metal-clad draw
out type switchgear. In such switchgear, circuit breaker is mounted on a carriage.
After opening of the breaker, the same can be lowered and the carriage can be
pulled out. For voltage of 33 KV and above, outdoor substation is generally
preferred. In such substation, circuit breakers, isolators, CT, PT, transformers, are
installed outside. Bus bars and connectors can be seen by naked eye. The bus
bars are supported on post insulators or strain insulators. The substation has
galvanised steel structures for supporting the equipment, insulators and the
incoming or outgoing lines.
Classification of Voltage Levels
Low Voltage
:
Below 1000 Volts …..LV
Medium high voltage
:
Between 1000 Volts and 33 KV …..MV
High Voltage
:
Above 33 KV and up to 132 KV …..HV
Extra High Voltage
:
220 KV, 400 KV …..EHV
Ultra High Voltage
:
760 KV …..UHV
3
SINGLE LINE DIAGRAM
132 / 33 KV SUBSTATION (single BB)
132 KV I/C
LINE 1
LINE PT
LA
ISOLATOR WITH E/SW
LINE CT (CT1)
LINE CB
BUS ISOLATOR
132 KV BUS
BUS ISOLATOR
BUS PT
HV CT OF TRAFO. (CT2)
HV SIDE CB
POWER TRANSFORMER
132 / 33 KV
LV CT OF TRAFO. (CT3)
LV SIDE CB
BUS ISOLATOR
33 KV BUS
BUS ISOLATOR
STATION
TRANSFORMER
FEEDER CT (CT4)
BUS PT
FEEDER CB
ISOLATOR WITH E/SW
33 KV O/G LINE FEEDER # 1
4
Above-mentioned Single Line Diagram is General layout of Switchyard
Equipments. Only Single Incoming 132 KV Line and Single 33 KV Outgoing
Feeder are shown. There may be ‘n’ number of Feeders as per design. It depends
upon the rating of installed Switchyard Equipments.
The CTs in circuit are used for different purpose as mentioned below:
CT1 – 132 KV Line Metering and Distance, Directional O/C & E/F Protection
CT2 – Transformer HV Metering & Differential, REF & Non-Directional O/C & E/F
Protection
CT3 – Transformer LV Metering & Differential, REF & Non-Directional O/C & E/F
Protection
CT4 – 33 KV Feeder Metering & Non-Directional O/C & E/F Protection
5
Substation Main Equipments and Its Functions
1) Power Transformer: To step up or step down voltage and transfer power from
one A.C. Voltage to another at the same frequency
2) Circuit Breaker: Automatic switching during normal or abnormal condition
3) Current Transformer: To step down the current for measurement / protection
4) Potential Transformer: To step down the voltage for measurement / protection
5) Isolator: Disconnection of circuit under no load condition
6) Earthing Switch: To discharge the voltage on dead lines to earth
7) Bus Section: For connecting Incoming / Outgoing Circuits
8) Lightning Arrester: To discharge lightning & switching over voltages to earth
9) Capacitor Bank: To Improve the power factor of the system & provide
compensation to reactive power absorbed by inductive loads, reduce the over
loading of the cables, transmission lines & transformers for the same load to be
handled
10) Protective Relay: To sound an alarm or to close the trip circuit of breaker so
as to disconnect a component during abnormal conditions (over load, under
voltage, unbalanced load, short circuits)
11) Battery Banks: To maintain the D.C. supply continuity during A.C. supply
failure for keeping equipment in operation for normal & abnormal conditions
12) Station Transformer: To supply of A.C. power for charging the batteries and
provide D.C. control supply for station equipments operations, for Illumination, for
spring charging motors of breakers, for cooling system of transformer
6
Some Common Terms used with meanings
1) ONAN
-
Oil Natural, Air Natural
2) ONAF
-
Oil Natural, Air Forced
3) OFAF
-
Oil Forced, Air Forced
4) WTI
-
Winding Temperature Indicator
5) OTI
-
Oil Temperature Indicator
6) PRV
-
Pressure Relief Valve
7) OSR
-
Oil Surge Relay
8) OLTC
-
On Load Tap Changer
9) RTCC
-
Remote Tap Change Control
10) MOG
-
Magnetic Oil Level Gauge
11) IDMT
-
Inverse Definite Minimum Time (For Relay)
12) NO
-
Normally Open Contact
13) NC
-
Normally Closed Contact
14) LILO
-
Loop In Loop Out (Used for defining Substation)
15) CRP
-
Control Relay Panel
16) TTB
-
Test Terminal Block
17) ACDB –
A.C. Distribution Board
18) DCDB –
D.C. Distribution Board
19) MB
–
Marshalling Box (For Breaker, Transformer control)
20) AVR
–
Automatic Voltage Control (For Tap Changing on RTCC Panel)
7
Detail description of Each Equipments, Its Testing & Maintenance
1) LIGHTNING ARRESTER
Lightning arrester gives protection to substation equipments by discharging
lightning & switching over voltages to earth. It consists of a series of spark gaps
and several non-linear resistances like thyrite, metrosil, etc. A non-linear resistor
is one whose resistance is not constant but inversely proportional to applied
voltage, it decreases rapidly as the voltage across it is increased, i.e. it has an
extremely low value when the high surge voltage appears & allows the flow of
heavy currents of the order of thousands of amperes & dissipates energy quickly
& recovers again, presents a high resistance value to the normal line voltage as
soon as surge has disappeared, so that any tendency of the arc to continue is
immediately suppressed. In a system which has its neutral solidly earthed, the
rated voltage of the arrester is usually taken as 80% of its maximum line to line
voltage. In an unearthed system it is taken as 100% of line-to-line voltage since
under fault conditions when one line is earthed, the arrester connected to the
other two lines would be subjected to full line-to-line potential.
Surge Counter
Lightning Arrester
8
Surge Counter
Testing: 1) IR Testing between Stack to stack & between each Stack to earth by suitable
megger.
2) Surge Counter Test - Apply 230V AC supply across the counter & check pointer
movement in clockwise direction.
Maintenance: 1) Insulator cleaning
2) Connections tightness
3) Checking of Earthing connections
4) Reading of leakage current on daily basis to be taken. If current shoots in red
zone, then that particular LA is to be replaced as early as possible.
9
2) INSTRUMENT TRANSFORMERS
These transformers are minimum oil type & hermetically sealed. They are
expected to be maintenance free during their service life. They transform the high
current or high voltage connected to their primary windings to the standard low
values in the secondary that feed the metering and protection apparatus. It also
isolates the secondary circuits from very high voltages of power system.
From the application point of view, these are divided into mainly two
categories 1) Metering 2) Protection type.
Metering Type – The specified performance of CT is to be maintained in the
range normally 5% to 120% of the rated current. The CT cores should be such
that it saturate at its instrument security factor (ISF) for safeguarding the
instrument from getting damaged under fault condition. The VT designed for
metering is required to perform as specified within the voltage range near to the
rated voltage normally 80% to 120% of the rated voltage.
Protection Type – Main requirement of performance of protection class CTs is
that its cores should not get saturated below its Accuracy Limiting Factor (ALF) up
to, which the primary current should be faithfully transformed to the secondary,
maintaining the specified accuracy. During fault conditions, the primary of CT
carry very high current and the first few cycles of wave have the D.C. component,
which may sature the core. Behaviour of the cores in such condition should be
such as to avoid getting magnetized & to come to normalcy (demagnetised stage)
soon after clearing fault.
Outdoor Type Instrument Transformer – These are used in Substations and
Power stations where high voltages are employed. While designing for their
performance, following factors should be considered.
A) Effect of atmosphere environment:-Use of porcelain insulators for external
isolation between Live and Ground. These insulators provide outer casing for all
the atmospheric conditions like rain, dust, chemical contamination, wind, sun, etc.
10
B) The insulation between primary & secondary windings has to be suitable for
withstanding the disturbance in the network system such as switching surges,
lightning surges, temporary over voltages, fault currents, over load currents, etc.
C) These transformers are normally oil filled with paper insulation and are
hermetically sealed to avoid ingression of moisture.
Instrument Transformer has the following major components:1) Primary Winding
2) Secondary Winding
3) Major Insulation
4) Insulator
5) Transformer Oil
6) Metal Tank
PT
CT
CT is connected in series with the supply line & PT is connected across the
supply line. The CT secondary should never be open circuited and no fuse should
be inserted. In a PT the secondary should never be short-circuited and a fuse is
used in PT secondary circuit.
11
Current Transformer: Types
a) Window CT: - This is constructed with no primary winding and is installed
around the primary conductor.
b) Bushing CT: - This is window CT specially constructed to fit around a bushing
and it cannot be accessed.
c) Bar CT: - It is window CT but has a permanent bar installed as a primary
conductor.
d) Wound CT: - This CT has a primary & secondary winding like a normal
transformer. This CT is rare and is used at very low ratios and currents, typically in
CT secondary circuits to compensate for low currents, to match different CT ratios
in summing applications, or to isolate different CT circuits.
The type of primary winding depends upon the type of CT insulation i.e. whether
Dead tank or Live tank (Inverted Type) Design.
Dead Tank: - In this design, the secondary core windings are housed in metallic
tank, which is lower part of the CT and solidly earthed. The leads of the primary
winding are brought at top chamber for termination. The primary winding in the
shape of ‘hair pin’ or ‘bolt’ is passed through the secondary cores and full
insulation is provided on primary windings.
Live Tank (Inverted): - In such design, the secondary cores and the primary
windings are assembled in the metal tank located at the top of the Current
Transformer. Here the secondary cores assembly is insulated fully for high system
voltage & primary winding is looped through the core assembly. The primary
winding can be single bar primary or multi-turn primary.
12
Hermetically Sealing: - The Instrument Transformer is supposed to be
maintenance free and hence there is no scope of filtering or change of oil during
its life. This makes it essential to hermetically seal the transformer to avoid
breathing of atmospheric air.
1) Sealing with Metallic Bellows: - It is fitted in expansion chamber mounted at
the top of the Instrument Transformer, which separates oil with any external
environment. This allows the expansion and contraction of oil volume, as the
bellow is free to expand and contract.
2) Sealing by Nitrogen Cushion: - Expansion chamber on top of the CT is
evacuated first applying vacuum and then vacuum is filled with dry Nitrogen. The
chamber is then sealed thus avoiding breathing with outer atmosphere.
If CT and Protective devices located within same switchgear, 5 Amp secondary
current is used. If CT lead goes out of the switchgear, 1 Amp secondary current is
preferred.
Accuracy Class: - It is the rated ratio accuracy in percent.
Accuracy Limit Factor (ALF): - It is the ratio of largest value of current to CT
rated current up to which CT must retain the specified accuracy.
Example: - CT - 5P20, 5 VA, ALF = 20
It means error < 5 % up to 20 times rated current for burden of 5 VA
Accuracy class 1% means max. Ratio error < 1% at rated current & burden.
CT Core Identification as per class: 1) Class - 0.2, 0.5, and 1.0: - Metering Core
2) Class - 5P10, 5P20, etc.: - Backup protection core (O/C & E/F Protection)
3) Class - PS: - Primary protection core (Differential, Distance, REF etc.)
13
CT Testing: 1) IR Testing –
a) Primary to earth by 5 KV megger
b) Secondary each core to earth by 500 V megger
c) Primary to secondary by 5 KV megger
d) Secondary core to core by 500 V megger
2) Polarity Test - For carrying out this test, we require one 1.5 V cell, DC
analogue ammeter.
P1
P2
S1 +
− S2
Analogue Ammeter
+
−
CELL
By making above connection, if there is positive deflection of ammeter, then
polarity is confirmed.
3) Ratio Test - Inject current in primary winding & measure induced secondary
current for different current readings and verify with CT Ratio.
4) Knee point check for PS class core - Inject 230 V variable AC voltage in
secondary core with ammeter in series. At certain stage, with 10% increase in
voltage, current shoots up almost 50%. This is the Knee point voltage. After
performing this test, Voltage is gradually reduced to Zero to demagnetise the CT.
5) Winding Resistance Test - Measure secondary winding resistance by microohm meter.
14
6) Tan Delta Measurement – For getting concept of Tan Delta (Tan δ ),
we consider the insulation of equipment as Capacitor.
Ideal Ic = V/Xc
Actual Ic
Loss
V Angle
δ
Xc
∼
φ Phase Angle
V
If the capacitor is good or perfect, it will pass only capacitive or charging current
on application of voltage. Ideal capacitive current Ic leads voltage by 90°. But in
practice, insulation has impurities & actual charging current vector departs from
the ideal Ic vector by a small angle (δ ) called the loss angle.
The loss angle (δ ) = 90 – Power factor angle
(φ )
Higher tan δ produces high dielectric loss that causes increase in temperature
of paper insulation. Increased value of Tan δ can be due to any of the following: a) Moisture in the insulation.
b) Contamination of oil.
c) Internal partial discharge.
CT Maintenance: a) Checking of Oil level & leakage, rectify the same immediately.
b) Checking of Insulation Resistance.
c) Power connection tightness.
d) Secondary connection tightness.
e) Cleaning of Bushings / Insulators.
f) Check the proper earthing of Body connection.
g) Check the earthing of CT Secondary core star points.
k) Check the working of stainless steel bellows.
l) Check the nitrogen pressure in case of Nitrogen filled CT.
15
Potential Transformer: - There are two types of PTs as mentioned below:
1) Electromagnetic Voltage Transformer – Its construction largely depends on
the rated primary voltage. Primary & secondary windings are wound on magnetic
core like in usual transformer. For voltages up to 3.3 KV, dry type transformer with
varnish impregnated taped winding is quite satisfactory. For higher voltages, it is a
practice to immerse the core and winding in oil. It is used up to 66 KV level.
2) Capacitor Voltage Transformer – For voltages above 66 KV, CVT is used. It
consists of a capacitive potential divider & inductive medium voltage circuit.
Primary voltage is applied to a series capacitor group. The voltage across
intermediate capacitor is taken to primary of auxiliary voltage transformer. The
secondary of auxiliary voltage transformer is taken for measurement or protection.
The inductive part is immersed in oil and sealed with an air cushion inside a steel
tank. Fuses are provided in secondary box. Voltage Factor of PT is maximum
system voltage, PT can withstand & is expressed in % i.e.120% continuous &
150% for 30 seconds.
PT Testing: 1) IR Testing –
a) Primary to earth by 5 KV megger
b) Secondary each core to earth by 500 V megger
c) Primary to secondary by 5 KV megger
d) Secondary core to core by 500 V megger.
2) Ratio Test - Inject A.C. variable voltage in primary winding & measure induced
secondary voltage at different voltages & verify the same with PTR.
16
PT Maintenance: a) Checking of oil level & leakage, rectify the same immediately.
b) Checking of Insulation Resistance.
c) Power connection tightness.
d) Secondary connection tightness.
e) Cleaning of Bushings / Insulators.
f) Check the proper earthing of Body connection.
g) Check the secondary fuse condition & replace if required by proper rating.
h) Check the working of stainless steel bellows.
i) Check the nitrogen pressure in case of Nitrogen filled PT.
17
3) ISOLATOR AND EARTH SWITCH
Isolator is the device, which makes & breaks circuits in no load condition.
Types of Isolators:
a) Centre Break Rotating Type Isolator.
b) Double Break Rotating Type Isolator.
c) Pantograph Type Isolator.
d) Tandem Isolator.
Earthing Switch is provided for safety purpose to work on Dead Lines and is
electrically & mechanically interlocked with Isolator.
Isolator Testing: 1) IR Testing – Phase to phase & Phase to earth by 5 KV megger.
2) Contact Resistance check - Measure contact resistance by suitable micro-ohm
meter.
18
Isolator Maintenance: 1) Checking of the male / female contacts for good condition and proper
connections.
2) Checking proper alignment of male & female contacts & rectify if required.
3) Cleaning of Insulators.
4) Lubrication of all moving parts on regular basis.
5) Tightness of all earthing connections.
6) In case of Isolator with Earth switch, check electrical and mechanical interlock
i.e. Isolator can be closed only when E/switch is in open condition & vice versa.
7) As Isolators are operated on No load, hence check the interlock with Circuit
Breaker, if provided i.e. Isolators can be operated when Breaker is in OFF
condition.
8) The motor operating mechanism box, in case of motor operated isolators,
should be checked for inside wiring, terminal connectors, etc.
9) Check the Panel indications i.e. semaphore & bulbs if provided (Isolator and
Earth switch - close and open condition) and rectify if required.
19
4) CIRCUIT BREAKER
Circuit Breaker is used to close or isolate the circuit in normal and abnormal
condition and to protect the electrical equipment against the fault. The parts of a
circuit breaker include –
1) Poles with interrupter, support porcelain, arc quenching medium, etc.
2) Operating mechanism
3) Support structure
4) Control circuit
SF6 Circuit Breaker
The part of the breakers assembled in one phase is called a pole. A circuit
breaker suitable for three-phase system is called a triple pole circuit breaker. All
the three poles operate simultaneously. Each pole comprises one or more
interrupters or arc quenching chambers. The interrupter is mounted on support
insulators. The interrupter encloses a pair of fixed and moving contact. The
moving contact can be drawn apart by means of the operating mechanism. The
operating mechanism gives the necessary energy for opening and closing of
contacts of the breakers. The arc produced by the separation of current carrying
contacts is extinguished by a suitable medium.
20
When a fault occurs in the protected circuit, the relay connected to the CT
actuates and closes its contacts. D.C. current flows from the source in the trip
circuit. As the trip coil of the breaker is energized, the circuit breaker operating
mechanism is actuated & it operates for the opening operation automatically. The
spring in the operating mechanism is charged by electrically or manually. Breaker
auxiliary switches are mechanically attached with the operating mechanism of
breaker. The contact changeover takes place as per breaker operation. Auxiliary
contacts are used for breaker operation circuit, indication circuit, and trip circuit
supervision circuit.
The Circuit breakers are classified on the basis of arc extinction medium:
(i) Bulk Oil type
(ii) Minimum Oil type
(iii) Air Blast type
(iv) SF6 Gas type
(v) Vacuum type
In short, difference of individual breaker is listed below:
1) Bulk Oil Circuit Breaker – Contacts are separated inside a steel tank filled
with transformer oil used for arc quenching.
2) Minimum Oil Circuit Breaker – Contacts are separated in an insulated
housing (interrupter) filled with transformer oil used for arc quenching. In the case
of MOCBs after certain number of tripping, oil is to be replaced as recommended
by the manufacturer. After 2 to 3 times of oil replacement, or after certain numbers
of serious faults, it is necessary to overhaul the complete breaker.
3) Air Blast Circuit Breaker – It utilizes high-pressure compressed air for arc
extinction.
21
4) SF6 Gas Circuit Breaker – Sulphur-Hexa-fluoride gas is used for arc extinction
in this breaker. It is must to monitor the SF6 gas pressure inside the breaker pole
and check periodically the contact resistance of each pole or the travel of each
pole. This is helpful to prevent the problem of bursting of poles. The SF6 breaker
has an advantage that the rate of restricting voltage is zero & hence the burning of
male / female contacts is less. Operating mechanism is of two types: 1) Movement of contacts is controlled by spring mechanism. (Spring Operated)
2) Movement of contacts is controlled by air pressure. (Pneumatic operated)
5) Vacuum Circuit Breaker – In this breaker, the contacts are housed inside a
permanently sealed vacuum interrupter. The arc is quenched as the contacts are
separated in high vacuum. For VCBs, the vacuum bottle is hermetically sealed
and as such no maintenance is required. However to ascertain the failure of
vacuum bottle, it is necessary to check the contact resistance of each pole or the
travel of each pole as specified by the manufacture. VCBs are generally used up
to 33 KV voltage systems.
Definition of Some Common Terms related with Circuit Breaker
a) Fault clearing time – It is the time elapsed between the instant of the
occurrence of a fault and the instant of final arc extinction in the circuit breaker. It
is the sum of relay time and breaker time.
b) Relay time – It is the time elapsed between the instant of occurrence of fault &
the instant of closure of relay contacts, i.e. closure of trip circuit.
c) Breaker time – It is the time elapsed between the instant of closure of trip
circuit and the instant of final current zero.
d) Anti Pumping of a circuit breaker – It blocks the repeat closing pulse when
breaker is already in closed condition.
22
e) Auto- reclosing of a circuit breaker – Auto-reclosing is provided to restore the
supply after interrupting a transient fault on overhead lines.
f) Rated short circuit breaking current – It is the highest value of short circuit
current, which a circuit breaker is capable of breaking under specified conditions
of recovery voltage and power frequency recovery voltage.
g) Rated short circuit making current – It may so happen that circuit breaker
may close on existing fault. The circuit breaker should be able to close without
hesitation as contact touch. The rated short circuit making current should be at
least 2.5 times the R.M.S. value of a.c. component of rated breaking current.
h) Operating sequence of a circuit breaker – The operating sequence denotes
the sequence of opening and closing operations, which the circuit breaker can
perform under specified conditions. The operating mechanism experiences severe
mechanical stresses during the auto-reclosure duty.
1) O-t-CO-T-CO
where O = opening operation, C = closing operation, CO =
closing followed by opening, t = 0.3 second for breaker to be used for rapid autoreclosure, T = 3 minute.
2) CO-t’-CO
where t’ = 15 seconds for breaker not to be used for rapid auto-
reclosure.
Maintenance of Circuit Breaker: a) Tightness of power connections & control wiring connections
b) Cleaning of Insulators
c) Lubrication of moving parts
d) Checking of contact resistance, close-open timing, Insulation resistance
e) Checking of gas pressure for SF6 circuit breaker (leakages if any)
f) Checking of air pressure for pneumatic operated breaker (leakages if any)
g) Checking of Controls, Interlocks & Protections like checking of pole discrepancy
system i.e. whether all three poles are getting ON – OFF at the same time
h) Cleaning of Auxiliary switches by CTC or CRC spray and checking its operation
23
5) CAPACITOR BANK
In any power utility, maintaining stable power supply at proper voltage is always a
problem. Due to lot of inductive load, the reactive power flow takes place in the
system which results into lowering of system
voltage and increase in
Transmission & Distribution losses. The HT capacitor provides an interim solution
in improving the power system stability, the voltage and power factor. HT
capacitor bank also compensate the losses occurring in the transmission lines.
Capacitor unit has one steel container, two bushings and several capacitor
elements enclosed in the unit. A single HV Capacitor may have a capacitance of 5
KVAr to 200 KVAr. Several identical units are mounted on Insulator racks and
connected in series parallel combination to obtain a High Voltage Capacitor Bank.
Before commissioning, megger the capacitor bank between phases and earth.
The megger reading for individual capacitor should not be less than 50 MΩ. For
more than one unit in parallel, minimum acceptable megger value can be derived
by dividing 50 MΩ by the number of units connected in parallel. Before switching
on capacitor, bus voltage, system incoming load current and power factor can be
noted. After energising, check that capacitor draws almost balance current in all
the 3 phases and is near to its rated value. Note the change in bus voltage, load
current and system power factor. Normally after capacitors are energised, there
will be little rise in bus voltage and some reduction in system load current and
improvement in power factor. In case load current increases instead of reducing, it
shows that capacitors connected are more than required for the load and in this
case the power factor shall be leading.
When Residual voltage factor (RVT) is used for unbalance protection, measure
open delta voltage, which should be negligible. In case, capacitors are connected
in double star with neutral CT, the current on the secondary side of neutral CT can
be measured, which should also be negligible.
24
Protection of Capacitor bank:
1) Fuse is provided for each capacitor in the bank. The fuses shall be external
type for 11 KV capacitor bank. The capacitor unit together with external fuse shall
be arranged in such a way to avoid bird faults by providing adequate clearance
between the body and the line terminal. Capacitor bank of voltage level more than
11 KV is provided with internal fuse type. In case of fault, the faulty element will
automatically go out of circuit.
2) Discharge resistors are provided within the capacitor unit to ensure safety after
de-energisation of capacitor (To reduce the residual voltage from crest value of
rated voltage to 50 volts or less within 5 minutes). The power loss in these
resistors is negligible.
3) Each capacitor bank is protected against lightning by gapless zinc oxide
arrester.
4) The capacitor protection equipment include over current, earth leakage and
protection to detect unbalance loading due to abnormal conditions.
Maintenance of Capacitors: Capacitors should be allowed to discharge through the discharge device provided
for the purpose before working on them. Never discharge capacitor by shortcircuiting its terminals, as it can get damaged this way. Following maintenance is
carried out on capacitor bank:
1) Cleaning of bushings
2) Tightness of connections of capacitor bank, series reactor
3) Checking value of capacitance & discharge Resistors
4) Checking earthing connections and tightness
5) Checking of all protections (Relays)
6) Checking of capacitors units for any leakage.
7) Checking of oil BDV of series reactor and NCT/RVT.
25
6) POWER TRANSFORMER
Transformer is one of the most important equipments in a power transmission and
distribution system. It does stepping up or stepping down the voltage and transfer
power from one A.C. voltage to another A.C. Voltage at the same frequency.
Transformer has Primary & Secondary windings housed in main tank filled with
insulated oil. Oil is used for providing insulation as well as cooling of windings.
1) The capacity of Transformer is expressed in Volt-ampere (KVA / MVA)
2) The transformation ratio K (constant) = Vs/Vp = Ns/Np
Where Vp, Np denote primary voltage & turns respectively. And Vs, Ns denote
secondary voltage & turns respectively.
If K > 1, then transformer is called step-up transformer
If K < 1, then transformer is called step-down transformer
For an ideal transformer, Input VA = Output VA
i.e. Vp x Ip = Vs x Is or Is/Ip = Vp/Vs = 1/K (where Ip & Is are Primary and
secondary current respectively). Hence currents are in the inverse ratio of the
(voltage) transformation ratio.
To calculate current of Primary & Secondary winding and transformation
ratio of 132 / 33 KV, 50 MVA Transformer:a) Primary Current in amp = Ip = VA / √3 x Vp, where Vp & Ip are primary voltage
and current respectively.
Hence Ip = (50 x 10*6) / (√3 x 132 x 10*3) = 218.69 Amp
b) Secondary Current = Is = VA / √3 x Vs, where Vs & Is are secondary voltage
and current respectively.
Hence Is = (50 x 10*6) / (√3 x 33 x 10*3) = 874.77 Amp
26
General view of Power Transformer :-
Main fixtures of Power Transformer and their functions are listed below: a) Buchholz Relay - This relay is designed to detect transformer internal fault in
the initial stage to avoid major breakdown. Internal fault in transformer generates
gases by decomposition of oil due to heat & spark inside the tank. These gases
pass upward towards the conservator tank, trapped in the housing of the relay,
thereby causing the oil level to fall. The upper float rotates & switches contacts
close & thus giving alarm.
In case of a serious fault, gas generation is more, which causes operation of lower
float & trips the circuit breaker. The gas can be collected from a small valve at the
top of relay for Dissolved Gas Analysis (DGA).
27
Checking the floats operation manually: a) Close the both valves. (From Transformer and main conservator side)
b) Drain oil from the buchholz relay.
c) Top float makes contact as the oil gets lowered and gives Alarm.
d) If oil is further drained, bottom float makes contact and gives trip signal.
After testing, both valves must be opened without fail and released the air from
relay. Alarm & Trip circuit can also checked by shorting contacts externally by link.
b) Oil Surge Relay - It is similar to Buchholz relay with some changes. It has only
one float & operates when oil surges reach and strike the float of OSR. It is used
with OLTC for detection of any damage or fault inside the tap changer and
prevents tap changer from damages in case of low oil level in OLTC tank.
Checking the float operation manually: a) This relay can be checked by pressing test switch provided on top side. Here
only one contact is provided which gives trip signal on operation of float. By
shorting contact externally by link, trip circuit can also be checked.
c) Explosion Vent - It consists of a bent pipe with bakelite diaphragm at both
ends. A protective wire mesh is fitted on the opening of transformer to prevent the
pieces of ruptured diaphragm from entering the tank. The wire mesh is also
provided at the upper end to protect upper diaphragm from any mechanical
damages. At the lower end, there is a small oil level indicator. When the lower
diaphragm ruptures due to excess internal pressure, the oil level rises in the vent
pipe & is visible through the indicator. In case the internal pressure developed is
not reduced to safe value after the bursting of lower diaphragm, upper diaphragm
gives away throwing the gas and oil outside and prevents further mechanical
damages.
28
d) Pressure Relief Valve - When the pressure in the tank rises above predetermined safe limit, this valve operates & performs the following functions: 1) Allows the pressure to drop by instantaneously opening the port.
2) Gives visual indication of valve operation by raising a flag.
3) Operates a micro switch, which gives trip command to breaker.
Checking the PRV operation manually: a) The operation of PRV can be done by lifting the plunger (Plunger operates
switch). By shorting contact externally by link, trip circuit can also be checked.
e) Oil Temperature Indicator - It is dial type thermometer, works on the vapour
pressure principle. The bulb, which is known as ‘Probe’ is exposed to the
temperature to be measured, is connected by a length of flexible tubing to a
borden gauge tube, which is known as 'operating bellow'. This bellow is filled with
a volatile liquid. The change in bulb temperature causes change in the vapour
pressure of the liquid & pointer moving on a dial calibrated in degree centigrade
indicates the consequent movement of the operating bellow. It has two pair of
contacts, one for Alarm & another for Trip. In general, oil temperature alarm is set
at 80°- 85° C and oil temperature tripping is set at 85°- 90° C.
Checking the OTI operation manually: a) The operation of OTI can be checked by tilting the float position. The first float
S1 is used for alarm and another float S2 is for trip signal.
Alarm & Trip circuit can also checked by shorting contacts externally by link.
f) Winding Temperature Indicator - It is also similar to OTI but has some
changes. It consists of a probe fitted with 2 capillaries. Capillaries are connected
with
two
separate
bellows
(operating/compensating).
These
bellows
are
connected with temperature indicator. Operating bellow is surrounded by heater
coil, which gets current from one WTI CT, when load on transformer increases,
corresponding current passes to the heater coil mounted on operating bellow. The
heater coil heats the operating bellow, which is filled with volatile liquid.
29
Due to this heat, vapour pressure of volatile liquid increases hence WTI shows
more temperature as compared to OTI. There are four mercury switches, 1
contact for Alarm, 2 for Trip circuit and 3 for cooler control and 4 as a spare.
In general, winding temperature alarm is set at 85°- 90° C and winding
temperature tripping is set at 90°- 95° C. The fan Auto ON operation is set at 60°
C and Fan auto OFF is set at 55° C.
Checking the WTI operation manually: a) The operation of Winding Temperature Indicator can be done by tilting the float
position. The first float S1 is used for alarm and another float S2 is for trip signal.
Fan auto operation can also be checked by float movement. Alarm / Trip circuit
can also be checked by shorting contacts externally by link.
g) Conservator - As expansion and contraction occurs in transformer main tank,
consequently the same phenomena takes place in conservator as it is connected
to main tank through a pipe. Conservator communicates with the atmosphere
through a breather, incorporating a dehydrator, which is connected to the breather
pipe. Other end of this pipe opens at the top in the conservator, just below the
conservator upper wall.
h) Breather - This is a special air filter incorporating a dehydrating material,
called, Silica Gel. It is used to prevent the ingress of moisture and contaminated
air into conservator. It consists of an inner metal cylinder filled with silica gel. Both
ends of this cylinder are enclosed by wire mesh screen. This cylinder is enclosed
in an outer casing of cast iron. Casing has 2 parts. The upper part is cover; where
as lower part is attached with an oil seal. When transformer breathes in, the air
enters which passes into the oil seal. The contamination, if any, is observed in this
oil. Then air passes through silica gel, where the moisture, if any, is observed by
the silica gel and pure and dry air goes to conservator tank of transformer. Normal
colour of Silica Gel is blue. If it turns to pink, then Silica Gel is to be reactivated /
replaced by fresh Gel.
30
i) Oil Level Indicator - It is also known as magnetic oil gauge (MOG). It has a pair
of magnet. The metallic wall of conservator tank separates magnets without any
through hole. Magnetic field comes out and it is used for indication. This
eliminates any chances of leakage. The driving magnet rotates and acquires the
position corresponding to height of oil level, as it is linked with a float. The float is
hinged & swings up and down with oil level. This rise or fall rotates driving magnet
with the help of bevel gear and pinion. Follower (Driven) magnet moves
accordingly and operates a pointer & a cam. The pointer reads oil level & cam is
set to operate a mercury switch to give low oil alarm as per the oil position.
Checking the MOG operation manually: a) Operation of MOG can be done by tilting the float position which gives alarm
signal. Alarm circuit can also checked by shorting contacts externally by link.
j) Radiators - Small Transformers are provided with welded cooling tubes or
pressed sheet steel radiators. But large transformers are provided with detachable
radiators plus valves. For additional cooling, exhaust fans are provided on
radiators. The hot oil in main tank goes up and enters in the radiators. After
cooling in radiators, either by natural air or forced air, oil again goes to main tank
from the lower valve and circulates continuously.
k) Bushings – It comprises a central conductor surrounded by graded insulation.
Bushing is necessary when a conductor is taken out through metallic tank or wall.
Oil filled bushing is used for 33 KV applications. For making bushing compact,
synthetic resin bonded condenser bushing is used for 33 and 66 KV applications.
For 132 KV & above voltages, oil impregnated paper condenser bushing is used.
It consists of a central conductor surrounded by alternate layers of insulating
paper & tin foil. The capacitor formed by alternate layers of tin foil and paper
insulation results in uniform electric stress distribution between conductor surface
and earthed flange. The bushing core is coated with suitable resin.
31
The assembly is enclosed in hollow porcelain and is provided with support flange
and top hood. The porcelain is filled with oil.
Creepage Distance (CD) – It is the shortest distance between two conductive
parts along the surface of the insulating material. CD requirement depends upon
rated phase to ground voltage and degree of atmospheric pollution.
Degree of Pollution
Recommended Min. CD
1) Clean area
16 mm / KV
2) Moderately polluted area
20 mm / KV
3) Industrial area
22 mm / KV
4) Heavily polluted/coastal area
25 mm / KV
l) Tap Changer - As load on the transformer increases, secondary terminal
voltage decreases. To maintain the secondary voltage, tap changers are used.
Tap changers are connected with H.V. winding (Primary winding).
Therefore in tap changers transformer, there are two windings in H.V. side, 1)
Main winding and 2) Tap winding. There are two types of tap changers.
A) Off Load Tap Changer - In this type, before moving the selector, transformer
is made OFF from both ends. Such tap changers have fixed brass contacts,
where taps are terminated. The moving contacts are made of brass in the shape
of either roller or segment.
B) On Load Tap Changer - In short we call it as OLTC. In this, taps can be
changed manually by mechanical or electrical operation without making off the
transformer. For mechanical operation, interlocks are provided for non-operation
of O.L.T.C. below lowest tap position and above highest tap position. Similarly for
electrical operation, limit switches are provided in circuit for non-operation of tap
change below lowest tap position and above highest tap position. For mechanical
operation, one hand interlock switch is provided in the circuit.
32
As soon as we insert handle, hand interlock switch opens out the electrical circuit
and no one can operate O.L.T.C. electrically.
RTCC (Remote tap change control cubicle) is used for tap changing by manually
or automatically through Automatic Voltage Relay (AVR) which is set +/- 5% of
110 Volt (Reference taken from secondary side PT voltage). During Auto tap
changing, Bell / Hooter will ring up thus gives information to substation operator
for tap changing.
Transition resistances are used in OLTC for avoiding momentarily interruption of
power supply during tap changing. At the time of tap changing, load current
passes through the transition resistances & no power interruption occurs during
tap changing.
Transformer Tap: - Tapping is provided in Primary winding. Hence by changing
the tapping, we can change secondary voltage as per requirement.
The transformer equation is: - V2/V1 = N2/N1
i.e. V2 = (N2 x V1)/N1
There is an Inverse relationship exists between secondary voltage & primary
turns. When primary turns are decreased i.e. Tap position is shifted from 3 to 4,
secondary voltage gets increased and if primary turns are increased i.e. Tap
position is shifted from 4 to 3, then secondary voltage gets decreased.
Parallel Operation of Transformers:
Before paralleling two or more transformers, the four principal characteristics of
those transformers should match as given below:
1) Same voltage ratio
2) Same percentage impedance
3) Same polarity
4) Same vector group
33
If two transformers of same output operating in parallel, the % impedance must be
identical, if Transformers are to share equally. If % impedance is not identical,
suppose T/F 'A' is having 4% impedance and T/F 'B' is having 2% impedance,
then load sharing will be,
Load A = L x ( Z2 / Z1+Z2 )
Where L is total combined output
Load B = L x ( Z1 / Z1+Z2 )
and Z is percentage impedance
So that A transformer will share only 1/3rd load & B transformer will share 2/3rd
load. Hence operating transformers in parallel, the output of the smallest
transformer should not be less than 1/3rd of the output of largest one.
When operating two transformers in parallel, one of the RTCC panels is kept on
Master mode and another one is kept on Follower mode so that simultaneously
tap changing is possible on both transformers. If transformers are not running
parallel, the control switch is kept on Independent Mode i.e. both transformers
taps can be separately changed.
Site Testing of Transformer:
1) Insulation Resistance Test –
a) Between HV & Earth.
b) Between LV & Earth.
c) Between HV & LV by suitable range of megger.
2) Voltage Ratio Test - This test is essential to check the output or the secondary
voltage on each tap position. By virtue of this test the problems in the OLTC can
be easily detected. 3 Phase, 440 V LT supply is applied to the primary side of the
transformer and the output volts at the secondary side for each tap position is
measured. If any break in voltage reading is observed during change of tap
position, then there is some problem in that particular tap.
34
3) Magnetic Balance Test - This test is carried out to check the balancing of the
induced voltages in the windings & flux distribution. Transformer is kept on normal
tap position and 3 Phase, 440 V LT supply is applied to the primary windings as
given below:
1) YNyn0 Transformer: - First the voltage is applied between R & N. Voltage will
be induced in between YN and BN. Voltages are noted & will be observed that:
In Primary side: - V RN = V YN + V BN = 2/3 + 1/3
On Secondary side: - V rn = V yn + V bn = 2/3 + 1/3
If the voltage readings on secondary are observed as above, then it can be
assumed that the flux distribution is balanced & proper. If the magnetic balance is
not correct, readings will be different and typical noise will be observed. This will
indicate that there is some problem in the core of the transformer.
Again apply voltage to YN, the result will be:
In Primary side: - V YN = V BN + V RN = 1/2 + 1/2
On Secondary side: - V yn = V bn + V rn = 1/2 + 1/2
Similarly apply voltage to BN, the result will be:
In Primary side: - V BN = V YN + V RN = 2/3 + 1/3
On Secondary side: - V bn = V yn + V rn = 2/3 + 1/3
Note: - In case of Dyn11 Transformer, voltage is applied on primary side between
first R and Y terminals (R Phase winding), next Y and B terminals (Y Phase
winding), and
B and R terminals (B Phase winding). Result will be same as
mentioned above for YNyno Transformer.
35
4) Vector Group Test - This test is carried out to check correctness of windings
connections. The Phase angle difference arises out of the internal connections of
the windings. A star / star transformer having the similar vector diagram for
primary and secondary side can be connected in two different ways internally.
In the first case there is 0° displacement between primary and secondary whereas
in the second case there is 180° displacement. In addition to this, a +30 or − 30°
displacement is possible in a 3 phase transformer when the vector diagram is
different i.e. delta/star OR star/delta type.
For parallel operation, secondaries must have same phase angle displacement
with respect to their primaries so that there may be no phase difference between
the terminals of the secondaries themselves.
A three-digit vector symbol is adopted to designate the vector group.
a) First letter in capital represents Primary winding connection - D: Delta & Y: Star
b) Second letter in small represents Secondary winding connection - d: Delta and
y: Star
c) Third digit represents the phase displacement between the primary and
secondary.
The convention employed is to describe it by the hour in a clock in which the HT
voltage is represented by the minute hand set to 12 o'clock position, and the LT
voltage is represented by the hour hand. Since 12 hours represents 360°of a full
circle, each hour represents a 30° phase difference. Thus ‘0’ represents no phase
difference, ‘1’ stands for minus 30°, ‘6’ for 180° and ‘11’ for plus 30° displacement
as referred to the standard counter clockwise vector rotation.
36
Vector Group Testing at the time of Commissioning or on repaired Job:
1) YNyn0 Transformer: - Keep the transformer on normal tap position. Short R &
r of windings. Apply 3 Phase L.T. voltage to primary windings. Measure voltages
on the secondary side.
R, r
R
r
n
n
b
b
B
y
Y
y
N
B
Primary
Secondary
Y
Vector Representation
Following conditions are to be satisfied:
a) V RN = V Nn + V rn
b) V Bb = V Yy
2) Dyn11 Transformer: - Keep the transformer on normal tap position. Short R &
r of windings.
R,r
R
r
y
n
n
b
b
B
y
B
Y
Y
Primary
Secondary
Vector Representation
Apply 3 Phase L.T. voltage to primary windings. Measure voltages on the
secondary side. Following conditions are to be satisfied:
a) V Bb < V By
b) V Yb = V Yy
37
5) Magnetizing Current Test - When a Power Transformer is charged, it is
generally presumed that it is to be charged on NO LOAD condition because it
draws magnetizing current containing high harmonics. Transformer may trip on
differential protection if it is not provided harmonic restraint protection. This current
inrush is due to the iron losses of the transformer. This current should be equal in
all three phases so that there would not be any spill current in the relay to trip the
primary circuit breaker of the transformer. The test is carried out at normal tap
position.
Apply 3 Phase L.T. voltage to primary windings through ammeters (ma)
connected in series of windings and keeping secondary winding open. It would be
seen that the current drawn by all the three phases would be same. The current is
drawn on account of the magnetizing of the core. (Iron loss) It can also be called
as no load current when the transformer is charged with rated primary voltage
applied across the primary, keeping secondary open.
6) Short Circuit Current Test – Short circuit test is carried out to check the
healthiness of windings. Apply 3 Phase L.T. voltages to primary windings &
secondary windings are shorted through ammeters of suitable range. If the
readings are equal in all three phases, transformer is supposed to be healthy.
Actually here the term “% Impedance" comes into picture.
The reduced voltage required to be applied across the primary of a transformer to
cause rated full load current to flow through this winding when secondary winding
is shorted, is known as impedance voltage. It is expressed as a percentage of the
rated voltage of former winding. In this case current flowing through secondary is
the full load current & is indicative of the copper losses.
38
7) Oil Test - The oil is used as insulation between windings & core and between
windings & tank. Without oil, the paper insulation of the windings could be
punctured early which in turn will result in failure of transformer. The oil facilitates
cooling of the windings and magnetic circuits. The oil protects windings and core
of transformer from the absorption of moisture. The test on oil is divided into two
different categories.
1) Physico chemical Testing: a) Density - It indicates the type of transformer oil whether paraffin base or
naphtha base.
b) Kinematics Viscosity - The oil should circulate freely in the equipment to
maximize heat transfer. A low viscosity oil fulfils this need. Viscosity of oil
increases because of oxidation taking place at all times. If viscosity increases by
15%, then oil needs replacement.
c) Flash Point – Flash point is a minimum temperature at which oil will support
instantaneous combustion (flash) but before it burns continuously. Flash point of
new oil should be fairly high.
d) Pour Point - It is the indicator of the ability of oil to flow at cold operating
conditions. It is the lowest temperature at which the fluid will flow when cooled
under prescribed conditions.
e) Neutralization Value - This indicates the presence of combined acids i.e.
organic & inorganic. The degradation of oil gives rise to acidic compounds and
formation of sludge. The acidity is given by its neutralization value, which indicates
the total acidity and is evaluated by milligrams of KOH per gram of oil. Acidity
content in oil should be low.
39
f) Water Contents - It is expressed in parts per million (ppm). Dielectric strength
of oil is very high when water content is low.
g) Inter Facial Tension - It is a measure of the molecular attractive force between
their unlike molecules at the interface. When oil oxidizes, the organic acid thus
produced are concentrated at the placing a drop and used oil on water surface,
water is spread rapidly over the surface in contrast to a new oil, which may float
as a lens on the water. It is considered that IFT gives an indication of degree of
slugging of oil as dissolved impurities in the oil tends to diffuse into the water,
which lowers the IFT.
2) Electrical Testing: a) Dielectric Strength - The BDV of oil is its ability to withstand electric stresses
without failure.
b) Resistivity - It is the measure of electrical insulating properties of oil. High
resistivity reflects low content of free flowing particles.
c) Dielectric Dissipation Factor (Tan δ ) - It is a measure to the ratio of the
power dissipated in the oil to the product of effective voltage and current. It is
tangent of loss angle & expressed in unit or percent. It determines the cleanliness
of oil & is related to aging characteristic of the oil.
40
Standard Values of New Oil as per IS : 335-1983
S/No.
Characteristics
1
Breakdown Value
a) > 145 KV
b) 72.5 to 145 KV
c) < 72.5 KV
2
Dissipation Factor at 90°C
(Tanδ )
3
Specific Resistance at 90°C
4
Water content ppm
a) > 145 KV
b) 72.5 to 145 KV
c) < 72.5 KV
5
Inter Facial Tension
6
Density
7
Flash Point
8
Pour Point
9
Total Acidity Test
41
Standard Values
60 KV (min.)
50 KV (min.)
40 KV (min.)
0.05
1 X 10 * 12 (Ω-cm)
15 ppm (max.)
20 ppm (max.)
25 ppm (max.)
0.03 N/m (min.)
0.89 g/cu.cm (min.)
140°C (max.)
- 6°C (max.)
0.03 mg KOH/g
(max.)
Dissolved Gas Analysis (DGA): - Transformer, in operation, is subjected to
various thermal and electrical stresses, resulting in liberation of gases from the oil
which is used as insulation media and coolant. The solid insulating materials like
paper, wooden support, pressboard, etc. cause degradation and form different
gases, which get dissolved in the oil. The most significant gases generated are
Hydrogen (H2), Methane (CH4), Ethylene (C2H4), Acetylene (C2H2), Propane
(C3H8), Propylene (C3H6), Carbon Monoxide (CO), Carbon Dioxide (CO2), and
Ethane (C2H6). The gas connected in the relay will help to identify the nature of
the fault. The greater the rate of gas collection, the more severe is the nature of
the developing fault.
Colour of the gas helps in finding the affected material.
Colour
------ Identification
White
----- Gas of decomposed paper and cloth insulation
Yellow
----- Gas of decomposed wood insulation
Grey
----- Gas of overheated oil due to burning of iron portion
Black
----- Gas of decomposed oil due to electric arc
Ratio Method used for Analysis of DGA results:-In this method three ratios of
gases are used. They are methane / hydrogen, ethylene / ethane, acetylene /
ethylene. If the ratio comes out more than one, it indicates abnormal deterioration
and less than one indicates normal aging.
Particulars
C2H2/C2H4
CH4/H2
C2H4/C2H6
a) Less than 0.1
0
1
0
b) 0.1 to 1.0
1
0
0
c) 1.0 to 3.0
1
2
1
d) More than 3.0
2
2
2
42
Interpretation of the faults according to the observed ratios of Gases
Characteristic
Fault
Ratio Code
Diagnosis
C2H2/C2H4
CH4/H2
C2H4/C2H6
0
0
0
1
0
0
Normal Aging
Discharge in gas filled
cavities due to incomplete
impregnation
1
1
0
As above but leading to
tracking or perforation of
solid insulation
1 to 2
0
1 to 2
Discharge of
high energy
1
0
2
Thermal fault of
low temp. less
than 150°C
Thermal fault
temp. 150 to
300°C
Thermal fault
temp. 300 to
700°C
Thermal fault
temp. > 700°C
0
0
1
0
2
0
0
2
1
0
2
2
No Fault
Partial
Discharge of
low energy
density
Partial
Discharge of
high energy
density
Discharge of
low energy
43
Continuous sparking in oil
between bad connections of
different potential or to
floating potential.
Breakdown of solid material
Discharge of power follow
through arcing, breakdown
of oil between winding or
between coils to earth
General insulated conductor
overheating
Local overheating of core
due to concentration of flux.
Increasing hot spot temp.;
varying from small hot spots
in core, shorting links in
core, overheating of copper
due to eddy currents, bad
contacts / joints (pyrolitic
carbon formation) up to core
and tank circulating
currents.
Actions during failure / tripping of Transformer –
The action to be taken depends upon the size of the transformer, operation of
protective relay, whether tripping is accompanied by loud noise, smoke or
expulsion of oil from the transformer, etc. Observe the transformer external
condition; look for any damage to the bushings, leads or cable box. Note the
temperature of oil & check if the oil level in the conservator is right. Take megger
readings between primary and secondary and also of each to earth. If everything
is right, proceed as noted below:
- The failure may possibly be due to a sudden and heavy overload, or short-circuit.
If a DO fuse has dropped out, check if its ampere rating is right. If incorrect,
replace by the correct size and energise the transformer, after switching off the
load. If everything is all right, close the secondary circuit; if fuse blows again, the
fault is obviously in the outgoing lines, which should be traced and rectified; if on
the other hand, the primary circuit fuse blows out, even when the load is
disconnected, an internal fault is indicated. This also apply, if an over current relay
alone has operated and tripped the breaker.
- If a differential relay operates when a transformer is first switched on, it may be
due to a switching surge. Check the harmonic restraint circuit and setting. If, on
the other hand, relay operates when the transformer is in service, it is a sure
indication of an internal fault.
Any tripping of buchholz relay requires to be carefully looked into. If the lower
assembly has tripped due to sudden evolution of large quantities of gas, a major
internal fault is to be inferred especially if either over current or differential or earth
fault relay has operated. If, on the other hand, the upper assembly has operated
due to slow release of gas it is necessary to find out its composition before any
conclusions can be drawn. If it is air only, there is no cause for worry, as air can
enter into the transformer in many ways.
44
When transformer is commissioned, it sometimes happens that the buchholz
relay upper assembly operates, after a few hours of run, due to the release of air
bubbles entrapped within the windings, such as when hand filling is employed for
filling of oil into the tank. If the accumulated gas is not air, an incipient fault is
indicated.
DGA would help in identifying the nature of the fault, and this should be done as a
routine measure. If the buchholz relay has tripped, without any gas being given
out, it may be due to electrical fault in the wiring.
- Thorough checking is required if the earth fault relay has tripped, or there is an
evolution of smoke or oil, and also PRV has operated in case of large transformer.
In such cases reclosing of breaker should not be permitted as it may cause further
extensive damage. Detailed testing of transformer is to be carried out and
compare the results with test certificate figures and consult the manufacturer.
In most of the cases, the cause of the fault can be found out if you carefully
observe the condition of windings by lifting the core and coil assembly. The
following notes may be of help in identifying the cause:
Lightning discharge or over voltage: This is characterised by breakdown of the
end turns close to the line terminal. There may be a break in the turns or end lead,
and also flash marks on the end coil and earthed parts close to it, but the rest of
the coils will be found to be healthy.
Sustained overloads: The windings in one or all the phases would show signs of
overheating and charring; the insulation would be very brittle and have lost all its
elasticity.
Inter-turn short: The same signs as for sustained overloads would be noticed,
but only on one coil, the rest of the coils being intact.
45
Dead short-circuit: This can be identified by the unmistakable, lateral or axial
displacement of the coils. The coils may be loose on the core; some turns on the
outermost layer may have burst outwards and broken as if under tension. If, in
addition to these signs, the windings are also completely charred, it is conclusive
evidence that the short-circuit has continued for an appreciable period, not having
been cleared quickly by the protective relays.
Visual checking of Transformer: - Check the colour of silica gel. If it is pink, reactivate or replace it. Also ensure
proper quantity of oil in breather oil cup.
- Check oil level in Conservator of Main Tank & OLTC. It should be > ½ level
marking.
- Check oil level in Bushings.
- Check for any oil leakage. Arrest leakages, if any.
- Check the working of OTI & WTI by taking hourly temperature readings. There
should be changes in readings as per loading of transformer and atmospheric
condition.
- Check the cooling system by making fans / pumps operation by manually.
- Check the tap position of RTCC panel and OLTC panel. It should have same
position number.
Check the humming noise & vibration of transformer. If any abnormality found, it is
to be referred to concerned manufacturer.
46
7) CONTROL & RELAY PANELS
Control or relay boards are built up by using requisite number of self-contained
sheet steel cubicles, comprising a front panel to carry the control apparatus & the
hinged or removable back cover to give access to interior wiring, cable
termination. This type is called as Simplex type panel. When panels are arranged
back to back in corridor formation, and door is then fitted at each end, are called
as Duplex panels. Depending upon the size of the substation the control and relay
board may incorporate the followings:
1) Indicating and metering instruments mounted on front.
2) Relays mounted on the backside in Duplex panel, flush mounting on front in
Simplex panel.
3) A mimic diagram representing main circuit connections is incorporated on the
front panel. It is a single line diagram incorporated on the front side of the control
panel. This diagram represents the actual physical position of various HT
electrical equipments in the sub-station yard along with status of equipments, ON
and OFF positions of various breakers and isolators through semaphore indication
or lamp indication.
47
4) The automatic semaphore indicators are used to denote position of switches.
5) Circuit Breaker control switch (TNC switch) is fitted on front. Normally switch is
on Normal (Centre) position. Handle is moved to the right or left to initiate close or
trip operations.
6) Indication lamps mounted for various purposes follow a standard colour code.
Red - C.B. or switch CLOSED
Green - C.B. or switch OPEN
White - Trip circuit healthy
Amber - Alarm indication i.e. CBs tripped on fault
7) Annunciation System – It gives alarm in case of any abnormality in the system.
Alarm bell rings and appropriate facia lamp flashes ON & OFF. Substation
operator has to ACCEPT the signal by pressing a button, which silences the bell
and causes the lamp to show a steady light. After taking remedial action, the
operator RESETS the alarm circuit by pressing another push button, the lamp
being simultaneously extinguished.
48
Colours for Internal Wiring
Red
- Phase connection, either directly connected to the primary circuit or
Connected to secondary circuit of CT and PT
Yellow
- Phase connection, either directly connected to the primary circuit or
Connected to secondary circuit of CT and PT
Blue
- Phase connection, either directly connected to the primary circuit or
Connected to secondary circuit of CT and PT
Black
- A.C. neutral connection, Star point connections of secondary circuit
of CT and PT, and connections in A.C. and D.C. circuit
Green
- Connections to earth
Grey
- Connections in D.C.circuit
Each wire should have a letter to denote its function. D.C. supply from +ve source
should bear odd number & from -ve source should bear even number.
CT Secondary Terminal – S2 of all protection & metering cores are shorted in CT
junction box. Only one common wire of S2 along with S1 wires of all 3 phases
CTs are brought to CRP. Earthing of S2 wires is done at one end. (CRP end)
In substation, various drawings are available namely:
a) Wiring Drawing: The routing of wires from various equipments in a control and
relay panel is shown in this drawing. The route of the particular wire as per its
purpose of application can be traced easily while attending any faults in the
particular circuit.
For reading of drawing it should be kept in mind that drawing is prepared
when isolator & breaker positions are OFF & spring of the breaker mechanism is
in deenergised condition.
b) Schematic Drawing: This drawing is a representation of various circuits such
as metering, protection, control, indication, annunciation, etc. in a control and
relay panels.
49
c) Layout Drawing: This drawing shows arrangement of various indoor and
outdoor equipments in a particular installation in a sequential order.
Common Ferrule Numbers used in wirings
A:
CT secondary connection for primary protection like Differential, Distance,
REF Relay). Small “a” used for PT secondary connection in PT terminal box.
B: Bus bar Protection (CT secondary connection). B for B phase indication.
C: Back up Protection (CT secondary connection for O/C & E/F Relay).
D: Metering (CT secondary connection).
E: Metering & Protection (PT secondary connection).
H: A.C. connection.
J: D.C. connection (Before Fuse).
K: D.C. connection for control (After Fuse).
L: D.C. connection for Indication (After Fuse).
M: Motor Supply (Spring charging Motor in Circuit Breaker).
N: RTCC (Tap Changer) connection. Also for denoting A.C. Neutral connection.
P: PT primary connection & DC circuit of Bus bar protection scheme.
R: R Phase Indication.
S: CT secondary connection in Terminal Box.
U: Circuit Breaker auxiliary contacts.
X: TB Numbering.
Y: Y Phase Indication.
50
8) STATION TRANSFORMER
This is a small distribution transformer located in the substation premises. It has
given protection through proper rating of D.O.Fuse. Incoming HT supply to the
transformer is tapped from LT bus of substation through Isolator. The output
voltage 440 Volt is terminated to ACDB through LT cables. The main purpose of
station transformer in substation is to provide auxiliary supply to various
equipments through A.C. Distribution Board (ACDB) via MCBs or Switch Fuse
units:
1) A.C. supply is used for battery charger, which converts A.C. to D.C. supply for
charging the batteries and parallel provides D.C. source for various controls of
substation equipments. In case of A.C. supply failure, batteries will take care of
D.C. supply continuity for equipment’s controls.
2) A.C. supply is used for OLTC for tap changing operation of transformer and
also cooling arrangement of transformer.
3) A.C. supply is used for spring charging mechanism of breakers.
4) A.C. supply is used for Office and Yard Illumination.
5) A.C. supply is used for Oil filtration, some miscellaneous welding work, and
Test supply for carrying out testing of various equipments in switchyard.
51
9) BATTERIES & BATTERY CHARGER
For controlling various operations of substation equipments, suitable D.C. supply
is required. In battery charger panel, A.C. 1 phase or 3 phases is given, which
converts A.C. to D.C. supply. This D.C. supply is given to various control panels of
substation and for charging the batteries through D.C.Distribution Board. (DCDB)
In case of A.C. supply failure, batteries provide D.C. supply for controlling the
operations
of substation
equipments
in normal or abnormal
conditions.
Battery capacity is expressed in ‘Ampere Hours’ which is the useful quantity of
electricity that can be taken from a battery at the specified rate of discharge before
its cell voltage falls to the specified value, which is equal to 1.75 volts multiplied by
the number of cells. Ampere hours is equal to the product of the specified
discharge current in amperes multiplied by the number of hours before the battery
discharges to the specified extent.
Precautions / Maintenance: - Batteries should be cleaned regularly.
- Cell voltages & Specific gravity is to be recorded as per schedule.
- Batteries should be charged in a well-ventilated place, so that the gases and the
acid fumes are blown away.
- Do not disturb any connection with charger on, as there is risk of sparking.
- If acid or electrolyte gets spattered into the eyes, wash them immediately with
large quantity of clean, cold water.
- Tighten connections periodically. Apply petroleum jelly to terminals to prevent
corrosion.
- Maintain level of the electrolyte – Add only the distilled water. Add electrolyte
only if some of the electrolyte spills out.
52
10) MEASURING INSTRUMENTS
a) Voltmeter: - Voltage in an AC circuit is measured by voltmeter. The voltmeter
is connected across the load or winding. For high voltage, voltage transformer is
necessary to step down the voltage for measurement. Voltmeter is connected
across the secondary circuit of PT. Voltmeter can be replaced on line by removing
fuses or keeping voltmeter selector switch in OFF position.
b) Ammeter: - Current in a circuit is measured by ammeter connected in series of
current path. If current is high, suitable current transformer (CT) is necessary to
step down current for measurement. Ammeter is connected in series of secondary
circuit of CT. Ammeter can be replaced by shorting CT secondary wires or
keeping ammeter selector switch in OFF position.
c) Energy Meter: -The Power in electrical circuit is measured by energy meter.
Energy is the total power consumed over a certain period and is measured in
kilowatt-hour (KWH). One kilowatt-hour is equal to the energy consumed when
power is utilized at the rate of one kilowatt for one hour. The term ‘unit’ used for
expressing consumption of electrical energy is equal to one kilowatt-hour, and all
tariffs for energy consumption are based on this unit. A registering mechanism in
the energy meter indicates the total energy consumption. Energy meters will
record correctly, if connections are made with due care to the polarity and the
terminal markings. Energy meters can be changed or replaced while in service by
use of T.T.B. (Test terminal block). In TTB, CT secondary can be shorted during
removal of Meter (avoiding open circuit of CT secondary) & PT supply can be
made OFF by disconnecting type arrangement or by removing fuses. Energy
meter records Import / Export energy parameters.
Import parameters are displayed by arrow in
in
direction.
53
direction and Export parameters
POWER TRIANGLE
Apparent Power
KVA (S)
Reactive Power
KVAr (Q)
φ
Active Power
KW (P)
Power Factor = Cos φ = (Active Power) / (Apparent
Power)
Active Power: The actual amount of power that produces the effective work is
called active or real power. It is measured in Watts.
Reactive Power: The power drawn by reactive load such as Capacitors and
Inductors in a system is called reactive power. It is measured in VAr (Volt amp
reactive).
Apparent Power: The total power demanded by the load is the product of current
and voltage. This power is referred as apparent power. It is measured in VA (Volt
amp).
Multiplication Factor of Energy Meter:
M.F. = [(Feeder CTR x PTR) / (Meter CTR x PTR)]
Case 1: - a) Feeder CTR = 600/1 A, Feeder PTR = 33000/110 V
b) Meter CTR = 300/1 A, Meter PTR = 33000/110 V
[(600/1) x (33000/110)]
M.F. of Energy Meter = ----------------------------- = 2
[(300/1) x (33000/110)]
54
Case 2: - a) Feeder CTR = 100/1 A, Feeder PTR = 33000/110 V
b) Meter CTR = 400/1 A, Meter PTR = 33000/110 V
[(100/1) x (33000/110)]
M.F. of Energy Meter = ----------------------------- = 0.25
[(400/1) x (33000/110)]
A Typical SECURE Energy Meter connection is shown below
SECURE MAKE
3 Phase 4 Wire Energy Meter
1
2
3
4
5
6
7
M1 R
L1
M2
Y
L2
8
9
10
M3 B
L3
N
R
R
R
Y
Y
Y
B
B
B
P
H
A
S
E
P
H
A
S
E
P
H
A
S
E
P
H
A
S
E
P
H
A
S
E
P
H
A
S
E
P
H
A
S
E
P
H
A
S
E
P
H
A
S
E
C
T
P
T
C
T
C
T
P
T
C
T
C
T
P
T
C
T
s
2
s
1
s
2
s
1
s
1
55
s
2
N
E
U
T
R
A
L
d) Earth Tester: - Resistance of the earth pit ‘E’ in following figure can be
measured directly with the help of an ‘earth tester’
Earth Tester
P1
C1
P2
C2
E
P
Potential
Electrode
Electrode
(Earth Pit)
C
Current
Electrode
E is the earth pit electrode under measurement; P & C are two auxiliary electrodes
of 15-20 mm diameter and 40 cm long bars. The electrode C1 is planted at a
distance of approx. 25 metres from E and P1 is fixed centrally between E and C1.
One reading of Pit resistance is taken by rotating handle of earth tester. Two more
readings are taken by shifting P1 a distance of 3 metres on either side of its
central position. The value is the resistance of Electrode E to the earth.
e) Insulation Tester (Megger): - Insulation resistance between an insulated
conductor (part) and earth is checked by megger. Phase conductor is connected
to the terminal marked ‘Line’ on the megger and the terminal marked ‘Earth’ is
connected to the earth continuity conductor or an efficient earth. The handle is
turned to indicate a steady reading on the instrument. A megger, with its handle
being turned gently, should record zero when its two leads are touched together,
and read infinity when its leads are held apart.
56
f) Oil Tester (BDV Tester): - Dielectric breakdown strength of transformer oil is
one of the most reliable tests for proving the condition of oil. Oil sampling is done
by taking due care. The glass bottle into which oil is drawn should be perfectly
clean, clear, transparent and dry. It should then be thoroughly rinsed with oil
known to be good. The sample of oil should drawn preferably be drawn from the
bottom of the transformer tank. As water is heavier than oil, it settles down at the
bottom. The first sample or two may be thrown away if it contains sludge or
droplets of water.
The gap between two electrodes is to be maintained / checked at 2.5 mm by
gauge and the test cup is cleaned properly. The cup is then filled with the sample
oil to be tested up to 1 cm above the electrodes. The cup top should then be
covered with a clean glass plate and allowed to rest for at least 5 minutes so that
all air bubbles may disappear. Any bubbles still standing on the surface may be
removed with a clean glass rod. Use thin rubber gloves if you can, so that the
sweat on your fingers may not cause any contamination of the oil. Carry out test
as per procedure until there is positive and final breakdown of the oil. The test is
carried out for six times on the same sample after a gap of at least 5 minutes. The
average of all six readings is the dielectric strength of oil under test.
57
Some Important Numbers used with their meanings
2:
Time Delay Relay or Timer
21:
Distance Protection Relay
27:
Under Voltage Relay
49:
Winding Temperature Indicator
50/51: IDMT Over Current Relay with Instantaneous element
50/51N: IDMT Earth Fault Relay with Instantaneous element
52:
AC Circuit Breaker
59:
Over Voltage Relay
62:
Pole Discrepancy Relay with timer
63:
Gas Operated Relay (Buchholz Relay)
64R:
67:
67N:
Restricted Earth Fault Relay
Directional Over Current Relay
Directional Earth Fault Relay
75:
P.T. selection Relay
80:
DC Supervision Relay
86:
Master Trip / Locking Out Relay
87:
Differential Relay
89:
Line Switch / Isolator (Electrically Operated)
94:
Anti-pumping Relay (For Breaker Control)
95:
Trip Circuit Supervision Relay
96:
Gas Pressure Relay (For Breaker Control)
58
Trouble Shooting Works: -
S/No.
1
Probable Trouble
CT Circuits
Noise in CT
2
Ammeter is not recording
1
PT Circuits
Voltmeter not showing correct
reading
2
1
Cause / Works to be attended
- Open circuit of secondary circuit
- Loose connections in secondary
circuit
- Ammeter switch fault causing open
circuit in CT secondary
- Ammeter may be faulty
- Ammeter switch is not making
contact
- Check fuses
- If above is OK, voltmeter may be
faulty
- Loose connections in PT circuit
- CT secondary circuit may be in
shorting position for one or two
phases
- PT circuit fuse may be blown
- Loose connections
- Energy meter may be faulty
Energy meters recording on
lesser side
D.C. Protection Circuits
Non working of trip healthy
indication
- Fusing of bulb or bulb may be fitted
loose
- Loose connections
- Resistance may be open circuited
- Misalignment of auxiliary contacts
of breaker
- D.C. fuse may be loose or blown off
- Trip coil is open
- Push button may be faulty
59
S/No.
2
Probable Trouble
D.C. Protection Circuits
Non tripping of breaker
Cause / Works to be attended
3
Non closing of breaker
- D.C. fuse may be loose or blown off
- Loose connections
- Close coil is open or burnt
- Misalignment of auxiliary contacts
of breaker
- No free movement of plunger of
close coil
- Mechanical trouble in breaker
- Spring might not have been
charged
- Non resetting of Master / Trip relay
- Non closing from remote end, if
Local / Remote switch on local
position
- Closing switch or Push button may
be faulty
- Check the circuit as per circuit
diagram
4
Tripping of breaker without
Indication
- Due to shorting of D.C Positive
- D.C. fuse may be loose or blown off
- Loose connections
- Trip coil is open or burnt
- Misalignment of auxiliary contacts
of breaker
- No free movement of plunger of trip
coil
- Mechanical trouble in breaker
- Trip switch or Push button may be
faulty
- Air pressure may be low in case of
pneumatic operated breaker. If low,
correct it
- Check the circuit as per circuit
diagram
- D.C. leakage
60
S/No.
5
Probable Trouble
D.C. Protection Circuits
Mal-operation of Relay
Cause / Works to be attended
6
Relay Flag not resetting
7
Spring charging motor does not - Either loose fitting of fuse and link
start
or blowing of fuse
- Loose connections
- Failure of A.C. or D.C. supply
- Misalignment or defective limit
switch of Spring charging mechanism
- Defective motor
- Defect in relay or setting, If relay is
defective, relay needs to be replaced
- Wiring connection problem
- Mechanical defect in the flag
mechanism
1
Annunciation Circuits
Non working of Bell
2
Continuous ringing of Bell
- D.C. leakage
- Disturbance in aux. relay contacts
adjustment
3
Non resetting of Bell
- Accept push button faulty
- D.C. leakage
- Aux. relay faulty
4
Non resetting of Indication
- Reset push button faulty
- D.C. leakage
- Either loose fitting of fuse and link
or blowing of fuse
- Loose connections
- Burning of Bell coil
- Disturbance in bell adjustment
- Aux. relay provided may not
working
- Sealing (hold on) supply getting to
the aux. relay through ‘accept’ push
button might have disconnected due
to faulty ‘accept’ push button
61
S/No.
1
2
Probable Trouble
Indication Circuits
Lamp not indicating for breaker
ON-OFF position
Semaphore not working
Cause / Works to be attended
- Either loose fitting of fuse and link
or blowing of fuse
- Lamp may be loose fitted or fusing
of lamps
- Loose connections
- Defective aux. switch contacts of
breaker
- Either loose fitting of fuse and link
or blowing of fuse
- Semaphore coil might have burnt
- Loose connections
- Defective aux. switch contacts of
breaker or Isolator/Earth switch, as
applicable
62