O & M OF POWER TRANSFORMERS in substations.ppt

PTR Operation and Maintenances - Existing practice

4. PREVENTIVE MAINTENANCE
•Maintenance may be defined as the upkeep of
the sub station electrical equipment in proper
working condition and efficient to derive the
following :
Reliable and efficient operation
Optimum utilisation
Availability
Reduced down time
Detection of premature faults
Minimising revenue loss etc,.
To meet the above requirement, the equipment has to be
checked, attended to, trouble-shot and operated under
specified conditions.
6/17/2024 4DEFINITION

5. PREVENTIVE MAINTENANCE
•The need for maintenance
During the operational life of the electrical
equipment, ageing occurs due to various
stresses which is incident on the equipment both
under normal and fault conditions and result in
deterioration of physical and chemical properties
of components making up the electrical
equipment.
The expected performance can not be obtained
from the equipment once it isaged.
6/17/2024 5THE NEED

6. PREVENTIVE MAINTENANCE
•The need for maintenance
The electrical equipment in service are
subject to the following stresses :
•Electrical stresses
•Thermal stresses
•Mechanical stresses
•Environmental stresses
•Combined stresses
6/17/2024 6STRESSES

7. PREVENTIVE MAINTENANCE
•Electrical Stress
The Insulation of electrical equipment experience
the following voltages :
•Continuous normal power frequency rated
voltage
•Temporary power frequency over voltages due to
voltage regulation, Ferranti effect, and long
duration power frequency over voltages.
•Lightning Impulse Voltage Waves (Surges)
•Switching Impulse Waves (Surges)
6/17/2024 7ELECTRICAL STRESS

8. PREVENTIVE MAINTENANCE
•Electrical stress
•Electro Dynamic forces under short circuit condition ;
The force experienced by parallel bus bars is
F = 2.04 * I
s
2
*L/d * 10
-2
Kg-f
I
s in KA, L
in metre d
in metre
The 90
0
bends tend to open out due to short circuit forces and the
transformer windings experience radial and axial forces
The electrical stresses can get intensified due to the presence of voids,
contaminants or cavities in the insulation.
For these reasons the electro dynamic forces cause ageing and
deterioration.
The stress experienced by electrical equipment for a given rated
voltage in decreasing order are :
Capacitors Cables Bushings Transformers
6/17/2024 8STRESS ON EQUIPMENT

9. PREVENTIVE MAINTENANCE
•Thermal Stress
Elevated temperatures may be reached during
operation due to dielectric losses, increased I
2
R
loss or by heat absorption from surroundings.
Temperature may increase abnormally due to
sustained short circuit current.
The cooling system failure in power transformers
also stress the windings thermally.
6/17/2024 9THERMAL STRESS

10. PREVENTIVE MAINTENANCE
•Mechanical Stress
There may be permanent mechanical
stress due to improper installation
Circuit breakers may experience vibration
stress during normal closing/opening and
during making/breaking under fault
The bus bar vibration in rigid bus bar
stresses the bus bar support mechanically
6/17/2024 10MECHANICAL STRESS

11. PREVENTIVE MAINTENANCE
•Environmental stress
Proximity to chemical industry, sea coast and
intentional damage caused by humans contribute
to ageing and deterioration
Environmental factors include pollution,
Radiation, Humidity, dust particles, moisture etc,.
•Combined stress
In most of the electrical equipment, normally
some of the stress factors as above will be
present
6/17/2024 11OTHER STRESS

12. PREVENTIVE MAINTENANCE
•The life of an electrical equipment depends on the
various stresses it undergoes during operation
•A large percentage of failures are due to deterioration of
insulation, loose contact and abnormal operating
conditions
•Failure Mechanism :
•Mechanical:-Noise, Vibration, Heat etc,.
•Hydraulic :-Fluid leakage, Sluggish operation, No
response on command etc,.
•Electrical:-I
2
R, Current Density, Defective design and
manufacture, Wrong operation etc,.
6/17/2024 12FAILURE MECHANISM

13. PREVENTIVE MAINTENANCE
•Warning Signals :
•Any abnormality will manifest itself by fore
warning signals like :
•Variation in sound
•Excess temperature
•Vibration
•Sparking
•Blown out fuses
•Frequent tripping
•Tripping before full load etc,.
6/17/2024 13WARNING SIGNALS

14. PREVENTIVE MAINTENANCE
•Approach to preventive maintenance
Many of the failures can be anticipated by proper
application of simple tests and timely maintenance
If the fault conditions leading to failure are detected in the
early stage itself, the extent of damage can be reduced
and the equipment can be reconditioned and put back
into service
Any abnormality will be followed by warning signals and
these shall be seriously taken note of and acted upon
6/17/2024 14APPROACH

15. PREVENTIVE MAINTENANCE
•Approach to preventive maintenance
The detection of incipient faults in electrical equipment
depends upon use of proper diagnostic tools, its effective
use, correlation and proper interpretation of test results
and observations based on experience, manufacturer’s
guidance etc,.
Proper maintenance of electrical equipment demands
that a maintenance schedule be prepared for each
equipment which include daily, weekly, monthly, half
yearly, yearly checks based on manufacturer’s
recommendations
6/17/2024 15APPROACH

16. PREVENTIVE MAINTENANCE
•Approach to preventive maintenance
Preventive maintenance calls for advance planning
based on previous experience, equipment
manufacturer’s guidance and preparation of equipment
specific maintenance schedule
Corrective or break down maintenance is carried out as
and when necessary only to low value and auxiliary
equipment, breakdown of which does not affect power
supply continuity
6/17/2024 16APPROACH

17. PREVENTIVE MAINTENANCE
•Approach to preventive maintenance
Condition based maintenance to asses the condition of
the equipment is carried out by conducting tests ON or
OFF the line. This will be very useful to prevent
equipment failure and other associated consequent
damages
Reliability tests are generally carried out on old
equipment by conducting “Remaining Life Assessment
Studies”. Based on economics, life extension techniques
are adopted without sacrificing reliability and availability.
The maintenance costs too are reduced6/17/2024 17CONDITION MONITORING

18. PREVENTIVE MAINTENANCE
•Responsibility of maintenance
personnel
Equipment failure analysis is the major responsibility of
maintenance personnel to prevent repeated failure and
provide inputs for necessary change in design
parameters, new equipment design, quality control plan,
erection and subsequent maintenance technique.
Reducing down time, Spares management to reduce
inventory and Documentation of maintenance activity
chronologically is also the responsibility of maintenance
personnel
6/17/2024 18RESPONSIBILITY

19. PREVENTIVE MAINTENANCE
•Dissipation factor
Charging current (I)
δ
θ
Voltage (V)
Dissipation factor = power(W)/VAr
= I*V*cosθ/I*V*sinθ
=sinδ/cosδ
=tanδ
6/17/2024 19TAN DELTA

20. PREVENTIVE MAINTENANCE
•Polarisation Index (PI)
= IR value@10 minutes / IR value@1 minute
( IR is Insulation Resistance in Mega ohm )
The insulation behaves as a dielectric and
hence when a voltage is applied across the
dielectric the total current drawn has the
following components;
Charging current, Resistive Leakage current,
Surface leakage current and Displacement
current
I
t= I
c+I
rl+I
sl+I
d
6/17/2024 20PI

21. O & M OF TRANSFORMERS
•Polarisation Index Values Acceptable
<1 Dangerous Wet
1.0-1.1 Poor Poor
1.1-1.25 QuestionableQuestionable
1.25-2.0 Fair Fair
2.0-4.0 Good Good
>4.0 Excellent Dry

22. O & M OF POWER TRANSFORMERS

23. O & M OF POWER TRANSFORMERS

24. O & M OF POWER TRANSFORMERS

40. •The loading of transformer is decided by the
permissible temperature rise of the winding and
oil.
•The load on the transformer varies according
to the load curve and is not steady.
•The output which can be obtained from a
transformer depends on the operating
conditions such as; Ambient temperature, Initial
loading, Cooling provision, Life expectancy etc.
6/17/2024 design 40

41. •Life expectancy reduces if the transformer is
overloaded for longer duration.
•Higher overloads are permissible for a shorter duration.
•Permissible duration of overloads depend depends
upon initial load, type of cooling and insulation class of
transformer.
•Main criteria for governing permissible overload is that
the average winding temperature should not exceed
95
0
C
6/17/2024 design 41

42. PERMISSIBLE DURATION OF OVERLOAD
(REFERENCE AMBIENT TEMPERATURE O F 35
0
C)
PERMISSIBLE TEMPERATURE AT RATED FULL LOAD FOR OIL IS 65
0
C
AND WINDING IS 80
0
C
OVER LOAD % 125 150 175 200 300
DURATION IN
MINUTES
125 45 15 10 1
6/17/2024 design 42

43. •For higher ambient temperature, the load on
transformer should be reduced.
•For lower ambient temperature, the load on the
transformer can be increased.
6/17/2024 design 43
LOADING ON THE BASIS OF AMBIENT TEMPERATURE
Type of cooling Percent of rated kVA
Decrease load for each
degree C higher than
ambient temperature
Increase load for each
degree C lower than
ambient temperature
Natural and water cooled1.5 1
Forced cooling oil and air1 0.75

44. 6/17/2024 Knowledge is power 44