5.1 QUESTIONS
1. What types of power transformers are
likely to be met with onshore and offshore?
2. Why are large oil-filled transformers often
fitted with a conservator?
3. What is the purpose of a Buchholz
relay?
4. How are large transformers cooled?
5. To which windings is a tapping switch
or tap changer connected? Why?
6. What is the advantage of silicone oil
over mineral oil?
7. Why is ‘Askarel’ used in many offshore
transformers?
8. What are the disadvantages of Askarel?
9. How
is the liquid level checked, and the integrity of the sealing monitored, in a
sealed transformer?
10. Why are LV cable boxes in transformers usually much
larger than HV boxes?
11. What
nameplate voltage ratio would you expect to see on a transformer used to
convert from a nominal 11 000V to a nominal 415V system?
12. What do you understand by a transformer’s
impedance Z? Give a typical value.
13. An
Askarel-filled sealed transformer is naturally cooled. What code letters would be used to describe
the cooling system?
14. What do you understand by a transformer
phase connection ‘Dy11’?
15. What precautions would you take before
operating an off-load tapping switch?
16. Describe
briefly the principle of an on-load tap changer. What types of mechanism are employed to
operate it?
17. What site tests would you expect to do on
an installed transformer?
18. What
is an auto-transformer? Where would it
be used? What are its properties as
compared with a double-wound transformer of the same rating and voltage ratio?
19. What
precautions would you take when connecting an auto-transformer into an earthed
system?
20. What material is used for cable conductors
on most installations?
21. What insulating materials are used for
power cables?
22. Why is steel wire not used for armouring
single-core cables?
23. What does the abbreviation HCL mean in a
cable description?
24. Name the precautions to be taken when
making a crimped cable termination.
25. Why are stress cones used when
terminating a high-voltage cable?
26. If
a 3-phase circuit is run with single-core cables between a non-hazardous and a
hazardous area, at which end must the armouring be bonded to earth?
27. Why are certain vital services designed
for operation on d.c.?
28. How are d.c. supplies to such services assured?
29. When should a battery be
boost-charged? Why? How is this done?
30. Why must ventilation always be on when a
battery is being charged?
31. What
do you understand by ‘central’ and ‘dedicated’ d.c. supplies? Give an example of both.
32. Why
are battery-supported a.c. supplies needed in certain cases? How are they achieved?
33. Why
is a battery, when being boost-charged, first given a constant-current charge,
then a constant-voltage charge?
34. What are the disadvantages of direct a.c.
measurements on high-voltage systems?
35. There
are two types of instrument transformers - ‘measurement’ and ‘protective’. What is their main difference?
36. Why
must a current transformer secondary never be fused? What are the dangers, and what precaution
must be taken when removing an instrument from a live CT circuit?
37. A CT with rated burden of 15VA is feeding a total
burden of 5VA through a 200 ft run of pilot leads with resistance of 0.15 ohms
per core per 100 ft. It is preferred to
use a CT with a 5A secondary; can this be done?
If not, what remedy would you propose?
38. What
class of accuracy would you expect to find generally used for measurement and
protective CTs and VTs in most installations?
What class is used with differential protection CTs?
5.2 ANSWERS
(Figures in brackets
after each answer refer to the relevant chapter and paragraph in the text.)
1. Oil-filled, sealed or
with conservator (not offshore).
Askarel-filled, sealed.
Dry type, encapsulated. (1.2)
2.
To allow expansion of the
oil with rise of temperature, while maintaining static oil pressure in the
tank. (1.2.2)
3. A Buchholz relay is
inserted in the pipe between the tank and conservator to:
(a)
trap gas bubbles and give a
‘gassing’ alarm
(b) to sense any surge of oil due to an
internal winding fault and to trip the circuit-breaker. (1.2.2)
4. Liquid-filled transformers (oil or
Askarel) have their windings cooled by thermo-syphon action whereby winding
heat is transferred to the liquid. The
liquid is usually cooled in tubes or radiators by natural convection, sometimes
assisted by forced ventilation. (1.2.2,
1.2.3)
Dry-type encapsulated transformers
are cooled by natural air circulation through the encapsulation. This may be assisted by forced fan
ventilation at the higher loadings. (1.2.4)
5. Tapping switch or tap changer operates
on the HV winding, which has lower currents to switch. (1.9)
6. Silicone oil is
non-flammable as compared with mineral oil. (1.2.2)
7. Askarel is used in offshore transformers because it too is
non-flammable and has
good heat-transfer properties (1.2.3)
good heat-transfer properties (1.2.3)
8. Askarel is toxic and risky to
handle. If spilt, it must all be
carefully recovered and disposed of ashore.
If allowed to fall into the sea, it would be destructive of marine life. (1.2.3)
9. By sight-glass on the side of the
tank. A pressure/vacuum gauge in the
space above the liquid will indicate if the sealing is faulty. (1.2.3)
10. Currents on the LV side are much greater than on the HV side
and may require many cables per phase. The LV cable boxes not only carry larger-section
conductors but may have to terminate many cables. (1.8)
11.
Approximately 11 000/435V
(no-load ratio), or 11 000 ±2½ ±5%/435V if tappings are shown. (1.3)
12.
Z is the impedance offered
to a current passing through a transformer.
It is usually expressed as a percentage, being that percentage of the
nominal applied voltage which, when applied to the primary windings with the
secondary windings short circuited, will give full-load rated current in the
secondary. (1.4)
13. LNAN. (1.6)
14. ‘Dy’
signifies a delta-connected high-voltage winding and a star-connected
low-voltage winding. If A, B, C are the
high-voltage terminals and a, b, c the corresponding low-voltage terminals,
then, taking the vector representing phase ‘A’ voltage as 12 o’clock, the
corresponding vector representing phase ‘a’ voltage is at 11 o’clock - that is,
the LV system leads 30o on the HV. (1.7)
15. Make sure that the
transformer is off load and isolated on both the HV and LV sides. (1.9.1)
16. An on-load tap changer changes the
tappings without breaking the current by using a ‘make-before-break’
method. The current in those turns which
are temporarily short-circuited during the transition is limited by introducing
resistance. To avoid the risk of the
changeover becoming stuck during transition, a ‘stored energy’ mechanism is used
which only starts the tap change when there is enough energy stored to complete
it without further outside power. The
storage of energy may be by spring or flywheel. (1.9.3)
17. (a)
Check for leaks, damage, signs of
overheating, earthing.
(b) Insulation resistance testing of HV and LV windings, to earth
and, if possible, between phases.
(c)
Checking liquid level and
effectiveness of sealing (if applicable).
(d) Simulate
operation of overtemperature or overpressure devices (also of Buchholz relay,
if fitted). (1.11.2)
18. In
an auto-transformer the secondary and primary sides share part of a common
winding in which the secondary and primary current oppose one another. This common part may therefore be of smaller
section and usually gives less heating.
It may be economically used where the voltage ratio is small - say 3:1
or less.
Compared
with its equivalent double-wound type, it is smaller and gives rise to less
heat. Its impedance is usually lower. It does not provide complete electrical
isolation between the two sides. (1.10)
19. Where one side is earthed, the earthed
line must be the one which is connected to the common primary/secondary
terminal in order that the earth may be applied to the other side also. If this is not done, the voltage of one LV line will be the same as
that of the HV side. (1.10)
20. Copper.
(2.1)
21. Polyvinyl chloride (PVC) or Ethylene
Propylene Rubber (EPR). (2.2.3)
22. Because of eddy-current heating. (2.2.5)
23. Hydrochloric Level.
(2.5)
24. Use the correct lug or ferrule and
correct crimping die. (2.6.2)
25. To control the electric stress where the
core screen ends. (2.6.2)
26. In the hazardous area. (2.6.3)
27. Because they must continue in operation
after failure of main a.c. power. This
means a supply from a battery, which in general requires operation of those
services by d.c. (3.1)
28. Power
is taken from an a.c. switchboard and is passed through a transformer-rectifier
(‘charger’) unit to provide the d.c.
required. A battery floats on the d.c. side ready to take
over the supply of d.c.
without interruption if the a.c. supply or the rectifier fails. (3.2)
29. After discharge, a battery would take a
fairly long time to recharge from the rectifier at the system’s
constant-voltage rate. This time is
shortened by ‘boosting’ - that is, by increasing the charge rate. Boosting should be done after an appreciable
discharge; also at 6- or 12-month intervals to maintain the condition of the
battery. (3.3)
30. At
top of charge a battery emits hydrogen and oxygen in an explosive mixture. Ventilation ensures that this gas mixture is
dissipated. (3.9)
31. Where several d.c. services, usually of a similar type,
are grouped to be supplied from a single D.C. Supply System, that system is
termed a ‘central’ one. Where a d.c. supply is provided
for a single equipment, that is a ‘dedicated’ system. Examples of central systems are: main
switchgear closing and tripping; fire and gas detection; communications
supplies. Examples of dedicated systems
are: gas-turbine or diesel engine starting; navigational aids; emergency radio. (3.6)
32.
Certain important services
such as process instrumentation require unbroken a.c. supplies. This is
achieved by providing a battery-supported d.c. system followed by an inverter to
convert the assured d.c.
power into a.c. (3.11)
33. If
the boost-charging voltage were first applied to a discharged battery, the
charge current would be so high that the battery might be damaged and the
rectifier overloaded. Current-limiting
circuits therefore ensure that the charge current cannot exceed a safe value -
this is the ‘constant-current’ mode.
After the battery emf has risen to the point where the charge current
will not exceed the safe value, the charge automatically becomes constant-voltage,
and the charge current tapers off (Fig 3.6). (3.8)
34. Instruments
and relays connected directly to the main system must be insulated to withstand
the full mains voltage. In HV systems
(6.6kV or 11kV) this is not practical.
Also current-operated instruments and relays must be able to carry the
full fault current of the main system - again not practical. Such devices are therefore operated through
instrument transformers (VTs and CTs). (4.2,
4.3)
35. Measurement instrument transformers must
maintain their specified accuracy over the normal working range of currents and
a little above; accuracy in the fault range is not important. Protective instrument transformers must have
their specified accuracy in the range of fault currents; accuracy in the normal
working range is not important. (4.4)
36. A high-resistance burden on a CT gives
rise to very high secondary voltages which could be a danger to personnel and
could cause insulation breakdown in the CT itself. An open-circuit is an extreme case of this. A blown fuse would cause an open-circuit;
therefore CT secondaries must never be fused.
When removing an instrument from a
live CT circuit, the CT secondary must first be short-circuited - preferably at
the CT secondary terminals - to prevent its becoming open-circuited when the
instrument is disconnected. (4.7)
37. Instrument burden = 5VA
Pilot leads burden = 15VA
Total = 20VA. This cannot be fed from a 15VA CT.
Either
substitute a 20VA CT, or else redesign the instrument system to work on 1A
instead of 5A. (4.8)
38. Measurement
CTs: Class
0.5
Protective
CTs: Class
5P
Measurement
VTs: Class
0.5
Protective
VTs: Class
3P
Differential
CTs: Class
X (4.4)
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