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Wednesday, December 12, 2012

CHAPTER 5 QUESTIONS AND ANSWERS



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)

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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