Three-Phase Wound Rotor Induction Motor roto rwith sliprings

Angle Type Battery Terminal

Bolt Brass Bolt

Brass Custom Fittings

Brass Fasteners

Brass Inserts

Appratus Mounted

Overhaul of Seawater Pump (Cummins QSC8.3)

Double Padmount Transformer

IMProcon Transducer

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 30^o 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)

CHAPTER 4 A.C. MEASUREMENTS

4.1       GENERAL

In a.c. power systems it is necessary continually to monitor the voltage, currents, power and similar quantities in the various parts of the system.  This is done by the use of instruments - that is by indicating voltmeters, ammeters, wattmeters etc.  The same measured quantities are also used to protect the system by means of relays, which are devices to detect when any of the quantities is going outside the predetermined limit.  They initiate whatever automatic action is necessary to restore the situation or disconnect faulty or overloaded apparatus.

Almost all electrical instruments and relays depend for their action on measurements of voltage or current or combinations of the two.  Measurements of frequency are obtained from analysing a voltage measurement.

The manner in which the various types of a.c. measuring instruments work is described in Chapter 8 of the manual ‘Fundamentals of Electricity 3’.  These include moving-iron, dynamometer and eddy-current types and also transducer-operated instruments.  In the following paragraphs it will be assumed that the appropriate type of instrument is used.

FIGURE 4.1

DIRECT MEASUREMENT

4.2       DIRECT MEASUREMENT

Voltage and current samples are taken either directly or indirectly from the conductors of the circuit to be monitored.  In the simplest case (direct measurement) the voltage is taken by tapping the main conductors.  The tappings must always be protected by fuses which, for a voltage-operated instrument or relay, are quite lightly rated, though still able to deal with the full fault capacity of the system.  In the 3-phase case a selector switch may be used to measure voltages between any desired phases, as shown in Figure 4.1 (a).

Direct measurement of current in a single-phase circuit is obtained by placing the instrument’s current-operated coil in series with a main conductor, shown in Figure 4.1 (b).  In the 3-phase case it is not possible to select phases for current measurement unless current transformers are used.  It would otherwise be necessary to break each phase to connect the ammeter, and this would not be acceptable.  Selection with the use of current transformers is shown under ‘Indirect Measurement’ in Figure 4.2.  Alternatively three separate ammeters may be used.

The currents in the separate phases can, however, be measured independently by use of a clip-on type ammeter (also known by the trade name ‘Tong Test’).  Different ammeter instruments can be plugged into the tongs to give current ranges from 10A to 1 000A.  On some types the range is altered by a switch on the tester.

Direct measurement has serious disadvantages.  In high-voltage systems the instrument or relay would have to be insulated up to the full system voltage, which for a normal sized switchboard instrument is not practical.  Current-operated instruments would not only have to be insulated up to the full system voltage, they would also have to carry the full normal current of the circuit and to withstand the extreme fault currents.  This, too, is not practical except for the lightest circuits.

4.3       INDIRECT MEASUREMENT

To overcome these objections indirect measurement is employed.  Transformers are used not only to scale down the quantities actually measured, but also to isolate the instrument or relay from the main system voltage.  Such transformers, which are designed specifically for this purpose, are known as instrument transformers.

Instrument transformers are of two types - ‘voltage transformers’ (VT) and ‘current transformers’ (CT).  They are shown diagrammatically in Figure 4.2 for both single-phase and 3-phase systems. For 3-phase there may be either three separate single-phase VTs (with their ratios adjusted for the star connection) as shown in the inset to the figure, or else a 3-phase unit, which is more usual. Current transformers are always provided as separate single-phase units.

The secondary voltages and currents may be chosen as desired, but in practice the VT secondary voltage is usually 110V line-to-line, and the CT secondary current 5A or 1A (see para. 4.7 for special caution when dealing with CT secondaries).

To select the phases between which voltages are measured, a 3-position selector switch is used, as in Figure 4.1, but connected to the VT secondaries.  Further positions may be provided to measure voltages between each phase and neutral.

To select the phases in which currents are measured, a special selector switch is used which inserts the ammeter into the CT secondary of the desired phase and at the same time allows the secondary currents of the other two phases to pass.  To avoid open-circuiting the CT secondaries, all contacts are of the make-before-break type.  This is shown in Figure 4.2(b), bottom right.

FIGURE 4.2

INDIRECT MEASUREMENT WITH INSTRUMENT TRANSFORMERS

A VT feeds, through secondary fuses (except in the earthed line), all voltage-operated instruments and relays in parallel, single- or 3-phase as required.  Current-operated instruments and relays are connected in series with the CT secondary whose phase is being used.  Fuses must never be used in a CT secondary circuit, for the reason stated in para. 4.7.

Instrument transformer secondaries must always be earthed.  With star-connected VT secondaries it is normal practice to earth one phase (usually the yellow) and not the star-point.  CT secondaries are normally commoned at some point, and it is usual to earth this common line, as shown in Figure 4.2(b).

4.4       INSTRUMENT ACCURACY

Since the purpose of instruments and relays is to monitor the actual conditions in the main power line, it is necessary that VTs and CTs reproduce those conditions, to a stepped-down scale, as accurately as possible.  That is to say their voltage ratio or current ratio must be correct and constant over their whole range of operation; they must not introduce undue phase shift while doing so (important for wattmeters); and they must reproduce unbalance conditions exactly.

The extent to which these conditions are met determines the accuracy class of the instrument transformer.  A distinction is drawn between ‘measuring’ and ‘protective’ types.  For measurements, the accuracy within, and a little above, the normal working range is important, but accuracy in the overcurrent and fault ranges of current does not matter.  On the other hand, a protective CT must deliver accurate currents in the fault range, whereas accuracy in the working range is unimportant. This gives rise to two different design concepts.

The classes of accuracy are laid down by British Standards (BS 3941 for VTs and BS 3938 for CTs).  For each type different ranges of accuracy are specified for measurement and for protective transformers according to the purpose for which they are to be used. The ranges are as follows:

VTs (BS 3941)			CTs (BS 3938)
Class	Voltage Ratio Error	Phase Displ	Class	Current Ratio Error	Phase Displ
Measurement  0.1  0.2  0.5  1  3	±0.1% ±0.2% ±0.5% ±1.0% ±3.0%	±5’ (angle) ±10’ ±20’ ±40’ not spec	0.1 0.2 0.5 1 3 5	±0.25 – 0.1% ±0.5 – 0.2% ±1.0 – 0.5% ±2.0 – 1.0% ±3% ±5%	±10’ – 5’ ±20’ – 10’ ±60’ – 30’ ±120’ – 60’ not spec not spec
Protective  3P  6P	±3% ±6%	±120’ ±240’	5P 10P	±1% ±3%	±60’ ±60’
Special			X	as specified

(Note:  These classifications replace the former A-B-C series, which is, however, still found on equipment installed before the change.)

Most indicating instruments on onshore and offshore switchboards are fed from VTs and CTs of Class 0.5, and most protective relays from VTs Class 3P and CTs Class 5P.  There are, however, exceptions (for example differential relays are fed from Class X CTs), and it is necessary to refer to drawings for particular cases.

If it is ever necessary to check or recalibrate a switchboard instrument or relay, it must always be done with instrument transformers of a class higher than those with which it normally runs.

4.5       VOLTAGE TRANSFORMER DESIGN

A voltage transformer is made basically like an ordinary open-type power transformer, with separate HV and LV windings. It is, of course, much smaller, having ratings in the range 15 to 200VA per phase. The loading on a VT (or CT) is termed ‘burden’, not ‘load’; an instrument transformer burden is always measured in volt-amperes, never in watts. At voltages up to those found in Shell installations, VTs are always dry-type, often embedded in synthetic resin. They are usually located inside the switchboards. On shore equipments, especially when associated with high-voltage oil circuit-breakers, VTs are often in oil-filled tanks (see Figure 2.1 of Part A of this manual).

The high-voltage VT primary fuses are of the HRC type. They have a low current rating but are capable of breaking the full busbar fault current of the HV system. They are located in the VT compartment and with some types are embodied in the VT itself.

Access to the high-voltage VT and its fuses is through the VT compartment door. This cannot be opened until the VT has been isolated. The manner of isolation varies with different manufacturers.

4.6       CURRENT TRANSFORMER DESIGN

A current transformer can take one of two forms. One type is wound like an ordinary transformer, with primary and secondary windings round a common core. As a CT steps current down, it steps voltage up. The primary winding, though connected in the system’s high-voltage system, is in fact the LV (high current) winding as far as the transformer is concerned, and the secondary is the HV (low current) winding. Wound-primary CTs are used where the primary current is low and where it is necessary to have several primary turns to achieve enough ampere-turns in the CT. The examples shown in Figure 4.3(a) and (b) are typical; burdens are in the range 5 to 30VA per phase. Wound-primary CTs must be able to withstand the full voltage and fault current of the main system on their primary windings.

FIGURE 4.3

TYPICAL CURRENT TRANSFORMERS

An alternative form of CT is known as the ‘bar’ or ‘ring’ type.  It has no primary ‘winding’ as such but uses the main conductor itself as a ‘one-turn’ primary.  The flux surrounding the conductor, due to the current it is carrying, links the closed iron core of the CT and induces voltage in the secondary winding, which is wound as a toroid around the circular core.  The secondary circuit is closed through its burden, and the current which flows in it is an exact scaled-down replica of the primary current in the conductor.

Bar-type CTs are generally used whenever the current ratio (e.g. 1 500/1A) is large enough.  They are also convenient in that several can easily be stacked over a single existing conductor.  It is very important that they be placed the right way up, otherwise the secondary terminal voltages and current flow will be reversed.  By convention the secondary terminal S1 always has the same polarity as primary terminal P1, or as that of the end of the bar emerging from the face marked P1.  This type of CT is shown in Figure 4.3(c).  Its construction is not limited by the fault current of the main system.

Another important difference between a CT and other types of transformer lies in its magnetisation.  The magnetising current, and therefore the flux, of a power transformer or a VT is constant and depends only on the applied voltage.  However a CT when it has no burden is effectively short-circuited, and no voltage is present, whatever the primary current; therefore there is no core flux.  If the burden is increased, so also is the voltage for a given current, as explained in para. 4.7, and this causes the magnetisation to increase.  Thus with a current transformer the magnetisation is variable not only with the current, but it also is increased depending on the burden connected.

In the limit, if the burden is increased beyond the rating of the CT, the core will saturate, and the current ratio of the CT will no longer hold; it will become inaccurate.  Moreover the iron losses will rise sharply and may cause severe overheating of the CT and possibly damage to it.

4.7       SPECIAL DANGERS WITH CURRENT TRANSFORMERS

When a CT secondary circuit is closed, a current flows through it which is an exact proportion of the primary current, regardless of the resistance of the burden.  In Figure 4.4(a) the secondary of the CT (assumed to have a ratio of 1 000/5A and to have 1 000A flowing in the primary) is carrying exactly 5A, and, since the secondary terminals S1 and S2 are short-circuited, there is no voltage between them.

If now the short-circuit be replaced by a resistance of, say, 0.5 ohm (as in Figure 4.4(b)), the same 5A will flow through, causing a volt-drop of 2.5V and a burden of 5 x 2.5 = 12.5VA.  If the resistance were increased to 5 ohms (as in Figure 4.4(c)), the terminal voltage with 5A flowing would rise to 25V and the burden to 125VA.  The greater the resistance, the greater would be the voltage and burden until, as it approached infinity (the open-circuit condition), so also in theory would the voltage (and burden) become infinite.  This cannot of course happen in practice because the CT would saturate or the terminals flash over due to the very high secondary voltage between them.  But it does show the danger of open-circuiting the secondary of a running CT.  Lethal voltages can be produced at the point of opening.  This is why CT secondaries are never fused.

The danger from an open-circuited CT is twofold.  It can produce lethal voltages and so is a very real danger to personnel.  The high voltage across the secondary winding could also cause insulation failure in that winding, leading at best to inaccuracy and at worst to burn out or fire.

Before ever an instrument or relay is removed from the secondary loop of a running CT (if such a thing had to be done), the wires feeding that instrument must first be securely short-circuited at a suitable terminal box or, better, at the CT itself.  Similarly, if a running CT is ever to be taken out of circuit, it must first be firmly shorted.  CTs with 1A secondaries are more dangerous than those with 5A, as the induced voltages are higher.

FIGURE 4.4

VOLTAGE AND BURDEN OF A CURRENT TRANSFORMER

To prevent this danger many CT secondaries are permanently short-circuited by a ‘metrosil’, which is a non-linear element with a high resistance at low voltages but which breaks down to almost a short-circuit at the higher and dangerous voltages.  It does, however, somewhat reduce the accuracy of the CT and is not always acceptable for this reason.

There is also a range of CTs designed to saturate if their burden becomes excessive, so that even on open-circuit their secondary voltage will not exceed about 100V.  It is not safe, however, to assume that such CTs are fitted in any particular case.

WARNING

WHENEVER POSSIBLE THE MAIN CIRCUIT SHOULD BE MADE DEAD BEFORE INTERFERING WITH CT SECONDARIES OR THEIR INSTRUMENTS OR RELAYS.

4.8       CALCULATION OF AN INSTRUMENT TRANSFORMER BURDEN

Instrument transformers are rated according to the burden that they can carry and still remain within their specified accuracy.  The burdens are always given in VA units (i.e. power factor is ignored), and all burdens are simply added together.  Manufacturers of instruments and relays similarly state the burdens of these devices in VA.  Thus, if a CT operates an ammeter (2VA), a current relay (3VA) and, say, the current coil of a kWh meter (4VA), the total burden on the CT of these three devices will be 9VA.

The burden imposed by long secondary pilot leads, however, cannot be ignored.  If, for example, the total resistance of a CT secondary run were 0.5 ohms (go and return) and the CT had a 5A secondary, the total volt-drop across the pilots would be 0.5 x 5 = 2.5V.  With 5A current flowing in them, the burden of the pilot leads would be 2.5V x 5A = 12.5 VA, and this would need to be added to that of the instruments (9VA above) to give a total burden on the CT of 12.5 + 9 = 21.5VA.  It must therefore have a rating sufficient to meet this total burden.  In general, pilot leads impose far less VA burden on a 1A current transformer than on a 5A.

FIGURE 4.5

CALCULATION OF CT BURDEN

In Figure 4.5 a 20VA CT with full-load secondary current of 5A supplies two ammeters, a current relay, a wattmeter and a kWh meter with VA burdens as shown.  The pilot leads have a resistance of 0.1 ohm per core.  Is the 20VA rating of the CT sufficient?

Total instrument burden = 2 + 2 + 3 + 2 + 4 = 13VA.

Total pilot load resistance = 2 x 0.1 = 0.2 ohm.

With 5A secondary current, volt-drop in leads is 5 x 0.2 = 1V.

Burden imposed by both leads = 5A x 1V = 5VA.

\Total burden on CT = 13 + 5 = 18VA.

As the CT is rated 20VA, it has sufficient margin.

The reader should work out for himself what would be the total burden if the CT had a 1A secondary.

4.9       LOCATION OF CTs AND VTs

Current and voltage transformers can be located anywhere desired where the primary conductors are available, but in HV switchgear they are usually incorporated in special chambers in the switchgear unit itself.  Figures 3.1 and 3.3 in Part A of this manual show views of typical HV circuit-breaker units, where the VT and CT chambers can be clearly seen.  The VT can be drawn forward to isolate it from the busbars.  Other manufacturers’ arrangements differ in detail, especially in the front or back access to the VT chamber.

4.10     INSTRUMENTS

A.C. instruments include voltmeters, ammeters, wattmeters, varmeters, power factor meters, frequency meters and synchroscopes.  Voltmeters, ammeters and frequency meters are almost all of the moving-iron or transducer-operated type, with an accuracy of 2% full-scale deflection.  Wattmeters and varmeters are of the dynamometer type, and power factor meters and synchroscopes have two sets of fixed coils and a moving-iron armature.  All voltage-operated coils (except those for 415V or 440V or less which may be direct-fed) are fed through VTs, and all current-operated coils through CTs at all voltages.

4.11     EXAMPLE -  INSTRUMENTATION FOR A GENERATOR

Figure 4.6 shows a typical set of instrumentation for an offshore high-voltage generator.  One complete set of indicating instruments is normally located on the electrical control panel in the Electrical Control Room; a second set is mounted on the generator local control panel.  A megawatt meter for each generator may also be mounted on the main control panel in the Platform Control Room if this is separate from the Electrical Control Room.  The generator circuit-breaker panel usually carries one ammeter and a voltmeter.

FIGURE 4.6

TYPICAL INSTRUMENTATION FOR A MAIN GENERATOR

Since wattmeter, varmeter, power factor meter and frequency meter movements tend to be expensive, an alternative which is being increasingly used is the transducer-operated instrument.  Here the VT and CT signals are fed into static electronic a.c./d.c. transducers, and a d.c. voltage signal is produced from each which faithfully represents the a.c. watts, vars, power factor or frequency.  These are led to simple d.c. voltmeter-type moving-coil instruments, but which are scaled in watts, vars, power factor or hertz.  Many such instruments can be connected in parallel. Figure 4.7 shows typical connections.  They can also be seen in Figure 4.6 where the transducers for wattmeters, power factor meters and frequency meters are indicated by the blocks with diagonal line.

FIGURE 4.7

TRANSDUCER OPERATED INSTRUMENTS

Where two or more such instruments are used from the same transducer, they are connected in parallel.  Some instruments have their transducer in the instrument case; others have the transducer in a separate box, especially if it operates more than one instrument.

Kilowatt-hour or megawatt-hour meters are fed through VTs and CTs whose connections are the same as for a wattmeter.  As kWh meters are often used onshore as a basis for financial charging, they sometimes operate through VTs and CTs of a higher standard of accuracy.

4.12     TARIFF METERING

In onshore installations, where the supply is taken from the National Grid, the Supply Authority tariff takes account of maximum demand and power factor as well as actual energy consumption in kilowatt-hours.  At the main substation therefore, where the supplies are taken, the Supply Authority installs sealed meters to record kilowatt-hours, kilovarhours and maximum demand.  In larger installations the maximum demand is measured in kVA; in smaller it is in kW.  It is averaged over each successive period of 30 minutes and then it resets.

A fuller description of how tariff metering is carried out will be found in the manual ‘Onshore Electrical Systems’, and methods of power factor control are described in the manual ‘Electrical System Control’.

These meters are the property of the Supply Authority, but they may be mounted on the consumer’s main switchboard.  Although installed for tariff purposes, they are of use to the consumer by enabling him to reduce load (if possible) to keep the maximum demand below critical level, and to take steps to improve the installation overall power factor.