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Wednesday, December 12, 2012

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.

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.

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