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:
(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’.
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