MEASUREMENT
Contents
Measurements -
General.......................................................................
Direct Measurement.......................................................................................
Indirect Measurement.....................................................................................
Instrument Accuracy.......................................................................................
Voltage Transformer Design.......................................................................
Current Transformer Design.......................................................................
Terminal Markings.........................................................................................
Special Dangers with Current Transformers..........................................
Burden.............................................................................................................
Calculation of an instrument transformer
burden.........................................
Location of CTs and VTs..............................................................................
Instruments...................................................................................................
CURRENT AND VOLTAGE TRANSFORMERS FOR
PROTECTION..
CURRENT TRANSFORMERS.......................................................................
Design...........................................................................................................
Operation......................................................................................................
Open-Circuited Current Transformer...........................................................
Short-Time Factor........................................................................................
Accuracy Limit Factor..................................................................................
Specification of Current Transformers..........................................................
Rated Secondary Current.............................................................................
Secondary Winding Impedance....................................................................
Primary Windings.........................................................................................
Application....................................................................................................
Effect of CT Magnetising Current on Relay Setting......................................
Quadrature or Air-Gap Current Transformers............................................
Summation Current Transformer.................................................................
VOLTAGE TRANSFORMERS......................................................................
Accuracy.......................................................................................................
Protection.....................................................................................................
Residual Connection.....................................................................................
Capacitor Voltage Transformers............................................................
Measurements - 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.
Direct Measurement
FIGURE 2.1 - 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 2.1(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 2.1(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 2.2. 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. Different ammeter instruments can be plugged
into the tongs to give current ranges from 10A to 1000A. 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.
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 2.2
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), 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 (refer to a later paragraph
in this section for special precautions when dealing with CT secondaries).
To select the phases between which voltages
are measured, a 3-position selector switch is used, as in Figure 2.1(a), 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 2.2(b), bottom right.
FIGURE 2.2 - 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 (see special
precautions). 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 2.2(b).
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. 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
|
CTs
|
||||
|
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%
|
±15' (angle)
±10'
±20'
±40'
not spec.
|
0.1
0.2
0.5
1
3
5
|
±10.25 - 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
|
|
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.
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 on
offshore installations most VTs are 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.
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.
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 2.3(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 2.3 - 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. 1500/1A) 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
2.3(c). Its construction is not limited
by the fault current of the main system.
FIGURE 2.4 - BAR TYPE CT SHOWING
CONSTRUCTION DETAIL
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 below, 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.
Terminal Markings
The terminals of a CT should be marked as indicated in Figure
2.4. The primary current flows from P1
to P2 and it is standard to put P1 nearer to the circuit breaker. The secondary current flows from S1 to S2
through the burden.
FIGURE 2.5 - CT
TERMINAL MARKINGS
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 2.6(a) the
secondary of the CT (assumed to have a ratio of 1000/5A and to have 1000A
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.
FIGURE 2.6 - VOLTAGE AND BURDEN OF A
CURRENT TRANSFORMER
If now the short-circuit be replaced by a resistance of, say, 0.5
ohm (as in Figure 2.6(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 2.6(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 burnout 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 shortcircuited 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.
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.
Burden
The load of a current transformer is called
the burden and can be expressed either as a VA load or as an impedance. In the former case the VA is taken to be at
the CT nominal secondary current. For
example, a 5VA burden on a 1A transformer would have an impedance of 5 ohms:
|
= 5V
|
impedance = =5 W
or on a 5A
current transformer:
|
= 1V
|
impedance = = 0.2 W
All burdens are connected in series and the
increase in impedance increases the burden on the current transformer. A current transformer is unloaded if the
secondary winding is short-circuited as under this condition the VA burden is
zero because the voltage is zero. The
errors of transformation depend on the angle of the burden as well as its
impedance.
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 usually 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 = 2.5 V. With 5A
current flowing in them, the burden of the pilot leads would be 2.5V x 5A =
12.5VA, 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.
In Figure 2.5 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?
FIGURE 2.7 - CALCULATION OF CT BURDEN
Total instrument burden = 2 + 2 + 3 + 2 + 4 = 13VA.
Total pilot load resistance = 2 x 0.1 = 0.2W.
With 5A secondary current, volt-drop in leads is 5 x 0.2
= 1 V.
Burden imposed by both leads = 5A x 1 V = 5VA.
\ Total burden on CT = 1 3 + 5 = 1 8VA.
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.
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. Manufacturers' arrangements
vary a great deal and the relevant manuals should be consulted before
attempting to locate any current or voltage transformers
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.
Since wattmeter, varmeter, power factor meter
and frequency meter movements tend to be expensive, an alternative which is
often used is the transducer-operated instrument. Here the VT and CT signals are fed into static
electronic ac/dc 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 2.8 shows typical connections.
FIGURE 2.8 - 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 also fed through VTs and
CTs whose connections are the same as for a wattmeter.
CURRENT AND VOLTAGE TRANSFORMERS FOR PROTECTION
CURRENT TRANSFORMERS
The current transformer is well established
but it is generally regarded as merely a device which reproduces a primary
current at a reduced level. A current transformer
designed for measuring purposes operates over a range of current up to a
specific rated value, which usually corresponds to the circuit normal rating,
and has specified errors at that value.
On the other hand, a protection current transformer is required to
operate over a range of current many times the circuit rating and is frequently
subjected to conditions greatly exceeding those which it would be subjected to
as a measuring current transformer.
Under such conditions the flux density corresponds to advanced
saturation and the response during this and the initial transient period of
short-circuit current is important.
It will be appreciated, therefore that the
method of specification of current transformers for measurement purposes is not
necessarily satisfactory for those for protection. In addition an intimate knowledge of the
operation current transformers is required in order to predict the performance
of the protection.
Current transformers have two important
qualities:
1.
They
produce the primary current conditions at a much lower level so that the
current can be carried by the small cross-sectional area cables associated with
panel wiring and relays.
2.
They
provide an insulating barrier so that relays which are being used to protect
high voltage equipment need only be insulated for a nominal 600V.
Operation
A representation of a ring-type current
transformer is shown in Figure 2.9. R2 is the
secondary winding resistance, Ie the magnetising
current and Rb, and Xb are the burden
resistance and reactance. The primary
ampere-turns must equal the sum of the secondary ampere-turns and the magnetising
ampere-turns.
N1I1
= N2 (I2 + Ie )
In practice Ie is small
compared to I2 and is therefore ignored in all CT calculations
with the exception of those concerned with ratio and phase angle error.
The magnetising current depends on the
voltage V2 which in turn depends on the product of the
secondary current and the impedance of the burden plus the CT secondary winding
resistance. That is, by Ohm's Law:
V2 =
I2 (R2 + Rb +jXb)
Note. The term (R2 + Rb +jXb) is not a simple
arithmetic sum as Xb is
90° out of phase with R2 and Rb
and so must be added by vectors. To
denote this the prefix "j", is used which literally
means "advance by 90°" The voltage I2Xb
is therefore 90° ahead of I2R2
and I2Rb
and Vb
= I2(Rb +jXb)
FIGURE 2.9 - EQUIVALENT CIRCUIT OF A RING-TYPE
CURRENT TRANSFORMER
If a vector diagram is drawn, Figure 2.9,
then the ratio error, which is the difference in magnitude of I1
and I2,
and 0, the phase angle error, become apparent.
The magnetising current Ie
lags V2
by 90°.
It can be seen that if the burden was wholly resistive then the ratio
error would be a minimum and phase-angle error maximum, whereas if the burden
was wholly reactive then the ratio error would be maximum and the phase-angle
error minimum.
FIGURE
2.10 VECTOR DIAGRAM OF A RING-TYPE CURRENT TRANSFORMER
Figure 2.11 shows a magnetising
characteristic for a 100/1A current transformer. It has been previously stated that Ie
is small compared to I2 up to and beyond the
knee-point of the characteristic. Hence
the ratio and phase-angle errors will also be small. This means that the primary-secondary current
relationship will be maintained to this point,
i.e. where the product I2 (R2 + Rb
+jXb) is 120V,
e.g.
if R2 = 1W and Rb
+jXb = 7.5 +j0W then linearity would be maintained up to a secondary current of
|
I2 = =
14.1A or 14.1 x CT rating.
Alternatively, if linearity is required up
to, say 20 x CT rating then the total impedance should not exceed
|
R2
+ Rb +jXb = =
6W
FIGURE
2.11 - CT MAGNETISING CHARACTERISTIC
Open-Circuited Current Transformer
If the impedance Rb
+jXb is very high then the voltage calculated from
I2 (R2 + Rb +jXb) would be very large, well above knee-point value and Ie would become significantly large in the ampere-turn balance equation
N1I1 = N2 (I2 + Ie) and I2 would be reduced. The limiting value is when the CT secondary winding is open-circuited and I2 = 0. All the input ampere-turns will he used as magnetising ampere-turns and will drive the current transformer Into saturation. As can be seen from Figure 2.11 the greatly increased magnetising current will not cause much increase to the average voltage. However, the change in flux from zero to the knee-point value is not accomplished in ¼ cycle but in perhaps 1/100 of this time. Thus the rate of change of flux and, therefore, the induced voltage during this period would be about 100 times the knee-point voltage. Insulation can be damaged by this high short-duration voltage and overheating caused by the great increase of iron losses.
I2 (R2 + Rb +jXb) would be very large, well above knee-point value and Ie would become significantly large in the ampere-turn balance equation
N1I1 = N2 (I2 + Ie) and I2 would be reduced. The limiting value is when the CT secondary winding is open-circuited and I2 = 0. All the input ampere-turns will he used as magnetising ampere-turns and will drive the current transformer Into saturation. As can be seen from Figure 2.11 the greatly increased magnetising current will not cause much increase to the average voltage. However, the change in flux from zero to the knee-point value is not accomplished in ¼ cycle but in perhaps 1/100 of this time. Thus the rate of change of flux and, therefore, the induced voltage during this period would be about 100 times the knee-point voltage. Insulation can be damaged by this high short-duration voltage and overheating caused by the great increase of iron losses.
Short-Time Factor
When a current transformer is used in a power
system it may be subjected to fault current many times larger than its primary
rating and, therefore, it must he able to withstand the effects of this current
for the time for which it is likely to persist.
The maximum current which it can carry without mechanical and thermal
damage is expressed as a multiple of its rated current and is known as the
short-time factor. For example, a
current transformer of ratio 200/5 which is capable of withstanding a current
of, say, 13,000 A would have a short-time factor of 65. Such a short-time factor would always be
associated with a period of duration of the current for example 3 s. Smaller currents would be permissible for
longer periods, the permissible time increasing as the square of the reduction
of current. Larger currents, however,
are not necessarily permissible for any period of time, since electromagnetic
forces have also to be considered.
Accuracy Limit Factor
When a current transformer is used to
energise a protective relay it must maintain its characteristic ratio up to
some multiple of its rated current. This
multiple, which depends on the type and characteristics of the protection, may
be 10, 20 or some even higher value and is known as the "Accuracy Limit
Factor".
The small ratio error introduced by the
magnetising current is often compensated for in the case of measuring current
transformers by Slightly modifying the ratio of primary to secondary turns from
the nominal ratio. For example, a 100/1
current transformer might have one primary turn and 98 secondary turns so that the transformation
ratio would appear to he 100 to 1.02 A, but when it is used to supply its rated
burden the secondary current is reduced from the above value to 1 ampere by the
magnetising losses.
Although the burden of a protective scheme is
only a few VA at rated current, if the accuracy limit factor is high the output
required from the current transformer may be considerable. On the other hand, it may be subjected to a
very high burden. For example, in the
case of overcurrent and earth-fault protection having elements of similar VA
consumption at setting, if the overcurrent elements are set at 100% an
earth-fault element set at 10% would have 100 times the impedance of the
overcurrent elements. Although
saturation of the relay elements modify this somewhat, it will be seen that the
earth-fault element is a severe burden and the current transformer is liable to
have considerable ratio error in this case.
For this reason it is not very much use applying turns correction to
current transformers used for protective purposes and it is generally simpler
and more satisfactory to wind them with turns corresponding to the nominal
ratio.
Specification of Current Transformers
A method of specifying current transformers
for protective purposes is detailed in BS3938.
In this specification they are defined in terms of rated burden,
accuracy class and accuracy limit.
Standard values of rated burden are:
2.5, 7.5, 10, 15
and 30 VA.
Two accuracy classes are quoted 5P and 10P
which gives a composite error at rated accuracy limit of 5% and 10%
respectively.
Standard accuracy limit factors are:
5, 10, 15, 20 and
30.
The method of describing a current
transformer is as follows: 15VA Class 5P20
which means that it is rated for a burden 5VA and will not have more
than 5% error at 20 times rated current.
It is frequently more convenient to refer
directly to the maximum useful voltage which can be obtained. In this connection, the knee-point of the
magnetisation curve is defined as that point at which an increase of 10% of
secondary voltage would increase the magnetising current by 50%. Design requirements for current transformers
for general protective purposes are frequently specified in terms of knee-point
voltage magnetising current at the knee-point or at some other point, and
secondary resistance. These are known in
general as 'Class X', current transformers.
Rated Secondary Current
Current transformers are usually designed to
have rated secondary currents of 0.5A, 1A or 5A. Most burdens will require a definite amount
of VA at rated current and consequently will have an impedance which varies
inversely as the square of the rated current, so that the value of the rated
secondary current does not appear to be important. Many burdens, however, are situated at some
distance from the corresponding current transformers and, as the wire size of
the interconnecting leads is usually large enough to carry the current produced
by a current transformer of any secondary rating, the leads introduce a
definite resistance and therefore more burden at the higher rated currents,
e.g. lead resistance 1 ohm at 1A
correspond to 1VA; lead resistance 1W at 5 A
corresponds to 25VA. Clearly in all
cases where leads may be appreciable there is a great advantage in using the
lower rated current transformer. Modern
practice favours the use of the 1A secondary windings.
Secondary Winding Impedance
Bearing in mind the high value of secondary
current which a protective current transformer may be required to deliver, it
is desirable to make the secondary winding resistance as low as practicable to
limit copper tosses and therefore heating.
In the case of wound primary-type current
transformers winding reactance also occurs, although its precise measurement
and definition is a matter of some difficulty.
Ring-type current transformers with a single symmetrical primary
conductor and a uniformly distributed secondary winding have no secondary reactance.
Primary Windings
To achieve a reasonable output from a current
transformer having a primary rating of 80A or less would require a large core
area and therefore it is more economical to increase the primary winding from a
single turn to two, three or more turns.
This of course necessitates an increase in secondary turns which
increases knee-point voltage for a given core area. The additional primary turns may be attained by
passing the primary conductor through a ring-type transformer a number of times
or it may be a specially constructed transformer with a primary winding.
Application
In specifying current transformers the
connected burden and mode of operation must be taken into account paying
attention not only to the wide range of devices which may be connected, but
also to the variation of impedance over the range of setting any relay. For example, the normal burden of an
overcurrent relay is 3VA at setting. The
normal setting range of the relay is 50% to 200% of nominal current. Therefore a 1A relay set to 50% would have a
setting current of 0.5 A and the voltage across the coil at this current would
be
|
V = = 6V
and the impedance would be
|
Z = = 12W
At a setting of 200% the setting current
would be 2 A, the voltage
|
V = = 1.5V
and the impedance would be
|
Z = = 0.75W
If the characteristic of the relay is to be
maintained up to 20 times the relay setting, then a knee-point voltage not less
than
20 x 6V = 120 V for a 50% setting
or 20
x 1.5V = 30 V for a 200% setting
would be required. The former is more onerous and therefore the
lowest setting must be taken into account when specifying the knee-point
voltage. There is, however, an alleviating
factor in that a relay operating at 20 times its setting will have saturated
magnetically and therefore the impedance will be reduced. The reduction for an overcurrent relay is
about half the impedance at setting which means that in the above case a
knee-point voltage of 60 V would be satisfactory
In many cases the current transformers
associated with the over-current protection must also cater for earth-fault
relays. An earth- fault relay having a
minimum setting of 20% would have voltage at setting of
|
= 15V
and the impedance would be
|
= 75W
The maximum earth-fault level may be
restricted to, say, twice the CT primary rating and therefore 10 times the
relay setting. The knee-point voltage
should therefore be greater than 10 x 15V = 150V, or allowing for saturation,
75V.
In this case the size is determined by the
earth-fault relay. A suitable current
transformer would be a 7.5VA Class 5P10.
This would produce a voltage of 7.5V at rated current when connected to
a 7.5W burden and would have only 5% error at
10 times rated current, i.e. at a
voltage of 10 x 7.5V = 75 V.
From the specification in the form 7.5 VA
Class 5P10, the knee-point voltage can be estimated. If it has a 5A secondary winding then at
rated current it would produce 1.5V across the rated burden and at 15 times rated
current 22.5V. As a rough guide the
knee-point voltage is the product of the VA rating and the accuracy limit
factor divided by the rated secondary current.
Class 5P is specified when phase-fault
stability and accurate time grading is required. When these are unimportant Class 10P is
suitable.
It may be that more than one relay is to be
connected to one set of current transformers in which case the total burden
must be calculated. It is generally
sufficient to add the burdens arithmetically but it should be borne in mind
some alleviation may be available by adding the burden vectorially in case of
difficulties in design.
It is not good engineering practice to
specify a current transformer which is substantially larger than necessary as
there is no advantage in performance and its cost would be higher and its
dimensions greater.
Effect of CT Magnetising Current on Relay Setting
The overall setting of a protection system is
affected by the magnetising current of the current transformers and, whilst the
effect may not be significant in the case of overcurrent relays, it can have
some effect on the overall setting of an earth-fault relay and can sometimes
have a profound effect on differential protection systems particularly where a
large number of current transformers are connected together. For example, a busbar zone protection scheme.
The primary operating current (P.O.C.) of a
protection System is the sum of the relay setting current and the magnetising
current of all the connected current transformers at the voltage across the
relay at setting multiplied by the CT ratio.
Quadrature or Air-Gap Current Transformers
A quadrature or air-gap transformer is merely
a current transformer with an air gap so that most of the primary ampere-turns
are used to magnetise the core. This means
that the flux, and therefore the secondary voltage, is proportional to primary
current. More correctly, the secondary
voltage is proportional to the rate of change of flux and therefore lags the
primary current by 90° hence the name quadrature current
transformer.
Summation Current Transformer
There are two applications of the summation
current transformer. One is the adding
together the secondary current from a number of current transformers and is
mainly used for measuring purposes. The
other is used in pilot-wire protection systems to convert the inputs from the
current transformers in each phase to a single output for comparison with a
similar output from the remote end via the pilot wires.
In the former case any input winding not in
use must be left open-circuited.
VOLTAGE TRANSFORMERS
The voltage transformer in use with
protection has to fulfil only one requirement, which is that the secondary
voltage must be an accurate representation of the primary voltage in both
magnitude and phase.
To meet this requirement, they are designed
to operate at fairly low flux densities so that the magnetising current, and
therefore the ratio and phase angle errors, is small. This means that the core area for a given output
is larger than that of a power transformer, which increases the overall size of
the unit. In addition, the normal three-
limbed construction of the power transformer is unsuitable as there would be
magnetic interference between phases. To
avoid this interference a five-limbed construction is used, which also
increases the size. The nominal
secondary voltage is sometimes 110V but more usually 63.5V per phase to produce
a line voltage of 110V.
Accuracy
Only in a few of the many protection
applications is the phase angle and ratio errors likely to be much
significance. However the likelihood of
a voltage transformer being provided solely for protection is small and
therefore the more stringent accuracies of instrumentation and metering are
usually required.
All voltage transformers are required by
British Standard to have ratio and phase-angle errors within prescribed limits
over a 80% to 120% range voltage and a range of burden from 25% to 100%.
For protection purposes accuracy of
measurement may be important during fault conditions when the voltage is
greatly suppressed. Therefore a voltage
transformer for protection must meet the extended range of requirements over a
range of 5% to 80% rated voltage and, for certain applications, between 120% and
190% rated voltage.
Protection
Voltage transformers are generally protected
by HRC fuses on the primary side and fuses or a miniature circuit-breaker on
the secondary side. As they are designed
to operate at a low flux density their impedance is low and therefore a
secondary side short-circuit will produce a fault current of many times rated
current.
Residual Connection
It is important that a voltage of the correct
magnitude and phase angle is presented to directional earth-fault relays and
the earth-fault elements of impedance relays.
As an earth-fault can be any one of the three phases it is not possible
to derive a voltage in the conventional manner.
The solution is to use the residual or broken delta connection as shown
in Figure 2.12
Figure
2.10 - BROKEN DELTA CONNECTION OF A VOLTAGE TRANSFORMER
Under three-phase balanced conditions the
three voltages sum to zero. If one
voltage is absent or reduced because of an earth-fault on that phase, then the
difference between the normal voltage and that voltage is delivered to the
relay. A secondary winding for this type
of connection is in addition to the normal secondary winding.
Capacitor Voltage Transformers
At voltages of 132kV or more, the cost of
electromagnetic voltage transformers is very high. A more economical proposition is the capacitor
voltage transformer. This is virtually a
capacitance voltage divider with a tuning inductance and an auxiliary
transformer as shown in Figure 2.11.
Any simple voltage-divider system suffers
from the disadvantages that the output voltage varies considerably with
burden. If, however, C2 is tuned with a
reactor, the burden can be varied over a wide range with very low
regulation. It is not feasible to
produce directly the usual 63.5 V as C2 would be impossibly large and therefore
a potential of around 12 kV is developed across C2. This is applied to the electromagnetic unit
and the 63.5 V derived from its secondary winding. This method also has the advantage that a
tapped winding can be provided to accommodate the fairly wide tolerances of
capacitors.
Figure
2.11 - CAPACITOR VOLTAGE TRANSFORMER
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