6 Current and Voltage T ransformers - ?? 6 • Current and Voltage T ransformers. ... • 6 •...
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Electromagnetic voltagetransformers 6.2
Capacitor voltagetransformers 6.3
Current transformers 6.4
Novel instrumenttransformers 6.5
6 C u r r e n t a n d V o l t a g eT r a n s f o r m e r s
Whenever the values of voltage or current in a powercircuit are too high to permit convenient directconnection of measuring instruments or relays, couplingis made through transformers. Such 'measuring'transformers are required to produce a scaled downreplica of the input quantity to the accuracy expectedfor the particular measurement; this is made possible bythe high efficiency of the transformer. The performanceof measuring transformers during and following largeinstantaneous changes in the input quantity isimportant, in that this quantity may depart from thesinusoidal waveform. The deviation may consist of astep change in magnitude, or a transient componentthat persists for an appreciable period, or both. Theresulting effect on instrument performance is usuallynegligible, although for precision metering a persistentchange in the accuracy of the transformer may besignificant.
However, many protection systems are required tooperate during the period of transient disturbance in theoutput of the measuring transformers that follows asystem fault. The errors in transformer output mayabnormally delay the operation of the protection, orcause unnecessary operations. The functioning of suchtransformers must, therefore, be examined analytically.
It can be shown that the transformer can be representedby the equivalent circuit of Figure 6.1, where allquantities are referred to the secondary side.
6 C u r rent an d Vol tageTran s f or me rs
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Rp Lp Rs Ls1/1
Figure 6.1: Equivalent circuit of transformer
When the transformer is not 1/1 ratio, this condition canbe represented by energising the equivalent circuit with anideal transformer of the given ratio but having no losses.
6.1.1 Measuring Transformers
Voltage and current transformers for low primary voltageor current ratings are not readily distinguishable; forhigher ratings, dissimilarities of construction are usual.Nevertheless the differences between these devices lieprincipally in the way they are connected into the powercircuit. Voltage transformers are much like small powertransformers, differing only in details of design thatcontrol ratio accuracy over the specified range of output.Current transformers have their primary windingsconnected in series with the power circuit, and so also inseries with the system impedance. The response of thetransformer is radically different in these two modes ofoperation.
6.2 ELECTROMAGNETIC VOLTAGE TRANSFORMERS
In the shunt mode, the system voltage is applied acrossthe input terminals of the equivalent circuit of Figure 6.1.The vector diagram for this circuit is shown in Figure 6.2.
The secondary output voltage Vs is required to be anaccurate scaled replica of the input voltage Vp over aspecified range of output. To this end, the winding
voltage drops are made small, and the normal fluxdensity in the core is designed to be well below thesaturation density, in order that the exciting current maybe low and the exciting impedance substantiallyconstant with a variation of applied voltage over thedesired operating range including some degree ofovervoltage. These limitations in design result in a VT fora given burden being much larger than a typical powertransformer of similar rating. The exciting current, inconsequence, will not be as small, relative to the ratedburden, as it would be for a typical power transformer.
The ratio and phase errors of the transformer can becalculated using the vector diagram of Figure 6.2.
The ratio error is defined as:
where:Kn is the nominal ratio
Vp is the primary voltage
Vs is the secondary voltage
If the error is positive, the secondary voltage exceeds thenominal value. The turns ratio of the transformer neednot be equal to the nominal ratio; a small turnscompensation will usually be employed, so that the errorwill be positive for low burdens and negative for highburdens.
The phase error is the phase difference between thereversed secondary and the primary voltage vectors. It ispositive when the reversed secondary voltage leads theprimary vector. Requirements in this respect are set outin IEC 60044-2. All voltage transformers are required tocomply with one of the classes in Table 6.1.
For protection purposes, accuracy of voltagemeasurement may be important during fault conditions,as the system voltage might be reduced by the fault to alow value. Voltage transformers for such types of servicemust comply with the extended range of requirementsset out in Table 6.2.
K VVn s
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0.8 - 1.2 x rated voltage0.25 - 1.0 x rated burden at 0.8pf
voltage ratio error phase displacement(%) (minutes)
0.1 +/- 0.1 +/- 5
0.2 +/- 0.2 +/- 10
0.5 +/- 0.5 +/- 20
1.0 +/- 1.0 +/- 40
3.0 +/- 3.0 not specified
Table 6.1: Measuring voltage transformer error limits
IsIsI XsXs sXs
IeII = exciting currentImIIII = phase angle error
IIII = secondary currentp
II = primary current
IpIpI XpXp pXpX
Ipp Figure 6.2: Vector diagram for voltage transformer
6.2.2 Voltage Factors
The quantity Vf in Table 6.2 is an upper limit of operatingvoltage, expressed in per unit of rated voltage. This isimportant for correct relay operation and operationunder unbalanced fault conditions on unearthed orimpedance earthed systems, resulting in a rise in thevoltage on the healthy phases.
Voltage factors, with the permissible duration of themaximum voltage, are given in Table 6.3.
6.2.3 Secondary Leads
Voltage transformers are designed to maintain thespecified accuracy in voltage output at their secondaryterminals. To maintain this if long secondary leads arerequired, a distribution box can be fitted close to the VTto supply relay and metering burdens over separateleads. If necessary, allowance can be made for theresistance of the leads to individual burdens when theparticular equipment is calibrated.
6.2.4 Protection of Voltage Transformers
Voltage Transformers can be protected by H.R.C. fuses onthe primary side for voltages up to 66kV. Fuses do notusually have a sufficient interrupting capacity for usewith higher voltages. Practice varies, and in some casesprotection on the primary is omitted.
The secondary of a Voltage Transformer should always beprotected by fuses or a miniature circuit breaker (MCB).The device should be located as near to the transformer
as possible. A short circuit on the secondary circuitwiring will produce a current of many times the ratedoutput and cause excessive heating. Even where primaryfuses can be fitted, these will usually not clear asecondary side short circuit because of the low value ofprimary current and the minimum practicable fuse rating.
The construction of a voltage transformer takes intoaccount the following factors:
a. output seldom more than 200-300VA. Cooling israrely a problem
b. insulation designed for the system impulsevoltage level. Insulation volume is often largerthan the winding volume
c. mechanical design not usually necessary towithstand short-circuit currents. Must be small tofit the space available within switchgear
Three-phase units are common up to 36kV but for highervoltages single-phase units are usual. Voltagetransformers for medium voltage circuits will have drytype insulation, but for high and extra high voltagesystems, oil immersed units are general. Resinencapsulated designs are in use on systems up to 33kV.Figure 6.3 shows a typical voltage transformer.
6.2.6 Residually Connected Voltage Transformers
The three voltages of a balanced system summate tozero, but this is not so when the system is subject to asingle-phase earth fault. The residual voltage of asystem is measured by connecting the secondarywindings of a VT in 'broken delta' as shown in Figure 6.4.
The output of the secondary windings connected inbroken delta is zero when balanced sinusoidal voltagesare applied, but under conditions of unbalance a residual
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0.25 - 1.0 x rated burden at 0.8pf0.05 - Vf x rated primary voltage
voltage ratio error phase displacement(%) (%)
3P +/- 3.0 +/- 120
6P +/- 6.0 +/- 240
Table 6.2: Additional limits for protection voltage transformers.
Voltage factor Time Primary winding connection/systemVf rating earthing conditions
Between lines in any network.1.2 continuous Between transformer star point and
earth in any network
1.2 continuous Between line and earth in an 1.5 30 s effectively earthed network
1.2 continuous Between line and earth in
1.9 30 sa non-effectively earthed neutral system
with automatic earth fault tripping
1.2 continuous Between line and earth in an isolatedneutral system without automatic earth fault