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TESTING AND COMMISIONING
DET310 CHAPTER 5 CURRENT TRANSFORMER AND VOLTAGE TRANSFORMER THE POWER TRANSFORMER

TESTING AND COMMISIONING
DET310 5.1 INTRODUCTION Current or voltage instrument transformers are necessary for isolating the protection, control and measurement equipment from the high voltages of a power system, and for supplying the equipment with the appropriate values of current and voltage - generally these are 1A or 5Α for the current coils, and 120 and 240 V for the voltage coils.

-continue- The behaviour of current and voltage transformers during and after the occurrence of a fault is critical in electrical protection since errors in the signal from a transformer can cause mal-operation of the relays. In addition, factors such as the transient period and saturation must be taken into account when selecting the appropriate transformer.

5.1 Voltage Transformers With voltage transformers (VTs) it is essential that the voltage from the secondary winding should be as near as possible proportional to the primary voltage.           In order to achieve this, VTs are designed in such a way that the voltage drops in the windings are small and the flux density in the core is well below the saturation value so that the magnetization current is small; in this way magnetization impedance is obtained which is practically constant over the required voltage range. The secondary voltage of a VT is usually 110 or 120 V with corresponding line-to-neutral values. The majority of protection relays have nominal voltages of 110 or 63.5 V, depending on whether their connection is line-to-line or line-to-neutral

Equivalent circuits VTs can be considered as small power transformers so that their equivalent circuit is the same as that for power transformers, as shown in Figure 1a. The magnetization branch can be ignored and the equivalent circuit then reduces to that shown in Fig 1b. The vector diagram for a VT is given in Figure.2, with the length of the voltage drops increased for clarity. The secondary voltage Vs lags the voltage Vp/n and is smaller in magnitude. In spite of this, the nominal maximum errors are relatively small. VTs have an excellent transient behaviour and accurately reproduce abrupt changes in. the primary voltage.

Figure 1.0

Figure 2

Errors When used for measurement instruments, for example for billing and control purposes, the accuracy of a VT is important, especially for those values close to the nominal system voltage.     Not withstanding this, although the precision requirements of a VT for protection applications are not so high at nominal voltages, owing to the problems of having to cope with a variety of different relays, secondary wiring burdens and the uncertainty of system parameters, errors should he contained within narrow limits over a wide range of possible voltages under fault conditions.      This range should be between 5 and 173% of the nominal primary voltage for VTs connected between line and earth.

Error VT= {(n Vs - Vp) / Vp} x 100%
-continue-  Referring to the circuit in Figure 1a, errors in a VT are clue to differences in magnitude and phase between Vp/n, and Vs. These consist of the errors under open-circuit conditions when the load impedance ΖB is infinite, caused by the drop in voltage from the circulation of the magnetization current through the primary winding, and errors due to voltage drops as a result of the load current IL flowing through both windings. Errors in magnitude can be calculated from Error VT= {(n Vs - Vp) / Vp} x 100% If the error is positive, then the secondary voltage exceeds the nominal value.

Burden The standard burden for voltage transformer is usually expressed in volt-amperes (VΑ) at a specified power factor. Table 1 gives standard burdens based on ANSI Standard C Voltage transformers are specified in IEC publication 186Α by the precision class, and the value of volt-amperes (VΑ). The allowable error limits corresponding to different class values are shown in Table 2, where Vn is the nominal voltage. The phase error is considered positive when the secondary voltage leads the primary voltage. The voltage error is the percentage difference between the voltage at the secondary terminals, V2, multiplied by the nominal transformation ratio, and the primary voltages V1.

Selection of VT’s Voltage transformers are connected between phases, or between phase and earth. The connection between phase and earth is normally used with groups of three single-phase units connected in star at substations operating with voltages at about 34.5 kV or higher, or when it is necessary to measure the voltage and power factor of each phase separately. The nominal primary voltage of a VT is generally chosen with the higher nominal insulation voltage (kV) and the nearest service voltage in mind. The nominal secondary voltages are generally standardized at 110 and 120 V. In order to select the nominal power of a VT, it is usual to acid together all the nominal VΑ loadings of the apparatus connected to

Table 1 Standard burdens for voltage Transformer
Characteristics for 120 V and 60 Hz Characteristics for 69.3 V and 60 Hz  design Volt- amperes power factor resistance(Ω) inductance (H) impedance (Ω) resistance W 12.5 0.10 115.2 3.040 1152 38.4 1.010 384 Χ 25.0 0.70 403.2 1.090 575 134.4 0.364 192 Υ 75.0 0.85 163.2 0.268 54.4 0.089 64 Ζ 200.0 61.2 0.101 72 20.4 0.034 24 ΖΖ 400.0 31.2 0.0403 36 10.2 0.0168 12 Μ 35.0 0.20 82.3 1.070 411 27.4 0.356 137 Table 1 Standard burdens for voltage Transformer

Table 2 Voltage transformers error limits
Class Primary voltage Voltage error (±%) Phase error (±min) 0.1 0.8 Vn , 1.0 Vn and 1.2 Vn 0.5 0.2 10.0 20.0 1.0 40.0 0.5 Vn 2.0 80.0 Vn 3.0 120.0 Table 2 Voltage transformers error limits

5.2 Current Transformers Although the performance required from a current transformer (CT) varies with the type of protection, high grade CTs must always be used. Good quality CTs are more reliable and result in less application problems and, in general, provide better protection. The quality of CTs is very important for differential protection schemes where the operation of the relays is directly related to the accuracy of the CTs under fault conditions as well as under normal load conditions.         CTs can become saturated at high current values caused by nearby faults; to avoid this, care should be taken to ensure that under the most critical faults the CT operates on the linear portion of the magnetization curve. In all these cases the CT should be able to supply sufficient current so that the relay operates satisfactorily.

Design CT conform to the normal transformer e.m.f equation where the average induced voltage is equal to the product of the number of turns and the number of turns and the rate of change pf magnetic flux, The normal design criterion is to limit the flux to the value where saturation commences – known as the knee point flux. The knee point voltage is Where, = flux density, B(tesla) x core area, s (m2 ) The knee point voltage is

Equivalent circuit An approximate equivalent circuit for a CT is given in Figure 3, where n2ZH represents the primary impedance ZH referred to the secondary side, and the secondary impedance is, ZL, Rm and Xm represent the losses and the excitation of the core.           The circuit in Figure 3 can be reduced to the arrangement shown in figure 4 where ZH can be ignored, since it does not influence either the current IH/n or the voltage across Xm. The current flowing through Xm is the excitation current Ιe. The vector diagram, with the voltage drops exaggerated for clarity, is shown in Figure 5. In general, ZL, is resistive and Ιe lags Vs by 90°, so that Ie is the principal source of error. Note that the net effect of Ie is to make I lag and be much smaller than ΙH /n, the primary current referred to the secondary side.

Figure 5: Vector Diagram of CT

CT Errors The causes of errors in a CT are quite different to those associated with VTs. In effect, the primary impedance of a CT does not have the same influence On the accuracy of the equipment  it only adds an impedance in series with the line, which can be ignored. The errors are principally due to the current which circulates through the magnetizing branch.           The magnitude error is the difference in magnitude between ΙH / n and IL and is equal to Ir the component of Ie in line with k (see Figure 7). The phase error, represented by θ, is related to Iq the component of Ie which is in quadrature with IL. The values of the magnitude and phase errors depend on the relative displacement between Ie and IL, but neither of them can exceed the vectorial error it should be noted that a moderate inductive load, with Ie and IL approximately in phase, has a small phase error and the excitation component results almost entirely in an error in the magnitude.

AC Saturation CΤ errors result from excitation current, so much so that, in order to check if a CT is functioning correctly, it is essential to measure or calculate the excitation curve. The magnetization current of a CT depends on the cross section and length of the magnetic circuit, the number of turns in the windings, and the magnetic characteristics of the material. Thus, for a given CT, and referring to the equivalent circuit of Figure 3, it can be seen that the voltage across the magnetization impedance, Es, is directly proportional to the secondary current. From this it can be concluded that, when the primary current and therefore the secondary current is increased, these currents reach a point where the core commences to saturate and the magnetization current becomes sufficiently high to produce an excessive error.

-continue- When investigating the behaviour of a CT, the excitation current should he measured at various values of voltage  the so-called secondary injection test. Usually, it is more convenient to apply a variable voltage to the secondary winding, leaving the primary winding open-circuited. Figure 4.8a shows the typical relationship between the secondary voltage and the excitation current determined in this way.        In European standards the point Κp on the curve is called the saturation or knee point and is defined as the point at which an increase in the excitation voltage of ten per cent produces an increase of 50 % in the excitation current

Burden The burden on a CT is a measured of the load expressed in volt amperes (VA) at the rated secondary current. For example, if the rated secondary current was 5A and the impedance was 2 ohm, the burden would be: (5 x 2) x 5 = 50 VA; ( V x A) Referring to Figure 6, the burden was increased to , say 1000 ohm, the current into the burden would be: 1000/11000 x 0.5 = 0.45 A’; The 0.45 A will be divided in proportion to magnitudes of the b urden and magnetizing impedance. Leaving 0.05 A to flow into the magnetizing impedance. The voltage across the burden would be: 0.45 A x 1000 ohm = 450 V;

-continue- If the burden is replaced by an open circuit, all the current would flow through the magnetizing impedance. The voltage across magnetizing impedance with burden open circuit: 0.5 x = 5000 V. Consider a fault current , say 8000 A, the voltage across the magnetizing impedance with secondary open circuited would be: 8000/2000 x 1000 = Volts. Noted that the voltage at the CT secondary increases with increasing burden, and rises to dangerously high levels if the secondary if open circuited. The flux of the CT rides so much as to cause saturation.

-continue- During saturation, voltage only appear for small portion of cycle across the secondary of the CT. The magnitude of peak voltage Vp developed by the CT under saturation is given by: Volts Where Vk= CT knee point voltage; Vf = secondary voltage if the CT is not saturated. For most applications, the CT must not be driven into saturation and therefore a low burden or short connection should be connected across the CT. Under normal operation: IpNp = IsNs This formula loss its relationship during saturation

5.2.5 Class and Type of CT CT can be classified into three(3 major) categories: Measurement CTs General Purposes CTs Class X CTs Measurement CTs Measurement CTs are required to maintained specified accuracy up to 120% of rated current, when the burden connected is equal to the rated output of the CT For example if the rated current is 5 A, rated output of CT is 15VA, the rated burden is

-continue- Thus, the accuracy limit are maintained up to 120% x 5A= 1.2 ohm Measurement CTs class are shown as: 0.1 (Lab / calibration function) 0.2 (Accurate revenue application) 0.5 (Revenue application) 1.0 (Normal application) 3.0 (Non revenue application) 5.0 (Estimate reading

5.2.5.2 General purpose Protection CTs
Protection class CTs are as below 5P5, 5P10, 5P15, 5P20, 5P30 10P5, 10P10, 10P15, 10P20, 10P30 First number – composite error / ALF P – protection type Second number – multiple of fault current Protection CTs are required to maintained their accuracy class up to several times its rated current. Accuracy classes 5P and 10P are intended to cover simpler froms of protection such as IDMT, instantaneous and earth fault, biased differential and etc. The 5P and 10P is known as “accuracy limit primary current’ and the ratio of

-continue- The 5 and 10 described as the percentage error between the ideal and the actual secondary current when the accuracy limit current flows. BS3938 has standardized the rated accuracy limit factors to 5, 10, 15, 20 or 30. For example: 30 VA, 5P10 Ct: Rated output= 30 VA, Class 5P, accuracy limit = 10 If the CT secondary rated current is 1A; The rated burden= since =30/1 = 30 ; A(1A x accuracy-limit factor) can flow before accuracy is lost. This is equivalent to an output of 102 x 30 =3000 VA;

-continue- The voltage before the CT lost its accuracy is: 10A x = 300 V; If now the burden is reduced to, say , the current which would flow before accuracy is lost would be 300 V/ =30 A; The rated accuracy factor has risen to 30. Thus accuracy limit factor and load (burden) are interrelated (inversely).

Class X CTs BS 3938 defines class X CTs as required for special purpose applications. The performances specification is defined in terms of the following characteristics: rated primary current. turns ratio (with an error not exceeding 1.25%) rated knee-point EMF at maximum secondary turns. maximum exciting current at rated-knee point EMF resistance of the secondary winding at 75 degrees Celcius. Class X Cts are usually applied when a high knee point is required to avoid saturation of the core. In protection context, they are usually categorised into two classes: Class A – designed to transform accurately without saturation up to a maximum fault current. Class B – for high impedance circulating current.

Figure 6:

TESTING AND COMMISIONING
DET310