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14、andards Association. Copyright 2013 IEEE. All rights reserved. viParticipants At the time this IEEE recommended practice was completed, the Protection and the secondary winding is connected to protective devices, instruments, meters, or control devices. Ideally, CTs change the magnitude of the curre
15、nt being measured without changing the phase angle or wave shape of the current. Practically, however, the output of CTs does contain some error and distortion, and dealing with these errors and distortion is one of the primary challenges in applying CTs. 4.1 Equivalent circuit for current transform
16、ers To understand the performance and application of CTs, it is necessary to start with an equivalent circuit. The circuit shown in Figure 1 is representative, although variations on this circuit may be found in various texts. Figure 1 Equivalent circuit for a CT In this equivalent circuit, Ipis the
17、 primary system current Isis the secondary current fed to the meters or relays N is the nominal turns ratio of the CT R+jZL is the impedance of the CT secondary winding and leads, and the secondary wiring to the loads (meters or relays) applied to the CT IEEE Std 3004.1-2013 IEEE Recommended Practic
18、e for the Application of Instrument Transformers in Industrial and Commercial Power Systems Copyright 2013 IEEE. All rights reserved. 4 Zbis the impedance of the load (meters or relays) Vsis the voltage across the CT secondary Ieis the exciting current drawn by the iron core of the CT There are seve
19、ral important relationships depicted in this equivalent circuit. First, the secondary current produced by the CT is not equal to the primary current, divided by the CT turns ratio. Instead, there is an error due to the need to supply exciting current to the CT core. This relationship can be expresse
20、d algebraically as eps INII (1) Secondly, the secondary voltage, Vs, is a function of the secondary current and the total secondary burden impedance, including both external elements (the impedances of meters, relays, and interconnecting wiring) and the internal impedance of the CT secondary winding
21、. LjRZIV bss Z (2) The third important relationship is that the exciting current, Ie, is a function of the CT secondary voltage, Vs. However, this is not a linear relationship, but rather is defined by a curve, called the CT secondary excitation characteristic, that represents the non-linear behavio
22、r of the iron core of the CT. Figure 2 depicts an idealized CT secondary excitation characteristic. Typically, the excitation characteristics are plotted on log-log paper. Therefore, even though some portions of the curve appear to be linear (i.e., a straight line), the relationship is actually non-
23、linear. Figure 2 Idealized CT secondary excitation characteristic The secondary excitation characteristic is derived from the hysteresis characteristic of the ferrous core of the CT. As Vsbecomes large, there comes a point when the CT essentially becomes unable to sustain further increases in Vs. Re
24、ferring to the equivalent circuit, further increases in Ip/N translate almost completely to increases in exciting current, Ie. At that point, the CT is said to be saturated. Practical secondary excitation characteristics are usually published by CT manufacturers in the form of excitation current ver
25、sus secondary rms voltage. The values are obtained either by calculation from transformer design and core-loss data or by testing a representative sample of the CTs produced by the manufacturer. The test is an open-circuit excitation current test on the secondary terminals, applying a variable sine
26、wave voltage at rated frequency and recording rms current versus rms voltage. A term that frequently appears in the technical literature is knee-point voltage. This is the voltage at the inflection point of the curve of Vsversus Ie(Figure 2). The working definition of knee-point voltage under IEEE S
27、td 3004.1-2013 IEEE Recommended Practice for the Application of Instrument Transformers in Industrial and Commercial Power Systems Copyright 2013 IEEE. All rights reserved. 5 IEEE standards is that it is the voltage at which a line tangent to the secondary excitation characteristic, when drawn on lo
28、g-log coordinates, is at an angle of 45to the horizontal. Under IEC standards, knee-point voltage is defined as the rated-frequency secondary voltage above which a 10% increase in voltage results in an increase of 50% or more in exciting current. Figure 3 is an example of a secondary excitation curv
29、e provided by a manufacturer for a specific CT design. Figure 3 Typical CT secondary excitation curve Detailed treatment of these issues is beyond the scope of this reference, but more comprehensive discussions can be found in other standards and in the technical literature. See B1, B2, B3, B4, B6,
30、B10, B11, and B12.5In particular, IEEE Std C37.110TMB7 includes an informative discussion of these topics. 4.2 Burden Burden, in CT terminology, is the load connected to the secondary terminals. It may be expressed in several ways: Voltamperes and power factor at a specified value of current, Total
31、ohms impedance and power factor, or Ohms of the resistance and reactive components. The term burden is used to differentiate the CT load from the primary circuit load. The power factor referred to is that of the burden and not of the primary circuit. 5The numbers in brackets correspond to those of t
32、he bibliography in Annex A. IEEE Std 3004.1-2013 IEEE Recommended Practice for the Application of Instrument Transformers in Industrial and Commercial Power Systems Copyright 2013 IEEE. All rights reserved. 6 4.3 Current transformer ratings and performance parameters 4.3.1 Continuous current ratings
33、 4.3.1.1 IEEE rating structure for current transformers The terms “rating” and “ratio” tend to be used interchangeably in day to day practice, although “rating” is the more precise term. The rating of a CT consists of a primary current rating and an associated secondary current rating. These ratings
34、 are related by the nominal transformation ratio of the CT, which is usually also the physical turns ratio of the transformer. The preferred syntax for ratings is: Primary current rating : secondary current rating Under IEEE standards, most CTs have a 5-ampere secondary current rating. Table 1, extr
35、acted from IEEE Std C57.13TM-2008, lists ANSI standard CT ratings for single ratio CTs. Table 2 lists standard ratings for multi-ratio CTs, while Table 3 lists standard ratings of CTs with dual ratios. Table 1 IEEE standard current transformer ratings for single-ratio CTs 10:5 800:5 15:5 1200:5 25:5
36、 1500:5 40:5 2000:5 50:5 3000:5 75:5 4000:5 100:5 5000:5 200:5 6000:5 300:5 8000:5 400:5 12 000:5 600:5 IEEE Std 3004.1-2013 IEEE Recommended Practice for the Application of Instrument Transformers in Industrial and Commercial Power Systems Copyright 2013 IEEE. All rights reserved. 7 Table 2 ANSI st
37、andard ratings for multi-ratio CTs (Typically found in bushing CTs on power apparatus) Current ratings (A) Secondary taps Current ratings (A) Secondary taps 600:5 50:5 100:5 150:5 200:5 250:5 300:5 400:5 450:5 500:5 600:5 X2-X3 X1-X2 X1-X3 X4-X5 X3-X4 X2-X4 X1-X4 X3-X5 X2-X5 X1-X5 3000:5 300:5 500:5
38、 800:5 1000:5 1200:5 1500:5 2000:5 2200:5 2500:5 3000:5 X3-X4 X4-X5 X3-X5 X1-X2 X2-X3 X2-X4 X2-X5 X1-X3 X1-X4 X1-X5 1200:5 100:5 200:5 300:5 400:5 500:5 600:5 800:5 900:5 1000:5 1200:5 X2-X3 X1-X2 X1-X3 X4-X5 X3-X4 X2-X4 X1-X4 X3-X5 X2-X5 X1-X5 4000:5 500:5 1000:5 1500:5 2000:5 2500:5 3000:5 3500:5
39、4000:5 X1-X2 X3-X4 X2-X3 X1-X3 X2-X4 X1-X4 X2-X5 X1-X5 2000:5 300:5 400:5 500:5 800:5 1100:5 1200:5 1500:5 1600:5 2000:5 X3-X4 X1-X2 X4-X5 X2-X3 X2-X4 X1-X3 X1-X4 X2-X5 X1-X5 5000:5 500:5 1000:5 1500:5 2000:5 2500:5 3000:5 3500:5 4000:5 5000:5 X2-X3 X4-X5 X1-X2 X3-X4 X2-X4 X3-X5 X2-X5 X1-X4 X1-X5 Ta
40、ble 3 ANSI standard ratings for CTs with dual ratios Double ratio with series-parallel primary windings (A) Double ratio with taps in secondary winding (A) 25 50:5 50 100:5 100 200:5 200 400:5 400 800:5 600 1200:5 1000 2000:5 2000 4000:5 25/50:5 50/100:5 100/200:5 200/400:5 300/600:5 400/800:5 600/1
41、200:5 1000/2000:5 1500/3000:5 2000/4000:5 Under IEEE standards, CTs are assigned a thermal rating factor. The actual thermal capacity of the CT windings is equal to: factorratingThermalamperesratingCurrentamperescapacityThermal u )()(IEEE Std 3004.1-2013 IEEE Recommended Practice for the Application
42、 of Instrument Transformers in Industrial and Commercial Power Systems Copyright 2013 IEEE. All rights reserved. 8 IEEE standards specify a variety of thermal rating factors, all for the case of a 30 C ambient, as well as curves that may be used to adjust the thermal capacity for other ambient tempe
43、ratures. The rating (ratio) of the CT as well as its thermal rating factor will usually be found on the CT nameplate as well as in manufacturers literature. Traditionally, those applications conceived and planned following IEEE standards employ CTs with 5A secondary ratings. European tradition favor
44、s CTs with 1A secondary ratings. As these traditions merge, the question of whether 1 A or 5 A ratings should be favored often arises. IEEE Std C37.110TMprovides guidance on comparing the technical performance of these two different ratings in order to make an informed decision about which tradition
45、 to follow in a specific application. The IEEE standards for CT continuous current ratings (such as IEEE Std C57.13TM) describe the methodology for specifying those ratings. Other standards may impose constraints on the ratings that are available for specific applications. For example, some circuit
46、breaker standards specify the minimum CT rating (ratio) that may be associated with circuit breakers as a function of the breaker voltage rating. 4.3.1.2 IEC rating structure for current transformers IEC standards define the secondary current rating of CTs to be 1 A, 2 A, or 5 A, with 5 A being the
47、preferred rating. However, there is a strong tradition in those parts of the world where IEC standards are applied to employ CTs with 1 A secondary current ratings. IEC standards also define a set of standard and preferred primary current ratings. Standard ratings are 10 A, 12.5 A, 15 A, 20 A, 25 A,
48、 30 A, 40 A, 50 A, 60 A, and 75 A and multiples thereof. Preferred ratings are 10 A, 15 A, 20 A, 30 A, 50 A, and 75 A and multiples thereof. IEC standards allow for multi-ratio CTs provided the lowest rating corresponds to one of the standard ratings. Unless otherwise specified, the actual thermal c
49、apacity of an IEC-rated CT, designated as Ith, is the primary current rating of that CT. It is possible to specify a thermal capacity, Ith, in excess of the nominal primary current rating. When this is done, the preferred thermal capacities are 120%, 150%, or 200% of the nominal rating. 4.3.2 Short-time current ratings There are two dimensions of short-time current ratings of CT
