Measuring current transformers

Specificalions for current transformers General requirements Measuring current transformers 2705 Pan 1/1992 Pan 2/1992 BS 7626/1993... 

These errors are valid only when the CTs are loaded by a minimum 25% of the rated VA burden, for CTs of class 1 and 50% for CTs of classes 3 and 5 and a primary currenl of not less than 5% or more than 120% of the rated current. The measuring CTs may not transform correcllv unless the above conditions are met. 

In some instances, it is required to measure the sum of the energy supplied along several feeders. This is the case, for example, when a factory site is fed by more than one cable and the tariff is based on the overall energy and maximum demand used. The usual method of doing this is to summate the currents in the individual feeders by installing separate current transformers in each and connecting their secondaries in parallel (Figure 17.3). 

The instmments discussed above (and many of the others which follow) act in response to the system voltage and/or current. In most cases, the values of these two parameters are very high, which presents problems in the design of the insulation and current-carrying capabilities of the instmment. In these instances, the instmment is supplied with a known fraction of the measured quantity using a voltage transformer or a current transformer, as appropriate. 

The current transformer is arranged with its primary winding in series with the supply (Figure 17.8). It thus carries the load current. The measuring instrument is connected across the secondary as shown. The ideal theoretical relationship between the currents and the number of turns on the primary and secondary is ... 

Current transformers used in transient current measurements must have a peak current rating at least equal to the maximum expected currents otherwise, current peaks are lost in the data due to saturation of the current probe. Figure 9.6 indicates how current probe saturation resulted in a flat-top current waveform and loss of vital information, making power quality analysis more difficult. 

Alternating current indicators are almost exclusively of the moving iron vane type because of their wide range and low sensitivity to frequency variations (less than the electrodynamic type). For current measurement above the range of moving iron vane and digital meters, a current transformer is the simplest and cheapest solution. When inserted in the circuit, it provides the proper ratio between the meter range and the measured current. 

Feist H, Koep M, Reich H. (1971) A current transformer and gated integrator for measurement of week currents from pulsed accelerators. Nud Instr and Meth 97 319-321. 

Electronic Measurements Ltd. at www.pemuk.com. These probes are not the usual current transformer type. The output from a Rogowski probe depends not on the instantaneous current enclosed, but on the rate of change of current. So, instead of just placing several turns around the wire to be sensed, as in a typical current transformer, the Rogowski probe effectively takes an air-cored solenoid and then bends that in a circle around the sensed wire (like a doughnut). Such probes are also considered virtually noninvasive. 

Many applications of this strategy are based on extensions of the concepts of impedance developed earlier in this chapter (41-43). However, the excitation waveform is usually an impulse in potential (rather than a periodic perturbation), and a transient current is measured. One records both E t) and i t) as observed functions. Then both are subjected to transformations, and comparisons are made in the frequency domain between E s) and i s). Ratios of the form i s)IE s) are transient impedances, which can be interpreted in terms of equivalent circuits in exactly the fashion we have come to understand. The advantages of this approach are (a) that the analysis of data is often simpler in the frequency domain, (b) that the multiplex advantage applies, and (c) the waveform E(f) does not have to be ideal or even precisely predictable. The last point is especially useful in high-frequency regions, where potentiostat response is far from perfect. Laplace domain analyses have been carried out for frequency components above 10 MHz. 

Current transformen A transformer-type of instrument in which the primary carries the current to be measured and the secondary is in series with a low current ammeter. A current transformer is used to measure high values of alternating current. 

The methods used for expressing the data fall into two categories, time domain techniques and frequency domain techniques. The two methods are related because frequency and time are the reciprocals of each other. The analysis technique influences the data requirements. Reference 9 provides a brief overview of the various mathematical methods and a multitude of additional references. Specialized transforms (Fourier) can be used to transfer information between the two domains. Time domain measures include the normal statistical measures such as mean, variance, third moment, skewness, fourth moment, kurto-sis, standard deviation, coefficient of variance, and root mean squEire eis well as an additional parameter, the ratio of the standard deviation to the root mean square vtJue of the current (when measuring current noise) used in place of the coefficient of variance because the mean could be zero. An additional time domain measure that can describe the degree of randonmess is the autocorrelation function of the voltage or current signal. The main frequency domain... 

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