Transformers Leakage reactance

The accuracy of a VT depends upon its leakage reactance and the winding resistance. It determines the voltage and the phase errors of a transformer and varies with the VA on the secondary side. With the use of better core material (for permeability) (Section 1.9) and better heat dissipation, one can limit the excitation cunent and reduce the error. A better core lamination can reduce the core size and improve heat dissipation. 

For ease of calculation, let us consider the impedance of the transformer as its leakage reactance, ignoring resistance and draw an equivalent circuit diagram as in Figures 24.25 (b) and (c) Assuming the length of the primary distribution line to be 15 km, the total line parameters will become... 

T) Leakage reactance of transmitter-end transformer. Resistance of overhead lines of section B-B. 

Example Figure 4.17 shows a 2000-kVA, 13.8-kV to 480/277-V transformer with a leakage reactance of 6.0% feeding a bus containing two 500-hp adjustable speed drives. A 750-kVAR Y-connected capacitor bank is installed on the 480-V bus for power factor correction. Perform an analysis to determine the conditions for resonance (consult Figure 4.18 for the transformer and capacitor connections and their respective voltages and currents) ... 

Currents now exist in both windings. Therefore a volt-drop must exist in each winding due to its leakage reactance (due to leakage flux) and its conductor resistance. The equivalent circuit of a single-phase transformer can be represented as in Figure 6.2. 

Figure 6.3 Leakage reactance in percent versus the MVA rating 50 Hz transformers.

Assume the leakage reactance Xfg of each generator unit transformer to be 0.08 per unit, and ignore the resistance. 

All but the smaller ratings of inverters used in the oil industry require a sinusoidal ouqtut waveform. The quality of the waveform is typically defiued as, being that no greater than 5% total harmonic distortion should be present. In order to achieve a sinusoidal output it is uecessary to include a filter iu the output circuit. The output of the inverter usually has a double wouud transformer so that the required line voltage is obtained. The filter is placed on the load side of the transformer, its leakage reactance of the transformer contributes to the filteriug process. 

A synchronous generator (and a synchronous motor) can be represented by many inductances and reactances to account for transformer-type induction, rotational induction, mutual coupling between windings, leakage and self-induction, magnetising and excitation induction and the effects of the pole-face damper windings. Extremely complex equivalent circuits have been developed for synchronous machines, see References 1 and 2 as examples. 

Open- and short-circuit tests were conducted on a 60-Hz, 500-W 120 6-V transformer. In the short-circuit test, the readings were 10 V, 4.16 A, and 24 W. The combined impedance of the coil resistance and the leakage inductance can be found using Eq. (10.35). The resistance and reactance of the transformer can then be found using Eq. (10.35). The resistance and reactance of the transformer can then be found using Eq. (10.36) and Eq. (10.37). 

---