Steady-State Operation With Suppression of Even-Harmonics

Suppression of the free even harmonics, in the control circuit of a saturable reactor in which the magnetization curve approaches the idealized shape of Fig. 7-9(a), leads to an almost rectangular wave form in the output current. Examples in which the even harmonics are suppressed are shown in Fig. 7-11. The saturable reactor in Fig. 7-11 (a) receives its premagnetization from a permanent magnet instead of from a winding carrying direct current. One application is that of a constant d-c source that receives its supply from an a-c voltage source and in which the output is rectified. The rectangular wave

form of the gate current makes for a low value of ripple in the d-c output. The permanent magnet in a saturable reactor, such as shown in Fig. 7-11 (a), limits premagnetization to only one value, and therefore confines control to practically only one value of output current.

A single-phase, constant-current arrangement making use of a high series impedance Z₀ in series with the control circuit is shown in Fig. 7-11(b); a constant d-c output circuit supplied from a 3-phase source with a high self-inductance L₀ in series with all three control circuits is shown in Fig. 7-11(c). The rectified output from a polyphase source has less ripple than from a single-phase source, which is one of the advantages of the 3-phase arrangement. Another advantage, in the case of large outputs, is that of placing a balanced load on the 3-phase supply.

The d-c current transformer shown in Fig. 7-11(d) is another example of saturable reactor operation with suppressed even-harmonics. This is an arrangement for measuring large values of direct current where the direct current to be measured is the premagnetized current. The alternating current supplied to the gate windings from the a-c source is rectified by means of the bridge-type rectifier, which supplies the d-c ammeter and which responds to the average value. The control winding, in the case of large values of direct current, consists of only one turn, i.e., one conductor linking the two cores as shown in Fig. 7-11(d). In this application the impedance of the d-c source is generally so high that the even-harmonics in the control circuit are suppressed. The d-c current transformer is particularly useful for measuring large values of direct current because it is smaller than the shunt that is ordinarily required in parallel with the d-c ammeter; the heat losses are also lower than in the case of a shunt. Moreover, there is the advantage that the measuring instruments are isolated from the d-c circuit.