Rectifier Transients

The shunt-tuned filter currents mentioned in the preceding section are transient. Since the tube current is cut off during each cycle, a transient current may occur in each cycle. When power is first applied to the rectifier, another transient occurs, which may be smaller or larger than the cyclic transient, depending on the filter elements. In reactor-input filters the transient current can be approximated by the formula given in Section 54 for a step function applied to the series circuit comprising filter L and C plus R_s. This circuit is valid because the shunting effect of the load is slight in a well-proportioned filter. In capacitor-input filters, the same method can be used, but here the inductance is the leakage inductance of the anode transformer. Therefore, equation 55 applies, except that the maximum step function voltage is E_pk.

Transients which occur when power is first applied differ from cyclic transients in that they are spasmodic. Power may be applied at any instant of the alternating voltage cycle, and the suddenly impressed rectifier voltage ranges from zero to E_pk. Starting transients are difficult to observe on an oscilloscope because of their random character. It is necessary to start the rectifier several times for one observation of maximum amplitude, and the trace is faint because it appears for a very brief time.

Excessive current inrush, which occurs when a power transformer is connected to a supply line, plagues rectifier design. The phenomenon is associated with core saturation. For example, suppose that the core induction is at the top of the hysteresis loop in Fig. 18 (p. 24) at the instant when power is removed from the rectifier, and that it decreases to residual value B_r for H = 0. Suppose that the next application of power is at such a point in the voltage cycle that the normal induction would be B_m. This added to B_r requires a total induction far above saturation value; therefore heavy initial magnetizing current is drawn from the line, limited only by primary winding resistance and leakage inductance. This heavy current has a peaked wave form which may induce momentary high voltages by internal resonance in the secondary coils and damage the rectifier tubes. Or it may trip a-c overload relays. The problem is especially acute in large transformers with low regulation. A common remedy is to start the rectifier with external resistors in the primary circuit and short-circuit them a few cycles later. Some rectifiers are equipped with voltage regulators which reduce the primary voltage to a low value before restarting.

A-c line transients may cause trouble in three-phase rectifiers, especially those having balance coils, by shifting the floating neutral voltage. Filters like that in Fig. 97 prevent such transients from appearing in the rectified output.

In some applications the load is varied or removed periodically. Examples of this are keyed or modulated amplifiers. Transients occur when the load is applied (key down) or removed (key up), causing respectively a momentary drop or rise in plate voltage. If the load is a device which transmits intelligence, the variation in filter output voltage produced by these transients results in the following undesirable effects:

1.  Modulation of the transmitted signal.

2.  Frequency variation in oscillators, if they are connected to the same plate supply.

3.  Greater tendency for key clicks, especially if the transient initial dip is sharp.

4.  Loss of signal power.

A filter which attenuates ripple effectively is normally oscillatory; hence damping out the oscillations is not practicable. Nor would it remedy the transient dip in voltage, which may increase with non-oscillatory circuits. The filter capacitor next to the load should be large enough to keep the voltage dip reasonably small. An approximation for transient dip in load voltage which neglects the damping effect of load and series resistance is

[57]

where ΔE_D is the transient dip expressed as a fraction of the steady-state voltage across R, and L, C, and R are as shown in Fig. 79(a). The accuracy of this approximation is poor for dips in transient voltage greater than 20 per cent.

Although the tendency for key clicks in the signal may be reduced by attention to the d-c supply filter elements, the clicks may not be entirely eliminated. Where key-click elimination is necessary, some sort of key-click filter is used, of which Fig. 98 is an example.

Fig. 98. Key-click filter.

This filter has inductance and capacitance enough to round off the top and back of a wave and eliminate sharp, click-producing corners. Figure 99 is an oscillogram showing a keyed wave shape with and without such a filter.

Fig. 99. Keyed wave shape with and without key-click filter.

In a choke-input filter, voltage surges are developed across the choke under the following conditions:

1.  Ripple Voltage. With large rectifier commutation angles, or with grid-controlled rectifiers, a surge occurs once each ripple cycle. In the limit, this surge equals the rectifier peak voltage.

2.  Initial Starting Surge. This surge adds to output d-c voltage. Under the worst conditions it raises the voltage at this point to twice normal and occurs every time rectifier plate voltage is applied.

3.  Keying or Modulation Transient. Surge value depends upon constants L, C, and R_L, and is limited by considerations of wave shape. This occurs each time the key is opened or closed, or load is varied.

4.  Short-Circuit Surge. If load R_L is suddenly short-circuited, it causes full d-c voltage to appear across the filter reactor until the circuit breaker opens. This occurs occasionally. Rectifiers are sometimes arranged so that, if the short circuit persists, the circuit breaker recloses 3 times and then remains open.

5.  Interruption of Reactor Current. This surge voltage is limited only by losses and capacitance of the circuit, and it may be large, as shown by Fig. 73. Unless the reactor is designed to produce this voltage, it occurs only through accident.

Conceivably, surges 1, 2, and 3 may occur simultaneously and add arithmetically. A reactor insulated to withstand surges 1 plus 2 plus 3 also would withstand surge 4. A reasonable value of peak surge voltage comprising these factors is 2 1/2 times the full d-c working voltage.

If surges 1 and 5 are too much for reasonable insulation, the reactor is protected by a gap or other means.

If a rectifier is disconnected from the supply line while the load is off, interruption of plate transformer peak magnetizing current may cause high voltages to appear at random in the windings in much the same way as reactor current interruption causes high voltages. This is especially true if the transformer operates at high core induction. The effect is partly mitigated by the arc energy incident to the opening of the disconnecting switch. But unless the plate transformer is insulated specifically to prevent dangerously high voltages, protective elements may have to be added in a rectifier subject to switching at light loads. The necessity for such protection may be estimated from exciting volt-ampere data plus the curves of Fig. 73.

Insufficient attention sometimes is given to the manner in which power supply lines are brought into buildings. This is particularly important where a rectifier is supplied by overhead high-voltage lines. Because of their relatively high surge impedance, lightning and switching surges occurring on such lines may cause abnormally high voltages to appear in a rectifier and break down the insulation of transformers or other component parts. The likelihood of such surges occurring should be taken into account before the transformers are designed.

Underground cable power lines impose much less severe hazards: first because they are protected from lightning strokes, and second because they have much lower impedance (about one-tenth that of overhead lines). Surges on these cables have much lower values compared to those on overhead lines carrying the same rated voltage. Protection against these surges varies with the type of installation.

The best protection of all is provided by an indoor power system with an underground cable connecting it to the rectifier. Good protection is afforded by oil-insulated outdoor surge-proof distribution transformers, stepping down to the rectifier a-c power supply voltage, with an underground cable between the distribution transformer and rectifier. No protection at all is provided when overhead lines come directly into the rectifier building.

With the trend to dry-type insulation, it is desirable to use lightning arresters on overhead lines where they enter the building. Because of their low impulse ratio, dry-type transformers require additional arresters inside the building. When a line surge is discharged by a lightning arrester, there is no power interruption.