Overvoltage Transient Waveforms

Overvoltage Transient Waveforms

The definition of a transient waveform is critical for the de­sign of overvoltage protection circuitry. An unrealistic voltage waveform with long duration of the voltage or very low source impedance requires a high-energy protection device, resulting a cost penalty to the end-user. IEEE Std. 587 defines two ov­ervoltage current waveforms to represent the indoor environ­ment recommended for use in designing protection devices. Table 1 describes the waveforms, open circuit voltage, source impedance, and energy stored in the protection circuitry.

1. Category I. The waveform shown in Fig. 7 is defined as ‘‘0.5 jU. s-100 kHz ring wave.’’ This waveform is repre-

sentative of category I indoor low-voltage (ac lines less than 600 V) system transients. This 100 kHz ring wave has a rise time of 0.5 ^s (from 10% to 90% of its final amplitude), with oscillatory decay at 100 kHz, each peak being 60% of the previous one. The rapid rate of rise of the waveform can cause du/dt problems in the semiconductors. The oscillating portion of the waveform produces voltage polarity reversal effects. Some semi­conductors are sensitive to polarity changes or can be damaged when unintentionally turned on or off.

Overvoltage Transient Waveforms

Overvoltage Transient Waveforms

Overvoltage Transient Waveforms

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Overvoltage Transient Waveforms

Figure 10. SCR crowbar overvoltage protection circuit for switching power supplier.

gation into the sensitive circuit; and (2) those that divert transients away from sensitive loads and so limit residual voltages. Attenuating a transient, that is, keeping it from propagating away from the source or keeping it from imping­ing on a sensitive load, is accomplished with series filters within a circuit. The filter, generally of low-pass type, attenu­ates the transients (high-frequency) and allows the signal or power flow (low-frequency) to continue undisturbed. Diverting a transient can be accomplished with a voltage-clamping de­vice or with a ‘‘crowbar’’ type device.

Filters. The frequency of a transient is several orders of magnitude above the power frequency (50/60 Hz) of an ac cir­cuit. Therefore, an obvious solution is to install a low-pass filter between the source of transients and the sensitive load. The simplest form of filter is a capacitor placed across the line. The impedance of the capacitor forms a voltage divider with the source impedance, resulting in attenuation of the transients at high frequencies. This simple approach may have undesirable effects, such as (1) unwanted resonance with inductive components located in the circuit resulting in high-peak voltages; (2) high capacitor in-rush current during switching, and (3) excessive reactive load on the power system voltage. These undesirable effects can be minimized by add­ing a series resistor (RC snubber circuit).

Voltage-Clamping Devices. A voltage-clamping device is a component having variable impedance depending on the cur­rent flowing through the device or on the voltage across its terminal. These devices exhibit nonlinear impedance charac­teristics. Under steady-state, the circuit is unaffected by the presence of the voltage-clamping device. The voltage-clamp­ing action results from increased current drawn through the device as the voltage tends to rise. The apparent ‘‘clamping’’ of the voltage results from the increased voltage drop in the source impedance due to the increased current. It must be clearly understood that the device depends on the source im­pedance to produce clamping. One is seeing a voltage divider action at work, where the ratio of the division is nonlinear (Fig. 9). The voltage-clamping device cannot be effective with zero source impedance. Table 2 lists various types of voltage – clamping devices and their features and characteristics.

Crowbar Devices. Crowbar-type devices involve a switch­ing action, either the breakdown of a gas between electrodes or turn-on of a thyristor. After switching on, the crow-bar de­vice offer a very low impedance path which diverts the tran­sient away from the parallel-connected load. These crowbar devices have two limitations. The first is their delay time, typ­ically microseconds, which leaves the load unprotected during initial voltage rise. The second limitation is that a power cur­rent from the steady-state voltage source will follow the tran­sient discharge current (called ‘‘follow current’’ or ‘‘power – follow’’).

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