Many commercial and industrial users of sensitive electronic and electrical equipment depend upon their power utility to supply power continuously at a reasonably constant frequency and voltage. An overvoltage or undervoltage condition (hereinafter referred to as a supply event) on the power lines feeding such high power consumers can lead to costly assembly and/or process line shutdowns and damage to sensitive electronic equipment. As a result, many medium-voltage power consumers make use of a voltage regulator to remove or substantially reduce the impact a supply event may pose upon their electronic and electrical equipment.
The conventional medium power voltage regulator consists of a booster transformer, a regulator transformer having a multi-tap secondary winding, electro-mechanical tap switches coupled between the booster transformer primary and respective taps of the regulator transformer secondary windings, and a mechanical crowbar switch connected across the booster transformer primary. The secondary of the booster transformer is connected in series with the power distribution line and a load (such as electronic equipment), and the primary of the regulator transformer is connected across the source side of the distribution line in advance of the booster transformer.
During normal line conditions, the crowbar switch is closed, causing the booster transformer to appear as a simple inductance in series with the load. Control logic monitors the load voltage, and closes one of the tap switches in response to a supply event at the load. The crowbar switch is then opened so that the voltage from the regulator transformer secondary appears across the primary of the booster transformer and becomes added to the source voltage. The particular tap switch to be closed is selected so that the voltage induced in the booster transformer secondary is of sufficient magnitude and polarity so as to counteract the supply event.
However, mechanical switches increase the maintenance costs of the conventional voltage regulator. Further, conventional voltage regulators suffer from poor response times (typically requiring several seconds to correct an undervoltage condition) due to the presence of the mechanical switches. Since industrial users of microprocessor-controlled equipment, and other power supply sensitive equipment, cannot tolerate large variations in supply voltage, the delay associated with the conventional voltage regulator is often unacceptable.
Due to the rapid response times of solid-state switches over mechanical switches, solid-state static voltage regulators (SVRs) have been developed recently as a replacement for the conventional mechanical voltage regulator. Once such voltage regulator is taught by Schoendube in U.S. Pat. No. 3,732,486, and consists of a booster transformer, a multi-tap shunt transformer, and a series of thyristor tap switches coupled between one end of the primary winding of the booster transformer and a respective tap of the shunt transformer. The other end of the primary winding of the booster transformer is connected to a half H-bridge circuit which allows the voltage regulator to operate either in boost or buck mode. The secondary winding of the booster transformer is connected in series between the input terminal and the load terminal, while the shunt transformer is connected between the input terminal and a voltage reference. The regulator includes a bypass thyristor switch connected across the booster transformer primary.
In operation, a line voltage is applied to the input terminal of the Schoendube voltage regulator. If the output voltage is within tolerance, the bypass thyristor switch is closed, thereby shorting the primary of the booster transformer and providing unity voltage gain. During undervoltage conditions, the H-bridge is configured for boost mode, and one of the tap switches is closed, causing the bypass thyristor to be commutated off and a voltage to be induced into the secondary winding of the booster transformer which adds to the voltage at the input terminal. Conversely, during overvoltage conditions, the H-bridge is configured for buck mode, and one of the tap switches is closed, causing a voltage to be induced into the secondary winding of the booster transformer which subtracts from the voltage at the input terminal. Although the voltage regulator taught by Schoendube provides a shorter response time than the conventional mechanical voltage regulator, the forced commutation of the thyristors can induce undesirable transients into the load.
Another voltage regulator with improved response time is taught by Flynn in U.S. Pat. No. 4,896,092, and consists of a booster transformer, an output transformer, and a switch matrix coupled between the output transformer and the booster transformer. The output transformer includes a primary winding, an output winding, and a multi-tap winding. The secondary winding of the booster transformer is connected in series with the input terminals and the primary winding of the output transformer, and the output winding of the output transformer is connected to the output terminals. The switch matrix comprises a series of triac switches each connected between the primary winding of the booster transformer and a respective tap of the multi-tap winding.
In operation, a line voltage is applied to the input terminals of the Flynn voltage regulator. A control circuit monitors the peak voltage at the output terminals each half cycle, and provides gating signals to the switch matrix to either boost the output voltage (when operating in boost mode) or reduce the output voltage (when operating in buck mode). However, Flynn does not address the problem of transients which might be induced into the load when the triacs are switched. Accordingly, there remains a need for a medium-voltage voltage regulator which provides a shorter response time than the conventional mechanical voltage regulator, and reduces the risk of transients being induced into the load when the load voltage is corrected.