1. Field of the Invention
The present invention is related generally to voltage regulators and more specifically to AC voltage stabilizers capable of providing a voltage regulated to within desired limits at high-power levels.
2. Description of the Prior Art
With the proliferation of input voltage-sensitive loads, such as computers, there is an increasing demand for AC line voltage stabilizers. These devices are connected between the utility line and the voltage-sensitive load. They typically provide a .+-.1% output or load voltage from a .+-.17% input or utility voltage.
For loads up to approximately 20 KVA there is a number of methods for providing the required stabilization. The vast majority of applications can be satisfied using a constant voltage transformer or one of its derivatives. However, for powers up to the 500 KVA range the constant voltage transformer is no longer viable. Alternative methods such as motor alternator sets are expensive, heavy, and pose special siting and maintenance problems. Other methods such as motor-driven variacs are generally too slow in operation; tap changing transformers may not provide sufficiently tight control.
Several static methods are available for providing the required regulation. Representative of these is U.S. Pat. No. 3,435,331. Disclosed therein is a voltage regulator utilizing a gapped booster transformer and a gapped filter transformer each having a winding comprised of a first and second portion. The first portion of each of the windings of the two transformers is connected in series with the source and the load. The first and second portions of the winding of the filter transformer are connected in series with a harmonic filter circuit across the turns of the booster transformer. The conduction of current through the second portion of the booster transformer is controlled by a pair of inverse parallel-connected silicon control rectifiers. The rectifiers are fired by a control circuit. This patent is characteristic of the prior art in that little or no protection is provided for the silicon control rectifiers by way of limiting the current flowing therethrough. It is also characteristic of the prior art in its use of harmonic filters for filtering the output wave form.