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Publication Date: 2010-Apr-06
Document File: 5 page(s) / 117K

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The Prior Art Database


A technique for the saturation control of a transformer by using an over-current sensor with an FET relay is disclosed. The technique described herein allows the transformer power source to be turned off on sensing an over-current in the system until the next zero crossing of the AC current when an over-current is detected.

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The invention generally relates to the transformer saturation control and more particularly to use of a current sensor and a field-effect transistor (FET) relay to control transformer saturation.


A transformer is a commonly used device that transfers electrical energy from one circuit to another through inductively coupled conductors or coils. Typically the coils are wound around a ferromagnetic core. A varying current creates a magnetic flux in the core. Often, in practice, the flux rises to a point, where magnetic saturation of the core occurs causing a huge increase in the magnetizing current and overheating the transformer. Typical situations that lead to transformer saturation include intermittent power, poor power connection, or a bumped power cord.

Generally, large transformers are employed in facilities that require good power with low source impedance, such as power distribution panels, hospital equipment close to a transformer, and hospital operating rooms provided with an Isolated Power System (IPS) monitored by a LIM (Line Isolation Monitor). Saturation of such large transformers leads to various problems including tripping of circuit breakers.

Generally various methods for transformer saturation control have been employed. One of the conventional methods used for transformer saturation control uses Negative Temperature coefficient (NTC) in the transformer primary circuit. An NTC is a thermally-sensitive resistor with a resistance that changes significantly and predictably as a result of temperature changes. The resistance of an NTC decreases as its temperature increases. During high current flow, NTCs in the transformer primary circuit is used to switch off the power supply, to prevent damage caused by inrush current. However, such

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NTC devices become hot and need to be cooled before providing protection for the next inrush event.

Another conventional method for controlling transformer saturation is to anticipate the conditions for transformer saturation from power variations. Power variations are anticipated using Digital Signal Processing (DSP). When transformer saturation is anticipated, a resistance is placed in the power line to limit the inrush current. However, when the system is under load, the resistance causes a voltage drop that also in turn causes system failures if the power supply cannot carry over the voltage drop. When resistors with bypass are used, there is no protection to the transformer.

Further, there are transformers that are designed not to saturate under ½ cycle drop. However such transformers are large and often bulky. In systems where the transformer needs to have good regulation, but cannot use large and bulky transformers, other methods need to be used to protect CB tripping due to transformer saturation.

Hence, there is a need for transformer saturation...