POWER TRANSFORMER AUTOMATIC CONTROL FOR TAP CHANGERS BASIC AND TUTORIALS

POWER TRANSFORMER AUTOMATIC CONTROL FOR TAP CHANGERS BASIC INFORMATION
What Is Automatic Tap Changer Controls For Power Transformers?


Automatic Control for Tap Changers. It is usual practice to use some sort of voltage measuring device to control the operation of the motor which drives the tap changer.

Such devices may be mechanical, balancing the force of a solenoid actuated by the voltage against weights or springs, or they may be an electrical network, usually a bridge circuit which balances against the voltage of a Zener dioide.

With either type of device, a voltage higher than a desired upper limit will start the tapchanger driving motor to change to the next lower tap voltage; similarly, a voltage lower than the desired lower limit will cause a change to the next higher tap.

The circuit usually includes a time delay to prevent tap changes, which would occur unnecessarily during very short time variations in voltage. It also may include a line drop compensator to facilitate maintaining the voltage within a given band at a point (load center) some distance from the transformer.

The line-drop compensator introduces a signal into the voltage regulating relay circuitry. This represents the voltage drop due to line impedance between the transformer and the load center.

The voltage-regulating relay (or contact-making voltmeter) should be adjusted so that the voltage bandwidth, or spread between voltages at which the raising and lowering contacts close, will be not less than the percentage transformer tap plus an allowance for irregular voltage variations.

For example, a tap-changing transformer with 11/4% taps should have a minumum voltage bandwidth of approximately 11/4% 1/2% 13/4%.

In addition, the voltage-regulating relay may contain a component for use when load tap-changing transformers are operated in parallel. In this case, the tap changers must be controlled so that they are approximately on the same tap position.

The component, a paralleling reactor, is used with external circuitry to detect, and generate a signal to minimize, circulating current that results when the tap changers are not on like positions.