Elimination of potential transformer in ANSI Type A voltage regulator

An ANSI Type A voltage regulator that eliminates the need for a potential transformer is disclosed. A control unit finds an output voltage by constantly monitoring the input voltage across the utility windings and the stored tap position. The value of the output voltage is further fine tuned by taking into account the effect of the impedance of the voltage regulator itself on the output voltage. The impedance is calculated using the instantaneous current through the regulator, the maximum rated current of the voltage regulator, the instantaneous voltage through the voltage regulator, the instantaneous Power Factor, and the tap position of the voltage regulator.

BACKGROUND OF THE INVENTION

The present invention relates to voltage regulators and, more particularly, to the use of the utility winding and a control unit in ANSI Type “A” Voltage Regulators to calculate the load voltage without the need of an embedded potential transformer.

A voltage regulator can be thought of as an autotransformer that regulates a secondary voltage. If there is a primary voltage that has a tendency to fluctuate, a voltage regulator will produce a constant secondary voltage. For instance, if a primary, or input, voltage fluctuates between 110 volts and 130 volts, the voltage regulator will maintain the secondary, or output, voltage at a constant 120 volts. Usually, a voltage regulator can increase or decrease its output voltage by up to 10% of its input voltage in ⅝% steps. The voltage regulator is equipped with a control unit which monitors the input and output voltages of the voltage regulator and moves the tap changer by the ⅝% steps to maintain a specified output voltage.

Typically, an ANSI load-side series winding, or Type “A,” voltage regulator uses a separate potential transformer to sense the load voltage and feeds that voltage to the control unit so that the control unit can change the tap position as needed.FIG. 1illustrates the typical physical connection of a voltage regulator100with an embedded potential transformer60. The potential transformer60is connected between the “L” and “SL” bushings. For example, the source voltage across the S and SL bushings may fluctuate between about 6900 volts and about 8300 volts. The load voltage is then stepped down by the potential transformer60to approximately 120 volts (or roughly between about 110 volts to about 130 volts). The control unit (not shown) then changes the tap position in response to the stepped down source voltage which results in the output voltage across the L and SL bushings of a constant 7620 volts.

FIG. 2illustrates a block diagram of the flow of information to the control unit in a typical embodiment of a voltage regulator that contains an embedded potential transformer. In block130, the voltage regulator feeds the input voltage to the control panel. In addition, in step140, the output voltage from the embedded potential transformer supplies the output voltage to the control panel. The control panel, in step150, in turn monitors the input and output voltages and adjusts position of the tap in order to adjust the output voltage as needed.

However, a need exists to simplify a voltage regulator by eliminating some of its components. By eliminating components of the voltage regulator, the material and manufacturing costs are reduced. In addition, the reliability of ANSI Type A voltage regulator increases with the reduction of components.

BRIEF SUMMARY OF THE INVENTION

According to the present invention, the utility windings and a control unit already present in voltage regulators will be used to sense the source voltage and calculate the load voltage in the voltage regulator without the need of a potential transformer. The utility windings provide the source, or input, voltage for the control unit. The control unit constantly monitors all tap changes as well as continuously stores the tap position electronically. The output voltage is calculated by the control unit by using the input voltage across the utility windings and the tap position in memory. To calculate a more accurate output voltage, the inherent impendence of the voltage regulator itself is considered in the calculation. The impedance of the voltage regulator is calculated using the instantaneous current through the regulator, the maximum rated current of the voltage regulator, the instantaneous voltage through the voltage regulator, the instantaneous power factor, and the tap position of the voltage regulator. The control unit, then in turn, may change the position of the tap in response to the load voltage.

In accordance with one embodiment of the present invention, the control unit software will be adjusted and reprogrammed for different modes of applications.

Accordingly, it is an object of the present invention to reduce the cost of material needed as well as the cost of manufacturing for the ANSI Type “A” voltage regulators by eliminating the need for the potential transformer component. By eliminating the potential transformer, reliability of the voltage regulator will increase due to the reduction of one active component in its assembly.

Other objects of the present invention will be apparent in light of the description of the invention embodied herein.

DETAILED DESCRIPTION

Referring toFIG. 3, is a schematic illustration of the physical layout of an ANSI Type A voltage regulator without an potential transformer according to one embodiment of the present invention. The input, or source, voltage is measured between the S and SL bushings, or across the utility windings310. The output, or load, voltage is calculated between the L and SL bushings. The windings and other internal components are mounted in an oil filled tank. The tap position changing mechanism is commonly sealed in the tank. The tap position changing mechanism is controlled by a control unit. In addition, the control unit keeps constant and accurate track of the current tap position.

Referring toFIG. 4, a block diagram illustrates the flow of information to and from a control unit in a voltage regulator without an embedded potential transformer according to one embodiment of the present invention. The control unit monitors the input voltage provided by the voltage regulator across the S and SL bushings, the tap position at all times, and the output voltage. The output voltage240is calculated from the output voltage algorithm230that uses the tap position supplied from the control unit220, the input voltage across the voltage regulator utility windings210, and from the calculated impedance of the voltage regulator itself250. The output voltage algorithm may be stored on any computer-readable medium accessible to the control unit. The control unit will notify the tap position changing mechanism to change the tap position in response to the calculated output voltage in order to maintain a consistent output voltage across the L and SL bushings. The control unit considers each step, or each tap position, as a ⅝% difference in output.

The control unit calculates an output voltage of the voltage regulator using a two step process. First, the control unit continuously monitors the tap changes as well as constantly stores the tap position electronically. Second, the output voltage is approximated by the control unit by using the input voltage across the utility windings as well as the stored position of the tap. The output voltage value is calculated by taking the instantaneous input voltage from across the utility windings and multiplying it by one plus the physical tap position that has been multiplied by the voltage difference of one tap position (1).
Vout=Vin*(1+(tap_pos*Vdiff. 1 tap pos.))  (1)

However, since the voltage regulator is an electrical device, it also consumes power and places load on the electrical system. Therefore, the impedance of the voltage regulator must also be considered in the calculation of the output voltage by the control unit to ensure a more accurate output voltage value. The impedance of the voltage regulator is found from using the instantaneous current through the regulator, the maximum rated current of the voltage regulator, the instantaneous voltage through the voltage regulator, the instantaneous power factor, and the tap position of the voltage regulator.

The calculated output voltage value can be summarized as equaling the output voltage value plus the voltage drop (2) due to the impedance of the voltage regulator. The voltage drop equals the instantaneous current multiplied by the impedance of the voltage regulator (3). Both the instantaneous current and the impedance are complex numbers.
Vcal. out=Vout+Vdrop(2)
Vdrop=I*Z(3)

The resistive component of the instantaneous current value equals the instantaneous current value multiplied by the absolute value of the instantaneous power factor (4). The instantaneous power factor is derived from fundamental voltage and current frequencies and is represented by the ratio of real power to apparent power. If the instantaneous power factor is less that zero, then the power factor is leading and reactive component of the instantaneous current equals the instantaneous current multiplied by the square root of one minus the square of the power factor (5). On the other hand, if the instantaneous power factor is greater than zero, the instantaneous power factor is lagging and the reactive component of the current equals the negative of the instantaneous current multiplied by the square root of one minus the square of the power factor (6).
Ires=I*|PF|(4)
Ireact=I*sqrt(1.0−PF2)  (5)
Ireact=−I*sqrt(1.0−PF2)  (6)

Assuming that the impedance percentage is known at a particular tap position, for example 0.6% at tap position16, the impedance is then calculated to be 0.6% multiple by the square of the input voltage divided by the KVA rating of the voltage regulator (7). The KVA rating on voltage regulators defines the load carrying or power capability and stands for kilovolt-amperes. Since the KVA rating equals the input voltage multiplied by the maximum rated current (8), the impedance equation reduces to 0.6% times the input voltage divided by the maximum rated current (9) or 0.6% of the input voltage across that utility windings divided by maximum rated current (10). Therefore, to find the impedance at any tap position, the impedance becomes 0.6% multiplied by the instantaneous input voltage across the utility windings divided by the maximum rated current multiplied by the tap position squared divided by sixteen squared (11).
Z=(0.006*V2)/KVA(7)
KVA=V*Imax(8)
Z=(0.006*V)/Imax(9)
Z=(0.006*Vin)/Imax(10)
Z=(((0.006*Vin)/Imax)*tap_pos2)/162(11)

Since the impedance is complex and mostly reactive, the resistive component of the impedance can be considered to equal one quarter the reactive impedance. Therefore, the reactive component of the impedance equals the calculated impedance or four times the resistive component of the impedance (12). Finally, the voltage drop is calculated to equal the resistive component of the impedance multiplied by the resistive component of the current minus the reactive component of the impedance multiplied by the reactive component of the current (13). The control unit can then use this value to determine accurately the output voltage in equation (2) and to notify the tap position changing mechanism when it is appropriate to change the position of the tap.
Zreact=4*Zres(12)
Vdrop=(Zres*Ires)−(Zreact*Ireact)  (13)