Arrangement and Method for Accurately Sensing Voltage of a High-Impedance Source and Supplying Power to a Variable Burden

1. Field of the Invention

The present invention relates generally to voltage sensors and battery chargers and more particularly to an arrangement for charging a battery or supplying other variable burdens from a high-impedance source while accurately sensing the source voltage of the high-impedance source.

2. Description of the Related Art

In electrical power distribution and transmission systems, it is necessary to obtain an accurate representation of the voltage of the alternating-current source at a particular location for a purpose such as the remote supervisory control of automated distribution switching installations. It is extremely desirable for such automated distribution switching installations to be self-contained and self-powered. Thus, it is desirable to utilize the alternating-current source to power the installation. However, since the voltage of the alternating-current source is in the range of 15 to 34.5 kv or higher, a potential transformer or voltage sensing device is utilized to obtain low voltage representations of the high voltage on a conductor and to provide power for operation of the switching installation. For example, one type of voltage sensor is described in U.S. Pat. No. 4,002,976. A rechargeable power source such as a battery is provided at the switching installation to store operating power obtained from the transformer or voltage sensor and to also provide the desirable feature of emergency operation in the absence of system voltage. It is also extremely advantageous to utilize a single voltage sensor to monitor the system voltage as well as to charge the battery. However, since the battery and the supplied installation load represent a highly variable burden, there is an inherent conflict in obtaining an accurate sensed voltage signal from the voltage sensor while the voltage sensor is supplying a variable burden. That is, the difference in burden between a fully charged battery and a fully discharged battery is so great that this varying burden causes inaccuracies in the sensed voltage. This is especially a problem when the voltage sensor is a high-impedance source since this type of supply has very poor voltage regulation; i.e., the output voltage that represents the sensed voltage varies rather greatly for small changes in the burden to the extent that the sensed voltage signal does not accurately represent the input voltages.

Accordingly, various approaches have been utilized to account for these adverse characteristics. For example, U.S. Pat. No. 3,771,043 supplies the variable burden with a constant burden regulator utilizing a constant current source and a shunt regulator. The high-impedance source is connected in parallel with the fixed load and the constant burden regulator is also connected in parallel to the system. Thus, the voltage to the fixed load is accurate and essentially unaffected by the variable burden. That approach attempts to maintain an accurate voltage measurement as the sensed parameter via the maintenance of a constant overall burden. That is, the additional constant current source and the shunt regulator present a relatively fixed burden, with the shunt regulator conducting a varying current dependent on the load presented by the variable burden load. This accomplishes the maintenance of an essentially constant voltage ratio at the output of the high-impedance source to obtain an accurate representation of the voltage of the source. While this arrangement is generally suitable for particular applications, it should be noted that the constant current source dissipates power to maintain a constant current supply and that any slight variation in the maintained constant burden has a direct effect on the voltage source due to its inherent nature as a high-impedance source.

Concerning other power supply arrangements where sensing is also accomplished, U.S. Pat. Nos. 4,567,540 and 4,571,658 are directed to a trip signal generator for a circuit interrupter wherein a representation of the current in a conductor is obtained from the voltage across a resistor in series with a power supply circuit. The power supply circuit utilizes a switching shunt regulator to maintain the voltage to the power supply regulator circuit within a predetermined range or ranges according to the particular sensed conditions. While that arrangement is useful for current sources, it is not suited to supply a variable burden while accurately measuring the voltage of a high-impedance source.

Accordingly, it is a principal object of the present invention to provide an arrangement to accurately sense the source voltage of a high-impedance source while supplying a variable burden load.

This and other objects of the present invention are efficiently achieved by the provision of a current-sensing element connected in series with a load circuit across a high-impedance source. The load circuit includes a variable burden load. A shunt regulator is connected with the variable load and is arranged to operate so that the combination of the shunt regulator and the variable burden present a controlled, nearly constant burden to the high-impedance source. The source voltage of the high-impedance source is sensed via the voltage across the series current-sensing element. While the output voltage is directly proportional to the burden, the output current is essentially unaffected by small variations in the burden. Thus, the source current through the series current-sensing element is utilized to accurately sense the voltage of the high-impedance alternating-current source while also supplying the variable burden load such as a battery, electrical circuitry, and a switch operator.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in conjunction with the accompanying drawing in which:

FIG. 1 is an electrical block diagram and schematic drawing of the present invention in an illustrative application;

FIG. 2 is an electrical schematic drawing similar to FIG. 1 and illustrating further details that can be employed to practice the present invention; and

FIG. 3 is an illustrative example of a specific current-sensing element for use with the arrangements of FIGS. 1 and 2.

Referring now to FIG. 1, the arrangement of the present invention is connected at circuit points 20,22 across a high-impedance source referred to generally at 24. The high-impedance source is represented by an ideal voltage source 16, i.e., an alternating-current source having no internal impedance, operating through an impedance 18 of high magnitude. A load circuit referred to generally at 26 and a current-sensing element 28 are connected in series circuit relationship across the circuit points 20,22. In an illustrative embodiment, a transformer, rectifier, and filter power supply stage 30 is connected in series with the current-sensing element 28 across the circuit points 20,22 of the source 24. The stage 30 is arranged to supply operating power across outputs 32,34 to a variable load 36 which, in a specific situation, includes a battery and additional circuit loads; e.g., a remote terminal unit (RTU), a radio transceiver, and an electrically powered switch operator for an automated distribution switch.

In accordance with important aspects of the present invention, a shunt regulator 38 is connected across the variable load 36 at the outputs 32,34 and the voltage of the source 24 is sensed via the voltage across the impedance element 28. The shunt regulator 38 is arranged to conduct so that the total combination of the shunt regulator 38 and the variable load 36 present a controlled, nearly constant burden to the outputs 32,34 of the stage 30. In turn, the stage 30 in combination with the current-sensing element 28 presents a constant burden to the source circuit at the terminals 20,22. In operation, the shunt regulator 38 senses the voltage at 32,34 and conducts to maintain a constant voltage thereacross as the burden or current drain of the variable load 36 varies. Accordingly, the burden to the source 24 is maintained very consistently such that the current through the fixed current-sensing element 28 accurately represents the voltage of the ideal voltage source 16. Thus, the current through the current-sensing element 28 varies only with the voltage of the ideal voltage source 16.

Due to the presence of harmonics, filtering of the sensed signal across the current-sensing element 28 is necessary where a sinusoidal signal is desired or where the phase angle is to be monitored. For example, a filter 29 is utilized to pass the components of the sensed signal at the frequency of the source 16 while attenuating other frequencies. Considering illustrative examples of the current-sensing element 28 and with additional reference to FIGS. 2 and 3, the current-sensing element 28 in one specific arrangement is a resistor 31 as shown in FIG. 2. In another specific arrangement and as shown in FIG. 3, the current-sensing element 28 is a current transformer 33.

It should be noted that in the arrangement of FIG. 1, the overall load circuit 26 can be considered a load supply circuit or a regulator circuit connected to supply a variable load. Additionally, the circuit can also be characterized as the portion 26 being a variable load with a shunt regulator being connected therewith. In the alternative, the circuit can also be characterized as the overall load circuit 26 including a variable load portion with the shunt regulator 38 being connected with the variable load portion.

Considering one specific application of the present invention in more detail and as illustrated in FIG. 2, the high-impedance source 24 is a voltage sensor 50 including a capacitor 52 connected in series with a primary winding 54 of a step-down transformer 56. A secondary winding 58 of the step-down transformer 56 is connected across the circuit points 20,22. When the voltage sensor 50 via a terminal 55 is connected to a conductor energized at a high ac voltage such as 25 kv, current flows through the capacitor 52 and the primary winding 54 of the transformer 56. The relatively high impedance of the elements 52 and 54 limits the current flow. In accordance with the step-down ratio of the transformer 56, e.g., 20 to 1, the voltage induced at the circuit points 20,22 is significantly stepped down and correspondingly the current is higher. The net result is a voltage at 20,22 that is representative of the voltage on the high-voltage conductor across 55,57 when the secondary winding 58 is connected to a constant burden. However, if the burden at 20,22 is varied, the voltage at 20,22 no longer accurately represents the voltage at 55,57 due to the voltage sensor being a high-impedance source. Accordingly, if a sensed voltage is desired that is accurate to within a few percent, it would be apparent that the voltage must be held constant at 20,22 via the control of the burden. However, the present invention illustrates that the current may also be used as an accurate representation of the voltage at 55,57 to the voltage sensor 50. Additionally, if the burden is controlled and held nearly constant, the accuracy of the sensed current signal as a representation of the source voltage is also increased.

While there have been illustrated and described various embodiments of the present invention, it will be apparent that various changes and modifications will occur to those skilled in the art. For example, it should be understood that the present invention is applicable to both direct current and alternating-current sources. For use with a direct-current source, the transformer, rectifier, and filter power supply stage 30 is deleted, such that the parallel combination of the shunt regulator 28 and the variable load 36 are connected in series with the current-sensing element 28 and the source 24. Accordingly, it is intended in the appended claims to cover all such changes and modifications that fall within the true spirit and scope of the present invention.