Abstract:
A means and method for measuring precise voltage phasors on medium-voltage alternating current (AC) distribution grids, using existing distribution transformers as voltage sensors. The errors introduced by the distribution transformers are minimized by taking into account the transformer&#39;s vector impedance, combined with measuring the transformer secondary current phasor. The invention includes a means and a method of measuring the distribution transformer&#39;s vector impedance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The invention disclosed herein was conceived and developed in part during work on Award Number DE-AR0000340, titled “Micro-Synchrophasors for Distribution Systems,” from the Advanced Research Projects Agency-Energy (ARPA-E) of the U.S. Department of Energy. 
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    The present invention is in the technical field of measurement of electric parameters. More particularly, the present invention is in the technical field of voltage phase angle measurements on an alternating current (a.c.) power grid. 
         [0005]    The voltage and current on an a.c. power grid have a fundamental frequency, often one of the following: 50 Hertz, 60 Hertz, or 400 Hertz. For many applications, it can be useful to measure the phase angle of the voltage fundamental frequency, or the phase angle of the current fundamental frequency, or both. Such a measurement can be made either relative to the phase angle at a different physical location, or relative to a fixed time base such as that provided by the Global Positioning System. The measured fundamental angle can be combined with the measured fundamental magnitude to form a phasor measurement. 
         [0006]    Phasor measurements can be equivalently expressed in polar coordinates, as an angle and a magnitude; or they can be expressed in Cartesian coordinates, typically for a.c. systems as a real and an imaginary component; or they can be expressed as a vector on a rotating Cartesian coordinate system that completes one rotation per nominal fundamental cycle; or they can be expressed in any other mathematically-equivalent way. 
         [0007]    One known phasor application for a.c. grids, well known to those familiar with the art, is the synchrophasor application, in which the voltage phasor, current phasor, or both are examined simultaneously at two or more separate physical locations on an a.c. grid that connects those two or more locations. In this known application, the difference between phasors at the two separate physical locations may, for example, provide useful information about the power flow between those two locations. 
         [0008]    In a.c. power grids, it is common to refer to voltages above 100,000 volts as high-voltage, and voltages between 1,000 volts and 100,000 volts as medium-voltage, and voltages less than 1,000 volts as low-voltage. High-voltage is generally used in an a.c. grid for transmitting bulk power over long distances; this application is often referred to as a transmission system. Medium-voltage is generally used in an a.c. grid for distributing power from a substation to a location that is closer to end-use; this application is often referred to as a distribution system. Low-voltage is generally used in an a.c. grid by end users or consumers, such as residences, factories, and commercial facilities. 
         [0009]    Typically, synchrophasor applications have been applied to high-voltage power transmission systems, even if the measurements themselves are made on local low-voltage locations. 
         [0010]    In those synchrophasor applications on transmission systems, the difference in phase angle between two separate physical locations can often be tens of degrees or more, and detecting interesting phenomena rarely requires a resolution better than about half a degree. Indeed, the IEEE Standard C37.118 (2011) for synchrophasor measurements only requires a Total Vector Error of 1% or better, which corresponds to approximately ±0.5°. 
         [0011]    In our Department of Energy ARPA-E Project DE-AR0000340, titled “Micro-Synchrophasors for Distribution Systems,” we have been investigating the application of synchrophasor measurements to medium-voltage distribution grids, as opposed to the traditional application to high-voltage transmission grids. Due to smaller inductances and shorter distances on distribution grids compared to transmission grids, the phase angle changes during interesting phenomena on distribution grids are much smaller. We have determined that, for distribution grid applications, a angular resolution for voltage phasors and current phasors of ±0.015° could be useful. 
         [0012]    Transmission grids generally operate at 100,000 volts or higher, and distribution grids generally operate at 1,000 volts to 100,000 volts. As is well known in the art, to measure a.c. voltage on these grids it is necessary to proportionally reduce the a.c. voltage to an acceptable level for electronic devices, which conventionally measure signals that are less than 1,000 volts. 
         [0013]    Typically, this proportional voltage reduction is done with transformers. One commonly-available type of transformer, which we will call a distribution transformer, is intended to supply a significant amount of power to a load, such as a group of residences or a factory, but can also be used for making phasor measurements. The medium-voltage primary winding of distribution transformers is connected to the distribution grid; the low-voltage secondary winding delivers power to consumers, and is at a level that can be measured by electronic devices. 
         [0014]    In general, we are interested in making phasor measurements that indicate the voltage magnitudes and angles on the distribution conductors, but as a practical matter we instead measure the voltage phasors on the secondary windings of a transformer. 
         [0015]    Consequently, any phase angle shifts that occur inside the transformer, between the primary winding and the secondary winding, will affect the accuracy and resolution of a voltage phasor measurement. 
         [0016]    Prior to the present invention, it was believed by those familiar with the art that high-precision medium-voltage phasor measurements, using generally available distribution transformers, would be impossible. It is well known to those familiar with the art that the voltage phasor on the secondary winding of distribution transformers, where the measurement would take place, is strongly affected by the uncontrolled loads that are supplied by the secondary winding of a distribution transformer. 
       SUMMARY OF THE INVENTION 
       [0017]    The present invention is a means and a method for making precise voltage phasor measurements on medium-voltage conductors of a distribution grid, using measurements that are made on the secondary of an existing distribution transformer that is supplying power to loads. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is an illustration of the environment of the present invention. 
           [0019]      FIG. 2  is a schematic view of the key elements of  FIG. 1 . 
           [0020]      FIG. 3  is a view of one implementation of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Turning our attention to  FIG. 1 , we see distribution grid conductors  1  mechanically supported by a power pole  2 . Mounted on the power pole  2  is a transformer  3 . The transformer  3  could, for example, be a single-phase transformer having a ratio of 100:1 for converting 24 kilovolts on its primary winding to 240 volts on its secondary winding. The medium-voltage primary winding of the transformer  3  is connected through primary conductors  9  and a fuse  10  to the medium-voltage distribution grid conductors  1 . The low-voltage secondary winding of the transformer  3  is connected through secondary conductors  4  to conductors  5  on a low-voltage distribution grid. In  FIG. 1 , we see an enclosure  6  for the present invention. The present invention, inside its enclosure  6 , makes use of current sensors  7  on the secondary conductors  4 , and also makes use of voltage sensing conductors  8 , to make measurements of voltage phasors and current phasors on the low voltage secondary of transformer  3 . These measured secondary voltage phasors and current phasors are used in the present invention to precisely determine the voltage phasors on the medium-voltage distribution grid conductors  1 , as further described below. 
         [0022]    Turning our attention now to  FIG. 2 , we see, in schematic form, many of the same elements that we saw in  FIG. 1 : distribution grid conductors  1 , a transformer  3 , primary conductors  9 , a fuse  10 , secondary conductors  4 , conductors  5  on a low-voltage distribution grid, an enclosure  6  for the present invention, current sensors  7  on the secondary conductors  4 , and voltage sensing conductors  8 . We also see a Micro Synchrophasor Instrument  31 , developed under the Department of Energy ARPA-E Award Number DE-AR0000340, which implements one possible embodiment of the present invention. The Micro Synchrophasor Instrument  31  has voltage inputs  35  with appropriate ratings for direct connection to the low-voltage secondary winding of the transformer  3 , and has current inputs  34  with appropriate ratings for using current sensors  7  to measure the current flowing on the low-voltage secondary conductors  4 . 
         [0023]    Turning our attention now to  FIG. 3 , we see an illustration of a Micro Synchrophasor Instrument  31  which implements one possible embodiment of the present invention. (The hand  37  in the illustration is shown to visually indicate approximate scale, and does not play any part in the present invention.) The Micro Synchrophasor Instrument  31  incorporates a display  33  and communications means  36 , both of which are useful but not critical to the present invention. The Micro Synchrophasor Instrument  31  also incorporates voltage inputs  35  for measuring the low-voltage phasors on the secondary winding of the transformer  3 , current inputs  34  for measuring the current flowing on the secondary conductors through current sensors, and computing means  32  for implementing the algorithm of the present invention, which is further described below. 
         [0024]    In the present invention, we use measurements on the secondary, low-voltage conductors of a distribution transformer to precisely determine the voltage phasors on the primary, medium-voltage distribution grid conductors, which is also the voltage on the primary of the transformer, using the method explained further below. 
         [0025]    We begin by making voltage phasor measurements and current phasor measurements on the secondary, low-voltage conductors, using any precise method known in the art. Combining these secondary voltage phasor measurements and secondary current phasor measurements with the effective ratio of the transformer primary winding to the transformer secondary winding, which can be found on the transformer nameplate, we implement the following equation to determine the parameter we want to measure: the phasor vector of the fundamental voltage on the primary winding of the transformer. 
         [0000]        {right arrow over (V)}   primary =α transformer ·( {right arrow over (V)}   secondary +( {right arrow over (I)}   secondary   ·{right arrow over (Z)}   transformer ))
 
         [0026]    V primary  is the phasor vector of the fundamental voltage on the primary winding of the transformer, which is the parameter of interest; 
         [0027]    α transformer  is the effective ratio of the transformer primary winding to the transformer secondary winding; 
         [0028]    V secondary  is the measured phasor vector of the fundamental voltage on the secondary side of the transformer; 
         [0029]    I secondary  is the measured phasor vector of the fundamental current on the secondary side of the transformer; and 
         [0030]    Z transformer  is the fundamental vector impedance of the transformer, as further described below. 
         [0031]    Note that this equation requires that we know the fundamental vector impedance of the transformer. 
         [0032]    In our invention, we measure the fundamental vector impedance of the transformer by observing the changes in our secondary voltage phasor measurements that occur simultaneously with changes in our secondary current phasor measurements. We approximate the relationship between those two measurements and the fundamental vector impedance of the transformer as follows: 
         [0000]    
       
         
           
             
               
                 Z 
                 → 
               
               transformer 
             
             ≅ 
             
               
                 Δ 
                  
                 
                     
                 
                  
                 
                   
                     V 
                     → 
                   
                   secondary 
                 
               
               
                 Δ 
                  
                 
                     
                 
                  
                 
                   
                     I 
                     → 
                   
                   secondary 
                 
               
             
           
         
       
     
         [0033]    ΔV secondary  is the measured phasor vector of a change in fundamental voltage on the secondary side of the transformer, and 
         [0034]    ΔI secondary  is the measured phasor vector of a change in fundamental current on the secondary side of the transformer. 
         [0035]    As shown in this equation, the fundamental vector impedance Z transformer  can be approximated by analyzing the measured vector change in secondary voltage that occurs approximately simultaneously with a detected vector change in the measurement of secondary current. It is an approximation of the fundamental vector impedance Z transformer  for two reasons. First, the measured fundamental vector impedance is, in fact, the vector impedance of the transformer summed with the vector impedance of the grid that is upstream of the transformer primary; however, we have determined by experiment that the transformer vector impedance is almost always at least an order of magnitude larger than the upstream grid&#39;s vector impedance. Second, there can be changes in measured phasor vector of the fundamental voltage on the secondary side of the transformer that are caused by external factors other than changes in the phasor vector in the fundamental current on the secondary side of the transformer, such external factors including voltage sags on the primary, transformer tap changes, and other well-known events that affect transformer secondary voltage. 
         [0036]    To minimize the effect of these kinds of external factors, in our invention the approximation of the fundamental vector impedance Z transformer  may be calculated directly as described above, or it may be further refined using one or more of the following three methods:
       Threshold: For the purposes of calculating Z transformer , changes in ΔV secondary  are ignored unless they occur simultaneously with a change in ΔI secondary  that exceeds some threshold in vector magnitude, or exceeds some threshold in some parameter associated with the current phasor such as its real component or its imaginary component. This threshold may be fixed, or it may be determined by an algorithm that adapts this threshold to a history of measurements.   Statistical correlation: For the purposes of calculating Z transformer , which is in the present invention is approximated by a ratio, a statistical correlation may be used to calculate the most likely ratio between a large number of measured changes in ΔI secondary  and a change in ΔV secondary . For example, the slope of a linear least-squares fit could be used; or a statistical process that gives more weight to points that have a large change in current phasor could be used; or any other statistical method known to those familiar with the art could be used to extract an optimal ratio from a collection of ΔI secondary  and ΔV secondary  pairs.   Intentional change in secondary current vector: the present invention relies on changes in ΔI secondary , which almost always naturally occur in distribution transformer loads. In one implementation of the present invention, a measurement of Z transformer  can be made by intentionally adding and removing load from the secondary of the distribution transformer. In another embodiment, the adding and removing are done in a timed pattern that permits extraction of relatively small signals from background noise using methods well-known in the art, such as fixed-frequency adding and removing combined with Fourier analysis; or random adding and removing combined with auto-correlation.       
 
         [0040]    It will be apparent to one of ordinary skill that the above description, which assumes a single-phase system, can be readily extended to three-phase systems. 
         [0041]    While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention.