Abstract:
A system and method for dynamically adjusting capacitance added in parallel to an electrical line input for improving efficiency of an electrical system. A microprocessor monitors in real time the current and voltage wave forms of a system and selects the optimum amount of capacitance from a bank of capacitors of different values. The system is implemented at the utility transformer to encourage adoption of the device by utility companies and customers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of co-pending U.S. patent application No. 12/455,335, “System and Method for Power Factor Correction”, filed on Jun. 1, 2009, which claims priority to U.S. Provisional Patent Application No. 61/057,571, “Active PFC”, filed on May 30, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     Electrical loads in residential and industrial establishments tend to be reactive and not passive. The power factor of an alternating current (AC) electric power system is defined as being equal to the ratio of the real power to the apparent power of the load. The power factor is a number between zero and one and represents the efficiency of the load. In an electrical power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. Thus, it is desirable to have a load with a power factor as close as possible to unity. Due to the inductive nature of the majority of household appliances and machinery used in industrial applications, the power factor for these inductive loads is low. It is known in the art to add capacitance in parallel with the line input to raise the power factor to unity. As a result, savings in power consumption can be realized. 
     Currently in the market, power factor controllers are available which monitor the lead and lag in the power and automatically adjust the power factor by applying more or less capacitance as needed. However, power factor correction devices currently known in the art are designed to be installed at the electrical panel or alternatively they may be installed directly to each of the individual loads that require power factor correction. For a residential application, the factor power correction controller is usually connected to the fuse panel where the electricity is distributed to different locations within the house. As such, in order for the electrician to install the power factor correction controller, the electrician must have access to the residence and approval from the homeowner. 
     Utility companies are motivated to increase the power factor at each residence that they service because a power factor below a value of one requires the utility to generate more than the minimum volt-amperes necessary to supply the real power (watts). A low power factor therefore increases generation and transmission costs for the utility company. However, the utility companies are deterred from installing residential power factor controllers because they must schedule the installation with the customer and they are concerned with possibility liability issues. Additionally, customers are reluctant to have a power factor controller installed due to the inconvenience associated with the installation inside their home. 
     Therefore, there exists a need in the art for a power factor controller that can be installed to correct the power factor at an individual residence which does not require that the installer have access to the fuse panel inside the residence. Additionally, there exists a need in the art for a power factor controller that can be installed to correct the power factor of a plurality of residences in combination. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention is provided a power factor correction system which includes a power factor correction controller electrically coupled to the secondary windings of a three-phase utility transformer. 
     In a particular embodiment, the power factor correction controller of the system includes a capacitor bank, comprising a plurality of capacitors and a power factor correction circuit coupled to the capacitor bank for determining an existing power factor of a load and for selectively coupling one or more of the plurality of capacitors in parallel with the load to adjust the power factor towards unity. 
     The load may be an individual residence or a plurality of residences depending upon the electrical distribution layout. 
     In a specific embodiment, the power factor correction circuit further comprises a signal conditioning circuit coupled across a first and second alternating current incoming power lines, the signal monitoring circuit to sense a voltage signal supplied to a load by the first and second alternating current power lines and to sense a load current signal drawn by the load. The signal conditioning circuit also includes circuitry for filtering a noise signal from the sensed load current signal. An analog to digital conversion circuit is coupled to the signal monitoring circuit, the analog to digital conversion circuit to receive the sensed voltage signal and the sensed current signal and to generate a digital pulse representative of the zero- crossing of the voltage signal and the current signal. A digital signal processing and communications circuit is coupled to the analog to digital conversion circuit, the digital signal processing and communications circuit to receive the digital pulse representative of the zero-crossing of the voltage signal and the current signal, to determine a phase angle representative of an existing power factor of the load and to compare the existing power factor of the load to a preset threshold phase angle to determine if the existing power factor is leading or lagging. A capacitor switching circuit is coupled to the digital signal processing and communications circuit and to the capacitor bank, the capacitor switching circuit to selectively couple one or more of the plurality of capacitors in the capacitor bank to be in parallel with the load to adjust the power factor of the load towards unity. 
     The power factor correction circuit determines a time delay from the digital pulses provided by the analog to digital conversion circuit and converts the time delay to the phase angle representative of the existing power factor of the load. 
     The system includes a direct current power supply coupled to the incoming electrical lines to provide an operating voltage to the signal conditional circuit, the analog to digital conversion circuit, the digital signal processing and communications circuit and the capacitor switching circuit. 
     In a specific embodiment, the capacitor bank includes a plurality of capacitors of varying sizes to allow for course and fine adjustment of the power factor of the load. Additionally, at least one capacitor from the bank is always coupled in parallel with the load to protect the load from voltage spikes on the incoming power lines. 
     The power factor correction circuit may also selectively decouple one or more of the plurality of capacitors in parallel with the load to adjust the power factor of the load towards unity as determined by the existing power factor of the load. The capacitors that are decoupled from the circuit are de-energized to prevent a current or voltage surge when the system reselects the capacitor. 
     To make an accurate determination of the existing power factor, the power factor correction circuit of the present invention further comprises circuitry to cause the power factor correction circuit to determine the existing power factor a plurality of times within a predetermined amount of time and to average the results of the plurality of determinations to determine the existing power factor. 
     Additionally, the power factor correction circuit further comprises circuitry to cause the power factor correction circuit to determine the existing power factor following each selective coupling of one or more of the plurality of capacitors. As such, the existing power factor is recalculated each time a capacitor from the capacitor bank is coupled or decoupled from the circuit. 
     Various utility transformers are known in the art. In a particular embodiment, the utility transformer is a delta-wye, three-phase, step-down utility transformer. However, this is not meant to be limiting and other transformers are within the scope of the present invention. 
     In accordance with the present invention, a method for correcting a power factor of a load circuit is provided which includes positioning a power factor correction controller between the secondary windings of a utility transformer and a load. After the power factor correction controller is positioned in parallel with the secondary windings of the utility transformer, the method of the present invention continues by sensing the power supply voltage signal at the transformer, sensing the load current signal, determining the existing power factor of the load from the sensed power supply voltage signal and the sensed load current signal and selectively coupling or decoupling capacitors in parallel with the load circuit to correct the power factor of the load circuit to approximately unity. 
     The existing power factor of the load is determined by identifying the zero-crossing of the sensed voltage signal to generate a digital pulse representative of the zero- crossing of the voltage signal, identifying the zero-crossing of the sensed load current signal to generate a digital pulse representative of the zero-crossing of the load current signal, determining a time delay between the voltage signal and the current signal by comparing the digital pulse representative of the zero-crossing of the voltage signal to the digital pulse representative of the zero-crossing of the load current signal, converting the time delay to a phase angle representative of the existing power factor of the load and comparing the phase angle to a preset threshold phase angle to determine if the existing power factor is leading or lagging. 
     To make an accurate determination of the existing power factor, the power factor of the load is determined a plurality of times, using the previously described method, within a predetermined amount of time and the results are averaged to determine the existing power factor of the load. Additionally, the existing power factor of the load is determined each time a capacitor is selectively coupled or decoupled from the load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view of the power factor correction system in accordance with the present invention. 
         FIG. 2  is a block diagram of an embodiment of the power factor correction controller in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In accordance with the present invention, a system and method are provided for correcting a power factor by measuring the power factor of the load and then electronically adding capacitance in parallel to the load to optimize the power factor toward a value of unity. The invention has the advantage that as appliances and loads are added and or subtracted from the system the value of the capacitance added to correct for the inductive component is adjusted in real time. The invention maximizes efficiency of the overall system and so minimizes the energy cost to the consumer. Additionally, with the present invention, the power factor correction system is electrically connected to the load at the utility transformer. As such, with the present invention, it is not necessary for the installer of the system to have access to the interior of the building. 
     In the electric utility industry, three-phase transformers are commonly employed to step-down electric power generated by the utility company for consumption at a residential or commercial building. These transformers are typically mounted on a utility pole at a location close to the residence. The transformer may service multiple residences. In a residential setting, the step-down transformer transforms the 7,200 volts into 240 volts, which is then sent into the residence over three wires. The three wires go through the electric meter at the residence, which measures the amount of electricity used. One of the three wires is the ground wire and the other two wires carry 120 volts each, 180° out of phase with each other. Many three-transformer configurations are known in the art. The delta-wye, three-phase, step- down utility transformer is commonly used in the distribution of residential electricity. 
     With reference to  FIG. 1 , the power factor correction system  15  for use with a three-phase utility transformer  20  is illustrated. The three-phase utility transformer includes three primary windings and three secondary windings. Power from the utility company is supplied to the three primary windings of the transformer. In accordance with the present invention, the power factor correction controller  15  includes three terminal wires  80 ,  85 ,  90 . Each one of the three terminal wires of the power factor correction controller is electrically coupled to one of each of the three secondary windings  95 ,  100 ,  105  of the three-phase utility transformer  20 . 
     Once the power factor correction controller  15  is coupled to the secondary windings of the transformer, the power factor correction controller  15  serves to correct the power factor for varying reactive loads. 
     With reference to  FIG. 2 , a block diagram of the power factor correction controller  15  is provided to illustrate the main components of the power factor correction controller  15  and their association with the incoming electrical lines and the load  30 . As previously mentioned, the power factor of the load  30  varies as the reactive elements present in the load  30  vary. 
     As shown with reference to  FIG. 2 , the varying reactive load  30  is coupled across the terminal wires  80 ,  85 ,  90  of the power factor correction controller  15 . The power factor correction controller  15  is coupled across the secondary windings  95 ,  100 ,  105  of the transformer  20  and is positioned between the secondary windings and the load  30 , such that the power factor correction controller  15  is in parallel with the load  30 . The system may also include a neutral ground wire  110  coupled to the secondary windings. 
     The power factor correction controller  15  includes a capacitor bank  45  coupled across the terminal wires  80 ,  85 ,  90 . The capacitor bank  45  is comprised of an array of capacitors of varying sizes to allow for course and fine adjustment of the power factor of the load  30 . Power factor correction is accomplished by automatically switching capacitors from the capacitor bank  45  in and out of the load circuit to achieve a power factor approximating unity. 
     Prior to correcting the power factor, it is necessary to first determine the existing power factor of the load. The existing power factor is determined by measuring the phase relationship between the incoming voltage and the load current. It is known that the power factor is the cosine of the phase angle between the load current and the incoming voltage sinusoidal waveforms. The power factor is equal to unity when the voltage and the current are in phase, and is equal to zero when the current leads or lags the voltage by 90°. 
     In a particular embodiment of the present invention, to determine the existing power factor of the load, the system senses the incoming voltage signal and the load current signal and a signal conditioning circuit  50  filters the load current signal to remove any noise in the signal. These signals are then fed to an analog to digital conversion circuit  55  which comprises a zero-crossing circuit for the sensed voltage signal and a zero-crossing circuit for the sensed current signal. The analog to digital conversion circuit  55  generates digital pulses representative of the zero- crossing of the voltage signal and the current signal which are then fed to the digital signal processing and communications circuit  60 . The digital signal processing and communications circuit  60  then determines a time delay from the digital pulses and converts the time delay to a phase angle representative of the existing power factor of the load. The voltage and current signals are measured and the phase angle is determined at several hundred cycles a second and the results are averaged to determine the existing power factor. The digital signal processing and communications circuit  60  is then used to compare the phase angle representative of the existing power factor against a preset threshold phase angle to determine if the existing power factor is leading or lagging. When the load is inductive, the load current lags the incoming voltage, and the power factor is said to be a lagging power factor. When the load is capacitive, the load current leads the incoming voltage and the power factor is said to be a leading power factor. The results of this comparison are communicated to the capacitor switching circuit  65  which then increments or decrements the capacitor bank  45 . The capacitor bank  45  is positioned in parallel with the load such that the incrementing and/or decrementing of the capacitors in the capacitor bank  45  results in the adjustment of the power factor to approximately unity. In this embodiment, capacitance is added in equal increments and the process of determining the existing power factor is repeated after each increment. The circuit elements of the power factor correction controller  15  are powered through a direct current power supply  70 . 
     The load on the circuit varies as different appliances and machinery are switched on and off. In accordance with the present invention, the power factor is measured and capacitance is electronically added to the system to optimize the power factor towards a value of one. This has the advantage that as appliances and loads are added and/or subtracted from the system, the value of the capacitance added to correct for the inductive component is adjusted in real time, therefore maximizing the efficiency of the overall system and so minimizing the energy cost to the consumer. 
     Additionally, because the power factor correction controller  15  is coupled to the output of the transformer  20 , the load may include multiple residences. As the load various at each of the individual residences, the power factor correction controller adds or subtracts capacitance in real time to maximize the efficiency of the residences in combination. 
     In the present invention, the measurement of the current waveform monitors the zero crossing point and therefore allows for the switching of capacitors in a momentarily cold state. This ensures that no large transients are added to the electrical network inside the establishment. 
     Additionally, once a capacitor has been switched out of the system it is de- energized to prevent a current or voltage surge from occurring when the system reselects it. 
     The system in accordance with the present invention, installed in a household or industrial environment, will maximize efficiency and hence maximize the reduction of energy costs. Additionally, the present invention can be sized to applications of varying load and varying power factor, by adjusting the value and number of capacitors incorporated into the device. 
     As most establishments are inductive in nature, in accordance with the present invention one capacitor is constantly selected. This provides for immunity to appliances in the establishment from external voltage spikes form naturally occurring phenomenon. (e.g. lightening and electrical storms). Incorporating this with an array of electronically switchable capacitors allows the system to optimize the power factor and provide voltage spike protection concurrently.