Patent Publication Number: US-10784677-B2

Title: Enhanced utility disturbance monitor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority from the U.S. Provisional Application No. 62/622,981, filed on Jan. 19, 2018, the disclosure of which is hereby expressly incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     Field 
     This disclosure relates generally to a system and method for detecting a voltage disturbance on an electrical line coupled to a utility and disconnecting a load from the utility if the disturbance is detected and, more particularly, to a system and method for detecting a voltage disturbance on an electrical line coupled to a utility and disconnecting a critical load from the utility if the disturbance is detected, where the method includes calculating a sliding window root mean squared (RMS) voltage of each phase of a three-phase utility power signal and a sliding window filtered RMS average voltage of each phase of the signal, and determining whether a difference between the sliding window RMS voltage and the filtered RMS voltage is less than or greater than a predetermined value. 
     Discussion 
     A power distribution utility provides three-phase electrical power on a power distribution network to deliver the power at the proper voltage for a number of loads, such as homes, businesses, manufacturing facilities, etc. The utility includes various power sources, substations, switching devices, feeder lines, lateral lines, circuit breakers, transformers, current and voltage detectors, etc. that operate to deliver the three-phase power to the loads in a controlled and stable manner. 
     Faults may periodically occur in the distribution network that create short circuits or near short circuits that may significantly increase the current flow to the fault location from the power source, and may cause electrical voltage disturbances throughout the network, where the voltage sags and decreases at a certain rate and to a certain level depending on the relative location of the fault and the load. Techniques are known in the art that detect the occurrence of such faults typically by detecting a high fault current, and open circuit breakers, reclosers, etc. at the appropriate location to disconnect or remove the fault from the network as quickly as possible so as to prevent damage to circuits and components. However, some of the loads in the distribution network may be critical loads where even a small and/or short disturbance in the voltage of the power signal provided to those loads could have significant consequences. For example, a critical load may be a factory where even a small loss of electrical power can affect machinery in the factory that can cause productivity loss, product damage, etc. Typically, critical loads require a power service that includes all three AC voltage phases that are provided by the utility. 
     It is known in the art to provide an uninterruptible power supply (UPS) system for these types of critical loads that includes a detector that detects a voltage disturbance on the electrical line from the utility as a result of a fault, a switch for disconnecting the critical load from the utility when a disturbance is detected, and a power supply, such as a bank of batteries, that provides power to the critical load when the switch is opened all in a quick and seamless manner so that the power supply to the load is not interrupted. U.S. Pat. No. 5,943,246, titled, Voltage Detection of Utility Service Disturbance, issued Aug. 24, 1999 to Porter, herein incorporated by reference, discloses one known technique for detecting a voltage disturbance that includes monitoring the instantaneous voltages of all three-phases from the utility, and calculating a sliding window one-half half AC cycle RMS voltage of each phase at a very high sample rate. The RMS voltage calculations are then compared to predetermined maximum and minimum voltage values, for example, +10% or −10% of a nominal voltage, such as 120 volts, and if the RMS voltage calculation of any of the phases exceeds those voltage values, then the system opens the switch to disconnect the critical load from the utility and connects the power supply to the critical load. 
     It is desirable to set the maximum and minimum voltage values and the calculation sample rate so that the system switches to the power supply very quickly as the voltage sags in response to a voltage disturbance, but not so quickly if the voltage disturbance is not significant enough, and thus is not a result of a fault. In other words, it is desirable to eliminate false positives where if there is a small glitch on the utility power that is not the result of a voltage disturbance caused by a fault, the utility is not disconnected from the critical load. 
     Although the RMS voltage monitoring technique referred to above has been affective in quickly identifying a voltage disturbance and switching to the UPS power supply without loss of power to the critical load and without a significant occurrence of false positives, improvements can still be made. For example, sometimes a critical load will be receiving normal power at a higher voltage than the nominal voltage, such as at 105% of the nominal voltage, where a 10% voltage sag as a result of a voltage disturbance would not be enough to open the switch and connect the power supply because the system is configured for a 10% sag from the nominal voltage. Thus, in this example, the voltage would need to drop 15% from its operating voltage to reach 90% of the nominal voltage to open the switch and connect the power supply, which could undesirably cause problems in the critical load. 
     SUMMARY 
     The present disclosure describes a system and method for detecting a voltage disturbance on an electrical line coupled to a utility that provides three-phase electrical AC power signals to a critical load. The method includes reading instantaneous voltage measurements at a high sample rate of each of the three-phase power signals, and calculating a sliding window actual root mean squared (RMS) voltage for each three-phase power signal over a first predetermined sample period. The method also includes calculating a sliding window filtered RMS average voltage for each actual RMS voltage over a second predetermined sample period to identify normal changes in the voltage of the three-phase power signals from a nominal voltage, and obtaining a difference between the actual RMS voltage and the RMS average voltage for each of the three-phase power signals. The method then determines whether any of the actual RMS voltages is greater than a first predetermined percentage of the nominal voltage, whether any of the actual RMS voltages is less than a second predetermined percentage of the nominal voltage, whether the difference between the actual RMS voltage and the RMS average voltage for any of the three-phase signals is greater than a third predetermined percentage, and whether the difference between the actual RMS voltage and the RMS average voltage for any of the three-phase signals is less than a fourth predetermined percentage. If any of these conditions is met, then a voltage disturbance is detected and a switch is opened to disconnect the critical load from the utility and a back-up power supply is connected to the critical load. 
     Additional features of the present disclosure will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an uninterruptible power supply (UPS) system that detects a utility voltage disturbance and transfers power from the utility to a back-up power supply; 
         FIG. 2  is a flow chart diagram showing a process for detecting a utility voltage disturbance by calculating RMS voltages of the utility power signal; and 
         FIG. 3  is flow chart diagram showing a process for detecting a utility voltage disturbance by calculating magnetic flux of an isolation transformer. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the disclosure directed to a system and method for detecting voltage disturbances on a utility service power signal is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses. 
       FIG. 1  is a schematic block diagram of an uninterruptible power source (UPS) system  10  that is coupled to a power line  12  that delivers power from a utility  14  to a critical load  16 . In this example, the critical load  16  is the type of load, such as a manufacturing facility or a factory, that would require power from all of the three-phase power signals that the utility  14  provides, which would be provided on three separate electrical lines, where the line  12  is intended to represent the combination of those lines. The UPS system  10  includes an input circuit breaker  18  provided in the line  12 , an output circuit breaker  20  provided in the line  12  and a fast response static switch  22  provided in the line  12  therebetween. A by-pass circuit breaker  24  is provided in a by-pass line  26  so that the circuit breakers  18  and  20  and the static switch  22  can be by-passed under certain operating conditions to directly provide power from the utility  14  to the load  16 . The static switch  22  operates to disconnect the utility  14  from the load  16  very quickly in response to detecting a voltage disturbance on the line  12  as a result of a fault in the utility  14 , and can have any suitable switching devices therein to perform that task including both electrical and mechanical devices, where electrical switching devices typically operate more quickly than mechanical switching devices. In one non-limiting example, the static switch  22  includes a set of silicon controlled rectifiers (SCRs) to provide the fast switching, such as at 1000th of a second. During normal operating conditions where no voltage disturbance has been detected, the circuit breakers  18  and  20  are closed, the by-pass circuit breaker  24  is open, and the static switch  22  is closed or is conducting. 
     The UPS system  10  includes a system controller  30  that controls the position of the circuit breakers  18 ,  20  and  24 , and the static switch  22  to connect and disconnect the utility  14  to and from the load  16  in the manner described herein. The UPS system  10  also includes voltage sensors  28  that measure the instantaneous voltages of each of the three-phase signals on the line  12  and provides those voltage measurement signals to the system controller  30 . The voltages are analyzed by the controller  30  in the manner described herein to identify a voltage disturbance on the line  12  and will cause the static switch  22  to open in response to the disturbance, and thus disconnect the load  16  from the utility  14 . At the same time that the switch  22  is opened, the system controller  30  switches on a power inverter  36  that receives a DC power signal from a secondary power source  38 . The secondary power source  38  would typically be a bank of batteries, but may be other types of power sources, such as fuel cells, flywheels, capacitors, etc. The inverter  36  converts the DC power signal from the power source  38  to an AC power signal that is stepped up in voltage by an isolation transformer  32  and provided on the line  12  to the load  16 . 
     As will be discussed in detail below, the present disclosure describes two fast and reliable techniques for disconnecting the critical load  16  from the utility  14  in response to detecting a voltage disturbance on the line  12 . It is noted that although the discussion herein refers to the UPS system  10  that disconnects the critical load  16  from the utility  14  by opening the switch  22  and providing power to the load  16  from the power source  38 , the described techniques may also be applicable to disconnecting a micro-grid from the utility  14  that may include a switch of the same type as the switch  22 . As is known in the art, a micro-grid could be a section of the utility  14  that includes one or more power sources, such as photovoltaic cells, diesel generators, battery modules, wind farms, etc., that provide power to a number of loads, where generally a micro-grid would cover a larger physical area than the critical load  16  and possibly could include one or more critical loads within it. 
     A micro-grid can be disconnected from the utility  14  in the event of a fault occurring in the utility  14 , where the various power sources in the micro-grid can then support the loads in the micro-grid. During normal operation, the micro-grid sources may be reducing the amount of power that the loads in the micro-grid are drawing from the utility  14 , or may be placing power onto the utility  14 . In the micro-grid embodiment, the inverter  36  may always be providing power onto the line  12  from the power source  38  in addition to the power that is provided by the utility  14 , where if the static switch  22  is opened, the power source  38  is the only power source for the micro-grid. 
       FIG. 2  is a flow chart diagram  50  showing one embodiment of a process for determining whether the controller  30  will open the switch  22  in response to a voltage disturbance and connect the inverter  36  to the line  12 , as discussed above. Each of the calculations and operations referred to in the diagram  50  is performed at a certain high sampling rate to provide the desired switching speed. In one embodiment, the various calculations are performed at 4800 Hz or 4800 times per second. For a 60 Hz AC signal, there would thus be 80 sample points or calculations per AC cycle, where a one cycle sliding window calculation would include the last 80 sample voltages. 
     The algorithm reads the instantaneous voltage at the sample rate referred to above for each of the three-phases at box  52 , where the three-phases are referred to herein as a, b and c, and the instantaneous voltages are referred to as V a_inst , V b_inst  and V c_inst . The algorithm then calculates a one-half AC cycle sliding window RMS voltage of each phase at box  54  at each sample point to provide RMS voltage values V a_RMS , V b_RMS  and V c_RMS . Since the sliding window is over a one-half cycle, each calculation at each sample point uses the last forty voltage measurements, where the algorithm squares each voltage measurement, adds the forty squared voltage measurements, divides the added voltage measurements by forty and takes the square root of that value to give the RMS voltage values. 
     At box  56 , the algorithm calculates a filtered RMS average voltage for each phase at each sample point to obtain average voltage values V a_ave , V b_ave  and V c_ave  of each phase over a certain time period. In this embodiment, the average voltage values are determined over a sliding window full AC cycle, or eighty sample points. For the filtering calculation, if the voltage of any of the phases increases or decreases, the algorithm does not immediately use that instantaneous increase or decrease in the voltage, but more slowly processes the change in the voltage, where some predetermined percentage of the increase or decrease in the voltage is used in the filtered RMS average voltage calculation. In other words, the algorithm monitors voltage changes on the three-phases so that if the normal operating voltage provided to the critical load  16  from the utility increases or decreases relative to the nominal voltage, that change is slowly determined to be the normal utility voltage so that temporary fluctuating changes in the voltage are not used as the normal voltage. As the voltage of the phases changes, the calculation of the RMS average voltage for each phase will eventually reach the new normal voltage after some predetermined time constant, such as five minutes. Therefore, if there is a voltage disturbance on the line  12 , the new normal voltage, if it exists, is also compared to the RMS voltages to determine whether to open the switch  22 . 
     The algorithm will open the static switch  22  or keep the static switch  22  open if any one of four conditions is met. The first condition is provided at decision diamond  58 , which determines whether any of the RMS voltages V a_RMS , V b_RMS  and V c_RMS  is greater than a predetermined percentage of the nominal voltage, such as 120 V. In this non-limiting embodiment, that percentage is 10% above the nominal voltage so the algorithm looks for RMS voltage values that are 110% of the nominal voltage. If this condition is not met at the decision diamond  58 , then the algorithm determines at decision diamond  60  whether any of the RMS voltages V a_RMS , V b_RMS  and V c_RMS  is less than a predetermined percentage of the nominal voltage, i.e., 90% of the nominal voltage in this example. If this condition is not met at the decision diamond  60 , then the algorithm determines whether the difference between the RMS voltages and the filtered RMS average voltages of any of the phases, namely, V a_RMS −V a_ave , V b_RMS −V b_ave  and V c_RMS −V c_ave , is greater a predetermined percentage, such as 10% of the average voltage, at decision diamond  62 . Thus, the calculation at the decision diamond  62  determines whether the voltage on any of the phases has increased by 10% or more above the actual voltage, i.e., the new normal voltage, provided by the utility  14 , which may be different than the nominal voltage. If the average RMS voltage is the nominal voltage, then a 10% voltage increase would have been determined at the decision diamond  58  before the algorithm reached the decision diamond  62 . If this condition is not met at the decision diamond  62 , then the algorithm determines whether for each phase the difference between the RMS voltage and the filtered RMS voltage, namely, V a_RMS −V a_ave , V b_RMS −V b_ave  and V c_RMS −V c_ave , is less than −10% of the average voltage at decision diamond  64 . If the filtered RMS average voltage is the same as the nominal voltage, and the voltage sag is more than 10%, then the algorithm would have detected this voltage disturbance at the decision diamond  60 . If this condition is not met at the decision diamond  64 , the algorithm ends at block  66 . 
     If any of the conditions at the decision diamonds  58 ,  60 ,  62  and  64  has been met, then a disturbance is detected and the algorithm either opens the static switch  22  or keeps the static switch  22  open to disconnect the critical load  16  from the utility  14 . Specifically, the algorithm determines at decision diamond  68  whether the switch  22  is open, meaning it had previously been opened because of one of the conditions of the decision diamonds  58 ,  60 ,  62  and  64  had been met, and if not, the algorithm opens the switch  22  at box  70 . If the switch  22  is open at the decision diamond  68  or is opened at the box  70 , the algorithm then runs UPS system or micro-grid power balance equipment at box  72 , and the algorithm ends at the block  66 . More specifically, when the critical load  16  or the micro-grid is disconnected from the utility  14 , the amount of power being provided to the critical load  16  or micro-grid by the power source  38  may need to be rebalanced so that the power required to operate the load  16  is controlled, where if the power source  38  is generating excess power, it can be used to charge batteries or perform other operations, and if the power source  38  is not providing enough power for the loads, it is controlled to increase its power output or additional power supplies are provided to the load  16 . 
     Based on this discussion, and the example values given, if the one-half cycle RMS voltage of any phase goes below 90% of the nominal voltage, a voltage disturbance is detected and the switch  22  is opened. Further, if the voltage was actually 105% of the nominal voltage, and the one-half cycle RMS voltage dropped 10% of that value to 95% of the nominal voltage, then a voltage disturbance would also be detected, which allows for a faster detection of the disturbance instead of waiting to detect the 90% voltage drop in the known processes. For example, if the system is running at 105% of the nominal voltage and the voltage falls to 70% at the peak of the voltage waveform, the system will go to 95% of the RMS voltage in 1.7 ms and to 90% of the RMS voltage in 4.8 ms. Therefore, by employing the additional calculation of determining the filtered RMS average voltage as discussed, the switch  22  is opened more quickly than in the known systems. Also, if the voltage was actually 95% of the nominal voltage, a voltage disturbance would be detected at 90% of the nominal voltage. Further, recovery is inherent in the algorithm since it looks for a change in the RMS voltage compared to the average voltage. Thus, if the voltage is 105% of the nominal voltage and drops to 94% of the nominal voltage, a voltage disturbance will be detected. If the voltage stays at 94% of the nominal voltage, the disturbance will be cleared later when the average voltage goes below 104%. In other words, in this example, the drop in voltage to 94% the nominal voltage is not an actual voltage disturbance, but is a reduction in the normal operating voltage. 
     In an alternate embodiment for determining whether there is a voltage disturbance on the line  12  from a fault in the utility  14 , the magnetic flux in the transformer  32  is analyzed. The voltage across the transformer  32  is the same as the voltage provided by the utility  14 , where the magnetic flux in the transformer  32  can be calculated as the integral of the volts across the transformer  32  at a rate, for example, of 4800 times per second, which is a straight forward calculation, where normally the voltage goes positive and negative overtime and averages to zero. During a fault, the voltage on the line  12  remains at or near zero for some time because of the fault. During this time, the transformer flux does not change because the voltage is low or close to zero. Since transformer flux is the integral of volt seconds on the transformer core, the flux accumulates as the integral of the voltage rather than the integral of the voltage squared. Therefore, by adding the voltage measurements of the utility  14  on the line  12  at consecutive sample points, the flux in the transformer  32  can be calculated and used to detect a voltage disturbance in some cases more quickly than by calculating the RMS voltage at the sample points as discussed above. For example, if the measurements and calculations are performed 80 times per AC cycle and the voltage goes to zero at the zero voltage crossing, one-half cycle RMS voltage takes 2.7 ms to drop from 100% at the nominal voltage to 90%. If the flux change algorithm looks for a change of 10% in the peak transformer flux, a 10% voltage drop can be detected in 1.25 ms. 
       FIG. 3  is a flow chart diagram  80  showing how this embodiment is performed by using transformer flux in the UPS system  10  to detect a voltage disturbance on the line  12  and open the switch  22 , where like elements to flow chart diagram  50  are identified by the same reference number. As with the process of the flow chart diagram  50 , each of the operations referred to in the diagram  80  is performed at a certain sampling rate to provide the desired switching speed and reduce the chance of false positives, such as 4800 times per second. 
     As above, the algorithm reads the three-phase instantaneous voltages V a_inst , V b_inst  and V c_inst  at the box  52 . Next, the algorithm calculates the transformer flux for each phase at the current sample point as flux values V a_flux , V b_flux  and V c_flux  at box  82 , where adding the measured voltages at two consecutive time periods operates as summing volt-seconds to obtain the flux value, as:
 
 V   a_flux   =V   a_flux (previous)+ V   a_inst   −K   p_flux   *V   a_flux_ave ,  (1)
 
 V   b_flux   =V   b_flux (previous)+ V   b_inst   −K   p_flux   *V   b_flux_ave ,  (2)
 
 V   c_flux   =V   c_flux (previous)+ V   c_inst   −K   p_flux   *V   c_flux_ave ,  (3)
 
where V a_flux (previous), V b_flux (previous) and V c_flux (previous) are the measured voltages at the previous sample time, K p_flux  is a proportional term defined by asymmetrical losses in the transformer core due to asymmetrical voltages on the core, and V flux_ave  is an average of the calculated flux over a sample period, i.e., one AC cycle, where K p_flux *V flux_ave  is a correction for a DC flux offset due to starting and measurement errors, and where if there is no offset, V flux_ave  will be zero.
 
     At box  84 , the algorithm updates the saved flux values for the last AC cycle of flux values V a_flux , V b_flux  and V c_flux  so that the previous sample flux values are available for equations (1)-(3) for the next sample calculation, and all of the last eighty flux values V a_flux , V b_flux  and V c_flux  are available to calculate a new average flux in equations (4)-(6) below and the flux values V a_flux , V b_flux  and V c_flux  from one cycle ago are available to calculate a flux error value V flux_err  in equations (7)-(9) below. The algorithm then calculates the average flux value V flux_ave  for each phase at box  86 , for example, by adding all of the saved flux values and dividing that value by eighty, which are used in equations (1)-(3) for the next sample calculations, as:
 
 V   a_flux_ave =OneCycleSlidingWindowFilter( V   a_flux ),  (4)
 
 V   b_flux_ave =OneCycleSlidingWindowFilter( V   b     flux   ),  (5)
 
 V   c_flux_ave =OneCycleSlidingWindowFilter( V   c_flux ).  (6)
 
     The algorithm then calculates the flux error value V flux_err  for each phase at the sample rate at box  88  as:
 
 V   a_flux_err   =V   a_flux (now)− V   a_flux (1 cycle ago),  (7)
 
 V   b_flux_err   =V   b_flux (now)− V   b_flux (1 cycle ago),  (8)
 
 V   c_flux_err   =V   c_flux (now)− V   c_flux (1 cycle ago),  (9)
 
where V flux (1 cycle ago) is the flux value calculated 80 sample points earlier.
 
     The algorithm then determines whether any of the flux error values V a_flux_err , V b_flux_err  and V c_flux_err  is greater than a predetermined percentage, for example, 10%, at decision diamond  90 . If the flux error value is not greater than the predetermined percentage at the decision diamond  90 , then the algorithm determines whether any of the flux error values V a_flux_err , V b_flux_err  and V c_flux_err  is less than −10%, and if not the algorithm ends at block  94 . If either of the conditions is occurring at the decision diamonds  90  and  92 , then the static switch  22  is opened at the decision diamond  68 . 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.