QUANTIFYING EXCESS VOLTAGE DROP IN THREE-PHASE AC CIRCUITS

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to three-phase alternating current (AC) circuits and, more particularly, to a system and method for detecting and localizing excess voltage drop in single or multiple phases of three-phase AC circuits.

[0002] In industrial facilities, electrical machines such as generators, AC motors, and/or transformers are used in various applications. As one example, induction motors are used for applications like pumping, cooling, material movement, and other applications where cost-efficient and robust motors are required. An electrical distribution system is used in conjunction with electrical machines in such applications, with the electrical distribution system including protection and control components such as breakers, contactors, starters, etc.

[0003] In providing connections within an electrical distribution system and at the electrical machine, it is recognized that an improper connection of protection and/or control components can lead to over-heated contacts that can lead to fire, equipment damage, and process downtime and efficiency loss. That is, excess voltage drops can occur in distribution circuits when electrical connections become loose or have less wire binding force. While even good joints/connections have some level of voltage drop (e.g., in the range of 3-10 mV) that is acceptable, bad joints/connections have an excess voltage drop in the range 100-300 mV or higher. Such voltage drops can lead to abnormal power dissipation at the connections and correspondingly to over-heated contacts or hot spots in the connections that can initiate fire and equipment damage. Even at an early stage, the voltage drops lead to energy losses as line drops, in effect reducing motor efficiency and motor life for example.

[0004] Often, the problem of improper electrical connections is addressed to a certain extent by performing preventative maintenance, like annual retorquing of connections or by periodic inspection of all the electrical connections when the distribution circuit is offline. With respect to inspection of the electrical connections, such inspections are typically performed via separate dedicated sensing devices that inspect the electrical distribution system using acoustic or temperature sensing. For example, infrared (IR) scanners are often used to perform a periodic thermal inspection of the electrical distribution system. While effective, the inspection of electrical distribution system by way of such IR scanners is an intensive procedure that can be quite costly. Additionally, the inspection of an electrical distribution system by way of an IR scanner does not provide for online monitoring of the electrical connections and are limited to line-of-sight monitoring.

[0005] It would therefore be desirable to provide a system and method that detects excess voltage drops in a distribution circuit in an efficient and cost effective manner, so as to identify improper electrical connections in the circuit. It would also be desirable for such a system and method to be non-intrusive, such that the detection of excess voltage drops in a distribution circuit can be performed while the circuit is online. It would still further be desirable for such a system and method to localize a detected excess voltage drops to a particular phase or phases of the distribution circuit and quantify the excess drop.

BRIEF DESCRIPTION OF THE INVENTION

[0006] The present invention provides a system and method for detecting excess voltage drops in three-phase AC circuits.

[0007] In accordance with one aspect of the present invention, an electrical distribution circuit includes an input connectable to an AC source, an output connectable to terminals of an electrical machine, the output configured to provide three-phase voltages and currents to the electrical machine, and a diagnostic system configured to detect an excess voltage drop (EVD) in the electrical distribution circuit, the diagnostic system comprising a processor that is programmed to receive measurements of the three-phase voltages and currents provided to the electrical machine, compute a negative sequence voltage from the three-phase voltages and currents, determine a localization reference phase angle for each phase based in part on the three-phase voltages and currents, and calculate an EVD in the electrical distribution circuit based on the negative sequence voltage and the localization reference phase angles.

[0008] In accordance with another aspect of the invention, a method for detecting excess voltage drop (EVD) in an electrical distribution circuit includes measuring three- phase voltages and currents provided to an electrical machine in the electrical distribution circuit by way of voltage and current sensors and causing a diagnostic system to calculate a fault severity index (FSI) based on the measured three-phase voltages and currents, wherein causing the diagnostic system to calculate the FSI further includes receiving the measured three-phase voltages and currents provided to the terminals of the electrical machine, computing a positive, negative, and zero sequence voltage and sequence current from the three-phase voltages, determining a compensated negative sequence voltage from the negative sequence voltage component, determining a localization reference phase angle for each phase based in part on the three-phase currents, and calculating the fault severity index (FSI) in the electrical distribution circuit based on the compensated negative sequence voltage and the localization reference phase angles. [0009] In accordance with yet another aspect of the invention, a diagnostic system configured to detect an excess voltage drop (EVD) fault in an electrical distribution circuit is provided. The diagnostic system includes a processor programmed to receive three-phase voltages and currents provided to terminals of an electrical machine connected to the electrical distribution circuit, compute positive sequence, negative sequence, and zero sequence voltage and current from the three-phase voltages and currents, determine a compensated negative sequence voltage from the negative sequence voltage component, determine a localization reference phasor for each phase based in part on the three-phase currents, identify an EVD fault in the electrical distribution circuit based on the compensated negative sequence voltage, and localize the EVD fault to one or more phases in the three-phase output based on a difference between a phase angle of the compensated negative sequence voltage and the localization reference phasors.

[0010] Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The drawings illustrate preferred embodiments presently contemplated for carrying out the invention.

[0012] In the drawings:

[0013] FIG. 1 is a schematic view of a three-phase electrical distribution circuit for use with embodiments of the present invention.

[0014] FIG. 2 is a schematic diagram of a delta connected motor and an EVD fault therein according to an embodiment of the invention.

[0015] FIG. 3 is a schematic diagram of a star connected motor and an EVD fault therein according to an embodiment of the invention.

[0016] FIG. 4 is a flowchart illustrating a technique for detecting excess voltage drops in three-phase AC circuits according to an embodiment of the invention.

[0017] FIG. 5 is a phasor diagram illustrating localization reference phase angle phasors according to an embodiment of the invention.

[0018] FIG. 6 is flowchart illustrating sub-steps of STEP 66 in FIG. 4 for localizing of an EVD to one or more phases in a three-phase AC circuit according to an embodiment of the invention.

[0019] FIGS. 7-9 are phasor diagrams illustrating an EVD in Phases A, B and C, respectively, of a three-phase AC circuit.

[0020] FIG. 10 is a phasor diagram illustrating an EVD in more than one phase of a three-phase AC circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The embodiments of the invention set forth herein relate to a system and method for detecting and localizing excess voltage drop in single or multiple phases of three-phase AC circuits.

[0022] Referring to FIG. 1, a three-phase electrical distribution circuit 10 (or "motor circuit") is shown according to an embodiment of the invention. The electrical distribution circuit 10 is connected between a three-phase AC input 12a- 12c and a load 14, such as an AC electrical machine, to provide protection to the electrical machine and to condition power from the three-phase AC input 12a- 12c for delivery to the machine. According to one embodiment of the invention, the electrical machine 14 is in the form of an induction motor 14, and thus is referred to hereafter in FIG. 1 as induction motor 14. However, it is recognized that the electrical machine 14 could also be a generator or transformer, for example, or any other load that might be driven by a three-phase power and useful in an industrial setting.

[0023] The electrical distribution circuit 10 includes an input 16 that is connectable to three-phase AC input 12a- 12c to receive power therefrom. The electrical distribution circuit 10 also includes a three-phase output 18 that is connectable to motor terminals 20 of the induction motor to provide three-phase voltages and currents to the induction motor 14. According to one embodiment, the output 18 can be connected to motor terminals 20 at a terminal box 22 of the electrical distribution circuit 10, for example.

[0024] As further shown in FIG. 1, a plurality of circuit components are included in electrical distribution circuit 10 that are positioned between the input 16 and the output 18 - with the circuit components providing protection from and control of voltage and current that is provided from the three-phase AC input 12a- 12c for delivery to the induction motor 14. A number of such protection and control components are shown in FIG. 1, but it is recognized that other components could also/instead be included in electrical distribution circuit 10 according to embodiments of the invention. In the embodiment of electrical distribution circuit 10 shown in FIG. 1, a motor control center (MCC) 24 is shown as being included in the circuit. The motor control center 24 may comprise an assembly of one or more enclosed sections having a common power bus and containing a number of motor control units - such as a number of motor starters. The motor control center 24 can also include variable frequency drives, programmable controllers, and metering. Associated with operation of the motor control center 24 are a number of protection components/de vices to protect the motor 14, provide short- circuit protection, and/or isolate the motor circuit. For example, fuses 26 and contactors 28 are provided in the electrical distribution circuit 10, such as in a local electrical panel 30, to provide for short-circuit protection and control of the induction motor 14. A circuit breaker 32 and disconnect switch 34 are also provided to provide short-circuit protection and isolation of the electrical distribution circuit 10.

[0025] It is recognized that the electrical distribution circuit 10 shown in FIG. 1 is merely illustrative of a motor electrical distribution circuit that may be associated with an embodiment of the present invention, and that three-phase AC circuits of various configurations and arrangements could instead be provided according to embodiments of the invention.

[0026] With respect to the electrical distribution circuit 10 shown in FIG. 1, a plurality of electrical connections, generally referenced as 36, is made in the circuit to connect components 24, 26, 28, 32, 34 therein to one another and to the induction motor 14. In making such connections, it is recognized that an improper connection of protection and/or control components can lead to over-heated contacts that can lead to fire, equipment damage, and process downtime and efficiency loss. That is, excess voltage drops (EVD) can occur in motor circuits when electrical connections 36 become loose or have less wire binding force, with such voltage drops leading to abnormal power dissipation at the connections and correspondingly to over-heated contacts or hot spots in the connections that can initiate fire and equipment damage, energy losses as line drops, motor efficiency loss, and motor life reduction.

[0027] Accordingly, a diagnostic system 40 is included in the electrical distribution circuit 10 to detect an EVD fault in the electrical distribution circuit 10, according to an embodiment of the invention. The diagnostic system 40 receives an input regarding the three-phase voltage and current provided to the induction motor 14. According to an exemplary embodiment, the diagnostic system 40 receives voltage and current measurements acquired from voltage and current sensors integrated into a motor starter(s) in the MCC 24; however, it is recognized that separate dedicated voltage and current sensors could be included in electrical distribution circuit 10 to acquire voltage and current data and provide it to diagnostic system 40. As shown in FIG. 1, a processor 42 in the diagnostic system 40 receives the measured three-phase voltages and currents and is programmed to analyze the data to identify an EVD fault in the electrical distribution circuit 10. In identifying an EVD fault in the electrical distribution circuit 10, the processor 42 calculates a fault severity index (FSI) having a magnitude that is an indicator of the amount of EVD at a bad joint and an angle that indicates the phase which has the bad joints and the EVD therein.

[0028] While the diagnostic system 40 is shown in FIG. 1 as being in the form of a standalone product/device, it is recognized that such a system could be incorporated into a protection and control component included in the electrical distribution circuit 10. That is, a processor 42 having a program/algorithm thereon that enables detection of an EVD fault in the electrical distribution circuit 10 can reside in an existing starter, relay, drive, breaker, motor control center and/or other motor control or protection product in the electrical distribution circuit 10. The diagnostic system 40 can thus provide online monitoring of the electrical distribution circuit 10 from a location of the distribution circuit or at a location remote from the distribution circuit.

[0029] Furthermore, while embodiments of the inventors are described here below with respect to the processor 42 of diagnostic system 40 being programmed to perform a technique to identify an EVD fault in the electrical distribution circuit 10, it is recognized that the term "processor" as used herein need not be a programmable device. That is, it is to be understood that the processor 42 (and the steps performed thereby), as described hereafter, also covers equivalent hardware and computing devices that perform the same tasks. [0030] In detecting the existence of an EVD in a three-phase motor circuit using the supply voltages and currents, it is recognized that an EVD leads to voltage unbalance or asymmetry at the motor terminals, which in turn leads to a current unbalance or flow of asymmetrical currents in the circuit. As a rule-of-thumb a 1% voltage unbalance in supply could lead to a 6-10 % unbalance in motor current, such that even small voltage drops in the range of 100-300 mV could lead to appreciable current unbalance leading to its detection. This amplification of current unbalance is due to the low value of negative sequence impedance of motor circuits, and it is recognized that this amplification gets stronger as the size of the motor increases and as the negative sequence impedance decreases

[0031] According to embodiments of the invention, for purposes of detecting the existence of an EVD in a three-phase motor circuit, the method of symmetrical components is employed to simplify the analysis of the motor circuit as it becomes unbalanced, either due to fault or are unbalanced already. The asymmetrical/unbalanced phasors (voltages and currents) are represented as three symmetrical sets of balanced phasors - the first set has the same phase sequence as the system under study (positive sequence, e.g., ABC), the second set has the reverse phase sequence (negative sequence, e.g., ACB), and in the third set the phasors A, B and C are in phase with each other (zero sequence). Essentially, this method converts three unbalanced phases into three independent sources, which makes asymmetric fault analysis more tractable. Using the sequence phasors of voltages and currents, a fault severity index (FSI) is calculated, the magnitude of which is an indicator of the amount of EVD at a bad joint and the angle of which indicates the phase or phases which has/have the bad joint(s).

[0032] Embodiments of the invention are provided for detecting the existence of an EVD in a three-phase motor circuit in either a delta connected motor circuit or a star connected motor circuit. An illustration of a delta connected motor circuit 44 is provided in FIG. 2. In FIG. 2, the supply line voltages V _a , Vb, V _c are illustrated, as are the phase voltages at the motor terminals V _aM , V _bM , V _cM that are present at the motor - with the motor terminal voltages that are present being determined in part by any voltage drop (EVD) present in the motor circuit 44 on one or more of the phases, which are indicated as VEVDa, VEVDb, V _E V _D C- An illustration of a star connected motor circuit 46 is provided in FIG. 3. In FIG. 3, the supply voltages V _a , V _b , V _c are illustrated, as are the phase voltages at the motor terminals V _AM , Vb _M , V _CM that are present at the motor - with the motor terminal voltages that are present being determined in part by any excess voltage drop (EVD) present in the motor circuit 46 on one or more of the phases, which are indicated as V _EV Da, V _EV Db, V _E VDC-

[0033] Referring now to FIG. 4, and with continued reference to FIGS. 1 -3, a technique 50 is illustrated that is implemented by the processor 42 of diagnostic system 40 to identify the presence of an EVD fault in a delta or star connected electrical distribution (motor) circuit 10, according to an embodiment of the invention. Performing of the technique 50 (and the steps included therein) is first described here below with respect to a delta connected motor as in FIG. 2 and will later be described with respect to a star connected motor as in FIG. 3.

[0034] In a first step of the technique 50, three-phase current and voltage measurements are received by the processor 42 at STEP 52. According to an exemplary embodiment, the processor 42 receives three-phase power data (current and voltage data) as measured in the MCC, with the voltages and currents from the MCC 24 being subsequently supplied to the terminals 20 of the electrical machine 14 - such as an induction motor, for example. Upon receiving the three-phase current and voltage measurements, the processor 42 then extracts the fundamental components of the three- phase power (fundamental components of the currents and voltages) at STEP 54 according to a known technique/method. STEP 54 is shown in phantom in FIG. 4 as it is recognized that determination of the fundamental components is optional for performing of the technique 50 - as detecting, localizing and quantifying an EVD fault in the electrical distribution circuit 10 can be performed without the fundamental components. However, it is recognized that the most accurate analysis of EVD faults is achieved via determination and use of the fundamental components, as is described here below. [0035] As shown in FIG. 4, the technique 50 continues with the computing of sequence components of the fundamental supply voltages and currents at STEP 56, with the sequence components of the voltages being identified as Vi, ₂ ,o and the sequence components of the currents being identified as I _lj2j0 , where the positive, negative and zero sequence components are identified by 1, 2, and 0, respectively. With respect to determining the voltage sequence components, it is recognized that Kirchoff s Voltage Law (KVL) can be applied to the connected motor circuit to describe the relationship between the supply line voltages, phase voltages at the motor terminals, and the EVDs in equation form according to:

where V _a M, VbM, V _CM are the phase voltages at the motor terminals, V _a , Vb, V _c are the supply line voltages, and as V _E vDa, V _E vDb, V _E V _DC are voltage drops in the phase A, B, C respectively representing an EVD.

[0036] The positive, negative and zero sequence components voltages can be obtained by applying a transformation T to Eqn. 1. According to one embodiment, a sequence transformation is utilized to yield:

2]·

[0037] According to one embodiment, the transformation T is defined according to: where a = 1Z120 = -0.5 + j—^- (i.e., a unit vector at an angle of 120 degrees).

[0038] Considering the line drops in each phase are equal (i.e., no additional unbalance seen by the motor/motor terminals), then Eqn. 2 can be rewritten as:

[Eqn. 3], where VOM, VIM and V ₂ M are the zero, positive and negative sequence voltages at the motor terminals and Vo, Vi and V ₂ are the zero, positive and negative sequence voltages due to supply unbalance.

[0039] Eqn. 3 can then be rewritten according to:

4] ·

[0040] Re-arranging the negative sequence equation to isolate the negative sequence voltage at the motor terminals, V ₂ M , yields: V _2M = V ₂ - ^{(l - a) * V _EVDa + {a ² - 1) * V _EVDb + {a - a ² ) * V _EVDc }

[Eqn. 5].

[0041] Rearranging Eqn. 5 yields:

(l - a) * V _EVDa + (a ² - l) * V _EVDb + (a - a ² ) * V _EVDc = HV ₂ - V _2M )

[Eqn. 6].

[0042] The negative sequence voltage at the motor terminals, V _2M , can be estimated using impedance values in the electrical distribution circuit 10, along with the negative and positive sequence currents. More specifically,

V — 7 * T - 7

y 2M ^~ ^2\ ^ji l ^"r 22 * T

L 2 [Eqn. 7], where Z ₂₂ is the negative sequence impedance of the motor, Z ₂₁ is the coupling impedance between the negative sequence voltage and the positive sequence current, l\ is the positive sequence current, and is the negative sequence current.

[0043] Thus, in order to estimate the negative sequence voltage at the motor terminals, V _2M , the technique 50 next estimates the negative sequence impedance Z ₂₂ and the coupling impedance between the negative sequence voltage and the positive sequence current Z ₂₁ at STEP 58. In estimating the impedances Z ₂₂ and Z ₂₁ , it is assumed that the electrical machine is operating under healthy conditions, when all the connections in the electrical distribution circuit 10 are proper (i.e., there is no EVD fault due to improper connections). Assuming initially that there is no EVD fault in the downstream path and that line drops from a sensing point to the motor terminals are similar for each phase, then the negative sequence voltage is equal to the negative sequence voltage at the motor terminals (i.e., V ₂ = V _2M ). SO, during initialization - assuming there is no EVD fault present on the motor downstream path - the impedance parameters Z ₂₁ and Z ₂₂ can be estimated using a nonlinear recursive fit. [0044] Upon estimation of the impedance parameters Z ₂₁ and Z ₂₂ , Eqn. 7 may be implemented to determine the negative sequence voltage at the motor terminals, V _2M - Upon determination of the negative sequence voltage at the motor terminals, V _2M , technique 50 continues as STEP 60 with the determination of the negative sequence voltage due only to EVD, V ₂ E _V D, - i.e., a "compensated negative sequence voltage." The compensated negative sequence voltage, V _2EVD , can be determined according to:

[0045] By substituting Eqn. 8 into Eqn. 6, Eqn. 6 can be rewritten according to:

(1 - Λ) * V _EVDa + (a ² - 1) * V _EVDb + (a - a ² ) * V _EVDc ) = 3 * V _{2EVD [Eq} n. 9], where

(1 - a) = 3 - 30, (a ² - 1) = 3 - 150, and(a - a ² ) = 3 - 90.

[0046] The above EVD characteristics equation, Eqn. 9, may be used to derive the conditions and relations to quantify and localize EVD faults in a single phase or multiple phases of the delta connected motor.

[0047] Referring still to FIG. 4, technique 50 continues at STEP 61 where it is determined whether the magnitude of the compensated negative sequence voltage, V _2EVD , is greater than a threshold voltage level, so as to enable a determination as to whether the compensated negative sequence voltage constitutes an EVD indicative of a high resistance connect fault in the distribution 10. It is recognized that the threshold voltage level can be set dependent on the severity at which an alarm is to be raised, and thus the threshold could be a pre-defined value in the program or could be determined from user settings or using user inputs. According to an exemplary embodiment, the voltage threshold is set at lOOmV - such that voltage drops greater than lOOmV are categorized as an EVD - although it is recognized that the threshold could be a higher or lower value. If it is determined at STEP 61 that compensated negative sequence voltage, V2EVD, is less than the threshold voltage, as indicated at 62, then it is determined that there is no EVD in the system, as indicated at STEP 63. The technique then loops back to STEP 52 with the receiving of additional three-phase current and voltage measurements by the processor 42, such that monitoring for an EVD is continued.

[0048] Conversely, if it is determined at STEP 61 that the magnitude of the compensated negative sequence voltage, V _E V _D , is greater than the threshold voltage, as indicated at 64, then the technique continues at STEP 65 with the calculation of a localization reference phase angle for the each phase. According to an exemplary embodiment, the localization reference phase angles computed at STEP 65 are derived in part by using the phase angle of the fundamental current flowing through each respective phase - either in line or in phase. Thus, for Phase A, the localization reference phase angle can be described according to either: ref (PlEVDa = ^~30 + Ψ faline ref ΨΐΕΥΌα ^~ + ψ j _aphase , where ⁽ Ρ/ _α η _ηε Άηά ψ/aphase ^ the angles of the fundamental component of the current flowing through Phase A in line and phase, respectively.

[0049] For Phase B, the localization reference phase angle can be described according to either: ref ^2EVDb ⁼ 1 50 + ψ _ϊίηε ref <P ₂ EVDb = -180 + φ _{βρ 3β} , where <P n _ne d ^< Pfi _p hase ^aY ^ the angles of the fundamental component of the current flowing through Phase B in line and phase, respectively. [0050] For Phase C, the localization reference phase angle can be described according to either: ref <P ₂ EVDc = ⁹⁰ + Ψ feline

ref <P ₂ EVDe = ⁶⁰ + Pfephase where ψ f _c u _n e ^an d f _cp hase are the angles of the fundamental component of the current flowing through Phase C in line and phase, respectively.

[0051] A diagram of the localization reference phase angles computed at STEP 65 is provided in FIG. 5. As seen therein, each localization reference phase angle is a fault reference current phasor 63. The fault reference current phasor 63 are spaced 120 degrees apart, with the exact angle of each phasor dependent on whether the current flowing in line or phase is measured/determined.

[0052] Upon determination of the localization reference phase angles at STEP 65, a localization and quantification of the EVD is next performed at STEP 66. According to embodiments of the invention, localization and quantification of the EVD at STEP 66 may be performed via a calculation and analysis of a FSI - with the FSI being a phasor having a magnitude that is an indicator of the amount of EVD and an angle that indicates the phase or phases on which the EVD is present. With respect to the magnitude of the FSI phasor, the amount of EVD is derived from the compensated negative sequence voltage, V _JM V _D , as described in Eqn. 8 - with the compensated negative sequence voltage being broken down by phase as described in Eqn. 9. With respect to the angle of the of the FSI phasor, the phase or phases to which the EVD is to be attributed to (i.e., localizing of the EVD) is determined by a comparison of a phase angle of the compensated negative sequence voltage to the localization reference phase angles for each phase. [0053] Referring now to FIG. 6, the localizing of the EVD to one or more phases in the electrical distribution circuit that is performed at STEP 66 is further illustrated as a series of sub-steps. In a first step, the phase angle of the compensated negative sequence voltage is compared to the localization reference phase angles for each phase of the distribution circuit - as indicated at STEP 67. A determination is then made at STEP 68 as to whether the phase angle of the compensated negative sequence voltage is within a pre-determined angular range of any one of the localization reference phase angles. According to an exemplary embodiment, a determination is made at STEP 68 regarding whether the angle of the compensated negative sequence voltage is within 15 degrees of any of the localization reference phase angles - i.e., plus or minus 15 degrees of angular range from each localization reference phase angle. It is recognized that the pre-determined difference between the phase angle of the compensated negative sequence voltage and the localization reference phase angles is not required to be within 15 degrees, as a different angular range could instead be selected.

[0054] If it is determined at STEP 68 that the phase angle of the compensated negative sequence voltage is within a pre-determined angular range of any one of the localization reference phase angles, as indicated at 70, then the technique continues at STEP 72 by localizing the EVD fault to the particular phase corresponding to the localization reference phase angle to which the angle of the compensated negative sequence voltage is within the pre-determined angular range to. Examples of the EVD fault occurring on each of Phase A, B and C - and the describing thereof by way of Eqn. 9 - are set forth here below for purposes of illustration.

[0055] In an occurrence where the EVD is present in Phase A - i.e., \[ref(p _2EVDa — /-V _2EVD < \5 ^° - then the EVD in Phases B and C are assigned as

[0056] As the magnitude of the EVD fault is being sought, the term (1-a) can be removed from Eqn. 10, such that the EVD fault in Phase A is calculated according to:

[0057] An illustration of a FSI phasor 74 - having a magnitude indicating an amount of EVD and an angle indicating that the EVD is in Phase A - is provided in FIG. 7. As shown therein, the FSI phasor 74 has an angle that is within the pre-determined range of +/- 15 degrees of the localization reference phase angle for Phase A.

[0058] In an occurrence where the EVD is present in Phase B - i.e., 5 ^° - then the EVD in Phases A and C are assigned as

V _EVDa = 0 and V _EVDc = 0 , such that Eqn. 9 becomes:

((a ² - 1)* V _EVDb ) = 3 * V _{2EVD [Eqn} . 11].

[0059] As the magnitude of the EVD fault is being sought, the term (a ² -I) can be removed from Eqn. 11, such that the EVD fault in Phase B is calculated according to: v EVDb \ I = J * 3 ^J *\ Yv 2EVD ■

[0060] An illustration of a FSI phasor 76 - having a magnitude indicating an amount of EVD and an angle indicating that the EVD is in Phase B - is provided in FIG. 8. As shown therein, the FSI phasor 76 has an angle that is within the pre-determined range of +/- 15 degrees of the localization reference phase angle for Phase B.

[0061] In an occurrence where the EVD is present in Phase C - i.e., \ ref ⁽ p _2EVDc — /. V _2EVD ^ < \5 ^° - then the EVD in Phases A and B are assigned as

V _EVDa = 0 and V _EVDb = 0 , such that Eqn. 9 becomes: ((a - a ² )* V _EVDc ) - 3 * V _{2EVD [Eqn.} ηγ

[0062] As the magnitude of the EVD fault is being sought, the term (a-a ² ) can be removed from Eqn. 12, such that the EVD fault in Phase C is calculated according to:

[0063] An illustration of a FSI phasor 78 - having a magnitude indicating an amount of EVD and an angle indicating that the EVD is in Phase C - is provided in FIG. 9. As shown therein, the FSI phasor 78 has an angle that is within the pre-determined range of +/- 15 degrees of the localization reference phase angle for Phase C.

[0064] Referring again now to FIG. 6, if it is instead determined at STEP 68 that the phase angle of the compensated negative sequence voltage is not within a predetermined angular range of any one of the localization reference phase angles, as indicated at 80, then the technique continues at STEP 82 by localizing the FSI (and EVD) to more than one phase of the electrical distribution system 10. In localizing the EVD to more than one phase, the FSI is resolved along multiple axes to quantify an amount of EVD in each of the phases to which the EVD has been localized. That is, the magnitude of the FSI is proportioned amongst two or more phases to determine the amount of EVD in each of the phases. Examples of the EVD fault occurring in differing combinations of Phases A, B and C - and the describing thereof by way of Eqn. 9 - are set forth here below for purposes of illustration.

[0065] In an occurrence where the EVD is present in Phases A and B - i.e., ref(p _2EVDa < Z. V _1EVD < ref(p _2EVDb - then the EVD in Phase C is assigned as

V _EVDC ⁼ 0■> ^suc h that Eqn. 9 becomes:

((l - a)* V _EVDa + {a ² - 1)* V _EVDb ) = 3 * V _{2EVD [Eqn. 13]>} [0066] In Eqn.13, rewriting VEVO _H and V ^~ EVOb in the complex domain gives:

V _E V _DU = V _E V _DU (cos^ _la +ysin#

^V EVDb = ^V EVDb {∞s<p _lb +jsm<p _lb ). and substituting these values into Eqn.13 results in:

[0067] Comparing the real and imaginary parts of Eqn.14 yields:

V _EV Da I

^V EVDa I ^Sin (^/a ^{~ 30} )+ \ ^V EVDb\ * ^[ΐί {<Ρ Jh ~ ¹⁵⁰ ) = ^3 * * ^S m( F _{2? 5} )

which when solved provide:

sin^-150-^ _e +30

15].

[0068] Eqn. 15 thus enables the FSI to be resolved along multiple axes - Phases A and B - to quantify an amount of EVD in each of the phases to which the EVD has been localized. [0069] In an occurrence where the EVD is present in Phases B and C - i.e., ref(p _2EVDb < .V _2EVD < ref(p _2EVDc - then the EVD in Phase A is assigned as

V EVDa ⁼ 0■ _> such that Eqn.9 becomes: )* ^V EVBb +{ ^a - ^a )* ^V EVDc) 2 EVD [Eqn.16],

[0070] Similar to that described above, F _E vDb and F _E V _DC can be rewritten in the complex domain and substituted back into Eqn.16, with the real and imaginary parts being compared and solved to provide:

sin( _/c +90-^+150)

111

[0071] Eqn.17 thus enables the FSI to be resolved along multiple axes - Phases B and C - to quantify an amount of EVD in each of the phases to which the EVD has been localized.

[0072] In an occurrence where the EVD is present in Phases A and C - i.e., ref(p _2EVDc < -V _2EVD < ref(p _2EVDa - then the EVD in Phase B is assigned as

^EVDb ⁼ 0■ _> ^su ch that Eqn.9 becomes:

^-a)*V _EPDa + {a-a ² )*V _EPDc )=3*V 2,EVD [Eqn.18], [0073] Similar to that described above, VEVO _H and EVD _C can be rewritten in the complex domain and substituted back into Eqn. 18, with the real and imaginary parts being compared and solved to provide:

19].

[0074] Eqn. 19 thus enables the FSI to be resolved along multiple axes - Phases C and A - to quantify an amount of EVD in each of the phases to which the EVD has been localized.

[0075] An example of an FSI phasor present on more than one phase of the electrical distribution system 10 is provided in FIG. 10 - with the illustrated example showing an FSI phasor 84 present on Phases A and B of the electrical distribution circuit. The magnitude of the FSI phasor 84 indicates an amount of the EVD, with the particular angle of the FSI phasor 84 being resolved to quantify the amount of EVD in each of Phase A and B.

[0076] As indicated previously, the technique 50 illustrated in FIG. 4 can be performed for a star connected motor circuit 46 as in FIG. 3. Thus, the technique 50 of

FIG. 4 is again now described here with specific regard to the determining of an FSI and accompanying EVD fault in a star connected motor. Technique 50 begins by receiving three-phase current and voltage measurements at STEP 52 and extracting the fundamental components of the three-phase currents and voltages at STEP 54. The technique 50 continues with the computing of sequence components of the fundamental supply voltages and currents at STEP 56, with the sequence components of the voltages being identified as Vi, ₂ ,o and the sequence components of the currents being identified as Ii,2,o, where the positive, negative and zero sequence components are identified by 1 , 2, and 0, respectively. With respect to determining the voltage sequence components, it is recognized that Kirchoff s Voltage Law (KVL) can be applied to the connected motor circuit to describe the relationship between the supply line voltages, phase voltages at the motor terminals, and the EVDs in equation form according to:

where V _A M, VbM, V _C M are the phase voltages at the, V _a , Vb, V _c are the supply line voltages, and as V _E vDa, V _E vDb, V _E VD _C are voltage drops in the phase A, B, C respectively respresenting an EVD. Eqn. 20 differs from Eqn. 1 (for a delta connected motor), as the phase voltage is analyzed rather than line voltages.

[0077] The positive, negative and zero sequence components voltages can be obtained by applying a transformation T to Eqn. 20. According to one embodiment, a sequence transformation is utilized to yield:

[0078] According to one embodiment, the transformation T is defined according to:

where a = 1Z120 = -0.5 + (i.e., a unit vector at an angle of 120 degrees).

[0079] Considering the line drops in each phase are equal (i.e., no additional unbalance seen by the motor/motor terminals), then Eqn. 21 can be rewritten as:

[Eqn. 22], where VO _M , V _IM and V _2M are the zero, positive and negative sequence voltages at the motor terminals and V ₀ , Vi and V ₂ are the zero, positive and negative sequence voltages due to supply unbalance.

[0080] Eqn. 22 can then be rewritten according to:

V _r 0M 0 0 V E, VDa

1

V _x - a ² a - 1 a *

IM V E, VDb

3 [Eqn. 23].

2M - a a ² - 1 a V E, VDc

[0081] Re-arranging the negative sequence equation to isolate the negative sequence voltage at the motor terminals, V _2M , yields:

1 ₂

V _IM ~ Vj _. ^{~ ~} (V _EVDa + V _EVDb + aV _EVDc ) |- _{Eqn 2} 4].

[0082] Rearranging Eqn. 24 yields:

V EVDa + ^{a V} EVDb + ^{a V} EVDc = ³ * ( ^V 2 ^{~ V} 2M ) [Eqn. 25]. [0083] As previously described, the negative sequence voltage at the motor terminals, V _2M , can be estimated using impedance values in the electrical distribution circuit 10. Thus, the negative sequence impedance Z ₂₂ and the coupling impedance between the negative sequence voltage and the positive sequence current Z ₂₁ are estimated at STEP 58 and, upon estimation of the impedance parameters Z ₂₁ and Z ₂₂ , the negative sequence voltage at the motor terminals, V _2M is determined.

[0084] Upon determination of the negative sequence voltage at the motor terminals, V _2M , technique 50 continues as STEP 60 with the determination of the compensated negative sequence voltage (i.e., negative sequence voltage due only to EVD), V _JM V _D - The compensated negative sequence voltage, V _JM V _D , can be substituted into Eqn. 25 to yield:

VEVD _CL ^{+ a} ^EVDb ^{+ a} ^EVDc ⁼ 3 * V _2EVD [Eqn. 26].

[0085] The above EVD characteristics equation, Eqn. 26, may be used to derive the conditions and relations to quantify and localize EVD faults in a single phase or multiple phases of the star connected motor.

[0086] Referring still to FIG. 4, technique 50 continues as STEP 65 with the calculation of a localization reference phase angle for the each phase. According to an exemplary embodiment, the localization reference phase angles computed at STEP 65 are derived in part by using the angle of the fundamental component of line current flowing through each respective phase. Thus, for Phase A, the localization reference phase angle can be described according to: ref ΨΐΕνΌα ^~ Ψ fa , where Ψ f _a is the angles of the fundamental component of line current flowing through Phase A. [0087] For Phase B, the localization reference phase angle can be described according to : ref VlEVDb ⁼ Ψβ , where ψ _β ^' & the angles of the fundamental component of line current flowing through Phase B.

[0088] For Phase C, the localization reference phase angle can be described according to :

where ^ _c is the angles of the fundamental component of line current flowing through Phase C.

[0089] The respective localization reference phase angles computed at STEP 65 are provided in FIG. 5. As seen therein, each localization reference phase angle is a fault reference current phasor 63, with the phasors being spaced 120 degrees apart.

[0090] Upon determination of the localization reference phase angles at STEP 65, a calculation of the EVD is next performed at STEP 66. A calculation of the EVD at STEP 66 may be described alternately as a calculation of an FSI - with the FSI being a phasor having a magnitude that is an indicator of the amount of EVD and an angle that indicates the phase or phases on which the EVD is present. With respect to the magnitude of the of the FSI phasor, the amount of EVD is derived from the compensated negative sequence voltage, V _IE V _D , as described in Eqn. 8 - with the compensated negative sequence voltage being broken down by phase as described in Eqn. 26. With respect to the angle of the of the FSI phasor, the phase or phases to which the EVD is to be attributed to (i.e., localizing of the EVD) is determined by a comparison of a phase angle of the compensated negative sequence voltage to the localization reference phase angles for each phase.

[0091] As shown in FIG. 6, the localizing of the EVD to one or more phases in the electrical distribution circuit begins with a comparison of the phase angle of the compensated negative sequence voltage to the localization reference phase angles for each phase of the distribution circuit - as indicated at STEP 67. A determination is then made at STEP 68 as to whether the phase angle of the compensated negative sequence voltage is within a pre-determined angular range of any one of the localization reference phase angles (e.g., +/- 15 degrees of any of the localization reference phase angles).

[0092] If it is determined at STEP 68 that the phase angle of the compensated negative sequence voltage is within a pre-determined angular range of any one of the localization reference phase angles, as indicated at 70, then the technique continues at STEP 72 by localizing the EVD fault to the particular phase corresponding to the localization reference phase angle to which the angle of the compensated negative sequence voltage is within the pre-determined angular range to. Examples of the EVD fault occurring on each of Phase A, B and C - and the describing thereof by way of Eqn. 26 - are set forth here below for purposes of illustration.

[0093] In an occurrence where the EVD is present in Phase A - i.e.,

A < 15 - then the EVD in Phases B and C are assigned as 0 , such that Eqn. 26 becomes:

EVD a = 3 * V 2, EVD [Eqn. 27].

[0094] As the magnitude of the EVD fault is being sought, the EVD fault in Phase C is calculated according to:

EVDa 2 EVD [0095] In an occurrence where the EVD is present in Phase B - i.e., < 15 - then the EVD in Phases A and C are assigned as

V _EVDA = 0 and V _EVDC ⁼ 0 , such that Eqn. 26 becomes: u ^r EVDb ^{J r} 2 ' EVD [Eqn. 28].

[0096] As the magnitude of the EVD fault is being sought, the a term can removed from Eqn. 28, such that the EVD fault in Phase B is calculated according to

V E, VDb = 3 * 2 EVD

[0097] In an occurrence where the EVD is present in Phase C - i.e., 5 ^° - then the EVD in Phases A and B are assigned as

V _EVDA = 0 and V E _} VDb ⁼ 0 , such that Eqn. 26 becomes: a ^u V ^v EVDc = 3 ^J * ^v 2EVD [Eqn. 29].

[0098] As the magnitude of the EVD fault is being sought, the a term can removed from Eqn. 29, such that the EVD fault in Phase C is calculated according to

EVDc ' ^{J y} 2 ~ EVD

[0099] Illustrations of FSI phasors indicating that the EVD is in Phase A, B or C are provided in FIGS. 7-9. As shown therein, each of the FSI phasors 74, 76, 78 has an angle that is within the pre-determined range of +/- 15 degrees of the localization reference phase angle for a respective one of Phase A, B or C.

[00100] Referring again now to FIG. 6, if it is instead determined at STEP 68 that the phase angle of the compensated negative sequence voltage is not within a predetermined angular range of any one of the localization reference phase angles, as indicated at 80, then the technique continues at STEP 82 by localizing the FSI (and EVD) to more than one phase of the electrical distribution system 10. In localizing the EVD to more than one phase, the FSI is resolved along multiple axes to quantify an amount of EVD in each of the phases to which the EVD has been localized. That is, the magnitude of the FSI is proportioned amongst two or more phases to determine the amount of EVD in each of the phases. Examples of the EVD fault occurring in differing combinations of Phases A, B and C - and the describing thereof by way of Eqn. 26 - are set forth here below for purposes of illustration.

[00101] In an occurrence where the EVD is present in Phases A and B - i.e., ref(p _2EVDa < . V _2EVD < ref(p _2EVDb - then the EVD in Phase C is assigned as

V _EVDC ⁼ 0■> ^suc h that Eqn. 26 becomes:

V ^v EVD a + ~ a ^u V ^y EVDb = ^J 3 * ^v 2EVD [Eqn. 30],

[00102] Rewriting Eqn. 30 in the complex domain and solving for real and imaginary parts yields:

sin(^ + 240 - ^J

[00103] Eqn. 31 thus enables the FSI to be resolved along multiple axes - Phases A and B - to quantify an amount of EVD in each of the phases to which the EVD has been localized. [00104] In an occurrence where the EVD is present in Phases B and C - i.e., ref( 2 _EVDb < ^-V _2EVD < reftp _2EVDc - then the EVD in Phase A is assigned as

V EVDa ⁼ 0■ _> such that Eqn. 26 becomes:

2 V ^¥ EVDb + ^τ a ^u V ^¥ EVDc 2EVD [Eqn. 32],

[00105] Rewriting Eqn. 32 in the complex domain and solving for real and imaginary parts yields:

sin( _c + 120 - 0^ - 240)

FSIn = \V EVDc 2EVD

33].

[00106] Eqn. 33 thus enables the FSI to be resolved along multiple axes - Phases B and C - to quantify an amount of EVD in each of the phases to which the EVD has been localized.

[00107] In an occurrence where the EVD is present in Phases A and C - i.e., ref ^r _2E vDc ^< V _JEVD ^{< re} f iEVDa - then the EVD in Phase B is assigned as V EVDb ⁼ 0 j such that Eqn. 26 becomes:

V ^y EVDa + ~ a ^u V ^v EVDc = 3 ^J * ^v 2EVD [Eqn. 34],

[00108] Rewriting Eqn. 34 in the complex domain and solving for real and imaginary parts yields:

FSI A = f EiVDa = 3*\V 2,EVD

sin(^+120-pj ^ ^η·35]

[00109] Eqn.35 thus enables the FSI to be resolved along multiple axes - Phases C and A - to quantify an amount of EVD in each of the phases to which the EVD has been localized.

[00110] An example of an FSI phasor present on more than one phase of the electrical distribution system 10 is provided in FIG.10 - with the illustrated example showing an FSI phasor 84 present on Phases A and B of the electrical distribution circuit. The magnitude of the FSI phasor indicates an amount of the EVD, with the particular angle of the FSI phasor 84 being resolved to quantify the amount of EVD in each of Phase A and B.

[00111] While examples are set forth above in which an EVD is localized to one or two phases, it is recognized that the EVD may be localized to all three phases of the distribution circuit. In cases where the EVD in each of the three phases varies greatly in severity, Eqn.9 (for a delta connected motor) and Eqn.26 (for a star connected motor) can be used to quantify the EVD in each phase - i.e., the negative sequence voltage can be analyzed. In cases where there is exactly equal (or approximately equal) EVD in all three phases, the EVD in each phase can be localized and quantified by using positive sequence equations according to:

(l-a ² )*V _EVDa +(a-l)*V _EVDb +(a ² -a)*V _EVD = 3HV _l -V _lM )

[Eqn.36] and y ^v EVD a ₊ ~ _a ^u y ^y EVDb + ' n ^u2 V ^v I EVDc 3 * (^ - V _lM ) [Eqn. 37], for delta and star connected motors, respectively.

[00112] In each case, the positive sequence voltage at the motor terminals, can be determined according to:

V - 7 * T + 7 * J [Eqn. 38], where Zn is the positive sequence impedance of the motor, Z ₁₂ is the coupling impedance between the positive sequence voltage and the negative sequence current, h is the positive sequence current, and is the negative sequence current.

[00113] Beneficially, embodiments of the invention thus provide a system and method for detecting, localizing and quantifying EVD faults in three-phase AC motor circuits by using the motor three-phase voltages and currents. As EVD in three-phase motor circuits leads to voltage unbalance at the motor terminals and consequent current unbalance, an EVD fault can be detected via the analysis and processing of measured three-phase voltages and currents. As a rule of thumb, a 1% voltage unbalance in supply could lead to 6-10% unbalance in motor current— and thus even small voltage drops in the range of 40-300 mV can lead to appreciable current unbalance leading to detection of the voltage drop, with the effects of noise factors like supply unbalance, motor faults and motor intrinsic unbalance being accounted for, such that an EVD fault can be accurately diagnosed. According to embodiments of the invention, the detected EVD fault can be localized and quantified in a single phase, two phases, or all three phases. Such detecting of the EVD faults, and the online monitoring of the electrical distribution circuit in general, can be performed from a location at the distribution circuit or at a location remote from the distribution circuit.

[00114] A technical contribution for the disclosed method and apparatus is that it provides for a processor-implemented technique for detecting, localizing and quantifying excess voltage drops in three-phase AC motor circuits. [00115] Therefore, according to one embodiment of the present invention, an electrical distribution circuit includes an input connectable to an AC source, an output connectable to terminals of an electrical machine, the output configured to provide three-phase voltages and currents to the electrical machine, and a diagnostic system configured to detect an excess voltage drop (EVD) in the electrical distribution circuit, the diagnostic system comprising a processor that is programmed to receive measurements of the three-phase voltages and currents provided to the electrical machine, compute a negative sequence voltage from the three-phase voltages and currents, determine a localization reference phase angle for each phase based in part on the three-phase voltages and currents, and calculate an EVD in the electrical distribution circuit based on the negative sequence voltage and the localization reference phase angles.

[00116] According to another embodiment of the present invention, a method for detecting excess voltage drop (EVD) in an electrical distribution circuit includes measuring three-phase voltages and currents provided to an electrical machine in the electrical distribution circuit by way of voltage and current sensors and causing a diagnostic system to calculate a fault severity index (FSI) based on the measured three- phase voltages and currents, wherein causing the diagnostic system to calculate the FSI further includes receiving the measured three-phase voltages and currents provided to the terminals of the electrical machine, computing a positive, negative, and zero sequence voltage and sequence current from the three-phase voltages, determining a compensated negative sequence voltage from the negative sequence voltage component, determining a localization reference phase angle for each phase based in part on the three-phase currents, and calculating the fault severity index (FSI) in the electrical distribution circuit based on the compensated negative sequence voltage and the localization reference phase angles.

[00117] According to yet another embodiment of the present invention, a diagnostic system configured to detect an excess voltage drop (EVD) fault in an electrical distribution circuit is provided. The diagnostic system includes a processor programmed to receive three-phase voltages and currents provided to terminals of an electrical machine connected to the electrical distribution circuit, compute positive sequence, negative sequence, and zero sequence voltage and current from the three- phase voltages and currents, determine a compensated negative sequence voltage from the negative sequence voltage component, determine a localization reference phasor for each phase based in part on the three-phase currents, identify an EVD fault in the electrical distribution circuit based on the compensated negative sequence voltage, and localize the EVD fault to one or more phases in the three-phase output based on a difference between a phase angle of the compensated negative sequence voltage and the localization reference phasors.

[00118] The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.