METHOD AND SYSTEM FOR SYNCHRONOUS MACHINE HEALTH MONITORING

BACKGROUND

[0001] Embodiments presented herein relate generally to electrical machines and more particularly to monitoring health of synchronous machines.

[0002] A synchronous machine is an AC electrical machine that operates at a speed synchronous with the power supply frequency. Synchronous machines include a stator that carries the armature winding, a rotor that carries the field winding, and a brush-slip ring assembly for exciting the field windings on the rotor. The field windings are excited by a DC supply voltage. The DC supply voltage may be provided from an external source (separately excited synchronous machines), or provided by a generator mounted on the rotor (self-excited synchronous machines). The field windings are typically insulated using, for example, insulating varnish.

[0003] During operation of the synchronous machine, the field insulation may degrade, due to various factors, such as, heat dissipation of the field winding, internal heating of the synchronous machine, partial discharge phenomena, dust, water (humidity, condensation, and unwanted submergence), mechanical forces, electrical disturbances, and so forth. Such degradation of the field insulation may cause short-circuits within the field windings, and short-circuits with ground. The winding insulation degradation eventually leads to complete breakdown of the winding insulation, and may result in catastrophic failure of the synchronous machine. To prevent such failure, early warning systems exist, that monitor winding failure degradation.

[0004] One such system relies on injecting a low frequency square wave signal into the field winding and identifying winding failure based on a measured response. However, such systems are typically restricted by a maximum field to ground resistance that can be measured for predicting failure. Further, such systems typically require the synchronous machine to be taken off line for testing. Such down-time may result in unwanted service outage and loss of revenue.

[0005] Some other known systems are based on a broadband frequency sweep signal injection. While such systems are not restricted by the maximum field to ground resistance limit, such systems typically require the synchronous machine to be taken off line for testing. Further, such systems require complex signal generators and significant computation capability for spectral analysis of response of the broadband frequency sweep signal.

[0006] Therefore, there is a need in the art for systems and methods that overcome these and other problems associated with known systems.

BRIEF DESCRIPTION

[0007] According to one embodiment, a method of monitoring a synchronous machine is presented. The method includes measuring a field voltage with respect to ground to monitor a self induced component of the field voltage. The method then generates a signature of the self induced component based on the measured field voltage. Finally, the method generates a field winding health indicator based, at least in part, on the signature of the self-induced component and a baseline signature.

[0008] According to another embodiment, a system for monitoring a synchronous machine is presented. The system includes a monitoring module for measuring a field voltage with respect to ground to monitor a self induced component of the field voltage. The system further includes a signal analyzer for generating a signature of the self induced component based on the measured field voltage. The system also includes a prognostic module for generating a field winding health indicator based, at least in part, on the signature of the self-induced component and a baseline signature.

[0009] According to yet another embodiment, a computer program product is presented. The computer program product includes a non-transitory computer readable medium encoded with computer-executable instructions, wherein the computer-executable instructions, when executed, cause one or more processors to: measure a field voltage with respect to ground to monitor a self induced component of the field voltage; generate a signature of the self induced component based on the measured field voltage; and generate a field winding health indicator based, at least in part, on the signature of the self-induced component and a baseline signature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 illustrates an example environment in which a synchronous machine monitoring system may operate, according to one embodiment;

[0011] FIG. 2 illustrates an example field voltage waveform including a self induced component, according to one embodiment;

[0012] FIG. 3 illustrates flux density distribution of a cross section of an example rotor, according to one embodiment;

[0013] FIG. 4 illustrates an example waveform of self induced component of field voltage, according to one embodiment;

[0014] FIGS. 5A-5C illustrate example stator structures, according to one embodiment;

[0015] FIGS. 6A-6C illustrate example waveforms of self induced component of field voltage, according to one embodiment;

[0016] FIG. 7 illustrates an example of synchronous machine monitoring system, according to one embodiment; and

[0017] FIG. 8 is a flowchart of an example method for monitoring a synchronous machine, according to one embodiment.

DETAILED DESCRIPTION

[0018] Embodiments presented herein describe techniques for monitoring health of synchronous machines. The techniques include monitoring field windings of synchronous machines by measuring a field voltage with respect to ground (referred to herein as the field to ground voltage) to monitor a self induced component of the field voltage. The self-induced component of the field voltage is generated due to inherent non-uniformities or asymmetry present in the synchronous machine structure. The inherent non-uniformities or asymmetry may include slot-tooth configuration, air gap eccentricity, bearing related faults, misalignment or axial non-uniformity conditions. Different characteristics of the self induced component of the field voltage, such as spectral information, amplitude information, resonance frequency condition and phase conditions, are then used to determine the health of the field winding. In one embodiment, the monitored spectral characteristics may be compared to expected, baseline, or healthy state spectral characteristics. For the purpose of this disclosure, the expected characteristic, baseline characteristic, and the health state characteristic may be used interchangeably. In various embodiments, the techniques described herein detect field winding insulation faults, such as field-ground faults, and inter-winding faults. The health monitoring of the synchronous machines, according to the embodiments presented herein, do not require the synchronous machine to be taken off-line. In other words, the health monitoring techniques described herein enable monitoring of online synchronous machines. Although the following description describes health monitoring for synchronous generators or alternators, die embodiments presented herein apply equally to other electrical machines as well.

[0019] FIG. 1 illustrates an example environment 100 in which a synchronous machine monitoring system may operate, according to one embodiment. The environment 100 includes a synchronous machine 110, an exciter and control unit 120, and a health monitoring system 140. The synchronous machine 110 may be connected to a load. Although FIG. 1 illustrates an electrical load connected to the synchronous machine 110, it should be appreciated that in implementations where the synchronous machine 110 is a synchronous motor, the load may be a mechanical load.

[0020] The synchronous machine 110 is an electromechanical energy conversion device where the rotor rotates at the same speed as the rotational speed of a rotating magnetic field. Example synchronous machines include synchronous generators, synchronous motors, and power factor compensators. The synchronous machine 110 may be switched between a motoring mode and a generating mode by changing the electrical connections. For instance, in mobile combustion engines, the synchronous machine 110 may be an integrated starter-generator. The synchronous machine 110 operates in the motoring mode, accepting electrical energy from an onboard battery to start the combustion engine. Once the combustion engine is fired up, control electronics switch the synchronous machine 110 to the generating mode, accepting mechanical energy from the combustion engine shaft, and generating electrical power. The synchronous machine 110 includes an armature winding 112 and a field winding 114. Typically, in low power and low torque applications the synchronous machine 110 may be of a rotating armature type including the armature winding 112 disposed on the rotor, and the field winding 114 disposed on the stator. In industrial applications involving high torque and high power, the synchronous machine 110 may be of a rotating field type including the armature winding 112 disposed on the stator, and the field winding 114 disposed on the rotor. The field winding 114 is connected to the exciter and control unit 120 via field terminals 116A and 116B.

[0021] The exciter and control unit 120 includes an exciter such as, but not limited to, a DC generator, a battery, a rectified AC supply, or a static exciter, to excite the field windings 114. A static exciter feeds back a portion of the AC from each phase of generator output to the field windings 114, as DC excitations, through a system of transformers, rectifiers, and reactors. An external DC source may be used for initial excitation of the field windings. The exciter applies an excitation voltage, herein referred to as field voltage to the field windings 114 of the synchronous machine 110, thereby causing a field current to flow through the field winding 114. Due to rotation of the field windings 114, the flux linked to stationary coils, disposed in a stator of the synchronous machine 110, varies in a sinusoidal fashion, causing a sinusoidal variation of voltage across the terminals of the stationary coils. The exciter and control unit 120 controls the operation of the synchronous machine 110. For example, the exciter and control unit 120 may control the field voltage and field current supplied to the field windings 114, so that the voltage at the output remains constant. Further, the exciter and control unit 120 may control the power delivered to the synchronous machine 110 or the power delivered from the synchronous machine 110. The exciter and control unit 120 may also control the power factor of the synchronous machine 110.

[0022] Prolonged use of the synchronous machine 110 may degrade lamination of the field winding 114 and may cause inter-turn faults, and ground faults. FIG. 1 illustrates an equivalent circuit for a ground fault 130. A ground fault is one where the lamination of the field winding 114 has degraded, thus causing a short circuit between the field winding 114 and the ground. Typically, the housing of the synchronous machine 110 is connected to ground for safety from electrical shocks. When the lamination of the field winding 114 degrades, a short circuit may result between the field winding 114 and the housing of the synchronous machine 110, thus causing the ground fault 130. The ground fault 130 may be modeled as a parallel connection of a resistance (representing a resistive component of the ground fault 130) and a capacitance (representing a capacitive component of the ground fault 130). The resistive component represents insulation resistance between the field winding 114 and ground, while the capacitive component represents the dielectric strength of the field winding insulation. In order to monitor the health of the insulation one may monitor the resistive and capacitive components or a parameter dependent on said components. For example, the resistive component will be very high in case of a healthy insulation, while as it ages, the insulation resistance decreases. In one embodiment, the health of the insulation is monitored through measurement of quantities that reflects these parameters, such as, but not limited to field to ground current and field to ground voltage.

[0023] Various embodiments presented herein may be applied to detect the ground fault 130 in the field winding. Embodiments presented herein are described for the rotating field type synchronous machine. However, it should be appreciated that the embodiments may apply equally to all types of synchronous machines.

[0024] The ground fault 130 may be detected by the monitoring system 140. The monitoring system 140 monitors the health of the synchronous machine using signal monitoring. The monitoring system 140 does not require injection of a probe signal to prognosticate the health of the synchronous machine. Inherent asymmetries in the synchronous machine 110 enable such an injection-free prognostic of the synchronous machine 110. FIG. 2 is an example graph illustrating a field voltage waveform plotted against time. FIG. 2 represents a field voltage signal 202 measured between the field terminal 116A and an arbitrary tap or point of the field winding f; and a field voltage signal 204 measured between the field terminal 116B and the arbitrary point f. Under ideal or theoretical conditions, the field voltage signals 202 and 204 would be a pure DC signal. However, practically (i.e. in real world applications) the illustrated field voltage signals 202 and 204 include a DC component, and a self induced AC component. The self induced AC component of the field voltage signals 202 and 204 represents the sub-harmonic oscillations caused due to inherent asymmetries in the synchronous machine 110.

[0025] Such asymmetry is caused due to factors such as inherent airgap eccentricity -the condition of an unequal airgap that exists between the stator and the rotor due to rotor sagging. Airgap eccentricity may be static or dynamic. Static eccentricity is characterized by a displacement of the axis of rotation where the position of the minimal airgap length is fixed in space. Static eccentricity is caused by the stator ovality or by the incorrect positioning of the rotor or stator at the commissioning stage.

[0026] Static eccentricity may cause dynamic eccentricity, too. Dynamic eccentricity refers to the effect of the rotor not rotating on its own axis, and the minimum airgap rotates with the rotor. Dynamic eccentricity may be caused by a bent shaft, mechanical resonance, bearing wear or misalignment, or even static eccentricity as mentioned previously. Therefore, the non uniform airgap of a certain spatial position is sinusoidally modulated and results in an asymmetric magnetic field. FIG. 3 illustrates an example rotor 300, with magnetic flux density and a flux contour plot, according to one embodiment. The rotor 300 includes 4 poles 302, 304,306, and 308 each having a field coil. FIG. 4 is an example graph of the self induced component waveforms of field voltage measured across two separate field coils wound on adjacent poles i.e. pole 302 and pole 304. As seen in FIG. 4 the two self induced component waveforms are substantially sinusoidal. The two self induced component waveforms exhibit a phase difference due to the spatial position of the field coils on the rotor 300.

[0027] The slot-tooth configuration in an electrical machine represents one type of a stator configurations. A slot-tooth stator comprises a number of teeth at a specified separation from each other on the inside of a stator back iron. The voids between the teeth are known as slots. The slots carry the stator windings. The slot-tooth structure in an electrical machine may be configured for mechanical support, protection from abrasion, and may include cooling channels or grooves in the stator back iron. Such channels or grooves introduce non-uniformity in the stator magnetic circuit. FIGS. 5A, 5B, and 5C illustrate sections of the stator having no grooves, one groove 502, and three grooves 504 A, 504B, and 504C, respectively. The configuration of such grooves affects the self-induced component of field voltage. For instance, FIGS. 6A, 6B, and 6C illustrate the self-induced component of field voltage for the stator configurations illustrated in FIGS. 5A, 5B, and 5C, respectively. Typically, the number of such grooves (or stator asymmetries in general), directly affects the magnitude of the self-induced component of field voltage.

[0028] Due to all these various factors that causes non-uniform airgap, the reluctance seen by the rotor at one particular instant is different from the next instant when it is rotating. The flux generated in the windings may be calculated using formula: Flux = MMF/ Reluctance, wherein MMF (Magneto Motive Force) is fixed. While reluctance seen by the rotor at one particular instant is different from the next instant when it is rotating because of non uniform air gap. Once the field pole encounters a change in flux, field coils see a change in flux linking with them. Using the formula, e = [-N *(Change in flux)/ (change in time)], it induces a voltage in the field coils apart from the enforced field voltage. The above generated EMF (electromotive force) is known as the self-induced component of the field voltage. The above stated method for calculating self-induced component in the field windings is applicable for any kind of non-uniformities or asymmetry in the synchronous machine embodiments.

[0029] In FIG. 1, the health monitoring system 140 is connected to the field winding 114 via the field terminals 116A and 116B. Referring now to FIG. 7, an example synchronous machine monitoring system is illustrated, according to one embodiment. The monitoring system 140 includes a monitoring module 710, a spectrum analyzer 720, and a prognostic module 730. The monitoring system 140 may also include a signature data store 740, an operating state monitor 750, and a resistance computation module 760.
The monitoring module 710 measures the field to ground voltage at field terminals 116A or U6B or both, to monitor the self induced component of the field to ground voltage. For example, in a fault detection mode, to detect presence of a ground fault 130 or an impending ground fault, the monitoring module 710 may measure the field to ground voltage at only one field terminal 116A or 116B. In a fault location mode, the monitoring module 710 may measure the field to ground voltage at both field terminals 116A and U6B, to locate the position of the ground fault 130 within the field winding 114. In the fault location mode, the monitoring module 710 may intermittently measure the field to ground voltage at both field terminals 116A and 116B in staggered synchronization, such that the monitoring module 710 monitors the field to ground voltage at field terminal 116A at one monitoring interval and then at field terminal 116B at the next successive monitoring interval. Alternatively, the monitoring module 710 may measure the field to ground voltage at both field terminals 116A and 116B simultaneously, at each monitoring interval.

[0030] As discussed in conjunction with FIG. 1, the self-induced component of the field to ground voltage is indicative of presence of the ground fault 130, the severity of the ground fault 130, and the location of the ground fault 130 within the field windings 114. The monitoring module 710 may measure the field to ground voltage continuously in the protection mode. Alternatively, the monitoring module 710 may measure the field to ground voltage intermittently, after fixed intervals of time.

[0031] In one example implementation, the monitoring module 710 includes a high precision resistor of a known value connected between the field terminal 116A or 116B and ground. The monitoring module 710 measures the voltage drop across the high precision resistor to measure the field to ground voltage. The monitoring module 710 measures the current flowing through the high precision resistor to measure the field to ground current.

[0032] The spectrum analyzer 720 then computes a field to ground spectral signature from the measured field to ground voltage. The spectrum analyzer 720 may identify spectral features from the field to ground spectral signature such as frequency components of the monitored field to ground voltage, the amplitudes of the identified frequency components, the phase difference between the field to ground voltage and the field to ground current, and so forth.

[0033] In one implementation, the spectrum analyzer 720 may include a Fourier transform module, for example, a Fast Fourier Transform module, to compute the field to ground spectral signature based on the measured field to ground voltage. The spectral signature includes the frequency content of the measured field to ground voltage, plotted against amplitude of the various frequencies. The spectrum analyzer 720 may then use, for example, curve fitting algorithms, or peak detection algorithms, to identify peaks of amplitude in the field to ground spectral signature.

[0034] Typically, for a synchronous machine 110 with a healthy field winding 114, the self induced component of field voltage exhibits a small amplitude. However, with degradation of the field winding insulation and an impending ground fault condition, the amplitude of the self-induced component of field voltage increases in magnitude.

[0035] The prognostic module 730 then compares the computed spectral signature with a baseline signature. The baseline signature may be identified from mathematical analysis, or simulations or baseline experiments. Alternatively, the baseline signature may be identified by the monitoring system 140 while the synchronous machine 110 is still new. The prognostic module 730 then generates a winding health indicator based on the difference in the computed spectral signature and the baseline signature for the field winding 114.

[0036] In one implementation, the prognostic module 730 may compare spectral features of the computed spectral signature, such as, but not limited to, the frequency components present in the spectral signature, the amplitude or intensity of the frequency components, and so forth. The prognostic module 730 may then compare one or more of the spectral features with the spectral features of the baseline signature. The baseline signature itself may be a data record with the values corresponding to the frequencies of the frequency components, and values corresponding to the values of the frequency components, arranged, for example, in ordered pairs.

[0037] The baseline spectral signatures may be stored in a signature data store 740. The signature data store 740 may store a baseline signature specific to a particular model of the synchronous machine 110. Alternatively, the signature data store 740 may store multiple baseline signatures, each corresponding to a particular range of models of the synchronous machine 110, tagged with structural parameters of the synchronous machine 110, such as, but not limited to, the number of poles, and structural discontinuities in the stator core. For instance, the structural discontinuities in the stator core may be in the form of grooves disposed at regular spacing in the stator core including cooling vents, mounting slots, and so forth. Such structural discontinuities and the number of poles influence the characteristics frequency of the self-induced component of the field voltage, such as, but not limited to, frequency, amplitude, and phase. The structural parameters may be stored as numbers that may be used as scaling factors for a base frequency of the self-induced component of the field voltage.

[0038] The base frequency of the self-induced component of the field voltage may be dependent on various operating parameters of the synchronous machine. Such operating parameters include, without limitation, operating speed of the synchronous machine. The signature data store 740 may include different baseline signatures for various operating speeds of the synchronous machine. Alternatively, the signature data store 740 may have stored thereon, the baseline signature associated with various scaling factors related to the operating speed of the synchronous machine. Another factor affecting the frequency of the self-induced component of the field voltage is stator current harmonics. The stator current harmonics are AC components of the voltage present in the stator winding. The stator current harmonics are generated due to varying load conditions. The stator current harmonics may be induced in the field windings 114 and may appear as a distinct frequency component in the self-induced field voltage.

[0039] The operating state monitor 750 may detect such structural parameters, and operating state parameters of the synchronous machine 110, via a network of sensors disposed on the synchronous machine 110. Such sensors may include any known sensors, such as, speed sensors, stator AC signal sensors, load sensors, and so forth. The structural parameters may be identified by an identification chip on the synchronous machine 110.

[0040] Once the ground fault 130 has been detected, the health monitoring system 140 may switch to the fault location mode. The fault may be located in any section of the field winding. The monitoring module 710 measures the field to ground voltage and the field to ground current at field terminal 116A. The spectrum analyzer 720 men identifies the frequency and amplitude of the self-induced component of the field voltage. The impedance computation module 760 then computes a first value of insulation impedance at the identified resonant frequency. In an identical manner, the process is repeated for the opposite side of the field windings 114. In other words, and the monitoring module 710 measures the field to ground voltage and field to ground current at field terminal 116B. The impedance computation module 760 then computes a second value of insulation impedance at the identified frequency of the self-induced component of field voltage. Knowing the value of the field winding resistance, the difference between the insulation impedances at the two field terminals 116A and 116B yields information as to the location of the ground fault 130. The location of the ground fault 130 is indicated as in terms of the percentage of field winding from one end of the field winding 114 at which the insulation failure has occurred.

[0041] It is to be understood that the winding health indicator generated by the prognostic module 740 may be viewed at a location remote from the synchronous machine 110. The prognostic module 740 of the monitoring system 140 may be coupled with an output device (not shown), for example, by means of wireless communication, in order to transmit the winding health indicator data generated by the prognostic module 740. Further, the winding health indicator may be categorized in different levels based on the deviation value of identified resonance frequency and the healthy state resonance frequency. For example, in case of very high deviation values, the winding health indicator may be represented with a red light and audible alarm This may be an indication that the field winding 114 may have undergone substantial insulation damage for which the field winding 114 may need immediate inspection.

[0042] In one example implementation, the various functions of the spectrum analyzer 720 and the prognostic module 730 are implemented as software instructions capable of being executed on a processor. In such an implementation, the software instructions may be stored on a non-transitory computer readable medium such as, but not limited to, hard disc drives, solid state memory devices, random access memory (RAM) linked with the processor, and so forth. The processor may be, for example, a general purpose microprocessor, a microcontroller, a programmable logic device, and so forth. An example computer system including such an implementation of the processor, may also include peripheral input devices such as a keyboard and a pointing device, peripheral output devices such as a visual display unit, and one or more network interfaces such as a Bluetooth ® adaptor, an IEEE 802.11 interface, an IEEE 802.3 Ethernet adaptor, and so forth. Alternatively, the processor may be implemented as a special purpose processor including the various modules hard-coded into the special purpose processor. Components of the computer system may be linked by one or more system busses. It should be appreciated that computer system described herein is illustrative and non-limiting. Other implementations of the computer system are within the scope of the present disclosure.

[0043] FIG. 8 is a flowchart illustrating a process 800 for synchronous machine health monitoring, according to an embodiment. At step 802 a field voltage with respect to ground is measured to monitor a self-induced voltage component. The self induced component in the field windings may result because of the inherent non-uniformities or asymmetry present in the motor structure.

[0044] At step 804, a signature of the self-induced component of the field voltage is generated, based on the measured field to ground voltage. The signature of the self-induced component can be generated using the signal analyzer 720. The signal analyzer 720 examines the spectral composition of electrical voltage waveforms. The signature of self-induced component is then supplied to the prognostic module 714 for generating a health indicator.

[0045] At step 806, the signature of self-induced component based on the measured field voltage is compared with at least a known baseline signature of the field ground fault. The signature of self-induced component is retrieved from signal analyzer 720, while the baseline signature is retrieved from signature data store 750. The baseline signature may be retrieved based on structural parameters of the synchronous machine 110, such as a number of poles, and structural discontinuities in the stator core. The baseline signature may also be retrieved based on operational parameters of the synchronous machine 110, such as the operating speed of the synchronous machine 110 and stator current harmonics. Differences in the signature of self-induced component and baseline signature may then be used for generating a health indicator, which indicates the presence of ground fault in the field windings of a synchronous machine. The health indicator may be a visual warning or an audio alarm or a combination of both.

CLAIMS

1. A method of monitoring a synchronous machine comprising:

measuring a field voltage with respect to ground to monitor a self induced component of the field voltage;

generating a signature of the self induced component based on the measured field voltage; and

generating a field winding health indicator based, at least in part, on the signature of the self-induced component and a baseline signature.

2. The method of claim 1 wherein the self induced component of the field voltage is generated based on at least one of structural asymmetry of the synchronous machine, non-uniformities in the magnetic circuit of the synchronous machine, and loading conditions of the synchronous machine.

3. The method of claim 1 further comprising:

identifying structural parameters of the synchronous machine, comprising at least one of a number of poles, and structural discontinuities in the stator core; and

identifying one of a plurality of known signatures as the baseline signature based on the structural parameters.

4. The method of claim 1 further comprising:

identifying operating parameters of the synchronous machine, comprising at least one of an operating speed, and a stator current harmonic signal; and

generating the field winding health indicator taking into account the operating parameters.

5. The method of claim I further comprising:

computing a ground fault resistance value based on the field voltage; and generating the field winding health indicator taking into account the ground fault resistance.

6. A system for monitoring a synchronous machine comprising:

a monitoring module for measuring a field voltage with respect to ground to monitor a self induced component of the field voltage;

a signal analyzer for generating a signature of the self induced component based on the measured field voltage; and

a prognostic module for generating a field winding health indicator based, at least in part, on the signature of the self-induced component and a baseline signature.

7. The system of claim 6, wherein the self induced component of the field voltage is generated based on at least one of structural asymmetry of the synchronous machine, non-uniformities in the magnetic circuit of the synchronous machine, and loading conditions of the synchronous machine.

8. The system of claim 6 further comprising a signature data store for storing a plurality of known signatures and at least one structural parameter associated with each of the plurality of known signatures, wherein at the structural parameter comprises a number of poles, and stator core structural discontinuity information.

9. The system of claim 6 further comprising an operating state monitor for identifying operating parameters of the synchronous machine comprising at least one of an operating speed, and a stator current harmonic signal; and wherein the prognostic module generates the field winding health indicator taking into account the operating parameters.

10. The system of claim 6 further comprising a resistance computation module for computing a ground fault resistance value based on the field voltage; and wherein the prognostic module generates the field winding health indicator taking into account the ground fault resistance.