Circuit breaker and method for sensing current indirectly from bimetal voltage and determining bimetal temperature and corrected temperature dependent bimetal resistance

A circuit breaker includes a bimetal electrically connected in series with separable contacts, and an operating mechanism structured to open and close the contacts. A temperature sensor distal from the bimetal includes an output having an ambient temperature signal. A trip circuit includes a first circuit having a first input electrically interconnected with a bimetal output to input a voltage representative of current, a second input electrically interconnected with the temperature sensor output to input the ambient temperature signal, a real-time thermal model structured to provide a corrected temperature dependent bimetal resistance as a function of the voltage and the ambient temperature signal, and an output including a current value which is a function of the ambient temperature signal and the corrected bimetal resistance. A second circuit includes an input having the current value and an output structured to actuate the operating mechanism in response to predetermined current conditions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to circuit interrupters and, more particularly, to circuit breakers including a bimetal in series with separable contacts. The invention also pertains to methods for determining bimetal temperature and/or bimetal resistance.

2. Background Information

Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.

In certain circuit breaker applications (e.g., without limitation, arc fault detection), the voltage across the thermal bimetal element is employed as an indirect measurement of the circuit breaker load current. Sensing current with a bimetal element is complicated by the variation of that element's impedance as a function of temperature. This variation results in inaccuracies in the measurement of the amplitude of the measured current. For example, the bimetal element's impedance can vary as much as about 70% with temperature over the normal operating range of the circuit breaker depending upon the type of bimetallic material used.

As is typical with most metals, the bimetal impedance has a positive temperature coefficient (PTC). In other words, resistance increases with temperature. If the design of the circuit breaker electronics assumes that the bimetal resistance is constant, then any resistance-temperature variation of the bimetal can introduce error in the current sensing. This variation can cause proportional inaccuracies in the measurement of the load current, thereby affecting performance of control algorithms implemented in the circuit breaker electronics. As bimetal temperature rises, the voltage drop across the bimetal increases for a given amount of load current. The net effect is that the load current appears larger than it really is. Depending on the control algorithm, the potential result could be an errant command to trip the circuit breaker.

U.S. Pat. No. 4,486,733 discloses an electrothermal responsive protective mechanism of the bimetal type having also an ambient temperature compensating bimetal element to maintain accurate operation of the protective mechanism despite ambient temperature changes.

U.S. Pat. No. 6,813,131 discloses a circuit breaker trip assembly including a bimetal element and a thermistor. The thermistor is a temperature-sensing device, which is adapted to respond to the temperature of the bimetal element, in order to measure the approximate temperature thereof. For example, if located in close proximity to the bimetal element, the thermistor will be subjected to less extreme temperatures since the power dissipated by the bimetal element is dispersed to the cooler ambient environment within the circuit breaker housing. Even in close proximity, the temperature sensed by the thermistor proximate the bimetal element, although less extreme, is still proportional to the actual temperature of the bimetal element. An amplifier input is referenced to the bimetal element output terminal. The bimetal impedance has a positive temperature coefficient, while the thermistor has a negative temperature coefficient. Responsive to the thermistor, the amplifier provides a negative gain with respect to the bimetal voltage. In turn, the trip assembly provides a trip signal as a function of the compensated amplifier output voltage.

There is room for improvement in circuit breakers including a bimetal in series with separable contacts.

There is also room for improvement in methods for determining bimetal temperature and/or bimetal resistance.

SUMMARY OF THE INVENTION

These needs and others are met by the present invention, which compensates for variations of bimetal electrical resistance with temperature when sensing circuit breaker current indirectly by bimetal voltage. Significantly, a bimetal temperature sensor that is in physical contact with or proximate to the bimetal is not required.

In accordance with one aspect of the invention, a circuit breaker comprises: a housing; separable contacts; a bimetal electrically connected in series with the separable contacts, the bimetal including a temperature dependent resistance and an output having a voltage representative of current flowing through the separable contacts; an operating mechanism structured to open and close the separable contacts; a temperature sensor distal from the bimetal, the temperature sensor including an output having a signal representative of ambient temperature; and a trip circuit cooperating with the operating mechanism to trip open the separable contacts, the trip circuit comprising: a first circuit including a first input electrically interconnected with the output of the bimetal to input the voltage representative of current flowing through the separable contacts, a second input electrically interconnected with the output of the temperature sensor to input the signal representative of ambient temperature, a real-time thermal model structured to provide a corrected temperature dependent resistance of the bimetal as a function of the voltage representative of current flowing through the separable contacts and the signal representative of ambient temperature, and an output including a current value which is a function of the voltage representative of current flowing through the separable contacts and the corrected temperature dependent resistance, and a second circuit including an input having the current value and an output structured to actuate the operating mechanism in response to predetermined current conditions.

The first circuit may comprise a processor structured to repetitively execute an iterative algorithm as the real-time thermal model.

The processor may be further structured to periodically input the voltage representative of current flowing through the separable contacts and the signal representative of ambient temperature.

The processor may be further structured on an initial iteration of the iterative algorithm to determine (a) an initial absolute temperature of the bimetal from the ambient temperature plus a predetermined value, (b) an instantaneous power dissipated by the bimetal, and (c) a temperature rise of the bimetal over the ambient temperature, and on a subsequent iteration of the iterative algorithm to determine (d) a subsequent absolute temperature of the bimetal from a subsequent input of the ambient temperature plus the temperature rise of the bimetal over the ambient temperature, (e) a subsequent instantaneous power dissipated by the bimetal, and (f) a subsequent temperature rise of the bimetal over the subsequent inputted ambient temperature.

The processor may be further structured to calculate the corrected temperature dependent resistance of the bimetal as a predetermined function of the subsequent absolute temperature of the bimetal.

The processor may be further structured to calculate the subsequent instantaneous power dissipated by the bimetal from the square of the voltage of the bimetal divided by the corrected temperature dependent resistance of the bimetal.

The processor may be further structured on another iteration after the subsequent iteration to determine (g) another absolute temperature of the bimetal from another input of the ambient temperature plus the subsequent temperature rise of the bimetal, (h) another instantaneous power dissipated by the bimetal for the another iteration after the subsequent iteration, and (i) another temperature rise of the bimetal over the other inputted ambient temperature.

The processor may be further structured to save the subsequent instantaneous power dissipated by the bimetal and the subsequent temperature rise of the bimetal for use by another iteration after the subsequent iteration.

As another aspect of the invention, a method of determining a temperature of a bimetal electrically connected in series with separable contacts and including an output having a voltage representative of current flowing through the separable contacts, comprises: sensing a temperature representative of ambient temperature; inputting the voltage representative of current flowing through the separable contacts; and employing a real-time thermal model to determine the temperature of the bimetal from the sensed temperature representative of ambient temperature and the voltage representative of current flowing through the separable contacts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As employed herein, the statement that a part is “electrically interconnected with” one or more other parts shall mean that the parts are directly electrically connected together or are electrically connected together through one or more electrical conductors or generally electrically conductive intermediate parts. Further, as employed herein, the statement that a part is “electrically connected to” one or more other parts shall mean that the parts are directly electrically connected together or are electrically connected together through one or more electrical conductors.

The present invention is described in association with an arc fault circuit breaker, although the invention is applicable to a wide range of circuit interrupters.

Referring toFIG. 1, a circuit breaker2includes a housing4, separable contacts6and a bimetal8electrically connected in series with the separable contacts6. The bimetal8includes a temperature dependent resistance10(FIG. 2) and an output12having a voltage14(FIG. 2) representative of current16(FIG. 2) flowing through the separable contacts6. An operating mechanism18is structured to open and close the separable contacts6. A temperature sensor (T)20is disposed distal from the bimetal8and includes an output22having a signal24representative of ambient temperature.

A trip circuit26cooperates with the operating mechanism18to trip open the separable contacts6. The trip circuit26includes a first circuit28providing a real-time thermal model function29and a second circuit30providing a trip function31(e.g., without limitation, arc fault detection (AFD)). The first circuit28includes a first input32electrically interconnected with the output12of the bimetal8to input the voltage14(FIG. 2) representative of the current16(FIG. 2) flowing through the separable contacts6, and a second input34electrically interconnected with the output22of the temperature sensor20to input the signal24representative of ambient temperature. As will be discussed below in connection withFIGS. 2-4, the real-time thermal model function29is structured to provide a corrected temperature dependent resistance36(FIGS. 3 and 4) of the bimetal8as a function of the bimetal voltage14(FIG. 2) and the signal24representative of ambient temperature. In turn, the thermal model function29provides an output38including a bimetal current value40which is a function of the bimetal voltage14and the corrected temperature dependent bimetal resistance36. The AFD trip function31includes an input42having the current value40and an output44structured to actuate the operating mechanism18in response to predetermined current conditions.

Examples of suitable arc fault detectors are disclosed, for instance, in U.S. Pat. No. 5,224,006, with a preferred type described in U.S. Pat. No. 5,691,869, which is hereby incorporated by reference herein.

The first circuit28includes a suitable processor (μP)46structured to repetitively execute an iterative algorithm (58ofFIG. 3or58′ ofFIG. 4) as the real-time thermal model function29. The μP46includes or cooperates with an analog-to-digital converter (ADC)48and is structured to periodically input the voltage14representative of current flowing through the separable contacts6and the signal24(e.g., without limitation, voltage) representative of ambient temperature. As will be discussed below in connection withFIGS. 2-4, the μP46measures bimetal voltage14and circuit breaker ambient temperature, employs the thermal model function29to estimate bimetal temperature50(FIGS. 3 and 4) and bimetal electrical resistance36(FIGS. 3 and 4), and derives the circuit breaker current86(FIGS. 3 and 4) from these quantities. In particular, the μP46senses the voltage24representing ambient temperature and the bimetal voltage14using the ADC48, and processes that information using the algorithm58ofFIG. 3or58′ ofFIG. 4to provide the circuit breaker current86.

The ambient temperature is within the circuit breaker housing4. The μP46employs the temperature sensor20(e.g., without limitation, a thermistor, disposed distal (i.e., a suitable distance in order to measure ambient temperature) from the bimetal8) to sense the circuit breaker internal ambient temperature. Then, the μP46employs the bimetal voltage14(FIG. 2) and a thermal model51(FIG. 2) of the bimetal8and the circuit breaker2to estimate the transient bimetal temperature rise52(FIG. 2) above the circuit breaker internal ambient temperature54(FIG. 2). The estimated transient bimetal temperature rise52and the internal ambient temperature54are employed to estimate the absolute bimetal temperature55(FIG. 2) and to determine the bimetal resistance10, which is then employed to estimate the actual circuit breaker current16.

The μP46preferably discrete-time samples the bimetal voltage14(FIG. 2) at a suitable sampling frequency that is equal to or greater than the Nyquist rate (i.e., rapid enough that the entire spectral content of the voltage waveform is captured). See, for example, the sampling period, Ts, of Equation 4, below.

Although a processor-based current sensing mechanism is disclosed, discrete digital electronic components and/or a continuous-time system (e.g., without limitation, employing analog electronics) and/or combinations thereof may be employed. Alternatively, other suitable current sensing mechanisms may be employed. One example includes analog/digital hybrid bimetal voltage sensing, in which the half-cycle integral or peak of the bimetal voltage is first determined with an analog circuit and then is digitally sampled.

FIG. 2shows a linked first-order continuous-time electrical model56and the thermal model51of the bimetal8and the associated circuit breaker2(FIG. 1). This model may be replaced by a more detailed model (Example 6, below) if increased accuracy is desired. The thermal resistance of the bimetal8, RΘ(° C./W), to the circuit breaker internal ambient temperature (° C.)54models transient and steady state heat loss from the bimetal8including conduction, convection and radiation. The thermal capacitance of the bimetal, CΘ(J/° C.), models transient and steady state temperature rise in the bimetal8due to power dissipation. The estimated instantaneous power dissipated by the bimetal8is given by Qbimetal(t) (W). ΔT(t) is the estimated temperature rise52of the bimetal8over the circuit breaker internal ambient temperature54versus time, t. ΔT(t) is determined by solving the continuous-time expression of Equation 1.

QCΘ(t) is the net instantaneous power transfer into the circuit breaker bimetal8.

Using Kirchhoff's law for heat flow, Equation 1 is rewritten to provide the expression of Equation 2.

Equation 3 provides a corresponding Laplace domain expression from Equation 2.

s is defined from £−1{ΔT(s)}=ΔT(t);

£−1is the Inverse Laplace Transform Operator; and

Equation 4 provides a substitution for Equation 3 to derive a discrete-time version of the above continuous-time model through backward rectangular integration.

Tsis sampling period (seconds) (e.g., without limitation, about 1 ms for a 60 Hz line cycle; a suitable period such that the sampling frequency is equal to or greater than the Nyquist rate);

z is the discrete-time system equivalent of the Laplace operator s found in continuous-time systems, and is defined from Z−1{ΔT(z)}=ΔT(n);

Z−1is the Inverse Z Transform Operator; and

Equation 5 represents the discrete-time equivalent of the thermal model51.

n is an integer, which is greater than or equal to zero;

ΔT(n) is the estimated temperature rise of the bimetal8over the circuit breaker internal ambient temperature (° C.) for sample n;

Qbimetal(n) is the estimated instantaneous power (W) dissipated by the bimetal8for sample n; and

ΔT(n−1) is the estimated temperature rise (° C.) of the bimetal8over the circuit breaker internal ambient temperature for sample n−1.

Relatively better performance in the discrete-time model51is achieved by employing a better integration method, such as trapezoidal integration. Equation 6 provides a suitable substitution for Equation 3.

Equation 7 represents a relatively more accurate discrete-time equivalent of the thermal model51.

Qbimetal(n−1) is the estimated instantaneous power (W) dissipated by the bimetal8for sample n−1.

FIG. 3shows the algorithm58employing the thermal model of Example 6. First, at60, integer n, which represents a sample number, is set to zero. Next, at62, a suitable initial bimetal temperature rise above ambient, ΔT(0), is set to value u for sample 0, which value is a predetermined value (° C.) (e.g., without limitation, zero; about zero; any suitable value). Then, at64, a suitable initial estimated instantaneous power dissipated by the bimetal, Qbimetal(0), is set to value v for sample 0, which value is a predetermined value (W) (e.g., without limitation, zero; about zero; any suitable value). Then, at66, the integer, n, is incremented. Next, at68and70, the voltage of the bimetal8, Vbimetal(n) (V), and the circuit breaker internal ambient temperature, Tambient(n) (° C.), are respectively measured through the ADC48(FIG. 1). Next, at72, the estimated absolute temperature 50 of the bimetal (° C.) for sample n is determined from Equation 8.
Tbimetal(n)=Tambient(n)+ΔT(n−1)  (Eq. 8)
wherein:

Tambient(n) is the absolute temperature of the circuit breaker internal ambient (° C.), for example, as measured by the thermistor20(FIG. 1); and

ΔT(n−1) is the previously estimated temperature rise of the bimetal8over the circuit breaker internal ambient temperature (° C.) for sample n−1.

fr(T) is a function (e.g., without limitation, derived from data obtained from the bimetal manufacturer; nearly a linear function; approximated by a linear function; a hash function) that represents or approximates the variation of bimetal resistance (Ω) with bimetal temperature (° C.). This provides the corrected temperature dependent bimetal resistance as a predetermined function of the absolute temperature of the bimetal8.

Next, at76, the estimated instantaneous electric current86flowing through the bimetal, ibimetal(n) (A), is determined for sample n from Equation 10.

Then, at78, the estimated instantaneous power dissipated by the bimetal, Qbimetal(n) (W), is determined from one of Equations 11 and 12.

Next, at80, ΔT(n) is determined based on the thermal model from Equation 7, above.

Finally, at82and84ΔT(n) and Qbimetal(n) are respectively saved for use in the next iteration (sample n+1), which is repeated beginning at step66.

Hence, the μP46determines the following on an initial iteration of the iterative algorithm58: (a) an initial absolute temperature of the bimetal8from the ambient temperature plus a predetermined value, u, at step62, (b) an instantaneous power dissipated by the bimetal8at step78, and (c) a temperature rise of the bimetal8over the ambient temperature at step80. Then, on the subsequent iteration of the iterative algorithm58, the μP46determines: (d) a subsequent absolute temperature of the bimetal8from a subsequent input of the ambient temperature plus the temperature rise of the bimetal over the ambient temperature at step72, (e) a subsequent instantaneous power dissipated by the bimetal at step78, and (f) a subsequent temperature rise of the bimetal over the subsequent inputted ambient temperature at step80. This process continues on a subsequent iteration of the iterative algorithm58to refine the absolute temperature of the bimetal8from another input of the ambient temperature plus the subsequent temperature rise of the bimetal8, another instantaneous power dissipated by the bimetal8, and another temperature rise of the bimetal8over the last input of the ambient temperature.

FIG. 4shows the algorithm58′ employing the thermal model of Example 5. This algorithm58′ is similar to the algorithm58ofFIG. 3except that step64, Qbimetal(0)=ν, and step84, save Qbimetal(n) for use in the next iteration, are not employed, and except that Equation 5 is used in place of Equation 7 at step80′.

A transient error in the estimated current86may arise if ΔT(0) (the assumed initial temperature rise of the bimetal8over ambient temperature at step62ofFIGS. 3 and 4) is different than the actual initial temperature rise of the bimetal8over ambient temperature. This error may be no worse than that of a conventional circuit breaker trip unit (which has no compensation for bimetal resistance variation with temperature). Furthermore, given time, the error will decay to zero. One way to address this error is to initially set ΔT(0) equal to zero, in order that the estimated bimetal resistance36, Rbimetal(n) is minimized and, thus, ibimetal(n)86must be equal to or greater than the actual bimetal current16(FIG. 2). This approach offers a way to err on the safe side, but could potentially cause circuit breaker nuisance trips in some instances.

If the thermal models of Examples 5 and 6 are not parameterized precisely, they may still yield useful information as long as the estimated bimetal electrical resistance36approximates, but does not exceed, the actual bimetal electrical resistance10(FIG. 2). In this case, the temperature-related error in the current measurement will be nonzero, but will be less than that of a circuit breaker with no temperature compensation.

If the estimated bimetal electrical resistance36(as calculated by the algorithms58,58′) becomes greater than the actual bimetal electrical resistance10(FIG. 2), then the estimated circuit breaker current86is less than the actual circuit breaker current16(FIG. 2). Hence, the circuit breaker2and bimetal thermal model parameters (RΘand CΘ) must be suitably chosen to ensure that this case does not occur.

The example trip circuit26includes an armature88, which is attracted by the large magnetic force generated by very high overcurrents to also actuate the operating mechanism18and provide an instantaneous trip function.

A trip signal90is generated at the AFD output44in order to turn on a suitable switch, such as the silicon controlled rectifier (SCR)92, to energize a trip solenoid94. The trip solenoid94, when energized, actuates the operating mechanism18to trip open the separable contacts6. A resistor96in series with the coil of the solenoid94(or the resistance of the coil if the resistor96is not used) limits the coil current and a capacitor98protects the gate of the SCR92from voltage spikes and false tripping due to noise.