Patent Description:
The invention deals with installation devices comprising electronic circuitry and one or more current transformers arranged such that it can measure the electrical current flowing in a main circuit comprising one or more conductors. The installation device may be employed for various purposes such as metering, control, monitoring, and protection of electrical equipment or electrical loads. Examples of such installation devices are electronic overload relays, circuit breakers, protective relays, motor starters, motor controllers, etc..

An important feature of installation devices is their operating current range. Wider current range allows the manufacturers of installation devices to cover a broad application range with less variants of the product, resulting in simplified offering and lower production and logistic costs. Wider current range is also desired by the user as it allows for enhanced application flexibility, simplified logistics, and lower inherent costs.

In installation devices concerned by the present invention sensing of the electrical current flowing in the main circuit is achieved by means of one or more current transformers. The secondary terminals of the transformers are connected to an electronic circuit where the secondary current is processed and employed to achieve the required functionalities of the installation device. The secondary current is usually rectified in order to simplify the sensing and processing circuits. In self-powered installation devices, such as electronic overload relays or circuit breakers, the secondary current of the transformers may also be used to power the electronic circuit. In such cases, a voltage regulator is also provided to power the electronic circuit with a regulated voltage, the voltage regulator being supplied by the secondary current of the transformers.

The current transformers employed to measure the current in the main circuit are typically the main components limiting the current range of the installation device, because their error increases both at small and high operating current values.

For optimum operation, a current transformer needs a certain amount of magnetic flux density in its ferromagnetic core in order to induce a voltage and consequently a current in the secondary circuit. This amount is called magnetization flux density and it depends on multiple parameters such as magnetic permeability and losses of the ferromagnetic core, dimensions of the ferromagnetic core, magnitude of the current in the main circuit, number of turns in the windings of the transformer, resistance of the secondary windings, and electrical load connected to the secondary terminals. The magnetization flux density causes a reduction of the secondary current and results in a current measurement error, called here magnetization error. The magnetization error increases with decreasing permeability of the ferromagnetic core of the transformer. Common materials employed in the ferromagnetic cores such as electrical steels feature low initial permeability at low magnetic fields and the permeability value increases with the magnetic field reaching a maximum value before approaching saturation. At high current values where the magnetization flux density approaches the saturation magnetic flux density Bs of the ferromagnetic material in the transformer core, a sharp drop of the magnetic permeability is caused leading to sharp increase of the current measurement error, called here saturation error. The saturation magnetic flux density Bs is a physical limit for any ferromagnetic material and ranges between <NUM> Tesla and <NUM> Tesla for common materials used in the construction of current transformers. The maximum operational current of a current transformer is thus limited by the saturation error and magnetic cores with high saturation magnetic flux density are preferred when measuring high currents is needed.

The magnetization and the saturation error can by decreased by reducing the total voltage load seen by the secondary side of the current transformer, which typically comprises the voltage drop over the resistance of the secondary winding of the transformer, the voltage drop over load resistors such as the shunt resistor employed to measure the secondary current, and other possible load voltages such as from rectifiers and voltage regulator. In self-powered installation devices, the rectifier and the voltage regulator circuitry cause a "continuous" load voltage which is almost constant with the secondary current causing a magnetization error whose absolute value is almost constant versus the measured current. The "continuous" load voltage is the dominant source for the magnetization error at low currents, where the voltage drop over resistors is usually negligible. The low initial permeability of common ferromagnetic materials employed in the core of current transformers is one additional factor causing the magnetization error to increase at low currents. The minimum current where current transformer can optimally operate is thus strongly limited.

The useable operation range of current transformers is thus limited both at low and high current values because of magnetization errors and saturation errors. The magnetization error can be reduced by employing ferromagnetic materials with high initial permeability in the core of the transformer. Example of such materials are Ni-Fe magnetic alloys with around <NUM>% Ni content, however, they feature relative high cost and moderate saturation magnetic flux density of around <NUM> Tesla. The saturation magnetic flux density of Ni-Fe alloys with approximately <NUM>% Ni content is improved up to around <NUM> Tesla but the initial permeability is lower, and the cost of the material is still high. Nanocrystalline magnetic alloys feature high initial permeability and lower cost than Ni-Fe alloys, however, the saturation magnetic flux density of the material is still limited at around <NUM> Tesla and the fill ratio of the magnetic cores is relatively low around <NUM>% to <NUM>% leading to an effective saturation limit of the transformer core typically below <NUM> Tesla.

Electrical steels are iron based alloys which contain up to <NUM>% silicon, most commonly around <NUM>%. Electrical steels feature the best saturation magnetic flux density out of commonly available magnetic materials at around <NUM> Tesla. Their cost is significantly lower than that of Ni-Fe alloys and nanocrystalline materials, however, their initial permeability is worse but sufficient for many applications. Due to the high saturation magnetic flux density, acceptable permeability, low cost and wide availability, electrical steels are often selected for the construction of current transformers.

The magnetization and saturation errors of current transformers can be further reduced by increasing the effective cross section area of the ferromagnetic core. Increasing the effective cross section area of the ferromagnetic core and reducing the resistance of the secondary winding results in increased material usage for the core and for the winding, which is typically made with Cu wire. This implies higher volume, mass, and cost of the current transformers resulting in little design flexibility. The design of current transformers requires thus a tradeoff between operating current range, accuracy, volume, mass, and cost. Compromising some merits is typically necessary.

Document <CIT> relates to a circuit breaker.

Document <CIT> relates to a current transformer.

Document "<NPL>" discloses an electronic error compensator,
Thus it is the objective of the present invention to provide an electrical installation device where the magnetization error of the current transformers is drastically reduced, at low cost and with easy implementation.

The objective is achieved by an electrical installation device with the features of claim <NUM>. So according to the invention the electrical installation device has a compensation signal circuit, said compensation signal circuit being operable to add a controlled compensation signal to the rectified secondary current.

The solution according to the present invention provides a method to reduce the magnetization error of current transformers by adding a controlled compensation signal to the rectified secondary current. The magnetization error causes a reduction of the raute mean square (rms) and of the DC values of the rectified secondary current by an approximately constant value, depending mainly on the construction of the current transformers and on the load conditions. For a given application it is thus possible to add a matching compensation signal with fixed magnitude to the rectified secondary current in order to strongly reduce the current error of the transformers. The useable operating current range of the transformers is thus significantly extended towards smaller current values. The advantage of the solution according to the invention by applying a compensation signal circuit is low cost and easy implementation due to using simple electronic circuitry. So the electrical installation device according to the invention features a wide current operation range combined with high accuracy, but at low cost of the installation device.

According to the invention, the compensation signal is a compensation current.

According to an advantageous embodiment of the invention, the compensation current has a fixed magnitude.

According to an advantageous embodiment of the invention, the compensation signal is a DC signal or a DC signal with an AC ripple.

According to an advantageous embodiment of the invention, the rectifier is a full wave rectifier with diodes.

According to an advantageous embodiment of the invention, the rectifier is a three phase full wave rectifier comprising <NUM> diodes.

According to an advantageous embodiment of the invention, the device further comprises a shunt resistor connected in the path of said rectified current to convert it to a voltage, and an amplifier to amplify the said voltage, and at least one biasing resistor connected between one input of the amplifier and a reference signal in order to provide the compensation signal. The secondary current is thus rectified and then converted to a voltage by passing it through a shunt resistor. The current shunt may be a variable resistor such as a potentiometer to allow tuning the response of the system with respect to a rated current value. The shunt voltage is amplified and then further processed by the electronic circuitry. The electronic circuit may be implemented on one or more circuit boards and the circuitry may be based on analog electronic components, or it may use digital and mixed electronic solutions. The electronic circuitry can according to an advantageous embodiment be highly integrated into one or more components such as a microcontroller, mixed signal microcontroller, DSP, and/or ASIC.

In self-powered installation devices, the secondary current of the transformers may according to another advantageous embodiment of the invention also be used to power the electronic circuit. In such cases, a voltage regulator is also provided.

According to an advantageous embodiment of the invention, at least one biasing resistor is connected between the positive input of the amplifier and a voltage reference source featuring a higher DC voltage level than any ends of the shunt resistor.

According to an advantageous embodiment of the invention, an attenuating resistor is connected between the positive input of the amplifier and that end of the shunt resistor with the higher DC voltage value.

According to an advantageous embodiment of the invention, a further biasing resistor is connected between the negative input of the amplifier and that end of the shunt resistor with the higher DC voltage value.

According to an advantageous embodiment of the invention, the resistors connected to the amplifier for setting the gain include a variable resistor.

According to an advantageous embodiment of the invention, the biasing resistors and/or the attenuating resistor include a resistor whose value can be adjusted in production.

According to an advantageous embodiment of the invention, a voltage regulator circuit is connected in series with the shunt resistor.

According to an advantageous embodiment of the invention, the voltage regulator circuit also provides the reference source connected to one of the biasing resistors.

The invention will be described in greater detail by description of five embodiments with reference to the accompanying drawings, wherein.

In the figures, same elements and functional groups or elements and functional groups with same or similar function have the same reference numerals.

<FIG> shows in an exemplary scheme an electrical installation device <NUM> according to the prior art, here in the example a three-phase electronic overload relay. It has on its input side three pairs of electrical contacts 1a, 1b; 2a, 2b; 3a, 3b operable to connect an electric main circuit. Inside the device <NUM>, there is a three-phase current transformer unit <NUM>, having for each phase a current transformer <NUM>, <NUM>, <NUM>. Each of the transformers <NUM>, <NUM>, <NUM> is placed with its primary side 4a, 5a, 6a in series with and coupled to the electrical contacts 1a, 1b, 2a, 2b, 3a, 3b. Next, there is a sensing circuit <NUM> coupled to the secondary sides 4b, 5b, 6b of the current transformers <NUM>, <NUM>, <NUM>. The sensing circuit <NUM> comprises a rectifying circuit <NUM> coupled to the secondary sides 4b, 5b, 6b of the transformers <NUM>, <NUM>, <NUM>. The rectifying circuit <NUM> is a <NUM>-phase full wave rectifier comprising six diodes 13a, 13b, 13c, 13d, 13e, 13f. Thus the sensing circuit <NUM> is operable to generate a voltage signal representative of the current in the main circuit.

Coupled to the sensing circuit <NUM>, there is a processing circuit <NUM> operable to initiate a responsive action with respect to the voltage signal representative of the current in the main circuit. The electrical installation device <NUM> in the example here is an electronic overload relay, having output terminals 16a, 16b operable to connect a tripping unit <NUM>, for example a contactor. An electronic overload relay would typically also comprise an auxiliary relay driven by the processing circuit to operate the output terminals 16a, 16b; for simplicity reasons, the said auxiliary relay in not shown in <FIG>. Additional output terminals for signaling may also be provided. So here in the example the responsive action of the processing circuit <NUM> is generating a tripping signal and delivering it to the output terminals 16a, 16b to be from there supplied to the tripping unit <NUM>. For doing so, the processing circuit <NUM> may comprise an integrator and one or more comparators in order to detect fault conditions in the main circuit or in the load connected to the main circuit and to initiate responsive actions such as tripping commands and signaling. The processing circuit may comprise analog or digital electronics and implement various functionalities such as thermal models, detection of phase imbalance, and self-test.

The sensing circuit <NUM> has a variable shunt resistor <NUM> and an amplifier circuit <NUM> with an amplifier <NUM>. The amplifier circuit <NUM> is connected to the variable shunt resistor <NUM> and is operable to amplify the voltage drop over the variable shunt resistor <NUM> and to deliver the voltage drop signal to the processing circuit <NUM>.

In the prior art the variable shunt resistor <NUM> is used as a nominal current value adjustment means. Adjusting the value of the variable resistor allows tuning the response of the system with respect to a rated current value. The variable resistor <NUM> delivers a voltage drop that is proportional to the rectified secondary current and inversely proportional to the nominal current adjusted by the user. So a high nominal current setting means that the trip signal shall be generated in response to a high current signal only. Consequently the variable shunt resistor <NUM> is tuned to a low resistor value, so that the voltage drop across it and the voltage input to the amplifying circuit <NUM> is low, or in other words, the voltage drop needed to trigger the trip signal is achieved at very high current values only. For a low nominal current setting it is vice versa.

For self-powering the electronic circuits in the electronic overload relay <NUM>, there is a voltage regulator <NUM> provided, fed from the rectifying circuit connected to the secondary side of the transformer unit <NUM>. The voltage regulator <NUM> is electrically connected in series to the shunt resistance <NUM>.

<FIG> shows the device <NUM> as of <FIG>, whereby the amplifier circuit <NUM> is explained in more detail, yet still exemplarily and schematically, using a block diagram. The amplifier circuit <NUM> has, here in the example, an operational amplifier <NUM>. A feedback resistor <NUM> is connected between the output side <NUM> and the inverting input <NUM> of the amplifier <NUM>. The non-inverting input <NUM> is connected to the one end <NUM> of the shunt resistor <NUM> which is connected to the voltage regulator <NUM>. An input resistor <NUM> is connected between the inverting input <NUM> and the second end <NUM> of the shunt resistor <NUM>. Insofar the exemplary schematic of the amplifier circuit is a conventional inverting amplifier circuit, wherein the gain is determined by the ratio of the two feedback resistors <NUM> and <NUM>.

<FIG> shows the amplifier circuit <NUM> in combination with a compensation signal circuit <NUM> according to a first embodiment of the invention. The compensation signal circuit <NUM> is operable to add a controlled compensation signal to the rectified secondary current. In the arrangement according to <FIG>, the biasing resistor R<NUM>, reference numeral <NUM>, is connected to a reference signal V<NUM> allowing to add a compensation signal to the signal coming from the shunt resistor RS. , reference numeral <NUM>.

The relation between the voltage and the secondary current passing through the shunt resistor Rs, reference numeral <NUM>, can be expressed as: <MAT>.

The compensation signal is created by injecting a signal into the input of the amplifier <NUM> that amplifies the voltage drop over the shunt resistor <NUM>. In the arrangement shown in <FIG>, the biasing resistor R<NUM> reference numeral <NUM>, is connected to a reference signal V<NUM> to add a compensation signal to the signal coming from the shunt resistor Rs. , reference numeral <NUM>. The output voltage is then given by the following equation: <MAT>.

The gain of the amplifier <NUM> is set by the input resistor R<NUM>, reference numeral <NUM>, and the feedback resistor R<NUM>. , reference numeral <NUM>. The equivalent compensation current referred to the secondary current passing through the shunt resistor is deduced to be: <MAT>.

In order to obtain a positive compensation current based on Eq. <NUM>, it is necessary to implement a reference signal V<NUM> smaller than V<NUM>, such that (V<NUM> - V<NUM>) is negative.

<FIG> shows an embodiment, where the biasing resistor R<NUM> , reference numeral <NUM>, is connected to a reference signal V<NUM> and the resistor R<NUM>, reference numeral <NUM>, attenuates the value of the reference signal, such that the compensation signal is equal to a fraction of (V<NUM> - V<NUM>).

The embodiment shown in <FIG> allows implementing a positive compensation current from a positive voltage reference V<NUM>, such that V<NUM>> V<NUM>. The biasing resistors R<NUM> , reference numeral <NUM>, is connected between the positive input of the amplifier <NUM> and the reference signal V<NUM>. The magnitude of the required compensation signal is usually small compared to the magnitude of the signal coming from the current shunt <NUM> and it is practical to use an attenuator for generating it. The attenuating resistor R<NUM>, reference numeral <NUM>, is connected between the positive input <NUM> of the amplifier <NUM> and the signal V<NUM> from one end <NUM> of the shunt resistor <NUM>, such that the compensation signal is equal to a fraction of (V<NUM> - V<NUM>) depending on the values of the resistors R<NUM> and R<NUM>. , reference numerals <NUM>, <NUM> respectively. The output voltage of the amplifier <NUM> for the arrangement from <FIG> can be expressed as in Eq. <NUM> or <NUM>: <MAT> <MAT>.

The equivalent compensation current referred to the secondary current passing through the shunt resistor <NUM> is given then by: <MAT>.

The compensation current from Eq. <NUM> corresponding to the circuit from <FIG> depends strongly on the gain setting of the amplifier.

<FIG> shows an arrangement where the compensation signal is implemented by biasing both the positive and the negative inputs of the amplifier <NUM>, employing the resistors R<NUM> and R<NUM> , reference numerals <NUM>, <NUM>, respectively.

The embodiment presented in <FIG> with a compensation circuit 33b provides a better decoupling between the gain setting of the amplifier <NUM> and the equivalent compensation current, by connecting a further biasing resistor R<NUM> , reference numeral <NUM>, between the negative input <NUM> of the amplifier <NUM> and the signal V<NUM>. The output voltage of the amplifier <NUM> can be expressed as below: <MAT>.

The equivalent compensation current can then be calculated as in Eq. <NUM>: <MAT>.

The values of the resistors R<NUM>, R<NUM>, and R<NUM> can be tuned to minimize the dependence between the value of the compensation signal and the value of the closed loop gain of the amplifier <NUM>. The arrangement form <FIG> provides great flexibility in adjusting the compensation signal and it only needs a positive voltage reference V<NUM>. A positive voltage reference is preferred as it allows implementing a solution using only unipolar power supply resulting in low cost and simplicity of the electronic circuit.

The value of the compensation signal could be fixed or variable. For example, it is possible to tune the value of the compensation signal for each device based on a calibration procedure. A one-time or multiple-times adjustable resistor <NUM> may be employed to that purpose for setting the value of biasing or attenuating resistors. Such arrangement is shown in <FIG>, where a potentiometer, P<NUM>, , reference numeral <NUM>, is employed to adjust the biasing of the positive input of the amplifier <NUM>. Other types of adjustable resistors may also be used such as rheostats, programmable resistors, or laser trimmable resistors.

A variable resistor such as a rheostat, potentiometer, or programmable resistor may also be provided to set the gain of the amplifier <NUM>, one possible example being given in in <FIG>. The feedback resistor R<NUM>, reference numeral <NUM>, in <FIG> is made a variable resistor and employed for setting the amplifier <NUM> gain. It may be controllable by the user, directly or indirectly, for adjusting the response of the installation device, for example with respect to a desired nominal current value of the main circuit.

The values of the biasing or amplifier input resistors connected to the ends of the shunt resistor <NUM> are typically significantly higher than the value of the shunt resistor <NUM> such that they divert a negligible amount of the secondary current from the shunt resistor <NUM>. However, the current diverted by the biasing and amplifier input resistors is deterministic and can be accounted for in the design of the circuit such that the accuracy of the solution is not affected.

An operational amplifier may be employed but other types such as difference or differential amplifiers are also possible. Additional components such as capacitors could be added to the circuit for filtering or other purposes.

The solution may include one or more current transformers and a half-wave or a full-wave rectifier. For applications featuring a main circuit with <NUM> phases, the solution would provide best performance by including <NUM> current transformers and a full-wave rectifier based on <NUM> diodes. A voltage regulator supplied by the rectified secondary current of the transformers is provided in series with the shunt resistor for self-supplied installation devices. The common connection between the voltage regulator and the shunt may be connected to the ground of the electronic circuit, which may be a floating ground.

The compensation circuit is connected to the voltage reference of the voltage regulator, as shown in <FIG> shows an electrical installation device <NUM> including three current transformers <NUM>, <NUM>, <NUM>, a full wave rectifier <NUM>, a voltage regulator circuit <NUM> in series with a shunt resistor <NUM>, an amplification circuit <NUM> with an amplifier <NUM> with adjustable gain using the adjustable feedback resistor <NUM>, and a compensation signal circuit 33b, said compensation signal circuit being designed and operational like the one described in <FIG>. Potential V<NUM>, ground potential, in <FIG> on the top left, is in <FIG> at the common connection between the voltage regulator <NUM> and the shunt <NUM>. Potential V<NUM> is at the connection point to the voltage regulator circuit <NUM>.

Additional electronic circuitry such as an ADC or a microcontroller may be used to implement the required functionalities of the installation device. The ADC and/or the amplifier may be integrated in the microcontroller or in other type of integrated circuit such as DSP or ASIC.

Claim 1:
An electrical installation device (<NUM>), operable to connect an electric main circuit, and to achieve device functionalities, said installation device (<NUM>) having
at least one current transformer (<NUM>) coupled to the electric main circuit,
the secondary terminals (4b) of the transformer (<NUM>) being connected to an electronic circuit,
said electronic circuit being operable to process the secondary current and to employ the secondary current for achieving device functionalities,
wherein the electronic circuit comprises a rectifier (<NUM>), with a plus terminal or voltage reference source (V<NUM>) and a minus terminal (V<NUM>), to rectify the secondary current, characterized in that
the electrical installation device (<NUM>) has a compensation signal circuit (<NUM>), said compensation signal circuit (<NUM>) being operable to add a controlled compensation signal to the rectified secondary current,
wherein the compensation circuit (<NUM>, 33b) comprises a voltage regulator circuit (<NUM>), an op-amp (<NUM>), a shunt resistor (RS, <NUM>), and five further resistors (R<NUM>, <NUM>; R<NUM>, <NUM>; R<NUM>, <NUM>; R<NUM>, <NUM>; R<NUM>, <NUM>), wherein
the voltage regulator circuit (<NUM>) is arranged between the plus terminal (V<NUM>) and a ground (V<NUM>),
the shunt resistor (RS, <NUM>), is arranged between the ground (V<NUM>) and the minus terminal (V<NUM>),
resistors R<NUM> (<NUM>) and R<NUM> (<NUM>) build a voltage divider between the plus terminal (V<NUM>) and the ground (V<NUM>), the mid of R<NUM> (<NUM>) and R<NUM> (<NUM>) being connected to a non-inverting input of the op-amp (<NUM>),
resistors R<NUM> (<NUM>) and R<NUM> (<NUM>) build a voltage divider between the ground (V<NUM>) and the minus terminal (V<NUM>), the mid of R<NUM> (<NUM>) and R<NUM> (<NUM>) being connected to an inverting input of the op-amp (<NUM>), and
the resistor R<NUM> (<NUM>) is arranged between an output of the op-amp (<NUM>) and the inverting input of the op-amp (<NUM>),
wherein the compensation signal is a compensation current (ISC) through the shunt resistor (<NUM>).