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Patent Issued for Method for Measuring an Alternating Current Which Is Generated Using Inverters, and Arrangement for Carrying Out the Method

May 7, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventor Hallak, Jalal (Vienna, AT), filed on March 29, 2007, was published online on April 22, 2014.

The patent's assignee for patent number 8705255 is Siemens Aktiengesellschaft (Munich, DE).

News editors obtained the following quote from the background information supplied by the inventors: "Inverters are conventionally used to transform energy from direct current sources into alternating currents and to feed them into AC power systems or to loads. The voltage waveform and the phase angle are matched to the alternating current in the power system so that direct current components and phase shifts are minimized. This is important, above all, in the case of public power supply systems, since in this case, precise feed guidelines are predefined by the network operators, with maximum permissible direct current components.

"In order to be able to regulate the inverter suitably, zero crossing in the power system is continually detected and is predefined as a zero crossing signal. Additionally, at the output of the inverter, the alternating current is measured as a controlled variable. Inaccuracies resulting from components can arise in the measuring circuits used, which include, for example, shunt resistors or current transformers. These can be, for example, an unwanted offset voltage in current transformers, or a temperature drift. Methods are therefore known from the prior art by means of which measuring circuits are adjusted.

"One method consists, for example, in manually adjusting each measuring circuit after its production. Other methods make use of microcontrollers which perform an adjustment procedure after each switching-on of the device.

"In measuring circuits which use shunt resistors to detect the current, the losses which occur at the shunt resistors must be taken into account.

"According to the prior art, therefore, in the case of inverters of high output, the use of current transformers is provided in order to measure the current at the output of the inverter. With these current transformers, in order to eliminate unwanted offset voltages, methods with manual adjustment during production are used. However, a method is also known wherein a microcontroller integrated into the measuring circuit is used for adjusting the current transformer at every switching-on of the device.

"However, with this method, no deviations due to temperature drift or residual magnetism of the current transformer core are rectified. Residual magnetism is caused by changes in the current direction, due to hysteresis effects in the current transformer core. Heat generated during operation, or external temperature changes, lead to temperature drift, which means that inaccuracies caused by temperature-dependent properties of the current transformer components arise in the current transformer."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventor's summary information for this patent: "It is an object of the invention, using a method of the type described in the introduction, to provide an improvement over the prior art. It is a further object of the invention to provide an arrangement for carrying out the method.

"This object is achieved with a method for measuring an alternating current which is generated using inverters and which is fed into an AC power system, wherein a zero crossing signal of the AC power system is predefined, wherein, triggered by the zero crossing signal, the measured alternating current is periodically adjusted in such a manner that an adjustment value that is assigned to the zero crossing signal is predefined, a measured value which is assigned to the zero crossing signal in the same manner being adapted to said adjustment value.

"This method also enables periodic adjustment, with an adjustment value, of the measurement signal detected with a measuring circuit, during operation. The adjustment is controlled by means of the zero crossing signal of the AC power system. By this means, it is achieved that the adjustment is carried out synchronously with the AC power system and also that deviations due to temperature drift or residual magnetism are compensated for.

"In an advantageous embodiment, an adjustment value equal to zero Ampere is assigned to each zero crossing of the power system current and each measured value detected at the zero crossing of the power system current is adapted to this adjustment value. The adjustment is then carried out at each zero crossing of the power system. Fixing of the adjustment value at zero Ampere also has the effect that the current measured at the output of the inverter is measured without any direct current component caused by inaccuracies. An accurate measurement output signal is therefore made available for the regulation of the inverter, so that feeding into the AC power system takes place synchronously with the power system current and without direct current components.

"Adjustment during zero crossing also uses a particular type of alternating current generated by the inverters. During reversal of the current direction, the current value remains constant for the duration of the reversing procedure. This duration is used for adjusting the measurement signal.

"It is also advantageous if, at each adjustment, a correction value is determined from the adjustment value and the detected measured value and if, until the next adjustment, the measurement signal corrected with the correction value is output as the measurement output signal. This represents a simple method for adjustment of the measurement signal, and can be carried out either by means of a discrete circuit or by means of a microcontroller.

"For an AC power system without phase shifting, that is without a reactive current component, it is advantageous to adjust the measured values with the adjustment value at the time points of the zero crossings. Feeding into the AC power system then takes place synchronously with the zero crossing signal without any direct current components.

"When the current generated by an inverter is fed into a separate network, a phase shift usually takes place between the power system voltage and the power system current due to non-correctable impedances. The measured values are then advantageously adjusted with the adjustment value at the time points of the zero crossing of the power system current.

"It is herein favorable to determine the time points of the zero crossings of the power system current by detecting the power system voltage and the phase shift, since detection of the power system voltage is, in any event, required within the inverter arrangement as a fixed point for the current feed.

"The problem is also solved with an arrangement which is configured to carry out the claimed method, wherein a current transformer is provided for measuring the alternating current and wherein the alternating current generated by the inverter is applied to the input of the current transformer and wherein a voltage exists at the output as the measurement signal and wherein the measurement signal is periodically adapted to the adjustment value, depending on the zero crossing signal. The continually adjusted current transformer operates almost loss-free, so that the overall efficiency of the inverter is improved compared with measuring circuits having a shunt resistor.

"In an advantageous embodiment of the arrangement, a microcontroller with at least two inputs is provided, wherein the measurement signal generated by the current transformer is applied to a first input and the zero crossing signal is applied to a second input and wherein, furthermore, a voltage value from the current transformer and assigned to the current value of zero Ampere is predefined for the microcontroller as the adjustment value and wherein, at an output of the microcontroller, a measurement output signal corrected with the correction value can be tapped off. The use of the microcontroller permits a simple arrangement which is easily adapted to different inverters with different current transformers.

"Depending on the number of items to be produced, it may also be advantageous if a Sample and Hold amplifier is provided, the measurement signal from the current transformer and the zero crossing signal being applied to the inputs of said Sample and Hold amplifier, and if a correction value is applied as the output signal, said correction value being fed to the positive input of an operational amplifier and if, in addition, the measurement signal is applied to the negative input of the operational amplifier, so that the corrected measurement output signal can be tapped off at the output of the operational amplifier. With this circuit constructed with discrete components, no microcontroller is needed."

For additional information on this patent, see: Hallak, Jalal. Method for Measuring an Alternating Current Which Is Generated Using Inverters, and Arrangement for Carrying Out the Method. U.S. Patent Number 8705255, filed March 29, 2007, and published online on April 22, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=55&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=2724&f=G&l=50&co1=AND&d=PTXT&s1=20140422.PD.&OS=ISD/20140422&RS=ISD/20140422

Keywords for this news article include: Electronics, Microcontroller, Siemens Aktiengesellschaft.

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Source: Electronics Newsweekly


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