Abstract: Embodiments test MOV arresters. One embodiment comprises applying a plurality of voltages to a high voltage lead of the MOV arrester, the plurality of voltages being changed between an initial test voltage and final test voltage (corresponding to a point of conduction of a reference MOV arrester); determining a resistive current through the MOV arrester for each of the voltages; determining information corresponding to the resistive current when the applied test voltage corresponds to the point of conduction of the reference MOV arrester; comparing the determined information with information associated with the point of conduction of the reference MOV arrester; determining that the MOV arrester has passed the test when the determined information corresponds to the information associated with the reference MOV arrester; and determining that the MOV arrester has failed the test when the determined information does not correspond to the information associated with the reference MOV arrester.

Abstract: A method of detecting and localizing a source of partial discharge in an electrical apparatus placed in an enclosure containing an acoustically-conductive fluid.

Abstract: A probe (10) for sensing movement of a body of magnetic material comprises a magnetically energisable pole piece (30), a closed loop electrical circuit (20) having a first end (25) which is wound around the pole piece (30) and forms a pole piece coil (25) inductively coupled to the pole piece, the arrangement being such that movement of a body of magnetic material (70) relative to the pole piece (30)induces a current in the closed loop circuit (20), and a second end (45), remote from the pole piece (30), which forms a transformer primary coil. A transformer secondary coil (65) is inductively coupled to the primary coil (45), and terminated by a load resistance and means for measuring an output signal from the secondary coil. The primary (45) and secondary (65) coils form a transformer such that a current in the primary coil induces a voltage across the secondary coil. One or more shorting turns (60) of a conductor are inductively coupled to the primary (45) and secondary (65) coils.

Abstract: A current transformer test device and a method implemented using the test device, is useful to verify proper CT installation and operation, and does not rely on the generator (or other system) to be in a state of relatively high assembly. The test device allows a current transformer to be tested by supplying an alternating current (AC) signal to a primary winding of the current transformer to thereby induce an AC signal in a secondary winding of the current transformer. The test device is used to simultaneously monitor the phases of the supplied AC signal and the induced AC signal so that a determination can be made as to whether the current transformer secondary winding is properly installed relative to the current transformer primary winding, and to simultaneously monitor the current magnitudes of the supplied AC signal and the induced AC signal so that the turns ratio of the current transformer can be determined.

Abstract: A method for transformer testing includes receiving a failing test result of a transformer, the test result being determined from a test taken during transformer manufacture, determining, via a knowledge-based system, a predicted root cause of the failure based on the test result and a knowledge base of transformer information, and determining, via the knowledge-based system, a suggested course of action for the failure based on the test result and the knowledge base of transformer information.

Abstract: The present invention is a method of and an apparatus for measuring resistances of at least one transformer winding (109) in configurations of connected transformers windings. The method is based on increasing driving power from the source of electrical energy (12, 102) until the current reaches a predetermined level, which is below the core saturation current level of the measured transformer or the measuring device current limit, and then controls the power of the source of the electrical energy to maintain predetermined current flowing through the at least one winding over a time period sufficient to cause a L di/dt contribution to a voltage drop across the at least one winding to decrease below a threshold level. The apparatus measures simultaneously the constant current flowing in the at least one winding and the voltage drop over the at least one winding after the predetermined time period has elapsed.

Abstract: A procedure to measure the grounding resistances of electrical installations by measuring the loop impedance. According to this procedure, using voltage generator transformers, a current is injected into the loop and this current is measured using current measurement transformers. Two transformers are united into a single transformer, used as impedance transformer, and the loop impedance is established by taking several measurements that allow determining and eliminating parameters of the transformer, by analysis of the measurement results.

Abstract: An apparatus (10) for detecting the occurrence of partial discharge in electrical equipment, such as high voltage transformers. The apparatus comprises a detecting means including an ultrasonic transducer (11) mounted at one end of an electrically insulating elongate rod (13). The ultrasonic transducer (11) is adapted to detect ultrasonic pulses or waves generated by the occurrence of a partial discharge in the electrical equipment. A radio frequency (RF) transducer (17) is also utilized to detect radio frequency (RF) pulses or waves also generated by the partial discharge. The rod (13) can be mounted in the wall of the electrical equipment and so provide a mechanism for adjusting the position of the detecting means within the electrical equipment. The apparatus is also removable from the electrical equipment and re-locatable to other equipment.

Abstract: An approved method and apparatus for detecting partial discharge events within a transformer comprises asserting a MEMS acoustical probe through the wall of the transformer to optically measure partial discharge events. In an enhanced embodiment, temperature compensation is also possible, and detection may be confirmed via an independent electromagnetic or other sensor.

Abstract: The present invention relates to a method of diagnosing a fault on a transformer winding by using frequency response analysis. The method comprises the steps of: measuring the impedance on said winding as a function of frequency, said measurement being represented in the form of a first voltage gain; comparing said impedance measurement with a reference measurement represented in the form of a second voltage gain, said comparison including a step of calculating three first parameters, each of said three first parameters being a correlation coefficient, between said first and second gains over three different frequency ranges. The method further comprises a step of determining the relative variation of at least a fourth parameter, said fourth parameter being a physical magnitude characteristic of said transformer, said relative variation being obtained by comparing said first and second gains.

Abstract: A method for evaluating the accuracy of a current transformer is presented. The current transformer has a primary winding and a secondary winding. A current is not circulated through the primary winding. A load is disconnected from the current transformer. A load impedance is measured of an internal instrumentation associated with the current transformer. The resistance in the secondary winding of the current transformer is determined. The load impedance is compared to a standard burden. A standard burden value is selected. The secondary voltage for a rated current is determined. The injection voltage is adjusted to the value of the secondary voltage. The exciting current and power are measured. The ratio error or the phase angle are then calculated by using the exciting current and power.

Abstract: A current sensor using the compensation principle, in particular for measuring direct and alternating currents, includes an entirely digital evaluation circuit for signal processing. This substantially lowers the cost of the control electronics, in particular through integration in an ASIC. The magnetic field probe is, for example, supplied with a square wave voltage of a predetermined frequency, the probe signal is recorded by one or two comparators and the pulse width is digitally measured by counters. Furthermore, the conventional analog final stage for the compensation current is replaced by a PWM end stage with a series connected sigma-delta modulator. The square wave can be synchronized with a defined synchronization signal, which can be derived, for example, from the processed measurement values.

Abstract: Testing a transformer by applying to the transformer a test signal, the frequency of which may be lower than the nominal frequency of the transformer. The voltage of the test signal may also be lower than the nominal voltage of the transformer. A number of frequency-dependent parameters are measured, particularly the eddy current resistance and the hysteresis curve of the transformer, in order to derive a simulation model which simulates the behavior of the transformer at different frequencies. Using this simulation model, it is possible to predict operating parameters of the transformer, such as the terminal voltage on the secondary and the terminal current in the secondary, during operation with a frequency deviating from the frequency of the test signal, particularly during operation with the nominal frequency of the transformer.

Abstract: The magnetic core of an a-c current transformer is optimized so that the current transformer is able to function with a primary electric current having a d-c current component. In the preferred embodiment, the magnetic core is optimized to have nonlinear incremental permeance so that the magnetic core operates with high incremental permeance when the d-c current component is not present (thereby providing current transformer operation with relatively high accuracy), and the magnetic core operates in a non-saturated manner with reduced incremental permeance when the d-c current component is present (thereby providing acceptable current transformer operation, but with reduced accuracy). An electronic assist means may be provided for improved current transformer accuracy. An alternate embodiment combines an electronic assist means together with a magnetic core that has low permeance.

Abstract: A preferred embodiment of a transformer includes a core, and a primary and a secondary winding positioned on the core so that the primary and secondary windings are inductively coupled when the primary winding is energized. The transformer also includes a sensor for measuring an operating parameter of the transformer, an audible signaling device, and an actuator for activating the audible signaling device when the operating parameter reaches a predetermined value.

Abstract: A monitoring system includes at least one partial discharge (PD) sensor. The PD sensor is configured to monitor a component of an aircraft wiring system and to acquire a monitoring signal. A method embodiment for monitoring an aircraft wiring system includes acquiring a number of monitoring signals for a number of components of the aircraft wiring system using a number of partial discharge PD sensors. The method further includes conveying the monitoring signals from at least one of the PD sensors to a data acquisition system.

Abstract: An internal fault indicator for an electrical device is triggered by a sudden increase in pressure as occurs when an insulation failure creates an electric arc. The heat released in the arc is transferred onto the surrounding volume causing localized overheating, vaporization and decomposition of the insulating material. The resulting pressure surge moves a diaphragm. The movement of the diaphragm releases a spring driven plunger from a barrel which extends through the housing of the electrical device. Prior to activation the plunger is held in an “armed” position by a retaining pin. Upon triggering, the plunger is pushed by the spring until it protrudes from the housing to provide a visual signal of the internal fault. A pressure relief valve may be integrated with the internal fault indicator.

Abstract: A preferred method for estimating conductor losses in a transformer having a first and a second winding includes energizing the first winding while the second winding is short-circuited by an electrical conductor so that power is supplied to the first winding and a portion of the power is dissipated due to a resistance associated with the electrical conductor. A preferred method also includes measuring the power supplied to the first winding, calculating the portion of the power dissipated due to the resistance associated with the electrical conductor, and subtracting the portion of the power dissipated due to the resistance associated with the electrical conductor from the power supplied to the first winding.

Abstract: The invention relates to a method, a computer program product, a computer program and a device (20) for the plausibility checking of voltage transformers (8) in an electrical switchgear (1) as well as a switchgear (1) with such a device (20). According to the invention, a topological sub-area (1a; 1b) with several galvanically connected voltage transformers (8) is recorded for an instantaneous topology of the switchgear (1), groups of voltage transformers (8) with the same measured signals are identified and, if more than one group is present, an indication or alarm signal is generated by the substation control system (2). Amongst other things, exemplary embodiments relate to: Criteria for the production of warning or alarm messages for problematic voltage transformers (8) and/or sub-areas (1a; 1b); presentation of the results of the plausibility test; coordination of the plausibility test with switching operations.

Abstract: A system and method for creating, editing, and/or executing a test program for testing a transformer is provided. The system includes an input that allows the user to select the desired test instructions and pre-existing sequences of test instructions to create or edit a test program having a sequence of test instructions. The processor executes the test program by generating commands that are performed in a predetermined order.

Abstract: The present invention relates to a method of diagnosing a fault on a transformer winding by using frequency response analysis. The method comprises the steps of: measuring the impedance on said winding as a function of frequency, said measurement being represented in the form of a first voltage gain; comparing said impedance measurement with a reference measurement represented in the form of a second voltage gain, said comparison including a step of calculating three first parameters, each of said three first parameters being a correlation coefficient, between said first and second gains over three different frequency ranges. The method further comprises a step of determining the relative variation of at least a fourth parameter, said fourth parameter being a physical magnitude characteristic of said transformer, said relative variation being obtained by comparing said first and second gains.

Abstract: An apparatus for detecting partial discharge in on-line high voltage electrical equipment containing a dielectric, such as high voltage transformers. The apparatus includes an ultrasonic transducer and an annular capacitive plate for detecting, respectively, the ultrasonic pulse and the radio frequency pulse generated by the occurrence of partial discharge in the equipment.

Abstract: Testing a transformer by applying to the transformer a test signal, the frequency of which may be lower than the nominal frequency of the transformer. The voltage of the test signal may also be lower than the nominal voltage of the transformer. A number of frequency-dependent parameters are measured, particularly the eddy current resistance and the hysteresis curve of the transformer, in order to derive a simulation model which simulates the behavior of the transformer at different frequencies. Using this simulation model, it is possible to predict operating parameters of the transformer, such as the terminal voltage on the secondary and the terminal current in the secondary, during operation with a frequency deviating from the frequency of the test signal, particularly during operation with the nominal frequency of the transformer.

Abstract: An offset signal for failure detection, that cannot be removed if an input signal line is broken is applied from an offset application circuit with output impedance higher than impedance of a differential transformer to an output signal of the differential transformer. A drive signal applied to a differential transformer is generated in digital form and an offset correction and a gain correction to output of the differential transformer are made in digital form for making controls necessary.

Abstract: The load current drawn from a power transformer is sensed to determine if and when the load current exceeds a predetermined threshold. The length of time the load current exceeds the threshold is also sensed. By monitoring the excess current flow and the length of time for which it flows, it is possible to anticipate a rise in the winding temperature of the power transformer and to initiate cooling before the windings have reached a critical temperature, as a function of the ambient temperature and/or the temperature of the transformer. Thus, systems embodying the invention include circuitry for sensing the current drawn from and/or by a transformer, determining when the current exceeds a predetermined value and timing circuits for sensing the length of time for which the excess current flows. The timing circuits are needed, in part, to differentiate between a transitory overload condition and a static, continuous, overload condition.

Abstract: A system and method for testing an on-line current transformer is provided. The current transformer includes a primary winding and a secondary winding. An operating current continues to flow through the primary winding during testing of the current transformer. A controllable load is applied to the current transformer secondary winding. The controllable load is varied over a range of load settings including a maximum current setting and a maximum voltage setting. At a plurality of load settings within the range of load settings, a current flowing through the current transformer secondary winding is measured. Also, a voltage across the current transformer secondary winding is measured. An actual excitation curve is generated from the measured currents and voltages corresponding to the plurality of load settings.

Abstract: A diagnosis apparatus (20) mounted to a separable transformer includes an arithmetic control section (27) for inputting, through a differential amplifier (24), a signal smoothing circuit 25 and a voltage detecting section (26), a mutual induction signal generated in a primary coil (11) of the separable transformer when dc power from a dc power supply section (21) and an ac signal from an oscillator (22) are applied to a capacitor (C) which cooperates with the primary core to form a resonance circuit. The arithmetic control section determines the presence or absence of a wire breakage or short-circuit failure in a secondary-side closed circuit of the separable transformer by comparing a frequency characteristic of mutual induction signal, determined based on mutual induction signals successively supplied thereto while the frequency of the ac signal from the oscillator varies, with frequency characteristics for normal, wire breakage and short-circuit conditions stored beforehand in a storage section (28).

Abstract: A method and apparatus for detecting a partial discharge in a voltage transformer, the transformer having a first voltage winding and a second voltage winding. A grounded electrostatic shield being operably attached to the transformer, preferably between the windings. A radio frequency current transformer is operably connected to the electrostatic shield wherein the partial discharge occurring within the voltage transformer is detected.

Abstract: An internal fault indicator for an electrical device is triggered by a sudden increase in pressure as occurs when an insulation failure creates an electric arc. The heat released in the arc is transferred onto the surrounding volume causing localized overheating, vaporization and decomposition of the insulating material. The resulting pressure surge moves a diaphragm. The movement of the diaphragm releases a spring driven plunger from a barrel which extends through the housing of the electrical device. Prior to activation the plunger is held in an “armed” position by a retaining pin. Upon triggering, the plunger is pushed by the spring until it protrudes from the housing to provide a visual signal of the internal fault. A pressure relief valve may be integrated with the internal fault indicator.

Abstract: A power fault testing apparatus for testing telecommunications equipment interfaces to Telcordia's 2nd Level AC power fault standards. The apparatus is powered by a three-phase 480 VAC, 600 A power service. A plurality of transformers, including a variable autotransformer and a fixed transformer, and load resistor banks are coupled to the power source in a network organized as a plurality of selectable and switchable power paths for interfacing with an Equipment Under Test (EUT) operable to be disposed in a test chamber. Power output of the fixed transformer is referenced to a single point ground positioned at a relay bank. The power to the EUT is referenced to building ground that is tied to the single point ground of the fixed transformer. The fixed transformer thereby operates as an isolation transformer, which allows for testing the EUT while operating under power.

Abstract: A system and method for conducting diagnostic services on live electrical equipment, such as electrical power transformers using a tap switch installed into each phase of the transformer. The tap switch is used along with the appropriate test and measurement equipment connected to a test and measurement system to perform the diagnostic service when the transformer is live and in-service. The tap switch allows the diagnostic service to be performed without the hazard of disconnecting the bushing tap in order to connect the bushing tap back to the test and measurement equipment. The tap switch provides for live diagnostic tests and measurements for the early identification of potential failures using diagnostic services, such as partial discharge (PD) and frequency response. Thus, live diagnostics can be performed on electrical power transformers while the transformers are in-service using a tap switch resulting in a reduction in down time and thus costs.

Abstract: An apparatus and method are used to determine equivalent individual phase voltages and phase currents from an alternating delta power source and load configuration. A delta-wye input transformer and a wye-delta output transformer are interconnected between the delta power source and the delta power load. Each phase of the delta power source is effectively connected across a corresponding wye configured coil. The delta-wye input transformer and the wye-delta output transformer are electrically connected to each other in a manner thereby providing three conductors through which the individual phase currents through the wye coils may be directly measured. The phase voltages are preferably measured across the wye coils of either the input transformer or the output transformer not intermixed.

Abstract: The invention relates to sensors for detecting a time-varying current by employing a plurality of coils with separate windings, disposed on separate toroidal cores, that are placed in close proximity of each other so that each coil responds independently to the same current. The current produces a time-varying magnetic field which in turn induces a plurality of voltages across the coils that can be combined to provide a resultant signal. Some embodiments of the invention employ coils with opposite windings to obtain signals with opposite phases and combine these signals through differential detection means to obtain a combined signal. One aspect of the invention relates to production of a wide-band or a selectable band-width sensor by preparing at least one coil to be dissimilar with respect to the others. In addition, the invention provides provisions for easy calibration of the sensor.

Abstract: The on-line winding test unit determines a characteristic signature of a monitored winding(s) in a transformer, generator, or the like. A sensor detects incoming pulses, originating elsewhere on the energy delivery system, applied to the winding. A sensor detects output pulses after each input pulse has propagated through the winding. Data corresponding to the input and output pulses are stored. A processor computes the spectral densities of the data records. The logic then computes the characteristic signature, H(f), for the winding such that H(f) equals the average of Gxy divided by the average of Gxx for the valid data. Coherence is used to determine a valid H(f). A comparison of the H(f)'s over elapsed time using an error function indicates winding deformation or displacement.

Abstract: A portable circuit breaker tester (10) for testing a current transformer (30, 32, 34, 36) of a circuit breaker (18) having a first terminal (30e, 32e) on a first end of the transformer and a second terminal (138) on a second end of the transformer has a test port (14) configured to be coupled to the first (30e, 32e) and second terminals (138) and a test circuit (68, 74, 96, 104, 146) coupled to the test port (14). The test circuit (68, 74, 96, 104, 146) is configured to provide a test current through the test port (14) to the first terminal (30e, 32e), to monitor the second terminal (138) through the test port (14), and to provide an output signal to an indicia (38, 40) based on whether the test current is received at the second terminal (138). The test circuit (68, 74, 96, 104, 146) is configured to perform first and second testing operations on the circuit breaker (18) through the test port (14).

Abstract: In order to spread a transformer having energy saving and environmental effects widely and quickly, a method for supporting received orders of a transformer is provided. In the method, a first response including a customer's desired inquiry relating to an installation date of a measuring circuit is issued from a sales department to a customer who accessed a web site of the sales department. A second response including the installation date of the measuring circuit is issued from a measurement department in charge of measurement of the transformer. Since cooperative communication is achieved between the sales department and the measurement department in charge of the transformer measurement, the system can quickly cope with the customer who accessed the web site of the sales department and a social demand of early realization of energy saving and CO2 reduction can be realized with highly excellent effects.

Abstract: A method for determining the DC current flowing through an AC power meter. The internal impedance of the distribution transformer is first sensed, then the DC voltage in the AC voltage waveform is measured and finally the measured DC voltage is converted to DC current.

Abstract: A method of monitoring contact burnoff in tap changers operating under load in which the load current is measured and for nominal variation of the voltage of the particular tap parameters are stored which are used to calculate the burnoff rate per contact per switching operation. From these values the cumulative burnoff rate of both the switching contact and resistance contact are determined and compared with limits or threshold values.

Abstract: This invention relates to a strobe charge apparatus for charging a capacitor using a flyback transformer.

Abstract: A method for determining the DC current flowing through an AC power meter. The internal impedance of the distribution transformer is first sensed, then the DC voltage in the AC voltage waveform is measured and finally the measured DC voltage is converted to DC current.

Abstract: A switching assembly for performing impulse measurement testing on each of a plurality of transformer terminals includes a first plurality of single pole, double throw switches and a switch controller for each switch. Each switch has a first position that connects a respective transformer terminal to one of a ground and a shunt and a second position that connects the terminal to an impulse generator. Preferably, the switch controllers are co-located to enable a user to change the position of any one of the switches from a central position.

Abstract: An offset signal for failure detection, that cannot be removed if an input signal line is broken is applied from an offset application circuit with output impedance higher than impedance of a differential transformer to an output signal of the differential transformer. A drive signal applied to a differential transformer is generated in digital form and an offset correction and a gain correction to output of the differential transformer are made in digital form for making controls unnecessary.

Abstract: Transformer winding movement is detected by providing an input sensor inside the transformer on an input winding lead and providing an output sensor inside the transformer on an output winding lead. This eliminates the frequency limitation imposed by the lead lengths required for prior art external measurement, making it possible to obtain useful measurements at much higher frequencies (20 MHz) than with the prior art techniques.

Abstract: The load current drawn from a power transformer is sensed to determine if and when the load current exceeds a predetermined threshold. The length of time the load current exceeds the threshold is also sensed. By monitoring the excess current flow and the length of time for which it flows, it is possible to anticipate a rise in the winding temperature of the power transformer and to initiate cooling before the windings have reached a critical temperature, as a function of the ambient temperature and/or the temperature of the transformer. Thus, systems embodying the invention include means for sensing the current drawn from and/or by a transformer, determining when the current exceeds a predetermined value and timing means for sensing the length of time for which the excess current flows. The timing means are needed, in part, to differentiate between a transitory overload condition and a static, continuous, overload condition.

Abstract: A system and method for testing an on-line current transformer is provided. The current transformer includes a primary winding and a secondary winding. An operating current continues to flow through the primary winding during testing of the current transformer. A controllable load is applied to the current transformer secondary winding. The controllable load is varied over a range of load settings including a maximum current setting and a maximum voltage setting. At a plurality of load settings within the range of load settings, a current flowing through the current transformer secondary winding is measured. Also, a voltage across the current transformer secondary winding is measured. An actual excitation curve is generated from the measured currents and voltages corresponding to the plurality of load settings.

Abstract: An internal fault indicator for an electrical device is triggered by a sudden increase in pressure as occurs when an insulation failure creates an electric arc. The heat released in the arc is transferred onto the surrounding volume causing localized overheating, vaporization and decomposition of the insulating material. The resulting pressure surge moves a diaphragm. The movement of the diaphragm releases a spring driven plunger from a barrel which extends through the housing of the electrical device. Prior to activation the plunger is held in an “armed” position by a retaining pin. Upon triggering, the plunger is pushed by the spring until it protrudes from the housing to provide a visual signal of the internal fault. A pressure relief valve may be integrated with the internal fault indicator.

Abstract: A system is disclosed for monitoring electrical partial discharge signals in electrical power apparatus. The apparatus includes a housing and a plurality of openings in the housing through which electrical power may pass via electrical power contact units. The system includes a plurality of sensor units, each being in communication with, and associated with, an electrical power contact unit at each opening in the housing. The sensor units are for sensing partial discharge signals at each opening in the housing and for producing sensor output signals. The system also includes a control unit for simultaneously and synchronously receiving the sensor output signals over an interval of time, and produces simultaneous synchronous output signals. The system also includes a processing unit for processing the simultaneous synchronous output signals.

Abstract: The invention relates to sensors for detecting a time-varying current by employing a plurality of coils with separate windings, disposed on separate toroidal cores, that are placed in close proximity of each other so that each coil responds independently to the same current. The current produces a time-varying magnetic field which in turn induces a plurality of voltages across the coils that can be combined to provide a resultant signal. Some embodiments of the invention employ coils with opposite windings to obtain signals with opposite phases and combine these signals through differential detection means to obtain a combined signal. One aspect of the invention relates to production of a wide-band or a selectable band-width sensor by preparing at least one coil to be dissimilar with respect to the others. In addition, the invention provides provisions for easy calibration of the sensor.

Abstract: An apparatus for monitoring the status of the insulation on the wire in a winding in a device comprises a surrogate sample of insulating material having a thickness and properties selected to fail before the failure of the insulation on the wires in the winding, and a detector for detecting and indicating failure of the sample of insulating material. A method of monitoring the status of the insulation on the winding in a dynamoelectric device includes associating a surrogate sample of insulating material with the winding, monitoring the surrogate sample, and providing an alarm when the surrogate sample fails, which is predictive of the failure of the insulation on the winding wire.

Abstract: There is described a fully automatic portable testing device which can be used generally for testing test-pieces to be operated at high voltages and/or high currents, such as current or voltage transducers or transformers for example. For this purpose, the portable testing device comprises a switched-mode power amplifier (3) whose output is connected to a matching transformer (4-7) having a plurality of secondary windings (5-7) which can be changed over. The test-piece on which measurements are to be made is in each case to be connected to the secondary windings (5-7) of the matching transformer. The portable testing device also comprises a control which in particular may have a digital signal processor (8) and/or a controller or computer (9) and which on the one hand controls the power amplifier (3) in a purposive way and preferably also performs an automatic changeover of the secondary windings (5-7) of the matching transformer, in order to generate a desired testing signal.