The present invention relates to a measurement system, arranged to measure at least one dielectric property of an element under test (EUT) selected from the relative permittivity , the capacitance C and the loss tangent tan 5, in particular over a frequency range, and comprising a voltage source system, that is arranged to provide a voltage signal, an electrical sensor system arranged to measure a current value Imeas through the EUT and a voltage value Vmeas across the EUT, and a process and control unit, arranged to process Imeas and Vmeas as a function of the voltage signal into an indication of said dielectric property as a function of frequency in said frequency range.

In the field of electrical power supply, the trend is shifting more and more from alternating (AC) current towards direct (DC) current. Semiconductor technology is nowadays able to provide compact power sources, as well as technology to change the supply voltage of or for a power supply. In use of semiconductor devices, the semiconductor parts and/or insulating materials are often exposed to transient surges, such as excess voltages or excess currents, due to switching on or off of the semiconductor devices. Most of the (overload will be at a line frequency (if any), such as 50 Hz or 60 Hz. However, there may be higher harmonics, in a sometimes rather broad bandwidth. More generally, dielectrical materials may be loaded with high field strengths, either due to these harmonics, but sometimes even based on the electrical field itself that is due to a high current. Both the high constant loading and the loading by the higher harmonics may give rise to effects such as dielectric heating, which in turn may lead to changes in dielectric properties or even dielectric breakdown.

It is thus important to know how a circuit part or dielectric material behaves under the influence of such loading with a high field strength and/or higher harmonics.

At present, dielectric materials may be tested by means of a well-known Schering bridge. This comprises an AC voltage source, arranged to supply a sine voltage with a frequency f and an amplitude V, as well as a number of circuit elements, such as a standard resistor, a variable resistor and a variable capacitance. The values of the variable components are varied until a balanced circuit is obtained.

A disadvantage of the known system is that it does not allow to the dielectric properties being known under all circumstances, in particular realistic applications. In turn, this leads to the application of dielectric materials much below their specification, or rather "possibilities", in order to be on the safe side.

It is therefore an object of the present invention to provide a measurement system of the kind mentioned above, that is able to measure dielectric properties of materials, and/or circuit elements or devices, in a wider range of circumstances, thereby enabling a better characterisation of the material and a wider range of applications for the material.

The invention achieves the object at least in part by means of a measurement system according to claim 1 , in particular a measurement system, arranged to measure at least one dielectric property of an element under test (EUT) selected from the relative permittivity , the capacitance C and the loss tangent tan 5, in particular over a wideband frequency range, and comprising a voltage source system, with a first voltage source that is arranged to provide a DC voltage Voan, and a second voltage source that is arranged to provide a wideband AC voltage signal VAC , an electrical sensor system arranged to measure a current value Imeas through the EUT and a voltage value Vmeas across the EUT, and a process and control unit, arranged to process Imeas and Vmeas as a function of VAC and Voan into an indication of said dielectric property as a function of frequency in said wideband frequency range, wherein the voltage source system is arranged to provide Voan and VAC to the EUT in parallel. Herein, the wideband AC voltage signal comprises a range of voltage signals within a frequency band.

The inventor has realised that measurements of the known system may only performed either at a constant, high voltage and with a broad frequency spectrum, or the other way around. For example, because it takes a certain time to determine the values of the variable components at a certain frequency there will not be an instantaneous measurement. But in all cases, the measurement signal will be a more or less high voltage sine wave. But because of this time delay during measurement, the changing high voltage will cause heating that is in fact not due to dielectric properties but only due to the high field strength of the basic signal, that would not be present in case of transient loads. And such constant high loading will not be present in practice. There is thus a kind of mixing between effects on the dielectric material due to the amplitude of the high voltage AC test signal, that would not be present in practice, and effects dus to the signal having a certain high frequency, at which the dielectric properties are to be determined. To counter this, the insight was used that such problems would not occur if the dielectric material would be loaded by means of a DC high voltage, overlaid with a broadband AC signal. The DC voltage ensures that the insulating properties will be measured correctly, because there will be no superfluous dielectric heating due to alternating high voltages, while the broadband AC signal allows to measure the property/properties in particular over more frequencies at a time without undue additional heating, since the amplitude of the AC signal may be very much lower than the amplitude, or rather magnitude, of the DC signal. Based on the measurement results, i.e. the values of Vmeas and Imeas as a function of the frequency, the dielectric properties may be calculated according to principles that are known per se. For example, if the capacitance C has been measured, and the geometry of the EUT is known, then one can calculate the relative permittivity according to s = C * d/A, wherein A = surface area of the EUT, and d = thickness of the EUT. Furthermore, tan 5 = resistive power loss/reactive power = ( Vm * n) I ( Vmeas * Imeas), complex. Such calculations are known to the skilled person, and will not be elaborated in detail.

Particular embodiments and further features and advantages are described in the attached dependent claims, as well as in the now following part of the description.

In embodiments, the voltage source system is in addition arrangeable to provide either Voan or VAcm. This allows for example to measure the DC voltage or field strength at which dielectric breakdown occurs. Alternatively, it allows to measure the properties at various frequencies of an electronic part in use thereof, in which it is supplied by an external power source. Thus, it is not the voltage source of the measurement system that provides a high voltage DC signal, but for example a charging station for an EV. In either case the voltage source system simply turning down one of the two voltage sources.

Advantageously, the first voltage source is an adjustable high voltage source, arranged to provide a Voan with a value between zero and a maximum DC voltage in the range of 0-15 kV, more specifically between 1 and 15 kV. This means that the DC voltage is variable, and can reach a maximum value. This maximum value can have any value upto 15 kV, although other, higher values are not excluded. With a value of upto 15 kV, most situations in e.g. consumer electronics are covered. Of course, the selected maximum DC voltage is preferably higher than the standard supply voltage for the EUT. Thus, in many cases the maximum DC voltage should be (much) higher than 230 V, such as 1 kV. For testing charging systems for EVs (electrica Ivehicles) that charge at high speed chargers with a charging voltage of 400 V or even 800 V, the maximum DC voltage should be (much) higher than that, such as a few kV or more.

The set-up or lay-out of the second voltage source is not particularly limited, as long as it is wideband, able to generate a plurality of frequencies in a short time, or preferably substantially simultaneously, such as pseudorandom binary sequences or the like. In favourable embodiments, the second voltage source is or comprises a maximum length sequence generator, in particular an m-sequence signal generator. This is a useful example of a generator of pseudorandom binary sequences, that comprise frequencies over all of the bandwidth, is spectrally flat and highspeed, and thus does not lead to unnecessary heating at any of the frequencies, and is easily controlled.

The frequency band of the AC voltage signal VAan is not particularly limited, as long as it reflects the range of frequencies that will, or may, play a role in practical situations. In embodiments, the wideband AC voltage signal VAan has signal frequencies within a band of from 100 Hz upto a maximum frequency of between 2.5 kHz and 25 MHz, such as in particular a band from 100 Hz to 250 kHz. In such a frequency band, the harmonics are included from the first one at 100 or 120 Hz, upto the highest relevant harmonics to be expected in everything but the most extreme situations. Furthermore, for most solid insulators, the primary relaxation frequencies may be detected in this band, by detecting local extreme values in the relative permittivity (or the measured capacitance) and/or the loss tangent tan 5. The relaxation frequencies are those where the material's polarisation can no longer follow the voltage signal, thereby leading to dielectrical heating up of the material. Thus knowledge of these relaxation frequencies is important is designing and assessing systems with dielectric materials.

The invention will now be elucidated with reference to some exemplary, non-limiting embodiments, and to the drawings, in which the only Figure 1 diagrammatically shows a measurement set-up with a measurement system according to the invention.

In Figure 1 there is shown diagrammatically a measurement set-up with a measurement system according to the invention, generally indicated with reference numeral 1 , and an element under test (EUT) 100, as indicated in the dashed box.

The measurement system 1 comprises a voltage source system with a high- voltage DC source 2, a wideband AC voltage source 3, a voltage sensor 4, a current sensor 5, a resistor 6, and a process and control unit 7. Furthermore, safety components are indicated as a guard resistor 8, a guard capacitor 9 and a guard circuit 10.

The EUT 100 is represented here, simplified, as a resistance 101 and a capacitance 102. In practice, the EUT may be a piece of insulator, or dielectric, material with a known geometry. It can also be a part such as a cable, connector et cetera.

Although various parts of the measurement system 1 have been shown as separate parts, they may be integrated. It is assumed that the high DC voltage is hardly influenced by any of the EUT or the rest of the measurement system, and thus that the DC voltage across the EUT is in fact substantially the same as the voltage output by, and set in, the high voltage DC source 2. If necessary, the DC voltage and the AC voltage could of course alternatively be measured by a general voltage sensor across the EUT. Furthermore, the current sensor 5 has been shown as, in fact, a voltage sensor that measures the voltage across a known pure resistor 6. Alternatively, current sensor 5 and resistor 6 may be replaced by a current sensor that measures the current directly.

The wideband AC voltage source 3 produces a variable voltage signal comprising many frequencies within a wide frequency band. An example of a useful wideband AC voltage source is a pseudorandom binary sequence generator. Such generators are designed to provide a voltage signal that is based on a binary sequence, generated by an algorithm, and often based on linear feedback shift registers. Examples are Gold sequences, and preferably the maximum length sequence, or m-sequence. It provides a wide frequency band and has a flat spectrum, thus causing little addition (dielectric) heating. Such an m-sequence generator may e.g. be the m:explore from applicant.

The process and control unit 7 is arranged to control the high voltage DC source 2 and the wideband AC voltage source 3, as well as receive the measured values and process same into an indication of one or more dielectric properties. In practice, the process and control unit 7 can start a measurement by initiating the high voltage DC source 2 to supply a high voltage, if desired settable and adjustable, to the EUT 100. The unit 7 then initiates the wideband AC voltage source 3 to supply the voltage signal, comprising the many frequencies, in the form of in particular a binary sequence.

The resulting voltage across the EUT, Vmeas, and the current through the EUT, Imeas, are then measured by the voltage sensor 4, the current sensor 5, respectively. The process and control unit 7 then analyses the signals, and calculates the values per frequency value of the input signal. If desired, the calculated values may also be displayed as a function of frequency, such as on a display on the process and control unit 7. The latter can also process the calculated values further into e.g. values of the capacitance C. Displayed values of C as a function of frequency may already give an indication of the frequencyAies where dielectric heating is highest. Such frequencies are relaxation frequencies, and are indicated by a local minimum of the capacitance C.

The unit 7 may furthermore calculate the relative permittivity , if the geometry and dimensions of the EUT are known. For example, for a disk with thickness d and surface area A, the s may be calculated as s = C * d I A. The value of s may of course also be displayed, additionally or alternatively. Relaxation frequencies will then show as local maxima of the relative permittivity. Finally, the loss factor tan 5 may be calculated from e.g. the input power and measured "power" (Vj _n * n) / (V_ neas * l_meas) - Herein, Vj _n and lj _n denote the momentary (measured or set) total input voltage signal and ~ current, while V_meas and /_ _me as denote the measured values as described above, respectively.

The guard resistor 8, guard capacitor 9 and/or guard circuit 10 are all optional, and provided only as safety measures, in case of e.g. failure or breakdown of the EUT. They do not influence the measurements proper.

The invention is in no way limited to the embodiments shown. The scope of the invention is rather determined by the attached claims.