VTSD 9562 – Limiter with Band-pass Filter

Schwarzbeck VTSD 9562: Bandpass and Limiter for Partial Discharge Measurements.
High voltage devices are very often tested according to the “Partial Discharge Method”. The high voltage equipment under test (for example a transformer) is therefore connected to a Partial Discharge Coupling Device.

The output of the Partial Discharge Coupling Device is connected to the input of an interference measuring receiver. The meter reading of the receiver depends not only on the amplitude, but also on the repetition rate of the pulse. Practical measurement shows, that the sensitivity of good receivers is completely required for a reasonable measurement. The power attenuators, which are included in some receivers for protection purposes, are sometimes switched off. In this case, the following attenuator cells are in danger, because they are less rugged. Resistors which are able to dissipate 1/4 watts or even 1/2 watts may be destroyed immediately by very short pulses with very high peak voltage, even though the average power is very low. The explanation for this burn out is the fact, that the thin resistive coating cannot deliver this “heat pulse” to the resistor’s body in a very short period of time. The resistive coating burns out on a narrow spot which is usually invisible. When the receiver is used without any input attenuation, even the diodes of the first mixer may be destroyed. A variety of limiters are already on the market to protect vulnerable receivers in standard EMI-measurement from potential overload. They usually use a combination of fixed attenuation and diode limiting. This combination helps insufficient receivers used in precompliance measurement. The additional attenuation is a minor problem when measuring interference voltage, because the limits are high. On the other hand the limiting diodes will to a certain degree produce signal distortion which can lead to wrong measurement. Being a reasonable compromise for EMI-measurement, this kind of limiter is not usable for partial discharge measurement.

The limiter VTSD9562 uses the fact, that the partial discharge measurement measures a pulse signal. A pulse signal corresponds with broad band spectrum in the frequency domain. While a sine wave signal concentrates its power in a single spectrum line (frequency), pulse spectrum spreads the power over a wide frequency range. For this reason, the frequency range is limited to 150 kHz – 30 MHz. A closer look to the corresponding time domain shows, that the pulses now have less amplitude and longer duration. Less amplitude means less destructive power for the receiver. Measurement precision is not influenced because of the fact that the pulse band width in the receiver is 9 kHz, about 1% of the band width of the band pass filter. The band pass filter uses a combination of high pass and low pass filters. Special toroid cores are used to keep intermodulation low. The diode limiter is placed after the filter clips voltages beyond +-2.5 V. This voltage is far beyond the level, which the receiver can process correctly under this condition. There should be no side effects for the measurement. The advantage of such a “double limiter” is better receiver protection without negative side effects on sensitivity and dynamic. The restriction in frequency range is negligible, because there is no partial discharge measurement outside this frequency range. On the other hand, using this limiter for conducted voltage measurement in the frequency range 9 kHz – 30 MHz is impossible.