Page 28 - Basic PD Theory

PD Data Types

winding. Using the 40 MHz cut-off frequency of the sensor and the 350 MHz upper frequency limitation of the instrument, the
frequency band of 40-350 MHz is monitored. As shown in Figure 25, this enables the detection of the fast rise-time PD pulses
without noise interference by maximizing the signal-to-noise ratio.

Magnitude (dB) Noise 80pF sensor High pass filter Test Instrument

20 30 40 50 60 70 230 240 250 340 350
Frequency (MHz)
Figure 25: Frequency Characteristics
High frequency signal pulses attenuate fairly quickly within the winding so only pulses occurring across voids in coils near the
high voltage end of the winding will be detected with high frequency testing, i.e., above 40 MHz. However, as stated before,
pulses across voids are highly dependent upon the applied voltage to ground, thus the coils near the high voltage end are the
ones likely to have PD. Since most failures occur in these coils, and the common failure mechanisms affect the entire winding,
high frequency testing is quite successful in detecting most failure mechanisms.

3.4.2 Disturbance Separation

3.4.2.1 Impedance Mismatch
One method of disturbance separation is to use the

A - Phase natural tendency of a high frequency pulse to distort
Circuit Ring Coil 30ê when it traverses from a conductor of one surge
100ê (characteristic) impedance level to another, or
Pulses halve in Pulses double in
magnitued magnitude impedance mismatch. The effect is a change in the
80pf magnitude (attenuation or amplification) and the
Capacitor rise-time (frequency) of the pulse. Since the typical
surge impedance of a circuit ring is about 100 Ω, and
that of a winding is of the order of 30 Ω, the
magnitude of a high-frequency pulse traveling across
this junction will be altered [Figure 26].
C - Phase B - Phase A pulse that originates in the winding will almost
Figure 26: Impedance Mismatch double in magnitude as it travels from a low
impedance coil to a high impedance circuit ring.
Meanwhile, pulses from the system (disturbances) will nearly halve in magnitude travelling from a high impedance circuit ring to
a low impedance coil. Because of this phenomenon, the high voltage leads of the sensing capacitors should be attached to the
winding as close as possible (less than 1 metre/3 feet) to the junction of the first coil and the circuit ring. Sensors in this position
will have the maximum sensitivity to machine PD and the maximum attenuation of disturbances.

3.4.2.2 Time-of-arrival
Perhaps the most effective means of separating PD-frequency disturbances is via the time-of-arrival method. Though this
method does require extra effort during installation, it ensures that PD pulses from outside the machine are classified as
disturbances (previously called noise). Two different installation configurations can be used for time-of-arrival separation:
differential and directional.

www.irispower.com

25 | P ag e