Page 27 - Basic PD Theory

PD Data Types

3.3.1 Portables
There are three basic types of Portable instruments: TGA-S (SSC installs), TGA-B (BUS installs) and PDA-IV (PDA
installs). These can be packaged individually or as a combination of any two. They require a Laptop computer as a
controller as shown in Figure 23.

3.3.2 Guards
There are three types of Guard instruments that include PD: TurboGuard (SSC installs), BusGuard
(BUS installs) and HydroGuard (PDA installs). The Guard instrument is continuous and requires a
dedicated remote computer connected via a network connection to function as a controller. This
system can be configured to function over a LAN or a WAN. Guard systems can include multiple
technologies, such as, PD, Endwinding Vibration, Rotor Flux, and Shaft Monitoring. In addition,
they have preconfigured smart triggers and data collection to facilitate data analysis.

3.3.3 Trac Instruments
Figure 24: Guard/Trac There are three types of Trac instruments for PD: BusTrac (Bus installs), HydroTrac (PDA installs)
System and PDTrac (Single-ended installs). The Trac instrument is continuous and connected via a
network, LAN or WAN connection to a remote computer used as a controller. Tracs are single
technology instruments used for trending of data and pattern analysis.

3.4 Noise and Disturbances
Most companies prefer a PD test that can be performed by their staff during normal machine operation. Electrical noise from
power tool operation, corona from the switchgear and radio frequency sources etc. are easily confused with PD from the
machine windings. This confusion can lead to healthy windings being misdiagnosed as deteriorated and thus lowering
confidence in the test results. A dependable on-line PD test should significantly reduce the influence of noise and disturbances,
thus leading to a more reliable indication of the insulation condition [2]
IEC 60034-27-2 defines that “Disturbances are electrical pulses of relatively short duration that may have many of the
characteristics of stator winding PD pulses – but in fact are not stator winding PD” [12]. Some of these disturbances are
synchronized to the AC cycle and some are not. Sometimes synchronized disturbance pulses can be suppressed based on their
position with respect to the AC phase angle. See Section 3.4.2 for more information about Disturbance separation.
“Noise” is defined to be “non-stator winding signals that clearly are not pulses” [12]. Electrical signals resulting from power tool
operation, corona from the switchgear and radio frequency sources are all examples of such noise. Noise may also be due to
electronic devices within the PD detection system itself. See Section 3.4.1 for more information about Noise minimization.

3.4.1 Noise Minimization

3.4.1.1 Frequency (Band-pass Filtering)
One of the most effective ways of minimizing noise is frequency domain separation, or to test at frequencies high enough that
only the higher frequency, over 50 MHz, partial discharge pulses will be detected and the lower frequency, below 40 MHz,
electrical noise pulses will be suppressed. The key is to select an appropriate frequency range such that the signal-to-noise ratio
(SNR) is so high that only PD signals and not noise are counted.
The sensing device defines the low-end frequency limit. The 80 pF sensors terminated into 50 Ω impedance limit the lower
cutoff frequency of detection to 40 MHz. Above this frequency, the noise signal is normally eliminated and therefore the Signal-
to-Noise-Ratio is high. Limiting the sensor to detect only high frequency signals reduces the total amount of PD energy found,
but more importantly, it also eliminates the need for an expert to discriminate between noise and PD. This creates a test that a
non-expert can perform and data analyzed.
The upper cutoff frequency is set by the test instrument. The test instrument has an input bandwidth of 0.1-350 MHz. It is
capable of detecting the fast rise-time initial pulses and the high frequencies (50-250 MHz) typical of PD originating in the

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