Product Literature

Principle Operation of GCM

The GCM is a sensitive real-time detector of submicrometer particles created by the incipient thermal decomposition of coatings and insulation.

Also referred to as a 'core monitor', because it was originally designed to respond to core overheating caused by circulating currents between laminations, the monitor's applications have been extended to provide early warning of all forms of overheating sufficient to produce thermal degradation.


Particle detection is by means of an ionization chamber thorough which the hydrogen cooling gas is circulated by the pressures produced by the generator fan. The Ion Chamber Detector (ICD) Figure 1 below, consists of an ionizing section and an ion collecting chamber contained in a pressure housing. The gas first passes through the ionizing section which contains a low level alpha source (Thorium 232).

The resulting ions then pass with the gas to the ion collecting chamber in which there is an electrode maintained at -10 volts. Because the ions are extremely small, they have a high ratio of charge to mass, giving them a high mobility when placed in an electric field. The -10V potential is sufficient to cause most of the ions to be attracted to a collecting electrode, where they produce the output current.
When particles are present in the gas (Figure 2), some of the ions will become attached to them. These particles, though invisible under a microscope, are many times larger than the ions.

Therefore, the charge to mass ratio of the particle-ion combination is very much reduced (by a factor of approximately a thousand), and the mobility is very low. This means that only a very few are attracted to the collecting electrode, resulting in a reduced output.
The volume of the ionizing section of the ion chamber is made large enough to establish ion-particle equilibrium, which requires several seconds. Total source strength is less than 0.1 Microcurie, far less than is used in home smoke detectors.

A typical GCM is shown in Figure 1. In this arrangement, a differential pressure gauge, which monitors the pressure drop across the ion chamber, is used to indicate flow (Indicator highlighted in Figure 4).
Ion chamber output current is amplified by an electrometer developed for this application to withstand the environmental conditions likely to be encountered. The amplified current is displayed on a meter and/or recorder, and is also used to activate alarm contacts.

The usual operating procedure is to adjust the flow to a given differential pressure indication, and then to adjust electrometer gain to produce an output of 80 per cent. The electronics will initiate the alarm verification sequence when ion chamber output drops below 50 per cent.
By comparing ion chamber output for filtered and unfiltered hydrogen, the system diagnostics can determine whether an alarm is real or due to equipment malfunction.

Switching of the alarm contacts will also open the solenoid valve ahead of the collector for a pre-set time interval, allowing a fixed volume of gas to flow through the collector. The collector contains a filter to trap submicrometer particles. The collector can be removed and the trapped materials analyzed to determine their source.

Overheating

High concentrations of submicrometer particles (pyrolysis products) are produced whenever any material within the generator is heated sufficiently to produce thermal decomposition. These 'hot spots' can lead to catastrophic failure (eg. Figure 5) if not caught in time.

When present in the hydrogen, these pyrolysis products are quickly detected by the GCM's highly sensitive ion chamber housed in the upper section of the cabinet. The GCM warns of impending failure much faster than temperature sensors such as RTDs or thermocouples.
Upon detection, an alarm verification sequence is initiated. If the alarm is confirmed, a verified alarm indication is given, alarm contacts are switched and a portion of the hydrogen flow is automatically passed through the sampling system where these particles are collected for laboratory analysis. Confirmation of an alarm is made quickly by the automatic alarm verification circuit that activates the solenoid valve in the filter/solenoid assembly.
Normally bypassed, all the hydrogen than passes through the filter which removes the particles. If the alarm is valid and thermally produced particles are present, their removal in this manner will cause the ion chamber output current (amplified by the electrometer) to return to its normal level, confirming their presence and the existence of overheating.

Gen-Tags Locate Hotspots

The usefulness of the generator condition monitor has been enhanced by the development of sacrificial coatings (GEN-TAGS) shown in Figure 6. These specially synthesized, chemically and thermally stable tagging compounds are incorporated in trace quantities in protective coatings applied to critical areas in the generator. These GEN-TAGS are designed to particulate at a lower temperature than the normal materials used in the generator and hence give an even earlier warning of overheating.


The use of several different coatings, along with the ability to trap the particles in a collector (for analysis), can greatly aid in the location of the area being overheated. (see Figure 7 & 8). Available in up to six different chemical signatures, GEN-TAGS early-warning hot-spot area distinction is now available for end windings, stator, rotor, bushings, and transformers.