MORTEROS Tipo A
3.16. CONTROL DE CALIDAD
insulation materials that are complex organic materials, which when degraded by heat or electrical action, produce a large number of gaseous, liquid and solid products. Insulation materials are natural or synthetic organic polymers and their thermal degradation is complex. As the temperature rises above its permitted value, circa 1608C, volatiles used as solvents in manufacture start to be driven off as gases. Then, heavier compounds in the resin reach their boiling point and the gases produced are heavier hydrocarbons, such as ethylene. As the temperature rises above 1808C, chemical decomposition of the resin starts. A supersaturated vapour of the heavier hydrocarbon decomposition products forms in the cooling gas close to the hot insulation. Rapid condensation of that vapour occurs as the cooling gas leaves the hot area, producing condensation nuclei that continue to grow until they reach a stable droplet size. The materials given off depend not only on the insulation material but also on the machine’s cooling gas. The insulation binder material, wood, paper, mica or glass fibre, can usually withstand much higher temperatures, but when 4008C is reached, they start to degrade and char, releasing gases such as carbon monoxide and dioxide, drawing oxygen from the air or from the degradation of the complex hydrocarbon in the resin. Pyrolysing activity therefore gives rise to a wide range of gases, liquid droplets and solid particles, which together form the smoke being driven off from the insulation.
Electrical discharge activity, within or adjacent to the insulation system, also degrades the insulation, releasing particulate and gaseous products. The very high temperature associated with sparking breaks down the insulation hydrocarbon compounds to form acetylene. It also breaks down the oxygen in the cooling gas, if it is air, to give ozone. Furthermore, continuous discharge activity gradually carbonises and erodes the insulating material to Figure 5 Comparison between measurements and the
predictions of a thermal image of an electrical machine Taken from [23]
a Comparison for 5.5 kW induction motor b Duty cycle for a
c Comparison for 7.5 kW induction motor d Duty cycle for c
produce, on a smaller scale, the degradation products that result from more widespread overheating.
Insulation degradation can be monitored chemically by detecting the presence of particulate matter in the coolant gas or by detecting gases, like carbon monoxide, ozone, or more complex hydrocarbon gases, like ethylene and acetylene.
5.1.1 Particulate
detection-core
monitors:
Detecting the smoke given off from degrading insulation is the simplest technique, since proprietary smoke detectors already exist using ion chambers to detect smoke particles. As the cooling gas of the machine enters the ion chamber, it is ionised by a weak radioactive source. The gas then flows through an electrode system carrying a polarising voltage. Free charges in the gas are collected on the electrode and flow through an external electrometer amplifier, which produces an output proportional to the ion current. When heavy smoke particles enter the chamber, their greater mass implies a lower mobility compared with gas molecules and, thus, the ion current reduces. Therefore the smoke is detected by a reduction in the amplifier output voltage. An ion chamber was designed to detect the products of heated insulation and this was applied to a large turbogenerator [25]. The primary impetus for this work was the need to provide early warning of core faults in large turbogenerators referred to in [4]. The lifetime of pyrolysed particles in the closed hydrogen cooling circuit of a large generator is 15 – 30 min after which the particulates are deposited onto the exposed surfaces of the machine. A single instance of insulation overheating should lead to a reduction of the core monitor ion current for this period of time. Fig. 6 shows typical core monitor responses. The sensitivity of the device depends upon the ion chamber design, but experimental figures for the monitor described in [25] show that it will produce a response ranging from 85% – 95% of full scale deflection when 100 cm2 of lamination insulation is pyrolised, depending on the material. However, the monitor has practical difficulties: 1. the output fluctuates with cooling gas pressure and temperature;
2. it responds to oil mist that may be present in the circuit of any hydrogen-cooled machine because of faulty hydrogen seals[26];
3. it is non-specific; that is, it cannot distinguish between the materials being overheated.
Items (1) and (2) affect the background signal from the monitor, which any signal because of damaging overheating must exceed. Fig. 7c shows a typical core monitor trace from a machine affected by oil mist. Item (3) affects the attitude of a machine operator to an alarm from the core monitor, since there will be less confidence in the monitor if it does not reveal the part of the machine where the detection originated.
A more advanced monitor, described in [27], overcame problems (i) and (ii) by using two identical ion chambers in series in the gas flow, with an intermediate particulate filter between them. The monitor displays the difference between the ion currents in the two chambers and, thereby, eliminates fluctuations because of pressure and temperature. It was suggested that oil mist is only produced by overheating and, thus, that its detection may be useful. The use of heated ion chambers was not initially encouraged, however, the current thinking is that heated ion chambers are essential for reliable detection. However, the oil mist content in a machine varies widely and can be high, in which case there can be frequent false alarms and, thus, the use of a heated ion chamber gives an advantage. To vapourise an oil mist, the ion chamber temperature must be raised above 1208C. The monitor described in [27] had heated ion chambers and the authors’ experience, using these set to 1208C, was that they gave an adequate protection against spurious oil mist indication but that they also reduced the number of droplets produced by overheating, causing a loss of sensitivity quantified in [28]
at 1208C as 20%,
The author is not aware of the core monitor being used on air-cooled machines, or machines without a closed cooling circuit, although, apart from the short time constant of the indication from the monitor, there seems to be no reason why it could not be used for these applications. Experience has shown that the core monitor cannot be relied upon, on its own, to give incontrovertible evidence of an incipient fault. It is a valuable device that does detect pyrolised insulation, but its indications need to be considered alongside those of other monitoring devices. The core monitor needs to be complemented by off-line techniques analysing the particulates causing the detection.
5.1.2 Particulate detection-chemical analysis:
Authors advocated taking particulate material samples when a core monitor indicates an alarm [25]. To collect a detectable amount of particulate matter within a short time, it is necessary to have a very large gas flowrate through the filter by venting the pressurised casing of the machine through the filter to the atmosphere. However, there is no agreement about the method of analysis, because the pyrolysis products contain large numbers of organic compounds and the resultant chromatograms are difficult to interpret,[29]andFig. 7. An alternative is to reduce the chemical information by using less sensitive techniques one of which makes use of the fact that many organic materials fluoresce when irradiated with ultra-violet light. The
resultant UV spectrum is less complex than a
chromatogram. Despite the techniques described here, there is, as yet, no conclusive way to identify material collected on a core monitor filter. A way out was sought by inserting tagging compounds in the machine, which when overheated give off materials with easily identifiable chemical compositions. This technique was used in the US in[25].
222 IET Electr. Power Appl., 2008, Vol. 2, No. 4, pp. 215 – 247
&
The Institution of Engineering and Technology 2008 doi: 10.1049/iet-epa:200702805.1.3 Gas detection:
The advantage of on-line gas analysis is that, because of the long residence times of overheating gases, earlier warning may be obtained of machine damage. The disadvantages are the complexity of the analysis and the difficulty of translating it into an electrical signal. Based on [30] a continuous monitor was devised for hydrogen-cooled generators, using a flame ionisation detector (FID) to measure the total organic content in the hydrogen. This detector is used for the detection of organic species and the generator cooling gas is introduced into a hydrogen/air flame, which forms part of a circuit that normally presents high resistance. When organic species are introduced, carbon ions are formed and the flame resistance decreases linearly with the amount of organic compound introduced. The device is sensitive andcan detect increases as small as 0.2 parts per million by volume (vpm) of methane (CH4) equivalent. Its sensitivity
is reduced by the presence of background levels of organic compounds that can be 10 – 50 vpm with a variability of +20%. However, one advantage over the core monitor is that it shows the trend of any increase.
An alternative to the FID detector is the photo ionisation detector, which contains an ultraviolet lamp that ionises the gas stream flowing past it. A potential is applied across electrodes in the detector and the conductivity is measured as in the FID, as shown in Fig. 8 alongside the core monitor indication. The device detects heavier hydrocarbon compounds in the gas stream and it has been shown that a fault on a large generator, involving 2 g of organic material Figure 6 Typical core monitor responses
a Machine with overheating conductor bar b Machine with a core fault
overheating, produced a 1 – 2 vpm deflection against background levels of 7– 10 vpm.
Recent work [31 – 33]has reverted to detecting the gases from faults using the more complex quadrupole mass- spectrometer.
On air-cooled machines overheating incidents produce a large volume of carbon monoxide and dioxide as well as light hydrocarbon gases. An instrument to detect
overheating by measuring the carbon monoxide
concentration was described in [4]containing a pump that drew air from sampled motors, to a commercial infra-red analyser measuring the carbon monoxide content. The air in a sealed motor enclosure should recirculate with a long residence time but leaks reduce this, diluting the carbon monoxide. However, the analyser detected carbon monoxide concentrations of less than 1 vpm. Calculations showed that 180 g of insulation heated to 3008C will introduce a 1.5 vpm rise in carbon monoxide concentration in the cooling air. Therefore the analyser had sufficient sensitivity to detect motor winding overheating.