Monitoring the air gap13 April 1998
Monitoring and diagnostic systems helped Hydro-Québec mitigate concrete growth problems at its Rapide-des-Iles hydro plant
The power station Rapide-des-Iles is located in the northwestern region of Canada’s province of Québec. The facility is owned and operated by Hydro-Québec, the largest hydroelectric power utility in Canada.
Rapide-des-Iles is composed of four Francis-type hydro generators rated at 40MVA. Units two, three, and four were commissioned in 1967, while unit one was inaugurated in 1973. The station is used for base load and all four generators regularly operate at full load, depending upon water availability.
‘Concrete growth’ is an alkali-aggregate reaction whereby cement and aggregates react. It leads to a slow and progressive increase in the overall volume of concrete, and it can cause deformation of embedded and attached structures. The problem was first diagnosed by Hydro-Québec at its Rapide-des-Iles plant in 1973-74. The growth had imposed intense constraints on the stator supports and was affecting stator roundness.
By the end of the 1970s, stator deformation had reduced the air gap close to critical values. Because of this, the rotor rims were deformed by the increased magnetic field. Two of the four generators soon encountered problems with their spider arm rotors, and Hydro-Québec was forced to take these generators off-line in the mid-1980s for safety reasons.
As a result, Hydro-Québec decided to install its newly developed air gap monitoring system in order to assess the effects of the alkali-aggregate reaction on all four stator-rotor structures.
On-line monitoring of the air gap in a hydroelectric generator provides sig-nificant and timely information about its structural condition. The vibrosystm air gap monitoring system (AGMS) makes it possible to monitor the behaviour of both rotor and stator structures dynamically (ie while the generator is in operation).
AGMS is based on capacitive technology developed at Hydro-Québec and licensed worldwide for commercialisation to VibroSystM. In the system, the air gap is measured by reading the modulated capacitive current which flows between a sensor which is cemented to the stator stack, and the rotor poles. The sensor is one of the plates of a capacitor, the other plates being represented by the rotor poles that pass in front of the sensor. The air gap between rotor and stator is the dielectric of the capacitor.
The modulated current signal is inversely proportional to the air gap dimensions; through electronic processing it is possible to convert the signal into a linear output response, which is the exact dimension of the air gap. By positioning enough sensors around the stator, it is possible to inform the user about the shape of the rotor, the shape of the stator, the minimum and maximum values of the air gap around the generator, the roundness and the eccentricity of the rotor and stator. Here the roundness is the index of how closely a figure resembles a circle: the smaller the roundness index, the more the figure resembles a perfect circle. The roundness of the rotor and stator are calculated with respect to the unit’s centre of rotation. The eccentricity of the rotor is the relative position of its geometric centre compared with the centre of rotation: the same applies to the stator.
The AGMS system has proven itself to be an efficient tool for monitoring structural problems related to loose rotor rim, weak rotor structure, uneven thermal expansion of rotor and stator, and structural change of stator foundation (concrete deformation). Monitoring these generator problems helps avoid the costly consequences of forced outages for breakdowns or the rotor-stator, and extends the safe operating period.
Problems Early AGMS data collected on each Rapide-des-Iles unit clearly showed weaknesses in the rotor rim between the spider arms. On unit four, the system also revealed two over-dimensioned replacement poles.
The air gap variation between each spider arm is about 50mils. For a nominal air gap of 450mils, this variation (11%) confirmed that the rotor rim had come loose over time. Con-sequently, the rotor round-ness as shown by the AGMS was outside prescribed standard values. The min-imum gap occurs at the end of each arm since these locations are rigid – experience with other installations around the world shows the same roundness problem with other rotors of the eight-arm spider design. To improve rotor uniformity and roundness, Hydro-Québec replaced the eight-arm spiders with new 14-arm ones and reinforced the spider assembly with a steel plate. In addition, Hydro-Québec machined 120mils off the two protruding poles on unit four to correct their over-dimensions.
New measurements taken on the units equipped with the 14-arm bracket showed a better-supported rim, in compliance with standard roundness values. With the AGMS permanently installed, Hydro-Québec was able to monitor dynamic rotor and stator structure behaviour and therefore ensure safe operation until 1994, at which time refurbishment works were planned on all four machines. As part of the refurbishment project, all four machines were disassembled and all stators replaced.
By comparing data gathered by the AGMS before and after the refurbishment, it was possible to evaluate the quality of the work performed, with regards to rotor-stator roundnesses and eccentricities as well as air gap (see table).
Hydro-Québec has now upgraded its AGMS to a ZOOM (Zero Outage On-line Monitoring) system. ZOOM measures air gap and several other user-defined parameters simultaneously, and records other machine status information. It has an open architecture which allows new and existing instrumentation to be mixed, minimising both the investment required and the installation downtime. All parameter data are integrated in a single database for correlation and in-depth machine analysis. This allows the company to monitor a wide range of additional parameters such as: vibration (combined generator guide and thrust bearing, turbine guide bearing, stator core, stator bar); temperature of the stator and bearings; magnetic flux; and on-line machine operating status (MW, MVA, rotor and stator currents and voltages).
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