Early warning of stator wedge failure in wind turbine generators

Background

A wind turbine generator holds its stator winding in place with wedges. Each stator wedge keys into the top of a stator slot, locks the coil bars in position and helps reduce harmonics. In many large generators the wedges are made from a ferromagnetic composite.

Stator wedges work loose over time. Electromagnetic forces, static and cyclic loads and differential thermal expansion all act on them, and a wind turbine generator sees far wider swings in load and temperature than a generator running steadily in a plant room. Once a wedge starts to move, the wear feeds itself: the wedge vibrates in its slot, wears faster and loosens further. A wedge that is eventually ejected leaves the winding unsupported, which can lead to winding damage, flashover and generator failure.

Challenge

The conventional stator wedge test is manual. The generator is taken out of service and an engineer taps each wedge with a hammer, judging its tightness from the acoustic response. On a wind turbine that means a shutdown, a technician working in the nacelle and, offshore, a vessel and a weather window. It is rarely practical to test often enough to catch a loosening wedge early.

Scheduled inspection also struggles with the shape of the failure. In the early stages there is little to find, and once deterioration accelerates the gap between a detectable fault and a failed generator is short. Repair after failure is expensive: the fault is usually unexpected, the crew may not know which parts are needed until they reach the turbine, and a catastrophic generator failure can damage other parts of the turbine.

Solution

A wind farm operator found that stator wedge wear debris does not simply settle beside the slot. As the rotor turns, it drives air around the generator’s closed cooling circuit, and a share of the wear debris is picked up and carried in that airflow. The quantity of debris in the air rises as the wedges deteriorate.

The operator’s engineers fitted a standard WearDetect sensor into the generator housing, with its detector end projecting into the circulating air. WearDetect uses a powerful permanent magnet to capture ferrous particles and reports how much debris the magnet has collected. In a gearbox that debris comes from the oil. In this generator the magnet collects ferromagnetic wedge debris from the cooling air instead, so the reading climbs as the wedges wear and can be trended in real time while the generator keeps running.

Two alert rules turn the reading into a maintenance decision: the accumulated debris passing a set threshold, and the rate of accumulation starting to accelerate. Either rule can raise an alert automatically, or the measurement can be presented as a value or a trend for our team to interpret.

A warning measured in months, not minutes

The operator describes stator wedge failure in seven stages. Wedge damage begins. Debris then appears in the cooling air. Later the wedges start to vibrate in their slots, followed by vibrational noise, heat, smoke and finally failure. The warning signs a turbine would normally show, such as noise and heat, arrive on the steep part of that curve.

WearDetect detects the second stage. With the alert threshold set to trigger after debris appears in the air but before the wedges start to vibrate, the operator estimates an early warning of around three to six months before failure, and potentially up to twelve. That is long enough to plan the intervention, order the parts and combine the work with other maintenance visits, rather than recover from an unplanned outage.

Installation

The operator used two mounting arrangements. Both keep the detector end raised above the floor of the generator housing, so the magnet sits in the moving air rather than in settled deposits.

Through the drive-end plate. The sensor body passes through an existing inspection hole in the drive-end plate, with the mounting fixed to the outside face. The sensor stays easy to reach for connection and inspection.

On an internal bracket. The sensor is fixed to an L-shaped bracket on an interior surface of the housing, fitted through an inspection hatch. This arrangement suits generator designs where a sensor in the drive-end plate could interfere with other instrumentation, such as the rotational speed sensor.

Other positions are possible depending on the generator, including the air outlet of an air-cooled generator or the recirculation circuit of an enclosed cooling system. WearDetect offers 4-20mA or 0-10V analogue outputs and CANbus or Modbus RTU digital outputs, so the reading can join an existing control or condition monitoring system and reach a remote team onshore.

What to consider before specifying

WearDetect captures ferrous particles, so this approach depends on the wearing parts being ferromagnetic. A generator with non-magnetic stator wedges will not produce debris the magnet can collect.

WearDetect is specified for use in oils, fuels and coolants. Fitting it in a generator air path is outside that standard application, so the generator design, the mounting position and the output format need discussing with our team before the sensor is specified.

The debris reading is an early indicator, not a diagnosis. It tells a maintenance team that stator wedge wear is under way and how fast it is progressing. An inspection still confirms the condition of the stator.

WearDetect also does its usual job elsewhere in the nacelle, trending ferrous debris in gearbox and bearing oil. See drivetrain and machinery condition monitoring for wind turbines, or the WearDetect product page for outputs, fittings and variants.