Electrical Diagnostics
Equipped with years of experience and the latest non-invasive diagnostic test techniques and technology, our electrical diagnostics engineers provide expert advice on the condition of generator stators, rotors and HV motors. As well as carrying out routine planned inspections, the team responds to emergency breakdowns when every minute of downtime counts.
Plant failures can result in expensive repairs and costly downtime. The best insurance is a planned monitoring and test regime that gives operators the best possible information to run their plant safely and reliably.
We offer a complete suite of low voltage and high voltage tests with full engineering back-up. We can also perform detailed generator rotor and core inspection tests without the need to remove the generator rotor. In addition, our mobile 30kV HV trailer transformer can test generator stators up to line voltage and be quickly mobilised to site.
As well as identifying insulation defects in generator stators, rotors and motors our tests can reveal other issues, such as vibration or problems with coolant water flow, which could lead to performance degradation or potential failures.
High Voltage diagnostic tests
These are non-destructive generator and motor tests which assess the condition of the stator insulation by using DC insulation resistance (IR/PI), AC loss tangent/capacitance, AC partial discharge measurements and high voltage withstand tests. The tests indicate contamination, ageing, delamination or isolated defects in the stator insulation.
Recurrent surge oscillograph (RSO) test
This test can be carried out statically or dynamically. The ‘off load’ technique identifies winding inter-turn insulation defects and winding insulation resistance to earth faults, on generator and exciter rotor windings. An inter-turn insulation fault on a rotor can produce localised heating, causing further deterioration of the insulation, which can lead to current carrying shorted turns and earth faults. This test is valuable during repairs to rotor winding inter-turn faults.
Flux probes (air gap search coils)
This ‘on load’ test of generator rotor field windings detects the presence of current carrying inter-turn insulation defects, by means of the output of flux probes (coils) installed in the air gap between the rotor and stator. The technique records the flux probe output and indicates the severity and position of any current carrying inter-turn defects. Current carrying shorted turns reduces the rotor field ampere-turns, requiring a compensatory increase in rotor current. Current carrying inter-turn shorts may also cause unbalanced magnetic fields and uneven heating of the rotor, resulting in vibration problems.
Stator and rotor inspections with the rotor in-situ
A visual inspection of the generator stator bore and rotor surface can be carried out using a camera system and robotic inspection vehicle. The inspections cover cooling vents, looking for debris, indications of fretting and partial discharge.
Electromagnetic core imperfection detector (EL CID) test
Designed to detect regions of core with damaged inter- lamination insulation on generator and large motor stators, this test can be carried out with the rotor removed from the stator or with the rotor in-situ, using a low profile robotic inspection vehicle.
Stator bar wedge tightness
Generator stator slot wedges fit into grooves near the tops of the stator core teeth. Loose wedges can lead to vibration of the stator bars, resulting in insulation damage and partial discharge between the bar surface and stator core when the conducting coating on the outside of the stator bars has worn away. An automatic hammer installed in a wedge tightness detector probe mechanically excites the wedge under test and the subsequent response values are processed to give a degree of tightness of the wedge.
End-winding modal analysis (bump test)
During operation generator stator windings are subjected to large electromagnetic forces and to withstand fault conditions have substantial stator winding restraint systems. In order to reduce the effects of vibration on the end windings, a modal analysis is carried out. If necessary,the end winding support structure is modified to change the natural frequency of the structure, thus reducing the risk of in-service failure and increasing the life expectancy of the stator winding.
Stator bar water flow checks
Direct water-cooled generator stator bars are made up of hollow section copper coils through which demineralised water is circulated to extract heat from the winding. The conductor bars are connected to the water inlet and outlet manifolds via flexible PTFE hoses. In each stator there are up to 50 small sub-conductors; these are vulnerable to blockage by debris in the water circuit which can lead to overheating and major damage. It’s therefore necessary to verify the correct water flow through each stator bar to ensure that no blockages exist in the winding. To guarantee the integrity of the system, the water flow through each individual PTFE hose is measured during a major outage of the generator, or when maintenance has been carried out on the cooling system. A portable ultrasonic flow-meter is used to measure the flow through each of the PTFE hoses as well as in the inlet and outlet pipes.
