Ultrasonic Testing for In Situ Generator Inspection

Ultrasonic Testing using Manipulators in Generators

The generator is one of the most crucial parts of a power plant, since it is where the mechanical energy is converted into electric energy. It is therefore essential that the generator is always in optimum condition. Among the critical components are the retaining rings at each end of the rotor. These are susceptible to damage as a result of specific chemical influences and mechanical or electrical forces. However, replacing the rings involves substantial costs. The DEKRA inspection system for retaining rings (KIRR) represents a cheaper yet equally effective alternative. The system has already been used to inspect over 1,600 generator retaining rings in 25 countries worldwide.

About Retaining Ring Inspection

A retaining ring has a long lifespan and often lasts for the entire life of the generator. However, the smallest of cracks in the retaining ring poses a risk to safety and could result in extremely costly damage. To avoid such disastrous consequences, many manufacturers recommend that the retaining rings are replaced periodically – an expensive exercise, which can cost up to a million euros. DEKRA’s KIRR offers a cost-effective alternative. The system detects and analyzes flaw indications in the retaining ring, enabling you to determine whether the retaining ring needs to be replaced. Furthermore, you can keep a close eye on newly detected flaw indications in retaining rings, which do not yet need replacing and monitor their condition during future operation. One crucial advantage is that it is no longer necessary to disassemble the rotor and the retaining rings for inspection purposes, which in turn yields cost savings of up to EUR 200,000. The KIRR system is adjustable to fit virtually all generators and large motors. This flexible system combined with our extensive experience in this field allows us to draw up the inspection plan on site, without detailed information being required beforehand. If required, the inspection can be performed within 24 hours. Ultrasonic crack detection is focused on the shrink-fit areas and wall thickness steps. Damage, such as stress corrosion cracking and top tooth cracking, is detected. In addition, eddy current scanning is added to accurately map the outer surface. The ring’s coating is left in place. If cracks are discovered, ring geometry and flaw data are used in an FEM computer model to calculate the residual lifespan. Furthermore, we establish the inspection interval for monitoring purposes.
DEKRA’s advanced ultrasonic testing experts are certified in accordance with ISO 9712 and Nordtest and our testing meets all the latest testing standards. Automated and semi-automated ultrasonic testing is carried out using the latest time of flight diffraction (TOFD), pulse echo and phased array (PA) techniques. We also provide customers with dedicated data evaluation software packages for the evaluation of ultrasonic testing data (ultrasonic testing signal and coordinates).
The KIRR System can be applied on Generators with a Ring Diameter of 600mm and upwards. In-Situ Test Characteristics:
  • Minimum gap between retaining ring and stator: 8mm
  • Technique: UT-TOFD, UT-TRL and ET scanning
  • Surface: 360° circle and 1-10mm pitch
  • Defect heights of 0,5 mm and upwards are detected
  • Typical test duration for two rings in-situ: 24 hours


Generators are crucial components in an industrial power plant and are designed to operate reliably for many years. One of the most expensive parts of a 3-phase generator is the stator, which is where the energy from the magnetic field is converted into electrical energy. The condition of the generator and its core deteriorates over time, increasing the likelihood of failures. Quantitative testing and periodic inspections of the generator core are necessary to avoid unplanned outages. ARGIS enables inspections to be carried out not only when the rotor has been removed, but also with the rotor still in place, thus saving you time, effort and money.
Major Savings, for In Situ Inspection with ARGIS:
  • Less manpower is required to prepare the generator for inspection (0,5-3 shifts instead of up to several weeks)
  • Reduction of outage costs up to EUR 300,000
  • Reduction of lost productivity due to outage time (which can amount to millions of euros)
  • Fewer risks of damage to the generator
  • A generator inspection can stay out of the critical path of an outage
  • High reproducibility
  • All data is stored for future comparison and data trending
  • Low flux leakage testing with excitation through rotor forging reduces dismantling of the generator
  • Better anticipation and planning possible for next major outage (plan repair work or postpone)
  • ARGIS can be combined with in-situ retaining ring testing (KIRR)
A chain containing motor drives and a docking station for the Generator Inspection Vehicle (GIV) is mounted around one of the retaining rings of the generator. The chain is positioned accurately in front of each slot so that the GIV can be inserted into a gap as small as 17 mm (0.67 inch) and up. The GIV then moves to the other end of the stator core and back in order to perform the LFCT, Wedge tightness test and visual inspections all in the same run to save time.
    Electromagnetic core flux leakage testing (ELCID or equivalent) is a standard feature. The step irons are measured with a separate module.