Minimize Risks with Robotic Generator Inspections and Comprehensive Testing

Historically, generator inspections require rotor removal (Figure 1). There’s a significant level of risk during this process due to possible generator damage during the disassembly, and it also requires additional outage time.

1. Removing the rotor from a generator is time-consuming and presents opportunities for potential damage. Robotic inspections can eliminate some of the risks. Source: Shutterstock

Robotic Inspections

But by combining robotic inspection technology and field service experts, plant managers can decrease outage duration and reduce risks related to rotor removal. For example, GE Vernova offers “state-of-the-art Robotic Generator Inspection Services” that are said to provide unparalleled advantages, which include:

■ Reduced downtime.

■ Lowered risk, since the rotor is not removed.

■ Reduced workforce needs for dismantling.

Among GE Vernova’s offerings is a generator air-gap robotic inspection with rotor in-situ. This offline inspection solution utilizes the company’s latest robotic tool technology, which can perform a complete generator air-gap inspection program. The inspection tool can visually inspect both the rotor and stator surfaces, as well as verify stator wedge tightness and complete a stator low-flux magnetic test, which is a non-destructive method for evaluating the condition of a rotating machine’s stator. The test uses a low level of excitation to detect faults in the stator core’s interlaminar insulation.

GE Vernova also has a generator retaining ring scanner, which is a robotic inspection tool engineered to detect stress corrosion cracks without the need to remove the retaining rings from the generator. This is another inspection that is performed offline, but it only requires minimal dismantling and can be carried out with the rotor in-situ or removed.

Generator Testing and Inspection Program

Meanwhile, GE Vernova offers a test and inspection program designed to thoroughly assess the condition of a generator during an outage. Based on decades of experience across one of the largest installed fleets, GE Vernova’s diagnostic experts provide customers with a detailed analysis and recommendations for more reliable operation as part of this program.

The inspection’s scope tries to match the allowed outage time, while accounting for the generator’s model to ensure all identified areas of concern are inspected. Examples of some electrical tests that might be done in the process include insulation resistance (IR), polarization index (PI), and high-potential (hipot) tests, which are used to assess the condition of a generator’s stator, rotor, and core plate insulation. More specifically, the tests provide the following information:

■ IR measures the electrical insulation’s resistance between the stator or rotor’s copper conductors and core. A high IR value is ideal, as it indicates that there is little leakage current. A low IR value indicates that the insulation has deteriorated.

■ PI is a variation of the IR test that measures the ratio of IR after 10 minutes to IR after 1 minute. A higher PI value indicates better insulation condition. A low PI value can indicate that the winding has absorbed moisture or been contaminated with dirt or oil.

■ Hipot testing determines if a generator’s stator winding insulation is strong enough to withstand electrical stresses during the machine’s expected life. Hipot testing is a standard factory acceptance test for new machines, but it can also be used to assess the condition of existing generators. Hipot tests can use AC at the power frequency, very low frequency (VLF) at 0.1 Hz, or DC. Typically, hipot tests increase voltage slowly to a specified level, then maintain that level for a minute. The winding either passes or fails. A failed hipot test indicates that the winding needs to be rewound or the failed component removed.

Other tests that might be performed in conjunction with GE Vernova’s generator testing and inspection program include stator low and high magnetic tests, and a PULSAR test.

Stator Low Magnetic Test. This test is performed at a relatively low magnetic flux density in the stator core. Its main purposes are to detect any interlaminar insulation faults or short circuits between laminations in the stator core, identify any localized heating issues in the core, and establish a baseline for future comparisons. The test typically involves energizing the stator core at about 3% to 4% of rated flux and using thermal imaging or other temperature measurement techniques to detect any hot spots.

Stator High Magnetic Test. This test is conducted at a higher magnetic flux density, usually close to the rated flux of the machine. Its objectives include verifying the core’s ability to withstand full magnetic stress without excessive heating, detecting any major core defects that might not be apparent at lower flux levels, and assessing the overall magnetic performance of the stator core. The high magnetic test often involves energizing the core to about 80% to 100% of rated flux, and monitoring temperature rise and power losses.

PULSAR Test.PULSAR test is a catchy name derived from pulsed stepped auxiliary winding ring test. It’s a specialized test developed to improve upon traditional electromagnetic core imperfection detection tests. The PULSAR test uses a pulsed excitation method instead of continuous sinusoidal excitation. It can detect both interlaminar faults and circulating currents in the stator core, and provides higher sensitivity and better fault localization compared to traditional methods. Notably, it can be performed more quickly and with less power consumption than conventional high-flux tests.

The PULSAR test works by applying short-duration, high-amplitude current pulses to an auxiliary winding wrapped around the stator core. It then measures the induced voltage in a search coil to detect any anomalies in the core’s magnetic properties.

Still more tests that GE Vernova performs on generators during maintenance periods include the stator cooling water flow test, stator end-winding vibration test (bump test), rotor field electrical test, rotor hydrogen seal pressure test, stator radial wedging test, and brushless exciter diode test. These tests, when used in combination, provide a comprehensive assessment of a generator’s condition and help ensure reliable long-term operation.

—Aaron Larson is POWER’s executive editor.