Rotating machines

Apply to
Stator and rotor windings, RTDs, insulated supports, insulated bearings

Needed instruments
Mega-ohmmeter

Available instruments
Megger MIT525-EU (2x)

Reference standard
IEEE Std. 43-2013

Description
Insulation resistance (IR) is defined as the ratio between the voltage applied to the insulation and the absorbed current. Because IR value is strongly dependent on temperature, the ratio between IR at 10 minutes and IR at 1 minute (from voltage application) is calculated, this ratio is the polarization index (PI), which does not depend on temperature.
The measurement is performed by applying to the insulation a negative DC voltage (amplitude dependent on the rated voltage of the winding) and measuring the absorbed current. The voltage application lasts 1 minute (IR only) or 10 minutes (IR and PI), in the second case IR is recorded every minute.
IEEE 43-2013 suggests minimum values for IR (reported at 40 ° C) and for PI: -IR> = “Machine rated voltage in kV +1” MΩ, PI> = 1.5 for insulation prior to 1970. -IR> = 100 MΩ and PI> = 2 for insulations after 1970.
If the test is passed subsequent ACt tests (Tangent Delta, Capacitance and Partial Charge) can be performed.

Apply to
Stator windings

Needed instruments
AC supply, measurement bridge, resonating inductor

Available instruments
Doble M4100
Omicron CPC 80 + CP TD1

Reference standard
IEEE Std. 286-2000
IEC 60034-27- 3

Description
The measurement of the dissipation factor (or tangent delta) and the capacity is performed by supplying the stator windings with a step voltage profile, each step is 20% of the maximum test voltage, increasing and decreasing. The maximum test voltage is usually equal to the rated phase-to-ground voltage of the machine. Tangent delta and capacitance tip-up are also computed, corresponding to the difference between the measured values at 100% and 20% of the test voltage.

The dissipation factor is a global measure of leakage losses, which may be due to polarization, conduction through insulating volume, conduction on the outer surface of the insulation. The value of these losses varies with the applied voltage, and with the time. Comparison of these values through the years allows to diagnose some aging and failure mechanisms.

Capacity measurement (and tip-up) allows diagnosing generalized aging phenomena, water absorption, and partial discharge.
The dissipation factor is also called tangent delta because insulation losses can be modeled by a capacitance in parallel with a conductance equivalent to the losses.

By supplying this circuit in alternating current it will be that due to the conductance G, the current angle will be different from 90 (capacitive current only), of an amount called delta, function of G itself.

Apply to
Stator windings (on-line and off-line)

Needed instruments
Partial discharges detector

Available instruments
Techimp Portable PDCheck 2.4
PDTech Deltamaxx

Reference standard
IEEE Std. 1434-2012
IEC 60034-27- 2

Description
Partial discharges are electrical discharges located inside the insulation or on the insulation surfaces, which only partially bridges the insulation distance. Their presence is indicative of a widespread damage to insulation, corona protection or field grading in the overhangs.

Off-line measurement is performed by applying to the windings the same AC supply used for tangent delta and capacitance at different voltage values (steps of 20% of the rated phase-to-ground voltage, up and down, also detecting values PD inception (PDIV) and extinction (PDEV). A capacitive coupler is connected to the line side of each phase (from the opposite side to the voltage supply) as a partial discharge sensor.
A measurement is also carried out by moving the capacitive sensor to the neutral side (also shifting the power supply) to detect any differences due to the different electrical stresses of the neutral bars during service.

On-line measurement is performed where permanent sensors are present, unlike the off-line measurement it does not change the voltage profile along the winding, but it can test the machine during its normal operating conditions, with all the degradation factors normally present (TEAM). Temporary sensor installation can be performed in case the machine is not equipped with permanent sensors (two outages are required for sensor installation).

The graph that collect PD measurement information is the Pattern (amplitude vs phase), which identifies the type of detected phenomenon.

Apply to
Stator (ohmic) and rotor (ohmic and impedance) windings

Needed instruments
Milli-ohmmeter, Insulation transformer, Variable transformer (Variac), Mutimeters, Speed-meter

Available instruments
Megger DLRO10HD
Trasformatore Belotti
Variac Belotti
Multimetri Fluke 189
Tachimetro Compact A2107

Reference standard
IEEE Std. 1434-2012
IEC 60034-27- 2

Description
The measurement of the ohmic resistance of the stator and rotor windings allows to check for any defects in brazings and shorted turns in the rotor windings. It is carried out using the volt-amp method, using integrated milli-ohmeters.

The static impedance of the rotor windings is measured by volt-amp method for different AC voltage values, that are applied to the winding by means of a Variac powered by an insulating transformer. The maximum applied voltage value corresponds to 70% of the rated voltage of the winding. In rotors of salient pole machines, the impedance measurement of the each pole is also performed at the maximum voltage value. This measurement allows to detect big insulation defects on rotor turns.

Dynamic impedance measurement is performed by supplying rotor winding from the collector rings, using the same method used for static impedance (volt-amp, insulation + Variac) for different rotation speed values. The measurement, when possible, is carried out for increasing or decreasing speeds. Current, voltage and speed (rpm) are continuously recorded. This measurement allows to check for shorted turns that are activated only during movement, as a result of centrifugal forces.

Apply to
Rotor windings

Needed instruments
Pulse generator, Oscilloscope

Available instruments
WEST Power pulse generator
Oscilloscopio Fluke Scopemeter 190-104

Description
The RSO test has the purpose to check the presence of shorted turns in the rotor windings and locate them, it also provides the possibility of locating grounded points in the winding. This measurement can be carried out rated speed and standstill conditions.
The measure is performed in two phases.

In the first phase, the pulse generator is connected to one of the rings and the impulse propagation time is calculated, ie the time required for the impulse to propagate through the entire winding. The rotor rings are, in this case, connected each to a channel of the oscilloscope.

In the second phase, the signal generator is applied alternately to both rings, their respective waveforms are saved and subsequently compared.

The test is successful if the traces of the two pulses are overlapped, unless small and normal deviations. A divergence between the two traces indicates the presence of a shorted turn, which can also be located along the winding, measuring the time between the beginning of the trace and the instant in which the traces begin to diverge.

Apply to
Rotor

Needed instruments
Oscilloscope
Multimeter
Current probes

Available instruments
Oscilloscopio Fluke Scopemeter 190-104
Multimetro Fluke 189
Sonda di corrente Fluke Flexi i2000

Description
During this measurement, the maximum value, the waveform and the spectrum of shaft voltages and currents are detected in the different operating conditions of the machine.

Knowledge of the frequency spectrum of shaft voltage allows to estimate the magnitude of defects in the rotor winding interturn insulation. The significant presence of equal harmonics is an indication of the presence of currents between shorted turns.
Rotor current measure allow you to check the current flow through the mass of the rotor body, bearings and seals.

The measurement of the voltage drop on the grounding device of the shaft, in addition to being functional to the measure of tree voltages, allows to check the effectiveness of shaft grounding.

Apply to
Rotor windings

Needed instruments
Oscilloscope

Available instruments
Oscilloscopio Fluke Scopemeter 190-104
Strumento WEST Power

Description
The measurement of the air gap flux is performed on machines that have the flux probe installed. This probe allows to visualize the waveform of the leakage flux in the airgap, from which information can be obtained about the presence of shorted turns in the rotor winding.

The main advantages of this measure consist in the fact that it is performed on-line, with all degradation factors acting normally on rotor windings without requiring any out of service (if the probe is already installed) and without having to change the configuration of the machine.

It is also the only method that allows exact location of the fault points. However, it would be advisable to carry out the measurements at different load values, as well as to consider that any bias in magnetic material reduce the sensitivity of the measurement itself.

The difference in the positive and negative spikes of the leakage flux generated by opposed slots (belonging to the same coil) of the waveform makes it possible to ascertain the presence of shorted turns.Measuring at different loads allows to move, along the polar circumference, the point of maximum sensitivity.

Apply to
Entire machine

Needed instruments
Vibration-meter
Fonometro integratore

Available instruments
Vibrotest 80
Bruel & Kjaer
Fonometro integratore

Reference standard
ISO 2954

Description
The measurement of mechanical vibrations is intended to assess the vibratory state of the machines for different operating conditions, to highlight possible imbalances and allow for appropriate maintenance work.

The vibration measurement will be carried out in addition to the shaft supports at different points, suitably located, which can guarantee a comparison with past and future measurements. Tests will be carried out at different loads, including no load and parallel with no load.

In cases where an electrical cause of vibrations will be verified, further tests, typically electrical measurements on the rotor, will be required.

Apply to
Entire machine

Needed instruments
Thermo-camera
Fonometro integratore

Reference standard
ISO 1680

Description
The acoustic pressure level relief is intended to check that the noise of the machine and its accessories is compatible with the presence of plant operators and visitors.

The thermographic detection of the machines during nominal induction magnetization tests or during service of the machine is used to highlight the presence of hot spots, useful for diagnosing possible electrical / magnetic problems and to plan maintenance works.

Apply to
Squirrel cage rotor of induction motors

Apply to
Stator windings

Needed instruments
HV DC generator
HV AC generator
Sphere Gap

Available instruments
Iris DC Ramp test 50
Doble M4100
Omicorn CPC 80

Reference standard
IEEE Std 95-2002
IEC 60034

Description
DC and AC Hipot tests are performed on the stator windings (each phase separately) to highlight the dielectric strength towards hearth and between phases.

DC high voltage test (up to 70 kV) is performed by connecting a DC high-voltage generator to the winding under test, interposing a sphere gap, able to limit any overvoltage. Overvoltage control is also possible within the HV generator itself.

The DC Leakage current test can be performed with different techniques, all possible with West Power’s proprietary instrumentation:

• Conventional, maximum applied voltage for 1 or 5 minutes, absorption of the absorbed current.
• Graded Time, the voltage is increased to steps of different duration to linearize the total absorbed current and allow more diagnostic information.
• Ramp, voltage is made to grow continuously and linearly, to compensate the capacitive current, and obtain useful information for diagnostics.

The AC high-voltage test is performed by connecting an AC high-voltage generator to the winding under test. The diagnostic content of this test is limited, allowing only to establish that the winding has passed / not passed the test, making it possible to put it / not put it back into service.

Apply to
Stator winding

Needed instruments
Hardness tester

Available instruments
Equotip 3
battitore D

Reference standard
–

Description

Checking the stator wedges hardness in pre-formed winding machines is a very important diagnostic test that allows for possible breakdowns that can cause considerable damage to stator winding and the core. In fact, due to time or poor installation, wedge loosening is a possible symptom of excessive vibration that can lead to disastrous failures over time.

The test consists of “tapping” with a calibrated hammer, equipped with an accelerometer, each single stator wedge (in 3 different points). The calibrated hammer applies a known force, the accelerometer measures the wedge “reaction”. This test substitutes or integrates the traditional test with the back of a key, providing objective results, independent of the beater’s ear.

A “map” of the stator wedges is then produced, in which each wedge has a color (green, yellow, red) depending on the hardness state.

The thresholds to which the color is assigned are agreed in advance with the customer.

Apply to
Magnetic core

Needed instruments
Thermo-camera
EL-CID instrument

Available instruments
Variatore

Description
The nominal induction magnetization tests (Loop test, Ring test) and reduced induction (EL-CID test) are intended to verify the integrity of the insulation of the magnetic laminations that make up the stator magnetic core.

The detection is performed by measuring the magnetic core overtemperature (Loop Test) or the magnetic flux generated by currents flowing between any shorted lamination.

Although the purpose of both tests is to detect any short-circuit between laminations, the technique used in the two tests is radically different, in Loop test thermal effect of shorted lamination is detected while with the EL-CID test short circuit currents are detected.

This fundamental difference allows the EL-CID test to be performed by exciting the magnetic core with a magnetic field value equal to 3-4% of the rated value, requiring a power supply with only few kVA of power, as opposed to the loop test which requires the nominal flux, having to map the temperature profile in the core. Performing a Loop-Test requires power supplies of several hundred kVA and several days, that is why the EL-CID test is performed much more frequently.

Apply to
Entire machine

Needed instruments
Video-endoscope

Reference standard
IEEE Std 62.2

Description
W.E.S.T. Power team can perform visual inspections on all types of alternators and, in general, rotating machines from 100 kW, with pre-wound windings.

The inspection covers all parts of the machine, both coupled with hydraulic turbines or thermal units.

Visual inspection with endoscope and mirrors, stator windings, end-winding supports systems, overhang insulation, conductive and semi-conductive paints in overhangs, ventilation ducts will be inspected using endoscope for detecting any damage to corona protection and obstruction of the same ducts due to material deposits. The stator core is inspected, detecting any deposits, oxidation due to loosening, moving blades, press-plate fingers. The rotor, when extracted, can be subjected to full inspection, check under the retaining rings to test the connections and insulation of the rotor end windings. The whole machine is carefully inspected in all its components.

Apply to
Static excitation systems & SFC

Needed instruments
Oscilloscope, Multimeter
Current clamp probe
Calibrator
Interface softwares
Drive per interfaccia SFC e sistema di eccitazione

Available instruments
PICO-Scope
Multimetro
Fluke clamp meter
Software and cables PLC SIEMENS e GE/FANUC
SSDTERM Software
T-Line Software

Description
Ordinary and extraordinary maintenance work. Troubleshooting. Damaged component replacement. PLC logic analysis. Courses at customer premises for SFC and excitation systems. Check performance and startup adjustments. Electrical measurements.

Apply to
Entire machine

Needed instruments
CTs
VTs
Voltage and current instruments
Shunt
Speed-meter
Oscilloscope

Reference standard
IEC 60034-4, CEI 2-5
IEEE Std 115

Description
Phase rotation check
The purpose of this test is to verify the correct cyclic sense of the phases, in relation to the marking of the terminals and the direction of rotation of the machine.

Symmetry of voltages and currents check
These surveys allow the verification of the degree of asymetry and impurity of the voltages and currents of the machine. Calculate (and / or measure) the values, for voltages and currents, of direct, inverse, and homopolar sequences;
Based on these values, the degree of dissymmetry of the voltage and current bays is determined.

Spectrum analysis of voltages
The spectrum analysis of the voltage waveform is performed, for the calculation of the voltage THD, total harmonic distortion.

Short-circuit tests
The three-phase short circuit characteristic at the rated speed, corresponding to the induction current graph vs to the excitation current, is traced.

No load tests
Core saturation test with relief of the no load curve at rated speed, the pattern of the no load voltage is traced vs excitation current.

Capability curve
Through the measurement of temperature, current, voltage, active and reactive power, the main points of the limit performance curves, also known as capability curves, are determined.

Apply to
Entire machine

Description
Slot contact resistance check (bars and coils)
The measurement of the contact resistance in the slot, between bars and the stator core, allows to verify the integrity of the bar corona protection, and possibly the need for better side fixing of the bars inside the slot.

Diode and brush control of excitation systems
Brushes and collector rings are checked to check for any stains, irregular wear and tear. The continuity of the diodes is measured in direct mode, and the voltage is held in reverse mode.

Check the thermal resistance (RTD) function
The correct indication of the RTDs installed on the machines is verified. The control will be done with machine at constant temperature.