FREQUENCY-RESPONSE ANALYSIS (AKA: BUMP TESTING)

Just as every bell has a unique tone when struck by a hammer, so do all generator winding components.  Every structure, in fact has its own characteristic modes of mechanical response when deformed after being struck.  While not very musical in nature, the response waveform gives the trained Technician valuable information on the integrity of the windings under test.

Bump testing can be done discretely (giving specific results for each specimen struck), or as a composite modal wave shape function where some of the specific information about individual specimens gets lost in favor of getting system-level resonant response information.  The two major design philosophies are split between GE & Westinghouse, which dates back as far as the companies respective founders, Edison and Westinghouse.

End winding resonance testing determines the mechanical resonant frequency of each of the series and phase connections.  Each of the series and phase connections are struck with an instrumented mallet equipped with an accelerometer, and the responding oscillation is detected by an accelerometer mounted on the specimen winding.  The data are sent to a Fast Fourier Transform processor where the stimulus and response are analyzed in the frequency domain.  Y(s) = H(s)*X(s) is a standard transfer function relationship where Y(s) is the output in the frequency domain (response), X(s) is the forcing function (how much energy you put into the system and at what frequencies), and H(s) is the transfer function that is the “fingerprint” function of the system under test.  This transfer function is displayed graphically as amplitude versus frequency, so that the resonant peak frequencies can easily be identified from a graph.

The resonant frequency of an object is the frequency where the system most efficiently accepts energy from an outside stimulus.  In the case of generator windings, the “system” is composed of the end windings, and the stimulus the twice-per-revolution mechanical force that acts upon the windings.  In the 60 Hz grid the forcing function will be 120 Hz; similarly for 50 Hz the forcing frequency will be 100 Hz.

In general, the current-carrying components of the generator must be mechanically designed in such a way that their resonant modes are not near this forcing frequency.  The word “near” means avoiding the band of -5% to +10% of the forcing frequency, or (115 – 135 Hz, for a 60-Hz unit).

Each OEM has certain parameters for determining acceptable levels of resonance.  Sidewinders applies the most sensitive criterion of all the OEMs, which is 0.20 g/Lb-f (read as: g’s per pound force).  Working through Newton’s Second Law of motion, F=ma, if you solve for units of [acceleration/force] you arrive with units of reciprocal mass (1/m).  The higher the mass, the lower this fraction; the lower this goes, the lower the amount of acceleration you will see.  This is a perfect moment to segue into the topic of tuning.

If you’ve ever played with a guitar, you quickly saw that the thicker, longer strings vibrated with the lowest frequency.  You probably also saw that if you tightened any string, the frequency went up.  You also noticed that if you put your finger down somewhere on one of the frets, the frequency went up.  We apply these ideas to bump testing as follows:

If a unit displays resonance, we first must characterize it as being above, or below the desired frequency band.  If above, we may apply “low tuning”, and as you might expect, if the results are below the desired range we would apply high tuning.

A way to low-tune a generator would be to either add mass, or increase the spacing between unsupported spans of the winding.  Sidewinders generally prefers to “high tune” machines, as this generally involves adding additional means of structural rigidity.  Ways to high-tune a winding system include:  adding additional blocking, applying wicking & flooding resins, replacing failed ties, adding nose rings, and in the case of certain Siemens-Westinghouse and MHI units, we can re-tension radial banding or radial studs.   All of these measures tend to drive frequency response upward.

FREQUENCY-RESPONSE ANALYSIS (AKA: BUMP TESTING)

Screenshots before (L) and after (R) Sidewinders made repairs due to bump test findings

Sidewinders has attended to many cases where a serious in-service failure occurred due to an unmitigated resonance issue that could have been detected and corrected by means of this testing modality.

Bump testing is a fast, economical way to gather valuable information about the condition of your generator and help predict and avoid costly failures!  Contact Sidewinders for a bump test quotation for your next planned outage.

Insulation Resistance and Polarization Index Testing of Generators

Megger & P.I. as it’s commonly referred to, is one of the quickest, safest, and simplest tests in the arena of generator electrical testing, but it is also one of the most useful.  It is this simplicity that often makes the test somehow seem less important than some of the “big ticket” tests such as Hi-Pot or ELCID.  Megger and PI is a very quick way to get an overall assessment of the health and cleanliness of an insulation system in just a few minutes’ time.

 

Sidewinders makes engineering evaluations based on the overall analysis of many tests and inspections considered as a whole, but the Megger & PI is considered the front line test when evaluating the condition of an insulation system.  The main goal of a test & inspection job is to verify that all insulating systems properly confines the flow of electricity within the conductors by means of Megger & P.I.; and that all conductors allow the unimpeded flow of current by means of DC resistance testing using a digital low resistance ohmmeter (DLRO).  Copper should pass current freely, and insulation should block the flow of current.

 

The megger test, as with most electrical systems, is best understood when an analogy is made to a piping system.  Take the common 50-foot long garden hose, for example.  Initially, the hose is empty and has zero pressure, and is closed at the far end.  The instant the faucet is turned on, the hose swells up, and you can hear and feel the water current rushing in as it charges the hose to the same pressure as the supply faucet, say 50 PSI.  If the hose doesn’t have any leaks, the current will stop once the hose has fully charged to 50 PSI.  If the hose has some microscopic leaks, there will be a small trickle current which could be measured as the leakage current.  Substituting voltage for pressure, and current for flow, we can make a direct analogy to an electric winding insulation system.  During the megger test, the winding will initially accept large current flows as the unstressed insulation becomes stressed up to the test voltage of say, 5000 volts.  Once the insulation reaches the test voltage, the insulation molecules continue to rotate and orient themselves to be parallel to the electric field lines of force.  After a period of ten minutes, the system is considered to have reached steady state, and any remaining current going into the winding is assumed to be purely due to leakage through imperfections in the insulation system.

Insulation Resistance and Polarization Index Testing of GeneratorsThe polarization index is a measure of how much the insulation system resistance improves with time.  Let’s return to the garden hose.  A perfect garden hose with the other end closed off would exhibit zero current 10 minutes after the pressure was turned on, and the hose could not expand any further.  If the other end was wide open, the flow at 10 minutes would be exactly the same as it was the moment the hose was turned on.  Taking the 10 minute flow divided by the 1-minute flow would give a ratio of 1.0.  In an electrical winding system, if there is a large amount of contamination, or a major breach in the insulation system, the leakage current would be large compared to the inrush charging current, and the ratio of the 10-minute resistance divided by the 1-minute resistance would be close to 1.0.

In a winding system, most OEM’s recommend a PI value of 1.25 or higher on a rotor winding, and 2.0 or better on a stator winding.  The reason for the difference lies in the fact that most generator rotors are an “open” insulation system, with naked conductors in the end winding potion and thus it is expected to tolerate lower insulation and P.I. values.  In stators, the voltages are far higher, and the insulation system is “closed”, that is, the entire length of the copper windings are entirely enclosed in insulation and thus we would expect higher insulation values.

 The polarization index is calculated as 

    \[PI  = \frac{R_{10}}{R_1}\]

where R10=the ten minute resistance reading, and R1 is the 1-minute reading.

In a perfect world, all winding systems would exhibit both high PI & high resistance values.  But we don’t live in a perfect world—we encounter units that are very old, facing demanding operating conditions, heavy contamination, and environmental effects such as humidity and intrusion of salt mist for seaside units, or industrial contamination such as sulfides or heavy metals for units in caustic environments.  Surface contamination and humidity are the most common causes of low resistance and/or PI values.  If we encounter low test values, the first step is to inspect the system and clean up any areas that appear to have contamination.  Low PI values are most often corrected by cleaning, followed by the application of dry heat for 12-36 hours.  Based on our experience, 95% of units will drastically improve after cleaning and dry out.  Units with electrical defects in the ground wall insulation will usually not improve after these efforts.  In these cases, further investigation is required.

As is often the case, marginal readings where the PI is good, and the Insulation Resistance is lower than we would like to see, we have OEM guidance which helps us make a tradeoff between the two factors.  For example, a unit has 2 gig-ohm resistance, but a low PI of 1.05, we could make the determination that the unit was suitable for service due to the excellent IR values and attribute the low PI to humidity.  Conversely, a resistance value of 25 meg-ohms and a PI of 2.6 would also constitute a unit that is safe to return to service.  In either of these cases, we would recommend performing additional testing to determine the nature of the defect and provide a plan for improving these readings.  Virtually anyone can perform the test; but it takes a highly trained and experienced operator to interpret the data and make the right call on what to do if the readings are less-than-perfect!

Sidewinders takes many factors into consideration, including but not limited to:

  • The OEM of the unit
  • The cooling technology (air-, hydrogen-, or water cooled)
  • The unit vintage
  • Plant-specific factors including environment, temperature, humidity, and altitude

The megger and PI test is one example of how Sidewinders’ experience and technical acumen can be instrumental in helping you ensure the maximum reliability of your generating assets.

HIRE THE EXPERTS –SIDEWINDERS, YOUR RELIABILITY PARTNER!

Insulation Resistance and Polarization Index Testing of Generators