1. Measurement of shaft voltage and shaft current on machines with sleeve bearings
Table of Content:
1 Objective............................................................................................................
2 Scope.................................................................................................................
3 Referenced Documents ....................................................................................
4 Definitions and Abbreviations............................................................................
5 Definitions .........................................................................................................
6 Measurement.....................................................................................................
6.1 General ..........................................................................................................
6.2 Conduction of test (Measuring points acc. to schematic diagram) ................
7 Assessment ......................................................................................................
8 Setting of shaft current protection......................................................................
9 Proceeding in case of deviations ......................................................................
10 Copyright..........................................................................................................
1 Objective
Description of shaft voltage and shaft current measurement.
Verification of proper functioning of bearing insulation.
2 Scope
This instruction is applicable to all machines with sleeve bearings of the cross function of
Generators
3 Referenced Documents
IEEE Std. 112-1991, § 9.4 (Induction machines)
IEEE Std. 115-1995, § 3.6 (Synchronous machines)
ETZ-A Bd. 85 (1964) H4, S. 106-112
Shaft voltages and currents, causes, effects and remedies).
4 Definitions and Abbreviations
TE Drive end resp. turbine end of a machine
EE Non-drive resp. exciter end of a machine
5 Definitions
Shaft voltage Ush is a low frequency AC voltage generated between the shaft ends.
Main causes are irregularities in the magnetic circuit, design features, elliptic
deformation of the stator, eccentricity of the rotor alignment (i.e. non-uniform air gap) etc.
In general Ush has its maximum value below rated voltage because of saturation effects.
Shaft currents (bearing currents) Ish are forced by Ush to flow through the shaft,
bearings, bearing supports, machine framework, and back to the other end of the shaft
in the case that there is no bearing insulation at all, or when it is defect or externally
short-circuited. Ish depends essentially on the ohmic resistance of this circuit (in the
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2. order of mOhms) and might damage the bearings and thus impair operational safety of
the machine.
6 Measurement
6.1 General
- Generally the EE- bearing of a machine is insulated against mass. In order to enable a
check of the bearing insulation also after final assembly some machines (bulb
generators always) are equipped with double bearing insulation (see diagram). Between
the bearing insulation a potential free tap is inserted. By means of this contact correct
functioning of the bearing insulation can be verified, with the machine at rest or in
operation. Furthermore safety against circulating shaft currents is significantly increased.
- Most frequently a shaft grounding brush is installed at the turbine end, which bridges
the oil film of the TE-bearing and grounds the shaft at TE-side in a defined manner (this
is to prevent static charging of the shaft, especially for steam turbine sets). In this case,
shaft voltage can be measured between the EE of the shaft and the frame (on ground
potential), provided there is good contact of the shaft ground brush (no pollution on the
running track, no oil etc.)
- Measurement of Ush is performed by connecting a low -range AC voltmeter (true
r.m.s., highimpedance), since Ush always contains harmonics. It is recommended to
take also an oscillogram of Ush, so as to have a fingerprint for further investigations in
case of later trouble.
- The machine is run at rated speed and with armature open circuited (unless otherwise
specified). Normally measurement of shaft voltage is performed together with the
measurement of the open-circuit saturation characteristic, in a voltage range from 10 to
120% rated voltage, or only at rated voltage respectively.
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3. - Before starting the measurement, check that there are no parallel paths that bridge the
bearing insulation (like pipes and conduits, armatures, grounding of devices that are
normally insulated etc.)
- Ish is measured by connecting an AC ammeter into the circuit (large range, w ith low
resistance leads and a regulating protective resistor in series to demonstrate flowing of
shaft current), respectively by means of a (permanently installed) shaft current
transformer.
Note:
In order to measure the true shaft current without introducing an error by the internal
resistance of the measuring instrument inserted into the current circuit, a magnetic
voltmeter (Rogowskicoil) may be used. It is not necessary that this voltmeter wrapps
completely around the shaft (the line integral of the magnetic field strength may also be
formed by splitting the circumferential path and summating the different readings).
6.2 Conduction of test (Measuring points acc. to schematic diagram)
Measurement of the shaft voltage (or shaft current respectively) is performed by
pressing an auxiliary brush, which is connected to a measuring device, onto the rotating
shaft, for safety reasons in trailing circumferential direction. Also make sure that the
brush is in good contact with the shaft surface.
The auxiliary brush is preferably metallic, for instance a fine-stranded cable, which is
protected by an insulating tube. Insulation has to be stripped for approx. 10 cm at the
cable end that is to be pressed against the shaft. At the other end a measuring lead can
be directly connected.
Measuring points are according to the schematic diagram (see above). With only one
insulated bearing and presence of a shaft ground brush, measurement of Ush is
performed from the EE of the shaft 2 to frame mass 4. With insulated bearings at both
ends, Ush is either measured between both shaft ends 1-2, or on the TE a temporary
shaft ground brush is mounted for the purpose of measurement. In this case
measurement can again be performed between EE and frame mass.
Demonstrate that bearing insulation is effective in the following way: bearing oil film is
bridged at both ends with brushes, whereby the measuring brush is connected to a
resistor (regulating able from 0 to 100 Ohms) in series with an ammeter. Flowing of shaft
current will be indicated, which will disappear when one of the brushes is lifted. This test
has to be performed very quickly just to prove that in normal service no shaft currents
will flow, as well as for setting and checking of an eventual shaft current protection
device (see below).
For bearings with double insulation, the insulation resistance is measured between the
potential tapping and frame mass 4, as w ell as between potential tap and shaft 2, in
order to verify that the bearing insulation is effective.
7 Assessment
For the amplitude of Ush no generally valid limit values can be given. In some cases
values up to approx. 70 V rms have been observed.
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4. Note:
According to literature, Ush < 300 mV will certainly not do any harm to sleeve bearings.
To produce breakdown of the bearing insulation, in general several volts are required.
8 Setting of shaft current protection
For the setting of shaft current protection, values between 200 and 1000 mA are
recommended. Frequently stray fields are present at the place w here the shaft current
transformer is installed. When limit values are set too low , this may in certain operating
conditions cause erroneous tripping of the shaft current protection. With the chosen
settings, the protection should not trip in any operational condition (i.e. from energizing
at no-load up to full load). However, try to set the lowest possible value between 200 and
1000 mA, where a trip just does not yet occur.
When setting in the range up to 1000 mA is not possible at all without causing erroneous
tripping, the design department must be contacted for a displacement of the shaft
current transformer.
9 Proceeding in case of deviations
Any nominal value, minimum value, maximum value, tolerance etc given in an instruction
is intended to serve for quality control purposes only. In the case of deviations, a
nonconformity report has to be written according to PNG Power.
10 Copyright
This Document is properly of PNG Power internal use only and must neither be copied.
Neither is it to be handed over nor in other way communicated to a third party.
Infringement will lead to prosecution.
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